Molecular multiphoton excitation

Quack M 1989 Infrared laser chemistry and the dynamics of molecular multiphoton excitation Infrared Rhys. 29 441-66... 

Equations (46) and (47) provide the required time dependence for an arbitrary number tt of periods of the field. Provided that the periodicity of the Hamiltonian is known, the Floquet approach outlined above is particularly useful for deriving accurate results for all the coherent mechanisms of multiphoton excitation outlined in Figure 2. In particular, direct multiphoton transitions in two-level models have been discussed by Shirley, using the Floquet approach. The Floquet-Liapounoff approach for the numerical treatment of many level molecular multiphoton excitation was introduced in Ref. 14. 

Results ftom the numerically exact direct integration have been extensively compared with results obtained in the framework of the QRA discussed in Section 2.4 for realistic conditions of molecular multiphoton excitation. It turns out that the QRA is excellently valid for most applications of interest, whenever intermediate near resonant levels exist. In such cases one will thus usually carry out the calculations in the framework of the QRA, checking the results at a few characteristic conditions against direct numerical results. 

The conmron flash-lamp photolysis and often also laser-flash photolysis are based on photochemical processes that are initiated by the absorption of a photon, hv. The intensity of laser pulses can reach GW cm or even TW cm, where multiphoton processes become important. Figure B2.5.13 simnnarizes the different mechanisms of multiphoton excitation [75, 76, 112], The direct multiphoton absorption of mechanism (i) requires an odd number of photons to reach an excited atomic or molecular level in the case of strict electric dipole and parity selection rules [117],... 

Many kinds of molecular systems pumped by a strong laser light show chaotic dynamics. Indeed, in a semiclassical model of a multiphoton excitation on molecular vibration, chaos was discovered by Ackerhalt et al. [85] and theoretically and numerically investigated in detail [86,87]. Moreover, the equations of motion that describe a rotating molecule in a laser field can exhibit a chaotic behavior and have been applied in the classical case of a rigid-rotator approximation [87,88]. 

Different types of chemical reactions involve different types of vibrational modes, e.g. dissociation reactions may be controlled by stretching vibrations, isomerizations by skeletal modes, and so on. The argument that infrared quanta are relatively energy-poor and infrared transitions generally have low absorption cross sections, especially if multiphoton excitation is required, limits the choice of suitable molecular transitions. With respect to these constraints the type of reaction chosen and described below was dissociation, involving molecules with maximal transition dipole moments, comparatively weak bonds to be broken, and vibrational excitation in the mid-infrared spectral range. 

In this contribution we demonstrate vibrationally induced molecular dissociation in two polyatomic molecules, namely chromium hexacarbonyl and diazomethane (Fig. 1), employing fs mid-infrared laser sources [1,2]. As will be shown below, in both cases the initially excited mode does not lead directly to reaction but provides the doorway to access the right combination of modes. Thus both reactions have three steps (1) Stepwise multiphoton excitation of the doorway mode, (2) sharing of energy with other modes, and (3) formation of... 

Xu, C., and Webb, W. W. (1997) Multiphoton excitation of molecular fluorophores and nonlinear laser microscopy, in Topics in Fluorescence Spectroscopy. J. R. Lakowicz (Ed.), Plenum Press, New York, pp. 475-540. 

C. Xu and W. W. Webb, Multiphoton Excitation of Molecular Fluorophores and Nonlinear Laser Microscopy. In Topics in Fluorescence Spectroscopy, Vol. V, J. R. Lako-wicz, Ed., 1997, p. 471. 

A) During the multiphoton excitation of molecular vibrations with IR lasers, many (t5 ically 10-50) photons are absorbed in a quasi-resonant stepwise process until the absorbed energy is sufficient to initiate a unimolecular reaction, dissociation, or isomerization, usually in the electronic ground state. 

The method discussed here is photofragmentation translational spectroscopy. In this method, the molecule of interest is expanded frcMn a nozzle into a vacuum, and then the expansion is collimated to form a molecular beam. The molecular beam is then crossed with the output of a pulsed CO2 laser which excites the molecule of interest above the dissoda-tion threshold, relying on infrared multiphoton excitation to induce decomposition. In order to dissociate, a molecule must absorb approximately 20 infrared photons. 

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