Collision Induced Emission

Abstract

A new continuous spectrum, associated with the weak forbidden mercury line $\lambda 2269.80$ A, has been found in a low pressure mercury discharge with a few centimeters pressure of argon or helium added. The continuum lies on the short wave-length side of the line, rising steeply to maximum intensity at about 2259A, and then falling more slowly to 10 percent near 2237A. It is attributed to radiation from excited mercury atoms in the ${}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ metastable state, perturbed by the close approach of rare gas atoms, at which time the thermal energy may be added to the energy of excitation to give a larger quantum. Since the total intensity of the continuum is many times that of the line, although the time during which an excited atom is appreciably perturbed is much less than the time between collisions, we must assume that during a collision the probability of emission is enormously increased. The $j$ selection rule, which normally inhibits the transition ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}-{}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$, evidently breaks down in the field of the colliding atom. This is shown to be reasonable on theoretical grounds.