Slow and velocity-tunable beams of metastableHe2by multistage Zeeman deceleration

Abstract

He2 molecules in the metastable a 3 Σ + u state 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, 77 K, and 10 K, the mean velocity of the supersonic beam was measured to be 1900 m/s, 980 m/s, and 530 m/s, with longitudinal velocity distributions corresponding to temperatures of 4 K, 1.9 K, and 1.8 K, respectively. The characterization of the population distribution among the different rotational levels of the a 3 Σ + u state by high-resolution photoelectron and photoionization spectroscopy indicated a rotational temperature of about 150 K for the beam formed by expansion from the room-temperature valve and a bimodal distribution for the beam produced with the valve held at 10 K, with rotational levels up to N = 21 being populated. A 55-stage Zeeman decelerator operated in a phase-stable manner in the longitudinal and transverse dimensions was used to further reduce the beam velocity and tune it in the range between 100 and 150 m/s. The internal-state distribution of the decelerated sample was determined by recording and analyzing the photoionization spectrum in the region of the lowest ionization threshold, where it is dominated by resonances corresponding to autoionizing np Rydberg states belonging to series converging to the different rotational levels of the X 2 Σ + u ground state of He + 2 . The deceleration process did not reveal any rotational state selectivity, but eliminated molecules in spin-rotational sublevels with J = N from the beam, J and N being 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 the a 3 Σ + u state of He2 is already reached at fields of only 0.1 T.

Multistage Zeeman decelerator for molecular-scattering studies

“…Specifically, formation of metastable He 2 molecules in the a 3 state (from here on He 2 *) is expected, as is also observed in the experiments by Motsch et al and Jansen et al that use a similar discharge source [51,52]. However, He 2 * is indistinguishable from He* in our detection system.…”

Precision measurement of the rotational energy-level structure of the three-electron molecule He2+

“…A supersonic beam of metastable helium molecules was produced in a source chamber by striking an electric discharge through an expansion of pure helium gas at the exit of a pulsed valve [42]. The body of the valve was cooled to a temperature of 77 K, resulting in a supersonic beam with a velocity of approximately 1000 m/s [43]. The molecular beam was collimated with a 2-mmdiameter skimmer before entering a second, differentiallypumped vacuum chamber containing a 55-stage Zeeman decelerator [43][44][45].…”

Zeeman deceleration of electron-impact-excited metastable helium atoms

“…Metastable species often possess one or more unpaired electron spins so that they become suitable systems of choice for Zeeman deceleration, an experimental technique relying on the fast switching of inhomogeneous magnetic fields to remove kinetic energy from a supersonic beam of paramagnetic species. Zeeman deceleration has already been successfully demonstrated for supersonic beams of Ne(3 3 P 2 ) [16,17], Ar(4 3 P 2 ) [18] and He 2 ( Σ + a u 3 ) [19]. Recently, metastable He and Ne atoms were successfully used in low-temperature merged-beam reactive scattering experiments (Penning ionization) involving bent magnetic guides [20,21].…”