Self-Broadening of Optical Double Resonance Lines in Cadmium

Collision broadening studies have been made of optical double resonance lines in cadmium. The results indicate that the collision frequency is velocity dependent, contrary to the usual theory of resonance self-broadening. This result supports the suggestion that the broadening results from interactions involving a number of intermediate states rather than only the initial and final states of the optical transition, because of the small oscillator strength ($f=0.002$) for transitions between the (5s)²¹S₀ ground state and the excited state in question (5s5p)³P₁. The cross section in this case has a 1v̄₁₂2/5 velocity dependence, so the collision frequency varies as v̄₁₂3/5. Measurements show that under this assumption the self-broadening cross section for cadmium is σcoll=(1.9±0.3)×10^-12cm²/v̄₁₂2/5, where v̄₁₂ is the average relative velocity in cm/sec. In the course of this work, the lifetime of the (5s, 5p)³P₁ state was redetermined and found to be (2.39±{}0.04)×{}10^-6 sec.