The effects of collisional quenching on photofragment emission (PFE) detection of vapor-phase $\mathrm{Hg}{\mathrm{Cl}}_2$ in combustion flue gas constituents are investigated. Exciting $\mathrm{Hg}{\mathrm{Cl}}_2$ via the $1{}^1\mathrm{\Pi }_u\leftarrow 1{}^1\mathrm{\Sigma }_g^{+}$ transition, time-resolved measurements of emission from the $\mathrm{Hg}(6^3{P}_1)$ daughter in buffer-gas mixtures of ${\mathrm{N}}_2$ , ${\mathrm{O}}_2$ , and ${\mathrm{CO}}_2$ indicate that the fragmentation pathway passes through a long-lived intermediate species, which we assign to $\mathrm{Hg}(6^3{P}_2)$ . Total quenching rate coefficients of $\mathrm{Hg}(6^3{P}_1)$ by ${\mathrm{N}}_2$ , ${\mathrm{O}}_2$ , and ${\mathrm{CO}}_2$ are consistent with values reported in the literature. In addition, total quenching rate coefficients for the intermediate $\mathrm{Hg}(6^3{P}_2)$ state are determined to be $1.72(\pm 0.08)\times {10}^{-10}{\mathrm{cm}}^3{\text{molecule}}^{-1}{\mathrm{s}}^{-1}$ and $2.90(\pm 0.37)\times {10}^{-10}{\mathrm{cm}}^3{\text{molecule}}^{-1}{\mathrm{s}}^{-1}$ for ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$ , respectively. An analysis of the impact of the collisionally dependent energy-transfer process that precedes the formation of $\mathrm{Hg}(6^3{P}_1)$ on the use of PFE to measure $\mathrm{Hg}{\mathrm{Cl}}_2$ concentration is presented.