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

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-d(2), benzene, and benzene-d(6). Ionization energies of 11.406 and 11.416 eV are obtained for the 1π(u) electron of C(2)H(2) and C(2)D(2), respectively. Vibrational levels of the ground state of the ion are observed with quantum intervals of 1855 cm(−1)(C(2)H(2)) and 1775 cm(−1)(C(2)D(2)). Ionization energies for the e(1)(g)(π) electron of C(6)H(6) and C(6)D(6) are determined to be 9.24(2) and 9.24(5) 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 11.5(3) eV for C(6)H(6) and at 11.5(9) eV for C(6)D(6). Results agree well with spectroscopic data.