GLOBAL FREQUENCY AND INFRARED INTENSITY ANALYSIS OF ${}^{12}$CH$_4$ LINES IN THE 900--4800 CM$^{-1}$ REGION

Abstract

A new global analysis of methane lines in the 900--4800 cm$^{-1}$ region has been performed thanks to new experimental data for both line positions and intensities. This implies three of the ${}^{12}$CH$_4$ polyads, namely the dyad (940--1850 cm$^{-1}$, 2 vibrational levels, 2 sublevels), the pentad (2150--3350 cm$^{-1}$, 5 vibrational levels, 9 sublevels) and the octad (3550--4800 cm$^{-1}$, 8 vibrational levels, 24 sublevels) and some of the associated hot bands. New FTIR spectra of the pentad and octad regions have been recorded with a very high resolution (better than $0.001$ cm$^{-1}$ instrumental bandwidth, unapodized) at 78 K using the Bruker IFS 125 HR Zurich prototype (ZP2001) spectrometer} {\bf 84}, 177--180 (2003).}. New intensity measurements were performed in the whole region at the Kitt Peak National Observatory. We also used previously recorded high-resolution Raman spectra} {\bf 156} 15--21 (1992) et J.-M. Jouvard, PhD Thesis, Dijon (1991).}. The effective Hamiltonian was expanded up to order 6 for the ground state, 6 for the dyad, 5 for the pentad and 5 for the octad. We obtain global root mean square deviations $d_{\mbox{\rm\scriptsize RMS}}$ for line positions $=1.4\times10^{-4}$ cm$^{-1}$ for the dyad, $6.0\times 10^{-4}$ cm$^{-1}$ for the pentad and $3.3\times 10^{-3}$ cm$^{-1}$ for the octad. This analysis represents a large improvement over the previous one} {\bf 208}, 1--13 (2001).} with $d_{\mbox{\rm\scriptsize RMS}}=0.041$ cm$^{-1}$ for the octad system.