V. A. Apkarian

Photogeneration of ions via delocalized charge transfer states. I. Xe2H+ and Xe2D+ in solid Xe

Delocalized charge‐transfer excitations in solid xenon multiply doped with atomic halogens (I, Br, Cl) and hydrogen are demonstrated to lead to charge separation by trapping of the positive charge. As evidence of such a concept we present the first vibrational spectra of Xe2H+ and Xe2D+, which are believed to be vibrationally bound ions.

Dynamics of molecular reactions in solids: Photodissociation of HI in crystalline Xe

The photodissociation of HI impurities in a crystalline Xe host is studied by molecular dynamics simulations. From the calculated trajectories, analyses are given for: The behavior of the HI molecule before dissociation, the motions of the fragments following photon absorption, and the sites and vibration dynamics of the H fragments long after dissociation. The main findings include: (i) The photodissociation yield as a function of temperature is not monotonic (0 at 0 K, 0.2 at 17 K, 0.1 at 35 K). (ii) The nascent H atoms, at early time (∼0.5 ps), exhibit well defined, high frequency (∼900 cm−1) vibrations in the cage. The H trajectories acquire increasingly a random walk character with the progress of time. (iii) H exit from the cage is virtually never direct. (iv) After relaxation to equilibrium, the product H atoms occupy dilated interstitial sites (nearest‐neighbor xenons displace by 0.3 and 0.6 A at the octahedral and tetrahedral interstitial sites, respectively). (v) The H atom dynamically distorts ...

Photodissociation of F2 and mobility of F atoms in crystalline argon

Experimental studies on the photodissociation of F2 in 12 K crystalline argon solids are reported, and compared with molecular dynamics (MD) simulations of the same system. At excess energies above 2.5 eV,the dissociation probability is near unity, in agreement with theory. At the longest experimentally accessible wavelength for photodissociation, 450 nm, which corresponds to a dissociation excess energy of 1.16 eV, the permanent dissociation probability is reduced to 0.05. This is an order of magnitude smaller than that predicted by the simulations. Possible sources for this discrepancy are discussed. Long range migration of F atoms, predicted by MD simulations, is demonstrated to occur both upon photodissociation of F2, and upon relaxation of argon fluorides. In the latter case, it is shown that with the initial impulse, the F atoms migrate on average a length of ∼70 A in the lattice. The thermal recombination of F atoms in both solid Ar and Kr are also characterized. The experimental studies are conduc...

Many‐body potentials and dynamics based on diatomics‐in‐molecules: Vibrational frequency shifts in ArnHF (n=1–12,62) clusters

The conjecture that limited basis diatomics‐in‐molecules type potentials may serve as an accurate representation of many‐body interactions is explored through molecular dynamics simulations of ArnHF (n=1–12,62). The important ingredient in the constructed potentials is the inclusion of ionic configurations of HF. Once the admixture between ionic and covalent configurations is calibrated by reference to an ab initio surface of the ArHF dimer, a single three‐body potential energy surface is defined, and used in subsequent simulations of larger clusters. The vibrational frequencies of HF, which are computed from velocity–velocity autocorrelation functions, quantitatively reproduce the cluster size dependent redshifts.

Charge transfer within He clusters

Mass spectrometric measurements of clusters containing N helium atoms and an NO molecule yield probabilities for charge transfer from He+ to NO that range from 0.8±0.1 for 〈N〉=540 to 0.02±0.02 for 〈N〉=15u2009000. From this we estimate that the He+ hole hops approximately 70 times before localization at a He+2 core.

A solid state rare gas halide laser: XeF in crystalline argon

Abstract Optically pumped laser action has been achieved over the XeF (B→X) and (C→A) transitions in crystalline argon. Conversion efficiencies as high as 30% are observed. Gain measurements indicate that the laser should be tunable over the broad C→A band of ≈80 nm.

Condensed phase laser induced harpoon reactions

Laser induced charge transfer reactions of halogens in rare gas solids and liquids provide na powerful means for the study of condensed phase dynamics. Many-body effects with nrespect to both electronic and nuclear coordinates, and cooperative interactions with nradiation fields, are some of the studied phenomena that are highlighted in this article.

Charge transfer and radiative dissociation dynamics in fluorine‐doped solid krypton and argon

The photodissociation of F2 in crystalline Kr is subject to only a minor cage effect—quantum yield of 0.5 at 308 nm, at T=12 K. Two‐photon‐induced harpooning in the same system leads to dissociation with near unit quantum efficiency; it is shown that this absorption is coherent with a giant cross section of 3(±2)×10−45 cm4u2009u2009s at 275 nm. Excitation and emission spectra of charge‐transfer transitions in solid Kr doped with F atoms, and solid Ar multiply doped with F/Kr are reported. The vertical excitation in F/Kr leads to a state delocalized over immediate neighbors—assigned to Kr+6F−. This state localizes on a subpicosecond time scale to the triatomic Kr+2F− configuration. The latter relaxes radiatively (τr=140 ns), to the repulsive wall of the ground state (∼1 eV above ground). The excitation in F:Kr/Ar solids shows the diatomic KrF(B←X) resonances. From the analysis of the spectrum, a harmonic frequency ω=75(±3) cm−1 is extracted for F atoms in the ground state, consistent with molecular‐dynamics simula...

Electronic spectroscopy of oxygen atoms trapped in solid xenon

The electronic spectroscopy of oxygen atoms trapped in solid xenon are reported. Excitation of the solid between 220 and 260 nm leads to two main emissions centered at 3.35 and 1.65 eV, both of which arise from the same upper state. The excitations are assigned to charge‐transfer absorptions in interstitial Oh sites; the emitting state, which has a lifetime of 227 ns, is assigned to Xe+O−(1Σ+). An avoided crossing between the ionic and covalent 1Σ+ states, and differential solvation of these states, explains the main trends of the spectroscopy of XeO in the gas and condensed phases. Implications regarding ionic and covalent many‐body surfaces are discussed.

CHARGE LOCALIZATION AND FRAGMENTATION DYNAMICS OF IONIZED HELIUM CLUSTERS

The dynamics of Hen+, n=3–13, clusters formed by electron impact ionization of the neutral is studied theoretically using mixed quantum/classical dynamics by both mean-field and surface hopping methods. Potential energy surfaces and nonadiabatic couplings among them are determined from a semiempirical, minimal basis DIM Hamiltonian. The dynamics of hole hopping, hole localization, and cluster fragmentation are described through trajectory data. He3+ clusters, with initial conditions given by the zero-energy quantum distribution of nuclear coordinates, dissociate through two-channels, He+He+He+ and He+He2+ with relative yields of 20% and 80%. The motif of hole localization on a pair of atoms, and subsequent dissociation of the initial pair with hole hop to a new pair is observed in trimers, and repeats in larger clusters. In the larger clusters, hole hopping among He2 pairs provides an additional, less important mechanism of charge migration. The coupled electronic-nuclear dynamics of triatomic units descr...