Laboratory Spectrum of the 111–202Rotational Transition of CH2

We have studied the N KaKc = 1 11 -2 02 rotational transition of the ortho-methylene radical CH 2 in its ground vibrational and electronic ( 3 B 1 ) state. Centered near 943 GHz, this pure rotational transition is split by fine-structure interactions into three components, which lie at 931, 945, and 956 GHz and are characterized by the quantum numbers J = 1-2, 2-3, and 0-1, respectively. In turn, each fine-structure component breaks up into a number of hyperfine components. A total of eight new hyperfine components were measured to a precision of about 20 kHz and analyzed as part of a global data set with previously known data at lower frequencies. Since the 1 11 -2 02 transition is energetically rather low (110 K) compared with the previously measured 4 04 -3 13 interstellar transition, it should be more widely observable in interstellar clouds, either through an atmospheric window from the ground or, in the near future, with SOFIA and the Herschel Space Observatory .