Reactive and Inelastic Collisions involving Molecules in Selected Vibrational States

State Selected Kinetics and Reaction Dynamics. — For many years chemical kineticists have sought to observe and understand the processes that bring about macroscopic chemical and physical changes at the level of individual molecular events. Unfortunately, the detailed microscopic information that can be extracted from the results of conventional 'bulb' experiments is necessarily limited, since the parameters that characterize the intermolecular collisions, such as relative translational energy, impact parameter, orientation, etc., have, under these conditions, a full spread of values in accordance with statistical laws. Over about the past 15 years therefore, increasing use has been made of experimental techniques which provide results whose connection with fundamental molecular collision dynamics is less obscured by the many 'layers' of averaging [see Section 2 below and Figure 1 in ref. l(a)] that play their part in determining the magnitude of the thermal rate constant for a chemical reaction, k(T), and its dependence on temperature. For example, molecular beam and 'hot atom' experiments can yield information about the excitation function, i.e. how the cross-section for reaction varies with collision energy, whilst i.r. chemiluminescence, chemical laser, and molecular beam techniques allow the experimenter to investigate how the energy that is released in an exoergic chemical reaction is distributed among the degrees of freedom of the separating products.

The experiments referred to in the second half of the previous sentence reveal something about the specificity of energy disposal in elementary exoergic reactions. The other side of this coin is the selectivity of energy consumption; for example, whether a reaction with a high activation energy is promoted more effectively by providing the reactants with excess translational energy or by providing the same energy to an internal degree of freedom. A measure of these selective energy requirements may be obtained by comparing the results of experiments which yield an excitation function with those where the rate of reaction is determined for selected internat quantum states of the reactants.

In practice, the Boltzmann laws actually impose some degree of state selection on a molecular system at thermodynamic equilibrium. This is because the separation of electronic and vibrational states is usually much greater than kT at low temperatures, so that the great majority of intermolecular collisions under these conditions must involve molecules in their lowest vibronic states. Photochemical methods provide the simplest means of disturbing the Boltzmann distribution over states and hence of studying the kinetics of processes involving species in excited states. The photochemical investigation of electronically excited species has, of course, been carried on for many years. However, the process of excitation alters the electronic structure of the atom or molecule that has absorbed light and the results of collisions involving these species cannot be directly related to those of the corresponding ground-state species since the chemical forces controlling the collision dynamics will be quite different. In relatively large molecules, for example cycloheptatriene, the energy supplied initially as electronic excitation can rapidly be transformed into vibrational excitation via a process of internal conversion. In this way, unimolecular processes can be studied as a function of internal energy supplied via photochemical activation. Such experiments are considered in Chapter 5.

Vibrational Photochemistry. — In contrast to electronic photochemistry, direct vibrational photochemistry has really only become possible quite recently with the development of powerful i.r. lasers capable of exciting molecules in their relatively weak vibration–rotation bands. The commonest such application has been to the study of vibrational energy transfer.' Molecules are promoted to excited vibrational levels by the absorption of pulsed laser radiation The requirement that frequencies emitted by the laser correspond with lines in the absorption spectrum of the molecule is most easily satisfied when the laser oscillates on lines in the (1,O) fundamental band of the molecule that one wishes to excite, although chance coincidences and tunable laser radiation have also been used. The rate of relaxation of molecules that have been excited in this way is followed by observing how the intensity of the vibrational fluorescence (Ifl) decays with time. In the simplest case, where relaxation occurs predominantly via collisions with a single component (Q) of the gas mixture,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

where kQ1, 0 is the rate constant for

BC(υ = 1) + Q -> BC(υ = 0) + Q (2)

The method of laser-induced vibrational fluorescence has yielded a great many results on the transfer of energy from molecules such as the hydrogen and deuterium halides, CO, NO, CO2, and other triatomic molecules to chemically stable collision partners. This subject has been reviewed more than once recently and will not be considered here. However, the technique is now being used in several laboratories to investigate the result of collisions between vibrationally excited molecules and potentially reactive species, particularly atomic free radicals such as H, N, 0, and halogen atoms. In many such cases chemical reaction, as well as energy transfer, is energetically possible. These alternative channels for removal of the excited molecules, which may be written as

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3a)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3b)

are not distinguished in a laser-induced vibrational fluorescence experiment, since this only provides a direct measure of the total rate constant for removal of BC(υ), i.e. k3 = k3a + k3b. To determine k3b it is necessary to observe one or other product directly and to relate its concentration to the initial concentration of the excited reactant. These three-atom systems are clearly the simplest in which one can study the effect of enhancing the vibrational energy of a molecular...