Role of metastable atoms and molecules of helium in excitation transfer to hydrogen atoms

The afterglow of a discharge in helium with a small admixture of hydrogen is studied spectroscopically (p=40 Torr, [e]≤1011 cm−3). The time-resolved measurements of intensities of the first four lines of the Balmer series are performed. The concentrations of metastable helium atoms and molecules are evaluated from the relative intensity of the absorption lines. The ratios of excitation transfer rates from atoms He(2 3 S 1) k 1(n) and molecules of helium He2(a 2sσ 3Σ u + ) k 2(n) to atomic hydrogen H*(n) are measured to be k 1(n=3)/k 2(n=3)=0.04±0.02 and k 1(n=4)/k 2(n=4)=0.01±0.02. The ratios of excitation rate constants k 2(n) corresponding to different states H(n) are measured to be k 1(n=4)/k 2(n=3)=0.023±0.01; k 1(n=5)/k 2(n=3)≤0.013; and k 1(n=6)/k 2(n=3)≤0.007.