David E. Moncton- X-ray Laser User Facility at Bates Laboratory

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Proposal to the National Science Foundation

for a Conceptual Design Study:

X-ray Laser User Facility

at

Bates Laboratory

Massachusetts Institute of Technology

Principal Investigator

David E. Moncton

Co-Principal Investigators Science Collaborators

William S. Graves Simon Mochrie Keith A. Nelson

Franz X. Kaertner Gregory Petsko Dagmar Ringe

Richard Milner Henry I. Smith Andrei Tokmakoff

Bates Senior Staff Contributors

Manouchehr Farkhondeh William M. Fawley James FujimotoJan van der Laan Hermann Haus Erich Ippen

Christoph Tschalaer Ian McNulty Denis B. McWhanFuhua Wang Jianwei Miao Michael PellinAbbi Zolfaghari Mark Schattenburg Gopal K. Shenoy

Townsend Zwart

April 2, 2003


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SUMMARY

Recent advances in accelerator, laser, and undulator technology have created the possibility of

constructing a national user facility based on an intense free-electron laser at extreme ultraviolet and x-ray

wavelengths. MIT is exploring the construction of such a facility at its Bates Laboratory site. The facility

would produce x-ray beams with peak brilliance some ten orders of magnitude greater than is presently

available from todays synchrotron sources, and pulse durations from 100 femtoseconds to less than 1femtosecond. The wavelengths produced will range from 0.3 to 100 nm in the fundamental, with

substantial power in the x-ray 3rd harmonic at 0.1 nm. The possibility of future upgrades to even shorter

wavelengths will be preserved in the design. Based on a 4 GeV superconducting linac incorporating a

number of extraction points, the complex will include the potential for twenty or more undulators and

x-ray beamlines.

In order to produce beams of the highest quality, various methods of seeding the electron beam

with high harmonics of laboratory lasers are currently under investigation, as is lasing by self-

amplification of spontaneous emission. A number of these methods will be exploited to produce radiation

sources matched to experimental needs. Advanced laser technology, used to seed the electron beam for

short pulse production, will also be used to produce the electron beam and for use in pump-probeexperiments. Thus, lasers will be an integral part of the facility.

As described in the body of the proposal, the scope of science possible with such a facility is both

broader and in some sense deeper than that pursued at todays synchrotron facilities or laser sources,

because it combines high power and coherence for the first time in the 100-0.1 nm range. The science that

is foreseen spans many disciplines including atomic and fundamental physics, condensed matter physics

and materials sciences, femtochemistry, structural biology, and various fields of engineering. The source

we propose, and the experimental methods it will spawn, will generally be qualitatively new and have

high impact in many fields of science and technology. The strength of the science and technology base in

the northeast region, and in particular at MIT, make this a superb location for such a facility.

But MIT is ideally suited to be the host institution for an equally important reason: the remarkableeducational opportunities that such an endeavor would create and sustain. Beyond the enormous impact

on graduate and post-graduate education of a facility with such a broad user science program, is the

opportunity to provide education in the underlying accelerator sciences and technologies. Such

technologies are becoming increasingly critical to society, but they are difficult, if not impossible to teach,

without the existence of working accelerators. A program at MIT would have immense impact in

developing a future generation with skills in this area. The presence of such a laboratory will also allow

enhancements of novel teaching methods for K-12 students and teachers. Many of these educational

opportunities will be exploited early in the proposed study by using infrastructure that currently exists at

Bates Laboratory.

The scale of the facility, with its technical infrastructure, is ideally matched to the 80-acre, 1.2 km

long Bates site. The scope of such a project has been under study at Bates by a small group for nearly ayear. The three-year study proposed here will, in the first 18 months, produce a conceptual design, an

R&D plan, a cost estimate and schedule, and a detailed scientific case, as integral parts of a proposal for

construction beginning in FY2007. During the balance of the proposal period, R&D will be undertaken, a

management plan developed, and the facility design advanced to the point necessary to begin

construction. It appears that the capital cost for the facility will be around $300M, assuming construction

beginning in FY 2007, and depending on the number of beamlines implemented with construction funds.

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TABLE OF CONTENTS

SUMMARY........................................................................................................................................... iii

INTRODUCTION ................................................................................................................................. 1

1 SCIENCE AND EDUCATION................................................................................................... 2

1.1 Overview of Source Properties........................................................................................... 2

1.2 Science ............................................................................................................................... 4

1.2.1 Nanoscale Dynamics with X-ray Transient Grating Spectroscopy....................... 5

1.2.1.1 Introduction........................................................................................... 5

1.2.1.2 The MIT/Bates X-ray Laser.................................................................. 6

1.2.1.3 X-ray Time-Resolved Transient Grating Spectroscopy........................ 6

1.2.1.4 Beamline Concept ................................................................................. 9

1.2.1.5 Proposed Work...................................................................................... 9

1.2.2 Nanoscale Dynamics with X-ray Photon Correlation Spectroscopy..................... 11

1.2.2.1 Introduction........................................................................................... 111.2.2.2 The MIT/Bates X-ray Laser.................................................................. 11

1.2.2.3 Beamline Concept ................................................................................. 11

1.2.2.4 Experiments with Photon Correlation Spectroscopy ............................ 12

1.2.2.5 Proposed Work...................................................................................... 14

1.2.3 Femtosecond Spectroscopy in Solution-Phase Chemical

and Biophysical Systems....................................................................................... 15

1.2.3.1 Introduction........................................................................................... 15

1.2.3.2 The MIT/Bates X-ray Laser .................................................................. 16

1.2.3.3 Time-Resolved X-ray Spectroscopy of Molecular

Dynamics in Solution............................................................................ 16

1.2.3.4 Beamline Concepts and Proposed Work............................................... 17

1.2.4 Trace Chemical Analysis in the Particle Counting Limit...................................... 20

1.2.4.1 Introduction........................................................................................... 20


1.2.4.3 Experimental Methods .......................................................................... 22

1.2.4.4 Proposed Work...................................................................................... 23

1.2.5 Structural Biology ................................................................................................. 24

1.2.5.1 Introduction........................................................................................... 25


1.2.5.3 Experimental Concepts ......................................................................... 27

1.2.5.4 Proposed Work...................................................................................... 32

1.2.6 Electron Dynamics with Attosecond Resolution................................................... 34

1.2.6.1 Introduction........................................................................................... 34


1.2.6.3 Experimental Concepts ......................................................................... 35

1.2.6.4 Proposed Work...................................................................................... 36

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TABLE OF CONTENTS (CONT.)

1.2.7 X-ray Microscopy With Atomic Resolution ......................................................... 37

1.2.7.1 Introduction........................................................................................... 37

1.2.7.2 X-ray Microscopy: Source Considerations........................................... 381.2.7.3 X-ray Microscopy: Real-Space Methods.............................................. 39

1.2.7.4 X-ray Microscopy: Reciprocal Space Methods .................................... 42

1.2.7.5 Conclusions........................................................................................... 45

1.2.8 Nanometer Lithography ........................................................................................ 46

1.2.8.1 Introduction........................................................................................... 46

1.2.8.2 Achromatic-Interferometric Lithography.............................................. 46

1.2.8.3 Zone-Plate Array Lithography.............................................................. 47

1.2.9 Status of Scientific Programs at Operating UV FELs ........................................... 48

1.2.9.1 Low Energy Undulator Test Line (LEUTL) at ANL ............................ 49

1.2.9.2 Deep Ultraviolet FEL at BNL............................................................... 49

1.2.9.3 Tesla Test Facility at DESY.................................................................. 511.3 User Program...................................................................................................................... 52

1.4 Education and Outreach Program....................................................................................... 53

1.4.1 Programs During Design Study............................................................................. 54

1.4.2 Planning for Programs During Facility Construction and Operation.................... 55

2 TECHNICAL CONCEPT SUMMARY ...................................................................................... 56

2.1 Facility Description............................................................................................................ 57

2.2 Injector ............................................................................................................................... 57

2.3 Linac................................................................................................................................... 59

