On the electronic structure of cleaved GaAs(110) surfaces exposed to formic acid

GaAs(110) surfaces cleaved in UHV and exposed to HCOOH have been studied by work function measurements (Kelvin method), electron energy loss spectroscopy (ELS) and by low energy electron diffraction (LEED). From the different changes of the work function on n- and p-type material information about intrinsic and extrinsic surface states is derived. In the loss spectra the adsorbed formate species causes a loss near 9 eV. The intensity of the loss near 20 eV generally ascribed to an excitonic transition from the Ga 3d core level into surface states is reduced only by a factor of two after saturation with HCOOH. This might be related to the c(2 × 2) superstructure observed in LEED, which suggests a saturation coverage of half a monolayer.     

Perturbation-enhanced quenching of laser-excited CS A1π(ν = 0) by atomic oxygen

Quenching of CS A¹Π(ν = 0) by O³ P is enhanced by perturbations due to á ³Σ⁺(ν = 10) and d³ Δ (ν = 4) The rotational-state dependence is linear in the singlet-triplet mixing coefficients and indicates an orientation-dependent interaction Symmetry correlations suggest that the perturbations promote formation of an attractive collision complex with quenching by intersystem crossing or chemical reaction.     

Symmetry adapted versus symmetry broken wavefunctions: the 1s core level ions of O+2

It is shown that, for O₂, in a MC SCF determination of the core ionization potentials employing the full Molecular point group, very few (N-1)-particle configurations are required in order to account for the symmetry breaking in the corresponding Hartree-Fock calculations.     

The Heitler-London interaction energy using the oscillator model

The oscillator model is used to compute the Heitler-London energy and the exchange potential for two interacting hydrogen atoms. With a single oscillator frequency it is not possible to obtain satisfactory results. Allowing the oscillator frequency to vary with internuclear separation enables reasonable values to be obtained.     

Picosecond absorption spectra of the second excited singlet state of a molecule in the condensed phase: Xanthione

The flourescence emission from the S₂ state of xanthione in benzene solution was time resolved using a Nd³⁺ glass mode-locker laser driven light gate technique. Using a separate optical apparatus, in conjunction with the mode-locked laser, transient absorption spectra were recorded at times subsequent to excitation. In both experiments excitation was provided by the 353 nm third harmonic of the neodymium laser. The exponential lifetime of the S₂ flourescence was found to be 12±3 ps. Two distinct transient absorption bands appear as a function of time after excitation. At early times a band centered at 620 nm is observed which we ascribe to absorption from S₂ while at longer times a band at 540 nm appears. This latter transient persists for up to 2 ns and we assign this band to absorption from T₁.     

Fluorescence from pyrene solubilized in aqueous micelles. A model for quenching by inorganic ions

When pyrene is solubilized within cetyl trimethyl ammonium bromide micelles the lifetime of its fluorescent state is very dependent upon the concentration of iodide ions within the surrounding aqueous phase. Quantitative examination of the kinetic data throws doubt on the existing model for quenching which requires pyrene to diffuse from within the hydrocarbon core to the peripheral region where encounter and quenching can occur. An alternative model is proposed which argues that the water-lipid interface penetrates deep into the micellar body close to the included pyrene such that quencher ions can encounter pyrene without the latter having to move appreciably. The observed effects of ionic strength add support to the new model.     

Production of atomic oxygen following flash photolysis of ClONO2

The photodissociation of ClONO₂ using a broad-band ultraviolet photolysis source has been investigated using time-resolved atomic absorption spectroscopy in the vacuum ultraviolet. The predominant atomic photolysis product is O(³PJ), the quantum yield for Cl(²PJ) production being less than 4%.     

Predissociation of the 3pπ D 1Πu+ state in H2, HD,and D2

Predissociation linewidths and lineshapes are reported for selected vibrational levels of the 3_pπ D ¹Π⁺_u state of H₂, HD, and D₂. We also discuss the effect of a perturbation by the 4_pδ B″B? ¹|gs_u⁺ state on the predissociation rate of the D state.     

Structure of non-uniform molecular fluids: Integrodifferential equations for the density-orientation profile

A family of integrodifferential equations are derived for the density -orientation profile, p(z₁ω₁), in a non-uniform molecular fluid. Three of these equations are generalizations of well-known results for simple (spherical molecule) fluids, while the remaining three apply only to molecular fluids. Applications to the gas-liquid interface are briefly discussed.     

A new approach to the measurement of the momentum densities in solids using an electron microscope

A new approach to the direct determination of electron momentum distributions in solids, using a combination of electron microscopy and electron spectroscopy, is described. The technique is several orders of magnitude more sensitive than currently available ones. The results of measurements on amorphous carbon are presented and used to demonstrate the potential of this new technique.