Nanoparticle over mirror (NPoM) cavities have been a source of extreme electromagnetic field confinement in sub-nm gaps, leading to extraordinary performance for surface-enhanced Raman scattering (SERS) spectroscopy. However, the performance of the overall NPoM cavity containing a large number of nanoparticles over metal has always been a point of concern. An NPoM cavity fabricated by trapping nanoparticles with Bessel beam illumination from a fiber optic negative axicon achieved five- and two-fold enhancements for excitation wavelengths of 785 and 532 nm, respectively, while it was three- and 1.75-fold in the conventional cavity. We investigated the photodimerization mechanism of 4-aminothiophenol (4-ATP) using SERS spectroscopy within an NPoM cavity. The research is focused on the laser-induced conversion or chemical enhancement (CE) of 4-ATP to 4,4′-dimercaptoazobenzene (DMAB), a process for understanding reaction dynamics at the nanoscale. Control experiments were performed by varying excitation laser power and integration time, revealing a higher optical-driven conversion of 4-ATP to DMAB at a higher laser power. Additional control experiments focused on the impact of oxidation on the Ag film substrate. It was found that an unoxidized Ag film is ∼91% less efficient for SERS compared with an oxidized film. X-ray photoelectron spectroscopy (XPS) confirmed the presence of oxygen in oxidized film although these results provide significant insights into the reaction dynamics. The study’s limitations include trapping dependency on nanoparticle characteristics, complexity of nanoscale interactions, narrow experimental conditions, and a focus on a single reaction pathway. These factors highlight areas for future research. Ultimately, this study advances the understanding of laser-induced reactions within NPoM cavities with SERS as a powerful tool for investigating molecular transformations, suggesting potential applications in sensing and plexciton coupling.