High Resolution Photoelectron Spectroscopy and Femtosecond Intramolecular Dynamics Using Supersonic Molecular Beams

High resolution helium Ialpha photoelectron spectroscopy of formaldehyde and ketene, and their deuterated compounds, are reported. The combination of a unique double-pass high resolution electron-energy analyzer and effective rotational cooling of the sample by supersonic expansion enable the spectroscopic characteristics of these molecular cations to be determined to a much higher accuracy than previously available. The vibrational autocorrelation functions are calculated from the high-resolution photoelectron spectra. They have shed considerable new light into the mechanism of the ultrafast intramolecular dynamics of the molecular cations studied. The present study reveals much more vibrational structural detail in the first electronic excited state of formaldehyde cations. The first electronic excited state of formaldehyde cations may have non-planar equilibrium geometry. Strong isotope effects on vibronic (vibrational) coupling are observed in the second electronic excited state of formaldehyde. Vibrational autocorrelation functions are calculated for all four observed electronic states of formaldehyde. The correlation function of the first electronic excited state of formaldehyde shows a rather slow decay rate on the femtosecond time scale. The ultrafast decay of the formaldehyde cations in the third electronic excited state implies that dissociation and intramolecular processes are the main decay path ways. The present spectra of the ground states of ketene cations have more fine structure than previously available. The AIEs of the first and the fifth excited states are determined unambiguously to a much higher accuracy. The doublet-like fine structures present in the first excited state of ketene implies the excitation of a 'soft' mode that was not observed before. The vibrational autocorrelation functions are calculated for four of the six observed electronic states. The dynamics of the ground states of the cations are characterized by a wave packet oscillating with small amplitude around the minimum on the upper PES. The decay dynamics of the first and the fifth excited states of ketene are characterized by ultra-fast intramolecular processes like predissociation.