The fragmentation dynamics of gas phase phenol molecules following excitation at many wavelengths in the range 279.145 > or = lambdaphot > or = 206.00 nm have been investigated by H Rydberg atom photofragment translational spectroscopy. Many of the total kinetic energy release (TKER) spectra so derived show structure, the analysis of which confirms the importance of O-H bond fission and reveals that the resulting phenoxyl cofragments are formed in a very limited subset of their available vibrational state density. Spectra recorded at lambdaphot > or = 248 nm show a feature centered at TKER approximately 6500 cm(-1). These H atom fragments, which show no recoil anisotropy, are rationalized in terms of initial S1<--S0 (pi*<--pi) excitation, and subsequent dissociation via two successive radiationless transitions: internal conversion to ground (S0) state levels carrying sufficient O-H stretch vibrational energy to allow efficient transfer towards, and passage around, the conical intersection (CI) between the S0 and S2(1pisigma*) potential energy surfaces (PESs) at larger R(O-H), en route to ground state phenoxyl products. The observed phenoxyl product vibrations indicate that parent modes nu16a and nu11 can both promote nonadiabatic coupling in the vicinity of the S0S2 CI. Spectra recorded at lambdaphot < or = 248 nm reveal a faster, anisotropic distribution of recoiling H atoms, centered at TKER approximately 12,000 cm(-1). These we attribute to H+phenoxyl products formed by direct coupling between the optically excited S1(1pi pi*) and repulsive S2(1pi sigma*) PESs. Parent mode nu16b is identified as the dominant coupling mode at the S1/S2 CI, and the resulting phenoxyl radical cofragments display a long progression in nu18b, the C-O in-plane wagging mode. Analysis of all structured TKER spectra yields D0(H-OC6H5) = 30,015 +/- 40 cm(-1). The present findings serve to emphasize two points of wider relevance in contemporary organic photochemistry: (i) The importance of 1) pi sigma* states in the fragmentation of gas phase heteroaromatic hydride molecules, even in cases where the 1pi sigma* state is optically dark. (ii) The probability of observing strikingly mode-specific product formation, even in "indirect" predissociations, if the fragmentation is driven by ultrafast nonadiabatic couplings via CIs between excited (and ground) state PESs.

The last few years have seen a surge in interest (both theoretical and experimental) in the photochemistry of heteroaromatic molecules (e.g. azoles, phenols), which has served to highlight the importance of dissociative excited states formed by electron promotion to sigma* molecular orbitals. Such excited states--which, for brevity, are termed pi sigma* states in this Perspective article--may be populated by direct photo-excitation (though the transition cross-sections are intrinsically small), or indirectly, by non-adiabatic coupling from an optically 'bright' excited state (e.g. an excited state resulting from pi* <--pi excitation). The analogous pi sigma* excited states in prototypical hydride molecules like H(2)O and NH(3) have long been recognised. They have served as test-beds for developing concepts like Rydbergisation, conical intersections (CIs) between potential energy surfaces, and for investigating the ways in which non-adiabatic couplings at such CIs influence the eventual photofragmentation dynamics. This Perspective article seeks to highlight the continuity of behaviour revealed by the earlier small molecule studies and by the more recent studies of heteroaromatic systems, and to illustrate the photochemical importance of pi sigma* excited states in many broad families of molecules. Furthermore, the dynamical influence of such excited states is not restricted to closed shell species; the Article concludes with a brief consideration of the consequences of populating sigma* orbitals in free radical species, in molecular cations, and in dissociative electron attachment processes.

The fragmentation dynamics of pyrrole molecules following excitation at many wavelengths in the range 193.3 o l phot o 254.0 nm have been investigated by H Rydberg atom photofragment translational spectroscopy. Excitation at the longer wavelengths within this range results in (vibronically induced) population of the 1 1 A 2 (ps*) excited state, but once l phot r 225 nm the electric dipole allowed 1 1 B 2 ' X 1 A 1 (p* ' p) transition becomes the dominant absorption. All of the total kinetic energy release (TKER) spectra so derived show a 'fast' peak, centred at TKER B7000 cm À1 . Analysis of the structure evident in this peak, particularly in spectra recorded at the longer excitation wavelengths, reveals selective population of specific vibrational levels of the pyrrolyl co-fragment. These have been assigned by comparison with calculated normal mode vibrational frequencies, leading to a precise determination of the N-H bond strength in pyrrole: D 0 ¼ 32850 AE 40 cm À1 , and the enthalpy of formation of the pyrrolyl radical: D f H 0 1(C 4 H 4 N) ¼ 301.9 AE 0.5 kJ mol À1 . The recoil anisotropy of the fast H atom photofragments formed following excitation to, and dissociation on, the 1 1 A 2 (ps*) potential energy surface (PES) is seen to depend upon the vibrational level of the pyrrolyl co-fragment. This observation, and the finding that the mean TKER associated with these fast H þ pyrrolyl fragments is essentially independent of l phot , can be explained by assuming that, upon N-H bond fission, the skeletal vibrational motions in pyrrole(1 1 A 2 ) molecules evolve adiabatically into the corresponding modes of the ground state pyrrolyl fragment. A second, 'slow' peak is increasingly evident in TKER spectra recorded at shorter photolysis wavelengths, and becomes the dominant feature once l phot r 218 nm. This component exhibits no recoil anisotropy; its TKER profile is reminiscent of that observed in many other dissociations that yield H atoms by 'statistical' decay of highly vibrationally excited ground state molecules. The form of the TKER spectra observed at these shorter excitation wavelengths is rationalised by assuming two possible decay routes for pyrrole molecules excited to the 1 B 2 (pp*) state. One involves fast 1 1 B 2 * 1 1 A 2 radiationless transfer and subsequent fragmentation on the 1 1 A 2 PES, yielding 'fast' H atoms (and pyrrolyl co-fragments)-reminiscent of behaviour seen at longer excitation wavelengths where the 1 1 A 2 PES is accessed directly. The second is assumed to involve radiationless transfer to the ground state, either by successive 1 1 B 2 * 1 1 A 2 * X 1 A 1 couplings mediated by conical intersections between the relevant PESs or, possibly, by an as yet unrecognised direct 1 1 B 2 * X 1 A 1 coupling, and subsequent unimolecular decay of the resulting highly vibrationally excited ground state molecules yielding 'slow' H atoms (together with, most probably, cyanoallyl co-fragments).

