COMPARISON OF LUMINESCENCE LIFETIMES FROM NATURAL AND LABORATORY IRRADIATED DIAMOND

Among gem diamonds, one of the most challenging identifications is distinguishing naturally irradiated diamond from laboratory irradiated diamond. In both cases, the GR1 optical center is created, which in high concentrations imparts a greenish color to the diamond. This research attempted to identify if time-resolved spectroscopy at the nanosecond scale would demonstrate substantive differences between the lifetime behavior of the GR1, in addition to the H3 and NV⁰ centers. Time-resolved luminescence has been used for several optical centers previously, but to our knowledge, this is the first study directly comparing data derived from natural and laboratory irradiated diamonds. All three centers showed nominally similar behavior and decay times and compare well with previously published values; however, the luminescence decay of treated diamonds showed more complex behavior with additional exponential decay components able to be resolved.

A suite of 26 naturally irradiated samples and 20 naturally sourced diamonds that were subsequently irradiated were examined. Luminescence lifetime measurements were performed at liquid nitrogen temperature using 12-watt white laser, a Horiba FluoroLog, a TBX-850 TE-cooled PMT and a Horiba FluoroHub-B. The decay curves contain both fast and slow decays and the data were best fit by using a series of two or three exponential decay components.

The H3 and NV⁰ centers have calculated components which show some variability in determined time constants between samples. As both these centers include nitrogen, the variability is likely caused by differences in the total or isolated nitrogen concentration. For both H3 and NV⁰ centers, nearly all of the laboratory irradiated samples show that a third component at longer times could be resolved in the data.

Compared with the decay behavior of the H3 and NV⁰ centers, the GR1 decay showed little variability in the values of the resolved components indicating that the lifetime of the GR1 center is not as susceptible to quenching. Most treated diamonds showed a longer time decay component, typically of 15-20 ns, that was not present in the natural samples. These differences in decay behavior indicate that lifetime decay behavior can provide an additional method to distinguish natural from laboratory irradiation in unknown samples.