Femtochemistry: Ultrafast Dynamics of the Chemical Bond

The following sections are included:

• General Introduction

• Contents

The following sections are included:

• Introductory Remarks

• Early Triggers: Concept of Coherence

• Ultrafast Lasers

• Supersonic Beams

• From Dynamics to Structures

• Significant Step in Time-Resolution

• Into Femtochemistry

• Femtochemistry

• Summary of Applications and Systems
  • Elementary Reactions
    • Dissociation Reactions - Direct Bond Breakage
    • Alkali Halide “Harpoon” Reactions: Resonance Motion along a Reaction Coordinate
    • Barrier Reactions: Saddle-Point Transition States
    • Bimolecular Reactions: Collision Complex/Intermediate
    • Rydberg-State Reactions: Rydberg Electron Dynamics
    • Surface Hopping Reactions: Two or More Potentials
    • Alignment and Geometry of the Transition State
    • Unimolecular and Bimolecular Reaction Times
  • More Complex Reactions
    • Isomerization: Twisting about a Double Bond (Trans/Cis and Cis/Trans)
    • Hydrogen-Atom Transfer: Motion in a Double Well
    • Consecutive Bond Breakage: Dynamics and Structure
  • Solvation Dynamics
    • Elementary Reactions in Solvent Cages: Dissociation and Recombination, and Predissociation
    • High Pressures: Gas to Liquid Interface Region
    • More Complex Reactions in Clusters: Proton Transfer, Acid-Base Reactions
  • Control: Population and Yield
    • Wave Packet Phasing: Multiple Pulse Excitation
    • Yield Control: Switch by Pulse Timing
  • Nonreactive Systems: Coherence and Structure
    • Bound I₂ Wave Packet and the Potential
    • Bound ICl Wave Packet and the Potential

• Closing Comments and Some References

Femtochemistry is concerned with the very act of the molecular motion that brings about chemistry, chemical bond breaking, or bond formation on the femtosecond (10⁻¹⁵ second) time scale. With lasers it is now possible to record snapshots of chemical reactions with sub-angstrom resolution. This strobing of the transition-state region between reagents and products provides real time observations that are fundamental to understanding the dynamics of the chemical bond.

One of the most fundamental problems in chemistry is understanding how chemical reactions occur; that is, how reagents make their journey to products. Traditionally, chemists start by studying the thermodynamics of a reaction, then its rate, and finally postulate its mechanism…

Some aspects of alignment and orientation have been considered for femtochemistry experiments. Elementary theoretical descriptions of the time evolution of alignment and angular momenta have been discussed and related to the radial and angular parts of the potential-energy surface. Applications to ICN unimolecular dissociation, H + CO₂ ‘oriented’ bimolecular reaction, and crossings between different potential-energy curves in alkali-metal halide reactions (M + X) are given. Emphasis is on the femtosecond dynamics of the transition-state region and the clocking of fragment separation in real time.

This article presents the progress made in probing femtosecond transition-state dynamics of elementary reactions. Experiments demonstrating the dynamics in systems characterized by a transition region and by a saddle-point transition state are reported, and comparison with theory is made.

In 1872 railroad magnate Leland Stanford wagered $25,000 that a galloping horse, at some point in its stride, lifts all four hooves off the ground. To prove it, Stanford employed English photographer Eadweard Muybridge After many attempts, Muybridge developed a camera shutter that opened and closed for only two thousandths of a second, enabling him to capture on film a horse flying through the air [see illustration at top right]. During the past century, all scientific disciplines from astrophysics to zoology have exploited high-speed photography to revolutionize understanding of animal and mechanical motions that are quicker than the eye can follow…

Transition state(s) (TS) of chemical reactions are fundamental in defining the region(s) of internuclear separation (R*) on the potential energy surface (PES) at which the reagent molecule is "passing on" to products. In contrast to the many successes in applying spectroscopies to the characterization of stable reagents stable products, TS spectroscopy has been very limited, because of the TS ultrashort lifetime (few vibrational periods) and the very low density of TS molecules that can be probed at R* Recently, elegant ideas of time-integrated emission, absorption, and scattering spectroscopy have been developed to infer the dynamics of the TS. Here, we offer a real-time technique that, because of its time resolution resolution (~40 fs), promises to provide direct information concerning the TS and the spectroscopy of reaction intermediates in the process of falling apart (dissociation) or forming a chemical bond (association)…

