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This paper extends the Wilson-Hamiltonian framework to compute the rovibrational spectra of diatomic molecules in strong magnetic fields, addressing the lack of experimental data in this domain. By incorporating full non-perturbative coupling between particle motion and magnetic fields, the authors derive electric quadrupole transition moment integrals and analyze spectral changes such as peak shifting and splitting. The findings provide the first fully quantum mechanical results for the rovibrational signature of \( ^1\mathrm{H}_{2} \) in extreme magnetic environments, offering insights crucial for experimental interpretation and astrochemical modeling.
Spectral changes in diatomic molecules under strong magnetic fields reveal unexpected phenomena like bond stiffening and symmetry-breaking, challenging existing theoretical frameworks.
In the absence of experimental data for molecular spectra in strong magnetic fields, high resolution and reliable computational spectra are required for the interpretation of spectra collected from highly magnetic astrophysical objects. In this paper, we extend the Wilson-Hamiltonian framework, recently implemented and benchmarked by us (\textit{J. Chem. Theory Comput.}, \textbf{21}, 9753 (2025) ), to a general three-dimensional framework suitable for computing the rovibrational spectra of diatomic molecules in strong uniform magnetic fields. The field-dependent electronic and nuclear Hamiltonians capture full non-perturbative coupling between particle motion and the field making the method applicable to all field strengths. The electric quadrupole transition moment integrals for rovibrational transitions in external static magnetic fields are formulated, implemented, and computed to yield spectra which respect the selection rules of the molecule-field system. The spectral changes with increasing field strength such as shifting, splitting, merging, appearance and disappearance of peaks are noted. Contributions from electrons and nuclei are studied individually, as well as in unison to reveal the underlying physics such as stiffening of the bond, emergence of a rotational barrier, field-induced coupling/decoupling of states, and symmetry-breaking in rotational and vibrational states. These results provide the first fully quantum mechanical computational results for the rovibrational signature of $^1\mathrm{H}_{2}$ in extreme magnetic field environments with accuracy suitable for experimental interpretation. The methodology developed herein has direct relevance for high-field spectroscopy and astrochemical modeling, both for providing computational data as well as for understanding the spectral impact of strong magnetic fields on electronic structure and nuclear motion in molecules.
