Influence of humidity on the characteristics of laser-induced air plasma

Abstract

The dependence of laser-induced breakdown spectroscopy (LIBS) on ambient conditions has not been clearly clarified as yet for element qualitative and quantitative analysis. To fill this gap, a laser-induced air plasma was produced by focusing a 1064-nm laser beam of Q-switched Nd:YAG pulse laser and the influence of relative humidity on the air plasma was systematically investigated by varying humidity at different laser irradiances. In contrast to the early findings, i.e., weak dependence of plasma properties on the humidity, some distinctive characteristics have been observed and explained. Examining the time-resolved emission spectra shows that the temporal evolution of spectral lines almost remains unchanged with the humidity and the influence of humidity on emission spectra can be neglected under a short gate width less than several hundred nanoseconds. As for the time-integrated emission spectra obtained at a higher laser irradiance, the increase in the emission intensity of H and O with the humidity is attributed to the increasing mole fraction of H and O and more laser energy that can be absorbed by the increasing water vapor. A faster growth of emission intensity of H and a slower increase of emission intensity of O respectively result from the relatively large increase of mole fraction of H and the relatively small increase of mole fraction of O with the humidity. Observations indicate that a shorter gate width or a lower laser irradiance is preferable in reducing the humidity influence and a certain magnitude of emission intensity of H and O spectral lines should be subtracted from the measured spectra for a more accurate and more reliable LIBS examination.