Determination of theortho‐/paradeuterium concentration ratio with femtosecond CARS

Abstract

The time-resolved femtosecond coherent anti-Stokes Raman scattering (CARS) method was used for the determination of para-and ortho-states in gaseous deuterium (D 2 ). The present investigations at the SINQ ('Spallations Neutronen Quelle') spallation neutron source at the Paul Scherer Institute (PSI) were performed for the development of an effective ultra-cold neutron (UCN) source. High concentrations of ortho-D 2 in UCN sources are mandatory and therefore flexible applications for the measurement of ortho-D 2 concentrations are of interest. Hence the non-equilibrium ortho-D 2 concentration in gas samples, taken from the liquid D 2 cold moderator of the SINQ, was measured with fs-CARS. It is shown that the sensitivity of fs-CARS due to the nuclear spin degeneracy is high, and that the method is an ideal tool for D 2 para-and ortho-state investigations. At a sample temperature of 298 K the ortho-D 2 concentration for thermalized D 2 was measured as 66.664(16)% of the total D 2 concentration, whereas the fraction of ortho-D 2 molecules in a specific sample from the spallation source was found to be 70.68(33)%.

Investigation of superexcited states in molecular deuterium

“…As shown in Ref. [25] the relative concentration of the ortho-D 2 molecules at the sample temperature of 298 K is 66.664(16)% of the total D 2 concentration.…”

“…As known, the ortho-D 2 and para-D 2 molecular states relate to different nuclear spin statistics and can hardly be converted into each other during experiment. Therefore, these states can usually be treated as two different types of molecules [25,27]. The effective rotational temperature inside the supersonic D 2 beam was less than 10 K providing that more than 99% of the ortho-para D 2 molecules were in their ground rotation states, that is J = 0 for the ortho-D 2 and J = 1 for the para-D 2 .…”

Effects of O2–CO2 polarization beating on femtosecond coherent anti-Stokes Raman scattering (fs-CARS) spectroscopy of O2

“…Therefore, single-shot measurements can be readily achieved with a substantial increase in the signal-to-background ratio over that possible with ns-CARS [7]. However, the increased laser bandwidth can simultaneously excite other species that have nearby Raman transitions; hence, the observed CARS signal may consist of interference beatings between various Raman transitions, which complicates the interpretation of the measured CARS signal [13][14][15][16][17]. In this case, the temporally resolved fs-CARS signal recorded will consist of a modulation pattern that results from the beating between the two or more Raman polarizations involved.…”

Preface to the second special issue on non‐linear Raman spectroscopy and related techniques

“…As a consequence, data evaluation is performed on an essentially background-free signal yielding high signal-to-noise ratios. Knopp et al 18 take advantage of the sensitivity and high dynamic range of femtosecond CARS to measure the para and ortho states in gaseous deuterium to characterize the ultra-cold spallation neutron source at the Paul Scherrer Institute in Switzerland. The high selectivity of time-domain methods against the non-resonant background is also exploited by Tran et al 19 In their work, femtosecond Raman-induced polarization spectroscopy is applied for the simultaneous determination of temperature and concentrations in a CO 2 -N 2 gas mixture up to 600 K.…”