FEMTOCHEMISTRY 83 CHIMIA 2000, 54, No.3

Chimia 54 (2000) 83-88 © Neue Schweizerische Chemische Gesellschaft ISSN 0009-4293

Femtochemistry

Majed Chergui*

Abstract: A brief review of the developments leading to the advent of femtochemistry is presented, along with the major achievements that have marked the past fifteen years since the birth of the field. Keywords: Biological systems· Condensed phases· Femtochemistry . Isolated molecules

The Nobel Prize for Chemistry for 1999 1. Historical Background where k is the Boltzmann constant, T is the was awarded to Ahmed Zewail, Professor temperature (in Kelvin) and Ea is the so- of Chemistry and Physics at the California The pioneering studies of A.H. Zewail called activation energy, i.e. the height of Institute of Technology, for 'his pioneer- and co-workers were the culmination of a the barrier up to a hypothetical state called ing investigation of fundamental chemi- century of efforts aimed atobserving chem- by Arrhenius, the 'activated complex' (see cal reactions, using ultra-short laser flash- ical reactions in real-time. The story starts Fig. 1). The Arrhenius equation has been es, on the timescale on which the reactions at the end of the XIXth century, more used with success by chemists and physi- actually occur. Professor Zewail's contri- precisely in 1889, when Svante Arrhenius cists to describe kinetic processes in a butions have brought about a revolution in presented a description of how chemical large class of media. chemistry and adjacent sciences, since this reactions vary as a function of tempera- In 1931, H. Eyring and M. Polanyi [2] type of investigation allows us to under- ture. His well-known formula describes developed the first potential energy sur- stand and predict important reactions' [1]. the temperature dependence of the reac- face for the H2 + H ~ H + H2 reaction and His works and discoveries gave birth to a tion rate k(T): J. Hirschfelder, Eyring and B. Topley [3] new branch of chemistry,femtochemistry, performed the first trajectory calculation a term he coined while celebrating his first k(T) = EafkT with femtosecond steps in 1936 using the results at a champagne party in 1987. then available computing power. This was Femtochemistry is concerned with the visualization of elementary processes in molecules. These involve the breaking and making of chemical bonds, which occur on the very short timescale of fem- Activated toseconds (1 fs = 10-15 s) to picoseconds (1 complexX* ps = 10-12 s). This can be better understood if one thinks of an elongation or a contrac- tion of a chemical bond, i.e. a motion that 1------corresponds to half a molecular oscilla- tion. Thus in H2 (the lightest molecule), the vibrational period is 7.6 fs. In the 12 molecule, the vibrational period is 160 fs. E'a On these short timescales the distances traveled are small, typically tenths of Ang- stroms, but these are crucial in chemical 1_Reactant---1A processes. To bring things to a more 'hu- man' scale, one femtosecond is to one second what one second is to thirty-two dE million years!

1 _ Fig. 1. Relation be- Product B tween the activation ·Correspondence: Prof. M. Chergui energies for the for- Institut de Physique de la Matiere Condensee ward (Eal and reverse Universite de Lausanne (Ea') reactions and CH-1 015 Lausanne-Dorigny Tel.: +41 21 6923678 Reaction coordinate the change (~E) in Fax: +41 21 692 36 35 internal energy in the E-Mail: [email protected] reaction. FEMTOCHEMISTRY 84 CHIMIA 2000,54, No.3 the birth of 'reaction dynamics', which Shortly before these developments, the packets). At that time, a temporal resolu- allowed one to think in terms of potential foundations of quantum mechanics were tion of seconds to at best milliseconds was energy surfaces superimposed by dynam- being laid down. In 1926, Erwin Schro- possible in chemistry by means of the ics: the path of a reaction from reactants to dinger introduced the idea of wave groups stopped-flow technique. The field had to products, through valleys and mountains, in order to make a natural connection wait several decades in order to reach the with the transition state at the saddle point. between quantum and classical descrip- required time resolution. The long-stand- Around that time, Eyring [4], M.G. Evans tions [6]. A year later, Ehrenfest published ing efforts to improve the latter were the and Polanyi [5] formulated the transition- his famous theorem [7], outlining the re- work of many