Preparation\nof High Vibrational States in the Entire\nMolecular Beam

Preparing highly excited molecules is of great interest\nin chemistry,\nbut it has long been a challenge due to the high laser power required\nwithin the narrow line width to excite a weak transition. We present\na cavity-enhanced infrared excitation scheme using a milliwatt laser.\nAs a demonstration, about 35% of CO molecules in a ground-state rotational\nlevel were excited to the highly excited <i>v</i> = 3 state\nin the entire pulsed supersonic beam, as confirmed by the depletion\nof molecules in the ground state. The method was also applied to excite\nHD molecules to the <i>v</i> = 2 state with a continuous-wave\ndiode laser. This work provides a universal approach to prepare molecules\nin a specific quantum state, paving the way to study the chemical\nreaction dynamics of highly excited molecules.