Laser mode-selective chemistry

Modem photochemistry (IR, UV or VIS) is induced by coherent or incoherent radiative excitation processes [4, 5, 6 and 7]. The first step within a photochemical process is of course a preparation step within our conceptual framework, in which time-dependent states are generated that possibly show IVR. In an ideal scenario, energy from a laser would be deposited in a spatially localized, large amplitude vibrational motion of the reacting molecular system, which would then possibly lead to the cleavage of selected chemical bonds. This is basically the central idea behind the concepts for a mode selective chemistry , introduced in the late 1970s [127], and has continuously received much attention [10, 117. 122. 128. 129. 130. 131. 132. 133. 134... 

Quack M 1991 Mode selective vibrational redistribution and unimolecular reactions during and after IR-laser excitation Mode Selective Chemistry ed J Jortner, R D Levine and B Pullman (Dordrecht Kluwer) pp 47-65... 

Mode-selective chemistry The use of laser beams to control the outcome of a chemical reaction by exciting specific energy states of the reactants. 

The reactions in Equations (22.1) and (22.2) were prepared by generating the H atoms with a microwave discharge and the HOD molecule in selected overtones by using laser excitation in the visible. The reaction in Equation (22.1) was prepared with four quanta in the H-OD stretch and produced nearly pure H2 -I- OD products. Conversely, when the HO-D stretch was prepared with five quanta, nearly pure HD -f OH products were found. The conclusion is that mode-selective chemistry can be realized by localizing vibrational... 

Bond- or Mode-selective Laser Chemistry. Suppose we wish to break a specific bond in a molecule or cause a molecule to rearrange in a specific way, and the desired transformation is not the one which will occur if the molecule is simply heated (i.e., it is not the weakest coordinate in the molecule). Can we, by selectively exciting with a laser the bond or motion in question, cause the desired transformation to occur in greater than thermal yield ... 

Energy Redistribution in Isolated Molecules and the Question of Mode-Selective Laser Chemistry Revisited, N. Bloembergen and A. H. Zewail, J. Phys. Chem. 88, 5459 (1984). 

My answer would be, using an analogy between molecules and human beings, that it is neither nice nor possibly easy to use brute force on the molecules. However, often the molecules may be in a state where they do not really know what they want to do. Then we might use some very mild means to seduce them to do what we would wish them to do. As an early example for such mild seduction I might quote the theoretical scheme for potentially mode selective infrared laser chemistry of ozone [1, 2], which predates some of the more widely publicized subsequent schemes using excited electronic states. 

Fig. 11.5 Various rates of laser photoexcitation or deposition of light energy into a molecule can provide various types of laser-induced chemistry from mode-selective photochemistry for very fast, subpicosecond rates of excitation to ordinary thermal chemistry for low-rate, millisecond excitation. The real values of relaxation times depend on the density of the irradiated substance.

Bloembergen, N., and Zewail, A. H. (1984). Energy distribution in isolated molecules and the question of mode-selective laser chemistry revisited. Journal of Physical Chemistry, 88, 5459-5465. 

We can indeed claim that this is an example of photoselective laser chemistry. The competition between relaxation and reaction of photoex-cited electrons in clusters represented in Fig. 14(b) is reminiscent of the competition in many laser-induced chemical processes, stimulated by the selective absorption of one or more photons, such as photodissociation, photoionization, isomerization, and so forth in polyatomic molecules, where the coupling of many vibrational modes provides energy randomization and relaxation on picosecond time scales. 

Laser excitation is not only a means for providing the necessary energy for chemistry to take place. We begin the discussion of the selectivity achievable by the excitation and the specificity of the detection for the special case of diatomic molecules. Then the spectrum is simpler in that there is only one vibrational mode as well as only one rotational constant. But once the molecule is electronically excited we will find it necessary to allow for the breakdown of the Born-Oppenheimer separation. 

Theory and calculations on the chemical reactions of polyatomic molecules are very active areas of research, " There are several reasons for this. The most modem experimental techniques using lasers and molecular beams are being applied to study the microscopic details of such chemical reactions including how different vibrational modes of polyatomic molecules influence reactivity," and measurements of the lifetimes of reaction complexes. State-selected experiments of this type require detailed quantum reactive scattering theory in their interpretation. Furthermore, there is a need for the accurate calculation of kinetic data such as rate constants of polyatomic reactions that are sometimes difficult to study in the laboratory but are important in areas such as atmospheric, combustion, and interstellar chemistry. 

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