The first observation of the free induction signals of OH radicals in the terahertz region E.N. Chesnokov, L.N. Krasnoperov, V.V. Kubarev and P.V. Koshlyakov EPJ Web Conf., 195 (2018) 06024 Published online: 23 November 2018 DOI: 10.1051/epjconf/201819506024