11
FEATURE ARTICLE Ultrafast Time-Resolved Transient Structures of Solids and Liquids Studied by Means of X-ray Diffraction and EXAFS Ivan V. Tomov, Dmitri A. Oulianov, Peilin Chen, ² and Peter M. Rentzepis* Department of Chemistry, UniVersity of California, IrVine, California 92697 ReceiVed: March 10, 1999; In Final Form: June 9, 1999 Picosecond and nanosecond transient structures have been observed directly using time-resolved X-ray diffraction and absorption. These experiments provide insight on the evolution of transient molecular structure on the atomic length scale during the course of a chemical reaction. Recent advances in the generation of short X-ray pulses and detectors have made time-resolved X-ray studies a reality. We discuss a few of the vast number of possible time-resolved structure studies in solids and fluids. Ultrafast relaxation dynamics of crystal lattice structures induced by picosecond and nanosecond laser pulses have been observed by means of time-resolved picosecond and nanosecond X-ray diffraction. Lattice deformation with 10 ps and 10 -3 Å resolution have been performed. The picosecond X-ray system, which we have used, is described, and its application to time-resolved ultrafast X-ray diffraction in crystals and EXAFS in liquids is discussed. 1. Introduction Since their discovery, X-rays have been the dominant source for the determination of structure. X-ray diffraction has been the most common and accurate method for the measurement of crystal structures from simple inorganic crystals, such as common salts to large biological molecules, such as DNA, hemoglobin, and rhodopsin. 1 X-ray absorption and lately EXAFS studies have made it possible to determine the structure around a specific atom in the liquid state. Lately, the wide use of synchrotrons has resulted in a number of EXAFS studies, which have helped considerably in the effort to elucidate the structure of liquids. 2 Electron and neutron diffraction has also been used; however, electrons, because of their short penetration depth, are more useful for surface characterization and gases. Time-resolved electron diffraction was first performed in 1988 in picosecond time-resolved studies of solids 3 and nanosecond time-resolved electron diffraction from gases. 4 Since then, several publications have appeared and now other groups are also active in this field. 5-9 Ultrafast laser spectroscopy has provided the decay and formation kinetics of many transient chemical and biological processes. However, the structures of excited states remain unknown although their identity as single or triplet states, etc., can be deduced from ultrafast optical spectroscopy. This article will be restricted to time-resolved X-ray diffraction and EXAFS, its aim being to introduce this rather new and potentially very important field of science. The vast majority of X-ray structure studies have been performed in the static regime. However, as in the case of lasers, which have been used since the 1960s, the means for time- resolved excited-state research, X-rays, have also started to become instruments for transient structure determination. 10 To understand, in depth, the dynamics of even the most well-known physical, chemical, and biological processes, such as bond dissociation and formation, protein folding and unfolding, liquid structures, phase transition, temperature or shock strain in materials, and even the excited-state structure of molecules, time-resolved X-ray studies have become rather mandatory. Normally X-ray structures are determined by averaging the data over the time of the experiment and obtaining a space average of all molecules in the sample. Transient studies, however, must account for the evolution of a structure, which may occur very fast and may even have a varying spatial and temporal distribution within the sample at a given time. Thus transient structure determination is far more challenging. The continuous development in technology has made possible the design, construction and use of time-resolved X-ray systems, which vary from the very powerful, versatile, and often used synchrotron sources, to small benchtop laboratory systems. Before we discuss time-resolved X-ray studies in detail, we will review briefly the basic principles of X-ray interaction with matter. 1,2 When X-rays are incident on a material they are scattered by the electrons of the atoms. The typical X-ray photon energy (5-100 keV) is significantly greater than the binding energy of the majority of the electrons although some inner shell electrons (i.e., K-electrons) may have comparable or even higher energy than the X-ray photon. As a first approximation, we may assume that the amplitude of the X-rays scattered from an individual atom is proportional to the total number of electrons in the atom, i.e., the atomic number. The scattered amplitude from an atom, the atomic form factor f can be calculated with reasonable accuracy. Absorption effects can also be included in the atomic form factor by considering it as a complex number. In crystals, the atoms are arranged in a periodic three- dimensional crystal lattice. The incident X-rays experience a ² Present address: Department of Chemistry, University of California, Berkeley, CA 94720. * Corresponding author. 7081 J. Phys. Chem. B 1999, 103, 7081-7091 10.1021/jp9908449 CCC: $18.00 © 1999 American Chemical Society Published on Web 08/04/1999

Ultrafast Time-Resolved Transient Structures of Solids and Liquids Studied by Means of X-ray Diffraction and EXAFS

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Page 1: Ultrafast Time-Resolved Transient Structures of Solids and Liquids Studied by Means of X-ray Diffraction and EXAFS

FEATURE ARTICLE

Ultrafast Time-Resolved Transient Structures of Solids and Liquids Studied by Means ofX-ray Diffraction and EXAFS

Ivan V. Tomov, Dmitri A. Oulianov, Peilin Chen, † and Peter M. Rentzepis*Department of Chemistry, UniVersity of California, IrVine, California 92697

ReceiVed: March 10, 1999; In Final Form: June 9, 1999

Picosecond and nanosecond transient structures have been observed directly using time-resolved X-raydiffraction and absorption. These experiments provide insight on the evolution of transient molecular structureon the atomic length scale during the course of a chemical reaction. Recent advances in the generation ofshort X-ray pulses and detectors have made time-resolved X-ray studies a reality. We discuss a few of thevast number of possible time-resolved structure studies in solids and fluids. Ultrafast relaxation dynamics ofcrystal lattice structures induced by picosecond and nanosecond laser pulses have been observed by meansof time-resolved picosecond and nanosecond X-ray diffraction. Lattice deformation with 10 ps and 10-3 Åresolution have been performed. The picosecond X-ray system, which we have used, is described, and itsapplication to time-resolved ultrafast X-ray diffraction in crystals and EXAFS in liquids is discussed.

1. Introduction

Since their discovery, X-rays have been the dominant sourcefor the determination of structure. X-ray diffraction has beenthe most common and accurate method for the measurement ofcrystal structures from simple inorganic crystals, such ascommon salts to large biological molecules, such as DNA,hemoglobin, and rhodopsin.1 X-ray absorption and latelyEXAFS studies have made it possible to determine the structurearound a specific atom in the liquid state. Lately, the wide useof synchrotrons has resulted in a number of EXAFS studies,which have helped considerably in the effort to elucidate thestructure of liquids.2

Electron and neutron diffraction has also been used; however,electrons, because of their short penetration depth, are moreuseful for surface characterization and gases. Time-resolvedelectron diffraction was first performed in 1988 in picosecondtime-resolved studies of solids3 and nanosecond time-resolvedelectron diffraction from gases.4 Since then, several publicationshave appeared and now other groups are also active in thisfield.5-9 Ultrafast laser spectroscopy has provided the decay andformation kinetics of many transient chemical and biologicalprocesses. However, the structures of excited states remainunknown although their identity as single or triplet states, etc.,can be deduced from ultrafast optical spectroscopy. This articlewill be restricted to time-resolved X-ray diffraction and EXAFS,its aim being to introduce this rather new and potentially veryimportant field of science.

