Optical characteristics of cylindrical exciplex and excimer lamps excited by microwave radiation

The object of investigation is the optical characteristics of Br₂, Cl₂, XeBr, and XeCl excilamps excited by microwave radiation. The power of the microwave discharge is determined from a pressure jump in the bulb of the lamp. Conditions for the effective generation of XeCl* and XeBr* exciplexes in cylindrical lamps are found. The efficiency of generation of these exciplexes equals 7.2 and 7.8%, respectively.     

Optical characteristics of plasma of I*<Subscript>2</Subscript>, Cl*<Subscript>2</Subscript>, Br*<Subscript>2</Subscript> halogen dimer barrier-discharge excilamps

Barrier-discharge excilamps operating in homonuclear chlorine, bromine, and iodine and their mixtures with inert gases have been studied. The spectral and energy characteristics of the barrier-discharge plasma have been obtained. The conditions have been determined at which the band D′ → A′ predominates in the spectra of molecules I*₂ (342 nm), Cl*₂ (257.8 nm), and Br*₂ (291 nm). The efficiencies of I₂, Cl₂, and Br₂ excilamps were found to be 1.6, 2, and 3.8%.     

Comparison of the interaction of Cl2 and Br2 with Cu(100)

The adsorption, reaction and etching of Cu(100) by Cl₂ was studied using temperature programmed desorption (TPD) and low energy electron diffraction (LEED), and the results were compared with recent results for Br₂. Although the general etching mechanism was the same for both gases (adsorption rate limited Cu halide formation followed by halide sublimation), significant differences between the behavior of Cl₂ and Br₂ were observed. The desorption of CuCl was characterized by a single zero order sublimation peak, independent of CuCl coverage, while limiting the CuBr coverage resulted in a desorption peak at temperatures lower than a prediction based on vapor pressure data of all known phases of CuBr. In addition, Cl₂ was found to be at least an order of magnitude less reactive than Br₂ towards halide formation. For both Cl₂ and Br₂, the halide formation rate reversibly decreased with increasing reaction temperature. However, for Br₂, but not Cl₂, annealing a chemisorbed halogen layer prior to further reaction irreversibly increased the halide formation rate. Structural differences between CuCl and CuBr on Cu(100) were also observed. For CuCl, LEED data suggested that highly faceted crystallites form at 325 K and remain stable until desorption, while LEED data for CuBr reveal a compressed epitaxial (111) layer that disorders below 400 K and then desorbs. The implications of these differences on etching and oxidation processes are discussed.     

Detection of vibrationally excited O2* by variable temperature photoelectron spectroscopy

Hot bands have been observed in the photoelectron spectra of the heavier diatomics Cl₂, Br₂, and J₂, at room temperature ^2–4. We report here the detection of hot bands in the photoelectron spectrum of O₂ at elevated temperatures using the technique of variable temperature photoelectron spectroscope ^{5, 6}.     

III: PROPERTIES OF COMPLEX SYSTEMS

The molecular structures, vibrational frequencies, dissociation energies and electron affinities of the Br₂/Br^? ₂, Br₂O/Br₂O^?, Br₂O₂/Br₂O^? ₂, Br₂O₃/Br₂O^? ₃, and Br₂O₄/Br₂O^? ₄ systems have been investigated using density functional theory (DFT) and hybrid Hartree-Fock/density functional theory. The methods used have been carefully calibrated against a comprehensive tabulation of experimental electron affinities, (Rienstra-Kiracofe, J. C., Tschumper, G. S., Shaefer, H. F., Nandi, S. and Ellison, G. B., 2002, Chem. Rev., 102, 231). Four different types of neutral/anion energetic difference are reported in this work: the adiabatic electron affinity (EA_ad), the zero-point corrected EA_ad (EA_zero), the vertical electron affinity (EA_vert), and the vertical detachment energy (VDE). The basis set used in this work is of double-zeta plus polarization quality with additional s- and p-type diffuse functions, and is denoted as DZP⁺⁺. The BHLYP method does well in reproducing the very limited experimental energetics, while the B3LYP method does well for the few known vibrational frequencies. The final predicted electron affinities with the BHLYP method are 3.41 eV (Br, experiment 3.36 eV), 3.02 eV (Br₂, experiment 2.6 ± 0.2 eV), 3.27 eV (Br₂O), 2.93 eV (Br₂O₂), 3.07 eV (Br₂O₃), and 2.54 eV (Br₂O₄). The global minimum structures for several of the larger dibromine oxides and their anions are unusual. For neutral Br₂O₂ the peroxide structure (BrOOBr) lies lowest, but for the anion a loosely bound C_s symmetry BrO-BrO^? structure lies lowest for the hybrid functionals, while a C₂ symmetry peroxide BrOOBr^? structure lies lowest for the pure functionals. Furthermore, the C₂ structures are found to exhibit an inverse symmetry breaking problem, and should be interpreted with caution. For neutral Br₂O₃, a chain structure Br-O₃-Br lies lowest, while the complex Br···O₃···Br^? lies lowest for the negative ion. For neutral Br₂O₄, a chain structure Br-O₄-Br lies lowest, while the complex BrO^?···BrO₃ lies lowest for the negative ion.     

