Time-Dependent Study of Vacuum-Ultraviolet Emission in Argon

A 250-keV pulsed electron accelerator was constructed for the excitation of noble gases, so that resonance radiation and continuous emission could be studied under conditions of good geometry with vacuum-ultraviolet spectroscopic techniques. For argon gas, time-dependent studies were carried out at the wave lengths of the ${}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$ resonance line, the ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ resonance line, the 1100-Å continuum, and the 1250-Å continuum over a wide range of gas pressures. The ${}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$ resonance line decays exponentially with a lifetime which seems to be governed by the escape of resonance radiation to the walls of the apparatus and by three-body destruction of resonance states. The 1100-Å continuum, on the other hand, decays exponentially with a lifetime more characteristic of two-body Franck-Condon collisions. The main continuum near 1250 Å reaches a maximum well after the pulse of exciting electrons is terminated and then decays with a lifetime which is very long at low pressure but decreases to a constant value of 2.8 μsec at high pressures(about 1000 torr). From the data we suggest that the ${}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$ states, which are richly populated with charged particles, are converted to argon metastable molecules and these in turn are converted by collisions to a molecule which radiates the 1250-Å continuum with the 2.8-μsec lifetime. Better fitting to the time-dependent emission at 1250 Å is obtained by also including the known channel starting with ${}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ metastable atoms which are converted to a radiating molecule by three-body collisions.