Selection rules for electric multipole transition of triatomic molecule in scattering experiments

In the electron or x-ray scattering experiment,the measured spectra at larger momentum transfer are dominated by the electric dipole-forbidden transitions,while the corresponding selection rules for triatomic molecules have not been clearly elucidated.In this work,based on the molecular point group,the selection rules for the electric multipolarities of the electronic transitions of triatomic molecules are derived and summarized into several tables with the variation of molecular geometry in the transition process being considered.Based on the summarized selection rules,the electron energy loss spectra of H_2O,CO_2,and N_2O are identified,and the momentum transfer dependence behaviors of their valence-shell excitations are explained.