Examples of Doppler-Free Two-Photon Spectroscopy

The first experiments on Doppler-free two-photon spectroscopy were performed on the alkali atoms [245-249] because their two-photon transitions can be induced by 

The isotope shift between different stable lead isotopes shown in Fig. 2.36 has been determined by Doppler-free two-photon absorption of a cw dye laser beam at X = 450.4 nm on the transition 6p Po lp Po [250] using the experimental arrangement of Fig. 2.32. 

Doppler-free two-photon transitions to atomic Rydberg levels [251] allow the accurate determination of quantum defects and of level shifts in external fields. Hyperfine structures in Rydberg states of two-electron atoms, such as calcium and singlet-triplet mixing of the valence state 45 and and Rydberg levels have been thoroughly studied by Doppler-free two-photon spectroscopy [252]. 

The application of two-photon spectroscopy to molecules has brought a wealth of new insight to excited molecular states. One example is the two-photon excitation of CO in the fourth positive system A TJ Ug and of N2 in the Lyman-Birge-Hopfield system with a narrow-band pulsed frequency-doubled dye laser. Doppler-free spectra of states with excitation energies between 8-12 eV can be measured with this technique [253]. 

For larger molecules, rotationally resolved absorption spectra could, for the first time, be measured, as has been demonstrated for the UV spectra of benzene C6H6. Spectral features, which had been regarded as true continua in former times, could now be completely resolved (Fig. 2.37) and turned out to be dense but discrete rotational-line spectra [254, 255]. The lifetimes of the upper levels could be determined from the natural linewidths of these transitions [256]. It was proven that 

The first experiments on Doppler-free two-photon spectroscopy were performed on the alkali-atoms [7.38-42] because their two-photon transitions can be induced by CW dye lasers or diode lasers in convenient spectral ranges. Furthermore, the first excited P state is not too far away from the virtual level in Fig.7.26b. This enlarges the two-photon transition probabilities for such near-resonant transitions. Meanwhile there are numerous further applications of this sub-Doppler technique in atomic and molecular physics. We shall illustrate them by a few examples only. 

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