Metastable densities and excitation processes in the He-Cd laser discharge

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Abstract
Helium singlet and triplet metastable densities have been measured by a line absorption technique in a 3 mm bore capillary tube over the pressure range 0.5-15 Torr and current range 10-200 mA. With constant discharge current the singlet and triplet densities show pronounced maxima of 2*1012cm-3and 9*1012cm-3respectively at about 2 Torr. At constant pressure the metastable densities saturate for currents above approximately 20 mA. Experimental values are in fair agreement with those calculated using known cross sections for production and loss mechanisms. The addition of cadmium vapour to simulate the conditions of laser oscillation at 4416 AA and 3250 AA in Cd II has two principal effects; cadmium vapour reduces the metastable populations-almost halving them when the optimum cadmium pressure for lasing is reached, and further the metastable densities now saturate at much higher currents (60-120 mAA). The observed optimum performance of the He-Cd laser with respect to discharge current, helium filling pressure and cadmium partial pressure is related directly to the behaviour of the helium metastable densities as these parameters are varied.