NEGATIVE-ION PHOTOELECTRON-SPECTROSCOPY OF HCF-, HCCL-, HCBR-, AND HCI- - PHOTOELECTRON ANGULAR-DISTRIBUTIONS AND NEUTRAL TRIPLET EXCITATION-ENERGIES

Photoelectron spectra and angular distributions are reported for the HCX(1A') + e <-- HCX-(XBAR 2A") and HCX(3A") + e- <-- HCX-(XBAR 2A") transitions of the halocarbenes (X = F, Cl, Br, and I). Taking photoelectron spectra at parallel and perpendicular laser polarizations with respect to the direction of the photoelectron detection allows us to distinguish the triplet transition from the overlapping singlet transition. Ab initio calculations used to simulate the Franck-Condon envelope for the triplet states combined with the experimental data predict that HCl has a triplet ground state. Best estimates for the triplet excitation energy based on these simulations are 14.9 +/- 0.4 kcal/mol (HCF), 4.2 +/- 2.5 kcal/mol (HCCl), 2.6 +/- 2.2 kcal/mol (HCBr), and -2 to -10 kcal/mol (HCI). Vibrational intervals of 850 +/- 60 cm-1 (HCCl), 725 +/- 70 cm-1 (HCBr), and 637 +/- 80 cm-1 (HCI) in the HCX(3A") + e- <-- HCX-(XBAR 2A") transitions are attributed to the C-X stretch of the neutral. Adiabatic electron affinities for the singlet states are found to be 0.542 +/- 0.005 (HCF), 0.535 +/- 0.005 (DCF), 1.210 +/- 0.005 (HCCl), 1.454 +/- 0.005 (HCBr), and 1.680 +/- 0.005 eV (HCI). The electron affinity of 3HCI is expected to lie between 1.25 and 1.59 eV. Asymmetry parameters are also reported for photoelectrons from F-, Br-, and I- (hv = 351.1 nm).