DYNAMICS OF S1 ACETONE STUDIED WITH SINGLE ROTORVIBRONIC LEVEL RESOLUTION

The spectroscopy and dynamics of selected vibronic bands of the S1 (np i) <-- S0 transition of acetone and acetone-d6 were studied with sing le rotational-torsional-vibronic (rotorvibronic) level resolution in a jet up to an S1 excess energy of 1300 cm-1. Using clean coherent exci tation with Fourier transform-limited nanosecond laser pulses quantum beats were observed in the time-resolved fluorescence and were attribu ted to coherently excited singlet-triplet eigenstates as identified by their magnetic properties. In the 8(1)23(0-) and the 8(1)23(0-) I(2,1 ) state of acetone, and the 7(1)I(1,0) state of acetone-d6, lifetimes increase with the rotational quantum number of the excited state N due to a breakdown of K selection rules for the spin-orbit interaction in this very asymmetric rotor. In the 8(1)23(0-) state, the rotorvibroni c levels of the methyl torsion tunnelling component with G symmetry ex hibit the longest lifetimes among the tunnelling components. Vibration al coupling of G torsional levels in the triplet manifold favoured by symmetry is proposed to selectively increase the density of coupling t riplet states and, hence, the lifetime of the eigenstates.