Measurements of Self‐Broadening in HCN

A high‐resolution directly recording grating spectrometer has been used to measure the line breadths of absorption lines in both the P and R branches of the 002 band of HCN. The observed line breadths, corrected for Doppler and instrument broadening, indicate that the earliest classical theory of Lindholm predicts the correct J dependence of pressure broadening between J=0 and J=25. The observed pressure broadening appears to be sensibly independent of J for J=27 to J=34. Hence the lines fail to become extremely narrow as J gets large, the line narrowing being a phenomenon required by existing theories which consider only J‐dependent terms. The high J half‐width, 0.0055 cm—1/cm Hg, is nearly equal to the term which must be added to Lindholm's earliest classical expression to bring it into agreement with observations at the band maximum. Present measurements agree well with the J=0 to J=1 microwave measurement by Smith et al. and also with the uncorrected line breadths measured at 580 mm Hg by Lindholm.