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J. Phys. Chem. 1993,97, 12421-12446
FEATURE ARTICLE Femtochemistrytg# Ahmed H.Zewail Arthur Amos Noyes Laboratory of Chemical Physics, California institute of Technology, Pasadena, California 91 125 Received: September 9, 1993
Contents I. Introductory Remarks 11. Early Triggers: Concept of Coherence 111. Ultrafast Lasers IV. Supersonic Beams V. From Dynamics to Structures VI. Significant Step in Time-Resolution VII. Into Femtochemistry VIII. Femtochemistry IX. Summary of Applications and Systems (A) Elementary Reactions (1) Dissociation Reactions - Direct Bond Breakage (2) Alkali Halide “Harpoon” Reactions: Resonance Motion along a Reaction Coordinate (3) Barrier Reactions: Saddle-Point Transition States (4) Bimolecular Reactions: Collision Complex/Intermediate (5) Rydberg-State Reactions: Rydberg Electron Dynamics (6) Surface Hopping Reactions: Two or More Potentials (7) Alignment and Geometry of the Transition State (8) Unimolecular and Bimolecular Reaction Times (B) More Complex Reactions (1) Isomerization: Twisting about a Double Bond (Trans/Cis and Cis/Trans) (2) Hydrogen-Atom Transfer: Motion in a Double Well (3) Consecutive Bond Breakage: Dynamics and Structure (C) Solvation Dynamics (1) Elementary Reactions in Solvent Cages: Dissociation and Recombination, and Predissociation (2) High Pressures: Gas to Liquid Interface Region (3) More Complex Reactions in Clusters: Proton Transfer, Acid-Base Reactions (D) Control: Population and Yield (1) Wave Packet Phasing: Multiple Pulse Excitation (2) Yield Control: Switch by Pulse Timing (E) Nonreactive Systems: Coherence and Structure
( 1 ) Bound 12 Wave Packet and the Potential (2) Boupd IC1 Wave Packet and the Potential X. Closing Comments and Some References
I. Introductory Remarks The Berlin Conference of March 1 4 , 1993, on Femtosecond Chemistry focused on the current advances made in experimental and theoretical studies in the field, particularly those relevant to molecular reaction dynamics. The Conference, which was masterfully organized by Professor Dr. J6rn Manz of the Freie UniversitBt, was appropriately held in Berlin, the birthplace of molecular reaction dynamics. Michael Polanyi and Henry Eyring, in 1931, developed, on the basis of Heitler-London theory, an empirical method for constructing a potential energy surface describing the reaction of three hydrogen ( H H2) atoms. Shortly afterwards, Eyring (1935) and M. G. Evans and Polanyi (1935) independently developed the important and famous work on the “transition state”, also known as the “activated complex”. Besides the excellent scientific presentations, the Conference brought us back in time to historical and human events of great magnitude and took us forward in time to new possibilities and contributions. The thoughtful introductions by J6m Manz and Gerhard Ertl, the rich historical development of flash photolysis by George Porter, the touching remarks on Michael Polanyi and Berlin by John Polanyi, and the visionary closing remarks by Jim Kinsey are examples of such moments. The notes and perspectives given here are the outcome of the lecture entitled “Femtochemistry-An Overview”. [At the Conference, I learned from George Porter that on the other side of the Atlantic, overview, a word used in the Americanvocabulary in such contexts, implies a “God1y”presentation. With deference to Lord Porter, I shall use “review” instead of “overview”!] At the Conference we reviewed thedevelopment from the early studies of intramolecular vibrational-energy redistribution, with a resolution of tens of picoseconds, to the current studies of reaction dynamics and transition states with time resolution of tens of femtoseconds. Rather than reviewing details here, which are discussed in the references given, we thought of reflecting on how the concepts and the methodology have evolved and what lies ahead (if it were possible to know!). To develop femtochemistry, and to study the elementary dynamics of reactions-bondbreaking/bond-making-several major advances and stepwise improvements were required, with a great number of people and incidents playing a crucial role. In the following pages we summarize the Caltech effort and the involvement of dedicated graduate students, postdoctoral fellows, research associates, and visiting scholars. We begin with our own struggles with the concept of coherence and with the development of the interface between ultrafast lasers and
+
t This article is dedicated to the memory of Richard B. Bernstein.
t Contribution No. 8829. This work was supported by grants from the National Science Foundation and the Air Force Office of Scientific Research.
0 1993 American Chemical Society
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12428 The Journal of Physical Chemistry, Vol. 97, No. 48, 1993 900
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Figure 1. Nanosecond resolution: coherent transients observed in gases and in a molecular beam of iodine using multiple pulse laser techniques. (a) Photon echo of iodine, detected on the fluorescence by three pulses ( 7 / 2 , *, and 7 / 2 ) ; (b) the incoherent resonance decay; (c) optical i"1 at different pressures; (d) optical TIin a beam; (e) coherent oscillations on the fluorescence; and (f) optical free-induction decay. [See: Acc. Chem. Res. 1980, 23, 360,and references cited therein] molecular beams. We conclude with some remarks on the importance of time scales. 11. Early Triggers: Concept of Coherence
Our beginning efforts in understanding coherence came about in 1976 when we initiated studies of the phenomenon of optical coherence using multiple laser pulses, in those days of nanosecond time duration. The coherence of interest was that of optical transitions which characterize what are called impurity molecules in solids. In chemistry, the work of Douwe Wiersma in Griiningen and our group at Caltech (Tom Orlowski, Dan Dawson, graduate students, and Kevin Jones, undergraduate student) showed that while the apparent inhomogeneous width of a transition is a few wavenumbers, the real homogeneous broadening of the dynamics is less by about 3 orders of magnitude. This point proved to be crucial later when we entered the field of molecular reaction dynamics.