When a chemical bond is broken in a direct dissociation reaction, the process is so rapid that it has generally been considered instantaneous and therefore unobservable. But the fragments formed interact with one another for times on the order of 10⁻¹³s after the photon has been absorbed. On this time scale the system passes through intermediate transition configurations; the totality of such configurations have been, in the recent literature, designated as “transition states.” Femtosecond transition-state spectroscopy (FTS) is a real-time technique for probing chemical reactions. It allows the direct observation of a molecule in the process of falling apart or in the process of formation. In this paper, the first in a series on femtosecond real-time probing of reactions, we examine the technique in detail. The concept of FTS is explored, and the interrelationship between the dynamics of chemical reactions and molecular potential energy surfaces is considered. The experimental method, which requires the generation of spectrally tunable femtosecond optical pulses, is detailed. Illustrative results from FTS experiments for several elementary reactions are presented, and we describe methods for relating these results to the potential energy surface(s).

Experimental results obtained for the dissociation reaction ICN* → [I⋯CN]‡* → I + CN using femtosecond transition-state spectroscopy (FTS) are presented. The process of the I–CN bond breaking is clocked, and the transition states of the reaction are observed in real time. From the clocking experiments, a “dissociation” time of 205 ± 30 fs was measured and was related to the length scale of the potential. The transition states live for only ~ 50 fs or less, and from the observed transients we deduce some characteristics of the relevant potential energy surfaces (PES). These FTS experiments are discussed in relation to both classical and quantum mechanical models of the dynamical motion, including features of the femtosecond coherence and alignment of fragments during recoil. The observations are related to the radial and angular properties of the PES.

The development of methods of generation of very short pulses of light with widths about or below 100 fs has enabled the measurement of the rate of direct photodissociation of a bond. The experiment measures absorption as a function of both time and the degree to which the probing light is off resonance with one of the fragments. A classical model with an exponential repulsion is used to relate experimental observables to the dissociation time and properties of the potential surface. The delicateness of the probe will improve as the photon energy approaches the threshold energy for dissociation.

Femtosecond transition-state spectroscopy (FTS) of elementary reactions [M. Dantus, M. J. Rosker, and A. H. Zewail, J. Chem. Phys. 87,2395 (1987) ] provides real-time observations of photofragments in the process of formation. A classical mechanical description of the time-dependent absorption of fragments during photodissociation [R. Bersohn and A. H. Zewail, Ber. Bunsenges. Phys. Chem. 92,373 (1988) ] forms the basis for the present scheme for relating observations to the potential energy surface. A direct inversion scheme is presented that allows the difference in the two relevant excited-state potential curves to be deduced from observed transients at different probe wavelength tunings. In addition, from the shape and dependence of the transients on pump wavelength, information on the lower of the two potential curves (i.e., that of the dissociating molecule) is obtained. The methodology is applied to the experimental FTS data (Dantus et al.) on the CN photofragment from the ICN photodissociation.

A comparison is presented between experiment and quantum and classical simulations of the time-dependent dynamics of the dissociation reaction of ICN via the A continuum. To illustrate the approach, second-order perturbation theory and a classical model (Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 373) are employed to calculate the real-time behavior of dissociating [I⋯CN]** species during the course of the reaction. In this way, the abilities of the two methods to describe accurately the reaction dynamics as revealed experimentally are evaluated: it is found that both quantum dynamical and classical mechanical treatments are capable of reproducing the essential temporal features probed by experiment and that, in this instance, classical equations of motion offer an adequate description of the process of nuclear separation. The difference between the relevant excited-state potential functions (over the long-range region) employed in the calculations is recovered from the simulated data by means of an inversion procedure (J. Chem. Phys. 1989, 90, 829) that relates absorption of the probing laser pulse to interfragment distance along the reaction coordinate. In addition, the variation in calculated transient behavior resulting from changes in the parameters describing the functional forms of the two potentials governing the observed reaction dynamics is examined in terms of their effect on characteristic reaction dissociation times, the lifetimes of transition-state configurations, and inversion to the difference between potential energy curves. Finally, comparison is made with analogous experimental and theoretical investigations of the fragmentation of the heavier Bi₂ molecule.

