Doppler-free techniques in analytical spectroscopy

Abstract

The application of Doppler-free spectroscopy techniques offers the possibility of spectrochemical analysis of isotopes. The basic requirements of Doppler-free spectroscopy are (i) the availability of tunable lasers with small line widths and (ii) the atomization of samples into vacuum or a low pressure atmosphere. With the exception of a few very light and heavy elements, the isotope line shifts are smaller than the Doppler widths. Therefore, the homogeneous line widths AVhom of atomic transitions have to be much smaller than the Doppler widths AvD of these lines and vacuum or low pressure atomization is required. The condition Avhom ~ AVD restricts the selection of atomizers which can be used in Doppler-free spectroscopy. Electrothermal atomization into vacuum or a low pressure noble gas, low-pressure discharge plasmas or analytical flames operating in the sub-atmosphere region (see e. g. [1]) are suitable methods. Basically, there are four Doppler-free techniques which, in principle, can be used in spectrochemistry: (1) excitation of species in an atomic beam at a right angle, (2) saturation spectroscopy, (3) off-resonance multiphoton spectroscopy and (4) resonant multiphoton spectroscopy (see e.g. [2]). Usually the techniques 1 3 use one laser, while technique 4 needs at least two laser systems. The restriction of method 1 is connected with the atomic beam atomization. However, the excitation is very efficient because the laser radiation is interacting with almost all atoms of the maxwellian velocity distribution in the beam. In Doppler-free off-resonant multiphoton spectroscopy we have again the interaction of all atoms with the laser radiation. However, because the atomic transition rates are very weak and dependent on I n (I: laser intensity, n: number of photons involved in the transition) the laser beam has to be focussed to get detectable signals. Therefore the interaction volume of laser radiation with the atomized sample is limited. In saturation and resonant multiphoton spectroscopy the lasers are interacting only with one atomic velocity group of the maxwellian distribution, which is a fraction of all analytes within the laser fields.