36 research outputs found

    Slow and velocity-tunable beams of metastable He2_2 by multistage Zeeman deceleration

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    Metastable helium molecules (He2_2^*) have been generated by striking a discharge in a supersonic expansion of helium gas from a pulsed valve. When operating the pulsed valve at room temperature, 77K, and 10K, the mean velocity of the supersonic beam was measured to be 1900m/s, 980m/s, and 530m/s, respectively. A 55-stage Zeeman decelerator operated in a phase-stable manner was then used to further reduce the beam velocity and tune it in the range between 100 and 150m/s. The internal-state distribution of the decelerated sample was established by photoionization spectroscopy.Comment: 10 pages, 7 figure

    Fluorescence-lifetime-limited trapping of Rydberg helium atoms on a chip

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    Metastable (1s)(2s) 3S1^3{\rm S}_1 helium atoms produced in a supersonic beam were excited to Rydberg-Stark states (with nn in the 273027-30 range) in a cryogenic environment and subsequently decelerated by, and trapped above, a surface-electrode decelerator. In the trapping experiments, the Rydberg atoms were brought to rest in 75~μ\mus and over a distance of 33~mm and kept stationary for times ttrapt_{\mathrm{trap}} in the 05250-525~μ\mus range, before being re-accelerated for detection by pulsed field ionization. The use of a home-built valve producing short gas pulses with a duration of about 20~μ\mus enabled the reduction of losses arising from collisions with atoms in the trailing part of the gas pulses. Cooling the decelerator to 4.7~K further suppressed losses by transitions induced by blackbody radiation and by collisions with atoms desorbing from the decelerator surface. The main contribution (60\%) to the atom loss during deceleration is attributed to the escape out of the decelerator moving traps of atoms having energies higher than the trap saddle point, spontaneous emission and collisions with atoms in the trailing part of the gas pulses causing each only about 20\% of the atom loss. At 4.7 K, the atom losses in the trapping phase of the experiments were found to be almost exclusively caused by spontaneous emission and the trap lifetimes were found to correspond to the natural lifetimes of the Rydberg-Stark states. Increasing the temperature to 100 K enhanced the trap losses by transitions stimulated by blackbody radiation

    Rydberg states of helium in electric and magnetic fields of arbitrary relative orientation

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    A spectroscopic study of Rydberg states of helium (nn = 30 and 45) in magnetic, electric and combined magnetic and electric fields with arbitrary relative orientations of the field vectors is presented. The emphasis is on two special cases where (i) the diamagnetic term is negligible and both paramagnetic Zeeman and Stark effects are linear (nn = 30, BB \leq 120 mT and FF = 0 - 78 V/cm ), and (ii) the diamagnetic term is dominant and the Stark effect is linear (nn = 45, BB = 277 mT and FF = 0 - 8 V/cm). Both cases correspond to regimes where the interactions induced by the electric and magnetic fields are much weaker than the Coulomb interaction, but much stronger than the spin-orbit interaction. The experimental spectra are compared to spectra calculated by determining the eigenvalues of the Hamiltonian matrix describing helium Rydberg states in the external fields. The spectra and the calculated energy-level diagrams in external fields reveal avoided crossings between levels of different mlm_l values and pronounced mlm_l-mixing effects at all angles between the electric and magnetic field vectors other than 0. These observations are discussed in the context of the development of a method to generate dense samples of cold atoms and molecules in a magnetic trap following Rydberg-Stark deceleration.Comment: 16 pages, 18 figure