The vast majority of X-ray structure studies have beenperformed in the static regime. However, as in the case of lasers,which have been used since the 1960s, the means for time-resolved excited-state research, X-rays, have also started to

become instruments for transient structure determination.10 Tounderstand, in depth, the dynamics of even the most well-knownphysical, chemical, and biological processes, such as bonddissociation and formation, protein folding and unfolding, liquidstructures, phase transition, temperature or shock strain inmaterials, and even the excited-state structure of molecules,time-resolved X-ray studies have become rather mandatory.

Normally X-ray structures are determined by averaging thedata over the time of the experiment and obtaining a spaceaverage of all molecules in the sample. Transient studies,however, must account for the evolution of a structure, whichmay occur very fast and may even have a varying spatial andtemporal distribution within the sample at a given time. Thustransient structure determination is far more challenging. Thecontinuous development in technology has made possible thedesign, construction and use of time-resolved X-ray systems,which vary from the very powerful, versatile, and often usedsynchrotron sources, to small benchtop laboratory systems.

Before we discuss time-resolved X-ray studies in detail, wewill review briefly the basic principles of X-ray interaction withmatter.1,2 When X-rays are incident on a material they arescattered by the electrons of the atoms. The typical X-ray photonenergy (5-100 keV) is significantly greater than the bindingenergy of the majority of the electrons although some inner shellelectrons (i.e., K-electrons) may have comparable or even higherenergy than the X-ray photon. As a first approximation, we mayassume that the amplitude of the X-rays scattered from anindividual atom is proportional to the total number of electronsin the atom, i.e., the atomic number. The scattered amplitudefrom an atom, the atomic form factorf can be calculated withreasonable accuracy. Absorption effects can also be includedin the atomic form factor by considering it as a complex number.

In crystals, the atoms are arranged in a periodic three-dimensional crystal lattice. The incident X-rays experience a

† Present address: Department of Chemistry, University of California,Berkeley, CA 94720.

* Corresponding author.

7081J. Phys. Chem. B1999,103,7081-7091

10.1021/jp9908449 CCC: $18.00 © 1999 American Chemical SocietyPublished on Web 08/04/1999

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three-dimensional arrangement of the electron density and thecontour map of this electron density is studied in X-rayscattering experiments. Therefore, an X-ray beam, whichimpinges on a crystal, sees a periodic structure of planes ofatoms and therefore the X-rays scattered by different atoms willinterfere with themselves. In the directions of constructiveinterference a well-defined pattern of scattered X-ray intensitiesis observed. Bragg’s law gives the condition for diffraction

whereλ is the X-ray wavelength,dhkl is the plane spacing, andΘΒ is the angle of incidence between the X-ray beam and thelattice plane. For monochromatic X-ray radiation the angle ofincidence must satisfy Bragg’s law. If a fixed direction and aX-ray beam with low divergency is used, then by rotating thecrystal different sets of diffracting planes can be studied (Braggdiffraction).

When polychromatic X-ray radiation is used, the crystal mayremain stationary and it will select and diffract radiation ofdifferent wavelengths in different directions in order to satisfyBragg’s law. This method, known as Laue diffraction, makespossible the collection of diffracted radiation from manydifferent planes simultaneously. To avoid overlap of spots, alow divergent beam with small cross section must be used. BothBragg and Laue diffraction methods are widely employed inX-ray crystallography. To obtain the entire structure of a crystal,a complete set of diffraction data is necessary and in this casethe Laue diffraction method allows for faster data collection.

In the case of noncrystalline materials, such as gases, liquids,and amorphous solids, where there is an absence of periodicarrangement of atoms, diffraction is also observed but with muchless efficiency. Nevertheless, information, on the structure maybe obtained. The average intensity of the scattered X-rays froman array of atoms, which may have all orientations in space, isdetermined by the Debye equation:

Herek ) 4πsin θ/λ, rmn is the distance between them- and thenth atom, andθ is the scattering angle. Since atoms have well-defined sizes and closest distances of approach, there is astructure relative to an origin at the center of an average atom.This type of structure is expressed in the form of a radialdistribution function. Modulation of the radial distributionfunction can give information regarding the interatomic distancesand molecular structure in liquids and amorphous solids.

X-ray absorption by molecules has been used to providestructural information of molecules, including chemical bonding,charge, and the oxidation-reduction state of atoms and mol-ecules. X-ray diffraction provides information regarding theglobal structure of molecules in the condensed phase. However,the spatial information obtained by X-ray absorption is limitedto the local environment of a particular atom. Absorption hasthe very desirable advantage of being very sensitive only to aparticular atom. Therefore, it provides the structure near theselected atom and does not favor largeZ atoms as in the caseof diffraction. It can be easily used, in principle, to study smallZ number molecules composed of H, C, N, O which are of greatimportance to chemistry and biology.

The absorption processes, which can give structural informa-tion, are extended X-ray absorption fine structure (EXAFS), nearedge resonances, and chemical shifts. EXAFS and near edge

resonances are displayed as modulation of the absorptionspectrum above the X-ray absorption edge. To understand thesemodulations in the spectrum we can think as follows: when anincident X-ray photon is absorbed by an atom, it, most likely,induces photoionization. The excess energy of the X-ray photonabove the ionization potential is carried away by a photoelectron.At very small kinetic energies (less than 50 eV) there is multiplescattering of the electron and interference effects between theelectrons of the atom and those of the near neighbor atoms. Afull molecular orbital treatment is necessary to describe theelectron near edge absorption spectrum. At higher kineticenergies the photoelectron can be treated as a spherical waveleaving the atom which scatters from the electrons of neighbor-ing atoms, and the resulting interference pattern is then afunction of a few atomic and molecular parameters. Thisinterference pattern is the EXAFS spectrum which is Fouriertransformed to yield structural information such as bond lengthsand angles.

The chemical shift of the absorption edge is very small (afew eV). It is the result of the charge screening effect of theouter electrons on the inner electrons in the atom. Chemicalenvironment changes the screening effect and results in acharacteristic chemical shift for a particular atomic arrangement.

2. Studies of Transient Structures by Means ofTime-Resolved X-ray Probing

In contrast to the static structure determination, to follow theevolution of the structure, we must take “snapshots” with theprobing pulsed X-ray radiation. The duration of the X-ray pulsesmust be practically as short as the lifetime of the transientstructure. The lifetime of the transient structures of interest varyfrom many seconds to femtoseconds. For example, folding andunfolding of proteins occurs in seconds while the dissociationof chemical bonds may require only femtoseconds. To coversuch a wide temporal range, various pulsed X-ray sources, anddetection devices have been designed and used. Figure 1 depictsthe time scales of typical transient phenomena, X-ray pulsesources, and detection techniques.

