Using a cw 465 nm Laser to Probe Metastable He2 Molecules

Abstract

A cw 465 nm laser has been built for sensitive detection of helium metastable molecules by resonant absorption on the \(a^{3}\Sigma _{u}^{+}(v=0)-e^{3}\Pi_{g}(v=0)\) transition band. The frequency-mixing radiation is obtained from commercial laser diodes in a periodically-poled KTP non-linear crystal. The 1.3-nm tuning range includes the main rotational lines of 3He2, 4He2, and 3He-4He dimers. Measurements of absolute molecular densities down to a few 109 cm−3 are reported in low pressure (1–400 mbar) room temperature He gas excited by a weak rf discharge. Unsophisticated detection techniques provide signals with good signal-to-noise ratios thanks to the narrow absorption linewidths (a few GHz, due to Doppler and moderate collisional broadenings) in the fully resolved spectrum. Prospects for use or upgrade of this blue laser to probe the broadened and shifted molecular lines in condensed He are discussed.