Pressure broadening coefficients of ν5 fundamental band lines of CH3I at 7 μm measured by diode laser absorption spectroscopy

We have measured the room temperature pressure broadening coefficients, γ, of over 100 lines in five Q-branches of the ν5 perpendicular band of methyl iodide (12CH3I) using tuneable diode laser absorption spectroscopy. The profiles of individual lines in the PQ2, PQ4, PQ5, PQ6 and RQ3 branches were recorded in a 1 m long White cell and at nitrogen or oxygen pressures up to 15 Torr. The lines were fitted to the Voigt profile to obtain the collision broadened line widths. Within individual Q-branches the broadening coefficients decreased monotonically with increasing J and for nitrogen broadening varied between 0.19 cm−1 atm−1 at low J and 0.12 cm−1 atm−1 at high J. The corresponding oxygen broadening coefficients were approximately 20% smaller. Self broadening coefficients were also measured for several of the Q-branches and found to be up to ∼4 times higher than the corresponding nitrogen broadening values.