In this article, we consider the femtosecond dynamics on model potentials describing antibonding (or nonbonding) and bonding systems. To obtain an analytical description of the dynamics, reaction times are defined with the help of a simple model. Applications to unimolecular and bimolecular reactions are discussed.

The time evolution of product-state anisotropy, through absorption or fluorescence, is developed and compared with the experimental observations for three reactions involving elementary and complex dynamics. The reactions are trans-stilbene– He → trans-stilbene + He, ICN → I + CN, and HgI₂ → HgI + I. Reagant rotation, applied torque, vibrational averaging, and dynamic axis switching are incorporated into the theoretical treatment. The theory is classical in approach, but it describes the experimental behavior on the femtosecond to picosecond time scale, which enables one to obtain the nature of the angular momentum in the final distribution, the structure of the final fragments, and the geometry of the initial complex prepared.

The photodissociation dynamics of some alkali halides are explored via the method of femtosecond transition-state spectroscopy (FTS). The alkali halide dissociation reaction is influenced by the interaction between the covalent and the ground state ionic potential energy surfaces (PES), which cross at a certain internuclear separation. Depending upon the adiabaticity of the PES, the dissociating fragments may be trapped in a well formed by the avoided crossing of these surfaces. Here, we detail the FTS results of this class of reactions, with particular focus on the reaction of sodium iodide: NaI* → [Na---I]^†*→Na + I. As in our first report [ T. S. Rose, M.J. Rosker, and A. H. Zewail, J. Chem. Phys. 88, 6672 (1988) ], we observe the dynamical motion of the wave packet along the reaction coordinate and the crossing between the covalent and ionic surfaces. The studies presented here characterize the effects of various experimental parameters, including pump and probe wavelengths, on the dynamics of the dissociation and its detection. Comparisons of the results with classical and quantum mechanical calculations are also presented.

Femtosecond probing of nuclear motion in an alkali halide (NaI) reaction reveals a persistent wave packet, recurring at long times up to 40 ps. The detection of both “free” Na atoms and “transient” [Na…I] ^≠* indicates non-exponential dynamics through the crossing zone of the covalent and ionic potential energy curves. These observations are related to the nature of the motion, and the level structure making the packet.

In chemical reactions, the dynamics of the transition from reagents to products can be described by the trajectories of particles (or rigorously, of quantum mechanical wave packets) moving on a potential-energy surface. Here we use femtosecond pulsed laser techniques to follow directly the evolution in space and time of such trajectories during the breakage of a chemical bond in the dissociation of sodium iodide. The bond breakage can be described in terms of the time evolution of a single reaction coordinate, the internuclear separation. As the velocities of the separating fragments are typically of the order of a kilometre per second, a time resolution of a few tens of femtoseconds is required^1,2 to view the motions on a molecular distance scale of less than an angstrom. The resolution obtained here permits the direct visualization of the wave packet's motion and provides snapshots of the trajectories along the reaction coordinate.

The dissociation reaction of HgI₂ is examined experimentally using femtosecond transition-state spectroscopy (FTS). The reaction involves symmetric and antisymmetric coordinates and the transition-state is well-defined: . FTS is developed for this class of ABA-type reactions and recurrences are observed for the vibrating fragments (symmetric coordinate) along the reaction coordinate (antisymmetric coordinate). The translational motion is also observed as a “delay time” of the free fragments. Analysis of our FTS results indicates that the reaction wave packet proceeds through two pathways, yielding either I(²P_3/2) or I*(²P_1/2) as one of the final products. Dissociation into these two pathways leads to HgI fragments with different vibrational energy, resulting in distinct trajectories. Hence, oscillatory behaviors of different periods in the FTS transients are observed depending on the channel probed (~ 300 fs to ~ 1 ps). These results are analyzed using the standard FTS description, and by classical trajectory calculations performed on model potentials which include the two degrees of freedom of the reaction. Quantum calculations of the expected fluorescence of the fragment are also performed and are in excellent agreement with experiments.

