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International Symposium on Molecular Spectroscopy (ISMS)

RYDBERG SPECTROSCOPY OF ZEEMAN-DECELERATED BEAMS OF METASTABLE HELIUM MOLECULES

Paul Jansen, Frédéric Merkt, Daniel Sprecher, M. Motsch

Published Jun 19, 2014• 0 citations
Publisher

Abstract

"Having three and four electrons, respectively, He$_2^+$ and He$_2$ represent systems for which highly accurate \textit{ab-initio} calculations might become feasible in the near future\footnote{W.-C. Tung, M. Pavanello, L. Adamowicz, \textit{J. Chem. Phys.} \textbf{136}, 104309 (2012).}. With the goal of performing accurate measurements of the rovibrational energy-level structure of He$_2^+$ by Rydberg spectroscopy of He$_2$ and multichannel quantum-defect theory extrapolation techniques\footnote{D. Sprecher, J. Liu, T. Kr\""ahenmann, M. Sch\""afer, and F. Merkt, \textit{J. Chem. Phys.} \textbf{140}, 064304 (2014).}, we have produced samples of helium molecules in the $a ^3\Sigma_\mathrm{u}^+$ state in supersonic beams with velocities tunable down to 100\,m/s by combining a cryogenic supersonic-beam source with a multistage Zeeman decelerator\footnote{M. Motsch, P. Jansen, J. A. Agner, H. Schmutz, and F. Merkt, \textit{arXiv:1401.7774}.}. The molecules are formed at an initial velocity of 500 m/s by striking a discharge in the pulsed expansion of helium gas from a reservoir kept at a cryogenic temperature of 10 K. Using rotationally-resolved PFI-ZEKE (pulsed-field-ionization zero-kinetic-energy) photoelectron spectroscopy, we have probed the rotational-state distribution of the molecules produced in the discharge and found vibrational levels up to $\nu'' = 2$ and rotational levels up to $N''=21$ to be populated. The molecular beam is coupled to a multistage Zeeman decelerator\footnote{N. Vanhaecke, U. Meier, M. Andrist, B. H. Meier, and F. Merkt, \textit{Phys. Rev. A} \textbf{75}, 031402(R) (2007).} that employs pulsed inhomogeneous magnetic fields to further reduce the beam velocity. By measuring the quantum-state distribution of the decelerated sample using photoelectron and photoionization spectroscopy we observed no rotational or vibrational state-selectivity of the deceleration process, but found that one of the three spin-rotation components of the He$_2$ $a ^3\Sigma_\mathrm{u}^+$ rotational levels is eliminated."

