Multiphoton electron detachment studies of C_7H_7− using a low-power infrared laser. Competition of radiative and collisional relaxation with vibrational excitation

Abstract

Infrared laser-induced electron detachment from C_6H_5CH_2−, C_6D_5CH_2− and cyclo-C_7H_7− has been investigated by ion cyclotron resonance spectroscopy. Anions are trapped at low pressure for up to several seconds and are uniformly irradiated with the unfocused output of a cw CO_2 laser. Electron detachment is monitored as a function of irradiation period, laser intensity, wave-length and neutral gas pressure. Infrared photodetachment spectra of C_6H_5CH_2− and C_6D_5CH_2− exhibit bands at 945 and 932 cm^(−1), respectively, while the spectrum of cyclo-C_7H_7− shows only slight dependence on laser frequency between 920 and 1080 cm^(−1). Modeling of the C_6H_5CH_2− photodetachment kinetics suggests that one step in the laser excitation process is rate limiting. Radiative relaxation does not appear to be significant at laser intensities greater than 10 W cm^(−2) and collisions with neutral molecules are only moderately effective in deactivating vibrationally excited anions.