Electron spectroscopy of autoionizing states of nitrogen atoms

During excitation of N₂, NO and N₂O molecules by fast neutral 1¹S helium atoms, highly excited molecular states are formed which yield, through predissociation, autoionizing nitrogen atoms. The energy of electrons released in autoionization was measured and the energy and plausible spectroscopic assignment of autoionizing levels was determined. The excited levels belong to Rydberg series converging to the N⁺(¹D) series limit. Besides molecular states derived by correlation of the final atoms, highly excited molecular parent states are also discussed.     

Luminescence of defect centres in Hg2Cl2

Abstract

Luminescence in Hg₂Cl₂, crystals excited with UV light is investigated in the spectral region 0.8–2.25 eV. Measurements are performed on as-grown samples and on samples previously exposed to UV light at RT. Six emission bands are found which depend on the concentration of the photochemical entities produced by irradiation of the crystals at RT. It is concluded that infra-red (IR) luminescence of Hg₂Cl₂ originates from crystal defects. The observed emission bands are tentatively attributed to the emission of (HgCI_xBr_3?x)^? and (HgCl_xBr_4?x)^2? complexes formed with residual Br impurities. Centres responsible for IR Hg₂Cl₂ emissions are excited: (i) via excitons of Hg₂Cl₂, (ii) via excited states of isolated Hg₂Br₂ molecules, and (iii) resonantly through the excitation bands of defect centres.     

Quadrupole interaction in the solid halogens—a new (not final) look

The nuclear quadrupole interaction in the molecular solids Cl₂, Br₂ and I₂ is reanalyzed with the help of density functional calculations. Though the present formalism overestimates the intermolecular interaction responsible for the solid–gas frequency shift and the asymmetry parameter η, a consistent picture is derived by appropriate corrections. The published experimental values of η for Cl₂ and Br₂ are found to be in error.     

Mean amplitudes of vibration of perhalogenated ethylenes

Mean amplitudes of vibration have been recalculated from spectroscopic data for C₂F₄, C₂Cl₄ and C₂Br₄. For C₂I₄ the calculated mean amplitudes are reported for the first time. The results for C₂Cl₄, C₂Br₄ and C₂I₄ agree perfectly satisfactorily with observed values from recent electron diffraction investigations of these molecules. Calculated values of generalized meansquare amplitudes for all the four molecules are also given.     

Isotope effects in Raman spectra of crystalline alkali-metal chlorates and bromates

Halogen and oxygen atom isotope effects have been observed in Raman spectra of single crystal NaClO₃, KClO₃, NaBrO₃, and KBrO₃. Orientational splitting of ³⁵Cl¹⁶O₂¹⁸O^? and ³⁷Cl¹⁶O₂¹⁸O^? ion vibrational modes was observed in KClO₃. Splitting of the v₁ mode of ⁷⁹Br and ⁸¹Br isotopes of singly substituted ¹⁷O and ¹⁸O bromate ions in NaBrO₃ and KBrO₃ is on the order of 1.5 cm^?1 in agreement with a calculation of isotopic frequencies. However, the predicted splitting between v₁ of the major isotopes, ⁷⁹Br¹⁶O₃^? and ⁸¹Br¹⁶O₃^?, was not observed, and the results of a band contour analysis suggest that a single k = 0 phonon is derived from the totally symmetric stretching mode of both species.     

He(I) photoelectron spectroscopy of anions in adiponitrile solution

He(I) photoelectron spectra are reported of solutions of salts in adiponitrile in which peaks characteristic of the anions are visible; the vertical ionisation energies of these peaks are 6.91 and 7.85 (I^?), 7.6 (Br^?), 8.1 (Cl^?), 7.0 (CNS^?), 8.1 and 6.8 (NO₂^?), and 7.6eV (SO₄^2?). The salts examined were tetra-n-butyl ammonium (NO₂^?, I^?, Br^?, Cl^?, CNS^?, SO₄^2?), N-methylpyridinium I^?, trimethylphenylammonium I^?, choline I^?, methyltrioctylammonium I^? and methyltriphenylphosphonium Br^?. The relationship between these spectra and the charge transfer to solvent spectra of the anions is discussed.     