2.4 Conventional Lasers and Seed Generation......................................................................... 59

2.5 Undulators .......................................................................................................................... 602.6 FEL Properties.................................................................................................................... 60

2.7 Photon Beamlines............................................................................................................... 63

2.8 Comparison to Other Sources............................................................................................. 64

2.9 Research and Development Program ................................................................................. 67

3 THE BATES LINEAR ACCELERATOR CENTER.................................................................. 69

4 INTERAGENCY AND INTERNATIONAL COOPERATION................................................. 70

4.1 Interagency Cooperation .................................................................................................... 70

4.2 International Cooperation................................................................................................... 71

5 DELIVERABLES AND PROPOSED SCOPE OF WORK ........................................................ 72

REFERENCES ...................................................................................................................................... 76

APPENDIX A TECHNICAL CONCEPT ........................................................................................... A-1

APPENDIX B PROJECT PLANNING.............................................................................................. B-1

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INTRODUCTION

The fields of laser and accelerator technology stand now at a point of remarkable opportunity: the

creation of highly coherent, powerful and ultra-short pulses of x-ray radiation ranging in wavelength from

100 to 0.1 nanometers. The x-ray laser has been the Holy Grail of both the laser and the x-ray research

communities since the invention of the laser. Now, not only can such a device be built, but it can also be

part of a very cost-effective national user facility providing many independent photon beamlines fordifferent areas of research. When contemplating the impacts of an x-ray laser on human knowledge and

technology, think of the independent impact of the x-ray and the laser as the starting point. All of that

scientific and educational benefit has been produced without any laser reaching the x-ray regime and

without any x-ray source producing coherent radiation. That is now possible for the first time, and it is so

compelling that in comparing the x-ray laser with current sources, a Global Science Forum sponsored by

the Organization for Economic Cooperation and Development (OECD) held in September 2001 [1]

concluded that, because of the vast increment in performance, it is very likely that entire new types of

scientific measurement and applications will be enabled. Now, eighteen months after that statement was

written, most experts would call it an understatement.

Three key elements of the facility we envision would make it unique. First, a 4-GeV linearaccelerator with superconducting radio frequency cavities would produce such high electron pulse rates

that twenty or more beamlines could be extracted to serve a large user community. Second, integrated

high-harmonic generation laser technology would seed the electron beam and generate photon beams with

high longitudinal coherence and pulse lengths significantly below 100 femtoseconds, perhaps below 1

femtosecond. Third, taking advantage of the ability of linear accelerators to extract beams at different

energies, we envision a facility spanning both the traditional extreme ultraviolet and x-ray wavelength

range. This approach provides for integration and synergy between the UV and x-ray communities and

the laser community, where scientists are anxious to move to wavelengths shorter than conventional

table-top technology can provide with high pulse power. Here, we propose a three-year study to develop a

construction proposal, optimizing the machine for the remarkable science and education opportunities it

will enable, and proceeding with the necessary preparations to be ready for construction in 2007.

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1 SCIENCE AND EDUCATION

The potential of x-ray laser radiation for science and education is extraordinary. One might

determine the structure of a single molecule with one x-ray pulse without the need for crystals; probe the

dynamics of atoms, molecules or of condensed matter on fundamental time scales and length scales

simultaneously; study the properties of matter at very high energy densities; improve technologies for

fabricating, inducing, and observing structure at the smallest length scales; and probe and exploit non-linear phenomena in the x-ray regime. It can be safely said that the x-ray laser promises to be a

comprehensive probe of spatial and temporal structure on all scales and at all resolutions relevant to all

forms of condensed matter. And it will be an exquisite tool to manipulate matter as well.

Educational opportunities are also exceptional for many reasons. The first is the scope of science

that the facility would support, with almost every experiment involving students as is common at todays

synchrotron radiation facilities. The success of the User Program discussed in Section 1.3 is essential in

this regard. Second, the technology associated with the facility spans many fields of physics and

engineering, thus providing a context for development of a curriculum in accelerator science and

technology, based on academic excellence at MIT and surrounding universities. Accelerators are

becoming increasingly important to society, but few schools offer substantial programs. Third, theexistence of a large-scale high-technology construction project would open many one-of-a-kind

experiences for students and teachers in a rich array of subjects from environmental planning and

architecture to project management. Finally, MIT has been a leader in educational innovation, including

the TEAL (Technology Enabled Active Learning) classroom [2], iLAB [3], the CMSE RET (Center for

Materials Science and Engineering Research Experience for Teachers) program [4], and the Cambridge

University collaboration [5], to name a few. These activities have reached well beyond traditional

graduate and undergraduate populations to include K-12 students and teachers, with a strong emphasis on

diversity. All of these experiences will be enhanced and new ones will be generated by the proposed

study, the eventual construction, and ultimately, the scientific operation of the x-ray laser facility.

1.1 OVERVIEW OF SOURCE PROPERTIES

Photon Beam Parameters. The extraordinary science potential of an x-ray laser source is a

direct consequence of the photon beam parameters and how they compare to other sources, as discussed

in detail in Section 2.8. The beams will have a tunable wavelength range from 100 nm to 0.1 nm

(wavelengths shorter than 0.3 nm will be obtained from higher harmonics); they will be fully coherent in

the transverse direction; they will be pulsed at a rate of 1 kHz, and each unseeded pulse will have a

duration up to 100-200 femtoseconds, contain up to 1 mJ of energy, and have a bandwidth of about 0.1%.

Of course it is the peak brilliance that is the strong suit of these machinessome ten orders of magnitude

greater than current sources and having pulse lengths 1000 times shorter. As we will see, a great deal of

the proposed science is driven by techniques exploiting these characteristics to access new regimes of

temporal behavior. However, the time-average brilliance properties also exceed third-generation

synchrotron sources by several orders of magnitude, and the flux is comparable at similar bandwidths.

Photon Degeneracy. High peak brilliance is a deeper concept than simply having a large number

of photons at once. For the first time, we can begin to think about an x-ray source having a large number

of photons occupying a single quantum state at one time. This parameter, called photon degeneracy, is

less than one for third-generation synchrotron sources at wavelengths of 0.1 nm and is only a few hundred

at 100 nm. For the x-ray laser, it will be of order 109. This increment of seven to ten orders of magnitude

in an important source parameter will have revolutionary scientific impact. The x-ray laser is not just a

better source than a traditional synchrotron; it is a qualitatively different kind of source. It would be more

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appropriate to call these sources first-generation x-ray lasers, than to call them fourth-generation

synchrotron sources. In fact, for most of the venerable experimental methods of hard and soft x-ray

research, it is essential that photons interact singly with the sample. Synchrotron facilities will continue to

be the workhorses of the field for some time to come. As will become apparent, the new experimental

methods appropriate for an x-ray laser have more in common with optical laser methods and many have

no analog at existing synchrotron sources.

Seeded Beams. In the preceding paragraph, we have described the unseeded beam parameters

essentially as a starting point for discussing the improvements obtained by seeding, as we propose with

high-harmonic generation from conventional Ti:sapphire lasers. The longitudinal structure of the

unseeded pulses is complex; the structure is not transform-limited and it is statistically noisy. Seeding has

the potential to reduce or eliminate these shortcomings, producing pulses that are transform-limited with

duration perhaps below 1 femtosecond, with correspondingly lower pulse power. The science we envision

will be greatly enhanced by, and in some specific cases entirely dependent on, these improved

characteristics. Successfully implementing this technology is challenging, but it will pay huge scientific

dividends. There appear to be no fundamental technical barriers. It looks straightforward at the longer

wavelengths, with increasing difficulty as one approaches the hard x-ray regime. The synergy generated

by the technical challenge, followed by the scientific pay-off of producing stable, reliable, transform-limited pulses approaching 0.1 nm wavelength, will be the intellectual driving force during the early

operation of this facility.

