Kinetic processes in high pressure gases: Excited state collisions

A dense sodium vapor in a high pressure buffer of argon has been simultaneously excited by short (4 ns) laser pulses from two lasers: the first tuned to one of the D line transitions at 589 nm and the second tuned to the photoionization threshold of the 3p states near 406 nm. The temporal evolution of the system was studied with and without the photoionizing laser pulses. At early times (about 100 ns) excited state populations are determined by energy transfer collisions between two laser-excited 3p atoms while the ion/electron density is controlled by superelastic heating of seed electrons followed by electron impact ionization of excited state atoms. At late times (about 1 microsecond) excited state populations are controlled by collisional-radiative recombination processes. Excitation transfer rates into the 4d, 5d, 6d, and 6s levels are measured. The optical emission spectra of a recombining Na plasma is observed. The shift and broadening of the spectral line emission due to electron collisions is observed and the electron density calculated from shift measurements. The results agree well with electron densities obtained in the same system from Saha plots of excited state densities.