Volume 133, number 3 CHEMICAL PHYSICS LETTERS 16 January 1987 VIBRONIC COUPLING AND INTRAMOLECULAR DYNAMICS OF PYRENE AS REVEALED BY THE So-+ EXCITATION SPECTRUM IN A SUPERSONIC JET Nobuhiro OHTA, Hiroaki BABA zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Division of Chemistry, Research Institute ofApplied Electnnty, Hokkaido University, Sapporo 060, Japan and Giancarlo MARCONI Istltuto di Fotochunrca e Radtazioni d’Alta Energia, FRAE-CNR, Via de’ Castagnolr 1. 40126 Bologna. Italy Received 9 October 1986; in final form 23 October 1986 The fluorescence excitation spectrum of pyrene obtained in a supersonic jet for the S,_,-+Sz transition shows a complicated structure due to the interaction of discrete levels of S, with the quasi-continuous levels belonging to S,. The intensity distribution pattern in this region has been evaluated from quantum-mechanically calculated quantities, such as the vibronic coupling inte- grals, and Franck-Condon factors deduced from independent experiments. The dynamics in pyrene following excitation into Sz are discussed. 1. Introduction The elucidation of the mechanism of interaction among different electronic states is important not only to understand various spectra, including absorption and emission spectra, but also to under- stand the mechanism of electronic relaxation pro- cesses in electronically excited states under collision- free conditions. The recent advent of the supersonic expansion method enables us to examine interstate interaction and intramolecular dynamics in poly- atomic molecules through the analysis of homoge- neous line broadening. Pyrene is one of the most interesting molecules to test the current theories on the mechanism which governs interstate interaction through the vibra- tional motion. Because of the limited energy separa- tion between the two lowest excited singlets S, ( ‘Bzu) and SZ ( ‘B1,), short and long axis polarized respec- tively, a complicated structure, called an intermedi- ate level structure, is observed in absorption in the spectral region corresponding to the S0+S2 transi- tion in biphenyl and fluorene matrices [ 11. Examples of this type of state interaction have been reported also for other aromatic molecules such as naphthalene [ 21, azulene [ 31, phenanthrene [ 41 and quinoxaline [ 51. Various attempts to disentangle the spectra of these molecules have been made by using the Green’s function technique [ 6,7] or by diagon- alizing the Hamiltonian matrix of the interacting states [ 2-4 1. All the methods used previously, how- ever, considered the off-diagonal elements as adjust- able parameters and succeeded in simulating the spectrum by trial and error procedures [ 2,6]. The analysis we present here for the excitation spectrum of pyrene differs from the previous ones in the fact that neither the vibronic coupling (VC) matrix ele- ment nor the Franck-Condon (FC) factor is adjusted to fit the experimental spectrum. In particular, the VC matrix element for each normal coordinate (Q) has been calculated quantum mechanically in the “orbital following” framework [ 81, a scheme which proved to be very useful in elucidating the two-pho- ton excitation (TPE) spectrum of the same molecule [ 91. Before we completed this work, we learned of a study on pyrene and its van der Waals complexes in a supersonic jet [ lo]. For the sake of completeness we report here the fluorescence excitation spectra in 222 0 009-2614/87/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)