Recently, the rapid dissociation dynamics of mercury diiodide has been studied by femtosecond transition-state spectroscopy (FTS). Such experiments provide a realtime picture of wave packet motions in two dissociation channels associated with the iodine atomic product in its ground and spin–orbit excited states. We present here a two-dimensional quantum mechanical treatment of the FTS experiment, which allows us to discuss the salient features of the wave packet motion and product state distribution. Possible refinements of the approach and their significance are also discussed.

Femtosecond time-resolved techniques with KETOF (kinetic energy time-of-flight) detection in a molecular beam are developed for studies of the vectorial dynamics of transition states. Application to the dissociation reaction of IHgI is presented. For this system, the complex [I⋯Hg⋯I]^‡* is unstable and, through the symmetric and asymmetric stretch motions, yields different product fragments: [I⋯Hg⋯I]^‡* → HgI(X²Σ⁺) + I(²P_3/2) [or I*(²P_1/2)] (1a); [I⋯Hg⋯I]^‡* → Hg(¹S₀) + I(²P_3/2) + I(²P_3/2) [or I*(²P_1/2)] (1b). These two channels, (1a) and (1b), lead to different kinetic energy distributions in the products. It is shown that the motion of the wave packet in the transition-state region can be observed by MPI mass detection; the transient time ranges from 120 to 300 fs depending on the available energy. With polarized pulses, the vectorial properties (transition moments alignment relative to recoil direction) are studied for fragment separations on the femtosecond time scale. The results indicate the nature of the structure (symmetry properties) and the correlation to final products. For 311-nm excitation, no evidence of crossing between the I and I* potentials is found at the internuclear separations studied. (Results for 287-nm excitation are also presented.) Molecular dynamics simulations and studies by laser-induced fluorescence support these findings.

In this paper, we discuss the experimental technique for real-time measurement of the lifetimes of the collision complex of bimolecular reactions. An application to the atom–molecule Br+I₂ reaction at two collision energies is made. Building on our earlier Communication [J. Chem. Phys. 95, 7763 (1991)], we report on the observed transients and lifetimes for the collision complex, the nature of the transition state, and the dynamics near threshold. Classical trajectory calculations provide a framework for deriving the global nature of the reactive potential energy surface, and for discussing the real-time, scattering, and asymptotic (product-state distribution) aspects of the dynamics. These experimental and theoretical results are compared with the extensive array of kinetic, crossed beam, and theoretical studies found in the literature for halogen radical–halogen molecule exchange reactions.

Femtosecond reaction dynamics of Rydberg states (6p ²E_1/2 and 7s ²E_3/2) in methyl iodide are reported. The observed ultrafast decay and the large isotope effect reflect the Rydberg–valence potentials crossing to yield CH₃+I. The dependence of the dynamics on the vibrational mode is studied with focus on the character of the mode and the coherent coupling. Molecular quantum dynamics simulations are also reported.

The elementary reaction dynamics of methyl iodide in two Rydberg states leading to an iodine and a methyl radical occur on the femtosecond time scale (M.H. Janssen, M. Dantus, H. Guo, and A.H. Zewail. Chem. Phys. Lett. 214, 281 (1993)). In this article, we consider the dynamics of this elementary process which involves both the Rydberg and valence states. Direct comparisons are made between theory and experiment with special focus on the following observations: large isotope effects, mode dependence of the predissociation rates, and coherence effects. The quantal molecular dynamics in two-dimensions show that the initial wave packet motion occurs along a vibrational mode involving the light atoms accompanied by transitions from the Rydberg state to the repulsive state; subsequent dynamics on the dissociative state lead to the C—I bond cleavage. The theoretical calculations also give the decay behavior of the Rydberg states with lifetimes in agreement with those observed in the femtosecond experiments. Moreover, the large isotope effect in observed predissociation rates of CH₃I and CD₃I has been successfully reproduced by the same model. The two-dimensional dynamics underscore the shortcomings of a one-dimensional picture in which the C—I serves as the sole reaction coordinate. The model presented here offers a viable mechanism for the dynamics of these Rydberg states.

