No abstract

We report the first detection of rotational transitions of methyl formate (HCOOCH 3 ) in the first torsionally excited state ( ). Recent progress on the assignment of laboratory spectra of methyl formate made it possible v p 1 t for us to assign about 20 unidentified lines in Orion KL from previous line surveys below 200 GHz to the first torsionally excited methyl formate. The rotational temperature and column density obtained in the first torsionally excited state were 44 ‫ע‬ 10 K and (8.6 ‫ע‬ 3.2) # 10 14 cm Ϫ2 , respectively. They were compared with those in the ground state.

The microwave spectrum of methyl formate has been observed in the 7−200 GHz region, and new 437 lines have been assigned to the first excited A torsional substate. Both excited state lines and ground state lines reported previously were analyzed simultaneously on the basis of an internal axis method Hamiltonian. A total of 3514 lines were fitted to a 10th-order reduced Hamiltonian model involving 67 molecular parameters to a 1σ standard deviation of 179 kHz.