What transient structural information would we hope to obtainwith time-resolved X-ray probing? Essentially any, given theappropriate equipment: from electron-phonon relaxation to theiron movement in hemoglobin. Here we will mention only afew examples from current research.

2dhklsin ΘΒ ) λ

Ia ) ∑m

∑n

fmfnsinkrmn

krmn

Figure 1. Time scale of transient phenomena, pulsed X-ray sources,and detection techniques.

7082 J. Phys. Chem. B, Vol. 103, No. 34, 1999 Tomov et al.

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On the atomic and molecular level, time-resolved X-ray datacould provide direct information allowing one to reconstructthe motion of atoms during a dynamic process.11 With anexperimental X-ray time resolution in the tens of femtosecondsrange, it may be possible to observe directly electron-phononinteraction and the evolution of chemical reactions, i.e., forma-tion and dissociation of chemical bonds.

In biology, static X-ray crystallography has been verysuccessful in determining the structure of many very importantbiological macromolecules such as proteins, enzymes, and DNA.To further understand the properties and behavior of thesemacromolecules under reaction conditions, one must know howtheir structural changes in time, i.e., identify the structures ofintermediate and transition states.12 This information in turn willmake the understanding of the reaction mechanism and activa-tion parameters of a particular reaction rather complete.

In material science the development of new and improvedtechnological materials requires increased sophistication instructural investigations, including in situ processing with hightemporal resolution.13-15 As technology advances, the use ofnonequilibrium and artificially structured materials in ultrafastswitching devices makes the understanding and knowledge ofthe temporal properties of these new materials mandatory.

These are a few examples of a wide field of time-resolvedstudies which are important for the advancement of basic scienceand technology. To achieve this ambitious goal a wide set ofexperimental X-ray data must be accumulated and theoreticalmodels used in order to determine the time-resolved evolutionof the transient structure of matter.

In the optical range by application of pump/probe techniques,in which an optical pump pulse initiates a chemical reactionand another delayed optical pulse probes intermediate spectraof the reacting system, a time resolution in the femtosecondrange has been achieved. However, optical measurements donot provide information regarding atomic and molecular posi-tions. Time-resolved X-ray studies are necessary in order tomeasure directly the short-lived transient structure of matter withatomic spatial resolution. Similar pump/probe transient tech-niques, developed for picosecond spectroscopy, can be appliedto time-resolved X-ray studies. Of course, short optical pulsesare needed for the initiation of the fast processes to be probedby X-ray pulses.

There are three components, which are critical for thesuccessful, execution of time-resolved X-ray experiments: apulsed X-ray source, reaction initiation, and detection systems.We will briefly discuss each of these components.

2.1. Pulsed X-ray Sources.There are three main types ofpulsed X-ray sources, which have been used in time-resolvedX-ray studies: electron impact sources, laser-produced plasma,and synchrotrons. There is extensive literature covering thecharacteristics and operational parameters of each source10,16,17

therefore, here, we will list only some of the most importantfeatures related to time-resolved studies.

(a) The First X-rays Were Generated by Electron Impact onMetal Anodes.In an impact source, the electrons are producedat the cathode, then accelerated by an electric field and finallyimpinge on the anode producing X-ray radiation. The spectrumof this radiation consists of a broad continuum and linescharacteristic of inner shell ionization of the anode atoms. In astandard, continuous X-ray tube the electrons are usually emittedfrom heated metal filaments. Submicosecond duration pulseshave been achieved by applying electrical pulses to the X-raydiode in which the electrons are generated by the field-emittingcathode. This technique allows for the production of X-ray

pulses with several tens of nanosecond duration. Shorter electronpulses with nanosecond and picosecond duration have beengenerated in photocathodes excited by ultrashort pulses. Insection 3 of this paper we will describe in more detail the laserpumped X-ray diodes for the production of nanosecond (ns)and picosecond (ps) hard X-ray pulses.

(b) Laser-Produced Plasma.When a powerful laser pulse isfocused on a solid target, the energy is deposited in a smallspot so fast that there is not sufficient time for heat diffusion totake place, and consequently hot plasma is produced. Thetemperature of the plasma can reach millions of Kelvin andtherefore it expands in the surrounding vacuum. This type ofplasma is known to emit a broad spectrum of electromagneticradiation including X-rays. The amount and properties of theX-ray radiation produced by a laser plasma depends on theplasma processes, but the duration of the X-ray pulses generatedclosely follows the duration of the excitation laser pulse. Thistechnique has generated nanosecond and picosecond X-raypulses and is expected to generate pulses in the femtosecond(fs) range.

(c) Synchrotrons Produce Electromagnetic Radiation byMoVing Charged Particles in a Circular Orbit at RelatiVisticVelocities. The radiation wavelength and power generateddepend on the electron energies and radius of curvature of thebending magnets. The spectral properties of the emitted radiationmay be further manipulated by inserting magnetic devices intothe ring. The output of the present day advanced synchrotronsis a “bunch” of high power X-ray pulses emitted in a narrowcone angle. The duration of a single pulse in the bunch may beas short as 50 ps.

To compare the capabilities of the various X-ray sources, a“spectral brightness” term is used which is defined as the numberof photons per second per unit area per unit solid angle per0.1% bandwidth. By far the brightest sources in the hard X-rayregion are the synchrotrons with brightness up to 1017 in the10keV range, which are several orders of magnitude higher thanthe X-rays generated in pulsed impact sources. However, thelaser plasma and X-ray diode can produce the shortest X-raypulses. Even though synchrotron facilities are the brightest andhave very wide wavelength ranges, they are also the mostexpensive and least flexible systems. Therefore benchtop-pulsedX-ray sources are also quite useful.

2.2. Reaction Initiation. The reaction, which generates thetransient structure to be studied, must be initiated by a shortlight pulse or other short pulse sources, which are synchronizablewith the X-ray probing pulse. In addition, both pump and probepulses must not damage the sample and must be reproducible.

Initiation can be achieved by various means. These includeshort laser pulses, temperature or pressure jumps, electrical ormagnetic pulses, and chemical diffusion. For the nanosecondand shorter time ranges, use of ultrashort laser pulses forinitiation seems to be the most practical choice. However, themore rapid the process, the higher the peak power needed forinitiation which may be limited by the damage threshold of thesample. In solid samples, excitation by optical pulses invariablyleads to crystal heating. In crystals with strong absorption thelaser energy is deposited in a thin layer near the entrance surfaceand then the energy is transferred into the bulk by heat diffusion.Therefore, strain and stress may be generated in the crystal.This effect is not observed and is not of importance in opticalspectroscopy; however, because of the lattice constant changecaused by the nonequilibrium heating of the sample it is veryevident and of importance for the transient structure of thecrystal. Strong absorption of the input laser radiation can also

Feature Article J. Phys. Chem. B, Vol. 103, No. 34, 19997083

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alter the quality of the crystal. Single-shot experiments requirepowerful X-ray pulses, which may be difficult to generate, andif used, the high flux of X-ray radiation may cause damage tothe sample. “Stroboscopic” experiments, where many shots areaccumulated may be more suitable, but require high repetitionrate systems with accurate synchronization between pump andprobe pulses. In addition, the structural processes studied mustbe reversible within the time period between excitation pulses,or a new sample must be supplied for every shot.