researchers. G. Porter and state theory, which gave an explicit ex- gime for the transition from a quantum to R. Norrish introduced the flash photolysis pression for the pre-exponential factor A a classical description: the quantum ex- technique which allowed millisecond time in the Arrhenius equation by invoking pectation values behave classically in the resolution [8]. By exposing a chemical statistical mechanics. The theory gave an limit of large quantum numbers. The use solution to heat, pressure or an electrical analytical expression for the rate constant of wave groups in physics (and obviously shock, M. Eigen achieved the microsec- with a 'frequency' for the passage through in chemistry) remained limited to a few ond temporal resolution [9]. The advent of the transition state. This frequency factor theoretical examples. the pulsed nanosecond laser in the mid is typically 1013 Hertz, the typical value Experimentally it was not possible to 1960s [10] and soon after of the picosec- for the frequency of molecular vibrations! synthesize wave groups (now called wave ond laser [11] brought about a million times improvement in resolution. Howev- er, even on the short picosecond timescale, molecular states already reside in eigenstates (the static limit) and there is only one evolution observable, the change ofpopulation with time of that state. Hence, with picosecond spectroscopy employed (e) in numerous applications in chemistry and biology, one is still concerned with kinetics, not dynamics. The advent of femtosecond laser technology, thanks to the work of C.V. Shank and co-workers [12] finally opened the door to femtochemistry, as one was now able to probe molecular motion in real-time. Indeed, as men- (f) tioned in the introduction, the vibrational periods of molecules are all larger than - 8 fs. A wavepacket can now be prepared, as the temporal resolution is sufficiently short to 'freeze' the nuclei at a given internucle- ar separation, i.e. the time resolution is (g) much shorter than the vibrational (and rotational) motions such that the wavepacket is prepared, highly localized, with the structure frozen on the excited-state potential surface. This ultrashort pertur- bation does not violate the uncertainty principle. On the contrary, a wavepacket is a coherent superposition of eigenstates and therefore, the system is coherently prepared. Because of this coherent synthesis, the transition from kinetics to dynam- (II) ics is possible, and the high temporal resolution implies a spatial resolution, so that one is able to monitor the evolution of the system at atomic resolution! Fig. 2. In 1894, Edouard-Jacques Marey developed a shutter camera to capture the motion of In kinetics one looks at the overall animals. In the above figure, he used a camera with 0.2 sec time resolution to analyse how a cat, population of substrates before the reac- released legs up, falls on its feet. While the cat is undergoing translational motion from (a)to (e), tion and of products after the reaction, and it performs a first isomerization process in (c) to (d) followed by a second one from (d)to (e).This conclusions are drawn about the reaction double isomerization preserves the symmetry of the cat, that reaches by (g) the ground in an mechanism. Femtochemistry on the other excited state! From (g) to (h), the cat relaxes (similar to vibrational cooling in ground state hand isconcerned withdynamics, asthanks molecules). The same principle was applied by A.H. Zewail and his group to capture the motion to the atomic-scale resolution, one can of molecules (or assemblies of molecules) while they undergo chemical change, except that his camera had a resolution of 0.000000000000001 sec! By analogy, the hands releasing the cat think of the molecular system as a set of correspond to the pump pulse in Zewail's experiments, while the shutter camera taking snap interaction spheres (the atoms) and reac- shots of its motion corresponds to the probe pulse. tions can be visualized directly. FEMTOCHEMISTRY 85

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Fig. 3. Example of a bimolecular reaction. In the ground state the IH..·C02 complex is weakly bound by hydrogen-bonding, the pump pulse dissociates the H atom from the HI molecule. The collision of the hydrogen atom with the carbon dioxide creates carbon monoxide, iodine and hydroxide. This reaction has been observed in real-time by Zewai! and co-workers using femtosecond spectroscopy [16].

