ELECTRONIC SPIN-ECHO STUDIES OF TRIPLET BIACETYL UNDER HIGH-PRESSURE

Optically detected two-pulse and three-pulse echo experiments have been performed under pressure to 50 kbar using a diamond anvil cell. In this paper we report the results on excited triplet neat biacetyl (BA) and BA-d6. Three-pulse echoes are easier to perform, and coherence has been maintained for durations longer than one triplet lifetime. In contrast, the two-pulse echo technique is prone to have spurious contributions, and it is besieged by the envelope modulation effect for BA-d6. Our data point to the importance of spectral diffusion caused by the bulk nuclei. Their flip-flop motion creates a fluctuating magnetic field on the triplet spin, destroying its coherence. As pressure increases, an increase in number density of such nuclei shortens the phase memory time (T(M)). A plot of T(M) vs relative volume produces a linear correlation. Our three-pulse echo lifetime shows a 40% increase from BA-h6 to BA-d6. This very small deuteration effect might suggest that electronic flip-flop, rather than nuclear spin flip-flop, is the dominant relaxing mode. However, a study of T(M) on different exciting light levels disproves this mechanism.