Rotational Structure of the ν3 Raman Band of Methane

A multiple reflection Raman tube and a high dispersion quartz spectrograph were used to investigate the fine structure of the ν3 band of gaseous methane. Sixty-eight maxima were measured, and evidence for all but one of the 15 theoretically predicted sub-branches was found. An evaluation of the rotational constants by combination relations for Raman and infrared frequencies gives three different B values for the upper vibrational state and surprisingly, three for the ground state. A more acceptable analysis, based on a single set of rotational constants for the ground state, B0=5.253 cm−1 and D0=1.9×10−4 cm−1, leads to B1 values for the various branches in the range 5.202 to 5.235 cm−1. The analysis gives ν0=3018.7 cm−1 and ζ1=0.056. The constants obtained for the ground state are also used in an analysis of the 2ν3 infrared band.