Molecular helium ion measurements find discrepancy between experiment and theory

Usually, the more particles in a molecule, the harder it is to make calculations of its properties. The molecular helium ion He₂⁺ consists of only two alpha particles and three electrons. This simplicity normally allows accurate calculation of its properties from first principles, or ab initio.

But Jansen et al. found a disagreement between ab initio calculations and experimental measurements of He₂⁺. The authors made the first accurate, high-precision measurements of the vibrational frequency and rotational structure of the first excited vibrational level of He₂⁺. Previously, it has been difficult to experimentally measure these values for He₂⁺ because it does not have a permanent electric dipole moment, precluding most standard infrared and microwave spectroscopy techniques.

The authors used pulsed laser radiation to record the spectra of He₂ molecules in Rydberg states, in which one electron is highly excited and which act like He₂⁺ ions. The highly excited electron moves in distant orbit, interacting weakly with the He₂⁺ ion. The authors used this excited electron to probe the vibrational and rotational motion of the ion. Next, the authors plan to use a continuous-wave laser to improve the precision and accuracy of their results.

Jansen said this discrepancy possibly indicates a flaw in the theoretical approach to calculating vibrational frequency and rotational structure and hopes that their findings will encourage theoreticians to perform new calculations. These precise measurements of this simple, three-electron system contribute to the fundamental understanding of bonding. It will also provide reference for scientists attempting calculations for other three-, four-, and five-particle molecules.

Source: “Fundamental vibration frequency and rotational structure of the first excited vibrational level of the molecular helium ion (He₂⁺),” by Paul Jansen, Luca Semeria, and Frédéric Merkt, The Journal of Chemical Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5051089 .