High-Resolution Excited-State Photoelectron Spectroscopy of the Lower Rydberg States of Jet-Cooled C 2 H 4 and C 2 D 4 R. A. Rijkenberg and W. J. Buma* Faculty of Science, Institute of Molecular Chemistry, UniVersity of Amsterdam, Nieuwe Achtergracht 127-129, 1018 WS Amsterdam, The Netherlands ReceiVed: October 3, 2001; In Final Form: January 7, 2002 High-resolution (2+1) resonance-enhanced excited-state photoelectron spectroscopy on supersonic jet expansions of pure C 2 H 4 and C 2 D 4 has been employed to investigate the spectroscopy and dynamics of the lowest excited 1 1 B 3u (π,3s) Rydberg state, as well as the 1 1 B 1g (π,3p y ), 1 1 B 2g (π,3p z ), and 2 1 A g (π,3p x ) Rydberg states of ethylene. In combination with ab initio calculations, this approach has for the 1 1 B 3u (π,3s) Rydberg state confirmed the majority of previous assignments in the two-photon excitation spectrum but, at the same time, has demonstrated that the hypothesis of a vibronic coupling between this state and the Jahn-Teller distorted 1 1 B 2g (π*,3s) state should be discarded. The resonances on which this hypothesis was based are in the present study shown to arise from excitation of the ν 3 (a g ) vibrational mode. For the 3p Rydberg manifold, the present study has for the first time been able to determine unambiguously the 0-0 transitions to all three (π,3p) Rydberg states and to disentangle the vibrational development of the practically degenerate 1 1 B 1g (π,3p y ) and 1 1 B 2g (π,3p z ) Rydberg states. Upon the basis of these assignments, it has been shown that the 2 1 A g (π,3p x ) Rydberg state is significantly more twisted around the central CC double bond upon excitation from the ground state than the other two (π,3p) Rydberg states. Although a priori argued to be difficult to detect, the present study does not give evidence for previous suggestions in the literature on the coupling of the 1 1 B 1u (π,π*) valence and the 1 1 B 3u (π,3s) or the 1 1 B 1g (π,3p y ) Rydberg states. However, the photoelectron spectra obtained for ionization via the lower Rydberg manifold of both C 2 H 4 and C 2 D 4 show unambiguously the absence of extensive coupling between the lower excited Rydberg states. I. Introduction Molecular systems containing rodlike organic building blocks functionalized by specific substituents at one or both of the termini positions constitute one of the primary areas of interest in the field of organic chemistry dedicated to the development of functionalized materials. The advantage of these organic materials is the possibility to fine-tune the chemical structure, and thus the desired property, by way of synthesis, making them highly suitable candidates within application areas such as photonics and molecular electronics. One such a class of materials with significant potential is built upon oligo(cyclo- hexilydenes). 1-3 Recent excited-state photoelectron spectro- scopic studies 4 on their structural unit, 1,1′-bicyclohexylidene, have strongly suggested that its peculiar spectroscopic proper- ties 5,6 ultimately find their origin in vibronic coupling interac- tions between the Rydberg manifold and the underlying continua of the valence state(s) of ethylene. 7-13 The spectroscopy of the lower excited states of ethylene, in particular in relation to vibronic coupling, has in the past been the subject of many experimental and theoretical studies. Despite the vast amount of knowledge that has been acquired by now, we found in our efforts to elucidate the spectroscopic properties of excited states in 1,1′-bicyclohexylidene in particular, and alkylated mono-olefins in general, that some rather fundamental questions on the composition of the wave functions of the lower excited Rydberg states were still unanswered. We have therefore reinvestigated these states with excited-state photoelectron spectroscopy. This spectroscopic technique is very powerful when nonadiabatic interactions between electronically excited states are considered. 14-16 The possibility to disperse the photoelectrons produced in the resonance enhanced multiphoton ionization (REMPI) process according to their kinetic energies enables the characterization of the intermediate state from which ionization occurs, e.g., is it a “pure” vibronic state or is it coupled to one or more other vibronically excited states. 14,16 A second aspect relevant for the present study is that excited-state photoelectron spectroscopy in principle should be able to unravel qualitatively, and in some cases quantitatively, the electronic character of the intermediate state from which ionization occurs, e.g., Rydberg versus valence character. 17 The coupling of electronically excited states of ethylene has been the subject of several theoretical investigations 7-13,18-20 discussing the interaction between the Rydberg manifold and the (π,π*) valence state. In this respect, especially the electronic properties of the 1 1 B 1u (π,π*) valence state have been the focus of a large number of theoretical studies employing a wide range of ab initio techniques. 7-12,18-32 Because the (π,π*) valence state and (π,3d π ) Rydberg state have the same nodal structure, Mulliken 7 argued that the unperturbed (π,π*) and (π,3d π ) wave functions cannot coexist for ethylene. Instead, the 1 1 B 1u valence state of ethylene consists of a linear combination of the (π,π*) and the (π,3d π ) wave functions, and the next 1 B 1u state is the (π,4d π ) Rydberg state. Buenker and Peyerimhoff 8 hold another view: according to them, the unperturbed (π,π*) and (π,3d π ) wave functions coexist separately, but they are subject to extensive configuration mixing in the planar conformation, * To whom correspondence should be addressed. E-mail: wybren@ fys.chem.uva.nl. Fax: (31)-20-525 6456/6422. 3727 J. Phys. Chem. A 2002, 106, 3727-3737 10.1021/jp013714k CCC: $22.00 © 2002 American Chemical Society Published on Web 03/14/2002