HOT BANDS IN OVERTONE ABSORPTION OF PYRROLE, METHYL ISOCYANIDE AND ISOBUTANE - PHOTOACOUSTIC SPECTRA AT 140-C

The intracavity laser photoacoustic spectra of the 4nu(N-H) and 4nu(C- H) overtones of pyrrole, the 5nu(C-H) of methyl isocyanide and acetoni trile, the 5nu(C-H) and 6nu(C-H) of isobutane, and the 6nu(C-H) of cyc lohexane were measured at temperatures up to 140 C (T140) and compared to the room temperature (T(rt)) spectra. Hot bands were identified by their temperature dependence in pyrrole, methyl isocyanide, and isobu tane. From the temperature dependence of the relative intensities of t he hot band compared to the main band, the following average wave numb ers of the thermally populated states were deduced: 340, 300, and 208 cm-1 for pyrrole, isobutane, and methyl isocyanide, respectively. Thes e values compare well to the bending motion frequencies in these molec ules. Large redshifts of the hot bands from the main peak were observe d, as expected for high overtone transitions due to large anharmonicit ies. The anharmonic constants determined from these shifts are - 25 cm -1 for the N-H in pyrrole and - 20 cm-1 for the C-H in methyl isocyani de and isobutane. In the case of the N-H absorption the hot band was c ompletely separated from the main transition. In the C-H overtone spec tra these hot band absorptions were not separated and result in an app arent increased width of the overtone transition.