Nature 495, 490–494 (2013)

Unlike atoms, when it comes to cooling, molecules can be very stubborn. Because of their complicated internal vibrational motion, even the simplest molecules are unaccommodating of laser cooling. Other ways of removing energy from molecules — such as sympathetic cooling, which relies on collisions with a partner more amenable to laser cooling — prove equally challenging because the molecules usually react with the cohabitating atomic species. But Wade Rellergert and colleagues have now demonstrated that ultracold atoms can efficiently quench molecular vibrational motion.

Rellergert et al. trapped barium monochloride ions and brought them into contact with a cloud of ultracold calcium atoms, that are held in a magneto-optical trap.

Surprisingly, the elastic collisions between the neutral atoms and the molecular ions do not result in a chemical reaction, but in the damping of the molecular vibrational motion. To check the outcome of the cooling, the authors used a new thermometric technique based on molecular photodissociation — the laser-induced dissociation of molecules in a certain vibrational state.

The degree of cooling demonstrated is modest, but further technical improvements promise highly efficient vibrational, and even rotational, cooling.