Low-temperature inelastic collisions between hydrogen molecules and helium atoms

Inelastic H(2):He collisions are studied from the experimental and theoretical points of view between 22 and 180 K. State-to-state cross sections and rates are calculated at the converged close-coupling level employing recent potential energy surfaces (PES): The MR-PES [J. Chem. Phys. 100, 4336 (1994)], and the MMR-PES and BMP-PESs [J. Chem. Phys. 119, 3187 (2003)]. The fundamental rates k(2-->0) and k(3-->1) for H(2):He collisions are assessed experimentally on the basis of a master equation describing the time evolution of rotational populations of H(2) in the vibrational ground state. These populations are measured in the paraxial region of supersonic jets of H(2)+He mixtures by means of high-sensitivity and high spatial resolution Raman spectroscopy. Good agreement between theory and experiment is found for the k(2-->0) rate derived from the MR-PES, but not for the BMP-PES. For the k(3-->1) rate, which is about one-third to one-half of k(2-->0), the result is less conclusive. The experimental k(3-->1) rate is compatible within experimental error with the values calculated from both PESs. In spite of this uncertainty, the global consistence of experiment and theory in the framework of Boltzmann equation supports the MR-PES and MMR-PESs, and the set of gas-dynamic equations employed to describe the paraxial region of the jet at a molecular level.     

Alkylation of Imidazole by Solid-Liquid Phase Transfer Catalysis in the Absence of Solvent

Phase Transfer Catalysis in the absence of solvent is described as a useful and general method for the selective N-alkylation of imidazole. In all cases high yields are obtained while quaternization is avoided.     

Inelastic collisions in molecular oxygen at low temperature (4 ≤ T ≤ 34 K). Close-coupling calculations versus experiment

Close-coupling calculations and experiment are combined in this work, which is aimed at establishing a set of state-to-state rate coefficients for elementary processes ij → lm in O(2):O(2) collisions at low temperature involving the rotational states i, j, l, m of the vibrational ground state of (16)O(2)((3)Σ(g)(-)). First, a set of cross sections for inelastic collisions is calculated as a function of the collision energy at the converged close-coupled level via the MOLSCAT code, using a recent ab-initio potential energy surface for O(2)-O(2) [M. Bartolomei et al., J. Chem. Phys. 133, 124311 (2010)]. Then, the corresponding rates for the temperature range 4 ≤ T ≤ 34 K are derived from the cross sections. The link between theory and experiment is a Master Equation which accounts for the time evolution of rotational populations in a reference volume of gas in terms of the collision rates. This Master Equation provides a linear function of the rates for each rotational state and temperature. In the experiment, the evolution of rotational populations is measured by Raman spectroscopy in a tiny reference volume (≈2 × 10(-4) mm(3)) of O(2) travelling along the axis of a supersonic jet at a velocity of ≈700 m/s. The accuracy of the calculated rates is assessed experimentally for 10 ≤ T ≤ 34 K by means of the Master Equation. The rates, jointly with their confidence interval estimated by Monte Carlo simulation, account to within the experimental uncertainty for the evolution of the populations of the N = 1, 3, 5, 7 rotational triads along the supersonic jet. Confidence intervals range from ≈6% for the dominant rates at 34 K, up to ≈17% at 10 K. These results provide an experimental validation of state-to-state rates for O(2):O(2) inelastic collisions calculated in the close-coupling approach and, indirectly, of the anisotropy of the O(2)-O(2) intermolecular potential employed in the calculation for energies up to 300 cm(-1).     

Observation of a Mott insulating ground state for Sn/Ge(111) at low temperature

We report an investigation on the properties of 0.33 ML of Sn on Ge(111) at temperatures down to 5 K. Low-energy electron diffraction and scanning tunneling microscopy show that the (3x3) phase formed at approximately 200 K, reverts to a new ((square root 3)x(square root 3))R30 degrees phase below 30 K. The vertical distortion characteristic of the (3x3) phase is lost across the phase transition, which is fully reversible. Angle-resolved photoemission experiments show that, concomitantly with the structural phase transition, a metal-insulator phase transition takes place. The ((square root 3)x(square root 3))R30 degrees ground state is interpreted as the formation of a Mott insulator for a narrow half-filled band in a two-dimensional triangular lattice.     

The Raman spectra and cross-sections of H2O, D2O, and HDO in the OH/OD stretching regions

We report the OH and OD stretching regions of the vapor phase Raman spectra of H₂O, and of a D₂O/HDO mixture, at room temperature. Also, the corresponding spectrum of H₂O at ∼2000 K in a methane/air flame is reported. These spectra are interpreted in terms of transition moments of the molecular polarizability, based on high-level ab initio calculations of the polarizability surface, and on variational wavefunctions considering the rotational-vibrational coupling in full. As a byproduct of this analysis several tables have been compiled including scattering strengths and assignments for individual rotational transitions of the three species. From these tables the Raman spectra in the OH/OD stretching regions can be simulated over the range of temperatures up to 2000 K for H₂O, and up to 300 K for D₂O and HDO.     

