Preparation of High Vibrational States in the Entire Molecular Beam

Preparing highly excited molecules is of great interest in chemistry, but it has long been a challenge due to the high laser power required within the narrow line width to excite a weak transition. We present a cavity-enhanced infrared excitation scheme using a milliwatt laser. As a demonstration, about 35% of CO molecules in a ground-state rotational level were excited to the highly excited v = 3 state in the entire pulsed supersonic beam, as confirmed by the depletion of molecules in the ground state. The method was also applied to excite HD molecules to the v = 2 state with a continuous-wave diode laser. This work provides a universal approach to prepare molecules in a specific quantum state, paving the way to study the chemical reaction dynamics of highly excited molecules.