With this powerful tool in hand, Ze- the molecule apart (dissociation). Out of In another experiment, Zewai 1and co- wail and co-workers were able to observe this process, new chemical species (the workers addressed an issue that had occu- the dynamical processes of bond break- fragments) are created. In order to clock pied chemists for many years, without ing, bond making and of the cornerstone their birth and to determine their order of reaching a clearcut answer. That is: why of reactivity, the transition state, with atom- appearance, another pulse travelling just a are certain chemical bonds more reactive ic-scale resolution. Swedish researcher S. few femtoseconds behind the first, hits the than others? What happens if a molecule Forsen of Lund University likened the fragments to give rise to emission of light has two equivalent bonds? Will they break situation, before the advent offemtochem- which will thus be detected by the fluores- simultaneously or sequentially? To tackle istry, to an audience seeing only the very cence spectrometer. The second pulse, this problem, they studied the dissociation beginning and the very end of Hamlet: 'the called the probe pulse, can be timed pre- of diiodotetrafluoroethane (C2F412) into main characters are introduced, then the cisely at different intervals to reveal how tetrafluoroethane and two iodine atoms curtain falls for a change of scenery, and, long it takes for various chemical species (Scheme 1)and found [15] that the two C- as it rises again, we see on the stage a to appear and in what order they do so. I bonds break sequentially: the first bond considerable number of bodies and a few cleavage takes about 200 fs, while the survivors. Not an easy task forthe unexpe- second follows on a timescale a 100 times rienced to unravel what actually took 2. From Small Molecules to longer. place'. Now the stage is set to see the Complex Organic Molecules Femtosecond spectroscopy has also whole action! been used to examine analogues of the With his ultrafast camera, Zewail and The first age of femtochemistry was activated complex involved in bimolecu- co-workers were able to capture each in- marked by the study of simple molecular lar reactions (see Fig. 3). In an experiment stant in the evolution of a molecular sys- systems, in order to establish the experi- [16], that has now become a textbook tem and generate a 'molecular movie'. mental methods and the theoretical con- example, Zewail and co-workers used The principle is very similar to that used cepts [13]. The experiment that gave birth molecular beams to produce a weakly more than a hundred years ago in the then- to femtochemistry was carried out by Ze- bound IH..·C02 complex between HI and nascent photography when shutter camer- wail and co-workers in 1987 on the disso- CO2, which is held together in the ground as with sub-second time resolution were ciation of cyanogen iodide (ICN), in which state by weak hydrogen bonding (Fig. 3). developed by E. Muybridge and by E.-J. the appearance of a free CN fragment was The HI bond can be dissociated by a fem- Marey to record the motion of animals found to occur in about 200 fs [14]. tosecond pulse, and the H atom is ejected (see Fig. 2). To take a picture of a simple chemical Scheme 1 reaction, Zewail and co-workers used two beams of femtosecond pulses and a fluo- rescence spectrometer. A first pulse of light (called the pump pulse) strikes the +21 molecule and energizes it. If the photon energy is sufficient, it eventually breaks FEMTOCHEMISTRY 86