2.3. Detectors.Most of the detectors used, in the time-resolved X-ray studies, have much slower response than thetime resolution needed for many of the processes studied.10

Although there are X-ray streak cameras with picosecondresolution, they require very high X-ray intensities and thereforethey have found very limited use. The most common detectorsfor hard X-rays fall into two categories: integrating detectorsand photon-counting detectors.

Integrating detectors are used most often, especially in X-raydiffraction. They measure some quantity, which varies with theX-ray intensity to which they are exposed. In the case of charge-coupled devices (CCD) this quantity is the amount of chargeaccumulated in a single element of the detector (pixel). In thecase of phosphor image plates, it is the number of F centerscreated in the phosphor and in X-ray film the degree ofblackening as a function of the numbers of X-ray photonsstriking the film. These types of detectors integrate the incomingX-ray radiation, are insensitive to the rate at which the X-rayphotons arrive and they can record large amounts of X-rayradiation as long as saturation is avoided.

CCDs are becoming rapidly the most important integrateddetector. Their performance characteristics, such as highdynamic range, versatile readout, data storage modes, and high-resolution two-dimensional geometry, have opened new pos-sibilities in data handling and processing. CCD detectors areused in two modes: direct and indirect detection of X-rayradiation. In the direct detection mode, the X-ray photons withenergies in the range up to 30 keV are absorbed by the siliconpixel chips; at higher energies the absorption efficiency of siliconbecomes too low to be useful. Indirect CCDs use a phosphorscreen in front that converts the incoming X-ray radiation intooptical photons which in turn are channeled to and detected bythe CCD. Frequently, fiber optic tapers are used as faceplatesbetween the phosphor and the indirect CCD detectors.

Photon counting detectors distinguish and record each incom-ing X-ray photon. In these detectors the limitation is determinedby the rate at which the photons arrive. Detection of a photonrequires some time during which the detector is insensitive tothe arrival of further photons. Although they will not countsimultaneously, arriving photons some of these detectors arecapable of determining the wavelength and position of eachphoton.

There is no perfect detector, all have strengths and weak-nesses, and the optimum choice depends on the exact nature ofthe measurements needed to be performed. Most of the X-raydetectors available measure intensity, but the phase informationis lost, and therefore, it is difficult to Fourier transform theexperimental data. This is the “phase problem” in X-raycrystallography.1

It is worth noting several of the assumptions on which thetime-resolved measurements are based: (1) during the X-rayprobing of the sample the structural changes should be suf-ficiently large to be detectable. As a rule of thumb the fasterthe process is, the smaller the changes. (2) Structural intermedi-ates may be detected, only if in the time scale of the experiment

there is an appreciable accumulation of intermediates and if theirlifetime is longer than the time resolution of the experiment.Also we have to consider the possibilities of several pathwaysleading to similar intermediate structures. (3) Time dependentchanges of the measured X-ray intensities must be reducible toelectron density distributions and atomic structure. Thermal andother conformational gradients will occur, and since they aretime dependent, care must be taken to either eliminate them ordecrease them to a large extent or their influence should beaccounted for in the interpretation of the results. Owing to thenonhomogeneous nature of the transient changes, there willalways be some degree of spatial and temporal averaging to bemade and therefore very often the transient data may not bedefined exactly as in the steady-state experiments. These aresome of the problems we encounter when processing theexperimental time-resolved transient structure data. Continuousimprovement of the data processing techniques is necessary forthe determination of the exact transient structures of complexmolecules.

3. Time-Resolved Studies with Nanosecond andPicosecond X-ray Pulses

We have developed a laser driven X-ray diode whichgenerates nanosecond and picosecond hard X-ray pulses.18,19

This system has allowed us to carry out a number of time-resolved X-ray studies. A laser driven X-ray diode is similar tothe conventional X-ray diode in which the thermal cathode isreplaced with a photocathode. Thus, by illuminating thephotocathode with short laser pulses, short electron pulses aregenerated at the cathode surface and then accelerated by highvoltage applied to the diode. When the accelerated electron pulseimpinges on the anode a X-ray pulse is generated (see Figure2). The operation of the X-ray diode with a photocathode issimilar to that of a conventional X-ray tube where the outputX-ray spectrum is determined by the anode material and theelectron energy. The duration of the X-ray pulse produced bya diode is determined by the duration of the electron pulseimpinging on the anode. To maximize the output X-ray flux,high peak current electron pulses have to be generated andsubsequently propagated through the X-ray diode. The spacecharge effects in the diode place a limit on the maximum outputflux, as well as on the shortest X-ray pulse duration that maybe emitted by the diode. Using this approach and ultrashort laserpulses X-ray pulses with nanosecond to femtosecond durationcan be generated. Because of isotropic space distribution of theX-ray radiation generated on the anode surface and lack offocusing optics for hard X-rays, only a small portion of the X-rayoutput can be collimated and used in experiments. A high

Figure 2. Experimental system for pump/probe time-resolved X-raystudies. A, anode; C, cathode; (s) laser pulses; (‚‚‚) electron pulses;(- -) X-ray pulses; (-‚-) diffracted X-ray pulses.

7084 J. Phys. Chem. B, Vol. 103, No. 34, 1999 Tomov et al.

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repetition rate system is therefore extremely important to makeup for this deficiency in X-ray flux. Our laser driven X-ray diodecan operate in the kHz range and its repetition rate is determined,mainly, by the pulse rate of the pump laser. The most efficientphotocathodes for optical radiation are the bialkali or semicon-ductor devices; however, they degrade at high photocurrentdensity and high repetition rates and also require high vacuumfor prolonged life. We have found that pure metal photocathodesare preferable for use in laser driven X-ray diodes. Usingpowerful ultraviolet (UV) laser radiation and low vacuumcompensate for their low efficiency and high work function.Our X-ray diode employs an aluminum photocathode and acopper anode; however, different X-ray wavelengths can beemitted by changing the anode material. The laser system, whichhas been used to drive the X-ray diode, generates 193 nm pulsesat a repetition rate of 300 Hz. The UV pulse duration was variedfrom ps to ns depending on the studies to be performed. In bothmodes of operation the UV pulse energy delivered by the lasersystem was higher than needed for driving the diode. The excessUV energy was used to form the pump pulse. To achieve this,the output of the laser system was split into two parts one usedto drive the X-ray diode and the other was used to pump thesample. This approach has another major advantage, which isthe very high degree of synchronization between the laser pumpand the X-ray probe pulses. The excellent synchronization makesit possible for us to accumulate the signal from many shots withhigh-resolution accuracy.