Femtosecond reaction dynamics of OClO in a supersonic molecular beam are reported. The system is excited to the A ²A₂ state with a femtosecond pulse, covering a range of excitation in the symmetric stretch between ν₁ = 17 to ν₁ = 11 (308–352 nm). A time-delayed femtosecond probe pulse ionizes the OClO, and OClO⁺ is detected. This ion has not been observed in previous experiments because of its ultrafast fragmentation. Transients are reported for the mass of the parent OClO as well as the mass of the ClO. Apparent biexponential decays are observed and related to the fragmentation dynamics:

Clusters of OClO with water (OClO)_n (H₂O)_m with n from 1 to 3 and m from 0 to 3 are also observed. The dynamics of the fragmentation reveal the nuclear motions and the electronic coupling between surfaces. The time scale for bond breakage is in the range of 300–500 fs, depending on ν₁; surface crossing to form new intermediates is a pathway for the two channels of fragmentation: ClO+O (primary) and Cl+O₂ (minor). Comparisons with results of ab initio calculations are made.

The real-time dynamics of hydrogen-atom-transfer processes under collisionless conditions are studied using femtosecond depletion techniques. The experiments focus on the methyl salicylate system, which exhibits ultrafast hydrogen motion between two oxygen atoms due to molecular tautomerization, loosely referred to as intramolecular “proton” transfer. To test for tunneling and mass effects on the excited potential surface, we also studied deuterium and methyl-group substitutions. We observe that the motion of the hydrogen, under collisionless conditions, takes place within 60 fs. At longer times, on the picosecond time scale, the hydrogen-transferred form decays with a threshold of 15.5 kJ/mol; this decay behavior was observed up to a total vibrational energy of ~7200 cm⁻¹. The observed dynamics provide the global nature of the motion, which takes into account bonding before and after the motion, and the evolution of the wave packet from the initial nonequilibrium state to the transferred form along the O–H—O reaction coordinate. The vibrational periods (2π/ω) of the relevant modes range from 13 fs (the OH stretch) to 190 fs (the low-frequency distortion) and the motion involves (in part) these coordinates. The intramolecular vibrational-energy redistribution dynamics at longer times are important to the hydrogen-bond dissociation and to the nonradiative decay of the hydrogen-transferred form.

When excited directly, complexes in transition states undergo nuclear motions characteristic of bound, quasibound, or unbound dynamics. In elementary reactions, the motions are on the femtosecond time scale and depend on the dimensionality of the potential.' For example, in the reaction ABA*^†→A+BA there are two relevant coordinates that describe the motion of the triatomic transition state species, ABA*^†: the symmetric and antisymmetric stretch modes, in addition to the bend. Near the saddle point, the motion is bound in the symmetric stretch and unbound in the antisymmetric stretch (reaction coordinate), and the complex could exhibit vibrational motion in the symmetric coordinate, perpendicular to the reaction coordinate. This quantized and bound motion is expected to remain at infinity (i.e., final products) with well defined coherence, as observed experimentally' and studied theoretically.…

Picosecond photofragment spectroscopy of the ultraviolet (UV) photodissociation of 1,2-diiodotetrafluoroethane reveals consecutive breaking of the two C–I bonds. Spin-orbit excited (I*) atoms show a prompt rise, in agreement with a direct mode dissociation of the first bond. Ground-state (I) atoms show a biexponential buildup, one component being fast while the other component is slow (30–150 ps depending on total energy), characteristic of the second bond breaking. The transient behavior of I atoms changes with the available energy. These results are interpreted in terms of a two step model involving a weakly bound radical. Simulations of transient behavior of I atoms, based on estimated internal energy distributions from the primary step and a model for dissociation rates as a function of energy, suggest that surface crossings are relevant to the dynamics and that the quantum yield of I atoms varies with excitation energy.