For studies in the gas phase, we decided to apply thesecoherence laser techniques to examine the optical dephasing due to elastic and inelastic collisions. The optical T I and T2, which have analogues in NMR, were measured on the incoherent decay (fluorescence). As discussed below, coherence is normally not observable by detecting incoherent emission; we used a prescribed pulse sequence capable of changing the coherence to population. These studies were performed at pressures of milliTorrs. To confirm our understanding of the meaning of dephasing at "zero pressure", we built a primitive, and our first, effusive beam apparatus, and, by 1917, we were examining coherence phenomena in molecular beams, both theoretically and experimentally (Orlowski, Jones, Rajiv Shah, postdoctoral fellow, and AI Nichols, undergraduate student). Iodine was one of our favorite molecules then, and remains so even at the present time. From these studies, we learned that coherence can be probed directly in real time in gases (and solids) and that incoherent
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The Journal of Physical Chemistry, Vol. 97, No. 48, 1993 12429
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Time (ns) Figure 2. Picosecond dynamics of IVR in a supersonic molecular beam of anthracene. Note the in-phase and out-of-phasecomplimentary nature of the transients, which indicate the energy redistribution among two vibrational states. There is a third vibrational state involved, as evidenced by the high-frequency oscillation. [See: Felker, P. M.; Zewail, A. H. Phys. Rev. Lett. 1984,53,501. Lambert, W. R.;Felker, P. M.; Zewail, A. H. J. Chem. Phys. 1981, 75, 5958.
decay (e.g. fluorescence) can be used to monitor such coherences provided that the laser pulse(s) is capable of forming a coherent superposition of states. For two states of a transition (say +a and + b ) , the coherent state can be written as
where the coefficients a ( t ) and b(t) contain in them the familiar quantum-mechanical phase factors, exp(-iEat/ h ) and exp(-iEbt/ h ) , respectively. With pulse sequences, we could directly monitor the behavior of the ensemble-averagedcoefficients,( a ( t )b*(t)), which contain information on the coherence decay (optical T2); they are the off-diagonal elements of a density matrix, Pab. The term ( a ( t ) a * ( t ) ) is the population of state and represents the diagonal element of the density matrix, pan;( a ( t ) a * ( t ) ) decays with the optical T I . Wewerethusable todemonstrate theopticalanalogue of N M R pulse techniques and to learn about coherence and the origin of optical dephasing in molecular systems of interest to chemical dynamics. At the time, this language of coherence was not popular and there was some feeling that it may be of no importance to chemistry. This situation was saved by the recognition of the relevance of coherence to dynamics as difficulties were encountered in deducing T Iand T2 from measurements of the line width of transitions. For two levels, the optical dephasing is related to the homogeneous width by
where T I ,and Tlbare the decay constants for the population of the a and b levels, respectively, and T $ is the pure-dephasing time constant. This expression was derived from a formal theoretical treatment of relaxation to clarify the meaning of dephasing vs energy relaxation (Jones). For example, if experimentalists manage to measure T2 by, e.g., a photon echo experiment, then this T2can be related to the lifetime of excited state b only if (T’z)-I 0 and the initial state (a) does not decay.
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Figure 3. Picosecond study of stilbene (and derivatives of 4-methoxy (MS),4-4’-dimethoxy (DMS),and 2-phenylindene (PI)). The microcanonical rate constant k ( E ) is plotted as a function of the excess vibrational energy in the first excited state. Note the presence of a threshold (barrier) for isomerization and the dramatic change in the position of the barrier and the rates as the structure changes. [See: Bafiares, L.; Heikal, A.; Zewail, A. H. J. Phys. Chem. 1992, 96, 4127. Syage, J. A.; Lambert, W. R.;Felker, P. M.; Zewail, A. H.; Hwhstrasser, R. M. Chem. Phys. Lett. 1982, 88, 266. Syage, J. A.; Felker, P. M.; Zewail, A. H. J. Chem. Phys. 1984,81, 4706.
Conversely, if the fluorescence decay of the b state is known, it provides no direct information on AVHif T$ is finite. Another difficulty is that T2 in ( 2 ) is for a homogeneous transition; Tz may or may not have any relationship to the apparent line width of the transition, which in most cases is inhomogeneous. Only if the line is perfectly homogeneous and T’2 type processes are absent can werelate theline width todynamical TI-typeprocesses. We summarized our findings (Figure 1) and approach in an Accounts of Chemical Research article entitled “Optical Molecular Dephusing: Principles of and Probings by Coherent Laser Spectroscopy”,which was published in 1980,and in a bookchapter with Mark Burns and Wing-Ki Liu (postdoctoral fellows)-see section X. One feature of this work which helped us later in the study of molecular reaction dynamics was the realization of the importance of the pulse phase in studies of coherence. With acousto-optic modulation techniques (Orlowski and Jones), it become possible to make pulse sequences with well-defined phases. This development (Warren Warren, postdoctoral fellow; Ed Sleva and Isaac Xavier, graduate students) took us into the domain of selective and prescribed pulse sequences which could then be used to enhance coherences or suppress them-the analogue of N M R multiple pulse spectroscopy. We were eager to extend these techniques to the picosecond time domain in order to study solids, but, for several reasons, our attention was diverted to gas-phase molecular dynamics. 111. Ultrafast Lasers
Our first picosecond (or subpicosecond on certain days!) laser was built following the design of Chuck Shank and Eric Ippen (1974) of a passively mode-locked (continuous wave, cw) and cavity-dumped dye laser with subpicosecond pulse duration. The effort started in 1976, and we had a real task building such a system (Shah and David Millar, graduate student). This laser system, however, was a new, pleasant experience in reliability and reproducibility, compared with the experience we had with glass lasers. The dye laser repetition rate was 100 kHz,and the pulse energy was 2 nJ. With this system, we focused our efforts on studies in the condensed phase, primarily solutions, with particular attention on energy transfer and rotational anisotropy
