Femtosecond photoelectron spectroscopy of trans-stilbene above the reaction barrier C. Dietl a , E. Papastathopoulos a , P. Niklaus a , R. Improta b , F. Santoro a,c , G. Gerber a, * a Physikalisches Institut, Universita ¨t Wu ¨ rzburg, Am Hubland, 97074 Wu ¨ rzburg, Germany b Istituto di Biostrutture e Biommagini del CNR, via Mezzocannone 6, I-80134 Napoli, Italy c Istituto per i Processi Chimico Fisici del CNR, Area della Ricerca del CNR di Pisa, via Moruzzi 1, I-56124 Pisa, Italy Received 14 September 2004; accepted 20 October 2004 Abstract Femtosecond photoelectron spectroscopy was employed to study the excitation of trans-stilbene above the isomerization reaction barrier. Apart from the S 1 contribution, evidence of a second electronic state is found based on two different transients measured across the photoelectron spectrum. Time Dependent Density Functional Theory calculations on S 0 ,S 1 ,S 2 and D 0 , together with simulations of the electron energy distribution support the experimental findings for selective photoelectron energies of the S 0 , S 1 ,... electronic states. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Photoelectron spectroscopy; Femtochemistry; Laser chemistry; Photochemistry 1. Introduction In the field of molecular laser spectroscopy, trans–cis isomerization about double bonds has been a subject of intense research in order to understand intramolecular rearrangements following absorption of ultraviolet (UV) laser radiation. In this context stilbene comprises of a prototype system, extensively studied both experi- mentally [1–6] as well as theoretically [7–12]. trans-stil- bene (see Fig. 1) exhibits a stable conformation in its ground electronic state due to a large barrier between the trans and cis configurations. Following absorption of a UV-photon of appropriate energy the transition to the first excited state can be induced. In earlier studies it has been well established that the topology of the ex- cited state involves a potential minimum and a barrier for the twisting motion about the ethylene bond [13]. Following the explanation proposed by Orlandi, Sie- brand and others [14,15], this barrier arises from an interaction of the S 1 (obtained by a pp * transition) and a higher excited surface that exhibits a minimum at the perpendicular (# = 90°) configuration of stilbene. When the excitation energy exceeds the 1200 cm 1 bar- rier the twisting motion about the ethylene bond brings the molecule toward the # = 90° configuration. Recently Leitner et al. [12] have obtained a theoretical estimation of the barrier of 750 cm 1 , actually lower than the experimental threshold of 1200 cm 1 , and they have shown that this value leads to reactions rates in good agreement with the experiment, if one corrects the RRKM theory by taking into account the finite time IVR (intramolecular vibrational distribution). When the molecule comes close to the S 0 /S 1 conical intersection, which has been recently located at CAS- SA(2,2)/6-31G * level of theory [16], it undergoes a non- radiative transition to the ground S 0 surface. As a result, the lifetime of the excited S 1 state lies in the range of sev- eral picoseconds and decreases as a function of the 0301-0104/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2004.10.028 * Corresponding author. Tel.: +49 931 8885715; fax: +49 931 8884906. E-mail address: gerber@physik.uni-wuerzburg.de (G. Gerber). www.elsevier.com/locate/chemphys Chemical Physics 310 (2005) 201–211