• HD⁺is made up of a hydrogen atom (protium) and a deuterium atom, both bound to an electron. It's the simplest type of molecular ion with different kinds of nuclei. Doing really accurate spectral experiments on HD⁺is super important for understanding how three - body systems work when they're bound together. The frequencies at which HD⁺vibrates and rotates are related to the masses of the nucleus and the electron. So, by putting together theory and experiments, we can use the very precise spectrum of HD⁺to figure out related physical constants. For example, we can find the "proton-electron mass ratio," which is a constant that doesn't have any units.

Molecular Hydrogen (HD⁺) Ions

 

• In recent years, preparing (ultra-) cold molecules and molecular ions has become a trendy research topic. The problem is that most molecular ions don't have a simple, repeating cycle for energy transitions. This is the biggest roadblock for directly using lasers to cool them down. However, sympathetic cooling, which relies on the Coulomb interaction between laser-cooled ions and other ions that cannot be directly laser-cooled, is a highly effective approach for cooling molecular ions. This method has substantially advanced the research on (ultra-) cold molecular ions.

A bi-component Coulomb Crsytal of Be⁺/HD⁺ ions