Infrared and Raman spectroscopy at high pressures

Pressure is accepted theoretically as a useful variable. However in a studies on liquid or solid samples, it is still relatively unusual for pressure to be used as an experimental variable. The reluctance of experimentalists to use this theoretically attractive variable is caused mainly by the technical difficulties associated with the use of sufficiently high pressures. In this talk I will try to show that in many cases the experimental limitations are no longer those introduced by the use of high pressures. High pressure spectroscopic studies clearly imply the use of high pressure spectroscopic cells. A brief account will therefore be given of the various types of high pressure optical cells which are currently being used for spectroscopic studies. Each individual high pressure spectroscopic study has its own special justification. However there are a few quite general observations that can be made which cover many of the specific objectives of individual high pressure spectroscopic studies. For example:(i) pressure induced frequency shifts carry unambiguous information about anharmonic terms in the relevant potential function (i.e. the potential V is a function of distance d. therefore pressure can be used to change d and study V.)(ii) all known materials undergo structural phase transitions if the form which is thermodynamically stable under ambient conditions is compressed to high enough pressures: these high pressure phases should be studied.(iii) as the application of pressure forces a material towards a phase transition, the spectroscopic study can be used to gain information about the approaching structural instability.(iv) virtually all infrared and Raman spectra contain examples of Fermi resonance which confuse the interpretation of the spectra and the effects of pressure are valuable aids to the correct assignment of the resonating levels.(v) pressure induced frequency shifts can often give extra information to help with the more reliable assignment of features within a spectrum.The above points will be discussed and illustrated by examples chosen mainly from recent work by members of the spectroscopy group at King's College London.