The Low-Lying States of He2

Area of Science:

• Molecular Spectroscopy
• Quantum Chemistry
• Atomic and Molecular Physics

Background:

• The Helium dimer (He2) is a fundamental van der Waals molecule, crucial for understanding interatomic interactions.
• Previous studies have investigated various electronic states of He2, but a comprehensive analysis of multiple states remained incomplete.
• Accurate molecular constants are essential for theoretical models and predicting He2 behavior under different conditions.

Purpose of the Study:

• To record and analyze the near-infrared emission spectrum of He2 at high resolution.
• To perform a global analysis of six excited electronic states of He2 by combining new and existing spectroscopic data.
• To derive improved molecular constants and equilibrium parameters for these states.

Main Methods:

• High-resolution Fourier transform spectroscopy was used to record He2 emission spectra in the 9000–15 000 cm−1 region.
• Spectroscopic data from previous infrared and laser measurements were integrated for a global analysis.
• Radiofrequency measurements were incorporated to accurately determine the fine structure of the a3Σ+u state.

Main Results:

• The c3Σ+g–a3Σ+u and C1Σ+g–A1Σ+u transitions of He2 were observed and assigned.
• A consistent set of improved molecular constants was derived for the c3Σ+g, b3Πg, a3Σ+u, C1Σ+g, B1Πg, and A1Σ+u electronic states (v = 0–2).
• The vibrational dependence of these constants was studied, yielding equilibrium parameters for the analyzed states.

Conclusions:

• This study provides the most comprehensive set of molecular constants for the investigated excited states of He2 to date.
• The derived equilibrium parameters enhance our understanding of the potential energy surfaces of these electronic states.
• The results serve as a benchmark for theoretical calculations and future spectroscopic investigations of van der Waals molecules.