Changes in the structure of materials, following pulsed laserirradiation, have been studied for the last two decades. Themotivation for these studies has been to understand ultrafastsystem disorder such as melting and induced phase transitions.Depending on the time scale of the experiments, insight intodifferent phenomena has been obtained. In the nanosecond andpicosecond time scale, crystal structure dynamics of thermallyinduced melting, shock propagation, strain, heat diffusion, crystalregrowth, and annealing have been determined.10

We have used our pulsed X-ray system to study the latticedynamics of metal and semiconductor crystals heated by nanoand picosecond laser pulses. The standard pump/probe set upis shown in Figure 2. The laser pulse absorbed by a thin layerof the sample induces heat, which in turn alters the latticestructure. The transient structure generates changes in thescattered X-rays recorded by the CCD camera. The recordedsignal is essentially a convolution of the material response withthe probing X-ray pulse. The time resolution that can be obtainedfrom these experiments depends not only on the duration ofthe pump and probe pulses but also on the material response,as well as the propagation time of these pulses in the bulk ofthe sample. It has been shown that the thickness of the sampleplaces a limit on the X-ray time resolution.20

It is known that the diffraction of monochromatic X-rayradiation from a crystal is governed by Bragg’s law. When evensmall changes in the interatomic spacing of a crystal occur, theBragg condition changes resulting in a shift in the diffractedangle. The relation between the angle shift∆θ and the latticespacingd is given by differentiation of Bragg’s equation

The angle shift, therefore, is a measure of the change in thespacing of the diffracting planes of the crystal. When the laserradiation is absorbed by a very thin surface layer the temperaturedistribution and the associated stress in the bulk of the crystalwill be nonuniform causing a distortion in lattice spacing. Figure3 shows a schematic diagram of an experiment where a

divergent monochromatic X-ray beam is diffracted by a singlecrystal in which the plane spacing has a gradient. When anonuniform lattice spacing distribution inside the crystal isinduced, the diffracted signal will be composed of signalsscattered over the range of angles corresponding to the latticespacing. If the divergence of the incoming X-ray beam coversall of these angles, then the recorded signal will contain all ofthe information about the distribution of the lattice space changeswhich will be obtained at different positions and at differenttimes. Experimentally, a whole rocking curve may be recordedwith a single shot by illuminating the crystal with a divergentbeam and using a CCD detector which can record the reflectedX-ray radiation over the entire scattered angle.

To calculate the angle dependent diffracted intensity twodifferent theoretical approaches are employed: dynamical andkinematical theory.1,21-23 These two theories describe the X-raydiffraction from perfect and ideally imperfect crystals, respec-tively. The probed depth of the X-ray radiation in a perfectcrystal is determined by its extinction length, which is definedas

Heree2/mc2 is the classical electron radius;λ, X-ray wavelength;Fh, structure factor for reflection; andV, volume of unit cell.Directional cosines of the incident and diffracted wave are:γo

) sin (φ+θ) and γh ) sin (φ-θ), hereθ is the Bragg angleandφ is the inclination of the reflecting plane to the surface.

Usually the extinction length is shorter than the absorptionlength. In a mosaic or imperfect crystal the scattered radiationis effectively decoupled from the incident radiation and theX-rays are attenuated by the photoelectric absorption. In practice,most crystals are somewhere between these two extremes;however, for good quality, semiconductor crystals, such as Siand GaAs and others with relatively small flows, the crystalmay be considered perfect and therefore the dynamical theoryis still applicable.

3.1. Picosecond Time-Resolved Diffraction.In the pico-second mode of operation, the laser system used for these studies

∆d/d ) - ∆θ/tanθ

Figure 3. Schematic diagram of Bragg diffraction from (a) singlecrystal and (b) deformed single crystal.

Am ) 2V x|γoγh|

λ|Fh| ( e2

mc2)-1

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was generating UV pulses with 1.8 ps duration and up to 1 mJenergy. About 100µJ of 193 nm radiation was used to pumpand saturate the X-ray diode. When a voltage of 75 kV wasapplied to the anode, the output X-ray pulse was measured tobe 8 ps.24 The photoemission from the metal cathode is in thesubpicosecond time domain; therefore, we may assume that theelectron pulse generated at the cathode surface has the durationof the excitation laser pulse. However, at high current densities,because of space charge effects the electron pulse is broadenedbefore it reaches the anode. Operation of the diode at low currentdensity allows for the generation of X-ray pulses with durationequal to the excitation laser pulse but at the cost of much smallerX-ray output flux.

The X-ray spectrum emitted by the diode with a Cu anodeconsists mainly of Cu KR, λ ) 1.54 Å, characteristic radiation.This radiation was measured with a large area CCD cameradesigned specifically for direct X-ray imaging. Our 16 bit X-rayCCD camera consists of a 2048× 2048 CCD chip (15µm pixel)interfaced to a Macintosh computer. The active area of the CCDis 30× 30 mm2; and when cooled below-100°C, it is capableof single-photon detection.

The X-ray pulses produced, by the system described above,were used to study the deformation of a gold (111) single-crystallattice heated by 1.8 ps, 193 nm, laser pulses. The experimentalsystem is shown in Figure 2. The 193 nm pulses generated bythe laser were split into two parts: one part, after passing avariable delay line, was focused by a 35 cm lens on the Aucrystal to a 3.5 mm diameter spot size having an average energydensity of 1.6 mJ/cm2. The rest of the 193 nm pulse energywas used to drive the X-ray diode. Two vertical slits formed aX-ray beam with 3 mrad divergence, which is enough to coverthe rocking curve of the crystal. With this set up about 105 CuKR photons per cm2 per second were impinging upon the crystal.Taking into account the geometry of the experiment, we findthat there is an experimental time resolution, imposed by theslit widths and their separation, anode takes off angle and crystalBragg angle. In these experiments, the estimated geometricalresolution is about 10 ps, which is longer than the X-ray pulseduration.

The gold crystal wasL ) 150 nm thick, grown on a 100µmthick mica crystal. Electron diffraction patterns showed a wellordered Au (111) crystal over several mm parallel to the surface.Thus we assume that there is a mosaic structure along thesurface, but along the thickness it is practically a single crystal.The diffracted X-ray radiation from the heated and unheatedarea of the crystal was detected simultaneously in a two-dimensional pattern by the CCD camera. In the measurementsreported here two consecutive exposures were carried out. Thefirst one with the selected part of the crystal under UV irradiationand the following without UV heating. Then a comparison ofthese two exposures was made.

