CH3OH laser line assignments revisited

Abstract

The CH3OH line frequencies calculated by diagonalizing the molecular Hamiltonian are affected by inaccuracies of the order of 10−3 ∼ 10−2 cm−1 or higher, the inaccuracy increasing with the values of the quantum numbers involved in the line. The main sources of inaccuracy are the computational approximations and the fact that the best available molecular constants have beer evaluated from best fits of microwave data, which usually involve low quantum numbers only. These inaccuracies and the richness of the methanol spectrum generate several ambiguities in the line assignments, in particular in the assignment of the far infrared (FIR) lines of the CH3OH laser. In order to overcome these ambiguities we have started a systematic investigation of the Fourier absorption spectrum of methanol in the regions between 8 and 80 and between 950 and 1100 cm−1. The peak precisions of our Fourier spectra are 2×10−4 and 5×10−4 cm−1 respectively for the two spectral regions (1). With such precisions and with definite assignments for the absorption lines it is possible to form loops of lines containing at least three absorption lines and closed by an investigated laser line. The balance of the frequencies involved in the loop must give zero within the experimental errors, this leading practically always to a unique choice for the assignment of the laser line. We have thus been able to prove or disprove several previous assignments, to make new assignments and to predict the frequencies of some possible FIR laser lines not yet observed.