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Journal of the Optical Society of America B

Two-photon ionization of cold rubidium atoms

Marco Anderlini, Emmanuel Courtade, D. Ciampini, Jörg Helge Müller, O. Morsch, E. Arimondo

Published Mar 1, 2004• 12 citations
Publisher

Abstract

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.

DOI
10.1364/josab.21.000480

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Physical Review A•May 20, 2004•10 citations
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.
By means of a simple theoretical model and experimental results, we analyse the two-photon ionization of cold rubidium atoms. We consider a two-step process in which ground state atoms are ionized by two blue (421 nm) photons, chosen to be quasi-resonant with the 5S → 6P transition. We show that for a good description of the process we have to take into account not only the three states coupled by the laser radiation, but also all the atomic states involved in the spontaneous emission cascade from the 6P to the 5S state, since optical pumping from 5S 1/2 ( F = 2) to the 5S 1/2 ( F = 1) states modifies the ionization efficiency when ionizing near resonance. The experimental and theoretical determination of ionization cross sections for excitations near resonance with the 6P 1/2 and the 6P 3/2 intermediate levels is presented.
We report the measurement of the photoionization cross section for the 5S1/2{5S}_{1/2}5S1/2​ state of rubidium, using atoms confined in a magneto-optical trap. A single-photon rate at $\ensuremath{\lambda}=266\mathrm{nm}$ was found by monitoring the decay of trap fluorescence after exposure to ionizing radiation from a quadrupled Nd:YVO4{\mathrm{N}\mathrm{d}:\mathrm{Y}\mathrm{V}\mathrm{O}}_{4}Nd:YVO4​ laser. In order to eliminate excited-state ionization, the photoionization and trapping lasers were alternately chopped, so that only ground-state atoms were ionized. We determine that the photoionization cross section at $\ensuremath{\lambda}=266\mathrm{nm}$ is $\ensuremath{\sigma}=1.7(2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}20}{\mathrm{cm}}^{2}.$
Photoionization of a cold atomic sample offers intriguing possibilities for observing collective effects at extremely low temperatures. Irradiation of a rubidium condensate and of cold rubidium atoms within a magneto-optical trap (MOT) with laser pulses ionizing through one-photon and two-photon absorption processes was performed. Losses and modifications in the density profile of the remaining trapped cold cloud or the remaining condensate sample were examined as functions of the ionizing laser parameters. Ionization cross sections were measured for atoms in a MOT, while in magnetic traps losses larger than those expected for ionization process were measured.
We describe a new technique for accurate measurement of absolute photoionization cross sections. By measuring the loss rateof atoms from a laser trap in the presence of ionizing light, we directly measure the photoionization rate. The only quantities requiring absolute calibration are the ionizing laser intensity and the fractional population in the relevant state. Our technique is capable of detecting extremely small ionization rates, which means that low-power cw sources can be used. We have applied this method to photoionization from the 5P(3/2) state of rubidium at wavelengths of 413 and 407 nm.The cross sections are 1.36(12) x 10(-17) and 1.25(11) x 10(-17) cm(2) , respectively.
The authors have used laser cooling and trapping techniques to investigate photoionization and cold collisions. With laser-trapped Rb, they have measured the photoionization cross section from the first excited (5P) level by observing the photoionization-induced loss rate of neutral atoms from the trap. This technique has the advantage that it directly measures the photoionization rate per atom. Knowing the ionizing laser intensity and the excited-state fraction, the measured loss rate gives the absolute cross section. Using this technique, the Rb 5P photoionization cross section at {approximately}400 nm has been determined with an uncertainty of 9%. The authors are currently attempting to extend this method to the 5D level. Using time-ordered pulses of diode-laser light (similar to the STIRAP technique), they have performed very efficient two-photon excitation of trapped Rb atoms to 5D. Finally, they will present results from a recent collaboration which combines measurements form conventional molecular spectroscopy (single photon and double resonance) with photoassociation collisions of ultracold Na atoms to yield a precise ({le}1 ppm) value for the dissociation energy of the X {Sigma}{sub g}{sub +} ground state of the Na{sub 2} molecule.
We observe destructive interference in the ground-state Rb two-photon ionization cross section when the single photon energy is tuned between the $5S\ensuremath{\rightarrow}5P$ and $5S\ensuremath{\rightarrow}6P$ transition energies. The minimum cross section is $5.9(1.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}52}\phantom{\rule{0.3em}{0ex}}{\text{cm}}^{4}\phantom{\rule{0.3em}{0ex}}\mathrm{s}$ and it occurs at a wavelength of 441.0(3)nm441.0(3)\phantom{\rule{0.3em}{0ex}}\text{nm}441.0(3)nm (in vacuo). Relative measurements of these cross sections are made at various wavelengths by counting ions produced when magneto-optically trapped Rb atoms are exposed to light from a tunable pulsed laser. This relative curve is calibrated to an absolute cross-section measurement at 532nm532\phantom{\rule{0.3em}{0ex}}\text{nm}532nm using the trap loss method. A simple calculation agrees reasonably with our results.