Gareth M. Roberts

Toward real-time charged-particle image reconstruction using polar onion-peeling.

A method to reconstruct full three-dimensional photofragment distributions from their two-dimensional (2D) projection onto a detection plane is presented, for processes in which the expanding Newton sphere has cylindrical symmetry around an axis parallel to the projection plane. The method is based on: (1) onion-peeling in polar coordinates [Zhao et al., Rev. Sci. Instrum. 73, 3044 (2002)] in which the contribution to the 2D projection from events outside the plane bisecting the Newton sphere are subtracted in polar coordinates at incrementally decreasing radii; and (2) ideas borrowed from the basis set expansion (pBASEX) method in polar coordinates [Garcia et al., Rev. Sci. Instrum. 75, 4989 (2004)], which we use to generate 2D projections at each incremental radius for the subtraction. Our method is as good as the pBASEX method in terms of accuracy, is devoid of centerline noise common to reconstruction methods employing Cartesian coordinates; and it is computationally cheap allowing images to be reconstructed as they are being acquired in a typical imaging experiment.

Direct Observation of Hydrogen Tunneling Dynamics in Photoexcited Phenol

The excited-state dynamics of phenol following ultraviolet (UV) irradiation have received considerable interest in recent years, most notably because they can provide a model for understanding the UV-induced dynamics of the aromatic amino acid tyrosine. Despite this, there has been some debate as to whether hydrogen tunneling dynamics play a significant role in phenols excited-state O-H bond fission when UV excitation occurs below the (1)ππ*/(1)πσ* conical intersection (CI). In this Letter, we present direct evidence that (1)πσ*-mediated O-H bond fission below the (1)ππ*/(1)πσ* CI proceeds exclusively through hydrogen tunneling dynamics. The observation of hydrogen tunneling may have some parallels with proton tunneling dynamics from tyrosine residues (along the O-H bond of the phenol moiety) in a wide range of natural enzymes, potentially adding further justification for utilizing phenols as model systems for investigating tyrosine-based dynamics.

Ultrafast Relaxation Dynamics Observed Through Time-Resolved Photoelectron Angular Distributions†

Time-resolved photoelectron imaging of the 7,7,8,8-tetracyanoquinodimethane (TCNQ) radical anion is presented. Photoelectron angular distributions (PADs) are qualitatively analyzed in terms of the simple s-p model that is based on symmetry arguments. The internal conversion dynamics from the first excited state (1(2)B(3u)) to the ground state ((2)B(2g)) may be observed through temporal changes in the PADs of the spectrally overlapping photoelectron features arising from photodetachment of the ground state and the excited state. A formulism for extracting the population dynamics from the β(2) anisotropy parameter of overlapping spectroscopic features is presented. This is used to extract the lifetime of the first excited state, which is in good agreement with that observed in the time-resolved photoelectron spectra.

Velocity-map imaging at low extraction fields

We present a velocity-map imaging (VMI) setup for photoelectron imaging that utilizes low electric extraction fields. This avoids any complications that could arise from electrostatic interactions between the extraction field and the molecular properties that are probed and has a minimal effect on the trajectory of ions in ion beam experiments. By using an attractive potential supplied to the detector, and keeping the electrodes at ground (zero) potential, we show that fringe fields between the VMI arrangement and the vacuum chamber can be eliminated, which is important in experiments on ions.

Excited states in electron-transfer reaction products: ultrafast relaxation dynamics of an isolated acceptor radical anion.

The spectroscopy and ultrafast relaxation dynamics of excited states of the radical anion of a representative charge-transfer acceptor molecule, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, have been studied in the gas phase using time-resolved photoelectron spectroscopy. The photoelectron spectra reveal that at least two anion excited states are bound. Time-resolved studies show that both excited states are very short-lived and internally convert to the anion ground state, with the lower energy state relaxing within 200 fs and a near-threshold valence-excited state relaxing on a 60 fs time scale. These excited states, and in particular the valence-excited state, present efficient pathways for electron-transfer reactions in the highly exergonic inverted region which commonly displays rates exceeding predictions from electron-transfer theory.

A Multipronged Comparative Study of the Ultraviolet Photochemistry of 2-, 3-, and 4-Chlorophenol in the Gas Phase.

The S1((1)ππ*) state of the (dominant) syn-conformer of 2-chlorophenol (2-ClPhOH) in the gas phase has a subpicosecond lifetime, whereas the corresponding S1 states of 3- and 4-ClPhOH have lifetimes that are, respectively, ∼2 and ∼3-orders of magnitude longer. A range of experimental techniques-electronic spectroscopy, ultrafast time-resolved photoion and photoelectron spectroscopies, H Rydberg atom photofragment translational spectroscopy, velocity map imaging, and time-resolved Fourier transform infrared emission spectroscopy-as well as electronic structure calculations (of key regions of the multidimensional ground (S0) state potential energy surface (PES) and selected cuts through the first few excited singlet PESs) have been used in the quest to explain these striking differences in excited state lifetime. The intramolecular O-H···Cl hydrogen bond specific to syn-2-ClPhOH is key. It encourages partial charge transfer and preferential stabilization of the diabatic (1)πσ* potential (relative to that of the (1)ππ* state) upon stretching the C-Cl bond, with the result that initial C-Cl bond extension on the adiabatic S1 PES offers an essentially barrierless internal conversion pathway via regions of conical intersection with the S0 PES. Intramolecular hydrogen bonding is thus seen to facilitate the type of heterolytic dissociation more typically encountered in solution studies.

