STUDY OF SI4 AND SI4- USING THRESHOLD PHOTODETACHMENT (ZEKE) SPECTROSCOPY

The threshold photodetachment (ZEKE) spectrum of Si4- is presented. Al though no transitions to the ground state of Si4 are observed, we obta in detailed information on the anion and several of the low-lying exci ted states of neutral Si4. The spectrum shows a long progression of we ll-resolved transitions between the D2h B-2(2g) rhombus anion and v2 v ibrational levels of the first excited D2h B-3(3u) neutral. The length and spacing of the progression is consistent with ab initio calculati ons performed by Rohlfing and Raghavachari [J. Chem. Phys. 96, 2114 (1 992)], but some of the sequence bands observed within the progression are not. We also observe transitions to the Si4 B-1(3u) state which is found at a lower excitation energy than predicted. The perturbed vibr ational structure in this band is attributed to vibronic coupling to a nearby electronic state which is ''dark'' with respect to ZEKE spectr oscopy. The ZEKE spectra are compared to the previously obtained photo electron spectra of Si4- as well as ab initio calculations on Si4- and Si4.