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Scheme 2 well as photochemical reactions. For ex- rotation of molecules [28] to bond-break- ample, a classic case of such a reaction is ing events [29]. Contrary to the expecta- the ring opening of cyclobutane to yield tion of an ultrafast energy randomization, ) ethylene or the reverse process (Scheme coherent nuclear motion inthe retinal chro- + 2). mophore [30], in a membrane protein [31] This provides a case study of a Wood- and for molecules in solutions or in crys- ward-Hoffman reaction. The reaction may tals [29][32-34] has been observed, mak- proceed directly through a transition state ing it possible to localize the energy along at the saddle point of an activation barrier specific reactive coordinates for a suffi- (Fig. 4a) or, it may proceed by a two-step ciently long time in the condensed phase. a) mechanism, starting with the breakage of The very first event in a chemical reac- one a-bond to yield tetramethylene (Fig. tion is the change of binding forces that [[J]t 4b). The heart of the problem lies in the originates from a redistribution of charge. competition between these two mecha- Therefore, in solvents, there is the need to nisms: a concerted one-step process ver- understand the dynamics of electronic sol- sus a two-step process with an intermedi- vation processes, whereby the environ- ate. Such intermediates are expected to be ment adapts to the new distribution of longer lived than transition states, such charges - a step that uses up part of the that the dynamics of their nuclear motion energy available for the reaction process. (vibration and rotation), unlike a concert- Ultrafast pump-probe spectroscopy has ed motion (translation), determine the recently been applied to pinpoint the de- outcome of the reaction. An Arrhenius tails of solvation dynamics in polar type treatment of the reaction is unable to [35][36], non-dipolar [37][38] and non- . . distinguish between the two schemes, and polar media [39]. Recently, the simplest of one actually only sees an activation barrier all solutes, the solvated electron, has been b) t U t (Fig. 4a). By combining femtosecond spec- used [40], and some of these studies were troscopy with time-of-flight mass spec- carried out using the ultimate time resolu- trometry and molecular beams, Zewail tion of 5 fs! and co-workers clearly established the The various steps of a biological func- existence of the diradical (Fig. 4b) as a tion can now be followed as the time distinct molecular species on a sub-pico- resolution is bridged from seconds to fem- second timescale [17]. toseconds. Major breakthroughs have re- Nearly all types of reactions in organic cently been achieved in our understanding Fig, 4. A classic case for a Woodward-Hoff- chemistry (Diels-Adler [18][19], Norrish of primary biochemical processes in pro- mann description of concerted reactions. The type [20], Bodenstein type [21] and many teins by a combination of femtosecond reaction may proceed in two ways. In a) the others [22-28]) have been scrutinized by spectroscopy and site-directed mutagene- reaction proceeds directly through a transition Zewail's group. They allow us to define sis. Molecular biology methods take the state at the saddle point of an activation barrier. In b) it proceeds by a two-step mechanism, the mechanistic concepts crucial to the role of chemical synthesis and many pro- beginning with the breakage of one a-bond to understanding of the nature of chemical tein systems can be manipulated at will produce tetramethylene as a diradical interme- bond changes. and obtained in a reproducible and unique diate. way. This has greatly contributed to make several biological systems the testing 3. From Condensed-Phase grounds of elementary reaction mecha- Chemistry to Biology nisms and dynamics. Such combinations towards the 0 atom of the neighboring of techniques are unraveling details of e02 molecule to form HOeO. Hence, the Most of chemistry, in particular pre- fundamental processes in biological phe- van der Waals molecule is a source of a parative chemistry, takes place in the con- nomena such as vision, photosynthesis, species that resembles the activated com- densed phase. Therefore, any chemical etc. [30][31][41-43]. plex of the reaction: change is affected by the motion of the Elementary chemical reactions Iikecis- surrounding