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Physical Review A

Measurement of the Rb(5D5/2) photoionization cross section using trapped atoms

B. C. Duncan, V. Sánchez-Villicaña, P. L. Gould, H. R. Sadeghpour

Published Mar 15, 2001• 37 citations
Publisher

Abstract

We report on measurements of the cross section for photoionization of Rb atoms in the highly excited ${5D}_{5/2}$ level at wavelengths ranging from 1064 to 532 nm (photoelectron energies of 0.175 to 1.34 eV). We efficiently populate the ${5D}_{5/2}$ level using coherent two-photon excitation of trapped atoms with pulses in the counterintuitive order. The absolute photoionization cross sections are then measured via the increased loss of atoms from the magneto-optical trap when it is illuminated by the photoionizing light. Our results are in good agreement with new calculations based on the valence-electron parametric potential method.

DOI
10.1103/physreva.63.043411

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We report on measurements of the cross section for photoionization of Rb atoms in the highly excited 5D5/2{5D}_{5/2}5D5/2​ level at wavelengths ranging from 1064 to 532 nm (photoelectron energies of 0.175 to 1.34 eV). We efficiently populate the 5D5/2{5D}_{5/2}5D5/2​ level using coherent two-photon excitation of trapped atoms with pulses in the counterintuitive order. The absolute photoionization cross sections are then measured via the increased loss of atoms from the magneto-optical trap when it is illuminated by the photoionizing light. Our results are in good agreement with new calculations based on the valence-electron parametric potential method.
We report the measurement of the absolute photoionization cross section for the 5P 3/2 state of 87 Rb at wavelength of 473 nm, which results in the photoelectron energies of 33 meV above the ionization threshold, using cold atoms confined in a vapor-loaded magneto-optical trap. The 87 Rb 5P 3/2 photoionization cross section at 473 nm is determined to be σ PI = 10.5 ± 2.2 Mb. Considering the spatial distribution of the trapped atoms, the average intensity of the ionization laser seen by an atom in the MOT instead of ionizing laser intensity I PI is used in our calculations for the photoionization cross sections. The excited state fraction is also accurately estimated using the latest experimental result.
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.
A series of experiments is performed on cold 85Rb atoms confined in a magneto-optical trap. A variation of Stimulated Raman adiabatic passage (STIRAP) is used to transfer population from the 5S 1/2 ground state to the 5D5/2 state with an efficiency exceeding 80%. This process relies on moderately intense pulses arriving in the counterintuitive order, i.e., the upper transition (5P → 5D) driven before the lower transition (5S → 5P ). There is reasonable agreement with computer simulations, excepting a marked discrepancy between measured intensities and those used in the simulations. With a large ensemble of 5D atoms available, the 5D photoionization cross section at four different laser wavelengths (1064, 788, 647 and 532 nm) is determined via trap loss measurements. The theoretical calculations agree well with the experimental results (17.7, 10.9, 7.6 and 5.2 Mb, respectively). An experiment to measure the 5S-5D collisional rate constant sets an order-of-magnitude upper bound of 10−10 cm3/s, assuming all atoms are 5S-5D pairs. ^
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.
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.