This paper will review briefly a recent experiment which measured the photodetachment probability for I-ions due to simultaneous absorption of two ruby quanta. We have calculated the two-quantum photoionization rate for atomic cesium as a function of wavelength. The "transition" from the initial state to the bound intermediate state is expressed in terms of oscillator strengths. The second "transition," from these excited valence states into the coulomb continuum is expressed as a cross section. We have observed what appear to be genuine two-photon ionization signals in three cases: 1) near resonance in the10P_{3/2}state using the second-harmonic ofQ-switched ruby, 2) away from resonances, using the 5300 Å second-harmonic ofQ-switched neodymium laser, and 3) away from resonances using the first anti-Stokes line at 5750 Å from ruby-pumped D2gas. Present experiments aim at scanning the interesting range near resonance, e.g., by thermally tuning the ruby.
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 theory of two-photon ionization of the alkali atoms is developed from the well-known perturbation formula for the second-order transition rate. Quantum defect Coulomb functions are used for both the negative and positive energy states. Spin-orbit splitting of the intermediate
p states is taken into account. The results are presented as dispersion curves of the transition rate for photon energies between the two-photon and one-photon thresholds. Cesium is of particular interest because the photon energy for the second harmonic of ruby laser light falls very near the
10p2P23 level. For a photon energy of 3.57 eV, the transition rate (in cgs units) is found to be 2.6\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}49}$\ifmmode\times\else\texttimes\fi{}
(photonflux)2.
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.
Experimental and theoretical results are presented for resonantly enhanced three-photon ionization of cesium atoms in the wavelength range from 830 to 950 nm. The accidental near resonance of the
6p2P23,21 levels at the one-photon energy and the near resonance at the two-photon level with the
6d2D23,25 states gives rise to large ionization signals and interesting anomalies in the relative intensities of the fine-structure levels. "Hybrid resonances" are observed which involve the production of
6p2P21 atoms from molecular cesium. One hybrid resonance involves ionization of the
6p2P21 state via an electric quadrupole transition ($6p ^{2}P_{\frac{1}{2}}\ensuremath{-}7p ^{2}P_{\frac{3}{2}}$). Both experimental and theoretical photoelectron angular distributions are compared for all of the observed resonances except for the
6p2P23 state where the signals were too weak for such a measurement. Agreement between experiment and theory is fair for three-photon ionization via the
6p2P21 state. Comparison of experiment and theory for the case of the
6d2D23,25 states indicates that hyperfine coupling during the laser-pulse duration may be important. The angular distributions for the hybrid resonances show the general behavior predicted by theory but the features appear broadened.
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.