The microwave spectrum of the SiCl radical

The microwave spectrum of ²⁸Si³⁵Cl, the most abundant isotopic species of the silicon monochloride radical, was observed in both the ${}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and ${}^2\text{Π}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ spin states of the ground vibronic state. The SiCl radical was produced in a flow cell by a dc discharge in SiCl₄. The observed transitions were $\text{J =}\text{7}\text{2}\text{←}\text{5}\text{2}\text{up to}\text{21}\text{2}\text{←}\text{19}\text{2}$ for both the spin states, and the observed frequencies were subjected to the least-squares analysis to yield accurate molecular constants as follows: B₀ = 7652.3048(23), D₀ = 0.007017(14), A_J = ?0.8392(16), p₀ + 2q₀ = 138.660(98), q₀ = 0.20(17), $\text{a + (b + c)}\text{2}\text{= 37.50(28)}$, $\text{a ? (b + c)}\text{2}\text{= 49.84(73)}$, b = 9(12), d = 46.40(94), and eQq₁ = ?23.13(96), all in MHz with 3σ in parentheses. The positive sign of the Λ-doubling constant, p₀, indicates that the contributions of ²Σ^? states dominate over those of ²Σ⁺ states. The spin and orbital averages of the unpaired-electron distribution were calculated from the observed hyperfine coupling constants and were discussed in terms of the electronic structure of the molecule.     

The adsorption of CO on evaporated iridium films,studied with infrared reflection-absorption spectroscopy

The adsorption of ^12CO on Ir films evaporated under ultrahigh vacuum (UHV) conditions was studied using infrared reflection-absorption spectroscopy (IRAS). Only a single absorption band was observed at 300 K, shifting continuously from the “singleton” value ~2010 cm^?1 at very low coverages to 2093 cm^?1 at saturation coverage. This band is attributed to CO adsorbed on top of the surface atoms. Synchronously with this shift the bandwidth at half maximum intensity $\text{Δv}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ decreases from ~30 to 8 cm^?1. The integrated peak area increases linearly with coverage up to a relative coverage (θ_r) of approximately 0.4, then the increase levels off and a maximum is observed. Upon continuing adsorption the intensity decreases slightly. In addition results are presented on adsorption at 300 K of ¹²CO?¹³CO isotopic mixtures. The coverage induced frequency shift is discussed in terms of a dipole-dipole coupling mechanism and it is concluded that intermolecular coupling can explain the shift (~83 cm^?1) observed. The decrease in intensity at coverages > 0.4 is attributed to the formation of a compressed overlayer with part of the CO molecules adsorbed in a multicentre position with different spectral properties. No infrared bands of nitrogen adsorbed at 78 K could be detected at pressures up to 6.7 kPa (1 Pa = 0.0075 Torr, 1 Torr = 133.32 Pa).     

The temperature programmed desorption of hydrogen from platinum and platinum-gold films

The thermal desorption of hydrogen from Pt and PtAu films has been measured in an ultra-high vacuum system by means of a mass spectrometer. On the average, hydrogen is more loosely bound on the alloys than on pure Pt. About 50% of the adsorbate is desorbed by pumping at 78 K from the alloys while only a very small percentage is desorbed from Pt at this temperature. After maximum coverage of Pt films by hydrogen adsorption three desorption peaks have been observed: γ (120 K), β₁ (200 K) and β₂ (330 K). The same peaks have been found for the alloys as well but the relative population of the various adsorption types was different. The relative peak heights vary with the alloy composition.     

The rotation-torsion contour of the 2668 Å origin band of toluene. High-resolution 295 K absorption spectrum

The 2668 Å vibrational origin band of toluene was measured and analyzed using an asymmetric-top/free-internal-rotor model. The change in the methyl group geometry between the ground (^eA′₁) and the excited ^eA″₁ ($\text{ππ}{}^1$) state was found to be slight. Rough values found for the changes in the phenyl frame rotational constants are similar to those for other monosubstituted benzenes with substituents lacking lone-pair electrons. The torsional structure in the toluene origin band is blue-degraded whereas the rotational structure in each Δm = 0 torsional manifold of transitions is red-degraded. Deconvolution of the torsional structure in the calculated contour shows that strong R-branch heads for transitions emanating from each torsional level of the ground state persist in the experimental contour for 0 ≤ |m| ≤ 12.     

Anisotropic photocorrosion of n-type MoS2 MoSe2, and WSe2 single crystal surfaces: the role 0f cleavage steps,line and screw dislocations

The photocorrosive behavior of the semiconducting transition metal dichalcogenides in contact with aqueous electrolytes is extremely anisotropic. Enhanced reactivity was observed at cleavage steps and on either line or screw dislocation cores emerging at the surface exposing mainly (101&#x0304;1) faces lying in 〈112&#x0304;0〉 directions. Corrosion patterns, however, could never be observed on smooth (0001) faces. The corrosion products of the molybdenum compounds are soluble, whereas WSe₂ formed WO₃ precipitates on the surface.     

