Intracollisional Interference in the Spectrum of HD Mixed with Rare Gases

We present the first observation and interpretation of an interference effect in the infrared absorption spectrum of HD-rare-gas mixtures. This effect, due to constructive or destructive interference between allowed and collision-induced dipoles during a collision, is referred to as "intracollisional interference." Is is manifested by the appearance of a narrow line at the $R_1\mathrm{}\left(1\right)\mathrm{}$ position which increases with rare-gas density. This interference has relevance to the determination of HD abundance in planetary atmospheres.