Kinetics of processes involving excited bromine (Br*(2P1/2)) and iodine (I*(2P1/2)) atoms during broad-band IBr photolysis

As demonstrated in [2], the observed many-fold acceleration of quenching of spin-orbit excited Br*(P-2(1/2)) atoms during broad-band IBr photolysis with increasing fraction df photodissociated IBr is easily accounted for in terms of formation of vibrationally excited IBrupsilon molecules by "self-quenching" of bound electron-excited states IBr* arising in this system: IBr*+IBr-->2IBr(upsilon) (upsilon <40), because at upsilon >7, IBrupsilon ensures Br* deactivation by a fast endothermic reaction with conservation of the spinorbit state of halogen atoms: Br*(P-2(1/2))+IBrupsilon(upsilon >7)-->Br-2+I*(P-2(1/2)). Based on the analysis of the plausible reactions involving spin-orbit-excited bromine and iodine atoms and vibrationally excited IBrupsilon molecules during broad-band photolysis of IBr, which is an active medium of a pulsed photodissociation bromine laser (Br*(P-2(1/2))-->Br(P-2(3/2))+hv(2.7 mu m)) considered as a promising candidate for a continuous laser with solar pumping, it is concluded that the lasing medium (IBr) decomposes to Bra and It by a chain mechanism through a series of fast exchange reactions (Br*+IBrupsilon(upsilon >7)-->Br-2+I* and I*IBr-->I-2+Br*. Considering the importance of the aforesaid results to the realization of the continuous lasing mode of a Br*-laser operated by IBr dissociation, this study was intended to gain experimental evidence to support this light-induced nonequilibrium pumping of IBr during its broad-band photolysis and to prove the dominant role of IBrupsilon in Br* quenching, excelling the role of another process commonly taken into account - Br* relaxation on other photolysis products. The analysis of the well known, yet unsuccessful (because lasing is interrupted very soon), attempts to sustain continuous operation of a Br-laser has revealed that IBrupsilon formation explains their failure which is associated not so much with fast decay of Br* by its reaction with IBrupsilon as with chain decomposition of IBr to Br-2 and I-2 capable of consuming several times more IBr that direct photodissociation of IBr. A few specific ways of muffling the effect of IBr* land, hence, IBrupsilon) formation during broad-band IBr photolysis to achieve continuous radiation of a Br-laser operated by IBr dissociation and pumped by solar radiation are proposed.