The rotation-torsion contour of the 2668 Å origin band of toluene. Calculation of the molecular beam fluorescence excitation spectrum

The rotation-torsion contour of the low-temperature molecular beam fluorescence excitation spectrum of toluene was analyzed within the framework of the asymmetric-top/free-internalrotor model. The features observed in the moderate-resolution spectrum are reproduced well by this calculation, suggesting that the zero barrier approximation in both the ground and excited states is acceptable even at the low temperatures achieved in the supersonic jet expansion. The values of the excited-state rotational constants are found to be the same as those obtained in the analysis of the high-resolution 295 K spectrum. Corroboration for sequence band assignments in the 295 K spectrum is also given.     

The optical spectrum of the doubly charged molecular nitrogen ion: Rotational analysis of the (0-0) and (1-1) bands of the D1Σu+-X1Σg+ transition of N22+: Comparison of observations with Ab-initio calculation results

Emission spectra obtained in the 1550–1650 Å region with a 10-m vuv spectrograph are conclusively assigned to the N₂²⁺ ion. The 1589-Å band, previously observed by Carroll, and a new band of the same system, have been rotationally analyzed. Ab-initio calculations have been performed which support the assignment of these two bands to the D¹Σ_u⁺-X¹Σ_g⁺ system. The calculations also explain the observed breaking-off points in the branch structure as well as weakening and broadening of the other expected bands. These phenomena arise from electron configuration changes and perturbation effects in the ground state.     

The electronic structures of some aromatic sulfinylamines

He(I) and He(II) photoelectron spectra of some sulfinylamines are reported and assignments made on the basis of CNDO/S LCAO MO calculations. Insight into the effects of several substituents on the electronic structures of the compounds is obtained. The difference between the actual electronic structure of o,o′, p-trimethyl sulfinylaniline and that interpolated from other molecules in the series is assumed to reflect a different conformation for this compound.The results of the MO calculations agree fairly well with the experimental data, although some discrepancies occur, concerning especially the π-donating capacity of a Cl substituent, and ionizations from chlorine lone pair orbitals.     

The reaction of nitrous oxide with rhodium

A clean rhodium filament at room temperature is highly reactive towards nitrous oxide. The oxygen atom of the N₂O molecule is adsorbed with a sticking probability of 0.45 whilst the nitrogen atoms appear in the gas phase as molecular nitrogen. The room temperature uptake of oxygen is about 5 × 10¹⁴ atom cm^?2 and is independent of nitrous oxide pressure in the range 3.5 × 10^?8 to 1.1 × 10^?6 torr. The adsorption curve is of typical form with an initial region of essentially constant sticking probability. For the first 80% of adsorption at room temperature the shape is satisfactorily accounted for if molecules are able to visit 4–5 adsorption sites whilst held in a weakly-bonded precursor state.     

The dipole moment of the FO radical

The electric dipole moment of the FO radical (${}^2\text{Π}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$) has been directly measured by observing its saturated absorption laser magnetic resonance spectrum (v = 1 ← 0) in the presence of a moderate electric field. The resulting dipole moment [μ(v = 0) = 0.0043(4) D, μ(v = 1) = 0.0267(9) D] is extremely small, and shows a large relative change with vibrational state. The sign of the dipole moment is the same for the two vibrational states. Previous failures to detect microwave or gas phase EPR spectra of FO are fully explained by this small dipole moment.     

The interaction of hydrogen with stepped surfaces of copper: A pairwise-additive model

A pairwise-additive model, with Morse functions as the two-body potentials, has been employed to study stepped surfaces of copper. In general, the hydrogen-atom-copper bond energy is increased when one or more copper atoms are added to the surface at sites different from that which the hydrogen atom is approaching. Approaches of the hydrogen atom to a step face produced hydrogen-copper binding energies larger than those found in the absence of the step. Variations in these general observations are noted for alterations of the approach site environment.     

