## 3 Results and Discussion
### Three-Dimensional Fluorescence Spectroscopy  
The 3D fluorescence spectra can provide fluorescence intensity information when the excitation wavelength (EX) and emission wavelength (EM) are changed simultaneously. In this paper, the contour fluorescence spectrum is used to analyze the test results of the samples. In order to more comprehensively display the 3D fluorescence characteristics of the samples, according to the excitation wavelength, it was divided into two wavelength ranges of 250-400 nm and 400-600 nm for testing and analysis. The yellow sample of R-Y-1 showed two fluorescence peaks in the fluorescence spectrum in the excitation wavelength range of 400-600 nm. One of the fluorescence peaks had an emission wavelength peak at 536 nm (excitation wavelength was 481 nm), and it was accompanied by a relatively wide band. The second fluorescence peak was at 457 nm (excitation wavelength was 403 nm) and was accompanied by a band of 365-510 nm (Figure 7a). The two fluorescence peaks of the other yellow sample R-Y-2 showed at 449 nm (EX = 400 nm) and 535 nm (EX = 490 nm). However, the yellow-green sample R-YG-1 had no obvious fluorescence peak in the excitation wavelength range of 400-600 nm and was mainly concentrated in the range of 250-400 nm. The fluorescence peak of this sample was located at 443 nm, accompanied by a wide fluorescence band of 260-500 nm, and the correspond  
Figure 6: PL spectra of diamond samples. ing excitation wavelength was 397 nm (Figure 7b). For the brown sample R-B-1, a more prominent fluorescence peak appeared at 450 nm, accompanied by a band of \(\sim\)250-625 nm (Figure 7c). The case of the purple-red sample R-P-1 was different from that of the previous four samples. Three fluorescence peaks appeared in the excitation wavelength range of 250-400 nm, which were located at 441 nm (EX = 394 nm), 676 nm (EX = 313 nm), and 665 nm (EX = \(\sim\)250) (Figure 7d). Based on the analysis of the data (Table 2), the fluorescence peaks of the 3D fluorescence spectra of HTHP-treated diamonds have the following characteristics. (1) Samples of different colors have fluorescence peaks in the range of 440-450 nm, which are often accompanied by a broad band ranging from 350 to 500 nm, and the excitation wavelength corresponding to the fluorescence peak is around 400 nm. The \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Samples** & **EM (nm)** & **EX (nm)** & **Accompanied Band** \\ \hline R-Y-1 & 536 & 481 & 500–590 \\ R-Y-1 & 457 & 403 & 365–510 \\ R-Y-2 & 535 & 490 & 475–610 \\ R-Y-2 & 449 & around 400 & 390–480 \\ R-YG-1 & 443 & 397 & 260–505 \\ R-B-1 & 450 & around 400 & \(\sim\)250–625 \\  & 441 & 394 & 365–545 \\ R-P-1 & 676 & 313 & 595–785 \\  & 665 & around 250 & 575–790 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The 3D fluorescence peak distribution of samples. Figure 7: The contour fluorescence spectra of the diamond samples. Sample R-Y-1 shows two fluorescence peaks (I, IV) in the excitation wavelength range of 400–600 nm **(a)**; the fluorescence peak of sample R-YG-1 is located at 443 nm (I) **(b)**; the brown sample R-B-1 shows a more prominent fluorescence peak at 450 nm (I) **(c)**; the purple-red sample R-P-1 shows three fluorescence peaks (I, III, IV) appearing in the excitation wavelength range of 250–400 nm (**d**). brown sample has special characteristics, accompanied by the band extending towards longer wavelengths. (2) The yellow samples have double fluorescence peaks in the excitation wavelength range of 400-600 nm. In addition to the first characteristic fluorescence peak, there is a second fluorescence peak at 535 nm, and it is accompanied by a 500-600 nm fluorescence band. The corresponding excitation wavelength is around 480 nm.