Analysis of dyad – dyad transitions of 12CH4 and 13CH4

Abstract

This work is a refinement of the (0100) and (0001) state description for both isotopes 12CH4 and 13CH4. We introduce recently observed dyad–dyad lines in addition to "pure rotational" and infrared transitions in a simultaneous fit of the ground-and upper-state parameters. Using a sixth-order-reduced Hamiltonian, significant improvements are achieved for the upper-state parameters that were previously retrieved from infrared transitions only. As a result, substantially more accurate predictions of the dyad – dyad spectrum of both isotopes are reported in the frequency range 0–300 GHz with a strength greater than 10−11 cm−2 atm−1 at 296 K.