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:
(1) previous studies of fundamental nonlinear optical processes such
as quantum mechanical interferences (now complete)
(2) 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 [4], collisional
broadening [13] 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 [5]
- 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).
