Ion formation during resonant action of intense IR laser radiation on monatomic molecules

The paper reports on the experimental observation of ion formation in a field of intense IR radiation from a pulsed CO2 TEA laser under conditions excluding the possibility of gas-kinetic collisions. Pulses of one microsecond duration and energy up to 1.5 joules were focused on a glass gas cuvette containing BCl3 at pressures ranging from 0.00001 torr to atmospheric pressure and furnished with flat circular electrodes forming a capacitor. A system of neutral filters enabled continuous tuning of radiation intensity from several kW to 50 MW per sq cm. The presence of charged particles in the laser-irradiated BCl3 was detected through the conduction current between the electrodes. The data are presented in the form of oscillograms showing ion currents for different pressures, and graphs showing the pressure dependence of threshold intensity and the intensity dependence of the total relative concentration of charged particles in the irradiated gas volume. The low pressures and short pulse time ensure the purely radiative mechanism of ion formation. The process is of low threshold (below about 10 MW per sq cm), and proceeds efficiently at intensities (about 50 MW per sq cm) considerably less than in the case of nonresonant photoionization.