The 13CH4 absorption spectrum at 80 K: Assignment and modeling of the lower part of the Tetradecad in the 4970–5470 cm−1 spectral range

Abstract

A 13C-enriched spectrum of methane (13CH4) was recorded at Doppler-limited resolution (0.0044 cm−1) at 80 K covering the entire Tetradecad region in the 4970–6200 cm−1. In this paper we present the spectral analysis of the lower part of the 13CH4 Tetradecad in the 4970–5470 cm−1 region. Starting with a non-empirical effective Hamiltonian derived by high-order Contact Transformations (CT) from the ab initio potential energy surface (PES), the effective Hamiltonian parameters have been determined for this region by extending the previous DAS (Differential Absorption Spectroscopy) spectral analysis for the upper part of the Tetradecad in the 5853–6200 cm−1 region. In total, 1387 rovibrational transitions were assigned belonging to five cold bands of the Tetradecad up to Jmax = 11. Their positions were fitted with an rms deviation of 1.6 × 10−3 cm−1. Measured line intensities for 737 transitions were modeled using the effective dipole transition moments approach with an rms deviation of about 10%. Finally, the observed transitions were incorporated to fit simultaneously the 13CH4 Hamiltonian parameters for the {Ground state / Dyad / Pentad / Octad / Tetradecad} system and the dipole moment parameters for the {Ground state - Tetradecad} system.