Vibration overtone hyperpolarizability measured for H₂.

Area of Science:

• Molecular Spectroscopy
• Quantum Chemistry
• Nonlinear Optics

Background:

• The second hyperpolarizability (γ) is a crucial molecular property influencing nonlinear optical phenomena.
• Accurate measurement of γ is essential for understanding molecular response to electric fields.
• Previous studies have focused on non-resonant contributions, necessitating investigation into resonant effects.

Purpose of the Study:

• To experimentally measure the second hyperpolarizability (γ) of the H₂ molecule.
• To investigate the resonant vibrational contributions to γ at specific overtone transitions.
• To compare experimental findings with theoretical predictions for γ.

Main Methods:

• Gas-phase electric field induced second harmonic generation (EFISHG) technique.
• Measurements performed at the one-photon resonance frequencies of H₂ 3-0 Q(J) overtone transitions (v, J = 0, J → 3, J for J = 0, 1, 2, and 3).
• Calibration using previously determined non-resonant γ values; measurement of pressure broadening and frequency shift.

Main Results:

• The resonant contribution to the second hyperpolarizability (γ) of H₂ was measured with 2% accuracy.
• Measured γ resonance strength exceeded theoretical predictions by 4%-14%.
• Pressure broadening and frequency shift data for the overtone transitions were obtained.

Conclusions:

• Experimental measurements provide valuable data for validating theoretical models of molecular hyperpolarizability.
• Discrepancies between experimental and theoretical γ values highlight the need for refined theoretical approaches.
• This study advances the understanding of resonant vibrational contributions to molecular nonlinear optical properties.