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arXiv:1601.01119 · Physical Review Letters

Photodissociation of Trapped Rb2+ : Implications for Simultaneous Trapping of Atoms and Molecular Ions

S. Jyothi, Tridib Ray, Sourav Dutta, Abdul-Rahman Allouche, Romain Véxiau, Olivier Dulieu, S. A. Rangwala

Published Nov 18, 2016• 34 citations
arXivHTMLPDF

Abstract

The direct photodissociation of trapped ^{85}Rb_{2}^{+} (rubidium) molecular ions by the cooling light for the ^{85}Rb magneto-optical trap (MOT) is studied, both experimentally and theoretically. Vibrationally excited Rb_{2}^{+} ions are created by photoionization of Rb_{2} molecules formed photoassociatively in the Rb MOT and are trapped in a modified spherical Paul trap. The decay rate of the trapped Rb_{2}^{+} ion signal in the presence of the MOT cooling light is measured and agreement with our calculated rates for molecular ion photodissociation is observed. The photodissociation mechanism due to the MOT light is expected to be active and therefore universal for all homonuclear diatomic alkali metal molecular ions.

DOI
10.1103/physrevlett.117.213002

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Bulletin of the American Physical Society•May 1, 1996•0 citations
The direct photodissociation of trapped ^{85}Rb_{2}^{+} (rubidium) molecular ions by the cooling light for the ^{85}Rb magneto-optical trap (MOT) is studied, both experimentally and theoretically. Vibrationally excited Rb_{2}^{+} ions are created by photoionization of Rb_{2} molecules formed photoassociatively in the Rb MOT and are trapped in a modified spherical Paul trap. The decay rate of the trapped Rb_{2}^{+} ion signal in the presence of the MOT cooling light is measured and agreement with our calculated rates for molecular ion photodissociation is observed. The photodissociation mechanism due to the MOT light is expected to be active and therefore universal for all homonuclear diatomic alkali metal molecular ions.
We have investigated trap loss in a dual-species magneto-optical trap (MOT) comprised of 85Rb^{85}\mathrm{Rb}85Rb and metastable 40Ar^{40}\mathrm{Ar}40Ar. We measure the trap loss rate coefficients for each species due to the presence of the other as a function of trap light intensity. We clearly identify both Penning ionization of Rb by Ar∗{\mathrm{Ar}}^{*}Ar∗ and associative ionization to form the molecular ion RbAr+\mathrm{Rb}{\mathrm{Ar}}^{+}RbAr+ as two of the trap loss channels. We have also measured the trap loss rate coefficient for the Ar∗{\mathrm{Ar}}^{*}Ar∗ MOT alone and observe production of Ar+{\mathrm{Ar}}^{+}Ar+ and Ar2+\mathrm{Ar}_{2}{}^{+}Ar2​+ ions.
This dissertation presents the findings of two experimental investigations in ultracold atomic and molecular physics: The study of the dual species trap loss in a rubidium - metastable argon magneto-optical trap and the photoassociative spectroscopy of ultracold metastable argon. The interspecies trap loss rate coefficients have been measured for ultracold collisions between 85Rb and 40Ar* in a dual-species magneto-optical trap (MOT) and the two rates have been found to be approximately equal over the range of intensities studied with values of β'Rb–Ar* = 3.0 ± 1.3 × 10-11 cm3/s and β'Ar*–Rb = 1.9 ± 0.9 × 10-11 cm3/s where β' Rb–Ar* is the trap loss coefficient for Rb in the presence of Ar* and β'Ar*–Rb is the reciprocal term. In addition, the trap loss rate coefficient for cold collisions in a metastable argon MOT alone have been measured with an average value of β Ar* = 5.2 ± 1.6 × 10-10 cm3/s. Using a quadrupole mass spectrometer, the production of Ar +, Ar+2, Rb+, and RbAr+ ions in the dual MOT have been observed, clearly identifying heteronuclear Penning and associative ionization as trap loss mechanisms. In the second experiment, the first ever investigation of the photoassociative (PA) spectroscopy of Ar* has been made. The exploratory study focuses on PA spectroscopy near the 4s[3/2]2 + 4p[5/2]3 asymptote of the Ar2* diatomic molecule over a 10 GHz range red detuned of the atomic resonance. With a range of probe laser intensities from ∼ 102–105ISat, 12 resonances have been observed. The spectra have been analyzed using the near-dissociation LeRoy-Bernstein method. Through this analysis, the spectra seem to best correlate with excitations to the 5g state.
We measure the collision-rate coefficient between laser-cooled rubidium (Rb) atoms in a magneto-optical trap (MOT) and optically dark Rb+{}^{+}+ ions in an overlapping Paul trap. In such a mixture, the ions are created from the MOT atoms and allowed to accumulate in the ion trap, which results in a significant reduction in the number of steady-state MOT atoms. A theoretical rate-equation model is developed to describe the evolution of the MOT atom number due to ionization and ion-atom collisions, and we derive an expression for the ion-atom collision-rate coefficient. The loss of MOT atoms is studied systematically by sequentially switching on the various mechanisms in the experiment. Combining the measurements with the model allows the direct determination of the ion-atom collision-rate coefficient. Finally, the scope of the experimental technique developed here is discussed.
Abstract We have studied the formation of the molecular ion Rb2+ and the atomic ion Rb+. These are created in laser excited rubidium vapor at the first resonance, 5s–5p and 5p-nl transitions. A theoretical model is applied to this interaction to explain the time evolution and the laser-power dependence of the population density of Rb+ and Rb2+. A set of rate equations which describe: the temporal variation of the population density of the excited states; the atomic ion density; and the electron density, were solved numerically under the experimental conditions of Barbier and Cheret. In their experiment the Rb concentration was 1×1013cm−3 and the laser power was taken to be 50–500 mW at vapor temperature = 450 K. The results showed that the main processes for producing Rb2+ are associative ionization and Hornbeck-Molnar ionization. The calculations have also showed that, the atomic ions Rb+ are formed through the Penning Ionization (PI) and photoionization processes. Moreover, a reasonable agreement between the experimental results and our calculations for the ion currents of the Rb+ and Rb2+ is obtained.
In this paper, we report the measurement of molecular formation rate constant of Rb2{\mathrm{Rb}}_{2}Rb2​ directly in a magneto-optical trap (MOT). The ground state molecules are detected by two-photon ionization, resonantly enhanced through the intermediate ${a}^{3}{\ensuremath{\Sigma}}_{u}^{+}\ensuremath{\rightarrow}{2}^{3}{\ensuremath{\Pi}}_{g}$ molecular band. We have measured the rate constant as a function of atomic density to conclude that the molecules Rb2{\mathrm{Rb}}_{2}Rb2​ are formed in the MOT by a photoassociation process caused by the trapping laser beams. We also measured the rate constant as a function of the trapping laser intensity. The results here presented are of importance to future experiments involving trapping of cold molecules.
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.