Light induced bistability in S2O6F2⇄2 SO3F: Theory and experiment

Abstract

Bistability is predicted and experimentally verified in an illuminated, gas phase reaction mixture of peroxydisulfuryl difluoride and fluorosulfate free radicals. The steady state solutions of simple dynamical equations, based on the assumption of spatial uniformity, are in good agreement with hysteretic effects observed in bulk absorption measurements: Sharp transitions between stable branches of different absorption occur as the laser power used to irradiate the system is slowly varied. The appearance of a ‘‘flame’’, previously reported by others and noted here, is explained by an extension of the model which enables calculation of spatial inhomogeneities. The results of the extended model, when bulk averaged, are similar to those for the uniform system; however, large scale structure and steep gradients, corresponding to a region of intense fluorescence, or flame, are predicted in the case of high absorption. Experimental measurements of absorption profiles confirm these predictions. A critique is made of prior work on bistability in the N₂O₄/NO₂ system.