Extended assignments of the 3ν₁+ν₃ band of methane

The 3ν₁+ν₃ band of methane has been investigated using a titanium sapphire laser (TSL) spectrometer. This band has recently been studied by others, and 31 transitions were assigned [Boraas et al, J. Chem. Phys. 100, 7916 (1994)]. In this work, the tone-burst modulation spectrum at liquid nitrogen (77 K) and room temperature (295 K) has been recorded to determine the rotational quantum number J" of the lower level of each transition. Stark modulation spectra were also recorded at room temperature by applying a 40 kV/cm electric field to identify the E symmetry and other species. Among the 269 transitions observed in the 77 K tone-burst modulation spectrum, we have assigned 215 transitions with respect to J", of which 49 are identified up to their tetrahedral components. We found a congested band structure that results from the large fraction of dark vibrational states that derive their intensity from a single bright state as a result of Coriolis interactions.