David Townsend

Direct current slice imaging

We report a new variation of the velocity map ion imaging method that allows the central section of the photofragment ion cloud to be recorded exclusively. The relevant speed and angular distributions for a molecular photodissociation or scattering event may therefore be obtained without need to utilize inversion methods such as the inverse Abel transform. In contrast to the recently reported “slicing” technique of Kitsopoulos and co-workers [C. R. Gebhardt et al., Rev. Sci. Instrum. 72, 3848 (2001)], our method makes no use of grids or pulsed electric fields which can distort the photofragment cloud and therefore compromise the resolution of velocity mapping. We find that by operating a multilens velocity mapping assembly at low voltages, the ion cloud stretches in the acceleration region owing to the kinetic energy release in the fragments. Furthermore, this inherent stretching is sufficient to allow the central section of the distribution to be recorded exclusively by application of a narrow time gate ...

Primary processes underlying the photostability of isolated DNA bases: adenine.

The UV chromophores in DNA are the nucleic bases themselves, and it is their photophysics and photochemistry that govern the intrinsic photostability of DNA. Because stability is related to the conversion of dangerous electronic to less-dangerous vibrational energy, we study ultrafast electronic relaxation processes in the DNA base adenine. We excite adenine, isolated in a molecular beam, to its ππ* state and follow its relaxation dynamics using femtosecond time-resolved photoelectron spectroscopy. To discern which processes are important on which timescales, we compare adenine with 9-methyl adenine. Methylation blocks the site of the much-discussed πσ* state that had been thought, until now, minor. Time-resolved photoelectron spectroscopy reveals that, although adenine and 9-methyl adenine show almost identical timescales for the processes involved, the decay pathways are quite different. Importantly, we confirm that in adenine at 267-nm excitation, the πσ* state plays a major role. We discuss these results in the context of recent experimental and theoretical studies on adenine, proposing a model that accounts for all known results, and consider the relationship between these studies and electron-induced damage in DNA.

Time-resolved photoelectron imaging of excited state relaxation dynamics in phenol, catechol, resorcinol, and hydroquinone

Time-resolved photoelectron imaging was used to investigate the dynamical evolution of the initially prepared S(1) (ππ*) excited state of phenol (hydroxybenzene), catechol (1,2-dihydroxybenzene), resorcinol (1,3-dihydroxybenzene), and hydroquinone (1,4-dihydroxybenzene) following excitation at 267 nm. Our analysis was supported by ab initio calculations at the coupled-cluster and CASSCF levels of theory. In all cases, we observe rapid (<1 ps) intramolecular vibrational redistribution on the S(1) potential surface. In catechol, the overall S(1) state lifetime was observed to be 12.1 ps, which is 1-2 orders of magnitude shorter than in the other three molecules studied. This may be attributed to differences in the H atom tunnelling rate under the barrier formed by a conical intersection between the S(1) state and the close lying S(2) (πσ*) state, which is dissociative along the O-H stretching coordinate. Further evidence of this S(1)/S(2) interaction is also seen in the time-dependent anisotropy of the photoelectron angular distributions we have observed. Our data analysis was assisted by a matrix inversion method for processing photoelectron images that is significantly faster than most other previously reported approaches and is extremely quick and easy to implement.

Negative frequency resonant radiation

Soliton resonant radiation emission is predicted to lead to a second mode that originates from the negative frequency branch of the dispersion relation. Measurements in both bulk media and photonic crystal fibres confirm our predictions.

Solvent induced conformer specific photochemistry of guaiacol

Using a combination of ultrafast solution- and gas-phase spectroscopies, together with high-level theory calculations, we demonstrate that we are able to track conformer-specific photodissociation dynamics in solution through solvent choice. We reveal this phenomenon in guaiacol (2-methoxyphenol), a key subunit of the natural biopolymer lignin. In cyclohexane, the first electronically excited (1)ππ* (S1) state in guaiacol relaxes with a time-constant of τ = 4.5 ± 0.2 ns, mediated through intersystem crossing to lower lying triplet (Tn) states and internal conversion and fluorescence back to the ground state (S0). In contrast, in methanol, a further relaxation channel is also present; the S1 state relaxes with a time-constant of τ = 2.9 ± 0.1 ns, which is now additionally mediated through coupling onto a dissociative (1)πσ* (S2) state and subsequent O-H bond fission, evidenced through the appearance of a spectral signature for the guaiacoxyl radical after ∼250 ps. With the aid of complementary calculations, we attribute this to the now absent intramolecular H-bond between OH and OMe moieties, which now favours intermolecular H-bonding to methanol, lowering the barrier to O-H dissociation and facilitating H-atom loss via tunnelling.

Ultraviolet relaxation dynamics of aniline, N, N-dimethylaniline and 3,5-dimethylaniline at 250 nm.

