Analysis of the rovibrational spectrum of 13CH4 in the Octad range

Abstract We have measured the infrared spectrum of methane 13 CH 4 from 1100 cm −1 (33 THz), below the fundamental range, to about 12 000 cm −1 (360 THz) in the high overtone region at temperatures ranging from 80 K to 300 K by high resolution Fourier transform infrared (FTIR) spectroscopy. With instrumental bandwidths between 0.0027 cm −1 (80 MHz) and 0.01 cm −1 (300 MHz) this provides close to Doppler-limited spectra, using the Zurich prototype spectrometer (ZP2001, Bruker 125HR) combined with a multipath collisional cooling cell. Using perturbation theory and an accurate empirically adjusted potential we have computed ro-vibrational energy levels of 13 CH 4 and 12 CH 4 in the same energy range. Exploiting the synergy between theory and experiment, we analyze here specifically the experimental spectra in the Octad range (∼3700–4700 cm −1 , or 110 to 140 THz), using the theoretical results to guide the fitting of parameters of a Dijon effective Hamiltonian theory. With the aid of the theoretical results it is possible to analyse the Octad of 13 CH 4 with much less effort than without such information. In the end 1144 purely experimental line positions were fitted with root mean square deviations d rms  ⩽ 2.6 × 10 −3  cm −1 (5548 data including theoretical results, with similar d rms ).