Fundamental Lengthscales and Timescales. Over the centuries, improvements in the optical

quality of photon sources have enabled measurements with improved resolution. With the development of

x-ray laser sources, an enormous step is now possible before physical limits are reached. Consider spatial

resolution. A laboratory x-ray tube enables x-ray images with about 0.1 mm resolution, typical of a

medical x-ray. The introduction of synchrotron sources moved that resolution to a few microns, and third-

generation undulator sources have the potential to resolve features below 100 nm. A concept exists for a

30 nm nanoprobe facility at Argonne National Laboratorys Advanced Photon Source. An x-ray laser

will have full transverse coherence, which is the equivalent of achieving the diffraction limit. In that limit,

a microscope is theoretically possible with resolution equal to the wavelength of the radiation.

In extracting static structure information, reciprocal space methods, commonly called x-ray

diffraction techniques, are perhaps even more important than real-space microscopy. The challenge of

extending diffraction methods to larger length scales is the conjugate of the problem, described above, of

extending real-space methods to shorter length scales. The full transverse coherence of an x-ray laser

beam will permit the perfect diffraction experiment, that is, one in which the resolution of the

diffraction peaks could potentially be the inverse of the size of the beam illuminating the sample. This is a

direct consequence of the uncertainty principle, and the concept of diffraction limit. The implications of

the situation are simple and powerful. With full transverse coherence, it will be possible to elect either

real space methods or reciprocal space methods over the entire range of spatial length scales (say 0.1 nm

to 0.1 mm) relevant to condensed matter. Indeed, in this limit, the most powerful approach is not to

separate imaging and diffraction, but to combine them in one general method referred to as coherentimaging. Implications for the study of non-periodic structures and, particularly, individual molecules, are

profound.

We now turn to an analogous discussion in the time domain, where the characteristics of the

beams we propose, particularly those seeded to approach or reach the transform limit, will be

revolutionary. Consider photoemission, or inelastic x-ray or neutron scattering. In these techniques, the

probe exchanges energy with the sample and the measurement of that energy transfer, as a function of

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momentum transfer, gives fundamental information on the dynamics of electrons, spins, phonons and

other quasi-particle excitations in condensed matter systems. The accessible region of energy resolution

for x-rays is from a few electron volts to 1 meV, with increasing difficulty at the best resolutions. For

neutrons, the available resolution range is shifted to smaller energies by a factor of 10 to 100, but the

limited flux of neutron beams means large samples are required. Neutron techniques cannot be widely

applied to microscopic samples, monolayers, or small particles containing a few atoms, molecules, or

clusters.

The x-ray laser would open a vast new dynamical regime by permitting studies to be carried out

in real time. There will be two distinct advantages of this capability. First, energy ranges accessible to

conventional inelastic x-ray and neutron scattering can be probed in real time, rather than in the energy

domain. In many complex systems, the dynamics are not well-described or understood in terms of

harmonic quasi-particle excitations whose excitation energy is greater than their inverse lifetime. In

many cases, the critical motions are large conformational changes involved, for example, in protein

function or chemical reactions. Second, and equally important, real-time methods will naturally extend to

times that correspond to energy resolution well outside the range of traditional energy-domain methods,

for example to nanoseconds (i.e. microvolts), or microseconds (i.e., nanovolts). Anticipating success in

seeding beams to get pulse lengths of 1 femtosecond, we will have a probe capable of accessingessentially all relevant timescales, from those of atoms and molecules to those in condensed matter in

virtually all forms. Not only that, but such a probe, to the extent that its wavelength approaches 0.1 nm,

will be generally sensitive to the structural character of the dynamics.

1.2 SCIENCE

Over the last decade, but with increasing activity in the last five years, the prospect of accelerator-

based x-ray laser sources has generated immense excitement in a growing scientific community. We

estimate that over 100 workshops have been held leading to the development of four 100-plus page

scientific cases for proposed new facilities: the Linac Coherent Light Source (LCLS) at Stanford, the

TESLA XFEL at the Deutsches Electron Synchrotron Laboratory (DESY), the SASE-FEL project at the

Berlin Electron Storage Ring for Synchrotron Radiation (BESSY), and the 4GLS project at the DaresburyLaboratory in the UK. High-level committees in the respective countries have favorably reviewed these

cases. Virtually all the work proposed in these documents can be undertaken at the proposed MIT x-ray

laser facility. Our website (http://mitbates.mit.edu/xfel/index.htm) contains these documents in their

entirety. Furthermore, beyond the paper studies, three demonstration facilities have been built to

produce radiation in the range of 100 nm providing the incentive for the development of real experimental

programs, all of which could also be carried out at the proposed MIT facility.

In consideration of all of this activity, we have concluded that the general case for the x-ray laser

has been made clearly and convincingly. Our goal in this document is not to repeat that work, but rather

to move toward the development of more specific experimental concepts and approaches that are driven

by compelling scientific opportunities. During the proposed study we intend to establish and execute a

process that will greatly expand the involvement of the external community, and result in conceptualdesigns for a specific set of about ten initial beamlines to be included in the construction proposal. This

process is described in more detail in Section 1.3 (User Program) and Section 5 (Deliverables and

Proposed Scope of Work). We want to emphasize here our intent that this process (1) be closely

integrated with the design of the machine so as to maximize the science it will enable, (2) be based on a

broad outreach activity involving many workshops and a public proposal solicitation process, and (3)

employ peer review for all proposals using well-established criteria to determine scientific merit.

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To this end, we present below eight contributions by scientists and faculty at MIT, Brandeis,

Yale, Argonne, and the Stanford Synchrotron Radiation Laboratory. It is essential that the project team

have a scientifically strong cadre of users with whom to interact on a regular basis as the technical

design of the facility is developed. It is also essential to have such a nucleus of interested and motivated

users to organize the planned workshops and generally provide leadership to the community to develop

and prepare the more detailed technical and scientific cases for each beamline included in the construction

proposal. Five of these contributions are proposals in themselves (their authors are collaborators on thisproposal) aimed at specific work that would be funded and performed under this study. The other three

contributions (Sections 1.2.4, 1.2.7, and 1.2.8) have a somewhat different flavor. They are concepts for

very innovative and in some cases speculative instruments in different wavelength ranges of the proposed

facility. In two cases, concept development and R&D are underway at Argonne and Stanford with

separate funding. For x-ray lithography, more work would be necessary before a specific proposal could

be made to develop realistic instrument concepts. But at this stage, we do want to think broadly and

entertain some high-risk concepts. Taken together, all of these ideas, we believe, give a flavor of some

extremely exciting possibilities. But not all science must wait for new machines to be constructed. We

also include a brief overview in Section 1.2.9 of science proposed or underway at existing 100 nm

sources.

1.2.1 Nanoscale Dynamics with X-ray Transient Grating Spectroscopy

1.2.1.1 Introduction

The development of time-resolved coherent laser spectroscopy has ushered in a new

understanding of condensed matter dynamics, including the time scales for electronic and vibrational

decoherence and relaxation, liquid-state molecular dynamics and chemical reactions, and collective

structural rearrangements in a variety of complex media. The technology and methods used have been

exploited extensively for practical applications as well, ranging from advanced materials characterization

and metrology to optical coherence tomography and other biomedical assessment techniques to the

coherent optics used routinely in photonic switching and communications. These applications, with their

myriad benefits to society, were developed and are being extended continually by the scientists andengineers whose graduate and postgraduate research was devoted to inventing time-resolved coherent

optics and spectroscopy.

Another scientific and technological revolution will be ushered in as ultrafast coherent optics and

spectroscopy are extended to wavelength scales 100 times shorter than those used today. Here we propose

a study of collective structural change in condensed matter using four-wave mixing or transient grating

techniques with coherent x-rays. Transient grating fringe spacings of = 1-100 nm, corresponding to

coherent scattering wavevectors q = 2/ extending to nearly the edge of the Brillouin zone, will be used

to examine acoustic modes, nonoscillatory density dynamics, polarization, and other order-parameter

responses on the same length scales. These are the mesoscopic correlation lengths whose fluctuations

underly the great majority of collective structural dynamics that we now measure through coherent

spectroscopy on much coarser length scales [68]. Thus our current measurements tell us a great dealabout the time scales for collective structural change, but little about the length scales or the connections

between the two.

