Abstract

The Ar₂ excimer band at 128 nm has generally been thought to arise from excimers in the closely spaced $A{}^3{\Sigma }_u^{+}\left(1_u\right)$ and $B{}^1{\Sigma }_u^{+}\left(0_u^{+}\right)$ states. We have investigated this designation spectroscopically by irradiating the dc discharge plasma of Ar₂ after supersonic expansion with the beam of a tunable titanium–sapphire laser to probe the 3p⁵4s manifold of atomic Ar and simultaneously monitoring the effects on atomic transitions as well as on the 128 nm band of Ar₂. Intensity increases and decreases of up to 5% were measured in the 128 nm band, depending on the wavelength of the incident radiation. The analysis of these results indicates that under the present discharge conditions, the 128 nm band is primarily due to excimers in the $A{}^3{\Sigma }_u^{+}\left(1_u\right)$ state, formed from atoms in the 4s[3/2]₂ level. It is also shown that the observed FWHM bandwidth of 8.7 nm can be explained by significant vibrational cooling of the excimer after expansion, with ≈80% population in the v = 0 level. Thus, when the laser was tuned to probe an absorption band of Ar₂ at 985 nm, and a decrease in intensity of the 128 nm band was recorded having the shape and bandwidth of the 128 nm band, this was proof that the 985 nm band originates from the $A{}^3{\Sigma }_u^{+}\left(1_u\right)\left(v=0\right)$ state.

© 1992 Optical Society of America