Cryogenic atomic hydrogen beam apparatus with velocity characterization

Abstract

Precision spectroscopy of hydrogen often relies on effusive thermal atomic beams, and the uncertainty in the velocity distribution of these beams can introduce systematic errors and complicate lineshape models. Here, we present an apparatus capable of high signal-to-noise studies of these velocity distributions at cryogenic temperatures for both ground state (1S) and metastable (2S) hydrogen using a simple time-of-flight technique. We also investigate how the cryogenic nozzle geometry affects these results.

Two-photon frequency comb spectroscopy of atomic hydrogen

“…The necessity of lower temperatures underlines once again the need of better light collection and dissociation ratio increase. Further improvement of the second order Doppler shift uncertainty can be made, if the velocity distribution is characterized as was for instance recently done in [62]. In our opinion, the second order Doppler shift is the most challenging systematics for an anticipated improvement of the 1S-3S measurement.…”

“…That is the case here since the PCV is very close to the nozzle (see the discussion in section 5.4). Therefore, based on the conclusions of [62], we believe a Maxwellian (i.e. q = 3) is expected.…”

“…This suggests that the true velocity distribution is close to a Maxwellian, at least for the evaluated low temperatures. This can be compared with a recent publication [62], which investigated the velocity distribution of a cryogenic hydrogen beam using a similar nozzle design to the one used here. In that publication, the distribution was found to significantly deviate from a Maxwellian (q ≈ 5) in contrast to the results here.…”

“…In total, 99.6% of the atoms undergo more than one wall collision. We expect this number of collisions to be sufficient for good thermalization based on the observations of [54,62] where nozzle designs with a similar number of collisions were studied. We note that, due to the nozzle's geometry, atoms traveling on-axis with the laser beam can only originate from collisions with other atoms or molecules and not from the cold walls.…”

“…Systematic Uncertainties Analysis ments in [62] were also measured far away from the nozzle with a small divergence angle so that it was likely only atom/atom or atom/molecule collisions within the nozzle could account for the on-axis beam. Further, the main conclusion of [62] was that one should expect a distribution close to a Maxwellian provided a few modest conditions are fulfilled -for instance, there should be a direct line of sight from a cold inner surface of the nozzle to the PCV. That is the case here since the PCV is very close to the nozzle (see the discussion in section 5.4).…”