    Metrology of Rydberg states of the hydrogen atom

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    We present a method to precisly measure the frequencies of transitions to high-nn Rydberg states of the hydrogen atom which are not subject to uncontrolled systematic shifts caused by stray electric fields. The method consists in recording Stark spectra of the field-insensitive k=0k=0 Stark states and the field-sensitive k=±2k=\pm2 Stark states, which are used to calibrate the electric field strength. We illustrate this method with measurements of transitions from the 2s(f=0 and 1)2\,\text{s}(f=0\text{ and } 1) hyperfine levels in the presence of intentionally applied electric fields with strengths in the range between 0.40.4 and 1.61.6\,Vcm1^{-1}. The slightly field-dependent k=0k=0 level energies are corrected with a precisely calculated shift to obtain the corresponding Bohr energies (cRH/n2)\left(-cR_{\mathrm{H}}/n^2\right). The energy difference between n=20n=20 and n=24n=24 obtained with our method agrees with Bohr's formula within the 1010\,kHz experimental uncertainty. We also determined the hyperfine splitting of the 2s2\,\text{s} state by taking the difference between transition frequencies from the 2s(f=0 and 1)2\,\text{s}(f=0 \text{ and }1) levels to the n=20,k=0n=20,k=0 Stark states. Our results demonstrate the possibility of carrying out precision measurements in high-nn hydrogenic quantum states

    Imaging-assisted single-photon Doppler-free laser spectroscopy and the ionization energy of metastable triplet helium

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    Skimmed supersonic beams provide intense, cold, collision-free samples of atoms and molecules are one of the most widely used tools in atomic and molecular laser spectroscopy. High-resolution optical spectra are typically recorded in a perpendicular arrangement of laser and supersonic beams to minimize Doppler broadening. Typical Doppler widths are nevertheless limited to tens of MHz by the residual transverse-velocity distribution in the gas-expansion cones. We present an imaging method to overcome this limitation which exploits the correlation between the positions of the atoms and molecules in the supersonic expansion and their transverse velocities - and thus their Doppler shifts. With the example of spectra of (1\mathrm{s})(n\mathrm{p})\,^3\mathrm{P}_{0-2}\leftarrow (1\mathrm{s})(2\mathrm{s})\,^3\mathrm{S}_1 transitions to high Rydberg states of metastable triplet He, we demonstrate the suppression of the residual Doppler broadening and a reduction of the full linewidths at half maximum to only about 1 MHz in the UV. Using a retro-reflection arrangement for the laser beam and a cross-correlation method, we determine Doppler-free spectra without any signal loss from the selection, by imaging, of atoms within ultranarrow transverse-velocity classes. As an illustration, we determine the ionization energy of triplet metastable He and confirm the significant discrepancy between recent experimental (Clausen et al., Phys. Rev. Lett. 127 093001 (2021)) and high-level theoretical (Patk\'os et al., Phys. Rev. A 103 042809 (2021)) values of this quantity

    Manipulating Rydberg atoms close to surfaces at cryogenic temperatures

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    Helium atoms in Rydberg states have been manipulated coherently with microwave radiation pulses near a gold surface and near a superconducting NbTiN surface at a temperature of 3K3 \text{K}. The experiments were carried out with a skimmed supersonic beam of metastable (1s)1(2s)11S0(1\text{s})^1(2\text{s})^1\, {}^1\text{S}_0 helium atoms excited with laser radiation to npn\text{p} Rydberg levels with principal quantum number nn between 3030 and 4040. The separation between the cold surface and the center of the collimated beam is adjustable down to 250μm250 \mu\text{m}. Short-lived npn\text{p} Rydberg levels were coherently transferred to the long-lived nsn\text{s} state to avoid radiative decay of the Rydberg atoms between the photoexcitation region and the region above the cold surfaces. Further coherent manipulation of the nsn\text{s} Rydberg levels with pulsed microwave radiation above the surfaces enabled measurements of stray electric fields and allowed us to study the decoherence of the atomic ensemble. Adsorption of residual gas onto the surfaces and the resulting slow build-up of stray fields was minimized by controlling the temperature of the surface and monitoring the partial pressures of H2_2O, N2_2, O2_2 and CO2_2 in the experimental chamber during the cool-down. Compensation of the stray electric fields to levels below 100mV/cm100 \text{mV}/\text{cm} was achieved over a region of 6mm6 \text{mm} along the beam-propagation direction which, for the 1770m/s1770 \text{m}/\text{s} beam velocity, implies the possibility to preserve the coherence of the atomic sample for several microseconds above the cold surfaces.Comment: 12 pages, 10 figure

    Precision Measurements in Few-Electron Molecules: The Ionization Energy of Metastable 4\mathbf{^4}He2\mathbf{{_2}} and the First Rotational Interval of 4\mathbf{^4}He2+\mathbf{{_2}^+}

