Predictions for methane spectra from potential energy and dipole moment surfaces: isotopic shifts and comparative study of 13 CH 4 and 12 CH 4 Micha¨ el Rey a,* , Andrei V. Nikitin b , Vladimir G. Tyuterev a a Groupe de Spectrom´ etrie Mol´ eculaire et Atmosph´ erique, UMR CNRS 7331, BP 1039, F-51687, Reims Cedex 2, France b Laboratory of Theoretical Spectroscopy, Institute of Atmospheric Optics, SB RAS, 634055 TOMSK, Russia Abstract In this paper, we focus on the calculations of dipole transition intensities and rovibrational spectra for 13 CH 4 and 12 CH 4 . Global variational calculations of methane spectra were performed using our recent potential energy and dipole moment surfaces, combined with the tensor formalism derived in previous works. Isotopic vibrational band center shifts due to the 12 C→ 13 C substitution were calculated up to the tetradecad range and compared with experimental values. These shifts were found to be quite irregular, but their variational predictions were very accurate, of the order ∼0.01 cm -1 , and could thus be used for a precise calculation of 13 CH 4 line positions. Rovibrational line intensities computed from the ab initio dipole moment surfaces agree very well with those available in the HITRAN 2008 database, at least for strong and medium lines. For both isotopologues, the first comparative study of theoretical spectra with J max = 20 is presented up to the octad range. Our preliminary results suggest that numerous 13 CH 4 bands which remain still unassigned could be identified and modeled using the proposed approach. 1. Introduction The spectroscopy of methane is of major importance both for the understanding of the nuclear motions in highly symmetric quantum systems and for various applications. In particular, precise knowledge of in- tensities of rovibrational and pure rotational transitions is crucial for the modeling of absorption/emission phenomena in various planetary atmospheres (Giant planets or Titan’s atmosphere [1]), brown dwarfs [2] and for other astrophysical [3, 4, 5, 6, 7, 8] applications. The measurements of the 13 C/ 12 C isotopic abun- dance at various parts of the Universe would benefit of the use of accurate spectroscopic data for methane isotopologues. Methane is also important in environmental sciences as it acts as a greenhouse gas [9] in the earth atmosphere. Consequently, there is a need to develop reliable methods for computing and modeling line positions and intensities for methane isotopologues , and more generally, for polyatomic molecules in wide spectral ranges. There exists two main theoretical approaches for the computation of the spectra of * Corresponding author Email address: michael.rey@univ-reims.fr (Micha¨ el Rey) Preprint submitted to Elsevier April 4, 2013