Combination of doppler-free polarization spectroscopy and magnetic rotation for the example of IBr

We describe a spectroscopic method which combines for the first time Doppler-free laser polarization spectroscopy with the magnetic rotation technique. This is achieved by the application of a modulating weak magnetic field to the overlap region of modulated pump and probe beam. By the double modulation, molecular levels with effectiveg-factors as low as 0.05 Gm_B can easily be detected and distinguished from diamagnetic levels. This is demonstrated for the molecule IBr where theB'0⁺ state is perturbed by a repulsive = 1 state which leads to increasingg-factors with increasing vibrational levels. With the normal polarization spectroscopy, several of these levels are not detectable within the manifold of overlapping lines from I₂ and Br₂ which are always present in this chemical system. The new method is well suited for characterizing perturbed molecular levels.Dedicated to Prof. Dr. Herbert Welling on the occasion of his 60th birthday