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.

When phenol is photoexcited to its S(1) (1(1)ππ∗) state at wavelengths in the range 257.403 ≤ λ(phot) ≤ 275.133 nm the O-H bond dissociates to yield an H atom and a phenoxyl co-product, with the available energy shared between translation and well characterised product vibration. It is accepted that dissociation is enabled by transfer to an S(2) (1(1)πσ∗) state, for which the potential energy surface (PES) is repulsive in the O-H stretch coordinate, R(O-H). This S(2) PES is cut by the S(1) PES near R(O-H) = 1.2 Å and by the S(0) ground state PES near R(O-H) = 2.1 Å, to give two conical intersections (CIs). These have each been invoked-both in theoretical studies and in the interpretation of experimental vibrational activity-but with considerable controversy. This paper revisits the dynamic mechanisms that underlie the photodissociation of phenol and substituted phenols in the light of symmetry restrictions arising from torsional tunnelling degeneracy, which has been neglected hitherto. This places tighter symmetry constraints on the dynamics around the two CIs. The non-rigid molecular symmetry group G(4) necessitates vibronic interactions by a(2) modes to enable coupling at the inner, higher energy (S(1)/S(2)) CI, or by b(1) modes at the outer, lower energy (S(2)/S(0)) CI. The experimental data following excitation through many vibronic levels of the S(1) state of phenol and substituted phenols demonstrate the effective role of the ν(16a) (a(2)) ring torsional mode in enabling O-H bond fission. This requires tunnelling under the S(1)/S(2) CI, with a hindering barrier of ∼5000 cm(-1) and with the associated geometric phase effect. Quantum dynamic calculations using new ab initio PESs provide quantitative justification for this conclusion. The fates of other excited S(1) modes are also rationalised, revealing both spectator modes and intramolecular vibrational redistribution between modes. A common feature in many cases is the observation of an extended, odd-number only, progression in product mode ν(16a) (i.e., the parent mode which enables S(1)/S(2) tunnelling), which we explain as a Franck-Condon consequence of a major change in the active vibration frequency. These comprehensive results serve to confirm the hypothesis that O-H fission following excitation to the S(1) state involves tunnelling under the S(1)/S(2) CI-in accord with conclusions reached from a recent correlation of the excited state lifetimes of phenol (and many substituted phenols) with the corresponding vertical energy gaps between their S(1) and S(2) PESs.

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...

Photodissociation dynamics of H 2 O at 121.6 nm have been studied using the H atom Rydberg ''tagging'' time-of-flight technique and by quasiclassical trajectory ͑QCT͒ calculations. Product kinetic energy distributions and angular distributions have been measured. From these distributions, rovibronic distributions of the OH radical product as well as the state resolved angular anisotropy parameters were determined. The dissociation energy D 0 0 ͑H-OH͒ is determined to be 41151 Ϯ5 cm Ϫ1 . Two clear alternations in the OH(X,vϭ0) rotational distribution have been observed, with each alternation corresponding to an oscillation in the anisotropy distribution. These oscillations had been attributed to the dynamical interference between the two conical intersection pathways. Further theoretical modeling in this work strongly supports this argument. Very highly vibrationally excited OH(X) products ͑up to vϭ9͒ have also been observed. These are ascribed to interconversion of H-O-H bending ͑H-H vibration͒ and O-H vibration in O-H-H geometries. The effect of parent rotational excitation on the OH(A) product state distribution and anisotropy distribution was observed for the first time. Experimental results also show clear evidence for the triple dissociation channel, O( 3 P)ϩ2H. Accurate branching ratios of different product channels have been determined. Results of detailed QCT calculations agree well with the experimental results in this work.

The photodissociation dynamics of 1 state ammonia molecules (both NH3 and ND 3 ) has been further investigated using the technique ofH(D) atom photofragment translational spectroscopy. The resulting NH2 (ND 2 ) fragments are observed to carry high levels of internal excitation, the precise disposition of which is sensitively dependent upon the parent v~ level excited. Dissociation from the v~ = 0 level of the 1 state yields ground state NH2 (ND 2 ) fragments, primarily in their zero-point level, but with high levels of rotational excitation specifically concentrated about the a-inertial axis; the population distribution over the energetically accessible product rotational levels with N <::::.Ka appears near to statistical. In contrast, dissociation from the parent v~ = 1 level yields a markedly inverted fragment internal energy distribution. These different energy disposals have been rationalized via classical traje~tory calculations employing the best available ab initio potential energy surfaces for the 1 and X states of the ammonia molecule. The energy disposal following excitation to the parent v~ = 2 and 3 levels is found to mimic that observed for, respectively, the v~ = 0 and 1 levels.These results provide clear evidence for the importance of anharmonic coupling (whereby an even number of bending quanta are redistributed into stretching motions) in promoting the fra~entation_o!parent levels ~ith v; ;;'2. The threshold energyJor producing electronically excIted NH2 (A AI) fragments IS 6.02 e V [ -6.16 e V for ND2 (A) ]. The present studies of NH3 photolysis suggest that this fragmentation channel opens at threshold and clearly indicate that branching into this channel occurs with much higher quantum yield than hitherto believed.