Photodissociation Dynamics in Isolated Molecules

The current foci of our long running interests in the photofragmentation dynamics of gas phase molecules are:

(i)  Identifying generic classes of photochemical behaviour – e.g. the central role of (n/π)σ* excited states in molecular bond fission processes.

(ii)  Exploring and predicting how photochemical outcomes are influenced (and can be tuned) by chemical substitution.

(iii) Extending such studies to ever larger systems, and comparing and contrasting photochemical behaviour in the gas and solution phase.

Two complementary photofragment translational spectroscopy (PTS) techniques are used to study gas phase photodissociation processes: H Rydberg atom (HRA) PTS – for fragmentations that yield H (or D) atom photoproducts – and velocity map ion imaging (VMI). The latter is more widely applicable, since it can (in principle) be used to investigate any product that can be ionised.  Against that, however, it lacks the ultimate kinetic energy resolution of the Rydberg tagging method.

Both PTS techniques employ (a) a jet-cooled pulsed beam containing the target molecule seeded in an inert carrier gas, (b) a nsec pulsed laser to photoexcite the molecule and (c) one (or more) additional lasers to probe (by Rydberg tagging, by resonance enhanced multiphoton ionization (REMPI) or by vacuum ultraviolet photoionization) the product of interest. The velocity distribution of this product is then determined by time-of-flight measurements or through analysis of the VMI image.

Conservation of momentum arguments enable us to convert this distribution to a spectrum of the total kinetic energy release (TKER) accompanying the fragmentation process, the analysis of which can yield the relevant bond dissociation energy and the internal energy (and angular) distributions of the unobserved partner fragment.  Such insights, allied with relevant electronic structure calculations and trajectory and/or wavepacket calculations, can provide uniquely detailed dynamical insights into the fragmentation process.

Ashfold HRAPTS and VMI instruments

Earlier HRA-PTS studies yielded unprecedented insights into the photofragmentation of many small hydride molecules, e.g. HCl, HBr, HI, HCN, H2O, H2S, NH3, PH3, HN3, C2H2, H2CO, HFCO, CH4, CH3SH, CH3NH2, allene, propyne, ketene, and the CH3 radical. The VMI studies similarly focussed on prototypical small molecules (e.g. halogens and interhalogens) as well as a range of small molecular cations (e.g. Br2+, H2O+, H2S+ and NH3+).

More recently, our focus has switched to larger systems – heteroaromatic molecules like imidazole, pyrroles, indoles and adenine, phenols, thiophenols and thioanisoles, and halogen containing species like iodocyclohexane, iodobenzene, fluorinated iodobenzenes and halophenols.

The types of insight revealed by such studies are illustrated by an example taken from a recent paper in Chemical Science. The first excited (S1) singlet state of phenol (PhOH) arises as a result of a π*←π excitation; it decays with a lifetime of ~2.1 ns. HRA-PTS studies show that O–H bond fission, by tunnelling through the barrier under the conical intersection (CI-1) between the S1(1ππ*) and S2(1πσ*) potential energy surfaces (PESs), is a significant contributor to this excited state decay. The resulting phenoxyl (PhO) radicals are formed in their ground (X) electronic state, implying efficient transfer through a second CI (CI-2, between the S2 and ground (S0) PESs) at longer O–H bond lengths.

The HRA-PTS data even allow us to identify the nuclear motion (an out-of-plane ring puckering motion of a2 symmetry) that promotes non-adiabatic coupling at CI-1. We have shown how the tunnelling rate can be tuned by substituting at the 4-position on the ring. Introducing a strong electron withdrawing group like CN ‘closes’ the O–H bond fission channel – reflecting the substituent induced destabilisation of the radical product, which leads to an increase in the O–H bond dissociation energy and in the relative energy of the S2 PES, thereby increasing the magnitude of the barrier under CI-1, and reducing the tunnelling rate. Such ideas have encouraged a ‘Hammett-like’ approach to modelling and predicting O–H bond fission rates following S1←S0 excitation of a wide range of 4-substituted phenols, up to and including L-tyrosine and di- and tri-peptides involving this amino-acid.

Ground and excited singlet state potential energy curves and PTS data for phenol photodissociation at 275 and 230 nm.

