VIBRONIC COUPLING IN THE GROUND CATIONIC STATE OF NAPHTHALENE - A LASER THRESHOLD PHOTOELECTRON [ZERO KINETIC-ENERGY (ZEKE)-PHOTOELECTRON] SPECTROSCOPIC STUDY

The two-color (1 + l') 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 a(g) vibronic levels show significant intensity in DELTAv= +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 DELTAv=0 propensity in the threshold photoelectron spectra occurs through excitation of the totally symmetric 8 mode having a(g) 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 for the fundamental vibrational frequencies of naphthalene in its ground cationic state to be determined and it has also allowed a more rigorous investigation of the vibronic coupling mechanism that occurs between the two lowest doublet cationic states. Moreover, an improved value for the adiabatic ionization energy of naphthalene of 65 687+/-7 cm-1 (8.1442+/-0.0009 eV) has been determined.