Properties of an air plasma generated by ultraviolet nanosecond laser pulses

Abstract

Temperatures and compositions of plasma environments created by laser pulses are wavelength-dependent, and they have not been determined as yet for air plasmas produced by ultraviolet radiation. This paper fills this gap and reports time-resolved spectral measurements of laser-induced sparks created in air using a Q-switched Nd : YAG operating at 355 nm with a pulse width of 7 ns. Temperatures are determined from Boltzmann analysis of emission from atomic oxygen lines at 715 and 777 nm. Electron number densities are inferred from Stark broadening of the H-alpha line (656 nm). The measurements have been carried out over a time interval of 90 ns–3 µs after the plasma formation that corresponds to the variation of the temperature and the electron number density from 27 000 K to 12 000 K and from 1.2 × 1018 cm−3 to 1.6 × 1017 cm−3, respectively. Simple rate estimates show that the plasma is likely to be in local thermodynamic equilibrium (LTE). Using a new approach to calculate time-dependent pressures, which employs blast-wave theory, we perform thermochemical computations of the plasma thermodynamic properties and compositions. The results strongly confirm the validity of the LTE assumption for the plasma investigated and yield electron number densities in very good agreement with the measured values.