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arXiv:1004.3812 · Physical Review A

State-selective detection of velocity-filtered ND 3 molecules

Benjamin Bertsche, Andreas Osterwalder

Published Sep 17, 2010• 29 citations
arXivHTMLPDF

Abstract

Translationally cold and slow ND${}_{3}$ is prepared by filtering the slow molecules from a thermal gas-phase sample using a curved electrostatic hexapole guide. This filter, like the curved quadrupole guide introduced by Rangwala et al. [Phys. Rev. A 67, 043406 (2003)] selects molecules by their forward velocity and effective electric dipole moment. Here we describe two main modifications with respect to previous work: (1) A segmented hexapole guide is used that produces a harmonic potential for the linearly Stark-shifted levels of ND${}_{3}$. A curved guide is combined with a straight hexapole guide, and independent high-voltage supplies are employed to allow for bandpass velocity filtering. (2) State-selective laser ionization is used to obtain time- and state-selective detection of the guided molecules. This enables the experimental determination of the rotational state population of the guided molecules.

DOI
10.1103/physreva.82.033418

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Translationally cold and slow ND3{}_{3}3​ is prepared by filtering the slow molecules from a thermal gas-phase sample using a curved electrostatic hexapole guide. This filter, like the curved quadrupole guide introduced by Rangwala et al. [Phys. Rev. A 67, 043406 (2003)] selects molecules by their forward velocity and effective electric dipole moment. Here we describe two main modifications with respect to previous work: (1) A segmented hexapole guide is used that produces a harmonic potential for the linearly Stark-shifted levels of ND3{}_{3}3​. A curved guide is combined with a straight hexapole guide, and independent high-voltage supplies are employed to allow for bandpass velocity filtering. (2) State-selective laser ionization is used to obtain time- and state-selective detection of the guided molecules. This enables the experimental determination of the rotational state population of the guided molecules.
Electrostatic velocity filtering is a technique for the production of continuous guided beams of slow polar molecules from a thermal gas. We extended this technique to produce pulses of slow molecules with a narrow velocity distribution around a tunable velocity. The pulses are generated by sequentially switching the voltages on adjacent segments of an electric quadrupole guide synchronously with the molecules propagating at the desired velocity. This technique is demonstrated for deuterated ammonia (ND3{}_{3}3​), delivering pulses with a velocity in the range of $20\ensuremath{-}100$ m/s and a relative velocity spread of $(16\ifmmode\pm\else\textpm\fi{}2)%$ at full width at half maximum. At velocities around 606060 m/s, the pulses contain up to 106{10}^{6}106 molecules each. The data are well reproduced by Monte Carlo simulations, which provide useful insight into the mechanisms of velocity selection.
Slow molecules are efficiently extracted from a thermal reservoir by exploiting the interaction between polar molecules and the field provided by an electrostatic quadrupole guide. Electrostatic velocity filtering is widely applicable, which is demonstrated by producing guided beams of ammonia ND3, formaldehyde H2CO, and the water isotopologs H2O, D2O, and HDO. Ultraviolet depletion spectroscopy in a cold guided beam of formaldehyde makes the population in individual rotational states visible and testifies their key role in velocity filtering. Enhanced Rayleigh scattering into an optical cavity is investigated as an alternative non-destructive detection method for cold molecules. For various room-temperature gases, the observed enhancement is well described by interference of electromagnetic fields scattered into the resonator by a classical driven dipole oscillator.
We report experiments in which a slow Rydberg positronium (Ps) beam was produced by velocity selection using a curved electrostatic quadrupole guide. Ps atoms in Rydberg-Stark states with principal quantum number n=14n=14n=14 were prepared by a two-color optical excitation process in a uniform electric field. Low-field-seeking Stark states were produced at the entrance of a 0.6-m-long quadrupole guide that includes a ${45}^{\ensuremath{\circ}}$ bend, and were detected at the end of the guide via their annihilation $\ensuremath{\gamma}$ radiation. The mean speed (kinetic energy) of atoms entering the guide was estimated to be $\ensuremath{\approx}180$ km ${\mathrm{s}}^{\ensuremath{-}1}$ (185 meV), whereas the mean longitudinal speed of guided atoms was measured via time of flight and found to be $\ensuremath{\approx}90$ km ${\mathrm{s}}^{\ensuremath{-}1}$, equivalent to a kinetic energy of 45 meV. The measured transport data are in broad agreement with Monte Carlo simulations, which are also used to establish the efficacy with which the arrangement we describe could be used to perform Ps-atom scattering measurements.
The measurement of the rotational state distribution of a velocity-selected, buffer-gas-cooled beam of ND3 is described. In an apparatus recently constructed to study cold ion-molecule collisions, the ND3 beam is extracted from a cryogenically cooled buffer-gas cell using a 2.15 m long electrostatic quadrupole guide with three 90° bends. (2+1) resonance enhanced multiphoton ionization spectra of molecules exiting the guide show that beams of ND3 can be produced with rotational state populations corresponding to approximately T(rot) = 9-18 K, achieved through manipulation of the temperature of the buffer-gas cell (operated at 6 K or 17 K), the identity of the buffer gas (He or Ne), or the relative densities of the buffer gas and ND3. The translational temperature of the guided ND3 is found to be similar in a 6 K helium and 17 K neon buffer-gas cell (peak kinetic energies of 6.92(0.13) K and 5.90(0.01) K, respectively). The characterization of this cold-molecule source provides an opportunity for the first experimental investigations into the rotational dependence of reaction cross sections in low temperature collisions.
The Stark interaction of polar molecules with an inhomogeneous electric field is exploited to select slow molecules from a room-temperature reservoir and guide them into an ultrahigh vacuum chamber. A linear electrostatic quadrupole with a curved section selects molecules with small transverse and longitudinal velocities. The source is tested with formaldehyde (H2CO)({\mathrm{H}}_{2}\mathrm{CO})(H2​CO) and deuterated ammonia (ND3).({\mathrm{ND}}_{3}).(ND3​). With H2CO{\mathrm{H}}_{2}\mathrm{CO}H2​CO a continuous flux is measured of $\ensuremath{\approx}{10}^{9}/\mathrm{s}$ and a longitudinal temperature of a few kelvin. The data are compared with the result of a Monte Carlo simulation.