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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)%.

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