We used a tunable cw dye laser to observe Doppler-free two-photon transitions to alkali-atom Rydberg levels. Real-time signals were obtained in an ionization cell of appropriate construction. The two-photon transition wavelengths to rubidium s levels were measured up to n = 50 with better than 1 x 10(-7) absolute accuracy and can be represented to within the experimental precision by a simple function of the principal quantum number, Other related transitions and future possibilities are considered.
Doppler-free two-photon absorption spectroscopy was applied to study highly-excited D states of Rb atoms. Using a cw dye laser as an excitation source, transitions from the ground 5S state to nd 2D states (n=11-32) were observed, the ionization energy of the highest state being only 117cm-1. These D doublets are not inverted. The fine structure splittings of these states do not vary as (neff) -3, nor as n-3. The isotope shift of the 85Rb, 87Rb ground states is found to be 160±12MHz. No pressure broadening was observed for these transitions within the instrumental resolution of 15 MHz.
The two-photon excitation and three-photon ionization of atomic cesium is investigated over the 6550-6950-\AA{} wavelength region with a tunable-dye-laser source having a 0.06-0.08-\AA{} linewidth and a space-charge ionization detector sensitive to a few ions per second. An online data-acquisition computer provides significant signal-to-noise recovery. The resulting dispersion curve for photoionization is interpreted in terms of the two-photon transitions from the ground
62S21 level to resonant
n2D and
n2S intermediate states, and represents, to the authors' knowledge, the first such two-photon absorption spectrum of Cs. Transitions from
n=9to13 have been recorded for the $6^{2}S_{\frac{1}{2}}\ensuremath{-}n^{2}D_{\frac{5}{2},\frac{3}{2}}$ series and from
n=11to14 for the $6^{2}S_{\frac{1}{2}}\ensuremath{-}n^{2}S_{\frac{1}{2}}$ series.
The multiphoton excitation of rubidium has been investigated over the 4600-6500-\AA{} wavelength region with a tunable dye-laser source having a linewidth better than 0.1 \AA{} and a space-charge ionization detector sensitive to a few ions per second. Multiphoton transitions have been observed to occur both through intermediate atomic states and through intermediate continuum states of the rubidium molecule. In the former case two-photon transitions have been observed from the
52S ground state of atomic rubidium to higher-lying
n2D levels for values of
n ranging from 9 to 34 and to
n2S levels for values of
n from 11 through 20. The fine-structure intervals of the
n2D levels for
n=9toc13 were measured together with the line-strength ratios and were found to be in good agreement with the predictions of a simple theoretical model. At the shorter wavelengths hybrid two-photon resonances were observed to be excited through resonant intermediate continuum states of
Rb2. As a result the dispersion curve for two-photon absorption in rubidium showed what appears to be resonant intermediate $p\ensuremath{\pi}(^{3}\ensuremath{\Sigma}_{u1})$ and
0g+ terms dissociating to give a
5P23 atom and resulting in the strong development of features corresponding to the $5^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}n^{2}D_{\frac{3}{2},\frac{5}{2}}$ part of the diffuse series, for $n\ensuremath{\le}50$ in absorption and the $5^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}n^{2}S_{\frac{1}{2}}$ part of the sharp series. Components to
322D and
302S were recorded to a precision of 0.3 ${\mathrm{cm}}^{\ensuremath{-}1}$ and quantum defects for these previously unobserved terms were derived. The corresponding hybrid two-photon resonances involving intermediate states dissociating to give a
52P21 atom were not observed in the wavelength interval available in this experiment.