Knowledge Bank

Van der Waals (vdW) vibrational excitation in the $A~$ electronic state of the propynal-Ar isomer, with Ar situated $\sim 4\backslash AA$ above the propynal plane, causes the average position of Ar to move with respect to the propynal (HCCCHO) monomer. This motion is large enough to cause the band type of the laser-induced fluorescence (LIF) spectra to change, i.e. from A-type to C-type, when the vdW bending modes gain one or more quanta of excitation. The other isomer of propynal-Ar, with Ar situated in the propynal plane in the HCO gulf, has also been observed. The isomeric structures are identified from rotational band analysis of the fluorescence excitation spectra and confirmed by holeburning spectroscopy. It is also found that Ar induces rapid intersystem crossing (ISC) from propynal $S1$ to $T1$ and the ISC rates depend on the orientation of the propynal-Ar interaction.