13C NMR Chemical Shifts Substituent Effects of (E)- and (Z)-N-ethyl-N-Methylamides

Carbon-13 NMR chemical shifts of a series of (E)- and (Z)-N-ethyl-N-methylamides [RC(O)NEtMe, R=H, Me, Et, i-Pr, t-Bu, CF₃, ClCH₂, Cl₂CH, Cl₃C, BrCH₂, Br₂CH, Br₃C and ICH₂] are reported. The α-carbon and carbonyl carbon chemical shifts are correlated with the empirical α-substituent effect and Charton's electrical parameter ([sgrave]_I), respectively. The N-alkyl carbon resonances were attributed mainly to the γ- and δ-effects of R.     

Chemiluminescence of alkaline earth monohalides in the collisions of ground state Ca, Sr, and Ba atoms with Cl2, Br2, I2, ICl,, ICl, and IBr

Formation of electronically excited alkaline earth monohalides MX^* in the reactions of ground state Ca, Sr, and Ba atoms with Cl₂, Br₂, I₂, ICl, and IBr was studied in a beam-gas arrangement. The MX^* spectra were observed for all the reactive systems with the exception of Ca, . The energy balance indicates that MX^* can be formed in a single-collision exchange reaction; also the dependence of MX^*-chemiluminescence intensity on halogen gas pressure is typical of a bimolecular process. The MX^*-chemiluminescence cross sections and lower limits of photon yields are estimated. Received 17 April 2000 and Received in final form 18 July 2000     

The Formation of Dimeric Centers of Copper,Iron and Gadolinium in Modified Pb<Subscript>5</Subscript>Ge<Subscript>3</Subscript>O<Subscript>11</Subscript>

It is discovered that the electron paramagnetic resonance (EPR) spectrum of the doubly charged copper centers occurs in single crystals of Pb₅Ge₃O₁₁ doped with gadolinium or iron after annealing in an atmosphere containing chlorine and bromine. Similar annealing of the crystals doped with copper in a chlorine and fluorine atmosphere leads to redistribution of the intensities of the EPR spectra of two types of Cu²⁺ centers. The influence of annealing on the ongoing intensity of spectra of the dimeric triclinic centers Fe³⁺–A, Gd³⁺–A (where A represent Cl^?, Br^?, O^2?, F^?) was the subject of this research. Consideration is given to the mechanisms for changing the charge state and association of copper center with defects.     

Dynamics of gas-phase reactions of muonium

Applications of variational transition state theory with semiclassical adiabatic transmission coefficients to the reactions of muonium with H₂, D₂, F₂, and Cl₂ are reviewed. In addition new calculations are presented for the Mu+Cl₂ and Br₂ reactions. The comparison of calculated rate constants and kinetic isotope effects (KIE's) with experiment is used as a test of the validity of semiempirical potential energy surfaces. Furthermore the VTST-plus-tunneling calculations allow an analysis of the KIE's that identifies those features of the potential which control the isotope effects.     

Experimental and ab initio structure of BrN02

Abstract

The ν₂ fundamental bands of different isotopomers of BrN0₂ (⁷⁹Br¹⁵N¹⁶O₂, ⁸¹Br¹⁵N¹⁶O₂, ⁷⁹Br¹⁴N¹⁸O₂, and ⁷⁹Br¹⁴N¹⁶O¹⁸O) located around 13 µm were recorded using highresolution Fourier transform infrared spectrometry. More than 8000 lines of all these isotopomers were reproduced using a Watson-type A-reduced Hamiltonian with a rootmean-square deviation of better than 7×10^?4 cm^?1 for the four isotopomers. Rotational and centrifugal distortion constants for the ν₂=1 states as well as for the vibrational ground states of these isotopomers were determined. For the first time, an analysis of the ground-state rotational constants obtained in this study combined with the constants obtained in our previous work on the ν₂ bands of ⁷⁹Br¹⁴N¹⁶O₂, and ⁸¹Br¹⁴N¹⁶O₂, has allowed us to calculate the r_m, structure of nitryl bromide. The structural parameters obtained were r_m(Br–N)=2.0118(l6) Å, r_m(N–O)=l.l956(12) Å and α(O–N–O)=131.02(12)Å. A new ab initio structure of nitryl bromide calculated at the CCSD(T)/SDBaug-cc-pVQZ level of theory is presented and was found to be in fair agreement with the experimental structure.     

Thermodynamic and structural properties of liquids modelled by ‘2-Lennard-Jones centres’ pair potentials

Molecular dynamics calculations have been carried out for model liquid systems of N (=108 or 256) molecules interacting through two Lennard-Jones (12–6) centres coinciding with the positions of the atomic masses (the ‘atom-atom’ pair potential). The objectives were (a) to study the dependence of the properties on the molecular anisotropy defined by the reduced distance l*=l/σ between the centres in the range 0·5–0·8; and (b) to compare the computed quantities with those of real liquids (F₂, Cl₂, Br₂, CO₂). This paper deals with thermodynamic and structural features. Time-dependent correlations will be treated in a future communication.