High-resolution time-of-flight measurements of H atom products from photolysis of phenol, 4-methylphenol, 4-fluorophenol, and thiophenol, at many UV wavelengths ( phot), have allowed systematic study of the influence of ring substituents and the heteroatom on the fragmentation dynamics. All dissociate by XOH (X ‫؍‬ O, S) bond fission after excitation at their respective S 1( 1 *)-S0 origins and at all shorter wavelengths. The achieved kinetic energy resolution reveals population of selected vibrational levels of the various phenoxyl and thiophenoxyl coproducts, providing uniquely detailed insights into the fragmentation dynamics. Dissociation in all cases is deduced to involve nuclear motion on the 1 * potential energy surface (PES). The route to accessing this PES, and the subsequent dynamics, is seen to be very sensitive to phot and substitution of the heteroatom. In the case of the phenols, dissociation after excitation at long phot is rationalized in terms of radiationless transfer from S 1 to S0 levels carrying sufficient OOH stretch vibrational energy to allow coupling via the conical intersection between the S 0 and 1 * PESs at longer OOH bond lengths. In contrast, H ؉ C 6H5O(X 2 B1) products formed after excitation at short phot exhibit anisotropic recoil-velocity distributions, consistent with prompt dissociation induced by coupling between the photoprepared 1 * excited state and the 1 * PES. The fragmentation dynamics of thiophenol at all phot matches the latter behavior more closely, reflecting the different relative dispositions of the 1 * and 1 * PESs. Additional insights are provided by the observed branching into the ground (X 2 B1) and first excited ( 2 B2) states of the resulting C6H5S radicals. photofragment translational spectroscopy ͉ nonadiabatic ͉ dissociation dynamics H eteroaromatic molecules such as pyrroles, imidazoles, and phenols are key components of the long-wavelength chromophores in nucleobases and aromatic amino acids (e.g., histidine, tryptophan, and tyrosine), which dominate the UVabsorption spectra of many biological molecules. *4 transitions are responsible for the strong UV absorptions, but these heteroaromatics also possess excited states formed by *4 electron promotions. Absorption to the 1 * states is very much weaker, but these states can still be populated by direct photoexcitation and/or radiationless transfer from 1 * (or 1 n *) states. Recent theoretical studies by Sobolewski et al. (1) alerted photochemists to the likely importance of 1 * states in promoting XOH (X ϭ N, O) bond fission in such molecules. In the case of phenol, the ground state correlates diabatically with an excited ( 2 B 2 ) electronic state of the phenoxyl radical after OOH bond extension, the 1 * state is bound with respect to R O-H , and the 1 * state ‡ correlates diabatically with phenoxyl products in their ground (X 2 B 1 ) state. Thus, a cut through the potential energy surface (PES) for the 1 * state along R O-H intersects both the 1 * and 1 PESs, as depicted in Fig. 1a. These crossings de...

H(D) Rydberg atom photofragment translational spectroscopy has been used to investigate the dynamics of H(D) atom loss C6H5SH(C6H5SD) following excitation at many wavelengths lambda phot in the range of 225-290 nm. The C6H5S cofragments are formed in both their ground (X(2)B1) and first excited ((2)B2) electronic states, in a distribution of vibrational levels that spreads and shifts to higher internal energies as lambda(phot) is reduced. Excitation at lambda(phot) > 275 nm populates levels of the first (1)pi pi* state, which decay by tunnelling to the dissociative (1)pi sigma* state potential energy surface (PES). S-H torsional motion is identified as a coupling mode facilitating population transfer at the conical intersection (CI) between the diabatic (1)pi pi* and (1)pi sigma* PESs. At shorter lambda(phot), the (1)pi sigma* state is deduced to be populated either directly or by efficient vibronic coupling from higher (1)pipi* states. Flux evolving on the (1)pi sigma* PES samples a second CI, at longer R(S-H), between the diabatic (1)pi sigma* and ground ((1)pi pi) PESs, where the electronic branching between ground and excited state C6H5S fragments is determined. The C6H5S(X(2)B1) and C6H5S((2)B2) products are deduced to be formed in levels with, respectively, a' and a'' vibrational symmetry-behavior that reflects both Franck-Condon effects (both in the initial photoexcitation step and in the subsequent in-plane forces acting during dissociation) and the effects of the out-of-plane coupling mode(s), nu11 and nu16a, at the (1)pi sigma*/(1)pi pi CI. The vibrational state assignments enabled by the high-energy resolution of the present data allow new and improved estimations of the bond dissociation energies, D0(C6H5S-H) < or = 28,030 +/- 100 cm(-1) and D0(C6H5S-D) < or = 28,610 +/- 100 cm(-1), and of the energy separation between the X(2)B1 and (2)B2 states of the C6H5S radical, T(00) = 2800 +/- 40 cm(-1). Similarities, and differences, between the measured energy disposals accompanying UV photoinduced X-H (X = S, O) bond fission in thiophenol and phenol are discussed.