Distributed luminescence from alkyl-capped silicon quantum dots

Orange luminescence attributable to a core of silicon atoms in alkyl-capped crystalline quantum dots excited at λa=355 and 405 nm is investigated as a function of applied intensity and time. The intensity of luminescence displays a linear power dependence on the intensity of the applied field, from which an exponent n=0.94±0.02 commensurate with single-photon absorption is derived. The dependence of luminescence on time is observed to be strongly nonexponential and is optimally accounted for by a probability density function which describes a continuous distribution of two decay times: the behavior is characteristic of a pair of elementary steps connected with light emission within a distribution of local environments, or a single rate process supported by two environments. Nonlinear least-squares fits to the time dependent luminescence formulated on this basis with a Gaussian, Lorentzian, or log-normal distribution of rates return most probable lifetimes T¯1=21±1 μs and T¯2=3.7±0.8 μs. The widths of the ...

Is UV-Induced Electron-Driven Proton Transfer Active in a Chemically Modified A·T DNA Base Pair?

Transient electronic and vibrational absorption spectroscopies have been used to investigate whether UV-induced electron-driven proton transfer (EDPT) mechanisms are active in a chemically modified adenine-thymine (A·T) DNA base pair. To enhance the fraction of biologically relevant Watson-Crick (WC) hydrogen-bonding motifs and eliminate undesired Hoogsteen structures, a chemically modified derivative of A was synthesized, 8-(tert-butyl)-9-ethyladenine (8tBA). Equimolar solutions of 8tBA and silyl-protected T nucleosides in chloroform yield a mixture of WC pairs, reverse WC pairs, and residual monomers. Unlike previous transient absorption studies of WC guanine-cytosine (G·C) pairs, no clear spectroscopic or kinetic evidence was identified for the participation of EDPT in the excited-state relaxation dynamics of 8tBA·T pairs, although ultrafast (sub-100 fs) EDPT cannot be discounted. Monomer-like dynamics are proposed to dominate in 8tBA·T.

Time-resolved spectroscopy of the dynamic Stark effect

This study reports on the ac Stark effect of nitric oxide electron-vibration (vibronic) levels dressed by an intense, bichromatic laser field. The aims of the work are to make quantitative measurements of the magnitude of the ac Stark shift of vibrational levels connected by the A2Σ+ ← X2Π1/2 transition and to map out the cycle-averaged level separation during the Stark field. In pump–probe experiments at intensities up to 30 TW cm−2, the ac Stark shift of the A2Σ+ vA = 2 ← X2Π1/2 vX = 0 transition is found to be Δ(S)2←0(800 nm) = 0.34 ± 0.05Up, where Up is the ponderomotive energy. By varying the time delay between component fields, the time dependence of the population in the A2Σ+ vA = 2 state is mapped out under different intensity conditions. Fluorescence monitoring of the upper level serves to identify the Stark-shifted transition and reveals that real population remains in the upper level after interacting with the short-pulse laser fields. Semiclassical calculations of NO irradiated by a bichromatic field indicate the direction of the A2Σ+ and X2Π1/2 level shifts and show that the population promoted to and retained in the A2Σ+ state is determined predominantly by intra-Rydberg state interactions rather than by multiphoton loss processes. The calculations also indicate that the magnitude of the ac Stark effect of the A2Σ+ ← X2Π1/2 transition depends on the spatial orientation of the molecule with respect to the linear polarization sense of the applied laser field.

E-field dependence of the ac Stark effect probed by a bichromatic laser field

In this paper we demonstrate, through numerical calculations, the possibility of determining the field dependence of the ac Stark effect through the interaction of an atom (or molecule) with a bichromatic ultrafast laser field. The time-dependent Schrodinger equation is solved for a single active electron confined to move in a square-well potential distorted by an intense Stark field in combination with a perturbative probe field which connects a pair of levels through photon absorption. The aim is to determine the field parameters that allow the dependence of the dynamic shift of the optically connected levels on the Stark field to be mapped out in real time. The calculations show that this can be achieved when the duration of the probe field is at least as short as the half-cycle period of the Stark field. An experimental realization of the proposed measurement scheme and its limitations are discussed. It is also possible, in principle, to adopt the ac Stark effect to determine the absolute phase of the carrier wave of an ultrafast laser pulse.