Dave Townsend

Bio: Dave Townsend is an academic researcher from Heriot-Watt University. The author has contributed to research in topics: Excited state & Ionization. The author has an hindex of 7, co-authored 19 publications receiving 269 citations.

Following the excited state relaxation dynamics of indole and 5-hydroxyindole using time-resolved photoelectron spectroscopy.

Abstract: Time-resolved photoelectron spectroscopy was used to obtain new information about the dynamics of electronic relaxation in gas-phase indole and 5-hydroxyindole following UV excitation with femtosecond laser pulses centred at 249 nm and 273 nm. Our analysis of the data was supported by ab initio calculations at the coupled cluster and complete-active-space self-consistent-field levels. The optically bright 1La and 1Lb electronic states of 1ππ* character and spectroscopically dark and dissociative 1πσ* states were all found to play a role in the overall relaxation process. In both molecules we conclude that the initially excited 1La state decays non-adiabatically on a sub 100 fs timescale via two competing pathways, populating either the subsequently long-lived 1Lb state or the 1πσ* state localised along the N-H coordinate, which exhibits a lifetime on the order of 1 ps. In the case of 5-hydroxyindole, we conclude that the 1πσ* state localised along the O-H coordinate plays little or no role in the relaxation dynamics at the two excitation wavelengths studied.

A Stark future for quantum control.

Abstract: We present an overview of developments using the nonresonant dynamic Stark effect within the fields of time-resolved molecular dynamics and quantum control, drawing on examples from our own recent ...

Manipulating dynamics with chemical structure: probing vibrationally-enhanced tunnelling in photoexcited catechol

Abstract: Ultrafast time-resolved velocity map ion imaging (TR-VMI) and time-resolved ion-yield (TR-IY) methods are utilised to reveal a comprehensive picture of the electronic state relaxation dynamics in photoexcited catechol (1,2-dihydroxybenzene). After excitation to the S1 (1ππ*) state between 280.5 (the S1 origin band, S1(v = 0)) to 243 nm, the population in this state is observed to decay through coupling onto the S2 (1πσ*) state, which is dissociative with respect to the non-hydrogen bonded ‘free’ O–H bond (labelled O1–H). This process occurs via tunnelling under an S1/S2 conical intersection (CI) on a timeframe of 5–11 ps, resulting in O1–H bond fission along S2. Concomitant formation of ground state catechoxyl radicals (C6H5O2(X)), in coincidence with translationally excited H-atoms, occurs over the same timescale as the S1 state population decays. Between 254–237 nm, direct excitation to the S2 state is also observed, manifesting in the ultrafast (∼100 fs) formation of H-atoms with high kinetic energy release. From these measurements we determine that the S1/S2 CI lies ∼3700–5500 cm−1 above the S1(v = 0) level, indicating that the barrier height to tunnelling from S1(v = 0) → S2 is comparable to that observed in the related ‘benchmark’ species phenol (hydroxybenzene). We discuss how a highly ‘vibrationally-enhanced’ tunnelling mechanism is responsible for the two orders of magnitude enhancement to the tunnelling rate in catechol, relative to that previously determined in phenol (>1.2 ns), despite similar barrier heights. This phenomenon is a direct consequence of the non-planar S1 excited state minimum structure (C1 symmetry) in catechol, which in turn yields relaxed symmetry constraints for vibronic coupling from S1(v = 0) → S2 – a scenario which does not exist for phenol. These findings offer an elegant example of how even simple chemical modifications (ortho-hydroxy substitution) to a fundamental, biologically relevant, UV chromophore, such as phenol, can have profound effects on the ensuing excited state dynamics.

Rotationally resolved photoelectron angular distributions from a nonlinear polyatomic molecule.

Abstract: We present, for the first time, rotationally resolved photoelectron images resulting from the ionization of a polyatomic molecule. Photoelectron angular distributions pertaining to the formation of individual rotational levels of $\mathrm{NH}_{3}{}^{+}$ have been extracted from the images and analyzed to enable a complete determination of the radial dipole matrix elements and relative phases that describe the ionization dynamics. This determination leads to the deduction of significantly different dynamics from those extracted in previous studies which lacked either angular information or rotational resolution.

Ultrafast Molecular Spectroscopy Using a Hollow-Core Photonic Crystal Fiber Light Source.

Abstract: We demonstrate, for the first time, the application of rare-gas-filled hollow-core photonic crystal fibers (HC-PCFs) as tunable ultraviolet light sources in femtosecond pump–probe spectroscopy. A c...

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Abstract: : The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Multireference Approaches for Excited States of Molecules.

Abstract: Understanding the properties of electronically excited states is a challenging task that becomes increasingly important for numerous applications in chemistry, molecular physics, molecular biology, and materials science. A substantial impact is exerted by the fascinating progress in time-resolved spectroscopy, which leads to a strongly growing demand for theoretical methods to describe the characteristic features of excited states accurately. Whereas for electronic ground state problems of stable molecules the quantum chemical methodology is now so well developed that informed nonexperts can use it efficiently, the situation is entirely different concerning the investigation of excited states. This review is devoted to a specific class of approaches, usually denoted as multireference (MR) methods, the generality of which is needed for solving many spectroscopic or photodynamical problems. However, the understanding and proper application of these MR methods is often found to be difficult due to their comp...

Time-resolved imaging of purely valence-electron dynamics during a chemical reaction

Abstract: The study of many fundamental processes in chemistry relies on the understanding of the dynamics of the valence electrons, which make and break chemical bonds. A laser method now provides direct information on the dynamics of the valence electrons—separate from any vibrational motion—during a polyatomic chemical reaction, without the need for strong laser fields that unavoidably influence the motions of these electrons.

High-energy pulse self-compression and ultraviolet generation through soliton dynamics in hollow capillary fibres

Abstract: Optical soliton dynamics can cause extreme alteration of the temporal and spectral shape of a propagating light pulse. This occurs at up to kilowatt peak powers in glass-core optical fibres and at the gigawatt level in gas-filled microstructured hollow-core fibres. Here, we demonstrate optical soliton dynamics in large-core hollow capillary fibres. This enables scaling of soliton effects by several orders of magnitude to the multi-millijoule energy and terawatt peak power level. We experimentally demonstrate two key soliton effects. First, we observe self-compression to sub-cycle pulses and infer the creation of sub-femtosecond field waveforms—a route to high-power optical attosecond pulse generation. Second, we efficiently generate continuously tunable high-energy (1–16 μJ) pulses in the vacuum and deep ultraviolet (110 nm to 400 nm) through resonant dispersive-wave emission. These results promise to be the foundation of a new generation of table-top light sources for ultrafast strong-field physics and advanced spectroscopy. Optical soliton dynamics in large-core hollow capillary fibres is demonstrated. The findings enable the scaling of soliton effects by several orders of magnitude to the multi-millijoule energy and terawatt peak power levels, and open up opportunities for new-generation table-top light sources for ultrafast strong-field physics and advanced spectroscopy.