Infrared multiple-photon excitation

A typical problem of interest at Los Alamos is the solution of the infrared multiple photon excitation dynamics of sulfur hexafluoride. This very problem has been quite popular in the literature in the past few years. (7) The solution of this problem is modeled by a molecular Hamiltonian which explicitly treats the asymmetric stretch ladder of the molecule coupled implicitly to the other molecular degrees of freedom. (See Fig. 12.) We consider the the first seven vibrational states of the mode of SF (6v ) the octahedral symmetry of the SF molecule makes these vibrational levels degenerate, and coupling between vibrational and rotational motion splits these degeneracies slightly. Furthermore, there is a rotational manifold of states associated with each vibrational level. Even to describe the zeroth-order level states of this molecule is itself a fairly complicated problem. Now if we were to include collisions in our model of multiple photon excitation of SF, e wou d have to solve a matrix Bloch equation with a minimum of 84 x 84 elements. Clearly such a problem is beyond our current abilities, so in fact we neglect collisional effects in order to stay with a Schrodinger picture of the excitation dynamics. 

Figure 10. Survival fraction of iron-methanol complexes Fe,(CDjOH) upon infrared multiple-photon excitation as a function of CO2 laser fluence at 956.2 cm. Notation n, m refers to number of iron atoms and methanol molecules, respectively, in complex. (From Zakin et al. )...

M. N. R. Ashfold and G. Hancock, Infrared Multiple Photon Excitation and Dissociation Reaction Kinetics and Radical Formation , in Gas Kinetics and Energy Transfer, ed. P. G. Ashmore and R. J. Donovan (Specialist Periodical Reports), The Royal Society of Chemistry, London, 1981, Vol. 4, p. 73. 

Two-photon excitation is preferable in 3D optical memory because the crosstalk between two adjacent layers is much reduced. Another advantage of two-photon excitation is reduction in multiple scattering. This reduction occurs because of the use of an illumination beam at infrared wavelength. 

The gas-phase photochemistry of haiogenated ethenes has been studied in the UV and VUV [60, 61], as well as in the infrared, using multiple-photon-absorption excitation with powerful CO2 laser sources [62-66]. Also, sensitized decompositions, for example using electronically excited Hg( P) atoms, have also been reported [67-69]. The net gas-phase photochemistry of these systems usually involves hydrogen halide elimination via three-and/or four-center transition states, with some evidence for simple bond fission producing halogen atoms in the case of Hgf Pj) photosensitization [70]. 

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]. 

Activation of the vibrational energy of ions can also be induced by the absorption of IR radiations. A popular type of IR radiation source is far-IR laser. In fact, many molecules have a broad IR absorption band. The most widely used IR source is a continuous wave (c.w.) CO2 laser, with the wavelength of 10.6 pm. This wavelength corresponds to an energy of 0.3 eV per laser photon. Because decomposition of a chemical bond requires >1 eV, laser excitation has to extended over hundreds of milliseconds to allow ions to absorb multiple IR photons. This method is known as infrared multiphoton dissociation (IRMPD). Another type of similar technique is black-body infrared radiative dissociation... 

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