A limitation on the high frequency modulation of gas discharge lamps is the duration of the radiative afterglow which is often dilated by self-absorption. Introducing a foreign gas into the discharge alters the absorption line shape and width by collisions, thus reducing self-absorption and the afterglow decay time. This is a general technique for extending the high frequency modulability. For the experiments, the effect of variable argon pressures on the self-absorbed Hg 2537-Å initial radiative decay time (τ) was measured from abruptly terminated discharges as independent functions of the mercury pressure (0.8–70 mTorr) and argon pressure (5–200 Torr). τ increases with the mercury density but is substantially reduced by the argon pressure in quantitative agreement with the theory of Holstein and Walsh and the concept that the initial decay is primarily limited by self-absorption for our range of variables. A detailed theoretical analysis indicates that there are several ways that additional argon reduces the Hg 2537-Å self-absorption: (1) the Hg 237-Å line gets broader simply because the additional argon atoms increase the Hgndash;Ar collision frequency; (2) adding argon causes the gas temperature to rise, and this drives the Hgndash;Ar collision frequency still higher; (3) the rise in gas temperature also causes an increase in the Hg 2537-Å Doppler width. Thus, a general technique for substantially increasing the modulability of a gas discharge lamp emitting self-absorbed radiation has been theoretically and experimentally demonstrated. These results are consistent with our previous analysis performed on measurements of the Hg 2537-Å intensity in these discharges relevant to fluorescent lamps. These phenomena may also be relevant in some gas lasers.
T. J. Hammond and C. F. Gallo, "Initial Afterglow of the Self-Absorbed Hg 2537-Å Radiation from Hg + Ar Discharges," Appl. Opt. 11, 729-734 (1972)