12430 The Journal of Physical Chemistry, Vol. 97, No. 48, 1993
Zewail
Rotational Motion in Real Time
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decay (Millar); the latter work was published in 1979 and was directly relevant to later studies in the gas phase. In 1979, with the arrival at Caltech of Ray Robbins (postdoctoral fellow), we decided to build a more versatile laser system that could allow us to more easily tune the wavelength and to study something different from dye molecules. We constructed a synchronously mode-locked (cw) dye laser. In this case, the mode-locking (active) was achieved by matching the cavity length to that of an argon-ion laser (repetition rate was 82 MHz and pulse energy was 1.8 nJ). Typically, 2-4-ps pulses could be achieved, and we soon learned (Millar) how to optimize the coherence width of the pulse and its substructure; the tunability feature was attractive. Millar and Robbins, in 1980, initiated the use of the mode-locked argon ion laser with single-photon counting detection to study the time-resolved fluorescence depolarization of DNA and RNA intercalated with ethidium bromide. The power of single-photon counting became apparent, and a new laser system, a synchronously pumped, cavity-dumped dye laser, was constructed for studies of gas-phase molecular dynamics, but now with the benefit of all the expertise of Robbins and Millar next door. IV. Supersonic Beams Stimulated by the work on coherence, and with the availability of picosecond pulses, we thought of an interesting problem relating to the question of intramolecular vs intermolecular dephasing. In large, isolated molecules (as opposed to diatomics), there are so-called bath modes and the question arose: Could these bath modes in isolated large molecules dephase the optically-excited initial state in the same way that the phonons of a crystal (or collisions in gases) do? This problem has some roots in the question of state preparation, and I was familiar with its relationship to the description of radiationless transitions through the work of Joshua Jortner and Wilse Robinson. My group and I initiated the construction of a supersonic molecular beam (Bill Lambert, graduate student) and soon Bill, who came to Caltech in 1977, was joined by Peter Felker (graduate student) in 1979; Peter devoted his initial effort to the picosecond system. Rick Smalley came to give a talk at Caltech in May of 1980 entitled "Vibrational Relaxation in Jet-Cooled Polyatomics" and spoke about the exciting work on naphthalene spectra. From the line width in the excitation spectra, he inferred the "relaxation time". At the time, the work by Don Levy, Lennard Wharton, and Smalley on cw (or nanosecond) laser excitation of molecules in supersonic jets was providing new ways to examine the
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Figure 5. Time-resolved rotational coherence spectra of stilbene (top) in supersonic beams. The recurrences give the rotational constants and the structure. [See: Baskin, J. S.;Felker, P. M.; Zewail, A. H. J . Chem. Phys. 1986,84,4708. Felker, P. M.; Zewail, A. H. J . Chem. Phys. 1987, 86, 2460. Baskin, J. S.; and Zewail, A. H. J . Phys. Chem. 1989, 93, 5701.1 (Bottom) Results from Felker's group at UCLA. The system is carbazole-(Ar)z: Structure of one isomer and the recurrences for the two types of isomers [see: Ohline, S. M.; Connell, L. L.; Joireman, P. W.; Venturo, V. A.; Felker, P. M. Chem. Phys. Lett. 1992, 193, 335. Felker, P. M. J . Phys. Chem. 1992, 96, 78441.
spectroscopyof molecules and van der Waals molecules. Listening to Rick and being biased by the ideas of coherence (section 11), I became convinced that the way to monitor the homogeneous dynamics is not through the apparent width but by using coherent laser techniques. This was further kindled by the need for direct measurement of energy redistribution rates; we were encouraged by Charlie Parmenter, after he had shown a chemical timing method of using collisions as a "clock" to infer the rate of energy redistribution. Building our first "real- supersonic molecular beam was fun as we did not know much about this kind of technology. In 1980, the molecular beam and picosecond system were interfaced with a nontrivial addition of a spectrometer to resolve fluorescence in frequency and time. Our thinking was to measure the rate of intramolecular vibrational-energy redistribution (IVR) in real time by observing the decay of the initial state prepared by the picosecond pulse. Instead, we observed the first vibrational coherent dynamics as quantum beat modulated decays. We waited to publish the work until we had made numerous and careful experimental checks, as we (perhaps I am to blame) did not believe the observation in view of previously known artifacts. The fact that the molecule anthracene was "too large" made us suspicious of its coherence behavior. This was further compounded by the skeptical beliefs of others. The work was published in the Journal of Chemical
:eature Article
The Journal of Physical Chemistry, Vol. 97,No. 48, 1993 12431 Reaction Rates of NCNO as a Function of Energy Pump-Probe schemer
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Vibrational Energy, E (wavenumbers)
Figure 7. Behavior of microcanonical rates with energy above threshold for the NCNO reaction. Note the “structure” in &(E)us E. The inset compares these results with calculations based on phase space theory (curve C), used to model product-statedistributions. Note the discrepancy between theory and experiment,especiallyat higher energies. The curves A and B are for modified theoretical calculations. [See: Khundkar, L. R.; Knee, J. L.; &wail, A. H. J. Chem.Phys. 1987,87,77. Klippcnstein, S . J.; Khundkar, L. R.; Zewail, A. H.; Marcus, R. A. J . Chem. Phys. 1988,89,4761.]
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Figure 6. Picosecond study of the state-to-state rates of the reaction NCNO CN + NO. (Top) The scheme for the initiation and probing of thereaction; (bottom)typical transientsobtainedat two excess energies abovethe reaction thresholdof 17 083 cm-’ [see: Khundkar, L. R.; Knee, J. L.; Zewail, A. H. J. Chem. Phys. 1987,87,77. Knee, J. L.; Khundkar, L. R.; Zewail, A. H. J. Phys. Chem. 1985,89,4659].
