Routes to control of H2 Coulomb explosion in few-cycle laser pulses

We have measured coincident ion pairs produced in the Coulomb explosion of H2 by 8-30 fs laser pulses at different laser intensities. We show how the Coulomb explosion of H2 can be experimentally controlled by tuning the appropriate pulse duration and laser intensity. For laser pulses less than 15 fs, we found that the rescattering-induced Coulomb explosion is dominated by first-return recollisions, while for longer pulses and at the proper laser intensity, the third return can be made to be the major one. Additionally, by choosing suitable pulse duration and laser intensity, we show H2 Coulomb explosion proceeding through three distinct processes that are simultaneously observable, each exhibiting different characteristics and revealing distinctive time information about the H2 evolution in the laser pulse.     

Ground and excited states of NH4: Electron propagator and quantum defect analysis

Vertical excitation energies of the Rydberg radical NH4 are inferred from ab initio electron propagator calculations on the electron affinities of NH4+. The adiabatic ionization energy of NH4 is evaluated with coupled-cluster calculations. These predictions provide optimal parameters for the molecular-adapted quantum defect orbital method, which is used to determine Einstein emission coefficients and radiative lifetimes. Comparisons with spectroscopic data and previous calculations are discussed.     

Structural defects cause TiO2-based photocatalysts to be active in visible light

Oxidation of TiO or Ti2O3 led to the formation of TiO2 with activity in visible light much higher than when TiN was used as a precursor, pointing out the importance of oxygen defects/vacancies for extension of activity of TiO2 into the visible region.     

Reynolds number effects on flow/acoustic mechanisms in spherical windscreens

There is a practical need to fully understand the mechanisms involved in the flow/pressure fluctuations around a screened microphone. A stream of uniform flow with low-frequency turbulence encountering a rigid, impermeable spherical windscreen is considered in this study. Pressure distributions on the surface of the sphere are determined by the flow structure. Pressure fluctuations at the center of the sphere are then calculated based on the integration of surface pressure distributions. Because of the low-frequency assumption, results from steady-state laminar flows can be used to investigate the Reynolds number effects on wind noise reduction. Three types of flow have been studied in this paper: an inviscid case, a low-Reynolds-number Stokes flow, and intermediate- and high-Reynolds-number flows. A Reynolds-number/wind-noise-reduction correlation shows that the wind noise reduction increases with decreasing Reynolds number.     

Rescattering double ionization of D2 and H2 by intense laser pulses

We have measured momentum spectra and branching ratios of charged ionic fragments emitted in the double ionization of D2 (and H2) molecules by short intense laser pulses. We find high-energy coincident D+ (and H+) ion pairs with kinetic energy releases between 8 and 19 eV which appear for linearly polarized light but are absent for circularly polarized light. The dependence on the polarization, the energy distributions of the ions, and the dependence on laser intensity of yield ratios lead us to interpret these ion pairs as due to a rescattering mechanism for the double ionization. A quantitative model is presented which accounts for the major features of the data.     

Mapping the potential energy surface of the tolylcarbene rearrangement in the inner phase of a hemicarcerand