In the liquid region not too far from the triple point the energy and pressure isochores are well represented by straight lines, the slopes of which increase with density and anisotropy. Thermodynamically consistent expressions for the energy and pressure as functions of density and temperature have been obtained for each system.

With Lennard-Jones parameters adjusted so as to secure the best overall fit, the agreement between experimental and computed thermodynamic properties is very satisfactory for F₂ (l*=0·505), quite good for Cl₂ and Br₂ (l*=0·608–0·63), but rather poor for CO₂ (l*=0·793). The ‘interatomic distances’ are close to the experimental values.

The static structural correlations are discussed in terms of the pair-correlation functions (pcf) g _A(r*) for the separation between ‘atoms’, the first few functions g_ll'm (R*) which arise from the expansion of the g(R*, θ₁, θ₂, φ₁₂) in spherical harmonics, and the pcf's for certain special near-neighbour configurations. The computed atom-atom structure factor is compared with the experimental data for liquid Br₂.

Mean square forces and torques have been evaluated and are related to some experimental results.     

LSP spectral changes correlating with SERS activation and quenching for R6G on immobilized Ag nanoparticles

In terms of chemical enhancement in Surface Enhanced Raman Scattering (SERS), we investigated the effect of halide and other anions to rhodamine 6G (R6G) adsorbed Ag particles that were immobilized on the substrates. The residual species on chemically prepared Ag particles such as citrate or a-carbon were thoroughly substituted by various anions, e.g., Cl⁻, Br⁻, I⁻, SCN⁻, CN⁻, or S₂O₃ ²⁻ anions, whose adsorption features are elucidated by the formation constants for AgX₂ ^(m−1)−, here X denotes the above anions. In particular, Cl⁻, Br⁻, or SCN⁻ ions activated SERS of R6G via intrinsic electronic interaction with Ag, whereas CN⁻, S₂O₃ ²⁻, or I⁻ anions quenched it due to their exclusive adsorption onto the Ag surfaces. We found that the activation process with the anions commonly yields a marked blue-shift of the coupled plasmon peak from ca. 650–700 to 500–550 nm in elastic scattering. It is rationalized by slight increase of the gap size between adjacent Ag nanoparticles by only ca. 1 nm based on theoretical simulations. This is probably caused by slight dissolution, oxidative etching, of the particles according to large formation constants of the complexes. Consequently, partly remaining negative charges on the Ag surface, and a slight increase in the gap size, providing huge electric field, facilitated R6G cations to adsorb on the nanoparticles, especially at the junction.     

Search for and study of phase transitions in some representatives of the APb2 X 5 family

Single crystals of KPb₂Cl₅, RbPb₂Cl₅, and RbPb₂Br₅ are grown and studied using optical polarization methods. The heat capacity of the crystals is investigated by differential scanning microcalorimetry, and the birefringence and the angle of rotation of the optical indicatrix are measured. The measurements are performed in the temperature range 270–640 K. It is found that KPb₂Cl₅ undergoes a first-order ferroelastic phase transition at T _0↑ = 530 K, T _0↓ = 528 K, and ΔH = 1000 ± 200 J/mol. The transition is accompanied by twinning and a change in symmetry mmm ? P2₁/c. The RbPb₂Cl₅ crystal remains monoclinic up to the melting temperature. The RbPb₂Br₅ compound belongs to the I4/mcm tetragonal modification and does not undergo structural transformations.     

Geometry,strength of binding and Br2 charge redistribution in the complex OC ··· Br2 determined by rotational spectroscopy

The ground-state rotational spectra of the six isotopomers ¹⁶O¹²C ··· ⁷⁹Br⁷⁹Br, ¹⁶O¹²C ··· ⁸¹Br⁷⁹Br, ¹⁶O¹²C ··· ⁸¹Br⁸¹Br, ¹⁶O¹²C ··· ⁷⁹Br⁸¹Br, ¹⁶O¹³C ··· ⁷⁹Br⁷⁹Br, ¹⁶O¹³C ··· ⁸¹Br⁷⁹Br, were observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. The spectroscopic constants B _O, D _J, χ _aa (Br_i), χ _aa (Br_o), M_bb (Br_i) and M _bb (Br_o), where i = inner and o = outer, were determined for each isotopomer. The complex is linear, with the weak bond between the C atom of CO and Br_i. The rotational constants were used to determine the distance r(C ··· Br_i) = 3.1058Å and to show that the Br—Br bond lengthens by ～0.005–0.01Å on complex formation. The intermolecular stretching force constant kσ = 5.0 Nm^?1 was obtained from D_J and the Br nuclear quadrupole coupling constants were interpreted to reveal that a fraction δ = 0.02 of an electronic charge is transferred from Br_i to Br_o when Br₂ is subsumed into the complex. Properties of the two series OC ··· XY and H₃N ··· XY, where XY = C1₂, Br₂ and BrC1, are compared.