The ground state of methane 12CH 4 through the forbidden lines of the ν3 band

Abstract

High resolution spectra of the ν 3 band of methane, 12CH 4, were recorded by using a “third generation vacuum Fourier interferometer”; a large pressure range (from 0.009 to 10 Torr) with a sample path fixed at eight meters was used, enabling observation of transitions with intensity ratios as low as 1 10 000 . More than 350 forbidden transitions of the ν 3 band, including about 125 transitions of the Q + branch, were unambiguously identified. Of the 277 transitions retained for computations, one-hundred have 11 ≤ J ≤ 16. From combination difference relations using pairs of transitions having the same upper state energy level (forbidden-allowed and forbidden-forbidden pairs were used), 276 independent differences between ground state energy levels could be determined with uncertainties of about 0.001 cm −1. These data yielded the following values for the ground state structure constants of 12CH 4 along with their standard deviations (in cm −1): β o hc =5.2410356±0.0000096 , γ o hc =(−1±0.00074) 10 −4 , π o hc =(5.78±0.18) 10 −9 , ϵ o hc =(−1.4485±0.0023) 10 −6 , ϱ o hc =(1.768±0.126) 10 −10 , ξ o hc =(−1.602±0.067) 10 −11 , Thus, for the first time, the scalar constant π 0 has been evaluated and ir values have been obtained for the two tetrahedral constants ϱ 0 and ξ 0; furthermore, these values are in very good agreement with the ones recently determined from radiofrequency data, i.e., in cm −1: ϵ o hc =(−1.45061±0.00014) 10 −6 , ϱ o hc =(1.7634±0.0068) 10 −10 , ξ o hc =(−1.5432±0.0040) 10 −11 From these values, the 276 differences can be reproduced with an overall rms deviation equal to 0.0009 cm −1. Finally, the ground state energies of 12CH 4 have been calculated for J ≤ 16.