An optical absorption (OA) band of interstitial dichlorine monoxide moleculeswith peak at 4.7 eV and halfwidth 0.94 eV is identified in F$_2$ laser-irradiated((ħω $\equiv$7.9 eV) synthetic silica glass bearing both interstitial O$_2$and Cl$_2$molecules.Alongside with intrinsic defects, this OA band can contribute to solarization ofsilica glasses produced from SiCl$_4$. Although only the formation of ClClO isconfirmed by its Raman signature, its structural isomer ClOCl may alsocontribute to this induced OA band. Thermal destruction of this band between300°C and 400°C almost completely restores the preirradiation concentration ofinterstitial Cl$_2$. An additional weak OA band at 3.5 eV is tentatively assigned to ClO$_2$ molecules. The strongly forbidden 1272 nm infrared luminescence band ofexcited singlet O$_2$ molecules is observed at 3–3.5 eV excitation, demonstrating anenergy transfer process from photoexcited triplet Cl$_2$ to O$_2$. The energy transfermost likely occurs between Cl$_2$ and O$_2$ interstitial molecules located in neigh-boring nanosized interstitial voids in the structure of SiO$_2$ glass network.