Molecular photodissociation

From the translational energy distributions obtained above, the quantum state distributions and the quantum state-specific anisotropy parameters can be determined. In a molecular photodissociation process, the photodissociation product detected at an angle in the center-of-mass... 

P. Andresen and R. Schinke, in Molecular Photodissociation Dynamics, eds. M. N. R. Ashfold and J. E. Baggott (Royal Society of Chemistry, London, 1987), Chap. 3. 

Quantum yield (d>) of molecular photodissociation in the gas phase is equal to unity according to the Einstein law. Frank and Rabinowitch [72] predicted the reduction of the quantum yield in a solution due to the cage effect. The quantum yield

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This chapter deals with the theory underlying the apphcation of wavepackets to molecular photodissociation and reactive scattering. The objective will be to derive and gather together the equations and theoretical methods needed in such calculations. No attempt will be made to reference aU calculations that have been undertaken in this very popular field. Several alternative related methods will be discussed, but it will not be possible to do full justice to all the different methods that have been proposed, many of which are being successfully used. 

A. Internal State Distributions of the Fragments of Molecular Photodissociation... 

H. R. Kim, W. J. Marinelli, and N. Sivakumar, Laser Studies of Molecular Photodissociation Dynamics, to appear in Proceedings Fourth Symposium Recent Advances in Laser Spectroscopy, May 20, 1983, Brooklyn Polytechnic Institute of New York, Wiley, New York. 

The confluence of theory and experiment achieved in recent years has greatly deepened our understanding of molecular photodissociation. At this point, however, it is important to underline that the cornerstone of realistic dynamical investigations is a multi-dimensional potential energy surface (PES). The interrelation between PESs on one hand and the various dissociation cross sections on the other hand is one prominent topic of this book and therefore we think it is useful to elucidate some qualitative aspects of PESs before we start with the development of the dynamical concepts. 

The classical theory of molecular photodissociation is simple to implement and it can be extended to systems with more than two or three degrees of freedom without severe problems. Moreover, it is invaluable for interpretation purposes. We can rigorously test the validity of the classical approach only by comparison with exact quantum mechanical... 

In order to model and to analyze time-resolved measurements with finite pulse lengths it is essential to construct the true wavepacket that the laser generates in the excited state and to explore how this wavepacket evolves in time. Chapter 16 closes our survey of molecular photodissociation. On the other hand, it brings us back to Chapter 2, namely to the question of how the photon beam excites the molecule. Section... 

Balint-Kurti, G.G. and Shapiro, M. (1985). Quantum theory of molecular photodissociation, in Photodissociation and Photoionization, ed. K.P. Lawley (Wiley, New York). 

Brouaxd, M., Martinez, M.T., Milne, C.J., Simons, J.P., and Wang, J.-X. (1990). Near threshold stereo-dynamics of molecular photodissociation The visible and near ultraviolet photodissociation of H2O2, Chem. Phys. Lett. 165, 423-428. 

Kulander, K.C., Cerjan, C., and Orel, A.E. (1991). Time-dependent calculations of molecular photodissociation resonances, J. Chem. Phys. 94, 2571-2577. 

Schinke, R. (1988a). Rotational distributions in direct molecular photodissociation, Ann. Rev. Phys. Chem. 39, 39-68. 

Schinke, R. (1989b). Dynamics of molecular photodissociation, in Collision Theory for Atoms and Molecules, ed. F.A. Gianturco (Plenum Press, New York). 

Zare, R.N. and Hershbach, D.R. (1963). Doppler line shape of atomic fluorescence excited by molecular photodissociation, Proc. IEEE 51, 173-182. 

Bersohn, R. (1980). Molecular photodissociation by an ultraviolet photon, IEEE J. Quantum Electron., QE-16, 1208-1218. 

Chamberlain, G.A. and Simons, J.P. (1975). Polarized photofluorescence excitation spectroscopy a new technique for the study of molecular photodissociation. Photolysis of H2O in the vacuum ultraviolet, Chem. Phys. Lett., 32, 355-358. 

Kupriyanov, D.V., Sevastianov, B.N. and Vasyutinskii O.S. (1990). Polarization of thallium atoms produced in molecular photodissociation experiment and theory, Z. Phys. D-Atoms, Molecules and Clusters, 15, 105-115. 

A second illustrative example of the utility of TRPES and TRCIS for studying complex molecular photodissociation dynamics that involve multiple electronic state is the case of the weakly bound cis-planar C2V nitric oxide dimer [174], The weak (Do = 710cm-1) 1 A ground-state covalent bond is formed by the pairing of two singly occupied ji orbitals, one from each NO(X2II) monomer. The very intense UV absorption spectrum of the NO dimer appears... 

As the excitation process in an external field can be regarded as being a nonadiabatic transition between dressed adiabatic states [32], effective laser control can be achieved by manipulating the parameters of these nonadiabatic transitions directly. Based on this idea, two control schemes have been proposed. The first one is a control scheme for the branching ratio during the molecular photodissociation, achieved by utilizing the phenomenon of complete reflection [24,43,44], The second is to control the population transfer by using a laser pulse with periodically swept parameters [24-29], In both cases the best parameters of the laser pulse can be easily estimated from the ZN theory of nonadiabatic transitions. 

Depending on the nature of the excited-state potential, wavepacket propagation following excitation may not be oscillatory. For example, in uni-molecular photodissociation from an unstable excited state (such as S2 or S3 in Figure 10), the wavepacket may simply move in one direction and never return. If the excitation pulse duration is short compared to the time required for photodissociation, then the photodissociation events in different excited molecules will be synchronized and wavepacket propagation can still be considered coherent. Experiments of this type will be discussed in the next section. 

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