Physics (December 198l), and the event signaled a major change in our research on molecular reaction dynamics. This initial work was followed by fruitful applications to a number of systems, spanning studies of IVR, radiationless transitions, and reaction rates. One of our first studies of reactions on the picosecond time scale, isomerization of stilbene, was stimulated when Robin Hochstrasser discussed with me his work on stilbene at room temperature. He felt that if we could resolve the low-frequency modes in the molecular beam, we would derive a great deal of information on the torsional potential. We resolved these torsional modes. Furthermore, we decided to study the rates as a function of energy and in the process found the barrier for twisting around the double bond (Jack Syage, postdoctoral fellow, Felker, and Lambert). Even now, stilbene remains a member of our molecular family and we are grateful to Robin. The following list highlights some of the work done in this initial period from 1981 to 1983 (Lambert, Felker, Syage, John Shepanski, postdoctoral fellow, Brian Keelan, graduate student, and Fida Al-Adel, visiting associate): (1) IVR in anthracene and stilbene (2) Trans-cis isomerization of stilbene (3) Quantum beats and radiationless transitions in pyrazine (4) Intramolecular hydrogen bonding in methyl salicylate ( 5 ) Intramolecular electron transfer in A-(CHz)3-@ systems ( 6 ) IVR and dissociation of intermolecular hydrogen-bonded complexes
(7)Restricted IVR: The phase-shifted quantum beats (8) Isomerization of diphenylbutadiene and styrene In continuation of this effort, we made new extensions covering the following topics: isomerization in isolated molecules vs in bulk solutions; nonchaotic multilevel vibrational energy flow; mode-specific IVR in large molecules; IVR dynamics in alkylanthracenes; isotope effect on isomerization of stilbene; charge transfer and exciton dynamics in isolated bianthracene; isotope effect on the intramolecular dephasing and molecular states of pyrazine; IVR dynamics in alkylanilines (the “ring + tail” system); mode-specific (non-RRKM) dynamics of stilbene-rare gas vdW complexes; solvation effect on intramolecular charge transfer; IVR dynamics in p-difluorobenzene and p-fluorotoluene (real time vs chemical timing); IVR dynamics in deuterated anthracenes; dynamics of interstate coupling in chromyl chloride; dynamics of IVR and vibrational predissociation in anthrac e n e A r , (n = 1, 2, 3); structural effects on the dynamics of IVR and isomerization in stilbenes; and dynamics of IVR and vibrational predissociation in n-hexane solvated trans-stilbene. Over the years, these studies were made possible by new (and some “old”) members of the group (Felker, Syage, A1 Adel, Keelan; Lutfur Khundkar, David Semmes, Spencer Baskin, Marcos Dantus, Larry Peng, Ahmed Heikal, graduates students; Dino Tinti, visiting associate; Todd Rose, Luis Bafiares, and Christoph Lienau, postdoctoral fellows). The research resulted in a series of publications. In the same laboratory (known as ‘036 Noyes”), we began the studies of rotational coherence phenomena, as part of the effort to understand the dynamics of energy redistribution, vibrational and rotational. We discovered the value of the phenomena in the development of a technique for structural determination of large molecular systems (Baskin and Felker) and for studies of coherence in dissociation reactions (Baskin and Semmes). This particular advance is highlighted in the following section (V). Theabovepicosecondstudiesofthedynamicsin isolatedsystems identified the different regions and types of IVR, gave the magnitude and selectivity of the coupling (by anharmonicity and/ or Coriolis) among vibrational states, and examined the nature of IVR and the microcanonical rates in a variety of reactions and
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12432 The Journal of Physical Chemistry, Vol. 97, No. 48, 1993
density of states is much less than that normally calculated by invoking all modes in the molecule-a nonstatistical IVR behavior. This behavior of IVR and its regions were found to be general in different systems. The groups of Stephen Wallace (Toronto), Michael Topp (Pennsylvania), Doug McDonald (Illinois), Joe Knee (Wesleyan), Jiirgen Troe (G8ttingen) and others have made important contributions to these studies. V. From Dynamics to Structures
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Figure 8. Femtosecond/molecularbeam apparatus for FEMTOLAND I. In this case, the laser system consists of a CPM oscillator and an amplifier(fourstages)pumpedbyaYAGlaser. Fortheother laboratories, differentarrangementsand lasers areused, but the general idea of interface is similar. The molecular beam is equipped with a time-of-flight (TOF) mass spectrometer (MS)and with a laser-induced fluorescence (LIF) arrangement. [See: Rosker, M. J.; Dantus, M.; Zewail, A. H. J . Chem. Phys. 1988,89,6113. Zewail, A. H.; Bernstein, R. B. In The Chemical Bond Structure and Dynamics; Zewail, A. H., Ed.; Academic Press: Boston, 1992;p 223. Khundkar, L. R.; Zewail, A. H. Annu. Rev. Phys. Chem. 1990, 41, 15.1 molecular systems. The concept of coherent energy flow within a single, isolated large molecule had been established (see, e.g., Figure 2)- Also, mode-selective and "nonkinetic" behavior were evident in studies of IVR and reaction rates, and comparison between theory and experiment illuminated our understanding of the behavior and the relationship to structures (see, e.g., Figure 3). Moreover, our appreciation of the concept of "the preparation of the system" was now firm-with coherent ultrashort pulses, it is possible to create a nonstationary state which evolves in time with the exact quantum rate of energy redistribution (dynamics). On the other hand, with cw lasers, one prepares the "eigenstates" of the system and there is no motion on the time scale of the experiment. We discussed these issues in one of a series of papers in the Journal of Chemical Physics. In the case of IVR, the coherent superposition, eq 1, is made of vibrational states on a single potential surface of a molecule. We could show how this coherent superposition, which leads to quantum interference effects, evolves in time, and how the phase changereflects the flow of energy (IVR) from theinitially prepared state to the final bath state(s). [Note that quantum beats per se are not evidence of energy redistribution!] The phase shift observed by detecting theinitialand final stateof the redistribution also elucidated, at least for us, the notion of motion and dynamics (Figure 2). This point helped our thinking of the wave packet preparation which was later invoked in the femtosecond experiments. The selectivity in the coupling indicated that the actual
