‘‘Gentle recoil’’ synthesis of free-radical clusters via unimolecular photolysis of closed shell complexes

Abstract

Thermal rate constants for R+N2H2→RH+N2H (R=H, OH, NH2) determined from multireference configuration interaction and variational transition state theory calculations

Cluster photofragmentation dynamics: Quasiclassical trajectory studies of Arn–H2S and Arn–SH (n=1,2)

“…6͑a͒ confirms that significant amounts of energy are tied up in van der Waals vibration. This is completely consistent with the experimental results, 28,29 which identified a long sequence of van der Waals stretching states in the Ar-SH LIF spectrum. Second, the vibrational energy distribution decreases to zero at the dissociation limit.…”

“…One can readily work through the ballistic model proposed previously to estimate the energetics of radical cluster formation, 28,29 focusing on hydride-containing precursors (H 2 X, e.g., XϭO, S͒ with an arbitrary additional subunit ͑M͒. For HX uncoupled from the rest of the cluster during the photolysis step, the departing H-atom carries away the majority of the excess energy, leaving XH with translational energy T XH :…”

Photodissociation of Ar–HCl: An energy-resolved study of the dynamics of total fragmentation into H+Ar+Cl

“…At a given excitation energy of the cluster, the available kinetic energy for fragment recoil is different for the two spin-orbit states of the Cl atom ͓smaller in the case of Cl( 2 P 1/2 )͔. Based on the gentle recoil mechanism, 24 the probability of formation of Ar-Cl radicals may significantly change if the two possibilities of radical products, Ar-Cl( 2 P 3/2 ) and Ar-Cl( 2 P 1/2 ) ͑with different binding energies͒ are considered in the calculation, instead of only the Ar-Cl( 2 P 3/2 ) one. In addition, interference between resonances associated with different excited electronic states may also change the hydrogen fragment distribution with respect to that calculated for a single electronic surface.…”

“…More recently, production of Ar-SH and Ar 2 -SH radicals has been directly observed with high efficiency after ultraviolet photolysis of Ar n -H 2 S (nр2) clusters. 24 Nesbitt and co-workers have proposed a ''gentle recoil'' mechanism 24 to explain the high production of open-shell complexes by photodissociation of the Ar n -H 2 S (nр2) clusters. Such a mechanism would also be valid in the photodissociation of Ar-HCl and other related Rg-HX clusters.…”

“…Even in those cases, however, the radical can survive if most of the recoil energy remains as translational energy of the radical center of mass, and only a relatively small amount ͑not enough to fragment the radical͒ is converted into internal excitation. The extensive vibro-rotational excitation observed in the Ar n -SH (nр2) radicals, 24 and predicted for the Ar-Cl radicals 15,23 seems to indicate that the direct photodissociation process follows a similar mechanism in the clusters Ar n -H 2 S (nр2) and Ar-HCl ͑and probably other Rg-HX systems͒. Thus direct dissociation of the H atom would lead both to total and partial fragmentation of the cluster, which in the case of Ar-HCl would yield HϩArϩCl, and HϩAr-Cl products, respectively.…”