## 3 Results and Discussion
### Photoluminescence (PL)
In addition, these dyes did not exhibit any PL when excited with band-pass filters with wavelengths greater than 585 nm. This confirms the conclusion that the D1, D2, and D3 dyes belong to the triarylmethane family  
Figure 14: PL spectra of TLC bands of (**a**) reference crystal violet and (**b**) crystal violet band for the RO4 sample (subgroup I) using the ten different preset optical excitation filters of the VSC. of dyes. On the other hand, the D4 dye which is suggested to be phthalocyanine did not express PL response under excitation of the different band-pass filters. In summary, the VSC studies resulted in the successful categorization of the 30 studied BP pen samples into four main groups based on the IRL characteristics. The TLC results revealed the presence of crystal violet as an essential dye component in all examined samples, along with some other dyes from the triarylmethane family of dyes. The crystal violet demonstrated weak PL using the first excitation filter (400-485 nm) and under filters beginning from the seventh to the tenth filter in the band-pass range of 585 nm-800 nm. The ink samples of Group I were distinguished by the presence of D1 in their corresponding TLC plates, which was prominently visible, particularly in RX1 and RO4 samples relative to the PE1 sample. The TLC analysis of the TO1 sample revealed the presence of crystal violet only, without any indication of the presence of D1 or other dyes. Nevertheless, the PL spectrum of the ink stroke of the TO1 sample, as well as the corresponding IRL images, indicated the presence of other dye components that match D1. This suggests that D1 dye exists in the TO1 sample in very low concentrations that are undetectable using TLC. The strong PL observed in Group I can be attributed to the presence of D1, as this dye exhibited a high PL signal throughout the entire excitation range, as shown in Figure 15a. Furthermore, D2 and D3 exhibited a higher IRL than crystal violet upon optical excitation using the first (400-485 nm) band-pass filter, and comparable IRL to that of crystal violet under excitation wavelengths greater than 585 nm. These results agree  
Figure 15: PL spectra of the TLC bands of (**a**) D1, (**b**) D2, and (**c**) D3 dyes separated from the solvated ink samples of the RO4, PR2, and PK1 pens, respectively. with the optical characteristics of the ink stroke samples containing these dyes (subgroup II and subgroup III). This strongly suggests that crystal violet and triarylmethane dyes (D2 and D3) are responsible for the IRL characteristics observed in both Group II and Group III samples [26; 38]. For samples of the subgroup IV which expressed no luminescence, IRL could be observed in the separated dyes, including crystal violet and D2. This could be explained by the presence of phthalocyanine dye (D4) within the ink matrix of those samples with absorption maxima centered around 670 nm, as shown in Figure 10. The absorption peak at 670 nm overlaps with the IRL wavelength of the triarylmethane dyes (660-690 nm) under excitation using the first five optical filters with band-pass in the range 400-585 nm, as shown in Figure 15. The overlap between the absorption of phthalocyanine and emission of triarylmethane dyes could result in photoluminescence quenching. The samples including the hand-written \"A\" letter using the solvated inks showed IRL characteristics similar to crystal violet. The solvation of different inks could result in diluted writing ink solutions, which in turn causes a decrease in the overall dye concentration. Among all dyes present, crystal violet expressed a higher relative concentration compared to other dyes. This marginalized the concentrations of the other dyes compared to crystal violet, which caused the IRL of crystal violet to appear as the most prominent dye in subgroups I, II, and III. For subgroup IV samples, the different solubility extents were reflected in the IRL properties of the corresponding written samples using the solvated samples.