Photolysis of p-tolyldiazirine (6) in the inner phase of a hemicarcerand with four butane-1,4-dioxy linker groups (5) in C(6)D(5)CD(3) at 77 K yields the 5-methylcycloheptatetraene hemicarceplex 5 circle 3b in 41% together with innermolecular reaction products resulting from an insertion of transient p-tolylcarbene (1b) into an acetal C-H or linker C-O bond of 5 and from the addition of 1b to an aryl unit of 5. The yield of incarcerated 3b increased up to 67% if 6 is photolyzed inside a hemicarcerand with deuterated spanners and butane-1,4-dioxy linker groups (d(48)-5). Hemicarceplex 5 circle 3b is not formed if the photolysis is carried out in CDCl(3). Incarcerated 3b is stable at room temperature in the absence of oxygen and is characterized by 1D and 2D NMR spectroscopy. In the presence of oxygen, 3b reacts quantitatively to yield toluene and CO(2). Upon heating solutions of d(48)-5 circle 3b in C(6)D(5)CD(3), 3b rearranges to 1b and m-tolylcarbene (18). Both tolylcarbenes immediately react with the surrounding host. From a product analysis and the measured rate constants for the thermal decomposition of d(48)-5 circle 3b in the temperature range 70-102 degrees C, the activation parameters for the 3b to 1b and 3b to 18 rearrangements are calculated (3b to 1b: DeltaG(373)++ = 27.3 +/- 1.4 kcal/mol, DeltaH(373)++ = 26.4 +/- 1.0 kcal/mol, TDeltaS(373)++ = -0.9 +/- 1.0 kcal/mol; 3b to 18: DeltaG(373)++ = 27.8 +/- 1.4 kcal/mol, DeltaH(373)++ = 19.7 +/- 1.0 kcal/mol, TDeltaS(373)++ = 8.1 +/- 1.0 kcal/mol). These values are compared with those calculated by Geise and Hadad at the B3LYP/6-311+G** level of theory (Geise, C. M.; Hadad, C. M. J. Org. Chem. 2002, 67, 2532-2540). The slightly higher inner phase activation free energy of the 3b to 18 rearrangement is explained through steric constraints imposed by the surrounding hemicarcerand on the transition state. The enthalpy-entropy compensation observed for the 3b to 18 rearrangement is discussed and interpreted as a result of a hemicarcerand and solvent reorganization along the reaction coordinate. It is taken as indirect evidence for the intermediacy of 2-methylbicyclo[4.1.0]hepta-2,4,6-triene in the 3b to 18 rearrangement.     

Upper bounds on a two-term exponential sum*

We obtain upper bounds for mixed exponential sums of the type where p^m is a prime power with m⩾ 2 and X is a multiplicative character (mod p^m). If X is primitive or p⫮(a, b) then we obtain |S(χ,f,p ^m)| ⩽2np ^{2/3 m}. If X is of conductor p and p⫮( a, b) then we get the stronger bound |S(χ,f,p ^m)|⩽np ^m/2. This paper is dedicated to Prof. Wang Yuan on the occasion of his 70th birthday.     

Kinematically complete charge exchange experiment in the Cs(+)+Rb collision system using a MOT target

Charge exchange is examined with unprecedented precision using the newly developed magneto-optical trap-target recoil ion momentum spectroscopy (MOTRIMS) technique. Initial and final state selective, charge exchange cross sections are obtained for 6 keV Cs+ colliding with rubidium in 5s and 5p states. For each charge transfer channel, cross sections differential in scattering angle are measured. These data are used to overturn previous long-standing conjecture as to the origin of oscillations seen in total charge exchange cross section measurements, and compare well with an enhanced Demkov model calculation.     

Extensions of Modules over Hopf Algebras Arising from Lie Algebras of Cartan Type

In this paper we prove that there are no self-extensions of simple modules over restricted Lie algebras of Cartan type. The proof given by Andersen for classical Lie algebras not only uses the representation theory of the Lie algebra, but also representations of the corresponding reductive algebraic group. The proof presented in the paper follows in the same spirit by using the construction of a infinite-dimensional Hopf algebra D(G) u( ) containing u( ) as a normal Hopf subalgebra, and the representation theory of this algebra developed in our previous work. Finite-dimensional hyperalgebra analogs D(G _r ) u( ) have also been constructed, and the results are stated in this setting.     

“Step-up” versus “step-down” scattering asymmetry in the neutralization of H on free-electron vicinal metal surfaces

The charge transfer between H⁻ and a free-electron vicinal metallic surface is studied using a wave-packet propagation method. We apply a statistical Thomas-Fermi-von Weizsäcker model with a local density approximation for the exchange-correlation energy to compute the ground-state electronic structure of the substrate. The long-range image charge effects in the electron transfer are included on a phenomenological level. We obtain the ion-survival probability from a rate equation for a set of realistic scattering trajectories of projectiles that are incident with a kinetic energy of 50 eV. Our calculations reveal a pronounced substrate orientation dependence of the charge transfer dynamics expressed in a “left-right” (or “step-up-step-down”) scattering asymmetry in the final ion-survival probability, which is caused by an enhancement of electron loss on the outgoing part of those ion trajectories which approach steps from below.