For chemical reactions in solution, the solvent exerts an important influence on the elementary processes of bond making and breaking. The solvent may, for example, enhance bond formation by trapping reactive species in a ‘solvent cage’ on the reaction timescale¹, or it may act as a ‘chaperone’ that stabilizes energetic species². Ultrafast reaction dynamics in solvent shells can be probed using laser spectroscopic techniques developed to resolve atomic motion on the femtosecond (fs) timescale³. Here we report on a study of the femtosecond dynamics of the dissociation of neutral iodine molecules encaged in clusters of around 40–150 argon atoms, which form a solvent shell^4–6. We find that, when dissociation occurs from the A-type excited electronic state of I₂, the iodine atoms exhibit coherent motion on a sub-picosecond (<10⁻¹²s) timescale, rebounding from the ‘frozen’ solvent cage and recombining. The ‘hot’ I₂ molecule is then cooled over by collisions with the argon atoms. We provide support for these interpretations using molecular-dynamics simulations. Dissociation from the B state, meanwhile, involves slower bond-breaking and slower recombination of the fragments—there is no coherent ‘rebound’ from the solvent cage. The dissociation pathway therefore depends critically on the timescale of bond breaking relative to that of solvent rearrangement.

The dynamics of I₂ dissociation and recombination (caging) in a macrocluster are reported, covering the femtosecond to picosecond time domain. Both the exit-channel and recombination dynamics are observed and related to solute-solvent interactions. The dissociation involves bond breakage on bound/repulsive potentials with concomitant evaporation of the solvent shell. The caging is accompanied by relaxation of the system and/or diffusion of the free iodine atoms prior to reforming the I₂ bond. Molecular dynamics are used to compare with experiments.

An extension of femtochemistry to the gas–liquid transition region is reported for an elementary reaction, the dissociation and recombination of iodine in supercritical argon reaching a density of 30.5 mol ℓ⁻¹. Three phenomena are systematically studied in the range 0 to 2000 bar of argon: the change of the coherent nuclear motion with solvent density, the rate of predissociation, and the caging by the solvent. The approach offers a unique way for examining the dynamics in real time from the gas to the liquid density limit.

Theoretical and experimental studies are discussed for the femtosecond wavepacket dynamics and reaction of iodine in compressed argon solution and in the collision-free gas phase limit. Classical and quantum calculations are compared with experiment and are used to demonstrate the influence of the solvent on wavepacket nuclear motion dephasing, on the caging and recombination of the separating iodine atoms, and on iodine vibrational relaxation. It is shown that these phenomena are observed on the time scale in which they occur.

In this Communication we report our first study of realtime reaction dynamics in finite size clusters. The reaction is of the type AH + S_n, where the proton transfer (bimolecular) dynamics is examined as the acid AH is solvated with different number of molecules, n = 1,2…etc This is in continuation of our effort to study reaction dynamics in real time,¹ but now extending the scope of the previous collisionless (solvent free) condition to a range where condensed phase effects can play a role. Of particular interest to us is the condition at which solvation induces vibrational relaxation and modifies IVR. Real-time studies of reactions in clusters offer great opportunities for obtaining the rates directly² and for examining these solvation processes under controlled conditions in molecular beams. Such stepwise solvation by beam methods has been advanced for a variety of systems spanning small molecules,³ large molecules,⁴ hydrogenbonded systems,⁵ and electrons…

Recently, we presented a formalism for extracting highly resolved spectral information and the potential of bound isolated systems from coherent ultrafast laser experiments, using I₂ as a model system [Gruebele et al., Chem. Phys. Lett. 166, 459 (1990)]. The key to this approach is the formation of coherent wave packets on the potential energy curve (or surface) of interest, and the measurement of their scalar and vector properties. Here we give a full account of the method by analyzing the coherences of the wave packet in the temporal transients of molecules excited by ultrashort laser pulses, either at room temperature, or in a molecular beam. From this, some general considerations for properly treating temporal data can be derived. We also present a direct inversion to the potential and quantum and classical calculations for comparison with the experiments.

