Resonance Enhanced Multi-Photon Ionization Detected mm-Wave Absorption: The 115 GHz Line of CO.

We investigate the potential for sensitively detecting mm-wave absorption through multi-photon ionization using the lowest rotational transition in CO (X$^1\Sigma$, J=1 $\leftarrow$ J=0) at 115~GHz as a diagnostic tool. This transition represents an almost perfect realization of a quantum mechanical 2-level system. In the experiment, the output of a powerful continuous mm-wave source with sub-kHz resolution propagates counter to a pulsed molecular beam causing Doppler shifted resonances. Absorption is detected by monitoring the involved rotational levels through resonance enhanced multi-photon ionization (REMPI). Observed polarization, saturation and Doppler effects demonstrate that the interaction with the mm-wave field is coherent and can be simulated well using the results for the 2-level system. The combination of these results indicates that the mm-wave field is approximated well by a gaussian beam with beam waist around 15~mm and constant effective electric field amplitude on the molecular beam axis. Time correlated frequency modulation in the form of a short rectangular pulse is used to follow the molecular trajectory from the source to the laser interaction region. The experiments clearly demonstrate a great potential of extending high-resolution molecular beam spectroscopy with mass specific REMPI detection into the sub-THz or THz regimes.