species. Because of collisions trans isomerizations and dissociations are H + 2 ~ [H OF HO+ 0 with solvent molecules, the time and ener- the basis of the function of many biologi- gy barrier of a reaction may change with cal photoreceptors (retinal in vision and in The study of large molecules has respect to those in free (i.e. gas phase) bacteriorhodopsin; linear tetrapyrroles in opened up new and fascinating horizons in conditions. The competition between sol- phytochrome; p-comaric acid in the pho- physical-organic chemistry and has al- vent fluctuation times and the timescale of toactive yellow protein, etc.) and enzymes ready changed our view of fundamental a reactive motion is crucial if one wants to (myoglobin, hemoglobin, cytochrome, reactions, in a way that was unthinkable avoid randomization of energy along a etc.). Ultrafast spectroscopy is providing just a few decades ago. A good example is specific reactive coordinate in the con- detailed insight [30][31][41][42] into the the case of the diradical hypothesis in densed phase. The interplay between these function of such systems. Indeed, the abil- organic chemistry. For the past 60 years, timescales (coherent versus diffusive) is ity to visualize vibrational motion in a the concept of diradicals as intermediates now being addressed with unsurpassed protein enables studies of the relation be- of reactions has been considered the ar- precision by femtosecond spectroscopy. tween nuclear motion and biological func- chetype of chemical transformation in This concerns all possible reactions rang- tion. As an example, it is known that many classes of thermally activated, as ing from barrier-crossing events [26], over hydrogen bonds bind the double-stranded FEMTOCHEMISTRY 87

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DNA helix and determine the comple- electron bunch to record its time-evolving tion of the electron and of valency, that mentary of the pairing. Until recently the diffraction pattern [46]. play such a crucial role in chemistry. The dynamics of hydrogen bonds and the en- Electrons have low penetration depths generation of attosecond pulses is in prin- suing tautomerization, which isthought to into matter, so that one is mainly limited to ciple feasible [51]. When it becomes real- disturb the genetic code, was not well gas-phase reactions, at least for the time ity, we will be at the threshold ofa new era understood. By carrying out femtosecond being. Another approach that delivers lo- in the study of atomic and molecular sys- pump-probe studies on model base pairs cal structural information is X-ray absorp- tems. (e.g. 7-azaindole), Zewail's group [43] tion (via EXAFS, the Extended X-ray The field of femtochemistry has un- identified different timescales of the struc- Absorption Fine Structure) which has the dergone a real explosion since its birth in tural relaxation in the initial pair, vibra- advantage of being applicable to disor- 1987. There is no doubt that Ahmed Ze- tional relaxation and cooling of the tau- dered as well as ordered media. In combi- wail's personality has played a crucial role tomer. These studies have yielded a de- nation with ultrashort X-ray pulses it is in this respect. His ability to communicate tailed molecular picture of the nuclear now being implemented to probe dynam- with strength and conviction the essence dynamics and are providing us with the ical processes in condensed-phase chem- and the beauty of the new physics and fundamentals of chemical reactivity in ical media [47]. chemistry he was unraveling with his ul- biological functions. These dynamical structural approach- trafast camera, has been decisive to the es, based on X-ray diffraction and X-ray evolution of the field. The far-reaching absorption, are opening the way to an discoveries he made, matched by his cha- 4. From Spectroscopy to Imaging imaging of time-evolving biological sys- risma and his enthusiasm have marked