Cross Relaxation Studies in Diamond

Abstract

A microwave double resonance experiment performed on the paramagnetic nitrogen centers in diamond shows that in this system cross relaxation occurs via a four spin flip mechanism which exactly conserves Zeeman energy. In this process, which was first postulated by Bloembergen and co-workers in their paper on cross relaxation, two spins of the center line make a downward transition while a spin belonging to each satellite makes an upward transition. Simple rate considerations for this process indicate that if a saturating microwave field is suddenly applied to one of the three lines of the nitrogen spectrum, a weak probing microwave signal at either of the two other lines should register a definite change in absorption in a time ${T}_{21}$. Specifically, if ${T}_{21}$ is much less than other relaxation times of the system, then setting the pump upon the center line should force the absorption at either satellite to drop to zero. Setting the pump at the position of one of the satellites, on the other hand, should reduce the center line absorption to $\frac{3}{5}$ its thermal equilibrium value but should increase the absorption measured at the other satellite by the factor $\frac{6}{5}$. This behavior was precisely observed at 1.6\ifmmode^\circ\else\textdegree\fi{}K, using a bi-modal cavity.By resolving the rate at which a satellite decays to zero when the pump is set on the center line, ${T}_{21}$ is measured for all five satellites in the three principal orientations: ${H}_{0}\ensuremath{\parallel}[100]$, ${H}_{0}\ensuremath{\parallel}[110]$, and ${H}_{0}\ensuremath{\parallel}[111]$. The measured anisotropy is discussed.It is shown that the four spin flip transition may be used in special cases to establish continuous wave maser operation by inverting the population of one of the satellite lines. Steady state inversion of one of the nitrogen satellites is incidentally observed in a number of diamonds.