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

Abstract

We have measured the infrared spectrum of methane 13CH4 from 1100cm−1 (33THz), below the fundamental range, to about 12000cm−1 (360THz) in the high overtone region at temperatures ranging from 80K to 300K by high resolution Fourier transform infrared (FTIR) spectroscopy. With instrumental bandwidths between 0.0027cm−1 (80MHz) and 0.01cm−1 (300MHz) this provides close to Doppler-limited spectra, using the Zürich 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 13CH4 and 12CH4 in the same energy range. Exploiting the synergy between theory and experiment, we analyze here specifically the experimental spectra in the Octad range (∼3700–4700cm−1, or 110 to 140THz), 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 13CH4 with much less effort than without such information. In the end 1144 purely experimental line positions were fitted with root mean square deviations drms⩽2.6×10−3cm−1 (5548 data including theoretical results, with similar drms).