Collision cross sections for deactivation ofH*lpar;nthinsp;equals;thinsp;2rpar;at 300deg;K have been measured by observations of the quenching of Lymanhyphen;agr;fluorescence in a discharge flow system. The quenchers (with cross section in Aring;2) are H2(84thinsp;plusmn;thinsp;8), D2(84thinsp;plusmn;thinsp;8), N2(62thinsp;plusmn;thinsp;6), He(thinsp;3). For H*plus;H2, less than 1percnt; of the total quenching produces ions. Care has been taken to work with systems optically thin in Lymanagr;so that the rate of escape of radiation, which is the reference to which the quenching rate is compared, is not reduced by resonance trapping.
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