When we measure the multiple time scales for density (i.e. structural) relaxation in viscoelastic

polymer liquids or polarization relaxation in mixed ferroelectric crystals, we are observing on coarse

length scales (far longer than the correlation lengths for these variables) the integrated outcomes of

fluctuations whose dynamics may in fact vary sharply with (mesoscopic) length scale. Thus, we are left to

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speculate: are the fastest components of polymer relaxation associated with motions of polymer end

groups, intermediate components with side chains, and slowest components with backbone and whole

molecule motions? If so, then how do we understand the similar hierarchy of time scales observed in the

structural relaxation dynamics of glass-forming van der Waals liquids that have no obvious corresponding

hierarchy of structural correlation lengths [912]? Are the fastest dielectric relaxation components in

mixed crystals like KnbxO3/Kta1-xO3 near ferroelectric phase transitions [1315] associated with the

smallest polarized nanodomain regions, those formed around isolated impurity ions? Are the intermediaterelaxation time scales associated with those nanodomains that have encountered each other and merged

together as the temperature has cooled toward Tc and the polarization correlation length around each

impurity ion has grown? Are the slowest time scales associated with clusters of randomly situated

impurity ions whose surrounding polar regions have merged? Or does collective relaxation in complex

systems of this sort involve sequences of steps that inherently give rise to complex dynamics? These

questions are of practical as well as fundamental interest, since the design of these materials, used widely

in ferroelectric DRAMS, capacitors, and piezoelectric actuators, for high-bandwidth applications is

intimately connected to the association between local structure and dynamics.

Unambiguous association of the time and length scales of collective structural rearrangements

requires direct and simultaneous measurement of both. This will be enabled through coherent opticalspectroscopy using x-ray wavelengths. At the same time, the development of coherent optical methods

that operate on nanometer length scales will open a wealth of new possibilities. Coherent x-ray machining

and lithographic fabrication of nanometer features in advanced devices, x-ray optical trapping and

organization of atoms, molecules, and nanomaterials into assemblies with nanometer spacings, x-ray

coherent scattering metrology of ultrathin films, and other advanced materialsa new world of

applications will emerge under the leadership of the scientists and engineers whose graduate and

postgraduate research involves development of ultrafast coherent x-ray optics and spectroscopy.

1.2.1.2 The MIT/Bates X-ray Laser

The proposed x-ray laser will provide unprecedented capabilities for coherent ultrafast x-ray

spectroscopy, coupled with an exceptional degree of access through construction of a beamline dedicatedfor the purpose. The anticipated output parameters are extraordinary: approximately 1 mJ energy per

pulse at x-ray wavelengths, with pulse duration in the range of 100-200 femtoseconds. The optical four-

wave mixing measurements that we have conducted over the years on viscoelastic materials and crystals

that undergo structural phase transitions [9,11,12,1624] have typically involved pulse energies of tens of

microjoules, focused to spot sizes of tens to hundreds of microns in samples that are essentially

transparent to the excitation light. The proposed facility will provide comparable energies at the sample,

and far higher intensities if desired since the x-ray spot could be focused to far smaller sizes. More likely,

comparable spot sizes will be used and the energy may need to be reduced. Transient grating experiments

can be conducted with nearly any hard x-ray wavelength, and the availability of various wavelengths may

be exploited to enlarge the range of coherent scattering wavevectors. The pulse duration only needs to be

short compared to the fastest acoustic oscillation period, which will be on the order of 0.5-1 THz. If

seeding is used to produce shorter pulse durations, they will also be suitable.

1.2.1.3 X-ray Time-Resolved Transient Grating Spectroscopy

The time-resolved four-wave mixing setup illustrated in Figure 1 yields a transient grating fringe

spacing and corresponding coherent scattering wavevector magnitude q given by the wavelength of

and the angle between the excitation pulses, = 2/q = /2sin(/2). As in conventional light scattering

spectroscopy, the measurement length scale is limited to the order of the light wavelength. Time-resolved

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optical four-wave mixing measurements have provided an effective means for probing structural

relaxation dynamics in both liquid and solid materials, including elucidation of dispersive responses

involving thermal, acoustic, or optic phonon-polariton modes [9,11,12,1624]. An example is provided in

Figure 2, which shows data from a polymer liquid recorded as the temperature is cooled such that the

characteristic structural relaxation time (an average value used for characterization of nonexponential

relaxation dynamics) moves through the range of the acoustic oscillation period, i.e. through the range

ac 1 where ac is the acoustic frequency given by the acoustic (transient grating) wavevectormagnitude qac and the acoustic velocity vac(ac) through the dispersion relation ac/qac = vac. The data

show clearly the acoustic anomalies (velocity dispersion and attenuation maximum) that occur around the

region where ac 1. The data also show slower, nonoscillatory components at lower temperatures that

reveal structural relaxation dynamics occurring on time scales slower than the acoustic oscillation period.

This method, known as impulsive stimulated thermal scattering (ISTS) is based on the sudden sample

heating resulting from the excitation pulses and the subsequent thermal expansion that launches the

acoustic and slower responses observed through coherent scattering of the probe light. Data of this sort

have been used extensively for study of supercooled liquid dynamics [9,11,12,1921].

In other studies, excitation of acoustic waves or coherent optic phonons through impulsive

stimulated Brillouin scattering (ISBS) or Raman scattering, respectively, has been used to generatematerial responses relevant to collective structural change including viscoelastic relaxation or structural

phase transitions [1619], again with the time-dependent responses probed through coherent scattering. In

general, coherent time-domain spectroscopy of collective modes has proved particularly valuable in cases

where the frequencies of the modes are low or the damping (or dephasing) rates are high, including the

overdamped regime, and where the low-frequency spectrum may contain several contributions from, for

example, structural or polarization relaxation in addition to acoustic or optic phonons. In these cases,

conventional light-scattering spectroscopy may be unsuitable since the frequency shift may be

prohibitively low, the Stokes and anti-Stokes lines may broaden and merge, or additional central peak

FIGURE 1 Setup for four-wave mixing. (a) A binary phase mask pattern is used to generate two excitation

pulses that are recombined at the sample. The variably delayed probe pulse also is split to generate a

reference field for heterodyne detection. (b) Adaptation of the setup for x-ray wavelengths. A crystalline

grating will be used to split the incoming excitation and probe beams, and a second crystalline grating will be

used to recombine the pulses at the sample.

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FIGURE 2 Four-wave mixing data from polypropylene glycol. As T is reduced, the viscosity increases andthe structural relaxation time scale lengthens, passing through the ac 1 range at around 260 K wherethe acoustic damping rate is strongest. At lower T, slow components of structural relaxation are observed

directly. At long times (0.11 ms), thermal diffusion between the transient grating peaks and nulls is

observed.

features due to relaxation processes may obscure the acoustic or optic phonon features. The advantages of

the time-domain approach are often realized in association with collective structural rearrangements (e.g.,

structural phase transitions or structural relaxation in viscoelastic fluids), since they are characterized by

slow and complex collective dynamics, often involving coupled low-frequency modes [68,2024].

Recent x-ray Brillouin scattering measurements [25], while revealing interesting behavior at high

wavevectors, also show clearly the difficulties posed by strongly damped or overdamped responses aswell as additional central peak spectral features.

In x-ray four-wave mixing measurements, we expect to be able to adjust the transient grating

spatial period through the region of characteristic structural correlation lengthsL, i.e., we expect to gain

full access to the wavevector regime where qL 1. Acoustic anomalies (maximum in the damping rate,

dispersion in the velocity) similar to those that occur when ac 1 should be observed, in this case

directly revealing the correlation lengths rather than the correlation times involved in structural relaxation.

