The multiphoton excitation of rubidium has been investigated over the 4600-6500-Å wavelength region with a tunable dye-laser source having a linewidth better than 0.1 Å 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 $5{}_{}{}^2{}_{}{}^{}S$ ground state of atomic rubidium to higher-lying $n{}_{}{}^2{}_{}{}^{}D$ levels for values of $n$ ranging from 9 to 34 and to $n{}_{}{}^2{}_{}{}^{}S$ levels for values of $n$ from 11 through 20. The fine-structure intervals of the $n{}_{}{}^2{}_{}{}^{}D$ levels for $n=9\mathrm{} \mathrm{to}c 13$ 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 ${\mathrm{Rb}}_2$. As a result the dispersion curve for two-photon absorption in rubidium showed what appears to be resonant intermediate $p\pi \left({}_{}{}^3{}_{}{}^{}\Sigma _{u1\mathrm{}}^{}{}_{}{}^{}\right)$ and $0_g^{+}$ terms dissociating to give a $5P_{\frac{3}{2}}$ atom and resulting in the strong development of features corresponding to the $5{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}\to n{}_{}{}^2{}_{}{}^{}D_{\frac{3}{2},\frac{5}{2}}^{}{}_{}{}^{}$ part of the diffuse series, for $n\le 50$ in absorption and the $5{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}\to n{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ part of the sharp series. Components to $32{}_{}{}^2{}_{}{}^{}D$ and $30{}_{}{}^2{}_{}{}^{}S$ were recorded to a precision of 0.3 ${\mathrm{cm}}^{-1\mathrm{}}$ and quantum defects for these previously unobserved terms were derived. The corresponding hybrid two-photon resonances involving intermediate states dissociating to give a $5{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ atom were not observed in the wavelength interval available in this experiment.

• Received 3 May 1976

DOI:https://doi.org/10.1103/PhysRevA.14.1662