Picosecond dynamics and photoisomerization of stilbene in supersonic beams. I. Spectra and mode assignments

摘要

In this and the following paper, we present a full account of our earlier report lsqb;Syageetal., Chem. Phys. Lett.88, 268 (1982)rsqb; on the spectra and picosecond dynamics of stilbene isomerization in supersonic jets. The jethyphen;cooled excitation and dispersed fluorescence spectra ofthyphen;stilbenehyphen;h12and hyphen;d12are reported and assigned for theButhinsp;larr;thinsp;Agelectronic excitation. The 000wavelengths forh12andd12(in excitation) are 3101.4 and 3092.5 Aring;, respectively. Previously unidentified low frequency modes (as low as 20 cmminus;1inS0for hyphen;h12) have been observed and tentatively assigned as outhyphen;ofhyphen;plane modes ofausymmetry inC2h. This indicates thatthyphen;stilbene has a propellerhyphen;like geometry involving phenyl rotation (i.e.,C2symmetry). A Franckndash;Condon analysis of the low frequency modes and particularly theag, ngr;25inhyphen;plane symmetric bend mode indicates that a large geometry change takes place upon electronic excitation possibly due to a delocalization of double bond character from theCendash;Cebond toCendash;Jgr; bond. The geometry change of the inhyphen;planeCendash;Cendash;Jgr; betweenS1andS0was determined from the Franckndash;Condon and a normal mode analysis to be 1.3deg;plusmn;0.3deg;. The rms amplitude of this bend motion for the symmetric ngr;25bend mode (for one quanta inS0) is Verbar;lang;Dgr;Jgr;rang;2Verbar;1/2=1.0plusmn;0.2deg;. Mostagmodes involving benzenehyphen;type vibrations (other than Cndash;H stretch modes) have been assigned. Dispersed fluorescence spectra exhibited a broad background indicative of IVR which increased rapidly withS1vibrational energy. The spectra were completely diffuse above 1200 cmminus;1which is consistent with the barrier for isomerization being at about 1100ndash;1200 cmminus;1. The excitation spectra show a rapid decline in intensity at higher energies due to the process of isomerization which competes with radiative decay. However, sharp (albeit weak) structure could still be discerned at energies well in excess of 2000 cmminus;1. In the accompanying paper, we present results on thedynamicsof isomerization and its dependence on mode mixing and the nature of the reactive surface (adiabatic vs diabatic).