As mentioned earlier, with picosecond resolution we studied rotational anisotropy in liquids; the connection to isolated systems in the gas phase was not clear. We felt that the concept of vibrational coherence outlined above should now be extended to cover the rotational motion, especially in large molecules. We began with thetheoretical aspectsof the problem (Felker,graduate student and later a postdoctoral fellow). In the process of formulating the theory, a new phenomenon was discovered: Even in large molecules with significant excitation of angular momentum states, there will be a well-defined time of rotational recurrences when the original alignment is recovered (Figure 4). The recurrence times give the fundamental rotational periods and, hence, the moments of inertia (structural analysis). We also examined the origin of rotational dephasing, and independently, Ann Meyers and Hochstrasser treated this same problem of dephasing theoretically. Our first experimental attempt to confirm the prediction of the theory was on stilbene (Baskin). Indeed, rotational coherence recurrences were observed, and the structure of stilbene was deduced (Figure 5). As is often the case, in retrospect, the generality of this behavior should have been obvious. Later we could simplify the formalism and even understand the phenomenon classically; the similarity to the coherence of the "early triggers (section 11)" also becameclearer. [At the time, there were already theoretical predictions to the effect that rotational states would be scrambled in large molecules and that these large molecules would show no coherence.] It is interesting to reflect on the origin of this phenomenon, which is the key to structural analysis. Recurrences are the consequence of coherence among rotational states and are only possible because of the fundamental nature of their quantized energy spacings; they are commensurate or nearly so. The coherent rotational state, prepared by a polarized pulse, reflects the superposition of frequencies contained in the observable, Z ( t ) :
where K and ai are constants. Now, if vi are uncorrelated, the Z(t) N K and recurrences will not be observed. If, however, vi are commensurate, with, e.g., vi = miuO, where mi is an integer and vo is a fundamental frequency, then for all i, the cosine term will be +1 whenever t = n/vo, n being an integer (all ai have the same sign). Thus, at these particular times, there will be constructive interferences which lead to recurrences. The fundamental frequency gives directly the rotational period(s) and, hence, the moment of inertia. Physically, this simple description indicates the power of themethod: Despite theexpected dephasing of different angular momentumstates due to their different periods (jh = Zw), there will be particular times when rephasing is possible. In this respect, the phenomenon has an analogy in the photon echo, discussed in section 11,which can be visualized with "runners on a track". Following the first theoretical and experimental studies, we decided to apply the method to different molecular systems and to elucidate their structures (Baskin, Felker, and Semmes). The technique is Doppler-free and has the advantage of ease of application as the systems get larger in size; the slower the rotation of the molecule, the longer the recurrence time. It is now termed "Rotational Coherence Spectroscopy", and, following its devel-
The Journal of Physical Chemistry, Vol. 97, No. 48, 1993 12433
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Time Delay (fs) Figure 9. Femtosecond transition-statespectra of ICN dissociation reaction. (Right) The potentials involved and the concept of probing at different interfragment R* separation. (Middle) The first transients observed. (Left) Refined set of data showing the systematic change as X(R*) was tuned. [See: Dantus, M.; Rosker, M. J.; Zewail, A. H. J. Chem. Phys. 1987,87,2395; 19%8,89, 6128. Zewail, A. H. Science 1988; 242, 1645. Roberts, G.; Zewail, A. H. J . Phys. Chem. 1991, 95, 7973.1
opment in 1986, it has been exploited to deduce more than 100 structures. Both ground-state and excited-state structures have been studied with applications including highly asymmetric (e.g., complexes of Tryptamine/water) and large (e.g., 30-40 atoms) molecular systems. Important contributions in this area have been made by the groups of Peter Felker (UCLA), Doug McDonald (Illinois), Michael Topp (Pennsylvania), Joseph Michl (Colorado), Jiirgen Troe (Gottingen), John Simons (Oxford), Yongqin Chen (Berkeley), and others. With the significant improvement in time resolution (following section), we were later able to extend the method of detection to include the pumpprobe methodology, with mass resolution by multiphoton ionization (Norbert Scherer, graduate student, Khundkar, and Rose), and with laser-induced fluorescence (Dean Willberg, graduate student; Jack Breen and Michael Gutmann, postdoctoral fellows). The method was also extended to the femtosecond regime, as discussed in section VIII. In addition to the above methods of detection; a number of valuable variant techniques were developed, and these are as follows: stimulated emission (McDonald), stimulated Raman-induced fluorescence depletion (Felker), fluorescence depletion (McDonald), ionization depletion (Felker), and coherent nonlinear methods (Chen). In a recent bookchapter with Felker (see section X),we have reviewed thecurrent status of the development and the various applications to molecular structure determination of small molecules, large molecules, molecular dimers, hydrogen-bonded systems, clusters, ions, and systems of biological interest.
VI. Significant Step in Time Resolution All of the studies mentioned in section IV invoked a single pulse of about 15-ps duration and a fast detector, typically resolving 40-80-ps dynamics. The pulses, which were tunable, were generated from the synchronously pumped, cavity-dumped dye laser. The real issue was how to improve the time resolution in the molecular beam. As one of our students (initials N.S.) mentioned at the time: “When are we going to do a real ps experiment!” We knew that a “pumpprobe” methodology was
needed, as its time resolution is limited only by the pulse widths of the two lasers. The difficulty, however, was in the nature of such beam experiments: (1) thedensity is relatively low compared to all other liquid- and solid-state experiments, (2) the problems associated with the spatial overlap of three beams (two lasers and one molecular), and (3) the limitation on the temporal resolution imposed by the geometry of molecular beam experiments. In considering these issues and the sensitivity of detection, we were happy to discover the advantages we would gain from multiphoton-ionization time-of-flight mass spectrometry developed by Dick Bernstein and independently by Ed Schlag (with nanosecond lasers) or from laser-induced fluorescence developed by Dick Zare for studies of product-state distribution. We considered both the spatial and temporal resolution theoretically, and set up a “start-up” experiment, first in gas cells of stilbene and then in a molecular beam. With twin picosecond dye lasers (in synchrony), in 1983 we achieved our first real-time study of molecular dynamics with a time resolution only limited by the pulse, a few picoseconds (Joe Perry, graduate student, and Scherer). In January 1984, we were joined by Joe Knee (postdoctoral fellow), and Joe was instrumental, even before arriving, in helping us build another beam machine equipped with both LIF and MPI-TOF. In this beam machine, stilbene (Scherer and Shepanski; in 1984) and pyrazine (Knee and Fuad Doany, postdoctoral fellow; in 1985) were our first two examples. With success in sensitivity and time resolution, we decided to tackle different types of chemical reactions, those proceeding by predissociation and those undergoing direct (or indirect) fragmentation. To examine elementary reaction rates on a state-to-state basis, we looked for prototype systems describing such predissociation and fragmentation. At nearby USC in the groups of Curt Wittig and Hanna Riesler, we found a wonderful system, the following reaction involving four atoms:
