15min:

G. BARRATT PARK, JOSHUA H. BARABAN, ADAM H. STEEVES AND ROBERT W. FIELD, Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139.

Stimulated Emission Pumping (SEP) spectra from the 3⁴ level of S₁ acetylene to the N_B=10 polyad of S₀ contain evidence of an unexpected interference effect. S₀ intrapolyad intensity distributions are as a rule governed solely by the fractionation of the (single) bright state. However, the intensity distribution in the SEP spectrum observed from 3⁴ deviates from the expected pattern. Reduced dimension DVR calculations on the S₁ surface predict a three-state interaction involving one of the S₁ interloper bands. According to the calculation, this cis interloper state engenders an indirect coupling between the trans 2¹3¹6² and 3⁴ zero order states, causing the eigenstates to lie substantially farther apart than one might expect, and lending intensity to the cis 3¹6¹ interloper band that appears between them. These predictions agree quite well with the experimental observations. The intensity distribution in the SEP spectrum observed from 3⁴ seems to contain an interfering combination of the bright state patterns seen in the 2¹3² and the 3²B² spectra. This type of indirect effect can yield much information about state mixing, and is particularly interesting in this case since its effects on the level structure cannot reasonably be accounted for by effective Hamiltonian models that rely solely on polyads.