Optogalvanic transients in the 1s2,4→2p1,3 excitations of radio frequency neon plasma

The optogalvanic effects (OGE) induced by pulsed laser excitation of Ne 1s2,4→2p1,3 transitions in a low power, ∼30 MHz radio frequency Ne discharge at ∼5 Torr are described. The polarity (sign) of the OGE signal is controlled by perturbations of the 1sj populations. The steady state 1s4 population is ∼101 times larger than the 1s2 population and the OGE signals for 1s4→2p1,3 excitations are correspondingly stronger than those for 1s2→2p1,3 excitations. The plasma temperature is found to be ∼1000 K. The excitations 1s2,4→2p3 are more efficient at signal production than the 1s2,4→2p1 excitations, which is contrary to prediction. The OGE signals are consequences of: (1) perturbation and reequilibration of the metastable 1s3 and 1s5 populations; (2) radiatively trapped 1s2→1S0 photons; and (3) collisionally induced 1s2, 1s4↔1s3, 1s5 energy transfer. The OGE signal components, both the ionization and photoacoustic constituents, are temporally coincident only when the immediate causative agents are trapped photons. When otherwise produced, the photoacoustic part is delayed relative to the ionization component by the time required for the acoustic wave to travel from the locus of excitation to the sensitive region(s) of the plasma. copyright 1999 American Institute of Physics