The fragmentation dynamics of gas phase phenol molecules following excitation at many wavelengths in the range 279.145⩾λphot⩾206.00nm 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 λphot⩾248nm show a feature centered at TKER ∼6500cm−1. These H atom fragments, which show no recoil anisotropy, are rationalized in terms of initial S1←S0 (π*←π) 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(π1σ*) potential energy surfaces (PESs) at larger RO–H, en route to ground state phenoxyl products. The observed phenoxyl product vibrations indicate that parent modes ν16a and ν11 can both promote nonadiabatic coupling in the vicinity of the S0∕S2 CI. Spectra recorded at λphot⩽248nm reveal a faster, anisotropic distribution of recoiling H atoms, centered at TKER ∼12000cm−1. These we attribute to H+phenoxyl products formed by direct coupling between the optically excited S1(π1π*) and repulsive S2(π1σ*) PESs. Parent mode ν16b is identified as the dominant coupling mode at the S1∕S2 CI, and the resulting phenoxyl radical cofragments display a long progression in ν18b, the C–O in-plane wagging mode. Analysis of all structured TKER spectra yields D0(H–OC6H5)=30015±40cm−1. The present findings serve to emphasize two points of wider relevance in contemporary organic photochemistry: (i) The importance of π1σ* states in the fragmentation of gas phase heteroaromatic hydride molecules, even in cases where the π1σ* 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.

High-resolution time-of-flight measurements of H atom products from photolysis of phenol, 4-methylphenol, 4-fluorophenol, and thiophenol, at many UV wavelengths (λ _phot ), have allowed systematic study of the influence of ring substituents and the heteroatom on the fragmentation dynamics. All dissociate by X H (X = O, S) bond fission after excitation at their respective S ₁ ( ¹ ππ*)– S ₀ 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 πσ* potential energy surface (PES). The route to accessing this PES, and the subsequent dynamics, is seen to be very sensitive to λ _phot and substitution of the heteroatom. In the case of the phenols, dissociation after excitation at long λ _phot is rationalized in terms of radiationless transfer from S ₁ to S ₀ levels carrying sufficient O H stretch vibrational energy to allow coupling via the conical intersection between the S ₀ and ¹ πσ* PESs at longer O H bond lengths. In contrast, H + C ₆ H ₅ O(X ² B ₁ ) products formed after excitation at short λ _phot exhibit anisotropic recoil-velocity distributions, consistent with prompt dissociation induced by coupling between the photoprepared ¹ ππ* excited state and the ¹ πσ* PES. The fragmentation dynamics of thiophenol at all λ _phot matches the latter behavior more closely, reflecting the different relative dispositions of the ¹ ππ* and ¹ πσ* PESs. Additional insights are provided by the observed branching into the ground (X ² B ₁ ) and first excited ( ² B ₂ ) states of the resulting C ₆ H ₅ S radicals.

The fragmentation dynamics of imidazole molecules following excitation at 193.3nm and at many wavelengths in the range of 210⩽λphot⩽240nm have been investigated by H Rydberg atom photofragment translational spectroscopy. Long wavelength excitation within this range results in population of the 1A″1(π1σ*) excited state, but the 2A′1←XA′1(π*←π) transition becomes the dominant absorption once λphot⩽220nm. The measured energy disposals show parallels with those found in recent studies of the UV photolysis of pyrrole [Cronin et al., Phys Chem. Chem. Phys. 6, 5031 (2004)]. The total kinetic energy release (TKER) spectra display a “fast” feature, centred at TKER ∼9200cm−1. The analysis of the structure evident in the fast feature reveals the selective population of specific in-plane stretching vibrational levels of the imidazolyl cofragment; these fragments are deduced to carry only modest amounts of rotational excitation. Comparison with calculated normal mode vibrational frequencies allows the assignment of the populated levels and a precise determination of the N–H bond strength in imidazole: D0=33240±40cm−1. The observed energy disposal can be rationalized using Franck-Condon arguments, assuming that the potential energy surface (PES) for the 1A″1(π1σ*) state has a topology similar to that of the corresponding π1σ* state of pyrrole. As in pyrrole, photoexcitation populates skeletal motions in the S1 state (in-plane motions in the present case) that are only weakly coupled to the N–H dissociation coordinate and thus map through into the corresponding product vibrations. A second, “slow” feature is increasingly evident in TKER spectra recorded at shorter λphot. This component, which exhibits no recoil anisotropy, is attributed to H atoms formed by the “statistical” decay of highly vibrationally excited ground state molecules. The form of the TKER spectra observed at short λphot is rationalized by assuming two possible decay routes for imidazole molecules excited to the 2A′1(π1π*) state. One involves fast 2A′1(π1π*)⇝1A″1(π1σ*) radiationless transfer and subsequent fragmentation on the 1A′1(π1σ*) PES, yielding fast H atoms (and imidazolyl cofragments)—reminiscent of behavior seen at longer excitation wavelengths where the 1A″1(π1σ*) PES is accessed directly. The second is assumed to involve radiationless transfer to the ground state, most probably by successive 2A′1⇝1A″1⇝XA′1 couplings, mediated by conical intersections between the relevant PESs and the subsequent unimolecular decay of the resulting highly vibrationally excited ground state molecules yielding slow H atoms.