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 /-t. Many lines of the P, 0, and R branches of the 1.66-ju band have been observed. These are well resolved and are of the correct structure and spacing for the F2 component of 2 vz. The lines for values of / up to 10 are sharp^ showing no indication of splitting due to interaction with neighboring states or to centrifugal distortion. The rotational constants obtained for this band are B' = 5.178, £3 = 0.0346. The latter value is lower than in the vz fundamental. Other bands observed are more, complex and irregular, presumably due to mutual interaction.