Vibration overtone hyperpolarizability measured for H₂

The second hyperpolarizability (γ) of the H₂ molecule was measured by gas-phase electric field induced second harmonic generation at the frequencies of the one-photon resonance for the 3–0 Q(J) overtone transitions (v, J = 0, J → 3, J for J = 0, 1, 2, and 3). The magnitude of the resonant contribution to γ was measured with 2% accuracy using the previously determined non-resonant γ for calibration. Pressure broadening and frequency shift for the transitions were also measured. A theoretical expression for the resonant vibrational γ contribution in terms of transition polarizabilities is compared to the observations. The measured γ resonance strength is 4%–14% larger than the results obtained from this theoretical expression evaluated using ab initio transition polarizabilities.