Experimental study of the spectral characteristics of laser-induced air plasma

摘要

The characteristics of laser-induced air, N_(2), and O_(2) plasma spectra are investigated spectroscopically. The study concentrates mainly on the temporal behavior of laser-induced plasma after breakdown. We used delayed spectra and spectra evolution for this study. Except for the general one-beam laser-induced breakdown experiment, a second laser beam was added to further probe the behavior of plasma during its decay. We report the experimental results of spectra composition, spectra time evolution, and spectra affected by a second laser beam. We determined that all the laser-induced air plasma spectra are from a continuous spectrum and some line spectra superposed on the continuous spectrum. The stronger short wavelength continuous spectrum is caused by bremsstrahlung radiation of electrons in the plasma, and the weaker long wavelength continuous spectrum is caused by electron and ion recombination. Line spectra originate from excited molecules, atoms, and their first-order ions, but no line spectra form higher-order ions. The results show that the temporal behavior of some spectra is a decay-rise-redecay pattern. With the two laser beam experiment we found that all the spectra intensities are enhanced by the second laser beam, but the response of various spectra to the delay of the second laser beam is quite different, in particular, the intensity increments of some spectra increase with the delay of the second laser beam. Some microscopic processes of laser-induced plasma obtained from the experimental results are discussed. These results are useful for a better understanding of some laser-induced air plasma related applications, such as laser-guided lightning and laser-induced breakdown spectroscopy.