FERMI RESONANCES AND LOCAL MODES IN STIBINE, SBH3 - A FOURIER INTERFEROMETRIC AND LASER PHOTOACOUSTIC STUDY OF THE OVERTONE SPECTRUM

The third stretching overtone region of a natural sample of stibine, S bH3, has been studied with high resolution infrared spectroscopy and t he fifth and the sixth overtone region with Ti:Sapphire ring laser int racavity photoacoustic spectroscopy. The third overtone consists of a local mode pair of bands (400A(1)/E) which have been rotationally assi gned both for (SbH3)-Sb-121 and (SbH3)-Sb-123 With a vibration-rotatio n model based on rectilinear normal coordinates. The vibrational depen dencies of the model parameters are explained well with a simple block diagonal vibrational model. An extension of the standard vibration-ro tation model is used to show that the upper state rotational energy le vel structures of both isotopic species are close to the rotational st ructure of an asymmetric rotor. High resolution laser spectrum of the fifth overtone consisting of a local mode pair of bands (600A(1)/E) sh ows severe perturbations in the upper state rotational structure. The (510A(1)/E) and (700A(1)/E) bands have been recorded with low resoluti on. All experimentally known vibration-rotation band origins of (SbH3) -Sb-121 have been reproduced well with a curvilinear internal valence coordinate system based Fermi resonance local mode model. The potentia l energy surface obtained agrees well with recent ab initio results. ( C) 1996 American Institute of Physics.