Motional Stark Effect in High Magnetic Fields: A New Technique for Sub-Doppler Spectroscopy

We have observed transitions between Rydberg states of He with sub-Doppler resolution, using a novel technique. Ordinary Doppler line shapes, observed at low magnetic fields, are completely altered at high fields by the quadratic Stark effect induced by velocity-dependent electric fields arising from atomic motion transverse to the magnetic field. The high-field line shape has an abrupt cutoff, corresponding to $v_{\perp }\simeq 0$ atoms, whose slope is primarily limited by the homogeneous linewidth.