Observation of the slower, nonacoustic dynamics observable on time scales longer than the

acoustic period (which is very fast at high q) with variable wavevector will provide a direct window into

the connection, if any, between the structural correlation length and time scales. If the broad distribution

of relaxation times, with average value , is associated with a distribution of correlation lengths withaverage value L, then as the transient grating wavevector is varied from the qL < 1 regime through the

qL 1 range, the dynamical response will progressively lose its slower relaxation components until, for

qL > 1, the structural relaxation dynamics are filtered out completely because the measurement is being

made on a length scale shorter than that of the observed structural relaxation processes. In crystalline

solids such as the mixed ferroelectrics mentioned earlier, very similar considerations hold since strain is

linearly (piezoelectrically) coupled to the polarization which is the order parameter [26]. Examination of

the soft optic phonon branch [7,17] (whose displacements give rise to the polarization) in the high-

wavevector range should also be revealing. If nanodomain polarization correlation lengths L are

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associated with the correlation times measured, the results should be analogous to those described above

for acoustic modes, namely a stiffening of the phonon response at high wavevectors as the qL 1 range is

reached and exceeded. Thus x-ray four-wave mixing measurements of samples that under collective

structural rearrangements will directly reveal the relevant time and length scales and the association

between them. Other related issues such as the wavevector range at which acoustic modes in disordered

or partially disordered materials become overdamped will also be addressed directly.

1.2.1.4 Beamline Concept

The details of the beamline to be constructed for x-ray coherent scattering experiments will be

formulated during the proposed grant period. However, the beamline will certainly need apparatus for

generation and delivery to the sample of excitation and time-delayed probe x-ray pulses. This could be

done using crystalline grating interferometers as shown in Figure 1. Reflective optics may also be used

for beam delivery to the sample. A separate beamsplitting step, prior to the one shown in Figure 1, is

necessary for generation of the probe pulse, which must be variably delayed and then directed to the

sample either through reflective or diffractive optics. The latter, as illustrated in the figure, permit

heterodyning of the coherently scattered signal field with a reference field that originates from the probe

beam. This methodology [27] is used extensively for optical four-wave mixing measurements.

The beamline will also require an amplified femtosecond laser system with which time-resolved

optical probing can be used following x-ray excitation. The optical wavelengths cannot be used for

coherent scattering off the x-ray generated transient grating, but they can be used to assess electronic and

other responses to x-ray excitation irrespective of length scale. An understanding of the x-ray excitation

process and what sample dynamics are initiated by it will be a crucial element in the development of

coherent scattering methodology at x-ray wavelengths.

1.2.1.5 Proposed Work

A study of the experimental feasibility and theoretical description of time-resolved four-wave

mixing measurements conducted with coherent x-ray wavelengths on collective structural dynamics is

proposed. This will include preliminary design of the experimental beamline at the proposed facility andestimates of the expected signal levels and time-dependences that might be observed. One of the science

collaborators on this proposal, Keith Nelson, has been involved in formulation of x-ray four-wave mixing

experiments in connection with the LCLS project. This project, if supported and brought to successful

operation, will provide the necessary coherent x-ray pulses, but will offer far more than the energy

required for the proposed experiments. Also its single experimental station will be used for a number of

other experiments as well as research on condensed matter of the sort proposed here.

Professor Nelson also is involved currently in an experimental collaboration with Professors

Henry Kapteyn and Margaret Murnane at the University of Colorado, attempting to conduct four-wave

mixing measurements at soft x-ray wavelengths generated through high harmonic generation of

femtosecond optical pulses [28]. These measurements, if successful, will provide access to a range of

wavevectors considerably wider than the range accessible through visible wavelengths, but still farsmaller than the range reached with hard x-rays, and confined by absorption to near-surface regions. The

measurements also will teach us much about how to conduct this class of experiment with coherent

x-rays. For example, the experimental setup depicted in Figure 1(b) is being tried, not with crystalline

gratings, but with 100-nm grating structures fabricated through electron-beam lithography [29].

Prof. Shaul Mukamel and his group at the University of Rochester has undertaken

[3032] theoretical treatment of x-ray four-wave mixing applied to the study of molecular response.

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However, the experiments of interest here deal explicitly with condensed matter collective responses that

have not been considered to date. Perhaps the most important questions deal with the mechanisms

through which such responses will be generated by the x-ray excitation pulses. What are the relevant

mechanisms through which hard x-ray excitation pulses will interact with the samples? At optical

wavelengths, there are essentially two excitation mechanisms that give rise to sudden, spatially periodic

stress in the sample, thereby inducing acoustic, and in some cases, slower density responses. In ISTS,

optical absorption and rapid thermalization produce a sudden temperature rise and a step-function appliedstress. In ISBS, the initial, spatially periodic stress takes the form of an impulse function, which drives a

transient acoustic response. The combination of driving forces and time-dependent responses has been

treated in detail, and both mechanisms have been exploited for study of acoustic behavior associated with

collective structural change [9,11,12,16,1924]. Following excitation through either mechanism, the time-

dependent density dynamics are monitored through coherent scattering of probe light.

At x-ray wavelengths, absorption of crossed excitation pulses leads to ionization at the

interference maxima (i.e., the transient grating peaks), producing local currents and resulting in a spatial

distribution that may be influenced by electron mobility. At the high transient grating wavevectors q that

will be reached, thermal diffusion from grating peaks to nulls (the rate of which increases as q2 in the

hydrodynamic limit) may be fast compared to some or all components of the density response, in whichcase the temporal profile of the applied stress may approach that of an impulse function rather than the

step-function profile exerted at optical wavelengths. Apart from this, ionization of molecules at the

grating peaks will lead to a separate step-function stress since the steady-state density will be different for

partially ionized regions of the sample than for pristine regions. These effects all arise from x-ray

absorption and are likely to be the dominant contributions to acoustic and other density responses.

Additional interactions that give rise to stress in a manner analogous to stimulated scattering also must be

considered. Excitation of phonons in crystalline solids through similar mechanisms will be treated as

well. For example, liberation of valence electrons by ultrashort pulses reduces screening within the unit

cell, inducing coherent optic phonon responses in semiconductors even with visible excitation

wavelengths [24,33]. X-rays should produce similar responses in insulating crystals as well. Finally, the

detection of time-dependent responses through coherent scattering of x-ray probe pulses must also be

treated.

Apart from the light-matter interactions, density dynamics at high wavevectors and short times

will be modeled approximately so that the time-dependent responses likely to be observed can be

simulated. The purpose of the effort proposed is not to undertake a full simulation of the collective

dynamics at high wavevectors, but to anticipate the types of time-dependent responses expected in view

of plausible material response functions and the different excitation mechanisms at play, and on that basis

to elaborate experimental strategies for observing the responses of interest in the most incisive manner.

Support for a postdoctoral associate in the Nelson group will be used to undertake the theoretical

and feasibility study described above. Professor Shaul Mukamel has indicated a strong interest in working

collaboratively on the theoretical effort. His experience in treating x-ray four-wave mixing in molecules

specifically [32] and nonlinear optical spectroscopy generally [34] will accelerate the effort markedly.Through treatment of the x-ray excitation and probing processes and consideration of the collective

sample responses of interest, we will be able to estimate reliably the signal levels and dynamical features

to be expected from x-ray four-wave mixing measurements conducted at the proposed accelerator, the

information content that the experiments can be expected to provide, and the theoretical modeling that

will be necessary in order to extract that information content. Using realistic parameters for the output

generated by the proposed system, we will be able to assess critically the feasibility and scientific value of

the experiments.

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1.2.2 Nanoscale Dynamics with X-ray Photon Correlation Spectroscopy


Complete understanding of a condensed matter system depends not only on knowledge of its

static properties and structure, but also on how it changes in time in response to time-varying applied

fields, or, simply, to thermally-induced spontaneous fluctuations. In addition, these sample dynamics maybe crucial for deciding how suitable a material may be for a particular application, and surely are key for

any materials processing required to achieve a useful final product. In many cases, we still do not

understand how molecular interactions and motions at the Angstrom scale give rise to often-complex

collective dynamics at mesoscopic (nanometer) and macroscopic (micrometer and longer) length scales.

