The Photon Collisions unit undertakes research using vacuum ultraviolet photons to probe the energy structure of atoms and molecules. This research has both an intrinsic interest, in the understanding of the internal energy structure of atoms and molecules, and an applied interest in the use of the data for atmospheric, photochemical and astrophysical calculations. The research program covers both experimental and theoretical aspects of photoabsorption, photodissociation and fluorescence excitation spectroscopy. At the present time research has concentrated on molecular oxygen as this molecule exhibits many textbook examples of quantum physics that are also applicable to other molecules, and because of its importance to the world's atmospheric photochemistry.
Close coupled spectroscopy
Coupled molecular electronic states are responsible for many complex features in observed spectra. For example, Rydberg-valence interactions are well known to cause significant anomalies in the photoabsorption spectra of O2 , NO and N2.
Coupled-channel Schrödinger Equation
We examine the effects of the interactions between electronic states of a molecule through a coupled-channel Schrödinger equation (CSE) model that is based on the techniques of atomic scattering theory.
The model accounts for the spectral features of:
- intensity borrowing
- resonance asymmetry.
Applications Half collision processes
- High temperature photoabsorption cross sections - plasma/shock tube
- Non-optical spectra
- Dissociative charge transfer spectra (DCTS)
- Electron energy loss spectra (EELS)
Ultra violet laser physics
The UV Laser Physics Laboratory in the Laser Physics Centre has two complementary experimental research programmes based around a high power, pulsed dye laser facility:
- previous studies of fundamental nonlinear optical processes such as quantum mechanical interferences (now complete)
- the application of these techniques to the high resolution VUV spectroscopy of atmospheric molecules.
Non linear optics
The nonlinear optics studies have centred on the generation of several multiphoton excitation pathways in atomic systems (mainly sodium), which interfere quantum mechanically to produce both constructive and destructive effects. These interferences can be used to enhance or diminish the nonlinear process being monitored, such as four wave mixing or multiphoton ionisation. The work in this area is now complete, and the resources have been diverted to other projects.Previous highlights of the nonlinear optics activities include (see publication list below) -
- the demonstration of interference between different four-wave mixing processes using separate bound state resonances, in which the sign and magnitude of the interference could be controlled by varying the laser detuning from resonance [1,2]
- experimental investigation of laser induced continuum structures
- the suppression of the AC Stark effect in two-photon resonant, three-photon ionisation, caused by the introduction of a second laser field which generates a competing four-wave mixing pathway
VUV molecular spectroscopy
One output of these nonlinear processes - the generation of narrowband, tunable radiation in the vacuum ultraviolet (VUV) spectrum - has been applied to the high resolution VUV spectroscopic study of atmospheric molecules, primarily diatomic oxygen. Highlights include:
- the use of stimulated Raman scattering, third harmonic generation, and four-wave mixing to carry out the highest resolution studies of photoabsorption cross-sections for oxygen in the VUV [3,7,8,9]
- a benchmark comparison of the performance of our sub-Doppler VUV absorption spectroscopy with laser induced fluorescence and Fourier transform spectroscopy (J. Chem. Phys. 109, 3856, 1998)
- the observation of rotational edges and shape resonances , collisional broadening  and interferences (Phys. Rev. A 55, 4164 1997) in the oxygen Schumann-Runge bands
- the first characterisation of isotopic differences in highly resolved rovibrational structure in O2 
- the discovery of two new states of oxygen (Phys. Rev. A 52, 2717, 1995; Phys. Rev. A 54, 3923, 1996)
Pioneering Lamb Shift Measurement in Helium In addition, these same four-wave mixing techniques have been used in a collaborative experiment at the US National Institute of Standards and Technology in Gaithersburg to measure, for the first time, the 1S - 2S transition in helium using Doppler-free two (120nm) photon VUV spectroscopy.This enabled a new determination of the helium 1S Lamb shift, in reasonable agreement with theory and with independent measurements (see Bergeson et al., Physical Review Letters 80, 3475, 1998).