Optical fluorescence from molecular gases and metallic vapors in argon and helium has been elicited by irradiating gas mixtures with soft X rays. Emission spectra from species of interest in toxicological and environmental studies, e.g., ${\rm NO}, {\rm NO}_{2}, {\rm CO}, {\rm SO}_{2}$ , and Hg, have been produced and characterized. Probable energy transfer processes accounting for the observed excitation are presented. These include non-selective direct routes involving collisions with photoelectrons, selective indirect routes involving collisions with excited atoms, e.g., sensitized fluorescence, Penning ionization (Jesse effect), and charge transfer, and routes involving relaxation phenomena, e.g., recombination and cascade. The development of an analytical technique for the determination of the previous molecular species, in addition to N2 and OH, as part per million level impurities in rare gases was found to be highly feasible. The determination of molecular pollutants in air ...