Ultrafast α -CC bond cleavage of acetone upon excitation to 3p and 3d Rydberg states by femtosecond time-resolved photoelectron imaging.

The radiationless electronic relaxation and α -CC bond fission dynamics of jet-cooled acetone in the S (nπ) state and in high-lying 3p and 3d Rydberg states have been investigated by femtosecond time-resolved mass spectrometry and photoelectron imaging. The S state was accessed by absorption of a UV pump photon at selected wavelengths between λ = 320 and 250 nm. The observed acetone mass signals and the S photoelectron band decayed on sub-picosecond time scales, consistent with a recently proposed ultrafast structural relaxation of the molecules in the S state away from the Franck-Condon probe window. No direct signatures could be observed by the experiments for CC dissociation on the S potential energy hypersurface in up to 1 ns. The observed acetyl mass signals at all pump wavelengths turned out to be associated with absorption by the molecules of one or more additional pump and/or probe photons. In particular, absorption of a second UV pump photon by the S (nπ) state was found to populate a series of high-lying states belonging to the n = 3 Rydberg manifold. The respective transitions are favored by much larger cross sections compared to the S  ←  S transition. The characteristic energies revealed by the photoelectron images allowed for assignments to the 3p and 3d states. At two-photon excitation energies higher than 8.1 eV, an ultrafast reaction pathway for breaking the α -CC bond in 50-90 fs via the 3d Rydberg state and the elusive ππ state was observed, explaining the formation of acetyl radicals after femtosecond laser excitation of acetone at these wavelengths.