Dissociation of polyatomic molecules

There can be a difference between the dissociation of polyatomic molecules and delayed ionization in the nature of the initial excitation. In ZEKE spectroscopy the state that is optically accessed (typically via an intermediate resonantly excited state) is a high Rydberg state, that is a state where most of the available energy is electronic excitation. Such a state is typically directly coupled to the continuum and can promptly ionize, unlike the typical preparation process in a unimolecular dissociation where the state initially accessed does not have much of its energy already along the reaction coordinate. It is quite possible however to observe delayed ionization in molecules that have acquired their energy by other means so that the difference, while certainly important is not one of principle. 

Dissociation of polyatomic molecules into free radicals and free atoms may be described in much the same terms as one uses for diatomic molecules, but a more elaborate picture is necessary. 

Beswick, J.A. and Gelbart, W.M. (1980). Bending contribution to rotational distributions in the photo dissociation of polyatomic molecules, J. Phys. Chem. 84, 3148-3151. 

Freed, K.F. and Band, Y.B. (1977). Product energy distributions in the dissociation of polyatomic molecules, in Excited States, Vol. 3, ed. E.C. Lim (Academic Press, New York). 

A second rotational effect comes into play when rotations are strongly coupled to the vibrations, via, for instance, coriolis interactions. In that case, the projection of the principle rotational quantum number, the K quantum number in symmetric top molecules, is no longer conserved. The energy associated with this quantum number then gets mixed in with the molecule s vibrational energy, thereby increasing the density and sums of states. When this happens we say that the A -rotor is active. If the T-rotor does not couple with the vibrations, it is inactive. We first discuss what happens when a diatom dissociates and follow that with the dissociation of polyatomic molecules. 

The dissociation of a diatom differs from that of polyatomic molecules in two ways. First, the dissociation has no real barrier and is given by a simple one-dimensional interaction potential which is often expressed as a Lennard-Jones or a Morse potential. Secondly, the product atoms have no angular momentum. Thus, the initial angular momentum of the diatom is converted exclusively into orbital angular momentum (which is relative translational energy) of the products. For this reason, the diatom dissociation is not an appropriate model for the much more complex dissociation of polyatomic molecules. Nevertheless, there are certain features that carry over. 

The kinetics of dissociation of polyatomic molecules, in particular CH4, stimulated by vibrational excitation in conditions of not very high non-equilibrium parameters y = (Tv — To)/To has been analyzed by Kuznetsov (1971). The CH4 dissociation (9-13) proceeds through vibrational excitation of CH4 molecules at any parameters y = (Tv - To)/Tq. The vibrational energy distribution is an essentially non-Boltzmann distribution in this case, and it is characterized by both temperatures, Tv and To, even when Tv > Tq. The rate coefficient kR(To, Tv) of the methane dissociation (9-13) in non-equihbrium conditions (Tv > To) can be expressed as follows (Kuznetsov, 1971) ... 

C.D. Cantrell (ed.). Multiple-Photon Excitation and Dissociation of Polyatomic Molecules. Springer Topics. Curr. Phys., vol. 35 (Springer, Berlin, 1986) ... 

Basics of IR multiple-photon excitation/dissociation of polyatomic molecules in the ground state... 

Let us summarize some features of the processes of IR MP excitation and dissociation of polyatomic molecules that are most important as far as industrial applications are concerned. 

The characteristics of the multiple-photon vibrational excitation and dissociation of polyatomic molecules can be conveniently considered in an order corresponding to the above qualitative picture of the three stages of this process. 

There were also some remarkable meetings abroad during various scientific schools and conferences. I recall with pleasure my talks with Professor Ali Javan at the 1975 Les Houches School on Laser Spectroscopy (Fig. 14.12), and with Professor A. Siegman during my visit to Stanford University (Fig. 14.13), where we discussed animatedly the effect of isotope-selective multiple-photon dissociation of polyatomic molecules by IR laser pulses. At regular international conferences on laser spectroscopy and atomic... 

The central problems that were being investigated at the Laser Spectroscopy Department of the Institute of Spectroscopy at the time were (1) control of the motion of atoms, their cooling, atom optics, etc., and (2) resonance multiple-photon excitation and dissociation of polyatomic molecules by high-power IR laser radiation. Scientists from many countries visited the institute in order to get acquainted with the latest results obtained in these fields. In particular, Professor S. Stenholm visited our institute more than once (Fig. 14.20). Among the outstanding scientists who visited... 

Evseev, A. V., Letokhov, V. S., and Puretzky, A. A. (1985). Highly selective and efficient multiphoton dissociation of polyatomic molecules. Applied Physics B, 39, 93-103. 

The recent discovery of "multiphoton dissociation" of polyatomic molecules, where molecules, such as SFg, can be dissociated by multiple absorption of infrared laser photons, has stimulated many theoretical [14.13] and experimental [14.14] investigations about the mechanism of this process. Since the first steps, namely the excitation of lower vibrational levels with moderate level density may be isotope selective, the multiphoton dissociation may turn out to become a cheap and efficient way of laser isotope separation. Infrared lasers, such as the CO2 laser, have a high conversion efficiency which makes CO2 laser photons inexpensive. For more detailed discussions of the various aspects of laser isotope separation see [14.15-17]. 

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