-
+
NCNO NC NO (4) We initiated the reaction by one pulse and monitored the C N
12434 The Journal of Physical Chemistry, Vol. 97, No. 48, 1993 ..
1
I
l
l
2
Experimental FTS of the reaction Nal + [Na--f
l
l
l
l
3
r(1-I),
l
l
A
l
l
l
l
l
l
Figure 10. Coherent wave packet (localization) and an eigenstatefor the 12 system at 10 fs and 3.8 ps, respectively. [See: Gruebele, M.; Zewail, A. H. J. Chem. Phys. 1993,98,883.]
radical by another pulse (Figure 6). The signal-to-noise ratio was unbelievably good (Knee and Khundkar). This type of work gave rise to a whole series of experimental and theoretical studies where now the microcanonical state-to-state rates (Figure 7 ) could be directly compared with unimolecular theories of reaction rates (RRKM, SACM, phase-space theory, etc.). The NCNO reaction was followed by others, including (later) the ketene dissociation to CH2 and CO (Earl Potter, graduate student, Martin Gruebele, postdoctoral fellow, and Khundkar). The ketene system was studied earlier by Brad Moore’s group to address issues related to product-state distribution and the applicability of SACM and phase-space theories. In collaboration with Rudy Marcus, we examined the influence of the location of the transition state on the energy dependence of the rates for several of these systems. During this period (and later), the scope of reactions studied gave us a satisfying feeling about the insights we were gaining concerning the fundamentals of the rates and the potential energy surfaces. With this new time resolution, reactions of different types were (and continued to be) studied (Knee, Khundkar, Scherer, Doany, Perry, Potter, Gruebele). These include the following: (1) Dissociation reactions of phenol-benzene and related systems (2) Sequential bond breakage of CF2I-CF21CF2-CF2 21 (3) Ground-state (local-mode) reactions of H202 20H (4) The reaction of NCNO N C + NO, mentioned above ( 5 ) Fragmentation of methyl iodide CH31 CH3 + I(I*) (6) The ketene reaction CH2=C=O CH2 + CO, mentioned above
+
-
-
Na + I
5
4
-
-
Zewail
-
We also studied molecular beams of isoquinoline to examine the effect of IVR on coupling between two excited potentials. With a similar pumpprobe methodology and a different beam apparatus (mentioned below), we later examined other types of reactions including acid-base reactions of naphthols in ammonia and water clusters, mass-selected (Breen, Peng, Willberg, Heikal, and Peijun Cong, graduate student), fragmentation of I2X,, where X is He, Ne, Ar, or N2 and n = 1 , 2 , 3 , 4 (Breen, Willberg, and Gutmann); and stilbene solvated argon or hexane, mass-selected (Willberg and Heikal). Wittig’s group are making important contributions to studies of reaction rates in real time using their new apparatus with the two-pulse arrangement. In their studies of NO2 and NO3 dissociation, they measured the picosecond rates at different
0 N1 I
2 4 Time delay/picoseconds
!Ma ...n
DtlMa-nrw-m
Nlrl
I
6
a d
1.a
Figure 11. Femtosecond transition-state spectra of the NaI reaction. (Top) The covalent-to-ionic resonance observed for motion along the reaction coordinate (b). The transient with quantized steps (a) was obtained when product free Na atoms were detected. (Bottom) The quantum calculations (right) showing the bifurcation of the wave packet to covalent and ioniccomponents and the resonance motion. The potentials are displayed to the left. [See: Rose, T. S.; Rosker, M. J.; Zewail, A. H. J. Chem.Phys. 1988,88,6672;J . Chem.Phys. 1989,9I, 7415.Engel, V.;Metiu, H.; Almeida, R.; Marcus, R. A.; Zewail, A. H. Chem. Phys. Lett. 1988, 252, 1.1
energies and observed (for NO2) clear “structures” relating to the open quantized channels of the transition state. The NCNO system also showed structures (see Figure 7), but not as pronounced.
VII. Into Femtochemistry To observe and study the consecutive bond breakage process of the two C-I bonds in CF21-CF21, we had to do a calibration experiment on “half the molecule”, namely, CF31or CH31. We did such an experiment and realized that the rise of the nascent iodine atom was within our picosecond pulse duration. We also detected C N from ICN in an effusive beam (and in bulbs) and observed the rise within the picosecond pulse (Kneeand Khundkar; in 1985). My thirst for a better time resolution increased! The technology of pulse compression became available, and we ordered, from Spectra Physics, a pulse compressor-a fiber optic arrangement. The company indicated that it would take them several months to build one, and that the only one available was a t Purdue University in the laboratory of Professor Duane Smith, one of the two first graduate students in our group at Caltech. I mentioned to Duane my excitement about the experiment, which was intended to monitor directly the elementary bond breakage in a molecule. ICN was chosen because the C N radical could be conveniently monitored by laser-induced fluorescence (LIF); we were encouraged by the positive experience we had had with C N from NCNO and with the earlier picosecond results on ICN. Also, ICN had been central to studies of dissociation reactions and to photofragment spectroscopy. The work of Kent Wilson and Curt Wittig had provided a measurement
The Journal of Physical Chemistry, Vol. 97, No. 48, 1993 12435
Feature Article
a
..