DOI
10.15278/isms.2014.rh06

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We report on the use of multistage Zeeman deceleration to generate beams of He2{\mathrm{He}}_{2}He2​ molecules in the metastable $a\phantom{\rule{0.16em}{0ex}}{}^{3}{\ensuremath{\Sigma}}_{u}^{+}$ state with velocities tunable down to 100 m/s. The metastable molecules are generated by striking a discharge in a supersonic expansion of pure helium gas from a pulsed valve held at cryogenic temperature. The velocity and internal-state distributions of the metastable He2{\mathrm{He}}_{2}He2​ molecules are measured for nozzle temperatures of 300, 77, and 10 K by high-resolution photoelectron and photoionization spectroscopy. The deceleration process does not exhibit any rotational state selectivity with rotational levels up to ${N}^{\ensuremath{'}\ensuremath{'}}=21$ being populated, but eliminates molecules in spin-rotational sublevels with ${J}^{\ensuremath{'}\ensuremath{'}}={N}^{\ensuremath{'}\ensuremath{'}}$ from the beam, where ${J}^{\ensuremath{'}\ensuremath{'}}$ and ${N}^{\ensuremath{'}\ensuremath{'}}$ are the total and the rotational angular momentum quantum number, respectively. The lack of rotational state selectivity is attributed to the fact that the Paschen-Back regime of the Zeeman effect in the rotational levels of He2{\mathrm{He}}_{2}He2​ is already reached at fields of only 0.1 T.
We use a multistage Zeeman decelerator to generate slow beams ($v\ensuremath{\simeq}100\phantom{\rule{0.16em}{0ex}}\mathrm{m}/\mathrm{s}$) of translationally cold, spin-polarized metastable $a\phantom{\rule{0.16em}{0ex}}^{3}\mathrm{\ensuremath{\Sigma}}_{u}^{+}\phantom{\rule{4pt}{0ex}}{\mathrm{He}}_{2}$ molecules and perform a precision measurement of their spin-rotation fine structure. The spin polarization results from the elimination of the high-field-seeking J=NJ=NJ=N spin-rotational component of each rotational level (rotational quantum number NNN) by the Zeeman deceleration process. By repopulating the J=NJ=NJ=N component from the $J=N\ifmmode\pm\else\textpm\fi{}1$ low-field-seeking components using radio-frequency radiation, we measured the spin-rotation fine structure of 13 rovibrational levels with v=0v=0v=0, 1 and N=1–21N=1\text{--}21N=1–21. The low beam velocity and the resulting long interaction times with the radio-frequency radiation were exploited to determine the transition frequencies with a precision of 300 Hz.
A modular multistage Zeeman decelerator consisting of an array of solenoids has been developed to control the velocity of cold, paramagnetic atoms and molecules in supersonic beams. The translational motion these atoms and molecules was manipulated with high precision by exploiting the interaction between their magnetic moments and timedependent inhomogeneous magnetic elds. Deceleration was achieved by pulsing high currents (up to 300 A) through the solenoids of the decelerator with pulse sequences calculated to preserve the phase-space density of the atomic and molecular samples while removing a preset amount of kinetic energy. For the operation of the multistage Zeeman decelerator, solenoids and electronic units were developed to generate strong magneticeld pulses (> 2 T) with rise and fall times of only a few microseconds. An accurate three-dimensional particle-trajectory simulation program has been developed to analyze the deceleration process and optimize its e ciency. This software was thoroughly tested by systematic comparison with experimental results and enabled, in combination with measurements, the complete characterization of the spatial and velocity distributions of the decelerated and trapped atoms for di erent sets of experimental conditions. The program was essential to achieve a complete interpretation of the dynamics of the deceleration process and to devise a strategy to e ciently load the decelerated atoms and molecules into magnetic quadrupole traps. In a rst experiment, hydrogen atoms seeded in a supersonic expansion of Kr have been decelerated from an initial velocity of 435 m/s to nal velocities as low as 107 m/s in a 12-stage Zeeman decelerator. The operation of the decelerator has been analyzed by comparing the results of numerical particle-trajectory simulations with those of independent measurements of the velocity distribution by ion time-ofight mass spectrometry following photoionization of the decelerated atoms. The velocity distribution of the decelerated atom cloud was found to have a half width at half maximum of 25± 12 m/s corresponding to a temperature of ∼ 40 mK. The phase stability of the multistage-Zeeman-deceleration process has been investigated by numerical particle-trajectory simulations and experimental measurements on deuterium atoms in a 24-stage Zeeman decelerator. A one-dimensional analytical model of the phase stability in a multistage Zeeman decelerator was adapted from a one-dimensional model of phase stability in a multistage Stark decelerator and compared with the results of oneand three-dimensional particle-trajectory simulations. The comparison, which included the analysis of the e ects of nite switch-on and switch-o times of the deceleration pulses, revealed that transverse e ects in the decelerator lead to a considerable reduction of the
The helium dimer in its metastable triplet state is a promising candidate to be the first laser-cooled homonuclear molecule. An ultracold gas of He2∗_2^*2∗​ would enable a new generation of precision measurements to test quantum electrodynamics for three- and four-electron molecules through Rydberg spectroscopy. Nearly diagonal Franck-Condon factors are obtained because the electron employed for optical cycling occupies a Rydberg orbital that does not take part in the chemical bond. Three possible laser cooling transitions are identified and the spin-rovibronic energy-level structure of the relevant states as well as electronic transition moments, linestrengths, and lifetimes are determined. The production of He2∗_2^*2∗​ molecules in a supersonic beam is discussed, and a laser slowing scheme to load a magneto-optical trap under such conditions is simulated using a rate equation approach. Various repumping schemes involving one or two upper electronic states are compared to maximize the radiative force. Loss mechanisms such as spin-forbidden transitions, predissociation, and ionization processes are studied and found to not introduce significant challenges for laser cooling and trapping He2∗_2^*2∗​. The sensitivity of the vibrational levels of He2+_2^+2+​ with respect to the static polarizability of atomic helium is determined and its implications for a new quantum pressure standard are discussed.
Cold samples of oxygen molecules in supersonic beams have been decelerated from initial velocities of 390 and 450 m s−1 to final velocities in the range between 150 and 280 m s−1 using a 90-stage Zeeman decelerator. (2 + 1) resonance-enhanced-multiphoton-ionization (REMPI) spectra of the 3sσ g 3Π g (C) two-photon transition of O2 have been recorded to characterize the state selectivity of the deceleration process. The decelerated molecular sample was found to consist exclusively of molecules in the J ′′ = 2 spin–rotational component of the X ground state of O2. Measurements of the REMPI spectra using linearly polarized laser radiation with polarization vector parallel to the decelerator axis, and thus to the magnetic-field vector of the deceleration solenoids, further showed that only the magnetic sublevel of the N′′ = 1, J ′′ = 2 spin–rotational level is populated in the decelerated sample, which therefore is characterized by a fully oriented total-angular-momentum vector. By maintaining a weak quantization magnetic field beyond the decelerator, the polarization of the sample could be maintained over the 5 cm distance separating the last deceleration solenoid and the detection region.