Spectroscopy of reactive collisions

A detailed understanding of reactive collisions is the basis for optimization of chemical reactions that is not dependent on trial-and-error methods. Laser-spectroscopic techniques have opened a large variety of possible strategies to reach this goal [1054]. Two aspects of spectroscopic investigations for reactive collisions shall be emphasized  

The experimental conditions for the spectroscopy of reactive collisions are quite similar to those for the study of inelastic collisions. They range from a determination of the velocity-averaged reaction rates under selective excitation of reactants in cell experiments to a detailed state-to-state spectroscopy of reactive collisions in crossed molecular beams (Sect. 8.5). Some examples shall illustrate the state of the art  

The first experiments on state-selective reactive collisions were performed for the experimentally accessible reactions 

The development of new infrared lasers and of sensitive infrared detectors has allowed investigations of reactions where the reaction-product molecules have known infrared spectra but do not absorb in the visible. An example is the endothermic reaction 

A typical experimental arrangement for such investigations is depicted in Fig. 8.23. In a flow system, where the reactive collisions take place, the levels (u, J) of the reactant molecules are selectively excited by a pulsed infrared laser. The time-dependent population of excited levels in the reactants or the product molecules are monitored through their fluorescence, detected by fast, cooled infrared detectors (Vol. l,Sect. 4.5). 

The first experiments were carried out in 1983 [13.94,95]. The H atoms were produced by photodissociation of HJ molecules in an effusive beam using the 4th harmonics of Nd YAG lasers. Since the dissociated iodine atom is found in the two fine-structure levels I(Pi/2) and I(P3/2) two groups of H atoms with translational energies Ejjj = 0.55 eV or 1.3 eV in 

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One of the attractive goals of laser spectroscopy of reactive collision processes is the basic understanding of chemical reactions. The fundamental question in laser chemistry of how the excitation energy of the reactants influences the reaction probability and the internal state distribution of the reaction products can, at least partly, be answered by detailed laser-spectroscopic investigations. Section 8.4 treats some experimental techniques in this field. 

One of the attractive goals of laser spectroscopy of reactive collision processes is the basic understanding of chemical reactions. The fundamental... 

Although theoretical techniques for the characterization of resonance states advanced, the experimental search for reactive resonances has proven to be a much more difficult task [32-34], The extremely short lifetime of reactive resonances makes the direct observation of these species very challenging. In some reactions, transition state spectroscopy can be employed to study resonances through "half-collision experiments," where even very short-lived resonances may be detected as peaks in a Franck-Condon spectrum [35-38]. Neumark and coworkers [39] were able to assign peaks in the [IHI] photodetachment spectrum to resonance states for the neutral I+HI reaction. Unfortunately, transition state spectroscopy is not always feasible due to the absence of an appropriate Franck-Condon transition or due to practical limitations in the required level of energetic resolution. The direct study of reactive resonances in a full collision experiment, such as with a molecular beam apparatus, is the traditional and more usual environment to work. Unfortunately, observing resonance behavior in such experiments has proven to be exceedingly difficult. The heart of the problem is not a... 

The study of reactive excited states of van der Waals complexes is the link between the laser-assisted collision and the photodissociation approach it brings the collisional problem into a much simpler photodissociation problem. Here, a cold complex which has a defined geometry is formed between the collision partners and optically excited to trigger the reactive process. This creates the photodissociation of a molecule with very weakly bound ground state. The van der Waals spectroscopy has already allowed the accurate determination of the interatomic potential [Na-Ar (Smalley et al. 1977 Tellinghuisen et al. 1979), HgAr (Breckenridge et al. 1985, 1994 Fuke et al. 1984)]. More complex collisional... 

An important aspect of this kind of experiment is the time evolution of the reactive excited complex. It can be expected that for a reaction near the threshold, fine tuning of the optical excitation should yield to drastic changes in the reactive decay time as the spectroscopy already shows for the Ca-FICl system (Keller 1991, Soep et al. 1991, 1992). Real time evolution of binary reactive collisions can be studied through van der Waals complexes, since time t = 0 is defined by the excitation laser, as well as the starting internuclear distance between reactants which is fixed by the ground state geometry. This approach has been used for... 

This experiment shows that the harpoon mechanism can be observed through the spectroscopic observation of the charge transfer state Hg+Cl2. Moreover, it shows the interest of starting from a fixed geometry to understand the spectroscopy of the reactive collision complex. 

The technical discussion begins with a case that is convenient for an introductory presentation, that of vibrational dynamics in an excited electronic state. Experiments of this kind have been extensively pursued in both the frequency (35) and the time (36) domains. The formalism can be generalized to include the higher-order processes, such as two-photon spectroscopies (Raman (23,37-42), stimulated emission spectroscopy (20,22), and higher-order ones (23,43)). At the heart of the formalism is the notion of a nonstationary state which evolves in time. Our point of view, which follows Heller (44) can therefore be usefully applied to other aspects of dynamics, including photodissociation (45) and reactive collisions (46). 

The two main sources of information about atomic and molecular structure and interatomic interactions are provided by spectroscopic measurements and by the investigation of elastic, inelastic, or reactive collision processes. For a long time these two branches of experimental research developed along separate lines without a strong mutual interaction. The main contributions of classical spectroscopy to the study of collision processes have been the investigations of collision-induced spectral line broadening and line shifts (Vol. 1, Sect. 3.3). 

Laser spectroscopy has contributed in an outstanding way to detailed studies of collision processes. Chapter 13 gives some examples of applications of lasers in investigations of elastic, inelastic, and reactive collisions. 

ABSTRACT. Laser and molecular beam techniques allow detailed study of many dynamical properties of single reactive collisions. The chemical scope of these methods is now very wide and includes internal state preparation of reactants, change of collision energies, state detection of products, and thus determination of state-to-state reaction rates. The great impact of laser spectroscopy on knowledge in the field of structure, molecular energy transfer and the mechanism of elementary chemical reactions is illustrated by two selected examples, i.e. studies in which laser-induced fluorescence (LIF) has been used to determine the specific impact parameter dependence of the Ca + HF -> CaF(X) + H reaction and the product state distributions for the reaction of metastable Ca with SF5. 

One example of a halogen atom with a halogen molecule reaction is known where adiabatic formation of spin-orbit excited P n halogen atom product on the 2 A surface appears to be efficient, namely the l P p ) + IBr -> IBr -1- Bri Pij ) reaction. Houston [16], Wiesenfeld and Wolk [22,77], Spencer and Wittig [94], and Gordon et al. [76] demonstrated a high correlation of product Br( Py ) with reactant Kp Py ). Wiesenfeld and Wolk [22,77] have determined branching ratios for the reactive vs. nonreactive channels by vacuum ultraviolet absorption spectroscopy and found that ca. 80% of IpPy f ) reactive collisions yielded... 

Classical Dynamics of Nonequilibrium Processes in Fluids Integrating the Classical Equations of Motion Control of Microworld Chemical and Physical Processes Mixed Quantum-Classical Methods Multiphoton Excitation Non-adiabatic Derivative Couplings Photochemistry Rates of Chemical Reactions Reactive Scattering of Polyatomic Molecules Spectroscopy Computational Methods State to State Reactive Scattering Statistical Adiabatic Channel Models Time-dependent Multiconfigurational Hartree Method Trajectory Simulations of Molecular Collisions Classical Treatment Transition State Theory Unimolecular Reaction Dynamics Valence Bond Curve Crossing Models Vibrational Energy Level Calculations Vibronic Dynamics in Polyatomic Molecules Wave Packets. 

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