the tems or chemical systems in the course of field throughout the world. There is no Structure is at the heart of biology and a biological function or a chemical trans- doubt that the scientific community will of chemical sciences. F. Crick, co-discov- formation. experience many new discoveries and the erer of the double-helix DNA structure emergence of new concepts from his group. used to say 'If you want to understand Ahmed Zewail has been a regular visitor function, study structure'. Femtochemis- 5. Outlook and Conclusions to Switzerland. His achievements have try has added the dimension of time to been recognized by the Honorary Doctor- structure and we should now speak of Femtochemistry has opened new ave- ate of the University of Lausanne in 1997 'time-dependent structures'. nues in chemistry, biology and material and the Paul Karrer Medal in 1998. In an optical pump-probe experiment, sciences that could not have been foreseen both the excitation and the probing rely on 20 years ago. In particular, the ability to an apriori know ledge ofthe spectroscopy view molecular dynamics also suggests Received: January 26, 2000 (energy, intensity and lineshape of bands) new ways of controlling reactions. The of the system under study. Unfortunately, outcome of a chemical reaction is an inter- [1] Nobel citation spectroscopic data are not easy to translate ference of molecular pathways and the [2] H. Eyring, M. Polanyi, Z. Ph)'s. Chem. to atomic coordinates, except in a few rare prospect exists now for fine-tuning the 1931, B1 2,279; M. Polanyi, .Atomic Reac- cases such as small molecules for which motion and reactivity of molecules, so that tions', Williams and Norgate, London, potential curves or potential surfaces are laser-customized chemistry may be devel- 1932. [3] J.O. Hirschfelder, H. Eyring, B. Tapley, J. available. On the other hand, techniques oped [48][49]. This fine tuning can be Chern. Ph)'s. 1936, 4, 1700. achieved by appropriately shaped femto- such as X-ray or electron diffraction have [4] H. Eyring, J. Chen!. Phys. 1935,3, 107;H. proven to be powerful tools for the struc- second pulses which can drive a system Eyring, J. Chern. Phys. 1935,3,492. tural determination of condensed matter along a well-defined reaction channel. A [5] M.G. Evans, M. Polanyi. Trans. Faradav and biological molecules with a high spa- complementary approach is the use of Soc. 1935,31,875; M.G. Evans, M. Pol;- tial resolution. Therefore, one could in high intensity pulses which, because of nyi, Trans. Faraday Soc. 1937,33,448. principle combine the direct inversion their high electric fields, can perturb the [6] W. Schrodinger, Ann. Phys. 1926, 79,489. [7] P. Ehrenfest, Z. Phys. 1927,45,455. advantages of these techniques and their molecular system in such a way that in- tramolecular couplings are created in the [8] R.G.W. Norrish, G. Porter, Nature 1949, high spatial resolution with the high tem- 164,658. poral resolution of femtosecond laser system, which otherwise do not exist in [9] M. Eigen, Disc. Faraday Soc. 1954, 17, pump-probe techniques, provided suffi- the unperturbed system [50]. These cou- 194. ciently short duration electron or X-ray plings open new routes to the outcome of [10] R.W. Hellwarth, in 'Advances in Quantum pulses can be generated. Time-resolved a chemical reaction. Electronics', Ed. J.R. Singer, Columbia X-ray diffraction was for a long time lim- Are we at the end of the race against University Press, New York, 1961. p. 334; ited to the second to nanosecond times- time? In March 1993, at the Berlin Confer- F.J. McClung, R.W. Hellwarth, J. Appl. ence on Femtochemistry, the 1967 Nobel Ph)'s. 1962,33, 828. cales [44]. Recently, picosecond and fem- [II] L.E. Hargrove, R.L. Fork, M.A. Pollack, tosecond resolution has been reached but Laureate Sir George Porter observed: 'The Appl. Ph)'s. Lett. 1964,5.4; O.P. McDuff, only in experiments describing the melt- study of chemical events that occur in the S.E. Harris, 1EEE J. Quant. Electr. 