Determination of the ortho‐/para deuterium concentration ratio with femtosecond CARS

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)%.