Isotope effects in the electronic spectra of ammonia using ab initio semiclassical dynamics

The experimentally observed isotope effects in the absorption spectra of ammonia are reproduced theoretically using the semiclassical thawed Gaussian approximation. As a single trajectory method, it can benefit from the on-the-fly ab initio implementation, in which the potential is evaluated locally. Whereas the standard global harmonic approximation fails due to the high anharmonicity of the first excited electronic state of ammonia, the experimental spectra of ammonia isotopologues (NH₃, NDH₂, ND₂H, ND₃) are very well reproduced by the semiclassical on-the-fly calculations. Moreover, the isotope effects – narrowing of transition band, shift in 0-0 transition, and decreasing of the peak spacing – are all captured by this semiclassical method. The activation of different normal modes upon excitation in the case of each isotopologue is further discussed in detail.