Research project: Dyke: Photoionisation Studies of Reactive Intermediates with Synchrotron Radiation

For example, measurements of ionization energies of reactive intermediates lead to determination of key thermochemical quantities such as molecular heats of formation and bond dissociation energies. This information is particularly useful in computer simulations and modelling of environments such as the earth’s atmosphere and flames. Also, molecular ions produced from reactive intermediates are important in plasmas, combustion and solution, and studying them via spectroscopic means, supported by electronic structure calculations, as proposed in this project helps to understand their structure and properties, and hence behaviour in these environments.

Three separate but inter-related types of investigation (CIS, PES, and TPES) will be performed in the photon energy region 10-40 eV. They will be described in the order in which they would normally be applied to a molecular reactive intermediate under study.

i). Angularly Resolved Photoelectron Spectroscopy (PES) and Constant-Ionic-State (CIS) Measurements
Angular distribution (b-parameter) measurements can determine the symmetry of the orbital the electron occupied before photoionization, the angular momentum of the free electron and the phase differences between the continuum waves of the outgoing electron. Using synchrotron radiation has the important advantage that b values for the vibrational components of a photoelectron band can be obtained as a continuous function of photon energy. This can be achieved via either angularly resolved photoelectron (PE) or constant-ionic-state (CIS) measurements. Recent measurements by the Southampton group on O21Δg , OH, N and S demonstrate that the β parameter can respond dramatically to resonant behaviour and angular distribution measurements have emerged as a powerful way of studying the dynamics of photoionization.

ii) Threshold Photoelectron Spectroscopy (TPES)
TPES experiments provide distinctly different but complementary information to CIS and PES measurements and offer the experimental advantages of very high resolution and very high collection efficiency. TPES experiments will be performed in this project by modifying the existing Southampton spectrometer (designed to study short-lived species) to extract threshold electrons using the penetrating field technique. A TPE spectrum is obtained by measuring the yield of threshold photoelectrons as a function of the scanned photon energy.

The TPE spectrum of a molecule is usually much more structured than the conventional PE spectrum because in TPES ionization can take place indirectly via autoionization of neutral Rydberg states lying just above the ionic thresholds of the molecule as well as directly (as in PES). The interpretation of these more structured spectra will be greatly assisted by the CIS spectra recorded as part of this project and by Franck-Condon simulations which will use computed potentials of the states involved.

Reactive intermediates which are currently being studied with these methods include IF, CF2, OH, and NO3.