Real-Time Laser Femtochemistry: Viewing the Transition From

SPECIAL REPORT Real-Tiime Laser Femtochemfstrv Viewing the transition from reagents to products Ahmed H. Zewall, California Institute of Technology, and products. Such a description is the goal of molecular Rlchard B. Bernsteln, University of California, Los Angeles reaction dvnamics. Some 3dyears old now, molecular reaction dynamics One of the most fundamental problems in chemistry is has matured into a major field of chemistry. It has had understanding how chemical reactions occur; that is, impact on photochemistry and laser chemistry; laser how reagents make their journey to products. Tradi- mass spectrometry and ultrasensitive detection; iso- tionally, chemists start by studying the thermodynam- tope separation; energy transfer and relaxation in gases ics of a reaction, then its rate, and finally postulate its and solutions; disequilibrium and transport phenome- mechanism. na; atmospheric, combustion, and plasma chemistry; The century old and well-known Arrhenius rate dynamics of gas-solid interactions and heterogeneous equation catalysis; cluster formation and cluster chemistry; and k(T) = A~-~"~~ even dynamic photobiology. The Pimentel Report, "Opportunities in Chemistry," systemizes a large body of experimental data, express- stresses chemical reaction dynamics as one of the most ing a reaction's rate in terms of the activation energy, important current research areas in chemistry. As fur- E,, and the pre-exponential A-factor. For gas-phase bi- ther evidence of the significance of the field, the 1986 molecular reactions, the equation conveys the essence Nobel Prize in Chemistry was awarded to Dudley R. of the reactive encounter: Molecules collide (expressed Herschbach, Yuan T. Lee, and John C. Polanyi for their by the A-factor, which is related to the rate constant for incisive experimental and interpretive work on the collisions), but they react only if collisions are suffi- dynamics of elementary chemical reactions. ciently energetic (the exponential term). Major advances in the experimental study of molecu- The rate constant, k(T), however, does not provide a lar reaction dynamics have come from the application detailed molecular picture of the reaction. This is be- of molecular beam and laser techniaues. In the sim- cause k(T) is an average of the microscopic, reagent- plest molecular beam experiments, a beam of reagent state to product-state rate coefficients over all possible molecules, say A, is directed toward coreagent mole- encounters. These might include different relative ve- cules B (in the form of a target gas or another beam) and locities, mutual orientations, vibrational and rotational the reactive scattering thatproduces product molecules phases, and impact parameters. We needed a way to C and D is observed. The relative kinetic energy of the describe, with some quantitative measure, the process reagents can be changed in these "single collision" itself of chemical reaction: how reagent molecules ap- experiments by varying the velocity of A with respect proach, collide, exchange energy, sometimes break to B. bonds and make new ones, and finally separate into For laser-molecular beam experiments, a laser excites 24 November 7, 1988 C&EN of reaction cross-sections and their angular depen- Photofragmentation of a triatomic dence as a function of collision energy are compared molecule occurs in femtoseconds with theoretical (usually computational) predictions, based on ab initio or semiempirical calculations of potential energy. At the very least, such studies reveal the strength and range of intermolecular forces. Much more has been learned from detailed collision experiments in which the vibrational and rotational states of both the reactant and product molecules are known. Such "state-to-state" experiments provide information on the role of precollision states in determining the states of the product molecules. The problem is that a major feature of reaction dynamics is still unseen experimentally, namely the transition state between reagents:and products. This state often lasts only picoseconds or less, and experiments on a longer time scale provide data that are effectively time-integrated over the entire course of the collision. As emphasized recently by Ian W. M. Smith of the University of Birmingham, U.K., a great deal is known about the "before" and "after" stages of the reaction but it is difficult to observe the "during" phase. Now chemical dynamics is becoming more ambi- tious: It is trying to describekransition-state chemistry. Until recently, this has been approachable only by theoretical studies. The goal now is to make direct, real- Photofragmentation of a triatomic molecule such as ICN is time observations of the process of reaction itself, even shown schematically