The application of femtosecond transition-state spectroscopy (FTS) to molecular iodine is reported. The real-time motion of wave packets prepared coherently in the bound B state is observed. In addition, the motion is probed near and above the dissociation limit for the reaction: I₂→I(²P_3/2)+I*(²P_1/2). FTS measurements of the dynamics on repulsive surfaces are also reported.

A simple procedure is described for the inversion of femtosecond temporal spectra to yield the potential. From observations by the femtosecond probe laser of the vibrational oscillations resulting from the coherent excitation of several vibrational states by the femtosecond pump laser over a range of excitation wavelengths (λ₁), one ascertains the energy dependence of the vibrational period τ. A fit of the data points τ(λ₁) is used in an analytical representation of a classical expression for the turning-point difference ΔR, from which one obtains V(ΔR) directly, without concern for the vibrational level structure. The method is applied to I₂(B³Π_0+u).

The experimental methodology for structural femtochemistry of reactions is considered. With the extension of femtosecond transition-state spectroscopy to the diffraction regime, it is possible to obtain in a general way the trajectories of chemical reactions (change of internuclear separations with time) on the femtosecond time scale. This method, considered here for simple alkali halide dissociation, promises many applications to more complex reactions and to conformational changes. Alignment on the time scale of the experiments is also discussed.

We report our first successful ultrafast electron diffraction from beams of isolated molecules, CCl₄,I₂, and CF₃I. To demonstrate the feasibility of studying reactions, we report results on the structure of CF₃ radical from the dissociation of CF₃I.

The time resolution of crossed-beam experiments is explicitly related to velocity mismatch and other beam parameters. Application of this general treatment to ultrafast electron diffraction reveals that a time resolution of approximately 1.4 picoseconds can be achieved under conventional conditions. It is possible to reduce this value by increasing the electron energy, varying the angle between the electron and laser pulses, and reducing the focal width of the laser and the size of the molecular beam.

Ultrafast electron diffraction (UED) is developed, in this and the accompanying paper, as a method for studying gas-phase molecular structure and dynamics on the picosecond (ps) to femtosecond (fs) time scale. Building on our earlier reports (henceforth referred to as 1–3), we discuss theoretical and experimental considerations for the approach. Specifically we show that the use of rotational and vibrational coherences can add a new dimension to structural determination of gas-phase species. In addition to the internuclear separations of the molecular sample, the spatial alignment reflected in the scattering pattern contains bond angles and rotational constants for both excited-state and ground-state species. Vibrational coherence effects are also observable, and the motion of the wave packet is revealed by the change of the diffraction pattern with time, thus yielding the molecular dynamics. UED provides the temporal evolution of the reaction coordinate directly and is well-suited for studies of global structure changes on this time scale. Paper 5 details our experimental studies with UED and the current time resolution of the apparatus.

This paper, the fifth in a series, is concerned with the experimental description of ultrafast electron diffraction and its application to several isolated chemical systems. We present a detailed description of the Caltech apparatus, which consists of a femtosecond laser system, a picosecond electron gun, and a two-dimensional charge-coupled device (CCD) detection system. We also discuss the analysis of the scattering patterns. Ultrafast diffraction images from several molecules (CCl₄, I₂, CF₃I, C₂F₄I₂) are reported. For our first study of a chemical reaction in a molecular beam, we show the change in the radial distribution function following the formation of CF₃ radical after dissociation of CF₃I. The total experimental temporal resolution is discussed in terms of the electron pulse width and velocity mismatch. The electron pulse was characterized temporally with a streaking technique that yielded the width as a function of the number of electrons per pulse. Experimental results show that the electron source produces picosecond (or less) pulses at densities of 100 electrons per pulse and 10-ps pulses at 1000 electrons per pulse. We also report our observation of a novel photoionization-induced lensing effect on the undiffracted electron beam, which we have used to establish time zero for UED when reactions are initiated by a laser pulse.

