Absorption spectra of methane in the near infrared

A grating spectrometer with a PbS cell for detector has been used for the measurement of the infrared absorption bands of methane in the region of 1.66 p,. Many lines of the P, Q, and R branches of the 1.66-p, band have been observed. These are well resolved and are of the correct structure and spacing for the F2 component of 2 V3. The lines for values of J up to 10 are sharp, showing no indication of splitting due to interaction with neighboring s tatcs or to centrifugal distortion. The rotational constants obtained for this band are B' = 5. 178, 13= 0.0346. The latter value is lower than in the V3 fundamental. Other bands observed are more, complex and irregular, presumably due to mutual interaction.