What is needed is a powerful and general means of monitoring the full dynamic response over a range of

length scales for materials of all sorts, from proteins and long-chain polymers to ferroelectrics and

glasses.


In recent experiments, it has been shown that the new technique of x-ray photon correlationspectroscopy (XPCS) is capable of studying the slow dynamics of strongly-scattering samples at smaller

length scalesof the order of 30 nmthan can be achieved with laser PCS [3541]. In principle, XPCS

yields the intermediate scattering function (IFS) for the electron density of a sample, S(Q,t), and thus

should be a general and powerful method for characterizing the small-scale dynamics of condensed

matter. However, the time and length scales that can be studied via XPCS even at third-generation sources

is limited by the source brilliance, so that to extend XPCS studies to shorter times and into the nanometer

range demands even more brilliant sources. Specifically, the coherent flux, required for XPCS, is directly

proportional to the source brilliance. Thus, the proposed MIT/Bates x-ray laser source represents an

extremely exciting opportunity for novel XPCS studies.

1.2.2.3 Beamline Concept

Generally the proposed 1 kHz time structure of the source must be carefully considered in the

design of XPCS experiments, particularly the duty cycle, which may range from 10 to 100%. Our goal,

under this proposal, is to develop some of the methods and instruments that will permit us to optimally

perform XPCS experiments largely independent of the details of the duty cycle. An XPCS beamline will

have the following key features. First, it will have an x-ray beam splitter/delay line that will separate a

single pulse into two and introduce a variable time delay between the arrival times of the pulses at the

sample. We envision the beam splitter using eight thin diamond crystals to give two more-or-less

symmetric legs, each with a variable x-ray path length. The difference in x-ray path length between the

two legs then determines the delay time between pulses via the speed of light - a maximum path length

difference of 3 m would allow a maximum delay time of 10 ns. Two equivalent legs permit the path

length and delay time to be nulled straightforwardly. Second, it will have a slit system that will allow the

beam size on the sample to be accurately controlled, while introducing as little extraneous slit scatteringas possible. Third, it will have the means to orient the sample and detector as desired. Importantly, in

order to resolve speckle, we anticipate an unusually large sample-to-detector distance of 10 m or more.

Finally, it will have a fast-CCD based x-ray detector, with sufficient resolution to resolve speckle and

sufficient speed to keep up with the x-ray laser repetition rate.

To realize XPCS at the proposed x-ray laser, there are several issues that must be addressed,

including sample damage issues, and the construction of the x-ray beam splitter and delay line. We will

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FIGURE 3 Schematic intensity autocorrelation functions for a polymer melt for four wavevectors, increasing

from A to D.

Another as-yet untested, defining aspect of the reptation model concerns the lineshape of the ISF

versus tfor times neard. The lineshape is predicted to consist of a sum of exponential decays [46,54,55].

Figure 4, for example, shows reptation-model-based predictions for the mode amplitudes and mode

relaxation rates for a highly asymmetric diblock copolymer [55].

Careful XPCS measurements at the proposed x-ray laser source will allow detailed lineshape

analyses to test these sorts of predictions. But beyond tests of existing theory, new XPCS experiments

have the potential to yield key insight into resolving the puzzle of what determines the entanglement

length in blends and melts.

In addition to the polymer melts and blends discussed so far, there are many other polymeric

systems, in which the polymer conformations can be quite different and can exhibit novel dynamics.

Thus, we can anticipate that XPCS studies of a variety of polymer systems extending down to nanometer

length scales will prove an exciting and fruitful research area.

The second method for implementing XPCS at the proposed x-ray laser facility is a significant

departure from established methods, but being able to study condensed matter dynamics on time scalesfrom 1 picosecond to 10 nanosecond timescales is the potential reward. Now, we must rely on an x-ray

FIGURE 4 Specific predictions for the relative mode amplitudes and corresponding relaxation rates for

compositional fluctuations within an asymmetric (f=0.05) diblock copolymer melt. Note that d=4t*.

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pulse splitter and delay line to create from one pulse of the x-ray laser two x-ray pulses with a variable

separation in time (t). The two pulses are then permitted to scatter in succession off of the sample, and

the scattered x-rays are detected by a CCD area detector.

Each split pulse will generate a speckle pattern (given that the pulse width is shorter than any

sample dynamics) with the second speckle pattern corresponding to the samples configuration at a time

tafter the first. If the samples configuration is essentially unchanged in t, the two patterns will be thesame. On the other hand, if the samples configuration has completely changed after a delay t, the two

speckle patterns will be uncorrelated. The speed of light (3108 ms-1) together with the reasonable range

of delay line lengths (0.3 mm to 3 m, say) implies that it may be possible to create pulses with t=10-12s

to 10-8 s.

Although this range of times is similar to the range that can be achieved with NSE, it is pertinent

to recall that only a handful of NSE instruments exist in the world, and that with XPCS it will be possible

to study much smaller samples of difficult-to-manufacture materials or the dynamics of thin films. One

exciting avenue of research, for which being able to study small sample volumes will be an important

asset, will be to study the dynamics of internal motions within protein molecules. XPCS studies

performed on protein crystals, where crystallographic order orients the vibrational/relaxational modes,may prove especially informative in assessing the key motions of these fundamental molecular machines.

It is not possible, however, to separately read out CCD images from pulses separated by

10-12-10-8 s. Nevertheless, the statistical properties of the intensity distribution in the combined image

reveal whether the two contributing speckle patterns are correlated or uncorrelated, and indeed the extent

of their correlation [5659]. Although the conclusions are the same for partial coherence, for clarity we

will consider a perfectly coherent incident x-ray beam. In the case of a sample with slow dynamics

compared to t, it is as though there is a single speckle pattern, and the variance of intensity distribution

relative to the mean intensity squared - that is, the contrast of the speckle pattern - is 1. By contrast, the

sum of two uncorrelated speckle patterns has a contrast of 0.5. If t is varied over the range of

characteristic times of the samples dynamics, one will go from a constrast of 1 for small tto a contrast

of 0.5 at large t. Clearly, the variation of the contrast versus tdetermines the characteristic relaxationtimes of the sample. Thus, the second method of carrying out XPCS involves measuring the speckle

contrast for different settings of the delay line.

1.2.2.5 Proposed Work

In both of the XPCS methods that we have outlined, it is necessary to read out batches of

100 successive CCD images each separated one from another by 1 ms. These raw data must then be

processed to yield either correlation functions (method 1) or the speckle contrast (method 2). Because of

the very high data rates, a development effort to realize the data acquisition and data reduction envisioned

is necessary. Under this proposal, we will develop a prototype fast CCD detector system suitable for both

methods.

To this end, the first item that we need is a CCD camera capable of 1 ms readout. This is state-of-

the art for current technology. Specifically, we propose to purchase a CCD camera under this proposal

the Dalstar 1M60 from Dalsathat can read out an array of pixels in 0.86 ms. Mochries group has

already shown that this relatively inexpensive, commercial camera may be straightforwardly modified to

become a successful detector for XPCS [60].

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In the context of XPCS experiments at the proposed x-ray laser64 1024 pixels per

1 millisecond repeated 100 times per secondthe overall data rate is about 6 Megabytes per second, or

22 Gbytes per hour, or 500 Gbytes per day. It would be theoretically possible to store these quantities of

data for later data reduction and analysis. Practically, however, it is clear that such a data storage rate

would be difficult to maintain for extended periods, and if it becomes necessary to interrupt data

acquisition to do data storage, some of the gain in the signal-to-noise ratio from parallel detection in many

channels is lost. In addition, in order to be able to make sensible decisions during the experimental run, itis far preferable to reduce the data in real time, so that the correlation functions are available during the

experiment. Anticipation of CCD cameras with yet faster readouts, leading to increased data rates, also

directs one towards real-time data reduction. Thus, we propose to develop a CCD detector system, based

on the Dalstar 1M60, read out by 4 frame grabbers in 4 fast computers, one for each of the cameras

4 taps. The computers, in turn, will calculate intensity correlations versus time (method 1) or the

speckle pattern contrast (method 2) in real time. The modular nature of the system means that it should be

straightforward to upgrade to a faster detector, or faster frame grabbers, or faster computers as they

become available and/or required.