T h e reaction mg~ +rIHg .€I -+ mg + I $0
I A+BC
Atom Tnnrhr R~ctlonr
ABC+A+BC
Unlmoloculrr Dluoclatlon
-
i
r
Figure 12. Generic potentials for the description of the different types of reactions studied.
of the anisotropy parameter /3 and, hence, inference of the time scale. All that we needed was a factor of 10-100 improvement in time resolution. Duane shipped the compressor and joined us for 2 weeks, and we observed the first ICN subpicosecond transient. In the same year, the paper was accepted and published (December 1985) in theJourna1 ofPhysical Chemistry (Scherer, Knee, and Smith). We did not resolve the transition states of this reaction; we only detected the rise of the product. The last paragraph in this paper summarized what it would be possible to do if the time resolution were further improved by another order of magnitude: "Since the recoil velocity is -2 X IO5 cm/s, the fragment separation is 10 A on the time scale of the experiment (-500 fs). With this time resolution, we must, therefore, consider the proximity of fragments at the time of probing, Le., the evolution of the transition state to final products." Several important factors influenced the fast entry to femtochemistry, all of which had to do with "being in the right place at the right time". The development in 1981 by Richard Fork, Ben Greene, and Shank of the colliding pulse mode-locked (CPM) ring dye laser took the pulse du:ation to the 90-fs regime. By 1985, when we were involved in the ICN experiment, 27-fs pulses were generated with the help of intracavity pulse compression. Soon after, in 1987, 6-fs pulses were obtained by amplification and extracavity compression. With such short pulses and with the help of the earlier (around 1970) development of continuum generation by Bob Alfano and Stanley Shapiro, continuously tunable femtosecond pulses could be generated, only (!) requiring from chemists the expertise in ultrafast lasers and nonlinear optics. The interaction with several colleagues a t the time was another important factor. All had great interest in our effort to study molecular reaction dynamics in real time; an encouraging and generous attribute characteristic of many colleagues in this field of reaction dynamics. Rudy Marcus, my colleague at Caltech, was influential in emphasizing "relevance to chemistry". Rudy and I collaborated over the years, and his interest, especially when we showed a non-RRKM behavior, was great. John Polanyi came to Caltech in 1982as a Fairchild scholar. John is a visionary who saw the importance of (cw) transition-state spectroscopy, and his paper in a book I edited (proceedings of the conference
b w
Fragment Probing -
i
n
t
I
I
I
I
I
-
Figure 13. (a) Molecular dynamics and potential energy surface for the reaction of IHgI. (Bottom) Two snapshots of the wave packet at "0 fs" and at 600 fs. Note the motion of the packet in the translational (asymmetric motion) valleys at this particular energy. [See: Bowman, R. M.; Dantus, M.; Zewail, A. H. Chem. Phys. Lett. 1989, 156, 131. Dantus, M.; Bowman, R. M.; Gruebele, M.; Zewail, A. H. J. Chem. Phys. 1989,91,7437. Gruebele, M.; Roberts, G.; Zewail, A. H.Philos. Trans.R. SOC.LondonA 1990,332,223. Zewail, A. H.Faraday Discuss. Chem. Soc. 1991,91,207.] (b) Femtosecond transition-statespectra of the IHgI reaction. Note the propagation of the wave packet coherence to final HgI product and the change in the period from 300 fs to 1 ps as HgI is probed at different final energies. Thecomplex [IHgI] ** was also probed (not shown), and it decays in -200 fs. [See: references given in the caption of (a), and Baumert, T.; Pcdersen, S.;Zewail, A. H. J . Phys. Chem., paper appearing in this issue on pp 12447-12459.1
in Alexandria) was on this subject, for the reactions H + Hz and NaI dissociation (wing emission). For some reason, we did not discuss femtosecond transition-state spectroscopy at this time, but instead we (with John providing all the notes in writing) were interested in stimulated emission in the NaI system and in field 'dressing" of the potentials. I do not know why I did not think of this system as the first one for femtosecond transition-state spectroscopy. This may have been due to the earlier experience we had with ICN picosecond experiments. Later, the potentials sent by John from Toronto were helpful in thinking of the NaI experiment.
Zewail
12436 The Journal of Physical Chemistry, Vol. 97, No. 48, 1993
FTS o f
molecular Iodine
I
Time Del87 (Is)
Figure 14. Femtosecond coherent wave packet motion (top) of molecular iodine on the bound B state. The initial preparation and the final probing are made according to the scheme given on the potentials (bottom, left). The results give the period of the motion and the anharmonicity. At longer times, rotational recurrences (not shown) were also observed. The figure, bottom right, shows the inversion of the femtoseconddata to give the potential and comparison with the high-resolutiondata. The separation of the time scales for uibrationaland rotational motions and the importanceof the energy uncertainty of the pulse in creating coherences make this case a key example for studies of nonreactiue systems. [See: Bowman, R. M.; Dantus, M.; Zewail, A. H. Chem. Phys. Lett. 1989,162,297. Dantus, M.; Bowman, R. M.; Zewail, A. H. Nature 1990,343,737. Gruebele, M.; Zewail, A. H. J. Chem. Phys. 1993, 98, 883.1
At nearby UCLA, the arrival of Dick Bernstein to the area was a real blessing. H e was extremely excited about the development and the possibilities for real-time studies of molecular reaction dynamics. Dick also came to Caltech as a Fairchild scholar in 1986, and in 1988, we wrote a review article together (published in Chemical & Engineering News). By writing this article with Dick, I learned an enormous amount about molecular reaction dynamics; the famous book he had written with Raphy Levine on the subject was our reference. We also had a genuine collaboration on bimolecular reactions (Scherer, Khundkar, Bernstein; in 1987). Dick came again to Caltech in 1990, but sadly he died before ending his sabbatical. A number of experiments, particularly the new direction of surface femtochemistry, were designed as part of our plan. Finally, the collaboration I had with Rich Bersohn, while he was at Caltech
as a Fairchild Scholar, was very enlightening. We discussed the classical picture of the femtosecond spectroscopy of dissociating molecules, and Rich and I wrote a theoretical paper on this subject. In addition to the above two factors, a piece of good fortune came our way at a time when funding was limited and when the establishment of femtosecond lasers and molecular beam technologies required a “quantum jump” in support. S h a d Mukamel invited me to a workshop in Rochester (October 1985) on intramolecular vibrational redistribution and quantum chaos. I spoke about “IVR and chemical reactivity”, and there in the audience were two program directors from the Air Force Office of Scientific Research: Larry Davis and Larry Burggraf. They requested a preliminary proposal immediately, and I sent one in October, followed by a complete proposal in January of 1986. This proposal was funded and approved in August of 1986 to
Feature Article
The Journal of Physical Chemistry, Vol. 97, No. 48, 1993 12437
start in November of the same year. Larry Davis saw to it that we could order the equipment needed as soon as possible (the good old days!) and made the necessary arrangements to do so. We did not have laboratory space to house the new equipment, but Caltech responded. Fred Anson, our Division Chairman, arranged for the space (which once housed the X-ray machines from the Linus Pauling era) and Murph Goldberger, our President at the time, provided funds for the renovation without delay. By Thanksgiving (1986), we entered the new laboratory, and the CPM laser was operational at the "femtosecond party", on Dec 11, 1986. We focused again on the ICN reaction, but this time on the femtosecond time scale.
