Appl. Magn. Reson. 11, 539-552 (1996) Applied Magnetic Resonance 9 Springer-Verlag 1996 Printed in Austria Raman Heterodyne Detected Magnetic Resonance: II. Magnetic Transitions of NV Centre at Level Anticrossing Region C. Wei, S.A. Hoimstrom, N. B. Manson, and J. P. D. Martin Laser Physics Centre, Australian National University, Canberra, Australia Received October 25, 1995; revised March 8, 1996 Abstraet. In the preceding paper [1] we reported both cw and coherent transient measurements carried out in EPR and NMR transitions within the 3A ground state of the nitrogen-vacancy cen- tre in diamond using the Raman heterodyne detection technique. In this paper we use these mea- surements to characte¡ the nuclear magnetic transitions near a level anticrossing situation. The level anticrossing causes a mixing of the electronic spin and nuclear spin wave functions which results in a greatly enhanced NMR transition moment. The amount of mixing not only affects the dipole moment but, correspondingly, the characteristic relaxation times. In this paper we report the measurement of these parameters in the nitrogen-vacancy centre asa function of applied Zeeman field strength and analyse the results using the spin Hamiltonian formalism. Furthermore, com- bined with the particular features of the Raman heterodyne technique, such a system represents an ideal testing ground for the nonlinear behaviour of strongly driven transitions. Some results are illustrated, including dynamic Zeeman splitting and gain without inversion. I. Introduction Level crossing/anticrossing is a simple but fundamental quantum mechanics problem which occurs when two states approach the same energy (in this work, due to the Zeeman effect) [2]. Level crossing occurs when the two approach- ing states do not interact. In this case they have the same energy at the cross- ing point and their wave functions remain unchanged. The more common and most interesting situation is one where there is ah interaction between the two states which causes a mixing of the wave functions. In this case a level anticrossing takes place and the two states do not attain the same energy. The wave function mixing is a maximum at the anticrossing point while the sepa- ration of the two states reaches a minimum. Despite its simplicity, there are many interesting phenomena associated with this situation and it has often been a subject of interest [3, 4].