The heating of solid materials with picosecond laser pulses,when neither melting nor vaporization were induced, has beenstudied theoretically in detail.25 We have calculated the heatdistribution in the gold crystal using the optical, thermal, andother properties of gold and mica, which are relevant for ourexperiment. From these data we find that for a 1.8 ps laser pulse,the diffusion length isLd ) 22 nm. This length represents thecrystal depth heated during the pulse illumination. From theabove calculations we find that 0.1 mJ of absorbed energy in aspot size ofS ) 0.1 cm2 will increase the temperature of thevolumeSLd of gold by about 190°C. The heat from this volumespreads within picoseconds to the rest of the 150 nm thickcrystal. According to the heat diffusion theory it takes about

90 ps for heat equilibrium to be established inside the SL crystalvolume. The heat dissipation from this volume may take threedirections: along the gold film, into the air, or through the micasubstrate.

The above estimates show that, for the first 100 ps after thelaser pulse, the UV irradiated area of the crystal is in anonequilibrium transient stage. Inside the crystal there is athermal strain associated with a heated surface layer. Since theheated spot is much larger than the crystal thickness, a one-dimensional strain distribution is reasonable to assume. Thestrain normal to the heated surface layer as a function of depthis calculated from

whereT0 is the temperature of the unheated part of the crystalandR(T) is the linear thermal expansion coefficient.η(h,k,l) isa factor that takes into account the one-dimensional nature ofthe strain.26

A temperature gradient generated in the crystal lattice willalter the X-ray diffracted pattern. In our experiments the X-raypulse is probing the entire thickness of the crystal. Theabsorption loss, for the diffracted X-rays, traveled the longestdistance through the gold crystal is about 30%. Therefore, therecorded diffracted pattern is an integration, over the probedcrystal volume, for the time of the X-ray pulse duration. Weused the theory of X-ray scattering from a one-dimensionallystrained crystal to calculate the diffracted X-ray intensity for agiven temperature distribution. The results for the calculatedcurves, at several different time delays and a heating UV pulsewith an energy of 120µJ, are presented in Figure 4 (solid line).Negative delay times mean that the peak of the X-ray pulsearrives on the crystal earlier than the UV heating pulse.

In the experiment described here, we have studied thediffracted X-ray signal as a function of the delay between theheating UV laser pulse and the probe X-ray pulse. For everydelay point two consecutive 1 h exposures were made. The firstone with UV radiation heating a selected part of the Au crystaland the second one without UV heating. Both X-ray patternsare compared to ensure that there is no change due to any effectsother than those induced by the laser pulse. To compare theexperimental results with the calculated ones, a fit to the

Figure 4. Time-resolved X-ray diffraction Bragg profile curves forAu(111) crystal. The solid lines represent calculated results; dots depictexperimental data.

Φ(z,t) ) ∫T0

T(z,t) R(T)η(h,k,l)dT

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experimental points is made using a Gaussian shape for thediffraction signal. The experimental data curves are normalizedto their peak value and compared with the correspondingnormalized theoretical curves. In this experiment, the time delaywas varied in 10 ps steps in the range of-40 to +100 ps andlarger steps at longer times outside this range. The experimentaland calculated results for several delay times are depicted inFigure 4. The delay time of-20 ps corresponds to a cold crystal;at t ) 0, actually half of the UV pulse energy has been absorbedby the crystal and the change of the rocking curve is time-resolved and clearly observed. While the temperature distributionin the crystal is inhomogenious the scattered X-ray signal is acombination of the signals from the heated and cold parts ofthe crystal. After 100 ps, thermal equilibrium has beenestablished and we observe a shift of the rocking curve towardlarger Bragg angles. In Figure 5, the shift of the peak of therocking curve as a function of delay time is shown. These dataare the average of five runs at delay times between-100 and500 ps. The transition through a thermally nonuniform crystallattice, in the first 50 ps, is clearly seen. In this transition time,the width of the rocking curve is also slightly larger than theone at equilibrium. The spread of the experimental points ispartially due to the shot to shot fluctuations in the UV pulseenergy which results from the jitter in the triggering of theexcimer amplifier. After about 100 ps and up to 500 ps, whichwas the longest delay used in this experiment, no change in theshift was observed. These measurements show conclusively thatour experimental system is capable of 10 ps time resolutionand can easily detect transient lattice structure deformationscaused by temperature changes of about 20°C.

3.2. Nanosecond Time-Resolved X-ray Diffraction.Pulsedlaser processing of materials especially semiconductors is done,more often, using nanosecond rather than picosecond pulses.Therefore understanding crystal lattice dynamics in the nano-second time scale has not only scientific but also technologicalimportance. For these reasons we have performed nanosecondtime-resolved X-ray diffraction experiments which were aimedat recording the evolution of lattice deformation in semiconduc-tors and metals such as GaAs(111) and Pt(111) crystals, afternanosecond-pulsed UV laser irradiation. In these studies, thelaser system generated 12 ns, 193 nm pulses at a repetition rateof 300 Hz and crystals of Pt and GaAs, cut with their surfaceparallel to the (111) plane were used.27 Both of these crystalsabsorb strongly 193 nm radiation, and because only 10 nm of

the surface is penetrated by the UV photons, the bulk of thecrystal is heated by diffusion. The energy of the heating laserpulse on the crystal surface was about 3 mJ corresponding toan energy density of 30 mJ/cm2.

(A) GaAs Crystal.28 Only very high quality GaAs(111)crystals were used in the experiment described here. Severalcrystals of varying sizes were cut from a 0.5 mm thick, 50 mmdiameter GaAs wafer. The output from the X-ray diode consistedof 12 ns Cu KR X-ray pulses which passed through two slitsand then were directed to the GaAs crystal at the Bragg angle.The divergence of the input X-ray beam was large enough tocover all the changes in the Bragg angle induced by the latticechanges occurring as a result of laser heating. The experimentalprocedure was similar to the one used in the picosecond X-rayexperiment described above. The energy of the heating UVpulse, as much as 3 mJ, was deposited in a 0.1 cm2 spot. Wenote that in our studies the maximum energy density was lessthan 50 mJ/cm2. This is several times smaller than the meltingthreshold of GaAs (a 225 mJ/cm2 melting threshold was reportedfor nanosecond pulses at 193 nm29). The size of the UV spoton the crystal was much larger than the X-ray penetration depthin the crystal; therefore, as previously, one-dimensional distribu-tion of the temperature and stress in the probed bulk of thecrystal was assumed.

