Doppler-force spectroscopy

Some very important surface properties of solids can be properly characterized only by certain wet chemical techniques, some of which are currently under rapid improvement. Studies of adsorption from solution allow determination of the surface density of adsorbing sites, and the characterization of the surface forces involved (the energy of dispersion forces, the strength of acidic or basic sites and the surface density of coul-ombic charge). Adsorption studies can now be extended with some newer spectroscopic tools (Fourier-transform infra-red spectroscopy, laser Raman spectroscopy, and solid NMR spectroscopy), as well as convenient modern versions of older techniques (Doppler electrophoresis, flow microcalorimetry, and automated ellipsometry). 

The value of the temperature in eqn (5.3) is nowadays referred to as the Doppler temperature or Doppler cooling limit. At a typical value of the natural hnewidth of an allowed transition 2y = 27t X 10 MHz, the temperature To is of the order of 100 pK. Because of the great promise that laser cooling and subsequent laser trapping of atoms held for laser spectroscopy, researchers at the Institute of Spectroscopy in Troitsk, Russia, launched experiments in this field. By the time the first successful experiment was conducted (Andreyev et al. 1981, 1982), the first theoretical work, summarized in a review of the manipulation of atoms by the light pressure force of a resonant laser (Letokhov and Minogin 1981a), had already been completed. 

The Doppler-free techniques, which have been explained in Chap.10, eliminate the bothering Doppler width and therefore already small collision broadening can be sensitively monitored. One example is given by the investigation of broadening or shifts of Lamb dips in saturation spectroscopy (see Sect. 10.2), which can be measured with an accuracy of a few kilohertz if stabilized lasers are used. This high accuracy allows detection of even small interaction forces such as those experienced by atoms or molecules without permanent dipole moments. The interaction potential at large internuclear distances can be described in these cases by a van der Waals potential V(r) = -ar where the constant a depends on the polarizability of the collision partners. 

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