ARTICLES Exploring the Time-Scales of H-Atom Detachment from Photoexcited Phenol-h 6 and Phenol-d 5 : Statistical vs Nonstatistical Decay Azhar Iqbal, † Michelle S. Y. Cheung, † Michael G. D. Nix, ‡ and Vasilios G. Stavros* ,† Department of Chemistry, UniVersity of Warwick, CoVentry CV4 7AL, U.K., and School of Chemistry, UniVersity of Bristol, Bristol BS8 1TS, U.K. ReceiVed: April 4, 2009; ReVised Manuscript ReceiVed: May 19, 2009 The prevalence of 1 πσ* states in the photochemistry of heteroaromatics is becoming increasingly clear from the recent literature. Photodissociation measurements have shown that following excitation of phenol molecules above the S 1 /S 2 conical intersection, H-atoms are eliminated with two distinct ranges of kinetic energy release. Those with high kinetic energy are attributed to direct dissociation while those with low kinetic energy are traditionally attributed to indirect dissociation or statistical unimolecular decay, both pathways giving electronic ground-state phenoxyl fragments. Using a combination of femtosecond pump/probe spectroscopy and velocity map ion imaging techniques, the time and energy resolved H-atom elimination in phenol-h 6 and phenol-d 5 , following excitation at 200 nm has been measured. At the lowest kinetic energies, the H-atom elimination from phenol-d 5 occurs in <150 fs, in sharp contrast to what one expects from a statistical decay process. This implies that these H-atoms are formed through a direct dissociation process yielding electronically excited phenoxyl fragments. Introduction Ab initio calculations by Sobolewski and Domcke 1 have fuelled a great deal of interest in the photofragmentation 2 of various biomolecules and their prototypical chromophores. The potential energy surfaces (PES) of these molecules contain a singlet 1 πσ* state that is repulsive with respect to the X-H coordinate (where X is the heteroatom, typically N or O). The repulsive character of this electronic state gives rise to nonra- diative decay in these molecules following absorption of UV radiation. The nonradiative processes may impart photostability, protecting these molecules from dangerous photoinduced reactions. 3,4 Phenol, the chromophore of the amino acid tyrosine, has been a prime focus in recent years, being a prototype molecule for developing a better understanding of electronic structure and photochemistry of other, larger heteroaromatic biomolecules. Sobolewski, Domcke, and co-workers 5-7 have shown that the low fluorescence quantum yield of phenol following excitation at the wavelength of interest here (200 nm) is primarily due to an excited singlet state of 1 πσ* (S 2 ) character, which is dissociative with respect to the stretching coordinate of the O-H bond. This dissociative state (S 2 ) lies below the upper 1 ππ* (S 3 ) state but intersects both the optically bright 1 ππ* state (S 1 ) and the ground state (S 0 ) through two successive conical intersections (CI, S 1 /S 2 , and S 0 /S 2 , respectively), leading to elimination of neutral hydrogen (see Figure 1a in Ashfold et al. 8 ). Due to the weak S 2 r S 0 transition, S 2 is not excited directly and is populated by radiationless transfer from the optically bright S 1 state or, at these higher energies, from another 1 ππ* (S 3 ) state that dominates the absorption when λ < 220 nm. 2,9 Following population of the S 2 state, the excited phenol molecule evolves toward the S 0 /S 2 CI with two possible photochemical fates. The molecule can eliminate an H atom from the heteroatom site directly via the repulsive 1 πσ* state or, alternatively, highly excited ground-state phenol molecules may be formed which can also release H atoms when sufficient energy becomes localized in the correct vibrational mode following intramo- lecular vibrational relaxation (IVR). These H-atom elimination pathways are commonly referred to as direct dissociation and statistical unimolecular decay respectively. 8 The ability to disentangle the contributions from the direct and statistical pathways to dissociation of these hydrides (X-H) is of considerable value as this can provide detailed information about the nature of the coupling of PES at the various CIs. This paper describes the application of time-resolved velocity map ion imaging (TRVMI) which enables one to clock the real-time H-atom elimination with energy dependence and thus establish a time constant for dissociation via the different pathways. Direct dissociation is known to yield H-atoms with large amounts of kinetic energy, due to the repulsive nature of the 1 πσ* state, while indirect dissociation typically leads to H-atoms with much less kinetic energy on average. Disentangling these two pathways is possible as VMI enables one to separate H-atoms with varying amounts of kinetic energy within the image. The work described herein shows that both high and low kinetic energy H-atoms are released on an ultrafast (<150 fs) time scale, in sharp contrast to what one would expect via an IVR mediated statistical pathway for the low kinetic energy H-atoms. This casts considerable doubt over the previously assigned statistical pathway for dissociation yielding these H-atoms, as determined * Author to whom correspondence should be addressed. E-mail: v.stavros@ warwick.ac.uk. † University of Warwick. ‡ University of Bristol. J. Phys. Chem. A 2009, 113, 8157–8163 8157 10.1021/jp9031223 CCC: $40.75  2009 American Chemical Society Published on Web 07/01/2009 Downloaded by NESLI CONSORTIA UK on July 17, 2009 Published on July 1, 2009 on http://pubs.acs.org | doi: 10.1021/jp9031223