The formation and loss of Se(1S) excited states have been examined using long‐pulse (0.7–1.0 μsec), broadband (13 nm FWHM)‐Xe*2‐ fluorescence photodissociation of carbonyl selenide. Experimental data are compared to results of a computer code that models the Xe*2 radiation transport and Se(1S) kinetics. The effective quantum yield for Se(1S) production over the wavelength interval 160–180 nm has been found to be near 0.6. The observed Se(1S) decay rates have been used in conjunction with the code to predict the effects of excitation pulse length on the efficiency of the selenium laser system. Laser oscillation has been demonstrated for the Se(1S0→3P1) transition in a long‐active‐length (54 cm) optical cavity. Oscillator performance was characterized as a function of OCSe pressure. The optimized laser output was 1.3 mJ, a value consistent with the Se(1S) kinetics and laser cavity conditions.