Second look at the excitation mechanism of the He--Hg+ laser

We have resolved the controversy in the literature concerning the dominant excitation mechanism of the cw mercury ion laser by obtaining the first direct experimental proof that thermal energy charge transfer is the dominant formation process for the excited 7p2P terms of Hg II which support cw laser oscillation of 6150 and 7944 A. Using a floating double probe we have monitored directly the helium ion density versus time and confirmed its one-to-one correspondence with the measured decay of spontaneous emission from the Hg II 7p2P levels. The measured value of the cross section, sigma (He+-Hg), is 1.3times10-14 cm2, amongst the largest ever observed in atomic physics, and confirms earlier work by Dyson