Theoretical atomic momentum spectroscopy of diatomic, triatomic and polyatomic molecules

We present a general theory of quantum chemistry-based atomic momentum spectroscopy (QC-AMS) for predicting electron-atom Compton profiles due to the intramolecular motion of each atom in diatomic, triatomic and polyatomic molecules. The atomic motion is assumed to be decomposable into normal-mode molecular vibrations and molecular rotations, and the latter are treated classically. An accuracy assessment of the general theory is performed through comparisons with the AMS Compton profiles of HD and NO, predicted by the full quantum chemistry-based AMS theory that is precise but can work only for diatomic molecules. Furthermore, a practical test is also performed through comparisons with experimental Compton profiles due to intramolecular H-atom motions in HD and CH₄. It is shown that the general theory is highly accurate, except at extremely low temperatures.