here. The lower surface represents a though the duration of the event may be a matter of strip of the ground state potential energy surface as a only femtoseconds (1 fs is 10-l5 second). function only of R, the separation between the atom and the Femtochemistry, or chemistry on the femtosecond diatomic fragment. The absorption of a photon (energy hv) time scale, can be defined as the field of chemical dy- excites the triatomic molecule from its ground state to its namics concerned with the very act of chemical trans- upper, repulsive potential energy surface at the start of the formation, the process of breaking one chemical bond reaction. After a few hundred femtoseconds, the photofrag- and making another. On this time scale the molecular ments are essentially free of one another. Experiments can dynamics are "frozen out," and the complete evolution measure the real-time progress of the reaction as the of the chemical event is observed. This time scale is triatomic reactant makes the transition to its diatomic and perhaps the ultimate one as far as chemistry is con- atomic products. Since the recoil velocity of fragments is cerned, but it is a mistake to infer that new studies will tyvicaily 1 km per second, in 100 fs the distance spanned is end at this time resolution. 1 A. [Adapted from M. Rosker, M. Dantus, and A. H. Zewaii, Femtochemistry requires ultrafast laser techniques Science, 241, 1200 (1988).] to initiate and record snapshots of chemical reactions with femtosecond time resolution. It is also advanta- geous to work with molecules in supersonic beams or expanded jets; the expansion extensively cools the re- one of the reagent molecules and thus influences the actant molecules, which simplifies their internal ener- reaction probability or initiates a unimolecular process gy distribution and makes it easier to do state-to-state by energy deposition in a molecule, say ABC. In this so- experiments. However, since collisions play no role on called "half-collision" process, the fragmentation of the femtosecond time scale, experiments also can be the excited ABC molecule into AB and C is the dynami- carried out in bulk gases. cal process studied. In such experiments, the observa- The objective of this article is to highlight recent tions are of the angular, velocity, and quantum state developments in femtochemistry and to make the con- population distributions of the products. nection between these real-time ultrafast techniques The quantum state distribution of product molecules and earlier methods devoted to studies of reaction dy- can be obtained using infrared or visible chemilumi- namics under collisionless conditions. Reaction d)- nescence, laser-induced fluorescence, or resonance-en- namics in collisional environments, including con- hanced multiphoton ionization. Product angular and densed phases, is beyond our scope. velocity distributions are best measured by rotatable Real-time femtochemistry of elementary reactions mass spectrometer detectors with time-of-flight analy- allows reaction dynamics to be viewed in a new light. sis of products. The task is to relate these observables to Because the experiments can utilize laser pulses as the potential energy governing the interaction be- short as 6 fs (as of the writing of this article!), the actual tween molecular or atomic fragments. Measurements progress of the passage through the transition states of November 7, 1988 C&EN 21 Special Report a reaction can be viewed directly. For photoinitiated In this and all subsequent classical trajectory simula- unimolecular and bimolecular reactions, one can ob- tions of elementary chemical reactions, it has been nec- tain snapshots from which the potential relevant to essary to use numerical integrations of the classical fragment separation and product formation cqn be de- equations of motion with subfemtosecond step sizes in duced with a distance resolution of about 0.1 A. order to accommodate the high-frequency molecular We are now seeing a revival of flash photolysis, be- vibrations occurring in the reactants. The hydrogen gun in 1949 by Ronald G. W. Norrish and George Por- atoms in Hz move with a vibrational period of about 7.6 ter at Cambridge University, in the U.K., but the in- fs; thus, if a single oscillation cycle is to be approximat- creased nine-orders-of-magnitude improvement in ed numerically with more than 10 steps, each step time resolution in femtochemistry allows one to ob- needs to be smaller than 1 fs. In a typical thermal serve the transition states and the fundamental dynam- energy atom-molecule collision, a 1 fs time interval ics of chemical bond rupture and bond formation, with corresponds to an atomic displacement of A, a step subangstrom resolution. size corresponding to less than 1%of

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