High resolution difference bands of ethane C2H6 from torsionally excited lower states: rotation-torsion structure of the ν2, ν11 and ν4 + ν11 vibrational states

A high resolution Fourier transform infrared spectrum of C2H6, measured at a pressure of 173.3 Pa and an optical path of 153.2 m, was analysed between 1050 and 1295 cm(-1). Extensive absorption due to the difference bands nu(11)-nu(4), and several rotation-torsion lines of the difference band nu(2)-nu(4), in the region of the x, y-Coriolis resonance of nu(2) and nu(11), were observed. This allowed a detailed rotation-torsion analysis of the upper states nu(11) and nu(2). The anomalous torsional structure, found in the non-degenerate vibrational state nu(2), can be explained as the effect of an Hamiltonian term accounting for a strong dependence of the torsional barrier height on the normal vibrational coordinate q(2). The value of the barrier height derivative partial derivative V-3/partial derivative q(2) is estimated to be 127 +/- 10 cm(-1). Also detected and assigned were 'hot' difference transitions belonging to the (nu(4) + nu(11))-2 nu(4) band, yielding information on the upper state nu(4) + nu(11). It is believed that transitions from 3 nu(4) to 2 nu(4) + nu(11) are also detectable in the investigated region.