Pressure broadening coefficients of transitions in the ν5 band of methyl bromide from fitting to Voigt and Galatry line profiles

Tuneable diode laser absorption spectroscopy has been used to measure the room temperature pressure broadening coefficients (γ) of rotational transitions in the v5 fundamental band of methyl bromide (12CH379Br and 12CH381Br) around 6.9 μm. Nitrogen, oxygen and self-broadening coefficients have been determined for 125 lines in the RQ1, PQ3, PQ5, PQ7 and PQ8 branches and 49 P and R branch transitions. Line profiles within Q branches were recorded at incremental pressures of nitrogen and oxygen up to 15 Torr and fitted to a Voigt profile to yield the broadening coefficients. The nitrogen broadened data for 14 lines, chosen from the five Q branches, were also fitted with Galatry profiles. The line profiles of the P and R branch transitions were recorded for total nitrogen and oxygen pressures of up to 300 Torr and fitted to both Voigt and Galatry profiles. Within individual Q branches, nitrogen broadening coefficients were found to decrease monotonically with increasing J from 0.14 cm−1 atm−1 at low J to 0.09 cm−1 atm−1 at high J. The corresponding values for oxygen were approximately 25% smaller. Self-broadening coefficients were found to vary between 0.48 and 0.16 cm−1 atm−1 with a similar J dependence to the foreign gas broadening for J > 20. However, between J = 2 and J ≈ 20 the broadening coefficient was found to increase with J. The magnitude of the pressure broadening coefficient for P and R branch transitions was found to closely follow the J dependence measured for the Q branch lines.