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I recently found a paper (for the curious, this one) that talks about observing the motion of a nuclear wavepacket in H2O, as initiated by tunnel ionization. This wavepacket should be thought of as a superposition of the different vibronic states of the molecule after ionization, and is therefore described by a Franck-Condon oscillation: the oscillation of the overlap between the ground-state nuclear wavefunction and the different eigenstates of the nuclear degrees of freedom.

That paper reproduces a figure from a 1975 paper (L. Karlsson et al. Isotopic and vibronic coupling effects in the valence electron spectra of H216O, H218O, and D216O. J. Chem. Phys. 62 no. 12 (1975), p. 4745) that experimentally observed the Franck-Condon oscillations in energy-resolved photoelectron spectra:

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The measurement clearly resolves the different peaks in the spectrum, their roughly equal spacing, and even a slight displacement for the different isotopic combinations. However, it is clear to me that the shape of the lines, particularly in such an old paper, must come from inhomogeneous broadening mechanisms. (I would by default blame the thermal velocity spread of the molecules prior to ionization, but I can't tell for sure.) I would therefore like to pose the question:

  • if one were to remove all sources of inhomogeneous broadening, can one observe natural lineshapes for Franck-Condon oscillations such as these?

  • If so, what are the physical mechanisms behind them and what theoretical tools and models exist to study them?

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