AN EXPERIMENTAL-STUDY OF COLLISION BROADENING OF ROTATIONAL SPECTRAL-LINES OF (CH3)-C-13 (CN)-C-13-N-15 IN THE GROUND AND N-NU-8, N=1 AND 2 VIBRATIONALLY EXCITED-STATES AT MICROWAVE-FREQUENCIES

A double modulation microwave spectrometer was used to determine the linewidth parameter for some rotational components in the n-nu(8) vibrations of methyl cyanide under C-13 and N-15 substitution. The linewidth parameters for self-broadening of the DELTA-J = 2 <-- 1 rotational components for the ground, nu(8), and 2-nu(8) vibrations were determined over a pressure range of 1-13 mTorr and at a temperature of 300 K. An experimental method is presented to correct for modulation broadening when high derivatives are used to extract the absorption profile information from the signals. The eighth derivative profile was explored to determine if the spectral line shape remains Lorentzian over a range of modulation levels from 10% of DELTA-nu to more than 100% of DELTA-nu. These tests showed that the ratio of inner to next inner peak separations, designated in this paper as DELTA, of the eighth derivative was the same as that for an assumed Lorentzian line shape. Thus, line shapes were assumed to be Lorentzian for theoretical analysis of the derivative profiles and comparisons made between experiment and theory on that basis. Dipole moments for vibrationally excited energy levels for the nu(8) and 2-nu(8) vibrations were calculated from the linewidth parameter data after all corrections had been made for modulation, Doppler, and other nonpressure broadening effects.