To calculate the rocking curves for the laser heated crystalwe use the dynamical equations for slightly deformed crys-tals.22,23 We divided the strained part of the crystal into anarbitrary number of parallel layers and an average strain(temperature) was assumed for each particular layer. This kindof calculation has been shown to be accurate for steady-stateX-ray diffraction.23 Additional approximations are necessary fora crystal under transient conditions. In this case, the calculationsare carried out by slicing the probing X-ray pulse into a set ofmicropulses with picosecond duration and then performing thecorresponding calculations for every micropulse. During thepassage of each micropulse, the temperature distribution andrelated strain in the crystal is assumed to be constant but evolveswith each subsequent micropulse. Thus, for each micropulsethe diffraction process may be treated as being in a steady-state condition. With this approximation, we have calculatedfirst the temperature distributionT(z,t) in the crystal and thenthe thermal strainΦ(z,t)associated with a specificT(z,t). Thenwe calculated the X-ray diffracted signal for each micropulseand the overall diffracted X-ray signal was obtained byintegrating over the entire X-ray pulse envelope. Figure 6 showsthe calculated rocking curves for GaAs(111) symmetric Braggreflection at different time delays. In practice the calculatedrocking curve needs to be convoluted with the instrumentalbroadening function. For our experimental setup the instrumentalbroadening function was derived from the static X-ray rockingcurve and then convoluted with the calculated Bragg profiles.Although the simulated rocking curves (Figure 6) showed someshape change and broadening around zero time delay, whenconvoluted with the instrumental broadening function therocking curve became more symmetric. The integration of therocking curve gives us the integrated intensity of the scatteredX-ray radiation. In Figure 7 (solid line) we show the calculatedintegrated reflecting power as a function of the delay betweenthe UV heating pulse and the probing X-ray pulse. As seen,the integrated reflected power increases around zero delay.Qualitatively we can expect such an effect, because weak heatingof the crystal generates layers of the crystal with differenttemperatures and therefore slightly different lattice spacings.This distortion in the lattice leads to an increase in the

Figure 5. Shift of the rocking curve peak as a function of delay time.

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acceptance angle for Bragg reflection and therefore increasesthe integrated reflectivity. We note that, in this case, the crystalis still practically perfect and the dynamical theory is applicable.As the crystal is disturbed more aggressively, by more powerfullaser pulses, the crystal structure may revert to a “mosaic” oneand then an even higher increase of the integrated reflectivityis expected. We estimate, using the expressions for ideallyperfect and ideally mosaic crystals,1 that the integrated reflec-tivity of a mosaic crystal is higher by a factor of 5 than that ofan ideally perfect crystal. These measurements suggest thattransient reflectivity may be used as a diagnostic tool fordetermining the changes in structure which the semiconductorcrystal experiences during processing.

For a GaAs(111) crystal the penetration depth determinedby the absorption of the X-ray radiation in the crystal is 6µmand the corresponding extinction length isAm(111) ) 1 µm.The angular width of the rocking curve for symmetric Bragg-(111) reflection from a GaAs(111) single crystal using Cu KRradiation is∆θ ) 38µrad. An angular shift of the rocking curveof this size can be achieved if the crystal temperature is increased

by about 20°C. Experimentally, we have detected small rockingcurve shape changes occurring only at the 10 ns delay timewhich are due to the temperature gradient in the crystal. Forthe other delay times, only the shift in the rocking curve andthe increase in the reflected intensity were clearly seen. Thecorresponding measured integrated reflectivity is shown inFigure 7 (points), where we see an increase in the integratedreflectivity in the time window of about 50 ns. Physically, thismeans that small deformations in the lattice structure lead toan increase in the acceptance angle and therefore a larger partof the incoming divergent beam will be effectively reflected.We note that the experimental points correspond to the averagereflectivity over 12 ns and the probed depth within the crystal.These data shows directly a histogram of the evolution of thetransient structure of the crystal and its eventual return to theoriginal lattice spacing. The experimental data also indicatesthat this small light energy flux, supplied by the laser pulse tothe crystal, causes only transient structural changes and strainand not a permanent change in the crystal structure. However,higher power laser pulses produce permanent changes in thestructure.

By fitting the calculated and experimental rocking curves, aprofile of the evolution of lattice deformation in the crystal overa 100 ns time window was obtained. The X-ray scatteringcalculations were fitted to the measured data using the thermallyinduced lattice strain as a function of depth. Since the GaAscrystal was assumed to satisfy the dynamical theory, thepenetration depth of the X-ray radiation is governed by theextinction length. Therefore, the fitting was limited to a depthof less than 1.5µm. The temperature profile for larger depthswas determined using heat flow calculations.25 Figure 8 showsthe results of the best fitting procedure for the lattice evolution.We see that the maximum expansion of the lattice spacing takesplace at the end of the heating pulse. For a heating energydensity of about 20 mJ/cm2 the lattice expansion in a 1µm layeris of the order of 8× 10-4 A.

(B) Platinum Crystal.30 Time-resolved X-ray diffractionstudies were carried out also on a Pt(111) crystal, 6.5 mm indiameter and 0.5 mm in thickness. The main difference betweenthe GaAs and Pt crystals was that while the GaAs was apractically perfect crystal, the Pt crystal had mosaic structure.From the thermal properties of Pt we calculate that for a 12 nsheating pulse the diffusion length is 776 nm. This lengthrepresents the crystal depth heated during single pulse illumina-tion, and it is comparable to the penetration depth (800 nm) ofthe X-ray radiation in the bulk of the Pt crystal. For a 2 mJ UV

Figure 6. Dynamical diffraction theory calculations of the rockingcurve at different delay times for a 12 ns X-ray pulse reflected from aheated GaAs(111) crystal.

Figure 7. Calculated integrated reflectivity of laser-pulse-heated GaAs-(111) crystal as a function of time. The dots represent experimentaldata.

Figure 8. Lattice spacing evolution within the GaAs(111) crystalheated by a 12 ns laser pulse.

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pulse energy deposited in a 0.1 cm2 spot, the temperature ofthe Pt crystal, in this area and diffusion depth, will increase by70 °C during laser pulse irradiation. Estimates suggest that forthe first 100 ns after laser pulse excitation, the UV irradiatedarea of the crystal is in a nonequilibrium transient stage. In thebulk of the crystal there is a thermal strain associated with theheated surface layer. The time necessary to establish the strainprofile is roughly equal to the heat diffusion length divided bythe speed of sound in the crystal. For this case it is shorter than1 ns; consequently, the time lag between the establishment ofthe temperature distribution and strain profiles in the crystalcan be neglected.

To calculate the amplitude of the diffracted X-rays we assumethe kinematical approximation, which is appropriate for themosaic Pt crystal structure. The time delay of the experimentwas varied between-15 and+50 ns. At each time delay thestrain distribution can be obtained by the best match of thecalculated and measured X-ray rocking curves. The experimentaland calculated results for the heat induced strain in the crystal,at several delay times is shown in Figure 9. The value measuredis the average strain over 12 ns and the probed depth (800 nm)of the crystal. The lattice spacing were found to suffer a transientchange from 2.2653 to 2.2695 Å, and the strain reached itsmaximum value 10 ns after laser irradiation. These studies showthat time-resolved X-ray experiments detect the strain causedstructural changes even in mosaic, imperfect crystals. Also theexperimental transient structure agrees very well with thetheoretical prediction for both perfect and imperfect crystals inboth nanosecond and picosecond ranges.

