High-order harmonic generation in the vanadium hexacarbonyl monoanion

HHG from [V(CO)₆]⁻  is investigated using TDDFT within the OEP formalism. The closed-shell anion provides a prototypical transition-metal complex featuring triply degenerate highest occupied molecular orbitals with strongly anisotropic spatial distributions arising from metal–ligand backbonding. The three HOMOs, derived primarily from vanadium d orbitals hybridized with CO π* orbitals, extend preferentially within the xy, xz, or yz planes, leading to markedly different spatial extents along the laser polarization direction.

Orbital-resolved HHG spectra demonstrate pronounced selectivity despite near-degeneracy of the HOMOs. Under linearly polarized mid- and long-wave infrared laser fields, the two HOMOs whose spatial distributions are elongated along the polarization direction dominate the HHG yield by several orders of magnitude, while the third HOMO contributes weakly. This result establishes orbital spatial extent, rather than orbital energy alone, as the primary factor governing HHG efficiency in this transition-metal complex.

Time-dependent analysis reveals that intense laser fields drive substantial redistribution of electron density away from the nuclear region, generating a transient many-electron potential that is strongly negative near the metal center and weakly positive at larger distances. This laser-induced potential dynamically shifts the HOMO energies to more negative values, effectively increasing the ionization potential and raising the tunneling barrier during the pulse. As a consequence, the high harmonic cutoff is significantly extended beyond standard semiclassical estimates, while overall harmonic yields are reduced due to suppressed tunneling.

Spectral decomposition further shows that below-threshold and mid-plateau harmonics receive substantial contributions from excited-state and continuum recombination pathways, producing spectral splitting and baseline elevation, whereas near-cutoff harmonics arise exclusively from ground-state recombination. These results demonstrate that HHG in [V(CO)₆]⁻ is governed by the interplay between orbital geometry and laser-driven many-electron dynamics, highlighting the sensitivity of strong-field emission to transition-metal electronic structure.