Time-resolved laser optogalvanic spectroscopy of iodine in a radio frequency discharge

Abstract

Pulsed laser optogalvanic (LOG) spectra of iodine vapor in a ∼32 MHz rf discharge were excited at 14 900–17 100 cm−1. Two distinct, time-resolved components were observed: a fast component, synchronous with the laser pulse, width ∼1 μs, followed by a slow component, width ∼100 μs, delayed relative to the laser pulse. The fast component exhibits atomic transitions of I(I) and I(II). The slow component reproduces the B̃←X̃ photoacoustic (PA) spectrum of molecular I2. The signal delay of the slow component accords with the velocity of acoustic waves in iodine vapor. The rf electrode region is the ‘‘sensitive’’ region where the acoustic wave generates the slow LOG signal. Two mechanisms of signal generation and propagation are involved. The fast signal originates in a two-step laser photoionization of plasma-excited atoms, the first-step being resonant, and/or in changes of the atomic collisional ionization rates. These processes occur on time scales shorter than the laser pulse and generate an ‘‘instantaneous’’ LOG signal by creating additional electron–ion pairs. The delay of the molecular LOG signal, which is mediated by the PA effect, indicates that local heating produces a negligible perturbation of the discharge impedance. This is contrary to common belief. The polarity of the slow LOG signal depends on the direction of the PA wave, suggesting that the signal is generated by an actual physical movement of charged species by the pressure wave. Thermal effects are involved, but only as precursors to the PA wave.