Next, we require computer code that carries out the necessary calculations. Writing, debugging,

verifying, and refining this software is an essential part of the work of this proposal. A key advantage ofour software-based data reduction scheme is that it will be possible to continuously improve, for example,

how g2 is calculated, if improved algorithms become available. A software-based implementation also

facilitates other methods of processing data should that be desired. Others have implemented CCD-based

PCS (multi-speckle PCS) in real time [61,62]. The distinction here that warrants a significant

development effort is that the raw data rates are hundreds of times higher than what was achieved

previously.

Finally, it is worth remarking that, although the camera system we propose is a prerequisite for

carrying out XPCS at the proposed x-ray laser, the real-time data reduction that we will implement will in

addition very greatly improve the user-friendliness of XPCS at third-generation rings, providing

experimenters with the same sort of immediate review of the data as is available in light-scattering

experiments. Thus, our proposed detector system development effort has the potential to greatly increasethe pool of scientists who may be interested in carrying out XPCS experiments now and in the future at

the proposed x-ray laser.

1.2.3 Femtosecond Spectroscopy in Solution-Phase Chemical and Biophysical Systems


In large part, the advances made in chemistry and other molecular sciences during the twentieth

century involved molecular structure: its description, determination, creation, and manipulation. Atoms

were described as the building blocks of molecules, and the construction of molecules from diatomics to

DNA was essential to describing their chemical, physical, biological, and material properties. Changes to

molecular structure through reactions (or mutations) are thus used to influence such properties orfunction. Through this development, much of molecular structure has also been largely described in time-

invariant or static terms. Part of the reason for this may be that many of the tools we use to characterize

structure are time-averaged measurements. However, every chemist inherently deals with structural

change, even at levels as simple as drawing arrows to indicate the motion of electrons in chemical

reactions. Increasingly, researchers in the molecular sciences are emphasizing the importance of

describing, watching, and controlling the time-evolution of molecular structure. This is of broad

importance, whether to watch and control changes in molecular bonding during a chemical reaction, to

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understand the folding of a protein into its physiologically active structure, or to describe structural

transformation or phase transitions in molecular materials.

For studies of molecular dynamics in solution there are no broadly applicable tools to characterize

how structure changes on appropriate timescales. For that reason, the study of solution-phase chemical

and biophysical problems would greatly benefit from new structurally sensitive experimental methods for

describing molecular and collective dynamics. Femtosecond x-ray spectroscopy and scatteringexperiments offer the promise of unique sensitivity to atomic, molecular, and collective structure with

time resolution exceeding that of nuclear motions. Such techniques thereby offer tremendous

opportunities for characterizing molecular dynamics in liquids and other amorphous systems. The

proposed MIT/Bates x-ray laser project represents the creation of a center of intellectual activity that will

move this under-developed area of science forward, training and developing scientific personnel in an

emerging discipline within the chemical and biophysical sciences.

For the exploratory work under this proposal, a variety of possible experimental applications of

femtosecond x-ray pulses to studies in the liquid phase will be evaluated, for their use in the study of

near-equilibrium and non-equilibrium dynamics in solution. These vary by the nature of the chemical

system studied, the x-ray interaction employed, and the type of distance scales to be probed. The resultsof these feasibility studies will form a primary input to the design of a beamline for such experiments. In

the following sections, a general overview of methods applicable to chemical problems in solution is

described, followed by a more detailed description of four experiments. These four examples represent

specific experiments whose feasibility can be evaluated quantitatively through modeling, and which

overlap with other complementary time-resolved spectroscopies.


Several aspects of the proposed x-ray laser make it a unique as a facility for the study of solution-

phase molecular dynamics. The pulse length of 100 fs for unseeded beams makes it useful to all but the

fastest studies that involve observation of the motion of nuclei, opening experimental possibilities over

time scales from femtoseconds to milliseconds. The ability to seed beams to pulse lengths of 1 fs or lesswill then cover all the relevant timescales. The extremely high peak brilliance of this source compared

with others is vital to time-domain spectroscopies. All such methods use multiple pulses to interrogate a

sample, and generally involve detecting small changes in what would otherwise be already small signals.

Perhaps the most powerful attribute of the proposed x-ray laser source is the broad tunability over the soft

to hard x-ray energy range. For chemists this translates into the ability to spectroscopically probe distance

scales from the very local with atom specific excitation, to the mesoscopic with scattering experiments

that interrogate the 0.1-100 nm distance scales relevant to the study of numerous complex condensed

phases and biological systems.

1.2.3.3 Time-Resolved X-ray Spectroscopy of Molecular Dynamics in Solution

Studies of molecular dynamics in solution can be broken into two broad categories. One set of problems involve studies of dynamics near equilibrium, such as the changing collective structure of

liquids, conformational fluctuations and large amplitude motions in biopolymers, and dynamical

heterogeneity in supercooled liquids and glasses. Perhaps more experimentally challenging is finding

ways of describing the complex structural changes accompanying non-equilibrium processes, such as the

changes in nuclear and electronic configurations during chemical reactions, or large-scale conformational

changes in biophysical processes, such as protein folding or binding of substrates. Whether near or far

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from equilibrium, a broad predictive understanding of these types of processes requires experimental

methods that probe the evolving structures in these systems.

Generating high-brilliance coherent femtosecond x-ray pulses opens the door to numerous

possible techniques that could be used to study solution phase problems. These techniques will require

two or more pulsed electromagnetic fields, of which one or more act initially as a pump, to prepare a

system by initiating a chemical process or perturbing the system from equilibrium, and one acts as aprobe, to detect a time-dependent change. These can be broadly classified as optical-pump/x-ray probe

and multiple-pulse x-ray experiments. Each in turn can involve time-resolved x-ray absorption

spectroscopy (XAS) on core level electrons, scattering, or diffraction processes. A system can also be

probed by x-ray emission, although with limited control over the time scale of the dynamics to be probed.

The spectroscopic theory for such experiments is rapidly gaining momentum, and in the perturbative

limit, can formally be related to third-order nonlinear spectroscopies [63,64].

Optical-pump/x-ray probe experiments can be used to follow photo-initiated chemical processes

or molecular relaxation processes with structural selectivity. Resonant optical pumping can be used to

photoinitiate chemical reactions varying from charge transfer to bond rearrangements. Near-infrared

pumping of solvent overtones can be used to induce rapid temperature jumps for protein folding studies.Nonresonant optical pumping can be used as an impulse perturbation to an equilibrium systema method

often used in study of collective relaxation in complex liquids. Femtosecond x-ray absorption or

scattering can then be used as a structurally sensitive probe of these initiated dynamics [65,66].

Multiple pulse or nonlinear x-ray absorption or scattering experiments can be used to follow the

dynamics initiated by an x-ray absorption or scattering event. For pulses resonant with the absorption

bands of core electrons, such experiments include techniques broadly used in the optical range such as

pump-probe or hole-burning experiments, and transient gratings, yet with the additional control over

molecular scale excitation wave-vectors [64]. Such experiments would be sensitive to the dynamics of

molecular relaxation processes viewed through the core electron levels. While the lifetimes of core level

holes are much shorter than the planned 100 fs pulse length [67], other relaxation and transport processes

can be probed [68]. To the degree that inhomogeneous or dynamic line broadening may be present, x-raytransient photochemical hole burning or pump-probe could be used to probe structural dynamics or

heterogeneity present in the broadening of absorption lineshapes. Transient grating experiments would

follow relaxation of excitations prepared with a well-defined wave-vector on distance scales

corresponding to molecular and collective structures. Coherent multi-pulse experiments detected with

wave-vector selective scattering offers the ability to follow molecular dynamics through their influence

on the interference between fields scattered from two delayed pulses. For the case of nonresonant

excitation and probing, this is an x-ray photon correlation spectroscopy that will allow the large-scale,

collective motions of liquids or macromolecules and the heterogeneity to be probed as a function of time

and wavevector [69].

