The tunable diode laser: a versatile spectroscopic tool

The tunable diode laser (TDL) has become a very versatile and indispensable tool for basic research and analytical applications alike. The fairly mono chromatic beam emitted by the TDL qualifies it to be used for high resoluti on spectroscopy in the Doppler and sub-Doppler domain. At the Cologne labor atories, TDLs have been used to carry out research in mid IR-spectroscopy i n four areas where precisely controlled wavelength stabilized lasers (simil ar to 1 part in 10(6)) are required. These four areas of fundamental resear ch are: (a) a wavelength stabilized TDL spectrometer, where an internally c oupled Fabry-Perot interferometer (icFPI) of 90 MKz free spectral range is used for wavelength stabilization, which enables a frequency accuracy of be tter than 1 MHz. Line profile measurements with pressure broadening and pre ssure shift parameters have been performed, e.g. on molecular hydrogen, H-2 , and other species; (b) an IR heterodyne receiver with a TDL as local osci llator (LO) and an achieved spectral resolution of lambda/Delta lambda > 10 (6) is used for measurement of the Earth's and other planetary atmospheres. Atmospheric ozone has been recorded; (c) an IR supersonic jet spectrometer with multireflection optics is used to study the low temperature spectra o f van der Waals (vdW) complexes, such as Ar-CH4, Kr-CH4, and Ne-CH4. Very r ecently, in collaboration with A.R.W. McKellar from the National Research C ouncil, Ottawa, Canada, the spectra of paraH(2)-CH4 have been recorded and analyzed; (d) the carbon cluster TDL spectrometer combines the advantages o f high resolution and high sensitivity laser spectroscopy with an intensive cluster source. The carbon clusters are produced by laser vaporization of a graphite rod target and flushed into the absorption cell by a supersonic jet expansion of He. Several carbon clusters have been recorded, e.g. C-9, C-13, and others the assignments of which are in progress, providing the po ssibility ro monitor the growth and structure of small carbon clusters. Eac h of these four research areas will be discussed in turn, by presenting fir st a short overview of the field, followed by a brief experimental outline. Each section will be closed by reviewing the highlights of the most recent results obtained at the Cologne laboratories. (C) 1999 Elsevier Science B. V. All rights reserved.