Multiphoton Ionization Spectroscopy of Polyatomic Molecules
A short review of the historical development of two-photon and multi photon processes is given. Some of the interesting work on multiphoton ionization (MPI) of atomic systems is briefly mentioned.
In order to fully utilize MPI in spectroscopic or dynamic applications, an expression for the observed current needs to be derived in terms of molecular quantities. Before this can be accomplished the following question must be answered: Are the populations of the levels determined by the kinetic equations or are they pulsating with time due to the off-diagonal elements (Rabi cycle) in the density matrix formalism? For two-photon states in a three-photon ionization process, when the rate of the three-photon ionization is determined by the rate of the two-photon absorption from the ground state, one can use the arguments presented by Eberly and Ackerhalt to justify neglecting pulsation effects. It is further shown that for the cases considered, steady-state population of the two-photon level can be reached in the early part of the 10 nanosecond dye laser pulse used. Under these steady-state situations, kinetic equations gave an expression for the ratio of the current produced by circularly to that produced by linearly polarized lasers of the same intensity. The polarization ratio is shown to assist in assigning the two-photon state if a) the one-photon ionization process from the two-photon state is faster than the nonradiative relaxation process from the two-photon state or b) the ionization from the two-photon state is slower than the orientation (rotational) relaxation of the initially photoselected molecules in the two-photon state. The method is tested on three molecules with two-photon states of known symmetries and is found to give excellent agreement with predictions. The method is then extended to assign the lowest energy Rydberg states of hexatriene, pyridine and pyrazine in the 6.2 eV region. In hexatriene, the Rydberg state is found to be of an 1Ag symmetry (promotion to the 3p Rydberg orbital perpendicular to the molecular plane). For the N-heterocyclics, the results suggest that the lowest ionization potential is that for a nonbonding rather than for a π electron.
KeywordsRydberg State Multiphoton Ionization Polarization Ratio Lower Ionization Potential Time Dependent Perturbation Theory
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