Vibronic coupling in the ground cationic state of naphthalene: A laser threshold photoelectron [zero kinetic energy (ZEKE)‐photoelectron] spectroscopic study

TLDR: In this paper, the two-color threshold photoelectron spectra of naphthalene in a supersonic free jet have been recorded via nine vibronic levels of the S1 electronic state up to about 1420 cm−1 above the S 1 band origin.

Abstract: The two‐color (1+1’) threshold photoelectron spectra of naphthalene in a supersonic free jet have been recorded via nine vibronic levels of the S1 electronic state up to about 1420 cm−1 above the S1 band origin. The threshold photoelectron spectra recorded via the S1 band origin and via totally symmetric ag vibronic levels show significant intensity in Δν=+1 transitions in nontotally symmetric vibrations having b1g symmetry indicating that the ionization transition gains significant intensity through a vibronic coupling mechanism between the two lowest doublet cationic states. The strongest departure from the expected Δν=0 propensity in the threshold photoelectron spectra occurs through excitation of the totally symmetric 8 mode having ag symmetry indicating that a considerable displacement occurs along the normal coordinate of this 8 mode upon ionization from the S1 state. The superior resolution of the threshold photoelectron technique over conventional photoelectron methods has allowed accurate values ...