Heavy-atom molecules collisions

One important rule for excitation in atom-molecule collisions comes from the Franck-Condon principle. When only small amounts of kinetic energy are converted into vibrational energy it seems reasonable that the non-adiabatic transition is governed by the Franck-Condon factor in the transition region. However, due to the interaction of the heavy particles these factors can be quite different from those for the undisturbed molecule. Bauer et al.15 show that the interaction matrix element which determines the transition probability can be written as a product of the electronic part of the matrix element and the square root of the Franck-Condon factor. 

The internal and external heavy atom effects, IHA and EHA, have attracted a considerable attention in the community of molecular spectroscopists. This is part of an old problem of understanding environmental effects from solvents or solid matrices on S-T absorption or on phosphorescence of solute molecules. For higher temperature studies the triplet decay is quenched either by collision or by vibrational interaction with the matrix or the solvent. The molecules subject to studies in this respect have mostly been aromatic molecules perturbed by molecular oxygen, nitric oxide or other paramagnetic molecules, molecules either with heavy atoms and/or forming charge transfer complexes. 

Fig. 9. Bond distances (see inset) vs. time, in fs, for the old and new bonds in the H2 + I2 —> 2 HI reaction in an Xei2s cluster for an impact velocity of 6 km/s. The Iruge disparity in the vibrational periods of the two reactants make for somewhat more complex dynamics because it takes a rather long time for the very heavy iodine molecule to move. It is therefore mostly the H atoms that move during the bond switching. Also, it is necessary for the reactants to be rotationally excited and the fast rotation of H2 is very evident in the oscillation of the H-I bond distances before the collision. Similarly, the rotational excitation of the product HI molecules is evident in the oscillation of the H-H bond distance ruound the I-I bond distance after the collision.

Figure 16.20 FAB and MALDI techniques, (a) The principle of fast-atom beam formation with xenon (b) The formation of fast atoms of argon in a collision chamber and subsequent bombardment of the sample by this atom beam, usually of 5-10 kV kinetic energy (c) MALDI or ionization by effect of illumination with a beam of laser generated light onto a matrix containing a small proportion of analyte. The impact of the photon is comparable with that of a heavy atom. Through a mechanism, as yet not fuUy elucidated, desorption and photoionization of the molecules is produced. These modes of ionization by laser firing are particularly useful for the study of high molecular weight compounds, especially in biochemistry, though not for routine measurements.

Physical applications An early application of relativistic molecular theory was to heavy atom collisions, and the production of supercritical fields involving highly stripped ions [234-237]. Studies have been made of parity- and time-reversal symmetry violation in diatomic molecules [74,238,239], and of parity violation in small chiral molecules [240-242]. 

If h(Vj - fj)energy transfer steps generally occur rapidly (<100 gas kinetic collisions where M is the parent gas). There is some slight evidence that indicates such intermode exchanges are somewhat slower (100-200 gas kinetic collisions) for molecules which contain all heavy atoms (no hydrogen or deuterium). 

As explained in Section 16.2.4. fluorescence takes place in competition with radiationless deactivation. Fluorescent emission is normally spherically isotropic and of longer wavelength than the exciting radiation. It is extinguished by heavy atoms in the molecule, by the excitation of free rotation about chemical bonds and torsional vibrations, by high concentrations because of increased collision probabilities, or by the presence of paramagnetic oxygen in the sample solution. This can affect reproducibility in quantitative measurements [28], [84], [127],... 

This facilitates the relative importance of radiationless decay by internal conversion or by quenching through collision with traces of impurities. Consequently, phosphorescence is rarely observable in fluid media. An important exception is in the case of ketones which have lowest energy - (mr ) triplet excited states (4). Here photon emission occurs at rates of 10 to 10 sec , fast enough to compete with solvent or impurity quenching if care is taken to deoxygenate the samples and purify the solvents. For molecules such as acetone, acetophenone, benzo-phenone, biacetyl and benzil, phosphorescence is readily observed in fluid solution at ordinary temperatures with (1/e) lifetimes of 50-500 ys. Heavy atoms promote phosphorescence rates. Dibromoacetonaph-thone (5), with a lowest (TnT ) triplet state is a useful phosphorescence probe of micellar systems. There is a whole literature on heavy-atom induced room-temperature phosphorescence applications in analytical chemistry (6),... 

Both heavy (atoms, ions, molecules) and light (electrons, photons) particles can be involved in collisions. Polyatomic molecules have internal degrees of freedom (vibrational and rotational motion of atoms) and, in this sense, they have an internal structure. 

Table C2.13.1 Collision processes of electrons and heavy particles in non-thennal plasmas. The asterisk denotes short-lived excited particles, the superscript m denotes long-lived metastable excited atoms or molecules.

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