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Stephen C. Foster

Stephen C. Foster

Associate Professor of Physical Chemistry

B.Sc. Manchester University, 1978
Ph.D. Dalhousie University, 1982
email: Dr. Foster
telephone: (662) 325-8854
CV

Research Interests:

Stephen Foster

Cavity Ring-Down Spectroscopy

CRDS can be visualized as a 3 step experiment:

1. Laser light is injected (and trapped) into an optical cavity (e.g. a pair of highly-reflective mirrors).

2. The light bounces back and forth between the mirrors (making many round trips before it all "leaks" out through the mirrors).

3. The light leaking out through an end mirror is monitored, and the time taken for the signal to fall from I0 to I0/e is recorded (the ring-down time).

In an empty cavity, the ring-down is long; only a tiny fraction of the light is lost each time the pulse hits a mirror. However, if an absorber is placed in the cavity, the light is removed (absorbed) more rapidly by that species and the ring-down shortens. If the ring-down time is plotted as a function of wavelength we generate a very sensitive absorption spectrum.

Most CRDS experiments have been performed on gaseous samples, but in this lab we use liquid samples. When the sample is in a container (rather than directly between the mirrors forming the cavity), the container windows reflect and scatter light and hence reduce the lifetime of a light pulse in the cavity. In this lab, we have developed methods to minimize these loses and the technique holds great promise as a new analytical method.

Photochemistry of Optical Upconvertors

Optical upconversion is the process of taking low energy (long wavelength) photons and converting them to higher energy (shorter wavelength). The most familiar example is the second harmonic generation used to convert two high-power Nd:YAG infrared photons (at 1064 nm) into one green photon (at 532 nm). Low-power upconversion has numerous applications. Silicon based solar photovoltaic cells most efficiently capture near-infrared photons and waste a large fraction of the energy available in visible photons. A better solar cell could be designed if the infrared and red photons in the solar spectrum could be converted to visible light. A wider band-gap photovoltaic could then be used to more efficiently capture visible wavelengths.

This lab, in collaboration with Henry's group, are exploring a process which allows the use of Ru-based sensitizers and substituted anthracene compounds to upconvert low-power red and near-infrared photons into green and blue wavelengths. We have demonstrated the upconversion of a low-power 633 nm (red) light to blue light near 400 nm. Work continues to develop new systems that will move the long-wavelength input limit into the near infrared.

Hollow Cathode Absorption Spectroscopy

Relative Intensity

An efficient way to study radicals and ions is to prepare them in a large hollow cathode discharge from a suitable stable precursor molecule. The discharge creates large numbers of these reactive species and they can be observed with an IR-laser. The spectrum shown is a small portion of the triply-degenerate deformation mode of the NH4+ ion. Each line corresponds to a change from a single rotational level in the ground state to a different rotational level in the excited vibrational state.