Note: This article is the second part of a Lubman's review entitled, "Analytical multiphoton ionization mass spectrometry." Part I, published in Muss Spectromefy Reviews, Volume 7 , Number 5, reviewed the theory and instrumentation aspects of this field. The present article reviews applications of multiphoton ionization (MPI) to various analytical problems in mass spectrometry.
IV. APPLICATIONS
A. BackgroundIn its early stages of development, MPI was used as a tool for spectroscopy. In 1975 Johnson et al. (1) and Dalby et al. (2) reported the initial observations of MPI in the molecular systems of NO and Iz, respectively. This was followed by the use of MPI spectroscopy in a study by Johnson in which the normally forbidden lEZg state in benzene was observed for the first time (3) in a two-photon resonance. The transition occurs because the first photon corresponds to g -+ p excitation of a nonresonant virtual state and the second photon to p + g excitation. Thus, multiphoton techniques allow the probing of states normally inaccessible in a one-photon resonance. Significant contributions followed in the work of El-Sayed (4) and Goodman (5), which dealt with the vibronic selection rules for molecular species and the dependence of the two-photon tensor on photon polarization. Other work on the use of MPI for the study of molecular spectroscopy soon followed with the work of Colson (6) and Robin (7) and later Compton (25)' who studied three-photon-induced ionization spectra of pyrrole and furan and observed newly reported Rydberg states.These early experiments were generally performed in simple ionization cells at relatively high pressure, often as high as 50-100 torr. The cells were generally constructed of a glass or quartz cylinder with two electrodes, one which served as the bias electrode and the other as the collector. The resulting current was usually amplified by a sensitive current amplifier and the signal displayed on a