Laser light absorption characteristics of a laser produced hydrogen plasma

Abstract

Light from a neodymium doped glass laser was focused in hydrogen gas at pressures from 20 millitorr to 62 atmospheres in order to produce optical breakdown of the gas. In the cases where breakdown was experienced, absorption of the remainder of the laser pulse by the resulting plasma was studied. It was found that the hydrogen plasma had some very distinct absorption characteristics; in the, absorption was very small at pressures slightly exceeding the threshold for optical breakdown, and very strong at pressures above one atmosphere. There was strong evidence of a frequency shift greater than 35 angstroms of the laser light as a result of its transit through the plasma. Photographs of the forward transmitted laser intensity pulses after breakdown were compared in time with a similar pulse in which no breakdown was experienced. An effort was made to discover evidence of anomalous absorption at the plasma frequency of the laser light. The data does not conclusively support anomalous absorption in the pressure range where the plasma frequency is approximately equal to the laser light frequency. There was evidence of increased absorption at lower pressures that might be a result of an anomalous heating mechanism.