Time-resolved photoelectron imaging was used to investigate the electronic relaxation dynamics of gas-phase aniline, N, N-dimethylaniline, and 3,5-dimethylaniline following ultraviolet excitation at 250 nm. Our analysis was supported by ab initio coupled-cluster calculations evaluating excited state energies and (in aniline) the evolution of a range of excited state physical properties as a function of N-H bond extension. Due to a lack of consistency between several earlier studies undertaken in aniline, the specific aim of this present work was to gain new insight into the previously proposed non-adiabatic coupling interaction between the two lowest lying singlet excited states S1(ππ(∗)) and S2(3s/πσ(∗)). The methyl-substituted systems N, N-dimethylaniline and 3,5-dimethylaniline were included in order to obtain more detailed dynamical information about the key internal molecular coordinates that drive the S1(ππ(∗))/S2(3s/πσ(∗)) coupling mechanism. Our findings suggest that in all three systems, both electronic states are directly populated during the initial excitation, with the S2(3s/πσ(∗)) state then potentially decaying via either direct dissociation along the N-X stretching coordinate (X = H or CH3) or internal conversion to the S1(ππ(∗)) state. In aniline and N, N-dimethylaniline, both pathways most likely compete in the depletion of S2(3s/πσ(∗)) state population. However, in 3,5-dimethylaniline, only the direct dissociation mechanism appears to be active. This is rationalized in terms of changes in the relative rates of the two decay pathways upon methylation of the aromatic ring system.

Photoionization dynamics probed by angle-resolved photoelectron spectroscopy of NH3(B̃ 1E″)

We show that the photoionization dynamics of a small excited polyatomic molecule can be partially elucidated from the measurement of photoelectron angular distributions (PADs) at an energy resolution of only around 20 meV following excitation of various selected rotational levels in an excited electronic state. To illustrate this we present PADs corresponding to the formation of vibrationally resolved NH3+ following the excitation of NH3 to selected rotational levels in its B 1E″ electronic state. We also present a preliminary time-of-flight photoelectron spectrum showing peaks corresponding to resolved rotational levels of the NH3+ ion.

Following the relaxation dynamics of photoexcited aniline in the 273-266 nm region using time-resolved photoelectron imaging

Time-resolved photoelectron imaging was used to investigate the relaxation dynamics of electronically excited aniline in the gas-phase following ultraviolet irradiation in the 273-266 nm region. We find that at all wavelengths studied, excitation is predominantly to the long-lived (>1 ns) S1(ππ(*)) state, which exhibits ultrafast intramolecular vibrational redistribution on a <1 ps timescale. At excitation wavelengths centred on resonant transitions in the aniline absorption spectrum that have previously been assigned to the higher lying S2(3s∕πσ(*)) state, we also see clear evidence of this state playing a role in the dynamics. However, we see no indication of any non-adiabatic coupling between the S1(ππ(*)) and S2(3s∕πσ(*)) states over the range of excitation wavelengths studied.

The effects of symmetry and rigidity on non-adiabatic dynamics in tertiary amines: a time-resolved photoelectron velocity-map imaging study of the cage-amine ABCO

The non-adiabatic relaxation dynamics of the tertiary cage-amine azabicyclo[2.2.2]octane (ABCO, also known as quinuclidine) have been investigated following 3p Rydberg excitation at 201 nm using femtosecond time-resolved photoelectron imaging (TRPEI). The aim of the study was to investigate the influence of the rigid and symmetric cage structure found in ABCO on the general non-adiabatic relaxation processes commonly seen in other tertiary aliphatic amines (TAAs). Our data is compared with TRPEI results very recently obtained for several structurally less rigid TAA systems [J. O. F. Thompson et al., Chem. Sci., 2016, 7, 1826-1839] and helps to confirm many of the previously reported findings. The experimental results for ABCO in the short-time (<1 ps) regime strongly support earlier conclusions suggesting that planarization about the N-atom is not a prerequisite for efficient 3p-3s internal conversion. Additionally, individual photoelectron peaks within our ABCO data show no temporal shifts in energy. As confirmed by our supporting quantum mechanical calculations, this demonstrates that neither internal conversion within the 3p manifold or significant conformational re-organization are possible in the ABCO system. This result therefore lends strong additional support to the active presence of such dynamical effects in other, less conformationally restricted TAA species, where photoelectron peak shifts are commonly observed. Finally, the extremely long (>1 ns) 3s Rydberg state lifetime seen in ABCO (relative to other TAA systems at similar excitation energies) serves to illustrate the large influence of symmetry and conformational rigidity on intramolecular vibrational redistribution processes previously implicated in mediating this aspect of the overall relaxation dynamics.

The role of phase in molecular Rydberg wave packet dynamics

The dynamics of Rydberg wave packets in NO are investigated in the regime where the electronic period is comparable with the rotational motion of the molecular ion core. The presence of a rotating molecular core manifests itself in the wave packet dynamics as a series of peaks separated by the rotational beat period TRot, but offset by ΔμTRot, where Δμ is the difference in quantum defect between the two dominant Rydberg series in the superposition. We rationalize this by treating the dynamics of a wave packet created from a coherent superposition of two interleaved Rydberg series as two separate electron wave packets, which interfere with one another when they overlap spatially. There is a periodic phase difference between the two wave packets that depends on the rotational energy of the core in each Rydberg series and also on the quantum defects. The resulting interference pattern in the Rydberg population manifests itself as peaks in the wave packet spectrum at the stroboscopic period.