VIII. Femtochemistry Collision Complex
The goal of the ICN experiment was to resolve in time the transition-state configurations en route to dissociation:
ICN*
-
[I-*CN]'*
-+ I
CN
(5) BrItI
Not only did we wish to monitor the final CN, free of the force field of iodine (1985), but also the transitory species [I-.CN] * *. Mark Rosker, a postdoctoral fellow and an expert on the CPM laser, arrived in September of 1986. But prior to his arrival, he, Dantus, and I ordered the necessary equipment, and when Mark arrived we were ready. Great enthusiasm surrounded the new laboratory-called FEMTOLAND I-with Marcos and Mark starting the new direction (Figure 8). The first transient surprised us, but after long and late hours of discussions and control experiments, it became clear that, indeed, the transition configurations or the final products could be monitored in real time. We submitted our first communication to the Journal of Chemical Physics (received June 3, 1987), and it was accepted on June 15, 1987. The referee of this paper was not only prompt, but also, in retrospect, visionary. The report wasultrashort: "It (the manuscript) has thesmell that theauthors are onto some very exciting new stuff .... This manuscript meets all requirements for a communication. It may turn out to be a classic. Publish with all dispatch." At the time, our thinking of the process of bond breakage was intuitive and relied on classical concepts. Since atoms and molecules have typical velocities on the order of 1 km/s, the elementary dynamics of transition-state structures a t a resolution of angstroms occur on the femtosecond time scale. This classical estimate of the femtosecond time scale for nuclear motion was evident in the trajectory calculations of Joe Hirschfelder, Eyring, Martin Karplus, Polanyi (John), and others (the unit of time in these calculations, which were ahead of experiments, was the atomic unit!). The estimate was also apparent from transitionstate theory of reaction rates:
[Br,.,I.,,I]* Reaction Coordinate
-
7
6
5
__,_.
. .. --
,
-
'.
4
,,
,
.
3
?
3
2
5
4 rar-11
6
7
A
0'
krcafrion
= ( k ~ / h(Q' ) I~r)e-~c'~~
(6)
where Qris the product of partition functions for the reactants, and Q*is that of the activated complex (less theone of the reaction coordinate). The upper limit of k T / h is about 1013 s-l. Without trajectory calculations, the basic experimental observations could be related to the femtosecond dynamics; the delayed appearance of the C N (on-resonance) and the build-up and decay of transition configurations (off-resonance) was understood using simple classical mechanics and even a helpful kinetic picture of A B C. Two papers (I and I1 of a series), published in the Journal of Chemical Physics, outlined the methodology of "femtosecond transition-state spectroscopy (FTS)" with applications to the ICN dissociation reaction (Figure 9). Classically, we spoke of the change in internuclear separation
--
0
i:
0
5
10
15
Time, p s Figure 15. (a) Potential energy surface for the reaction of Br + 12. [See: b.] (b) Trajectories of the motion for the reaction of Br + 12. Shown at the bottom are the changes with time of the different coordinates. [See: Gruebele, M.; Sims, I. R.; Potter, E. D.; Zewail, A. H. J . Chem. Phys. 1991, 95, 7763. Sims, I. R.; Gruebele, M.; Potter, E. D.; Zewail, A. H. J . Chem. Phys. 1992, 97,4127.1
with time
and for a given potential V, we obtained R ( t ) and r ( R ) ,and also did the reverse.
Zewail
12438 The Journal of Physical Chemistry, Vol. 97, No. 48, 1993
TABLE 1 FEMTOSECOND CHEMISTRY 0" %rim Cmfaarr)' Femtochemislry in Cases I A H Zcwail Y Chcn 1. H.Glovnii
0
1.2
3.6
1.4
I
4.8
Dilhction I b d i u l Lmm)
S Y Lee R Schinke G. Sic&
M Shmro
Femlochemislry in C a w 11 I L Knee H D Mcycr RMBovmrn HK-i C A h.c . b. r u. b -. Dmiel ... A .. S . .t.u .c..
L WLe
C. Wimg
I I
Femlochemistry Irom Spectroscopy E 1. Hclicr R. D.Levine J. C. Poiinyi 1. L. K ~ Y Y 0.M. Ncvmrk P.R. Bmki I P S8m.r W H Miller
I I 1 From Femlochemistry into New Domains G. P m r
I
I
P. M. F d k n
I
S H.Lm M.0u-k '
I
Fcmtochcmistr) lrom Casea lo Solutions Femlochemistry in Clusters 11
F G. A m u
A. W. Cullcmui
R. 0 Onbn J A. Syigc
I L. Bowman W. C.Linckrgcr
T Elvcrvr
I
i HGem
