Helium induced pressure broadening and shifting of HCN hyperfine transitions between 1.3 and 20 K

Abstract

We have measured the helium induced pressure broadening and shifting of the distinct hyperfine components of the j = 1 <-- 0 and j = 2 <-- 1 transitions of HC14N at temperatures between 1.3 and 20 K. The HCN molecules were cooled to these temperatures using the collisional cooling technique. As a test of this cooling technique we measured the Doppler contribution to the spectral lines, and these measurements confirm that the molecules are at the same temperature as the walls of the spectroscopic cell. We observed that the hyperfine components of the 2 <-- 1 transition have distinct broadening coefficients that differ from one another by as much as 5%. The measured differences are in reasonable agreement with theoretical predictions. We have also performed molecular scattering calculations on three He-HCN potential energy surfaces in order to compare our results with theoretical expectations. At the lowest temperatures these calculations predict broadening coefficients that are considerably larger than the measured coefficients. We have previously found a similar discrepancy for two other molecules at these low temperatures, and we discuss possible experimental and theoretical origins for this persistent discrepancy.

The submillimeter: A spectroscopist’s view

“…One of the most interesting results of this work is a long-standing and persistent difference between theory and experiment for temperatures below about 5 K. It is unclear if the cause of this is experimental or theoretical, although careful consideration to both has been paid [75,76]. Fig.…”

“…As an example, Fig. 14 shows the pressure broadening cross-section for collisions between HCN and He as a function of temperature [75]. Above about 20 cm À1 ($30 K) the cross-section is essentially flat, as befits a semi-classical interaction with many open collision channels.…”

Ab initiocomputation of the broadening of water rotational lines by molecular hydrogen

“…[9] and [10]; hereafter DDP10). Other comparisons were very successful for simpler symmetries [11][12][13][14][15].…”

“…It must be strongly emphasized that both approaches are equivalent. Quite often, since inelastic cross sections or rate coefficients are made available in the literature, to get an estimate of σ (4) is truncated: only the first two terms, the inelastic cross sections, are used, sometimes yielding reliable results [24] and sometimes not [13], depending on the structure of the scattering amplitudes (see the following).…”

Searching for chameleon-like scalar fields with the ammonia method

“…For instance, a typical interstellar molecular cloud with the kinetic temperature T kin ∼ 10 K and the gas number density n H 2 ∼ 10 5 cm −3 has the local density of ρ space ≈ 3×10 −19 g cm −3 . Laboratory vacuum spectrographs usually operate in the pressure range from 1 to 10 mtorr and at the temperature regime of T kin ∼ 10−40 K (e.g., Willey et al 2002;Ronningen & De Lucia 2005;Ross & Willey 2005). This gives ρ lab ≈ 3 × 10 −9 g cm −3 if T kin = 10 K, P = 1 mtorr, and the vacuum chamber is filled by H 2 .…”