1967, ing of materials and the recovery of an femtosecond timescale is the ultimate QE3,101. ordered structure [45]. On the other hand, achievement in half a century of develop- [12] C.V. Shank, in 'Ultrashort Laser Pulses', using time-resolved electron diffraction, ment and, although many future events Ed. W. Kaiser, Springer-Verlag, Berlin, Zewail's group managed a 'tour de force' will be run over the same course, chemists 1988, p. 5; W. Kaiser, Science 1986,233, are near the end of the race against time' . 1276. by imaging for the first time, a chemical [13] A.H. Zewail, Chem. & Engin. News 1988, reaction as it evolves from reactants to If this is true as far as nuclear dynamics is 66,24; L.R. Khundkar, A.H. Zewail, Ann. products. He used an ultrashort laser pump concerned, sub-femtosecond pulses or at- Rev. Phys. Chern. 1990,41, 15; A.H. Ze- 18 pulse to trigger the dissociation of diiodot- tosecond pulses (1 attosecond = 10- sec- wail,J. Phys. Chem. 1993,97,12427; A.H. etrafluoroethane (C2F412) and an ultrashort onds) may prove useful to probe the mo- Zewail, Scientific American 1990,262,40. FEMTOCHEMISTRY 88 CHIMIA 2000, 54. No.3

[14] M.J. RoskeI', M. Dantus, A.H. Zewail, J. [38] P.V. Kumar and M. Maroncelli, J. Chel1l. [46] lC. Williamson, J. Cao, H. Ihee, H. Frey, Chern. Phys. 1988, 89, 6113; M. Dantus, Phys. ]999, 103,3038. A.H. Zewail, Nature 1997, 386, 490; J. M.J. RoskeI', A.H. Zewail, J. Chern. Phys. [39] C. Jeannin, M.T. Portella-Oberli, F. Vigliot- Cao, H. Ihee, A.H. Zewail, Proc. Natl. 1988,89,6128. ti, S. Jimenez, B. Lang, M. Chergui, Chel1l. Acad. Sci. USA 1999,96, 338. [15] L.R. Khundkar, A.H. Zewail, J. Chern. Phys. Letters 2000, 316, 51. [47] C. Bressler, M. Chergui, P. Pattison, M. Phys. 1990,92,231. [40] M.F. Emde, A. Baltuska, A. Kummrow, Wulff, A. Filipponi, R. Abela, SPIE 1998, [16] N.F. Scherer, L.R. Khundkar, R.B. Bern- M.S. Pshenichnikov, D.A. Wiersma, Phys. 3451,108; M. Saes, C. Bressler, M. Cher- stein, A.H. Zewail, J. Chern. Phys. 1987, Rev. Lett. 1998, 80, 4645; A. Kummrow, gui, R. Abela, to be published. 87, 1451; N.F. Scherer, C.N. Sipes, R.B. M.F. Emde, A. Baltuska, M.S. Pshenichni- [48] J.J. Gerdy, M. Dantus, R.M. Bowman, A.H. Bernstein, A.H. Zewail. J. Chern. Phys. kov, D.A. Wiersma, J. Phys. Chel1l. 1998, Zewai], Chern. Phys. Lett. 1990, 171, I; E. 1990, 92, 5239. A102, 4172; C. Silva, P.K. Walhout, K. D. Potter, J.L. Herek, S. Pedersen, Q. Liu, [17] S. Pedersen, J.L. Herek, A.H. Zewail, Sci- Yokoyama, P.F. Barbara, Phys. Rev. Lett. A.H. Zewail, Nature 1992, 355, 66. ence 1994, 266, 1359. 1998,80, ]086; K. Yokoyama, C. Silva, D. [49] B. Kohler, V.V. Yakov]ev, J. Chei, J.L. [18] B.A. Horn, J.L. Herek, A.H. Zewail. J. Arn. Hee Son, P.K. Walhout, P.P. Barbara, J. Krause, M. Messina, K.R. Wilson, N. Sch- Chern. Soc. 1996, //8,8755. Phys. Chern. 1998, A102, 6957; A. Baltus- wentner, R.M. Witnell, Y.P. Yan, Phys. [19] E.W.-G. Diau, S. De Fey tel', A.H. Zewail, ka, M.F. Emde, M.S. Pshenichnikov, D.A. Rev. Letters 1995, 74, 3360; C.J. Bardeen, Chern. Phys. Lett. 1999, 304, 134. Wiersma, J. Phys. Chern. 1999, A103, V.V. Yakovlev,K.R. Wilson, S.D. Carpen- [20] S. Kim, S. Pedersen, A.H. Zewail,J. Chern. 10065. tel', P.M. Weber, W.S. Wan'en, Chen!. Phys. Phys. ]995, /03,477. [41] K.C. Hasson, F. Gai, P. A. Anfinrud, Proc. Left. 1997, 280, 151;C.J. Bardeen, J. Che, [21] P.Y. Cheng, D. Zhong, A.H. Zewail, Chern. Natl. Acad. Sci. USA 1996,93,15124; T. K.R. Wilson, V.V. Yakovlev, V.A. Apkar- Phys. Lett. 1995,237,399; P.Y. Cheng, D. Ye, N. Friedman, Y. Gat, G.H. Atkinson, ian, c.c. Martens, R. Zadoyan, B. Kohler, Zhong, A.H. Zewail, J. Phys. Chern. 1995, M. Sheves, M. Ottolenghi, S. Ruhman, J. M. Messina, J. Chem. Phys. 1997, 106, 99, 15733. Phys. Chern. 