1.2.3.4 Beamline Concepts and Proposed Work

Building on the afore-developed general concepts, four types of experiments are discussed here

for their use in solution phase molecular dynamics, from the collective structure in liquids to chemical

reaction dynamics in solution and rapid protein folding events. The goal of this project is to investigate

the feasibility of these experiments theoretically and computationally, and provide input to the design of

beamlines for such experiments. The goal is to undertake specific investigations and to communicate a

conceptual and theoretical framework with which to describe and model such femtosecond x-ray

experiments. The feasibility studies will also permit subsequent investigations into the experimental

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systems that lend themselves to femtosecond x-ray studies and complementary optical and infrared

experiments.

X-ray Probed Optical Kerr Effect Spectroscopy (XOKE). The femtosecond Kerr effect

spectroscopies used in the optical regime have been widely used to study collective structural relaxation

processes in molecular liquids, supercooled liquids, and other amorphous condensed phases [7072].

OKE experiments in two-pulse or transient grating configurations have proven effective at characterizingthe time scales for collective dynamics by measuring the decay of a transient orientational or spatial

anisotropy induced by an optical pulse [73]. The lack of microscopic spatial selectivity makes separating

processes such as reorientation, collective librations, density fluctuations, and heterogeneous relaxation

difficult to distinguish.

The selectivity of OKE experiments to collective liquid relaxation can be extended to

characterizing the distance scales associated with relaxation processes, when combined with x-ray

probing. Figure 5 shows a schematic of this XOKE experiment. By exerting a torque on the liquid, a

strong femtosecond optical pulse creates an orientational anisotropy that decays with collective relaxation

processes. A femtosecond x-ray probe pulse can be scattered off this anisotropy to follow the relaxation

as a function of wave-vector, revealing the time scale of relaxation over different spatial dimensions.Probing with wavelengths in the 0.3 to 30 nm range effectively allows all relevant distance scales for

intermolecular liquid motions to be probed. This is a fundamental step in revealing the nature of

collective relaxation processes in liquids and other amorphous condensed phases. Particularly in

supercooled liquids, wave-vector dependent relaxation experiments will throw insight into whether non-

exponential relaxation behavior arises from heterogeneous dynamics in single nanometer scale domains.

Fragile supercooled organic liquids and isotropic phase liquid crystals are both excellent

candidates for probing structural correlation lengths and wavevector-dependent dynamics [71,74].

Theoretical evaluation of this experiment can work with both atomistic and hydrodynamic descriptions of

the signals, revealing how molecular and collective reorientation as well as density fluctuations are

observed in the experiment. Also the role of the polarization of the fields involved on the motions

observed should be addressed [72].

Photoinitiated Chemical Reaction Dynamics in Solution. Femtochemistry refers to the study

of optically initiated chemical reaction dynamics, which have most often been performed on simple

chemical reactions with femtosecond electronic spectroscopy of the low-lying valence states involved

FIGURE 5 An optically induced anisotropy in a liquid sample is probed by time-delayed small angle x-ray

scattering using a femtosecond x-ray pulse. The change in the scattering function at different wavevectors for

a given time delay S(q,) will gradually return to an isotropic shape with collective relaxation processes.

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[75]. In the case of photodissociation of small molecules, the knowledge of potential energy surfaces and

the ability to directly relate the wavelength of observation to a point on the reaction coordinate allows

much to be said about the dynamics of the nuclei and electronic states involved [76]. For increasingly

complex reactions, other structurally selective probing methods are required. Time-resolved x-ray and

electron diffraction experiments are beginning to be used in this context to study the gas phase reaction

dynamics of small molecules [77], or for probing of phase transitions in crystalline substances [78,79].

While scattering experiments in an isotropic solution may have some use in following the

reactants, transition states, products, time-resolved XAFS provides an alternate approach that gives local

information on changes in electronic and nuclear structure [79,80]. The applicability of this method has

recently been demonstrated. XAFS probing of optically induced charge-transfer reactions have shown the

ability of such methods to probe the structural changes accompanying excited state processes, [65,81].

Such techniques have broad applicability to the study of electron and proton transfer processes,

isomerization, photodissociation and more complex reactions in solution. XAFS would also be a sensitive

probe of the structural changes accompanying optically initiated spin state transition in iron complexes

[82].

We plan to evaluate is the use of x-ray probes of femtosecond UV photodissociation of CO frommetal carbonyls in solution. Photodissociation of metal dicarbonyls and hexacarbonyls in hydrocarbon

solvents have been widely studied and show rich dynamical behavior and solvation effects [83]. Also,

these lend themselves to complementary femtosecond infrared vibrational spectroscopies [84]. It is of

interest to not only follow the time-scale for the leaving of the ligand and solvation of the fragments, but

to determine what femtosecond x-ray spectroscopy can say about the relative geometries of fragments and

solvent during such a process. Femtosecond XAS or XAFS probing of the metal L-edge and K-edge of

the O will be tested as a structural probe of metal and ligand during dissociation and solvation.

Transient Photochemical Hole Burning of X-ray Absorption Line Shapes. Little is known

about the dynamic or static line broadening mechanisms for x-ray absorption features. Since the lifetime

of the core hole is exceedingly fast (


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larger scale, questions exist regarding the heterogeneity of denatured states, the degree of native structure

and compactness of intermediates, and how these are reflected in the topology of protein folding free

energy landscapes. These types of questions can be addressed by directly observing folding process and

heterogeneity at a molecular level on all relevant time scales. Probing of protein folding or denaturing

processes in solution with femtosecond x-ray scattering would open up the possibility of probing local

and large-scale structures on the now largely inaccessible time-scales between 100 fs and 1 s that govern

the initial collapse of denatured states [88,89].

In crystalline systems, time-resolved x-ray diffraction has been demonstrated as effective in

probing the localized dynamics of ligands and sidechains following picosecond excitation of

photoinitiated protein dynamics [90]. The more conformationally flexible, large amplitude motions in

solution have been probed through time-resolved small angle x-ray scattering (SAXS) on s to second

time scales, [66,91]. Such measurements have been used to follow the change in the radius of gyration for

the protein. To access the formation of protein structure in solution on various distance scales and over

shorter time scales, time-resolved SAXS can be used to probe folding induced by an optical pulse [89,92].

Perhaps the most general approach is folding initiated by a nanosecond temperature-jump experiment on a

cold-denaturing protein. A nanosecond near-infrared pulse can be used to rapidly heat a sample through

solvent overtone absorption, allowing the nanosecond to microsecond time scales of protein folding ordenaturing to be studied [93]. For a single excitation pulse, a broad range of time scales can be probed

with a sequence of micropulses from the x-ray laser pulse train. Equivalently, binding experiments can

also be initiated with optical pulses [94]. For instance, the binding of divalent zinc or calcium ions can be

initiated by optically releasing these ions from a photolabile ligand [95]. Binding of the ions by proteins

or peptides could be probed either with SAXS or XAS. As with the XOKE experiments, analysis of the

induced scattering change as a function of time and wave-vector can potentially be used to establish the

timescales for formation of secondary and tertiary structure and the degree of compactness of the protein.

It is possible that the analysis of transient and temperature-dependent equilibrium scattering patterns can

be used to quantify structural heterogeneity of the folding proteins.

1.2.4 Trace Chemical Analysis in the Particle Counting Limit


Trace particulate analysis is at once an important and a difficult area of forefront analytical

science research. The importance is exemplified by, but not limited to, the needs of the semiconductor

industry to analyze devices whose features will shortly approach 100 nm and for which one impurity atom

can dramatically affect device performance. The surface composition of submicron sized particulates is

crucially important in assessing their environmental impact. Similarly, microbiologists attempt to measure

the three-dimensional location and abundance of single altered molecules contained in a single cell. In the

future, where nanotechnology promises to become an important tool, such measurements are likely to

become even more important. The difficulty in the trace measurement of such samples arises in

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David E. Moncton- X-ray Laser User Facility at Bates Laboratory