RESONANT MULTIPHOTON IONIZATION OF LARGE MOLECULES IN SUPERSONIC BEAM ENVIRONMENTS

[摘要] Time-of-flight detected resonant two photon ionization (R2PI) in supersonic beams is investigated. The advantages of the method are the high sensitivity, spectral selectivity and the lack of extensive fragmentation in the mass spectra. To obtain these properties the molecular system must meet several restrictions. Stable intermediate electronic states, which are greater than half of the adiabatic ionization energy generally are detected with these advantages, with a single photon field supplying the energy to populate the intermediate state and to form the photoions. Modulation of the ionization efficiency is observed when a rapid decay channel is available to the intermediate state, which is no longer coupled to the photon field. These properties are illustrated by the behavior of aniline, bromobenzene, and metal carbonyl systems. Two-color ionization removes many of the restrictions inherent to one-color R2PI. High efficiency is maintained for a broader class of molecules by allowing the ionization photon to provide the energy spanned by the intermediate state and the ionization continuum levels. Experiments which scan the ionization laser energy, provide information regarding the nature of the ionization step in benzene and napthalene. The predominantly step-like direct ionization structure observed, indicates that this is the dominant ionization process. Thus the relevant selection rules governing the propensity of an intermediate state to ion transitions are largely of a vibrational nature. The two-color ionization method has been specifically applied to studies of collision-free vibrationally excited triplet state levels formed in the molecular beam. By varying the time delay between the pump and ionization lasers, the decay of excited state populations can be monitored. This time evolution data has been unavailable through conventional methods due to the experimental difficulties imposed by the long-lived, non-fluorescent nature of these states. As the decay rate of these states are highly sensitive to excess vibrational energy, the presence of collisional perturbation tends to distort the true non-radiative decay rates. In the molecular beam environment, the decay of triplet levels, isoenergetic with initially pumped S(,1) vibronic states, have been monitored as a function of the excess vibrational energy in several aromatic systems. . . . (Author;;s abstract exceeds stipulated maximum length. Discontinued here with permission of school.) UMI

[发布机构] Rice University