Two-photon ionization cross sections from the rubidium ground state have been measured with both a cw 421-nm laser and a combination of cw 421- and 1002-nm lasers. The measurements were performed within a high-vacuum magneto-optical trap while the trapping lasers were switched off, exploiting the long trap lifetime and also the trap laser confinement. The two-photon cross sections were determined for the blue laser near resonance with the 6P1/2 and 6P3/2 states and compared with the estimates of a theoretical model. In near resonance with the 6P states, large two-photon photoionization cross sections were measured.
We have studied two-photon resonant three-photon ionization of atomic sodium vapor. A dye laser tuned to either the 4d or 5s two-photon allowed transition was used to excite the atom and a second more intense laser (λ = 7000 Å) was used for ionization. The ion yield was measured as a function of resonant laser intensity and detuning, the nonresonant laser intensity, and the temporal overlap of the exciting and ionizing laser pulses. We have found1 that at high laser intensities the photoionization yield decreases as the nonresonant laser intensity is increased. When tuned to the 5s level a value of ~−0.6 was obtained for the exponential index even with the resonant laser tuned to maximize the ion yield. A theoretical model including the effects of Stark shift of the ground state, power broadening of the intermediate level due to saturation of the two-photon transition and by rapid transitions to the ionization continuum was used to predict the experimental results. Good agreement between theory and experiment required the use of a photoionization cross section for the 5s level which was ~20 times greater than the accepted value. Results for photoionization from the 4d level are also presented.
The efficient photoionization of a Bose-Einstein condensate requires the creation of ions with the smallest possible transfer of atoms from the condensate into the thermal phase. The spontaneous decay from excited states into atomic states with momentum different from the initial one should be reduced. We investigate theoretically the two-photon ionization of a rubidium condensate using near resonant excitation to the 6P state and second photon at 421 nm or 1002 nm into the continuum. Different ionization schemes with coherent control of the first excitation and reduction of the spontaneous decay are presented.
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.
We present non-perturbative ab initio calculations of resonant two-photon ionization in which the ground state of a complex atom is resonantly coupled by a laser field to two Rydberg states, each of which belongs to a different Rydberg series. As an example, we have studied the resonant two-photon ionization of neon via the 5s and 4d Rydberg states using the R-matrix-Floquet theory. We find that for weak fields, where the Rydberg states are not overlapping, interference effects are already present which lead to a suppression of ionization for a particular laser frequency. For strong fields, the field induced coupling between the two Rydberg states can strongly modify the weak-field behaviour. We give a qualitative explanation of these phenomena by considering the dressed states of a simple three-level model atom coupled to two continua. Finally we show, by varying the intensity, how the Floquet eigenstates corresponding to the ground state and the two Rydberg states of neon are adiabatically connected to the field-free states. © 1995 IOP Publishing Ltd.