Benjamin J. Whitaker

Very high time-of-flight mobility in the columnar phases of a discotic liquid crystal

1,4,8,11,15,18,22,25-Octaoctylphthalocyanine shows the highest time-of-flight (“long-range”) hole mobility so far reported for the columnar phase of a discotic liquid crystal. Unlike most other high long-range mobility columnar discotics, there is no clear evidence from x-ray diffraction of high order.

Differential cross sections for state‐selected products by direct imaging: Ar+NO

State‐selected differential cross sections have been obtained by directly imaging the products of collisions in crossed molecular beams. The new technique allows final state resolution and simultaneous detection of all scattering angles. The method has been used to study inelastic collisions between Ar and NO(2Π1/2, υ=0, J=0.5) at a collision energy of 0.21 eV. Rotational rainbows in the product angular distribution are directly observed to change in position as a function of the final rotational state; the peak of the angular distribution moves toward the backward hemisphere and the angular distribution broadens with an increase in final rotational quantum number. The method relies on multiphoton ionization of the product but is otherwise generally applicable to reactive as well as inelastic collisions.

Implementation and initial deployment of a fieldinstrument for measurement of OH and HO2 in thetroposphere by laser-induced fluorescence

An instrument to detect atmospheric concentrations of the hydroxyl (OH) and hydroperoxyl (HO 2 ) radicals has been developed using the FAGE (fluorescence assay by gas expansion) technique. The instrument is housed in a mobile laboratory and monitors the OH radical via on-resonance laser-induced fluorescence (LIF) spectroscopy of the A 2 Σ + (v′=0)–X 2 Π i (v ″ =0) transition at ca. 308 nm. Ambient air is expanded through a 1 mm nozzle to low pressure where it is irradiated by the laser pulse at a repetition rate of 7 kHz, with the resultant fluorescence being detected by gated photon counting. HO 2 is monitored by chemical conversion to OH by the addition of NO, with subsequent detection using LIF. Following laboratory and field calibrations to characterise the instrument sensitivity, detection limits of 1.8×10 6 and 2.1×10 7 molecule cm -3 were determined for OH and HO 2 respectively, for a signal-to-noise ratio, S/N, of 1 with 150 s signal integration time. The instrument was deployed for the first time during the ACSOE field campaign at Mace Head, Eire, for which illustrative results are given.

The imitation game - A computational chemical approach to recognizing life

When is an artificial cell alive? A Turing test–like method may provide the answer.

An investigation of the 355 nm photodissociation of NO2 by state‐resolved photofragment imaging

The 355 nm photodissociation of NO2 cooled in a supersonic beam has been investigated by state‐resolved photofragment imaging. The NO and O(3PJ) photofragments were state‐selectively ionized and projected onto a two‐dimensional, position‐sensitive detector to obtain speed and angular distributions. The speed distribution of the O(3P2) fragment displays two peaks corresponding to oxygen produced in coincidence with NO(υ=0) and NO(υ=1). The angular distributions for the O(3P2) and for the NO in several vibrational and rotational levels can be characterized by an anisotropy parameter of β=1.2±0.3. This value, while higher than that measured previously, is consistent with a dissociation lifetime on the order of 200–400 fs and with the colder rotational temperature of the current beam experiment. The rotational distributions of the NO product are found to be in good agreement with other recent measurements.

Non-adiabatic effects in chemistry revealed by time-resolved charged-particle imaging

Recent advances in photoelectron and photoion imaging techniques are reviewed. The general background to photofragmentation spectroscopy and, in particular, the information that can be extracted from laboratory and molecular frame photoproduct angular distributions are briefly discussed. Recent technological advances to the photofragment imaging experiment first introduced by Chandler and Houston, such as velocity mapping and event counting, are also described. The main focus of the review is devoted to time-resolved imaging applications in which femtosecond pump?probe techniques are combined with charged particle imaging. We survey the recent literature and describe a number of applications; in particular we show how pump?probe photoelectron imaging can be used to follow intersystem crossing in molecules such as pyrazine, and we also show how the transfer of nuclear coherences can be followed with this technique. We discuss how time-resolved pump?probe photofragment imaging can be used to follow unimolecular decomposition of free radicals with small reaction barriers and we show how this can lead to non-statistical behaviour as a result of restricted rovibrational coupling. Finally, we describe recent applications of pump?probe Coulomb explosion imaging which has the potential to image directly the nuclear wavefunction throughout the course of a chemical reaction. The review closes with some brief conclusions and pointers for future work.

Two-dimensional imaging of state-selected photofragments: the 355 nm photolysis of NO2

Abstract Two-dimensional photofragment imaging has been applied to the 355 nm photodissociation of NO 2 in a supersonic beam. The NO fragments are state-selectively ionized and projected onto a two-dimensional position-sensitive detector. The original velocity distribution is reconstructed by an Abel transform of the observed image of the fragments. The speed distribution of a single rovibrational state of NO consists of a single peak as expected from conservation of momentum and energy. The anisotropy parameter, β, of the NO photofragments is found to be 1.40±0.20, which is significantly larger than previously reported values measured with effusive molecular beams. This discrepancy is explained by the effect of the rotation of the parent molecule.

The decay dynamics of photoexcited argon cluster ions

Following the photoexcitation of argon cluster ions, Ar+n for n in the range 4–25, kinetic energy release measurements have been undertaken on the fragments using two quite separate techniques. For Ar+4–Ar+6, fragment ion kinetic energy spectra were recorded at 532 nm in a crossed beam apparatus as a function of the angle of polarization of the laser radiation with respect to the incident ion beam. Only Ar+ from Ar+4 was observed to exhibit a polarization dependence together with a comparatively high kinetic energy release. The principal fragment ion Ar+2 was found both to emerge with a low kinetic energy release and to display no dependence on the angle of polarization of the radiation. In a second series of experiments, mass and kinetic energy resolved cluster ions were photodissociated in the entrance to a time‐of‐flight (TOF) device of variable length. The subsequent deflection of all ions allowed for time resolved measurements to be undertaken on the neutral photofragments. Following the absorption o...

Fast photomultiplier tube gating system for photon counting applications

A normally “on” linear-focused 14-stage end-window photomultiplier tube (PMT) (Electron Tubes Limited 9893Q/100B), designed for fast photon counting, has been gated through control of the voltage applied to the first dynode. The gating circuit reduces the gain of the PMT during a laser pulse, in order to discriminate against the detection of scattered light, and then increases the gain promptly to observe extremely low levels of laser-induced fluorescence (LIF). An extinction factor for the laser scattered photons of >105 was observed, and has enabled count rates for photons due to LIF as low as 1 Hz to be measured for a laser pulse-repetition frequency of 7 kHz. The rise of the PMT gain is monitored directly by observation of the fluorescence using time-resolved photon counting, and the PMT turn-on time is 30 ns. No significant distortion of the temporal profile of the fluorescence was observed during PMT turn-on. The system, which can also be operated in ungated mode, is rugged and reliable, and has bee...

AOPDF-shaped optical parametric amplifier output in the visible

Time shaping of ultra-short visible pulses has been performed using a specially designed acousto-optic programmable dispersive filter of 50% efficiency at the output of a two-stage non-collinear optical parametric amplifier. The set-up is compact and reliable. It provides a tunable shaped source in the visible with unique features: a 4-ps shaping window with preserved tunability over 500–650 nm, and pulses as short as 30 fs. Several-μJ output energy is easily obtained.