Study of Si4 and Si4− using threshold photodetachment (ZEKE) spectroscopy

Abstract

The threshold photodetachment (ZEKE) spectrum of Si4− is presented. Although no transitions to the ground state of Si4 are observed, we obtain detailed information on the anion and several of the low-lying excited states of neutral Si4. The spectrum shows a long progression of well-resolved transitions between the D2h 2B2g rhombus anion and ν2 vibrational levels of the first excited D2h 3B3u neutral. The length and spacing of the progression is consistent with ab initio calculations performed by Rohlfing and Raghavachari [J. Chem. Phys. 96, 2114 (1992)], but some of the sequence bands observed within the progression are not. We also observe transitions to the Si4 1B3u state which is found at a lower excitation energy than predicted. The perturbed vibrational structure in this band is attributed to vibronic coupling to a nearby electronic state which is ‘‘dark’’ with respect to ZEKE spectroscopy. The ZEKE spectra are compared to the previously obtained photoelectron spectra of Si4− as well as ab initio calculations on Si4− and Si4.