Exploration of the origin of anomalous dependence for near-threshold harmonics in ${{\rm{H}}}_{2}^{+}$ on the ellipticity of driving laser fields

The anomalous dependence of near-threshold harmonics in the molecular ion on the ellipticity of the driving near-infrared laser field is studied theoretically based on accurate solution of the time-dependent Schrodinger equation in prolate spheroidal coordinates with the help of the generalized pseudospectral method. For these harmonics, the maximum radiation energy corresponds to a non-zero ellipticity of the driving field. Our analysis reveals that the origin of the phenomenon lies in the near-resonant excitation of π-symmetry molecular orbitals. The excited states responsible for the anomalous ellipticity dependence of different near-threshold harmonics are identified. The effect is confirmed at the equilibrium internuclear separation R = 2 a.u. as well as for stretched molecules at R = 3 a.u.