Low-energy positron scattering: modelling the formation of bound [M-e+] states in deformed polyatomic molecules

Quantum calculations are carried out to evaluate the scattering length A0 for low-energy positron collisions with gaseous acetylene molecules which are being 'distorted' in their vibrational mode during the scattering. As the molecular structure undergoes increasingly larger distortions of that bending mode, the clear signature of a metastable state (change of sign for A0) appears at bending deformation of about 16 degrees from the linear geometry. The effects of such state on the size of the annihilation parameters of the title molecule are evaluated and the implications for larger polyatomic are discussed. (authors)