Collision induced rotational spectrum of allene

Abstract

The far infrared collision induced absorption spectrum for collisions between allene and allene molecules is presented. The anisotropy and large size of allene is shown to dominate the observed rotational spectrum. Only a small fraction of the observed absorbance results from the leading multipole moment (quadrupole), uniform field and isotropic polarizability and interaction potential. An upper limit to the quadrupole moment of allene, Θ = 2·45 × 10-39 C m2, is determined from the low frequency wing of the collision induced absorption spectrum. The anisotropy of the molecular interaction potential, anisotropic polarizability, overlap distortion, field gradient and multipolar terms following the leading quadrupole are shown to contribute higher frequency absorption. Higher frequency collision induced absorption is also calculated when the multipolar expansion to describe the field is replaced by a model with point charges on the allene nuclei. The implication of the point charge model is that the multipol...