Preparing State-Selected Molecular Beams with high Efficiency using Quantum Cascade Lasers

We demonstrate the ro-vibrational state-selective preparation of CO molecules ($v$ = 0, J = 0 $\rightarrow$ $v$ = 1, J = 1) in an internally cold molecular beam using a continuous-wave, narrow-linewidth distributed feedback type quantum cascade laser, with an output wavelength of 4.65 $μ$m. Using a modulation-free wavelength stabilization scheme, we show that population transfer to near saturation levels can be obtained with sufficient long term stability, as needed for state-resolved scattering experiments. We show preliminary results of CO molecules prepared in the $v$ = 1 state, scattering from a clean Cu(100) surface. This study showcases the promise of QCL based excitation for state-selected molecular beam based scattering experiments, especially involving transitions in the mid-infrared domain ($λ$ > 4.5 $μ$m) which are otherwise difficult to access using conventional sources.