Ground and excited singlet state potential energy curves and PTS data for phenol photodissociation at 275 and 230 nm.

The figure above shows ab initio calculated (CASPT2) potential energy curves for S0, S1 and S2 states of phenol plotted as a function of RO–H, along with representative TKER spectra derived from H atom TOF spectra measured when exciting the S1(v=0) level (i.e. well below the energy of the S1/S2 CI (CI-1) and at 230 nm (i.e. directly to the S2 state, at energies well above CI-1). The structure in such spectra reveals the vibrational energy disposal in the partner (phenoxyl in this case) radical.

The photofragmentations of thiophenol (PhSH) and substituted thiophenols show many dynamical similarities with those of the phenols, but the S–H bond is weaker, the S2((1n/π)σ*) PES thus lies relatively lower in energy, and the energy barrier under CI-1 in thiophenol offers little impediment to S–H bond fission. The energy separation between the ground (X) state and first excited (A) state of the PhS radical is less than half that separating the equivalent states of PhO and, in contrast to the phenols, we observe both X and A state PhS radical formation in the UV photolysis of all thiophenols investigated to date.  The branching between X and A state products is determined by the competition between ‘adiabatic’ and ‘diabatic’ motion in the vicinity of CI-2, which we observe to vary both with excitation wavelength and with ring substituent in ways that can be understood on the basis that diabatic evolution is most likely when the breaking S–H bond lies in the plane of the ring.  The S1(1ππ*) state of thioanisole decays on a nsec timescale – i.e. 4-5 orders of magnitude more slowly than the corresponding excited state of thiophenol.  Nonetheless, CH3 radical products (i.e. evidence of S–CH3 bond fission) are observed and can be investigated by VMI methods.  Image analysis reveals a near total population inversion between the PhS A and X state products. We interpret this finding by invoking a substantive role for a spin-changing intersystem crossing fragmentation pathway, which reveals itself in the thioanisoles only because the alternative spin-allowed coupling via the analogue of CI-1 is so inefficient.


This research has benefited greatly from consistent support from EPSRC (e.g. via Programme Grants EP/G00224X/1 and EP/L005913/1).  The VMI programme in particular has further benefited from collaborations with the groups of Kitsopoulos (FORTH, Heraklion), Parker (Nijmegen), Whitaker (Leeds), Soep (Saclay) and Brouard and Vallance (Oxford) via the successive EU networks IMAGINE, PICNIC and ICONIC, and with Photek Ltd (through the above EU Networks, and via a Knowledge Transfer Partnership, no. KTP008481)


Representative publications:

Symmetry matters: photodissociation dynamics of symmetrically versus asymmetrically substituted phenols, T.N.V. Karsili, A.M. Wenge, B. Marchetti and M.N.R. Ashfold, Phys. Chem. Chem. Phys. 16, 588-98, (2014).

O–H bond fission in 4-substituted phenols: S1 state predissociation viewed in a Hammett-like framework, T.N.V. Karsili, A.M. Wenge, D. Murdock, S.J. Harris, J.N. Harvey, R.N. Dixon and M.N.R. Ashfold, Chem. Sci. 4, 2434-46 (2013).

UV photolysis of 4-iodo-, 4-bromo- and 4-chlorophenol: competition between C–Y (Y = Halogen) and O–H bond fission, A.G. Sage, T.A.A. Oliver, G.A. King, D. Murdock, J.N. Harvey and M.N.R. Ashfold, J. Chem. Phys. 138, 164318 (2013).

Controlling the electronic product branching at conical intersections in the UV photolysis of para-substituted thiophenols. T.A.A. Oliver, G.A. King, D.P. Tew, R.N. Dixon and M.N.R. Ashfold, J. Phys. Chem. A 116, 12444-59 (2012).

Tunnelling under a conical intersection: application to the product vibrational state distributions in the UV photodissociation of phenols. R.N. Dixon, T.A.A. Oliver and M.N.R. Ashfold, J. Chem. Phys. 134, 194303 (2011).

πσ* excited states in molecular photochemistry. M.N.R. Ashfold, G.A. King, D. Murdock, M.G.D. Nix, T.A.A. Oliver and A.G. Sage, Phys. Chem. Chem. Phys. 12, 1218-38 (2010).