We have observed quantum beats in the phosphorescence of tetramethylpyrazine-d 12 in a durene host crystal where the two upper spin components (тz, тy) decay via a common radiative channel. The initial phase coherence required to make the beats observable was created by a 1/2π microwave pulse resonant with the тz-тy transition at 857 MHz. When, after a few microseconds, the phase coherence is lost it can be restored by a second microwave pulse and the beats then appear as an echo signal: from this experiment one obtains a dephasing time T 2=1·2 μs.
In brown diamond, the long-lived emission at 2.818 eV is known to be due to a localized center in the photoexcited triplet state. In this paper, the emission intensity of the 2.818-eV center is studied as a function of the strength of an externally applied magnetic field. The cross-relaxation (CR) with N-V centers,
P1 centers, and g=2 doublet spins of unknown origin, as well as level anticrossing (LAC), is found to affect the phosphorescence intensity for certain magnetic-field strengths and orientations. The CR dynamics is studied optically by measuring the phosphorescence transients induced by microwave pulses resonant with a spin transition in the photoexcited triplet state. Under CR conditions, spin population relaxation in the excited triplet state becomes faster and deviates from a single-exponential recovery behavior. The triplet-state kinetics including CR effects is studied theoretically, and a double-exponential recovery is predicted. Based on an analysis of the kinetics, the experimental microwave recovery transients under the CR conditions are satisfactorily simulated. The CR rate is estimated to be (5.0\ifmmode\pm\else\textpm\fi{}1.5)\ifmmode\times\else\texttimes\fi{}
102 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for the CR with the N-V center and (8.0\ifmmode\pm\else\textpm\fi{}2.5)\ifmmode\times\else\texttimes\fi{}
102 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for the CR with the g=2 doublet species.