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    Molecular helium represents a benchmark system for testing ab initio\textit{ab initio} calculations on few-electron molecules. We report on the determination of the adiabatic ionization energy of the a\,^3\Sigma_u^+ state of He2_2, corresponding to the energy interval between the a\,^3\Sigma_u^+ (v=0v''=0, N=1N''=1) state of He2_2 and the X^+\,^2\Sigma_u^+ (v+=0v^+=0, N+=1N^+=1) state of He2+{_2}^+, and of the lowest rotational interval of He2+{_2}^+. These measurements rely on the excitation of metastable He2_2 molecules to high Rydberg states using frequency-comb-calibrated continuous-wave UV radiation in a counter-propagating-laser-beam setup. The observed Rydberg states were extrapolated to their series limit using multichannel quantum-defect theory. The ionization energy of He2_2 (a\,^3\Sigma_u^+) and the lowest rotational interval of He2+{_2}^+ (X^+\,^2\Sigma_u^+) are 34301.207002(23)±0.000037sys\pm 0.000037_{\mathrm{sys}} cm1^{-1} and 70.937589(23)±0.000060sys\pm 0.000060_{\mathrm{sys}} cm1^{-1}, respectively

    Multistage Zeeman deceleration of metastable neon

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    A supersonic beam of metastable neon atoms has been decelerated by exploiting the interaction between the magnetic moment of the atoms and time-dependent inhomogeneous magnetic fields in a multistage Zeeman decelerator. Using 91 deceleration solenoids, the atoms were decelerated from an initial velocity of 580m/s to final velocities as low as 105m/s, corresponding to a removal of more than 95% of their initial kinetic energy. The phase-space distribution of the cold, decelerated atoms was characterized by time-of-flight and imaging measurements, from which a temperature of 10mK was obtained in the moving frame of the decelerated sample. In combination with particle-trajectory simulations, these measurements allowed the phase-space acceptance of the decelerator to be quantified. The degree of isotope separation that can be achieved by multistage Zeeman deceleration was also studied by performing experiments with pulse sequences generated for 20^{20}Ne and 22^{22}Ne.Comment: 16 pages, 15 figure

    New method to study ion-molecule reactions at low temperatures and application to the H2+_2^+ + H2_2 \rightarrow H3+_3^+ + H reaction

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    Studies of ion-molecule reactions at low temperatures are difficult because stray electric fields in the reaction volume affect the kinetic energy of charged reaction partners. We describe a new experimental approach to study ion-molecule reactions at low temperatures and present, as example, a measurement of the H2++H2H3++H{\rm H}_2^+ + {\rm H}_2\rightarrow {\rm H}_3^+ + {\rm H} reaction with the H2+{\rm H}_2^+ ion prepared in a single rovibrational state at collision energies in the range Ecol/kB=5E_{\rm col}/k_{\rm B} = 5-60 K. To reach such low collision energies, we use a merged-beam approach and observe the reaction within the orbit of a Rydberg electron, which shields the ions from stray fields. The first beam is a supersonic beam of pure ground-state H2_2 molecules and the second is a supersonic beam of H2_2 molecules excited to Rydberg-Stark states of principal quantum number nn selected in the range 20-40. Initially, the two beams propagate along axes separated by an angle of 10^\circ. To merge the two beams, the Rydberg molecules in the latter beam are deflected using a surface-electrode Rydberg-Stark deflector. The collision energies of the merged beams are determined by measuring the velocity distributions of the two beams and they are adjusted by changing the temperature of the pulsed valve used to generate the ground-state H2{\rm H}_2 beam and by adapting the electric-potential functions to the electrodes of the deflector. The collision energy is varied down to below Ecol/kB=10E_{\rm col}/k_{\rm B}= 10 K, i.e., below Ecol1E_{\rm col}\approx 1 meV, with an energy resolution of 100 μ\mueV. We demonstrate that the Rydberg electron acts as a spectator and does not affect the cross sections, which are found to closely follow a classical-Langevin-capture model in the collision-energy range investigated. Because all neutral atoms and molecules can be excited to Rydberg states, this method of studyingComment: 39 pages, 10 figure
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