Photon spectroscopy, requirements

One very important aspect of two-photon absorption is that the selection ndes for atoms or synnnetrical molecules are different from one-photon selection ndes. In particular, for molecules with a centre of synnnetry, two-photon absorption is allowed oidy for g g or u u transitions, while one-photon absorption requires g-f u transitions. Therefore, a whole different set of electronic states becomes allowed for two-photon spectroscopy. The group-theoretical selection ndes for two-photon spectra are obtained from the synnnetries... 

All forms of spectroscopy require a source of energy. In absorption and scattering spectroscopy this energy is supplied by photons. Emission and luminescence spectroscopy use thermal, radiant (photon), or chemical energy to promote the analyte to a less stable, higher energy state. 

A second consideration is the ambient environment required for analysis. All techniques using particles require moderate or high vacuum conditions, i.e. pressures of 10-5 torr or lower. However, techniques using only photons work in any fluid environment, including liquids, with the provision, of course, that the wavelengths of interest are not appreciably absorbed or emitted by the environment. Thus in situ studies of such phenomena as catalysis and corrosion in typical ambient environments are possible with photon spectroscopy. Further, in many cases the surface film to be studied will be quite air stable and analysis outside of a vacuum chamber can result in a great savings in time and effort. 

Doppler-free two-photon spectroscopy offers several attractive features to study Rydberg series at high resolution. Since two photons of equal frequency are used, only one laser is required, resulting in a simplified setup. Furthermore, the splittings and linewidths observed depend on the initial and final states only. Consequently, spectra obtained by this technique generally exhibit excellent resolution and can easily be analyzed. 

At this point it is of interest to compare coherent two-photon with stepwise excitation, although the most suitable experimental scheme depends on the particular problem under study. In the case of Doppler-free coherent two-photon spectroscopy, only the initial and final state contribute to the recorded splittings and linewidths. Therefore, this technique offers better resolution, and generally simple spectra are obtained which are straightforward to interpret. On the contrary, the hyperfine structure of the intermediate state complicates the spectra recorded by resonant, two-step excitation, and a more involved analysis is required to deduce hyperfine structures and isotope shifts. Furthermore, the lifetime broadening of the intermediate state (At, )... 

The second, most frequently used method reduces the linewidth of molecular transitions instead of the density of lines. This is achieved by applying Doppler-free spectroscopic techniques such as saturation spectroscopy, polarization spectroscopy or Doppler-free two photon spectroscopy. Another way of reducing the Doppler-width uses a collimated molecular beam which is crossed perpendicular by a laser beam6. All these methods require single mode tunable lasers to fully utilize the Doppler-free resolution. 

---