One of the main goals of chemists is to understand the "alchemy" that leads to the building and breaking of molecules. There are many different ways of approaching this goal. One of these is photochemistry, the cracking of molecules by adding energy in the form of light to break bonds in the molecules. The resulting bond breakage is in most cases limited by statistical thermodynamic laws. With sufficiently brief and intense laser radiation properly tuned to specific resonances, we hope to bypass the statistical laws and break molecules precisely where we want to break them. Intellectually this is a challenging problem; if we succeed, laser selective chemistry may also have application in various areas of pure and applied chemistry and, perhaps, in medicine…

New experiments on the dynamics of collisionless energy redistribution in molecules indicate possibilities for laser-selective chemistry with (sub)picosecond pulses.

In this paper, we report experimental results on the locking of dephasing of molecules in the gas phase. The locking of inhomogeneous dephasing of the B ← X transition of iodine is achieved by using multiple-pulse and phase-coherent laser excitation, instead of a single-pulse excitation. We detect the locking using echo techniques. The locking concept is extended to the problem of intramolecular dephasing, emphasizing the potential for controlling selectivity in laser chemistry.

In this series of papers we report on the generation and application of multiple pulse phase coherent sequences in optical spectroscopy. In this paper the effects of intense pulse trains on systems with only two resonant energy levels are analyzed, with particular attention to the effects of extreme inhomogeneous broadening and population depletion to nonresonant levels. It is shown that these effects, which are present in virtually all optical systems, make the simple gyroscopic model of optical coherent transients invalid. Exact calculations show, e.g., that a two-pulse photon echo is not maximized by a 1:2 length ratio for the pulses; that the maximum excited state population is not created by a 180° pulse; and that three equal pulses are almost as effective as a 1:2:1 ratio for producing three pulse echoes. The role of pulse phase is extensively analyzed. Pulse sequences are proposed and experimentally demonstrated which permit optical phase sensitive detection and measurement of ground state relaxation parameters. The experimental results are based on an extension of the acousto-optic modulation and fluoresence detection techniques of Zewail and Orlowski [Zewail et al., Chem. Phys. Lett. 48, 256 (1977); Orlowski et al., ibid. 54, 197 (1978)]. The relative merits of fluorescence and transverse polarization detection are discussed, and fluorescence detection is shown to be more generally useful for these new sequences. Finally, composite pulse trains are shown to be capable of substantially increasing the signal available from highly inhomogenously broadened transitions. In paper II we extend the treatment to multilevel systems with some emphasis on solid state applications.

A novel observation of photon locking—the optical analog of spin locking—is reported, demonstrating the applicability of phase-coherent pulse sequences. The experiments are reported for the optical transition of iodine gas at 589.7 nm using the pulse sequence . Locking decay rates are presented as a function of pressure and compared with optical dephasing (echo-decay) rates.

Femtosecond selective control of wave packet population is reported for molecular iodine. It is shown that both population and phase control of the packet motion can be observed by a 2-D pulse sequence of variable delay times and phase angles. Extension to other type of control experiments is also discussed.

THE critical stage in a chemical reaction—the progression through the transition state from reagents to products—occurs in less than a picosecond (10⁻¹² s). Using laser pulses of femtosecond (10⁻¹⁵ s) duration it is possible to probe the nuclear motions throughout formation and break-up of the transition state^1,2. The coherence and very short duration of these femtosecond pulses provides a means to influence the course of the reaction during this stage if the time resolution is made sufficiently short. Here we describe a demonstration of such control of a chemical reaction on the femtosecond timescale. Using two sequential coherent laser pulses, we can control the reaction of iodine molecules with xenon atoms to form the product XeI by exciting the reactants through the transition state, in a two-step process. The yield of product XeI is modulated as the delay between the pulses is varied, reflecting its dependence on the nuclear motions of the reactants.

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