Doppler-free two-photon absorption experiments have been preliminarily carried out to investigate the highly excited D states of Rb atom using a cw tunable dye laser.The two-photon absorption signals with big signal to noise ratios have been obtained by means of a space-charge limited ionization detector equipped inside a Rb cell.The sharp slope of the first derivative curve of the absorption signal shows the entire possibility of frequency stabilization of a dye laser in terms of ultra-violet wavelength references via two-photon absorption technique.The obtained experimental results are compared with the results which have been performed so far by other researchers, and quantitative discussions are made.
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.
Doppler-free two-photon transitions to Rydberg states in alkali metals have been recommended as a potential source of reference lines or secondary standards for wavelength calibration, because they cover a wide range of the visible spectrum.1 Observation of spectral lines for the two-photon transitions was made possible by use of a thermionic diode detector cell similar to that originally designed by Harvey.2 The Pyrex cell enclosing the diode detector and containing natural Rb was placed in a two-chamber oven so that the temperature of the cell body and that of the sidearm Rb supply could be controlled independently.