Isomeric structures, large amplitude intermolecular motions and electronic relaxation of the propynal–Ar complex Original Research Article

Fluorescence excitation spectra near the 000 and 901 Ã←X̃ bands of propynal are observed in a supersonic expansion of a mixture of propynal and Ar in He. Propynal and Ar concentration dependence studies are used to assign several observed features to propynal–Ar and propynal–Ar2. The combined results of holeburning spectroscopy and rotational band contour analysis of the propynal–Ar features enables one X̃ state and two à state complex structures to be characterized. The vector R connecting the center carbon of propynal and Ar is perpendicular to the C–C–C axis for all three structures. The angle χ that R makes with respect to the oxygen side of the plane of propynal equals 68° and R=3.6 Å for the X̃ state and one of the two à state geometries. For the other à state geometry, χ=23° and R=3.8 Å. It is most likely that there is a large amplitude motion by Ar around the C–C–C axis associated with the zero point level of the X̃ state complex. A pair-wise atom potential calculation for propynal–Ar yields two local minima quantitatively corresponding to the experimentally observed geometries with reasonable binding energies. It is also found that Ar induces rapid intersystem crossing (ISC) from the propynal S1 to T1 electronic states and the ISC lifetimes (<8 ns) depend upon the orientation of the propynal–Ar interaction.