Far-wing excitation study on the transit region of Hg ³P ₁→³P₀ intramultiplet process in collisions with N₂ and CO

Laser-pump and probe approach has been applied to the far wings of Hg ³P₁ - ¹S₀ resonance line broadened by collisions with N₂ and CO to measure excitation spectra for the formation of Hg(6 ³P₀) and Hg(6 ³P ₁). The excitation spectra are highly asymmetric with the red wing being much more extended than the blue wing. The absolute ratio of nascent yields of Hg ³P₀ to ³P₁ is determined as a function of the excitation wave number. From these measurements, it is found, commonly for Hg-N₂ and Hg-CO systems, that (a) the nascent product ratio, Hg(³P₀)/Hg(³P ₁), grows on the red-wing surface (the A state) with increasing shift, Δv, of the excitation wave number from the line center and finally surpasses unity; (b) the blue-wing surface (the B̃ state) gives mostly Hg(³P₁) but has a small chance to give Hg( ³P₀). Time constant τ₀ for the Ã→³P₀ process of Hg-N₂ is found to change from 17 to 35 ns as the absorption distance R_c between Hg and N₂ changes from 3.6 to 4.7 Å. From these values of τ₀, the transition probability P(Ã→³P ₀) for a single approach of Hg-N₂ to the turning point region is estimated to be about 3.7×10^-5. The transition probability P(B̃→³P₀) is about 270 times larger than P(Ã→³P₀). CO is about 20 times more effective than N₂ for the B̃³P₀ process. The R_c dependence of τ₀ can be qualitatively explained by the vibrational frequencies of the bound à state and the Franck-Condon factor between the bound à state and the free (repulsive) ã state arising from Hg(³P₀)+N₂. These findings suggest that the direct Ã→ã transition is realized in these Hg-N ₂ and Hg-CO collisions. This gives a remarkable contrast to Hg-atom collisions, where the A→³P₀ process is parity-forbidden due to the 0⁺ and 0^- characters of the A and a states, respectively. The coupling mechanisms for the Ã→ã and B̃→ã transitions in Hg-N₂ collisions are discussed in detail. The theoretical estimate of the Ã→ã transition probability is made to be compared with the experimental value.