title: Dynamical interrogation of the hydration cage of bromine in single crystal clathrate hydrates versus water.
abstract: We report transient grating measurements carried out on single crystals of bromine clathrate hydrates and on bromine dissolved in water. In all cases, excitation into the B-state of Br2 leads to prompt predissociation, followed by cage-induced recombination on the A/A' electronic surfaces. In liquid water, the vibrationally incoherent recombinant population peaks at t=1 ps and decays with a time constant of 1.8 ps. In the hydrate crystals, the recombination is sufficiently impulsive to manifest coherent oscillations of the reformed bond. In tetragonal TS-I crystals, with the smaller cages, the recombination is fast, t=360 fs, and the bond oscillation period is 240 fs. In cubic CS-II crystals, the recombination is slower, t=490 fs, and the visibility of the vibrational coherence, which shows a period of 290 fs, is significantly reduced due to the larger cages and the looser fit around bromine. The mechanical cage effect is quantified in terms of the recombination time-distribution, the first three moments of which are associated with size, structural rigidity, and anelasticity of the cage. In the crystalline cages, the distribution is symmetric about the mean: mean time tm=300 fs, 400 fs and standard deviation sigma=70 fs, 100 fs, in TS-I and CS-II, respectively. The finding is consistent with the assignment of occupied cages: principally 5(12)6(2) polyhedra in TS-I and 5(12)6(4) polyhedra in CS-II. In liquid water, with diffuse cages, the distribution characterized by tm=555 fs and sigma=400 fs, is strongly skewed (gamma1=1.88) toward delayed recombination-the effective liquid phase hydration shell is larger than that in a hydrate phase, structurally disordered, and anelastic. Information about dipolar disorder, comparable in all three media, is extracted from electronic predissociation rates of the B-state, which is sensitive to the symmetry in the guest-host interaction.
authors.affiliation: Department of Chemistry, University of California, Irvine, CA 92697, USA.
authors.forename: I U
authors.initials: IU
authors.lastname: Goldschleger
authors.forename: G
authors.initials: G
authors.lastname: Kerenskaya
authors.forename: V
authors.initials: V
authors.lastname: Senekerimyan
authors.forename: K C
authors.initials: KC
authors.lastname: Janda
authors.forename: V A
authors.initials: VA
authors.lastname: Apkarian