Anomalous hyperpolarizability dispersion measured for neon

Measurements of electric-field--induced second-harmonic generation are presented which show that the hyperpolarizability ({γ}) of the Ne atom decreases with increasing frequency at infrared frequencies. Such negative dispersion of {γ} at frequencies far below the first resonance is unprecedented. The new data, when combined with the results of ab initio calculations indicate that the electron-correlation contribution is 40% of γNe. This raises the question of the adequacy of Hartree-Fock calculations of the nonlinear optical properties of other atoms and molecules.