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Physical Review A

Absolute Rb one-color two-photon ionization cross-section measurement near a quantum interference

T. Takekoshi, George M. Brooke, Brian M. Patterson, R. J. Knize

Published May 20, 2004• 10 citations
Publisher

Abstract

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 $441.0(3)\phantom{\rule{0.3em}{0ex}}\text{nm}$ (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 $532\phantom{\rule{0.3em}{0ex}}\text{nm}$ using the trap loss method. A simple calculation agrees reasonably with our results.

DOI
10.1103/physreva.69.053411

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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→5P5S\ensuremath{\rightarrow}5P5S\ensuremath→5P and 5S\ensuremath→6P5S\ensuremath{\rightarrow}6P5S\ensuremath→6P transition energies. The minimum cross section is 5.9(1.5)\ifmmode×\else\texttimes\fi10\ensuremath−52cm4s5.9(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}5.9(1.5)\ifmmode×\else\texttimes\fi10\ensuremath−52cm4s 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.
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.
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.
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 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.
Stimulated by a recent measurement of coherent control in photoionization of atomic barium, we have calculated one- and two-photon ionization cross sections of the aligned 6s6p 1P16s6p\text{ }{^{1}P}_{1}6s6p 1P1​ state of barium in the energy range between the 5d3/25{d}_{3/2}5d3/2​ and 5d5/25{d}_{5/2}5d5/2​ states of Ba+{\text{Ba}}^{+}Ba+. We have also measured these photoionization spectra in the same energy region, driving the one- or two-photon processes with the second or first harmonic of a tunable dye laser, respectively. Our calculations employ the eigenchannel RRR-matrix method and multichannel quantum defect theory to calculate the rich array of autoionizing resonances in this energy range. The nonresonant two-photon process is described using lowest-order perturbation theory for the photon-atom interactions, with a discretized intermediate-state one-electron continuum. The calculations provide an absolute normalization for the experiment, and they accurately reproduce the rich resonance structures in both the one- and two-photon cross sections, and confirm other aspects of the experimental observations. These results demonstrate the ability of these computationally inexpensive methods to reproduce experimental observables in one- and two-photon ionization of heavy alkaline earths, and they lay the groundwork for future studies of the phase-controlled interference between one- and two-photon ionization processes.
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.