Photodissociation dynamics potential surfaces

Schinke R and Huber J R 1993 Photodissociation dynamics of polyatomic molecules. The relationship between potential energy surfaces and the breaking of molecular bonds J. Rhys. Chem. 97 3463... 

The preferable theoretical tools for the description of dynamical processes in systems of a few atoms are certainly quantum mechanical calculations. There is a large arsenal of powerful, well established methods for quantum mechanical computations of processes such as photoexcitation, photodissociation, inelastic scattering and reactive collisions for systems having, in the present state-of-the-art, up to three or four atoms, typically. " Both time-dependent and time-independent numerically exact algorithms are available for many of the processes, so in cases where potential surfaces of good accuracy are available, excellent quantitative agreement with experiment is generally obtained. In addition to the full quantum-mechanical methods, sophisticated semiclassical approximations have been developed that for many cases are essentially of near-quantitative accuracy and certainly at a level sufficient for the interpretation of most experiments.These methods also are com-... 

Vibrationally mediated photodissociation (VMP) can be used to measure the vibrational spectra of small ions, such as V (OCO). Vibrationally mediated photodissociation is a double resonance technique in which a molecule first absorbs an IR photon. Vibrationally excited molecules are then selectively photodissociated following absorption of a second photon in the UV or visible [114—120]. With neutral molecules, VMP experiments are usually used to measure the spectroscopy of regions of the excited-state potential energy surface that are not Franck-Condon accessible from the ground state and to see how different vibrations affect the photodissociation dynamics. In order for VMP to work, there must be some wavelength at which vibrationally excited molecules have an electronic transition and photodissociate, while vibrationally unexcited molecules do not. In practice, this means that the ion has to have a... 

B. N. Fu, B. C. Shepler, and J. M. Bowman. Three-state trajectory surface hopping studies of the photodissociation dynamics of formaldehyde on ab initio potential energy surfaces, J. Am. Chem. Soc., 133 7957-7968 (2011). 

Finally, one of the most exciting recent developments is the observation of fluorescence or Raman scattering from a molecule during the fragmentation process (19). This technique allows one to probe both the upper and ground potential surfaces, giving directly the information that is the ultimate basis for study of photodissociation dynamics. 

Amatatsu, Y., Morokuma, K., and Yabushita, S. (1991). Ab initio potential energy surfaces and trajectory studies of A-band photodissociation dynamics CH3I —> CH3 + I and CH3 +1, J. Chem. Phys. 94, 4858-4876. 

The sudden changes in the adiabatic wavefunctions near avoided crossings make it more convenient to use diabatic potential energy surfaces when simulating photodissociation dynamics. The adiabatic potentials, usually constructed from electronic structure calculation data, should therefore be transformed to diabatic potentials. The adiabatic-diabatic transformation yields diabatic states for which the derivative couplings above approximately vanish. The diabatic potential energy surfaces are obtained from the adiabatic ones by a unitary orthogonal transformation [22,23]... 

The photodissociation dynamics of di- and triatomic molecules are well established. The great difficulty lies in obtaining highly accurate potential energy surfaces and their diabatic couplings. This can be an especially challenging task if many electronic excited states are available in the Franck-Condon region. 

E. Baloitcha, G.G. BaKnt-Kurti, Theory of the photodissodation of ozone in the Hartley continuum Potential energy surfaces, conical intersections, and photodissociation dynamics, /. Chem. Phys. 123 (1) (2005), Art. No. 014306. 

In the gas phase, results on the photodissociation of ICN to yield I -F CN were recently reported [90,91]. Optical absorption of a 120-fs, 307-nm pulse produced an excited state that is unstable with respect to dissociation. The dynamics were probed by determining the time-resolved excited-state absorption strength (measured by laser-induced fluorescence) at various wavelengths. Data are shown in Figure 13. The appearance and decay of unstable intermediates along the dissociation pathway was noted. The clarity of this experiment lies in the simplicity of the sample, a small molecule in the gas phase. In conjunction with resonance Raman spectra [92, 93] and other results, these results may make possible a rather complete determination of dissociation dynamics and of the reactive potential surface. 

Considerable work has already been carried out using ab initio calculations to predict the photodissociation dynamics of gas-phase metal carbonyls (45). This is a fertile area for computational work, given the extensive experimental results available, which include the use of ultrafast methods to characterize the short time behavior in photoexcited states. There is considerable evidence that surface crossings, especially of a spin-forbidden nature, play a considerable part in the dynamics. Much of the theoretical work so far has focused on reduced-dimensionality models of the PESs, which have been used in quantum mechanical smdies of the nonadiabatic nuclear dynamics, in which spin-forbidden transitions are frequently observed (45). Here, too, the potential benefits to be derived from a proper understanding of the spin-state chemistry are considerable, due to the importance of light-induced processes in organometallic and bioinorganic systems. 

Keywords Quantum chemistry Potential energy surfaces Wave packet propagations Photodissociation dynamics... 

Photochemical Reactions.—The use of correlation diagrams in chemical dynamics has been discussed in a review article,510 and the problem of potential-surface crossing in diatomic511 and polyatomic molecules has been widely considered in several papers.512 A paper has appeared concerned with the quantum-mechanical expression to describe the relaxation processes in a chemically reacting gas under monochromatic (laser) excitation.513 Other quantum theories of molecular photodissociation have also been published.514 These are too extensive for detailed consideration here, but in general provide solvable models for small-molecule reactions. 

Most recently, the photodissociation in the state has been investigated by quantum scattering calculations on the potential surface computed using the effective Hamiltonian method of Freed and co-workers [68], In this study, the dynamics is assumed to be electronically adiabatic in nature. Theoretical absorption spectra as well as the branching ratios and photofragment translation energy distributions corresponding to various... 

The aim of the present paper is to demonstrate that the field of photodissociation dynamics is in a state where the origin of product motion is at least qualitatively understood. The origin of product motion is either initial motion in the parent molecule, or it is generated in the fragmentation step by forces that are introduced by anisotropies in the excited state potential surfaces. In a few fortunate cases, photodissociation processes can be understood almost quantitatively. One of these cases is the photodissociation of water in the first absorption band in the VUV, which will be used here as a model system to discuss features that are important in other fragmentation processes as well. The whole discussion in this paper is kept on a qualitative level and several simplifications are made to stress the basic ideas and not the finer details. 

An impressive example of large rotational excitation is the photodissociation of water in the second absorption band (the B-state), where OH rotational states are populated up to N=45. In contrast, in the first absorption band of the same molecule, very little rotational excitation is found in the OH product, indicating an extremely small anisotropy in the excited state potential surface. This demonstrates, that the rotational state distribution in the products is very sensitive to the featurels of the excited state potential surface, in this case to its anisotrbpy with respect to Y. The large difference in the rotational distributions for the same molecule demonstrates also that dynamics and not kinematical constraints are responsible for this effect. 

As mentioned earlier, a useful concept when considering photodissociation is to think of the process as a half collision. The fragments are thought of as departing from a point on a potential surface that would have been reached by a collision between the fragments, had they been travelling towards each other with the appropriate impact parameter, etc. We shall make use of this concept in the discussion that follows, but a more detailed account of recoil dynamics will be given in Part 5 on bimolecular collisions. 

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