VOLUME 106, NUMBER 39, OCTOBER 3, 2002
© Copyright 2002 by the American Chemical Society
ARTICLES Substituent Effects in Molecular Electronic Relaxation Dynamics via Time-Resolved Photoelectron Spectroscopy: ππ* States in Benzenes Shih-Huang Lee,†,‡ Kuo-Chun Tang,† I-Chia Chen,*,† M. Schmitt,§ J. P. Shaffer,§ T. Schultz,§ Jonathan G. Underwood,§ M. Z. Zgierski,§ and Albert Stolow*,§ Department of Chemistry, National Tsing Hua UniVersity, Hsinchu, Taiwan, ROC 30043, Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan, ROC 10764, and Steacie Institute for Molecular Sciences, National Research Council of Canada, 100 Sussex DriVe, Ottawa, ON, Canada K1A 0R6 ReceiVed: May 1, 2002
We study the applicability of femtosecond time-resolved photoelectron spectroscopy to the study of substituent effects in molecular electronic relaxation dynamics using a series of monosubstituted benzenes as model compounds. Three basic types of electronic substituents were used: CdC (styrene), CdO (benzaldehyde), and CtC (phenylacetylene). In addition, the effects of the rigidity and vibrational density of states of the substituent were investigated via both methyl (R-methylstyrene, acetophenone) and alkyl ring (indene) substitution. Femtosecond excitation to the second ππ* state leads, upon time-delayed ionization, to two distinct photoelectron bands having different decay constants. Variation of the ionization laser frequency had no effect on the photoelectron band shapes or lifetimes, indicating that autoionization from super-excited states played no discernible role. From assignment of the energy-resolved photoelectron spectra, a fast decaying component was attributed to electronic relaxation of the second ππ* state, a slower decaying component to the first ππ* state. Very fast electronic relaxation constants (