4. Time-Resolved EXAFS

For liquids, X-ray diffraction is not well suited because thediffracted signal originates from all atoms in the liquid solution;small Z number atoms do not scatter with high efficiency andthe periodic order of the crystal is absent. There have been ofcourse X-ray diffraction studies of pure liquids, but to ourknowledge no time resolved data have been reported. Mostchemical and biological reactions and systems of interest andimportance take place in solutions. Even in concentratedsolutions, the solvent concentration is 100 to 10000 times largerthan the molecule of interest to be studied therefore its diffractedsignal may be easily masked by the diffracted “noise” signalof the solvent. In addition the short range order of the solution

and the fact that many of the important molecules are composedof low Z number atoms, such as C, H, N, and O, make X-raydiffraction a rather difficult technique to be used for thedetermination of transient structures in pure liquids or solution.We have used our system, in the nanosecond mode of operationand presently in picosecond range, to record diffraction signalsfrom a number of liquids and powder material.

However, to overcome some of the diffraction disadvantageswe are applying time-resolved EXAFS techniques for thedetermination of the evolution of transient structures in liquids.EXAFS, to our knowledge, has never been used before forultrafast time-resolved studies. EXAFS spectroscopy measuresthe absorption spectra in the vicinity of the absorption edge ofa selected atom in the liquid or solid state. By selecting theenergy of the probing X-ray continuum to be in the region ofthe X-ray edge of a particular atom, the structure of the firstfew coordination layers around this atom can be measured. Thestructural information obtainable by EXAFS consists of bonddistances and angles. EXAFS may also provide a measure ofdisorder in bond distances caused by any means includingoptical and thermal pulses.

The majority of the reported results in EXAFS studies havebeen obtained by means of point by point measurement.2 Thisapproach has limitations when applied to time-resolved studiesof molecular systems where structural changes are fast, becauseeach data point of the spectrum is collected at a different time.Also the time needed to collect a large number of points requiresproportionally long exposures. To eliminate these disadvantageswe have utilized a dispersive method for EXAFS measure-ments.31 Some of the most important advantages of this approachare

1. The entire EXAFS spectrum of interest is recordedsimultaneously. Therefore, fluctuations in the incident X-raybeam intensity do not influence the quality of the EXAFSspectra. In addition the time of X-ray exposure is greatlyreduced.

2. There are no moving parts in this spectrometer, and theX-ray path length is kept short; therefore, less powerful X-raysources than synchrotrons can be successfully employed.

3. Use of a large size X-ray CCD makes possible thesimultaneous recording of both the EXAFS spectrum of thesample and the incident X-ray radiation reference signal.

Figure 10 shows a schematic representation of our time-resolved EXAFS experimental system. A 20 cm diameter Si-(100) crystal oriented for the nonsymmetric (511) reflection isused for the dispersion of the X-ray radiation signals. Both

Figure 9. Evolution of strain in a Pt crystal as a function of timecalculated using the kinematical model (solid curve). Points representthe X-ray rocking-curve experimental data.

Figure 10. Schematic diagram of the dispersive X-ray spectrometerfor time-resolved studies.

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tungsten and molybdenum anodes have been used, in the X-raydiode, to generate the X-ray continuum. A X-ray CCD detector(1242× 1152 pixels, 22.5µm pixel) was situated 43 cm awayfrom the crystal. With this experimental system a 1000 eVEXAFS spectrum in the 13.5 keV, Br edge, region is spread 44mm along the Bragg angle direction. In our pulsed X-ray systemthe CCD records about 500 eV of the spectrum simultaneously.In the course of the experiment, one-half of the X-ray beampasses through the sample while the other half propagatesthrough air only and is used as a reference. This arrangementmakes possible the simultaneous recording of both the sampleand reference X-ray spectrum. Using W Lγ lines for calibration,this dispersive system achieved better than 8 eV resolution at13.5 keV and 5 eVfor the 9 keV Cu edge. Figure 11 shows theEXAFS spectra of Cu metal foil and Cu2+ ion in a solution ofCuBr2 in water, recorded by our EXAFS spectrometer.

The time resolution measurements are performed by thepump/probe method described above. Using our time-resolvedEXAFS system, we have recorded, first, the spectrum of CBr4

dissolved in alcohol, then exposed this solution to pulsed UVradiation and again recorded the EXAFS spectrum 10 ns afterexcitation. These preliminary results (Figure 12) display thestructural changes in the EXAFS spectrum before and afterphotodissosiation of the CBr4 molecule. We have shown thatwe can obtain EXAFS spectra, in liquids, with nanosecondresolution and record transient structures. We are in the processof measuring this, CBr4, and other molecular reactions in theliquid phase, with picosecond resolution, using our EXAFSsystem. These experiments will reveal directly their intermediateexcited state structures and bond length changes as function oftime. To our knowledge this is the first description ever of fasttime resolved EXAFS experiment. One of our goals is to

measure directly time-resolved transient structural changestaking place in a molecule and its immediate neighborhoodduring and after illumination with ultrashort UV pulses such asdissociation or dipole formation in a molecule and the subse-quent solvent structural reorientation. We use the same ultrashortUV pulses to both excite the molecule and pump the X-raydiode. Therefore, we achieve here also excellent synchronizationbetween the laser pump and X-ray probe pulses, which ismandatory for accurate time resolution in the picosecond andsubpicosecond range. To avoid thermal effects the solution iscirculated. The above experiment is just one example and ispresented for the purpose of showing that indeed we can performpulsed EXAFS and therefore time-resolved experiments withnanosecond and picosecond resolution. This method, we believeopens a new, vast and as of now unexplored, area for the studyof transient structure evolution in the liquid or solid state usingany chemical and biological molecules of interest.

5. Conclusion

The field of time-resolved X-ray spectroscopy has madesignificant progress in the past decade. The recent advances insource technology have stimulated a wide variety of novelexperiments using both synchrotrons and smaller laboratory sizesystems. These facilities make it possible to measure directlythe histogram of the transient structures and states occurringduring the course of a chemical or biological reaction.

Most important, these time-resolved X-ray facilities aremaking possible the direct detection and assignment of ultrafasttransient structures during the course of a photophysical,chemical, or biological process which have never been seen withany other method. Time-resolved X-ray diffraction and absorp-tion promise to provide real time “snapshots” of the structureevolution of ultrashort excited states and intermediates whichwere impossible to detect previously by other experimentalmethods.

Acknowledgment. This work was supported in part by theNational Science Foundation Grant CHE-9501388 and the W.M. Keck Foundation.

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