Geoffrey Duxbury
FRSE, EmeritusProfessor of Chemical Physics

Research

My research field, gas phase high-resolution molecular spectroscopy of small molecules, spans two main areas, electronic spectroscopy, and infrared spectroscopy.

The work on electronic spectra is targeted at understanding the complex coupling between electronic, vibrational and rotational motion in small free radicals and ions. Its main thrust has been the theoretical development of methods for treating the breakdown of the Born-Oppenheimer separation of electronic and nuclear motion in triatomic molecules, the Renner effect. It is based upon a method of partially decoupling electronic and nuclear motion developed with R.N. Dixon, and developed further in collaboration with J. Rostas and  Ch. Jungen and A. Alijah. It has been successfully applied to calculate the energy level pattern, complex spin-orbit interaction, and anomalous life times observed in the electronic spectra of CH₂ and NH₂.

The research on infrared spectroscopy has two main themes, the development of new experimental techniques, primarily laser based, to allow new insights on molecular behaviour to be developed, and the applications of molecular spectroscopy related to the atmosphere of the earth. My earliest work on instrument development and spectroscopic exploitation was the development of electric resonance (laser Stark) spectroscopy, including a novel type of double resonance. Using this system, we measured laboratory spectra of one of the key interstellar molecules H₂CS. We also developed one of the first efficient optically pumped submillimetre lasers.

Since then we have been involved in a number of developments in the use of diode laser spectrometers, first of all using lead salt lasers, and following on from this near infrared diode lasers, and most recently the development of a novel spectrometer based upon the use of pulsed quantum cascade (QC) lasers. QC lasers were developed in Capasso’s group at Lucent Technologies in the USA in 1994. We started our research on developing spectrometers using these lasers in 1998. Although other groups, principally in the USA, had developed spectrometers using pulsed QC lasers, we were able to develop a new type of spectrometer based upon the use of electrical pulses of a duration of between 100 nanoseconds and two microseconds during which a complete portion of a spectrum may be recorded. This led to an invitation to present our work at a SPIE Symposium in 2002, and more recently to an invited review article in “Chemical Society Reviews”.

Our work on spectroscopic applications of infrared spectroscopy was initially geared to the detection of one of the ozone destroying species, chlorine nitrate in the atmosphere. Following on from this participated in one of the definitive studies of the methyl radical. We were also involved in the EU SWAGG program to measure the absorption cross sections of the new greenhouse gases, which are likely to lead to enhancement of global warming in the future. As part of this, we extended the use of the collision cooling method for studying the rotational structure of the heavy hydrofluorocarbon molecules. Most recently, we have been applying QC laser spectrometers to measure trace quantities of gas in the atmosphere both in the natural atmosphere and in car exhausts. We have also used our spectrometer to study some of the chemical processes, which occur within the plasma in a diamond deposition system in Professor Ashfold’s group at Bristol University.

Some of the most exciting uses of QC lasers have been the unexpected discovery that our spectrometer may be used to study the non-linear behaviour of molecules exposed to a strong radiation field. This has led to our observation of rapid passage effects in low pressure gases and of free induction decay. The combination of the high power and tuneability of the QC lasers have led to our observation of these effects, which at present we are studying in detail.

Highlights

Marlow Medal, Faraday Division, Royal Society of Chemistry, 1975

1/7/1972 to 30/9/1972  Visiting Scientist, Ottawa  National Research Council of Canada

17/6/2004 to 13/7/2004  Visiting Scientist, Ottawa  National Research Council of Canada

1/4/1984 to 5/8/1984  Professeur Associé, UER de Physique  Université de Lille

1/2/1996 to 31/7/1996  JILA Visiting Fellowship  University of Colorado

20/4/1998 to 15/5/1998   Visiting Professor, Lab. Photophys Molec.  Université de Paris-Sud

16/5/2005 to 27/5/2005  Visiting Professor, Lab. Aimé Cotton  Université de Paris-Sud