Surface chemistry experiments speed upJohanna L. Miller Citation: Phys. Today 66(10), 15 (2013); doi: 10.1063/PT.3.2134 View online: http://dx.doi.org/10.1063/PT.3.2134 View Table of Contents: http://www.physicstoday.org/resource/1/PHTOAD/v66/i10 Published by the AIP Publishing LLC. Additional resources for Physics TodayHomepage: http://www.physicstoday.org/ Information: http://www.physicstoday.org/about_us Daily Edition: http://www.physicstoday.org/daily_edition

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www.physicstoday.org October 2013 Physics Today 15

In studying a chemical reaction on asurface, it’s useful to know not only theidentities of the desorbed products but

also their three-dimensional velocitydistributions. The distribution of kineticenergies provides information about theenergy released in the reaction. And thedetails of any anisotropy in the productvelocities offer clues into the transition-state geometry and forces imparted tothe products.

Ion-imaging techniques developedfor gas-phase chemistry, which captureproducts on a 2D position-sensitive de-tector, can help to measure velocity dis-tributions. But until recently, limita-tions on imaging speeds have meantthat only one product could be detectedat a time. Repeating the experiment tolook for every possible product—and areaction involving organic moleculescan have many—is tedious. And for asurface reaction, it requires the prepa-ration of new, clean surfaces, which cancomplicate comparisons of data fromdifferent repetitions.

Over the past several years, a teamof chemists and physicists at OxfordUniversity and the Rutherford Apple-ton Laboratory (RAL) developed a newultrafast sensor—called PImMS, forpixel imaging mass spectrometry—thatallows the imaging of multiple prod-ucts in a single experiment.1 So far, it’sbeen used mostly for gas-phase chem-istry. Now Michael White and col-

leagues, at Brookhaven National Labo-ratory and Stony Brook University,have applied PImMS to surface chem-istry.2 And the technique shows prom-ise for many more applications.

Imaging ionsThe basic concepts of ion imaging dateback decades.3 A chemical reaction isinitiated, typically by a pulsed laser,and the products are ionized and accel-erated by an electric field toward thedetector. Microchannel plates convertthe incident ions into bursts of elec-trons, and a phosphor converts the elec-trons into an optical signal, which canbe captured by a camera, typically aCCD. In 1997 André Eppink and DavidParker modified the technique by refin-ing the ion optics, a series of chargedelectrodes that focus all product mole-cules with the same initial velocity vector onto the same spot on the detec-tor, even if they originate in differentplaces.4 That velocity-map imagingtechnique, which allowed the reactionto be carried out in a larger region with-out loss of image resolution, proved ex-traordinarily useful to the field of gas-phase chemical dynamics.

Acceleration in an electric field sep-arates the ionized products by theirmass-to-charge ratio, and almost all theions are singly charged, so differentchemical species arrive at the detectorat different times; in that respect, the

Surface chemistry experimentsspeed up

Figure 1. Instruments for imaging a surface reaction, shown schematically. Pulsedlaser light at 335 nm initiates a reaction among molecules adsorbed on the titaniumdioxide surface. Desorbed products are ionized by 95-nm radiation, focused by theion optics, and accelerated toward the detector. As they traverse the time-of-flight(TOF) tube, products spread out in space and become separated by their mass-to-charge ratio. Each ion incident on the microchannel plates creates a burst of electrons. The phosphor converts the electron burst into an optical signal, which isimaged by the camera. (Adapted from refs. 2 and 5.)

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From ARPESto X-rayDiffraction

A new camera makes it possible to collect angular information aboutall products of a reaction at once.

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16 October 2013 Physics Today www.physicstoday.org

search and discovery

technique resembles time-of-flightmass spectrometry. But the differentproducts are separated by at most a fewmicroseconds, whereas a commercialCCD has a time resolution of tens ofmilliseconds. By time-gating the instru-ment’s electronics, one can use a CCD tocapture an image of a single productmass, but not to separately image morethan one mass in a single cycle.

Enter PImMSOxford chemists Claire Vallance andMark Brouard were experienced inboth ion imaging and mass spectrome-try, and the PImMS collaboration arosefrom their desire to combine the two.They teamed up with Oxford physicistAndrei Nomerotski (and later RichardNickerson), who brought expertise indetectors for particle physics, and Renato Turchetta of RAL, a specialist insensor design.

Unlike a CCD, the PImMS cameradoesn’t capture a series of full-frameimages. At the desired 25-ns frame rate,most of those images would be blank.Instead, each time one of the camera’s5184 pixels detects a flash of light, itrecords a time-stamped data point, (x, y, t). The electronics that produce thetime stamps—more than 600 transistorsper pixel—are contained in the pixelsthemselves.5 Such complex circuitryneeds to be shielded from parasitic col-lection of the charge carriers generatedby the incoming photons. That shieldingis achieved by strategically placing im-plants of highly doped semiconductor, atechnology known as INMAPS (isolatedn-well monolithic active pixel sensors)from Turchetta’s group.6 INMAPS isalso being deployed in particle-physicsdetectors.

