We theoretically investigate the distinct roles of magnetic dipole and electric quadrupole interactions in photoelectron momentum distributions (PMDs) arising from the strong-field multiphoton ionization of atomic hydrogen by linearly polarized, 800-nm laser pulses. By applying a Power-Zienau-Woolley-type transformation to the standard minimal-coupling Hamiltonian, we circumvent the inherent challenges associated with isolating individual nondipole contributions within conventional velocity-gauge formulations. Our ab initio simulations reveal that while both terms significantly influence the PMD, the electric quadrupole interaction becomes dominant at higher laser intensities. Crucially, we find that the magnetic dipole term induces a positive (forward) shift of the distribution peak, whereas the electric quadrupole term drives an even more pronounced negative (backward) shift. The resulting net peak shift is negative, establishing a gauge-independent physical mechanism that explains recent experimental observations.

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• Atomic & molecular processes in external fields
• Light-matter interaction
• Multiphoton or tunneling ionization & excitation
• Strong electromagnetic field effects