A theoretical analysis on the vibronic spectra with mass analyzed threshold ionization spectroscopy

We have theoretically studied the absorption vibronic spectra with the resonance two-photon (R2P) and non-resonance two-photon (NR2P) mass analyzed threshold ionization (MATI) spectroscopy. The theory uses the inverse Born-Oppenheimer approximation (IBOA) to establish a proper basis set. To analyze the MATI vibronic spectra, we have calculated the Franck-Condon factors involved in the vibronic transitions. Several experimental spectra are analyzed using this theory with emphasis on the importance of the resonance intermediate states. The long vibrational progression in a MATI spectrum can be partly attributed to the result of including the anharmonic correction in the calculated Franck-Condon factors. The experimentally observed isotope effect is also analyzed.