This technique employs a variable magnetic field to bring molecular energy levels into resonance with a fixed laser frequency. A Fabry-Perot resonator is transversely pumped by a CW CO2 laser and an intra-cavity beamsplitter is used to select parallel or perpendicular laser polarisation (with respect to the magnetic field). A discharge occurs in the sample region of the cavity located the between pole faces of a large (typically 0 - 2 T) magnet. The intra-cavity laser power is measured as a function of the magnetic field and a small sinusoidal field is superimposed on the d.c. field so that lock-in detection can be utilised. This technique is particularly suitable for open-shell molecules, where the energy levels tune significantly when a magnetic field is applied. The first transitions to be observed using this technique were in the lowest vibrational level of the
state of HBr+[28]. It has also been applied to the a
state of OH+ in order to study the J = 3-2 transition in the ground vibrational state [29]. A general review of laser magnetic resonance can be found in [30].