Excitation mechanisms in He–Cd and He–Zn ion lasers

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Abstract
Experimental evidence from pulsed afterglow studies is presented which demonstrates that thermal energy charge transfer is the primary source of excitation for the high-lying Zn II and Cd II laser levels, with configurations of the type 3d 10nx and 4d 10nx, respectively, whereas the main source of excitation of the Zn II and Cd II laser levels, with configurations of the type 3d 94s 2 and 4d 95s 2, is Penning ionization. The total velocity-averaged cross sections for thermal energy charge exchange and for Penning ionization are measured to be of order 10−15 cm2. These large cross sections account for the relatively high efficiency and low threshold currents characteristic of He–Zn and He–Cd ion lasers. The intensity of spontaneous emission from laser levels excited in a flowing afterglow by charge-transfer reactions is broken down into the component contributions: direct charge-transfer excitation, radiative cascade, and collisional cascade excitation. The results are used to estimate the values of the individual cross sections for direct charge transfer into Zn II and Cd II laser levels. The potential of thermal energy charge transfer as a general excitation scheme for gaseous lasers is illustrated by several new proposals for new cw laser transitions in the region between 2000 and 4000 Å.