EXPERIMENTAL AND THEORETICAL CHARACTERIZATION OF THE S-1-S-0 TRANSITION OF BENZO[A]PYRENE

We use fluorescence excitation and two-color resonance-enhanced multip hoton ionization (REMPI) spectroscopy to characterize the S-1-S-0 tran sition of benzo[a]pyrene, B[a]P. In addition, two-color REMPI with a t unable ionization laser provides an improved measurement of the molecu lar ionization potential, 7.103+/-0.004 eV. Ab initio calculations are performed to determine the vibrational frequencies for the 90 vibrati onal modes in the S-1 and S-0 electronic states. Our results indicate that the single-excitation configuration interaction (CIS) level of th eory provides an excellent representation of excited-state frequencies for this large molecule. Virtually all of the vibronic bands between similar to 380 nm and 397 nm, as well as numerous bands between 373 nm and 380 nm, are assigned with the aid of the calculated vibrational f requencies. In total, 27 of the 61 a' symmetry vibrational modes and 5 of the 29 a '' vibrational modes in the S-1 state have now been assig ned. (C) 1996 American Institute of Physics.