2000 (in press) 8486. [22] A.H. Zewail, Chern. Phys. Lett. 1995,233, [42] M.H. Vos,M.R. Jones,l-L. Martin, Chel1l. [50] E. Charron, A. Suzor-Weiner, F.H. Mies, 500. Phys. 1998, 233, 179. in 'Femtochemistry', Ed. M. Chergui, [23] P.Y. Cheng, D. Zhong, A.H. Zewail, Chern. [43] A. Douhal, S.K. Kim, A.H. Zewail, Nature World Scientific, Singapore, 1996, p. 332; Phys. Lett. 1995,242,368. 1995,278,260; T. Fiebig, M. Chachisvilis, T. Seidemann, in 'Femtochemistry', Ed. [24] J.J. Breen, L.W. Peng, D.M. Willberg, A. M. Manger, A.H. Zewail, A. Douhal, I. M. Chergui, World Scientific, Singapore, Heikel, P. Cong, A.H. Zewail, J. Chel1l. Garcia-Ochoa, A. de laHoz Ayuso,J. Phys. 1996, p. 370. Phys. 1990, 92, 805. Chern. 1999,103,741. [51] A.E. Kaplan, Phys. Rev. Lett. 1994, 73, [25] S.K. Kim,J.J. Breen, D.M. Willberg, L.W. [44] C.D. Surridge, Structural Biology (Nature) 1243; S.D. Vartak, N.M. Lawandy, Optics Peng, A. HeikaI, lA. Syage, A.H. Zewail, 1994, 1, 664; A. Bianconi, A. Congiu- Comm. ]995, 120, ]84. J. Phys. Chern. 1995,99,7421. Castellano, P.J. Durham, S.S. Hasnain, S. [26] A.A. Heikal, S.H. Chong,lS. Baskin, A.H. Philips, Nature 1985, 318, 685; D. Mills, Zewail, Chel1l. Phys. Lett. 1995,242,380. Science 1984, 223.811; D.M. Mills, in [27] D. Zhong, S. Ahmad, A.H. Zewail, J. Arn. 'Handbook of Synchrotron Radiation' ,Eds. Chern. Soc. 1997, 119, 5978. G.S. Brown, D. E. Moncton, Noth-Hol- [28] J.S. Baskin, M. Chachisvilis, M. Gupta, land, Amsterdam, 1991, 3; P. Chen, LA. A.H. Zewail, J. Phys. Chern. 1998, A102, Tomov, P. Rentzepis,J. Phys. Chern. 1999, 4158. B103, 7081; V. Srajer, T. Teng, T. Ursbya, 129] Q. Liu, C. Wan, A.H. Zewail, J. Phys. C. Pradervand, Z. Ren, S. Adachi, W. Chern. 1996,100, 18666. Schildkamp, D. Bourgeois, M. Wulff, K. [30] R.W. Schoenlein, L.A. Peteanu, R.A. Moffat, Science 1996, 274, 1726; L.X. Mathies, C.V. Shank, Science 1991, 254, Chen, P.L. Lee, D. Gosztola, W.A. Svec, 412; S.L. Dexheimer, Q. Wang, L.A. Pete- P.A. Montano, M.R. Wasielewski,J. Phys. anu, W.T. Pollard, R.A. Mathies, C.V. Chern. 1999, B103, 3270. Shank, Chern. Phys. Lett. 1992,188,61. [45] F. Raksi, K.R. Wilson, Z. Jiang, A. Ikhlef, [31] M.H. Vos, J.-C. Lambry, SJ. Robles, D.C. C.Y. Cote, J.-c. Kieffer, J. Chern. Phys. Youvan,J. Breton, J.-L. Martin, Proc. Natl. 1996, ]04, 6066; C. Rose-Petruck, R. Jime- Acad. Sci. USA 1992, 88, 8885. nez, T. Guo, A. Cavalieri, C.W. Siders, F. [32[ R. Zadoyan, Z. Li, P. Ashjian, C.C. Mar- Raski, J.A. Squier, B.C. Walker, K.R. Wil- tens, V.A. Apkarian, Chern. Phys. Letters son, C.P.J. Barty, Nature 1999, 398, 310; 1994,218, 504; R. Zadoyan, Z. Li, c.c. W.P. Leemans, R.W. Schoen]ein, P. Vol- Martens, V.A. Apkarian, 1. Chel1l. Phys. beyn, A.H. Chin, T.E. Glover, P. Balling, 1994,101,6648. M. Zolotorev, K.J. Kim, S. Chattopadhyay, [33] U. Banin, S. Ruhman, 1. Chern. Phys.1993, C.V. Shank, Phys. Rev. Lett. 1996, 77, 98, 439]; I. Benjamin, U. Banin, S. Ruh- 4]82; R.W. Schoen]ein, W.P. Leemans, man, 1. Chern. Phys. 1993, 98, 8337; U. A.H. Chin, P. Voltbeyn, T.E. Glover, P. Banin, R. Kosloff, S. Ruhman, Chern. Phys. Balling, M. Zolotorev, K.J. Kim, S. Chatto- 1994, /83,289. padhyay, C.V. Shank, Science 1996, 274, [34] N. Pugliano, A.Z. Szarka, S. Gnanakaran, 236; W.P. Leemans, R.W. Schoenlein, P. M. Treichel, R.M. Hochstrasser, J. Chern. Voltbeyn, A.H. Chin, T.E. Glover, P. Bail- Phys. 1995, 103,6498; N. Pugliano, A.Z. ing, M. Zolotorev, K.J. Kim, S. Chatto- Szarka, R.M. Hochstrasser,J. Chel1l. Phys. padhyay, C. V. Shank, J. Quant. Electr. 1996, 104, 5062. 1997,33, 1925; C. Rischel, A. Rousse, L [35] R. Jimenez, G.R. Fleming, P.V. Kumar, M. Uschmann, P.-A. Albouy, J.-P. Geindre, P. Maroncelli, Nature 1994, 369, 47]. Audebert, J.-c. Gauthier, E. Forster, J.-L. [361 L. Reynolds, J.A. Gardecki, SJ.V. Frank- Martin, A. Antonetti, Nature 1997, 390, land, M.L. Horng, M. Maroncelli, J. Phys. 490; A.M. Lindenberg, I. Kang, S.L. Chern. 1996, 100, 10337. Johnson, T. Misalla, P.A. Heimann, Z. [37J D.S. Larsen, K. Ohta, G.R. Fleming, J. Chang, J. Larsson, P.H. Bucksbaum, H.C. Chern. Phys. 1999, 11J, 8970. Kapteyn, H.A. Padmore, R.W. Lee, J.S. Wark, R.W. Falcone, to be published.