Shiou-Min Wu

Imaging the dynamics of gas phase reactions

Ion imaging methods are making ever greater impact on studies of gas phase molecular reaction dynamics. This article traces the evolution of the technique, highlights some of the more important breakthroughs with regards to improving image resolution and in image processing and analysis methods, and then proceeds to illustrate some of the many applications to which the technique is now being applied--most notably in studies of molecular photodissociation and of bimolecular reaction dynamics.

Photodissociation of vibrationally excited OH/OD radicals

This paper describes a joint experimental and theoretical study of the photodissociation of vibrationally excited hydroxyl radicals. OH and OD radicals produced in a pulsed electric discharge supersonic beam are state-selected and focused by a hexapole and then photo-dissociated by a single laser tuned to various H/D or O atom (2 + 1) resonance enhanced multiphoton ionization (REMPI) wavelengths between 243 nm and 200 nm. The angle velocity distributions of the resulting O+ and D+ photofragment ions were recorded using velocity map imaging. Photodissociation to the O(3PJ) + H(2S) limit is shown to take place by one-photon excitation to the repulsive 1 2Σ− state. The experimental data shows that vibrationally excited OH/OD which are formed in the discharge are dissociated, and a vibrational temperature of ≈2000 K was estimated for the beam source. An analysis in the high-energy recoil sudden limit is used to predict the O(3PJ) fine structure branching ratios and alignment information in the molecular and laboratory velocity frame of the imaging experiment. The measured and predicted fine structure branching ratios and alignment parameters agree well at all dissociation wavelengths, supporting the model for photodissociation in the sudden limit regime. Several aspects of the experiment such as OH pre-alignment and orientation, ion-recoil, and Doppler-free imaging are discussed.

Velocity map imaging study of OCS photodissociation followed by S(1S) Autoionization at 157 nm

Atomic sulphur ions (S+) were observed directly by crossing a carbonyl sulphide (OCS) molecular beam with a F2 laser. In this study both S+ ion and electron images were measured using the velocity map imaging technique. The results imply that S+ is produced from the well-known photodissociation of OCS at 157 nm leading to the dominant S(1S) + CO(1Σ+) channel, and then the excited S(1S) atom is directly ionized by another 157 nm photon. Correlated vibrationally resolved angular distributions and internal energy distribution of the CO coproducts are reported here and compared with previous studies. This experiment yields strong and sharp S+ images which may be useful for calibrating any imaging or laser ionization apparatus when using a 157 nm laser. A number of technical aspects such as corrections for partial slicing and imperfect laser polarization are described. Abstraction of product angular distributions using both polarized and unpolarized photolysis lasers is also demonstrated using velocity map imaging.

Angular momentum polarisation in the O(D-1) products of O-2 photolysis via the B-3 Sigma u(-) state

The first fully allowed spectroscopic transition in O2 is the transition comprising the well-known Schumann–Runge bands and continuum. We report a velocity-map imaging study in which the O(1 D) angular momentum polarisation and O(3 P) spin–orbit branching ratios arising from this process have been measured. We show that direct 157 nm excitation into the Schumann–Runge continuum leads to extremely strong angular momentum polarisation in the O(1D) product. Comparison with previous studies indicates that this is a general feature of dissociation via the B state. The fine structure branching ratios in the co-fragment O(3P J=2,1,0) were measured to be 88.5 ± 1.6 : 9.7 ± 1.4 : 1.9 ± 0.4. Based on a consideration of the Massey parameter for the system, the data were modelled using theoretical calculations based on adiabatic and diabatic models of the dissociation. While both models were able to describe some aspects of the dissociation accurately, neither was able to predict both the fine structure branching ratios of the O(3P) products and the O(1D2) alignment. We have also investigated O(1D2) alignment arising from 203.8 and 205.5 nm photodissociation via the state of O2 vibrationally excited to v=6–11. As in the 157 nm photodissciation of vibrationally ground state O2, strong polarisation of the O(1D2) photofragments is observed.