Multiphoton ionization of rubidium

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 5²S ground state of atomic rubidium to higher-lying n²D levels for values of n ranging from 9 to 34 and to n ²S levels for values of n from 11 through 20. The fine-structure intervals of the n ²D levels for n=9 toc 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 Rb₂. As a result the dispersion curve for two-photon absorption in rubidium showed what appears to be resonant intermediate pπ(³Σ_u1) and 0_g⁺ terms dissociating to give a 5P_{3 / 2} atom and resulting in the strong development of features corresponding to the 5²P_{3 / 2}→n²D_{3 / 2,5 / 2} part of the diffuse series, for n≤50 in absorption and the 5²P_{3 / 2}→n²S_{1 / 2} part of the sharp series. Components to 32²D and 30²S were recorded to a precision of 0.3 cm^-1 and quantum defects for these previously unobserved terms were derived. The corresponding hybrid two-photon resonances involving intermediate states dissociating to give a 5²P_{1 / 2} atom were not observed in the wavelength interval available in this experiment.