Absolute cross sections for H(2s) formation in electron capture by protons in gases

Absolute cross sections for the electron capture process H + +X to H(2s)+X + have been determined for protons in He, Ne, N 2 and O 2 at 16 keV and for Ar and H 2 within the range 12-28 keV. A nitric oxide filled photoionisation chamber has been used to record the Lyman alpha radiation emitted by electric field quenching of fast metastable H(2s) atoms formed in single collisions. Unlike previous measurements by Andreev (1966) full account has been taken of the polarisation of the quench radiation. The present results, which involve smaller uncertainties, are found to agree on average to within 4% of the cross sections at 16 keV reported by Andreev.