Terahertz laser velocity modulation spectroscopy of ions

Velocity modulation spectroscopy has been investigated in the terahertz region, employing pure rotational transitions of ArH + and rotation-tunneling transitions of H3O + to study the competition between pressure broadening and Doppler broadening on the lineshapes, neutral suppression, and modulation efficiency. Velocity modulation is demonstrated to be effective to frequencies as low as 60 cm � 1 , yielding S/N values for ArH + (770/1) that surpass published terahertz FM results by an order