Vallance and Brouard did the firsttest experiments of PImMS on gas-phase systems. Last fall, Nomerotskiwent to Brookhaven to work on the de-tectors for the Large Synoptic SurveyTelescope. There he met with White,who expressed an interest in the PImMScamera. White had long known aboutthe impact that ion imaging had had onthe study of gas-phase reactions, and hewanted to try it out on surface reactions.He’d made some preliminary attemptsusing a conventional camera, but withlimited success. The PImMS camera,which allowed imaging of all productmasses simultaneously, was just thething he’d been looking for.

Surface chemistryFor their proof-of-principle experiment,White and his group looked at photooxi -

dation of 2-butanone (CH3COCH2CH3)on titanium dioxide, a material knownfor its ability to catalyze the degrada-tion of organic pollutants into water,carbon dioxide, and smaller organicmolecules. Previous studies of that re-action had identified strong signals at27, 28, and 29 atomic mass units, corre-sponding to the organic fragmentsC2H3, C2H4, and C2H5. But it wasn’t clearwhether each of those fragments wasreally produced in the surface reactionor whether some of them might be theresult of larger fragments falling apartwhen ionized. Such dissociative ioniza-tion is a pervasive phenomenon thatcomplicates the study of almost all or-ganic photochemical reactions.

White and colleagues prepared asingle-crystal surface of TiO2, dosed itwith O2 and 2-butanone, and irradiatedit with pulsed 335-nm laser light to ini-tiate the reaction. With their ion opticsset up for velocity-map imaging, theycould irradiate the whole centimeter-square surface and still obtain angularinformation on the desorbed products.The products were ionized with 95-nmradiation, as shown in figure 1, and ac-celerated toward the detector and thePImMS camera.

Some of their data are shown in figure 2, with the detector’s two spatialdimensions collapsed into one for easeof display. Signals appear, as expected,at masses 27, 28, and 29, with the lower-mass signals appearing larger on thedetector plane. That spatial size pro-gression was entirely consistent withdissociative ionization: C2H5 radicalsare produced in the surface reaction,and some of them, when ionized, loseone or two of their hydrogen atomsalong with an electron. With the mo-mentum imparted to them by the escap-ing H atoms, the C2H3 and C2H4 frag-ments end up with broader transversevelocity distributions than their parentC2H5 radicals.

But the smoking gun was in the C2H3signal. The spatial distribution of dataat mass 27 is so broad, compared to thefinite size of the ionizing region, that itcould not result from C2H3 radicals de -sorbed directly from the surface. It mustbe due to dissociative ionization.

Moving aheadWhite and colleagues plan to continueto use PImMS to study surface photo-chemistry. The camera’s time resolutionand multimass capabilities will allowthe study of velocity and spatial prod-uct distributions in complex polyatomicsystems. The researchers’ long-termgoal is to combine PImMS with ultra-fast lasers to watch surface reactions inreal time.

Photochemistry—in the gas phase oron a surface—is not the only possibleuse for the PImMS camera. As Vallanceand Brouard emphasize, it can be usedfor any particle-imaging applicationthat requires nanosecond to microsec-ond time resolution. For example, by re-focusing the ion optics so that each pixelcollects all particles with a commonpoint of origin rather than a commonvelocity vector, one can obtain mass-resolved images of surfaces, includingbiological specimens.7

Johanna Miller

References1. A. T. Clark et al., J. Phys. Chem. A 116, 10897

(2012).2. M. D. Kershis et al., J. Chem. Phys. 139,

084202 (2013).3. D. W. Chandler, P. L. Houston, J. Chem.

Phys. 87, 1445 (1987).4. A. T. J. B. Eppink, D. H. Parker, Rev. Sci.

Instrum. 68, 3477 (1997).5. J. J. John, J. Instru. 7, C08001 (2012).6. J. A. Ballin et al., Sensors 8, 5336 (2008).7. M. Brouard et al., Rev. Sci. Instrum. 83,

114101 (2012); see also http://pimms.chem.ox.ac.uk.

TIME OF FLIGHT (µs)

DE

TE

CT

OR

PL

AN

E

CH3+

H O2+

C H2 (3−5)+

O2+

11.1 13.1 15.1 17.1 19.1

Figure 2. Pixel imaging mass spec-trometry data on the photooxidationof 2-butanone on a titanium dioxidesurface. The two dimensions of the de-tector plane are collapsed into one forease of viewing. (Adapted from ref. 2.)

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