Mass Spectrometric Study of Photoionization. I. Apparatus and Initial Observations on Acetylene, Acetylene-d 2, Benzene, and Benzene-d 6.

Abstract

A windowless vacuum ultraviolet monochromator and mass spectrometer are combined for the study of photoionization processes in the energy range 2000 to 600 A (6 to 21 eV). Details of the apparatus and techniques of operation are given and results are reported for an initial study of acetylene, acetylene-d2, benzene, and benzene-d6. Ionization energies of 11.406 and 11.416 eV are obtained for the 1πu electron of C2H2 and C2D2, respectively. Vibrational levels of the ground state of the ion are observed with quantum intervals of 1855 cm−1(C2H2) and 1775 cm−1(C2D2). Ionization energies for the e1g(π) electron of C6H6 and C6D6 are determined to be 9.242 and 9.245 eV, respectively. Quantum intervals for vibrational levels of the ground state ions are apparently equal for the two isotopic molecules and estimated to be 800 cm−1. A second onset of ionization is observed at 11.53 eV for C6H6 and at 11.59 eV for C6D6. Results agree well with spectroscopic data.