<Superscript>206</Superscript>Pb/<Superscript>207</Superscript>Pb ratios in dry deposit samples from the Metropolitan Zone of Mexico Valley

²⁰⁶Pb/²⁰⁷Pb isotope ratios of dry deposit samples in the Metropolitan Zone of Mexico Valley (MZMV) were determined and correlated with some contemporary environmental material such as gasoline, urban dust, etc., as possibile pollution sources, the latter presenting different signatures ²⁰⁶Pb/²⁰⁷Pb ratios were determined in samples “as is” by ICP-MS, using an Elan-6100. A standard material NIST-981 was used to monitor accuracy and to correct mass fractionation. The calculated enrichment factors of lead (taking rubidium as a conservative endogenous element) show its anthropogenic origin with percentages higher than 97.65%. ²⁰⁶Pb/²⁰⁷Pb ratio in dry deposit samples ranges from 0.816 to a maximum of 1.154, following a normal distribution. Arithmetic mean was 0.9967±0.0864 lower than those of possible pollution sources: 1.1395±0.0165 for gasoline, 1.071±0.008 for industrially derived lead and, for the more radiogenic natural soil and urban dust values ranging from 1.2082±0.022 to 1.211±0.108. The possible origin of lead in gasoline used prior to 1960 is discussed.     

Evaluation of heavy metal and elemental composition of particles in suspended matter of the Upper Course of the Lerma River

The metal distribution of Cr, Mn, Fe, Cu, Zn, and Pb, and the elemental composition, morphology and relative abundance of particles have been evaluated in suspended matter of the Upper Course of the Lerma River in Mexico. The metal concentrations in the suspended matter decrease in the sequence: Fe > Mn > Zn > Cu = Cr > Pb. The Fe and Mn concentrations in the suspended matter are produced principally by natural contributions whereas the Cu, Zn, Cr, and Pb concentrations are produced principally by anthropogenic contributions. In general, the particle groups decrease in the following sequence: aluminosilicate > silica > sulphur > metals > calcium > manganese > iron > calcium carbonate.     

Inelastic collisions in molecular nitrogen at low temperature (2 < or = T < or = 50 K)

Theory and experiment are combined in a novel approach aimed at establishing a set of two-body state-to-state rates for elementary processes ij --> lm in low temperature N(2):N(2) collisions involving the rotational states i,j,l,m. First, a set of 148 collision cross sections is calculated as a function of the collision energy at the converged close-coupled level via the MOLSCAT code, using a recent potential energy surface for N(2)-N(2). Then, the corresponding rates for the range of 2 < or = T < or = 50 K are derived from the cross sections. The link between theory and experiment, aimed at assessing the calculated rates, is a master equation which accounts for the time evolution of rotational populations in a reference volume of gas in terms of the collision rates. In the experiment, the evolution of rotational populations is measured by Raman spectroscopy in a tiny reference volume (approximately 2 x 10(-3) mm(3)) of N(2) traveling along the axis of a supersonic jet. The calculated collisional rates are assessed experimentally in the range of 4 < or = T < or = 35 K by means of the master equation, and then are scaled by averaging over a large set of experimental data. The scaled rates account accurately for the evolution of the rotational populations measured in a wide range of conditions. Accuracy of 10% is estimated for the main scaled rates.     

Interaction of atomic hydrogen with the beta-SiC(100) 3 x 2 surface and subsurface

We investigate clean and atomic hydrogen exposed beta-SiC(100) 3 x 2 surfaces by synchrotron radiation-based Si 2p core-level photoemission spectroscopy. The clean 3 x 2 surface reconstruction exhibits three surface and subsurface components. Upon hydrogen exposures, those surface and subsurface components are shifted to lower binding energies by large values, indicating significant charge transfer to the surface and subsurface regions, in excellent agreement with the recently discovered H-induced beta-SiC(100) 3 x 2 surface metallization. In addition, the interaction of hydrogen results in a large reactive component at Si 2p supporting an asymmetric charge transfer in the third plane below the surface, in agreement with previous experimental investigations. However, the results are inconsistent with recent ab initio theoretical "frozen" calculations predicting H atom to be in a bridge-bond position.     

Giant alkali-metal-induced lattice relaxation as the driving force of the insulating phase of alkali-metal/Si(111):B

Ab initio density-functional theory calculations, photoemission spectroscopy (PES), scanning tunneling microscopy, and spectroscopy (STM, STS) have been used to solve the 2sqrt[3]×2sqrt[3]R30 surface reconstruction observed previously by LEED on 0.5 ML K/Si:B. A large K-induced vertical lattice relaxation occurring only for 3/4 of Si adatoms is shown to quantitatively explain both the chemical shift of 1.14 eV and the ratio 1/3 measured on the two distinct B 1s core levels. A gap is observed between valence and conduction surface bands by ARPES and STS which is shown to have mainly a Si-B character. Finally, the calculated STM images agree with our experimental results. This work solves the controversy about the origin of the insulating ground state of alkali-metal/Si(111):B semiconducting interfaces which were believed previously to be related to many-body effects.