Suppression of Penning ionization by orbital angular momentum conservation

Katrin Dulitz, Tobias Sixt, Jiwen Guan, Jonas Grzesiak, Markus Debatin, and Frank Stienkemeier

Phys. Rev. A 102, 022818 – Published 27 August 2020

Abstract

The efficient suppression of autoionizing collisions is a stringent requirement to achieve quantum degeneracy in metastable rare gases. Here, we report on the suppression of Penning ionization in collisions between metastable He and laser-excited Li atoms. Our results show that the suppression is achieved by the conservation of both the total electron spin and $Λ らむだ$ , i.e., the projection of the total molecular orbital angular momentum along the internuclear axis. Our findings suggest that $Λ らむだ$ conservation can be used as a more general means of reaction control, in particular, to improve schemes for the simultaneous laser cooling and trapping of metastable He and alkali-metal atoms.

Received 4 September 2019
Revised 7 May 2020
Accepted 23 July 2020

DOI:https://doi.org/10.1103/PhysRevA.102.022818

Physics Subject Headings (PhySH)

Chemical reactions Interatomic & molecular potentials

Atomic, Molecular & Optical

Authors & Affiliations

Katrin Dulitz ^*, Tobias Sixt, Jiwen Guan, Jonas Grzesiak, Markus Debatin , and Frank Stienkemeier

Institute of Physics, University of Freiburg, Hermann-Herder-Strasse 3, 79104 Freiburg, Germany

^*katrin.dulitz@physik.uni-freiburg.de

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Vol. 102, Iss. 2 — August 2020

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Images

Figure 1
Schematic drawing of the experimental setup (not to scale). FC: Faraday cup, ion-TOF: ion-time-of-flight detector.
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Figure 2
Measured ion yields for electronic-state-selected ${He}^{*}$ -Li collisions (obtained using method 1) at $δ でるた$ = $- 12$ MHz as a function of time delay with respect to the valve trigger (thick lines). In the legend, ${He}^{s}$ and ${He}^{t}$ are abbreviations for the He( $2^{1} S_{0}$ ) and He( $2^{3} S_{1}$ ) states, respectively. Li(S) and Li(S, P) denote Li( $2^{2} S_{1 / 2}$ ) and a mixture of Li( $2^{2} S_{1 / 2}$ ) and Li( $2^{2} P_{3 / 2}$ ), respectively. The thick dash-dotted blue and dashed green curves are scaled with respect to the singlet-to-triplet ratio in the ${He}^{*}$ beam to aid the comparison between relative signal intensities. The integration time window used for the calculation of rate coefficients is indicated by the gray shading. Traces of MOT laser stray light (thin gray lines), recorded using a fast photodiode, illustrate the shutter closing characteristics for the two shutter time delays at which the traces were taken.
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Figure 3
Experimentally measured rate coefficient ratios $k_{i}^{\exp} / k_{1}^{\exp}$ (where $i$ = 3, 4, 6; see Table 1 for notation), obtained using method 1, as a function of Li( $2^{2} P_{3 / 2}$ ) population relative to Li( $2^{2} S_{1 / 2}$ ) (markers). The solid lines and the shadings are weighted means of the rate coefficient ratios and the resultant standard deviations, respectively. The uncertainties are statistical only ( $2 σ しぐま$ ). The ratios $k_{i, S, Λ らむだ} / k_{1, S, Λ らむだ}$ , which are calculated from the ratio of autoionizing states to the total number of states assuming electron-spin and $Λ らむだ$ conservation, are shown as dashed lines for comparison.
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Figure 4
Measured ion yields for state-selected ${He}^{*}$ -Li collisions as a function of time delay with respect to the valve trigger (thick lines). In contrast to Fig. 2, Li was optically excited to the $2^{2} P_{1 / 2}$ state using method 2. Therefore, the label “Li( $S, P$ )” in the legend denotes a mixture of Li( $2^{2} S_{1 / 2}$ ) and Li( $2^{2} P_{1 / 2}$ ). The timing characteristics of the laser light for Li optical excitation, measured using a fast photodiode, are illustrated by a thin gray line.
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Figure 5
Proposed coupling scheme for He( $2^{3} S_{1}, 2^{1} S_{0}$ ) $+$ Li( $2^{2} S_{1 / 2}, 2^{2} P_{1 / 2, 3 / 2}$ ). Correlations which lead to quartet states of the molecule are indicated as dotted lines. The dashed and solid lines represent correlations to molecular states of $^{2} Π ぱい$ and $^{2} {Σ しぐま}^{+}$ symmetry, respectively. In our model, only states of $^{2} {Σ しぐま}^{+}$ symmetry lead to Penning ionization. Correlations shown in red color are tentative only.
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Figure 6
Effective long-range potentials (solid lines with markers) for all He( $2^{3} S_{1}, 2^{1} S_{0}$ )-Li( $2^{2} S_{1 / 2}, 2^{2} P_{J}$ ) channels ( $J = 1 / 2, 3 / 2$ ), in which the maximum of the barrier is equal to the collision energy. The legend labels denote the following states: a: [He( $2^{3} S_{1}$ )-Li( $2^{2} P_{J}$ ) $^{2, 4} Π ぱい$ ], b: [He( $2^{3} S_{1}$ )-Li( $2^{2} S_{1 / 2}$ ) $^{2, 4} Σ しぐま$ ], c: [He( $2^{1} S_{0}$ )-Li( $2^{2} S_{1 / 2}$ ) $^{2} Σ しぐま$ ], d: [He( $2^{1} S_{0}$ )-Li( $2^{2} P_{J}$ ) $^{2} Π ぱい$ ], e: [He( $2^{3} S_{1}$ )-Li( $2^{2} P_{J}$ ) $^{2, 4} Σ しぐま$ ], f: [He( $2^{1} S_{0}$ )-Li( $2^{2} P_{J}$ ) $^{2} Σ しぐま$ ]. In addition to that, the interaction potential $\sum_{n} - C_{n, N} / R^{n}$ and the centrifugal potential ${(μ みゅー v_{rel} b_{N, \max})}^{2} / (2 μ みゅー R^{2})$ for system f are also shown as dashed and dash-dotted lines, respectively. The collision energy in the experiment is indicated by a dotted horizontal line.
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