High-sensitivity measurements of 12CH3D pure rotational lines in ground and excited vibrational states in the subTHz region

Pure rotational transitions of 12CH3D having intensity down to 7.9 × 10−30 cm/molecule were studied with a high sensitive spectrometer with radio-acoustic detection of absorption. Among the 14 measured spectral lines, 11 correspond to the rotational transitions within the first three excited states of 12CH3D. The theoretical description was carried out using an effective polyad model expressed in irreducible tensor form including a Triad of interacting bands. An accurate set of ground state rotational and centrifugal distortion constants was determined using a simultaneous weighted fit of experimental transitions available in the literature together with the new subTHz measurements. The pressure broadening and shifting parameters of the JK = 10–00 ground state rotational lines were measured. The shift coefficients of the ground state rotational lines QR(J,K) with Jmax = 5 were estimated using the data for the lines of excited vibrational bands and confirmed our experimental results. The information on pressure shifting allowed evaluating possible systematic errors of determining line positions.