The autocatalytic decomposition of acetic acid on Ni(110)

The flash decomposition of CH₃COOH was studied on a clean nickel (110) surface following adsorption at 30° C in order to access the applicability of chemical reaction rate theory to a homologous series of reactants on a well-defined surface. As was observed for formic acid, acetic acid adsorbed at 30° C to yield gaseous H₂O and to form islands of adsorbed anhydride intermediates; the decomposition proceeded by a two-dimensional auto-catalytic mechanism to form H₂, CO₂, Co and surface carbon. The decomposition of the anhydride was rate determining for the formation of CO₂ and H₂. The rate of decomposition was well described by the equation governing the formic acid decomposition on the same surface. The activation energy for this first order decomposition was determined to be 28.2 $\text{kcal}\text{gmol}$ and the pre-exponential factor, v, was found to be 6.4 × 10¹⁴ s^?1 with a fraction of initiation sites of 0.004. These values were nearly the same as those observed for the decomposition of HCOOH, suggesting identical intramolecular mechanisms for the unimolecular decomposition of the adsorbed intermediates. The relative values of v for the decomposition of HCOOH, DCOOH and CH₃COOH indicated that the motion of the H, D or CH₃ group was involved in the rate-limiting step.     

Ion-enhanced gas-surface chemistry: The influence of the mass of the incident ion

There are many examples of situations in which a gas-surface reaction rate is increased when the surface is simultaneously subjected to energetic particle bombardment. There are several possible mechanisms which could be involved in this radiation-enhanced gas-surface chemistry. In this study, the reaction rate of silicon, as determined from the etch yield, is measured during irradiation of the Si surface with 1 keV He⁺, Ne⁺, and Ar⁺ ions while the surface is simultaneously subjected to fluxes of XeF₂ or Cl₂ molecules. Etch yields as high as 25 Si atoms/ion are observed for XeF₂ and Ar⁺ on Si. A discussion is presented of the extent to which these results clarify the mechanisms responsible for ion-enhanced gas-surface chemistry.     

Surface chemistry of the halogen-metal interface: The interaction of chlorine with ruthenium (100)

Chlorine adsorbs on Ru(100) with high efficiency; UPS, Δφ, TD, AES and LEED data suggest that the low pressure interaction ceases with the formation of a dense chemisorbed layer of atomic Cl. This overlayer desorbs exclusively as Cl(g) with an activation energy of ~ 220 $\text{kJ}\text{mol}$. The failure of the system to grow bulk halide and the absence of molecular chlorine and ruthenium-chlorine species in the desorption products are discussed in thermochemical terms. A series of four ordered surface structures evolves with increasing coverage, and structural models are proposed which are in good quantitative accord with the results from Auger spectroscopy. As coverage deceases, it seems likely that the overlayer relaxes by the growth of ordered vacancies rather than by uniform expansion along a preferred direction.     

The vapor-phase infrared and Raman spectra of p-difluorobenzene (h4) and (d4)

This new study of the vapor-phase infrared and Raman spectra of p-difluorobenzene (h₄) and (d₄) includes assignments from the high-resolution electronic spectrum, from fluorescence spectra, and one assignment from the two photon electronic spectrum. A new set of frequencies and assignments has been made for the fundamentals of this important intermediate-sized molecule. The most important changes concern the assignments previously made for the lowfrequency a_u mode (ν₈), two members of the b_2u class (ν₁₉ and ν₂₀), and one very important change for the lowest frequency fundamental of the b_3u (out-of-plane) class (ν₃₀), which has been observed directly in the far infrared. The results are all corroborated by the assignments of the perdeuterated analog where changes in previous assignments of Fermi resonance pairs have also been made. Some interesting differences between the Raman spectra of the vapor and the liquid perproto compounds have not been adequately explained in this study.     

The adsorption and condensation of oxygen on aluminium at low temperaturf

The interaction of oxygen with an Al(111) surface at 30 K leads initially to dissociative adsorption, followed by the physisorption of molecular oxygen. The latter is characterised by HeI and HeII UPS spectra similar to the gas phase. The physisorbed molecules partly convert into the dissociated phase on standing in vacuum. There appears to be no difference between the UPS spectrum of a condensed layer of oxygen and that of the physisorbed species.     

Structural and reactive properties at the metal-halogen interface: The interaction of bromine with chromium (110)

The chemisorption and subsequent reaction of bromine on Cr(110 has been studied by Auger spectroscopy, LEED, Δφ, and thermal desorption measurements. For gas doses of < 7.5 × 10¹⁸ molecules m^?2, very efficient dissociative chemisorption leads to a series of well-ordered, out-of-registry compression structures. Uniquely, however, the overlayer falls back into registry at saturation coverage; at this point the appearance of glide symmetry indicates that the three-fold coordinated adsorption sites are occupied exclusively. Bromine → metal charge transfer occurs during adsorption (in contrast to Cr(100)). On raising the temperature at low coverages, the surface phase decomposes by evaporation as CrBr molecules; at higher coverages the desorption product switches to CrBr₂. Continuous growth of bulk CrBr₂ sets in at high gas exposures, this corrosion reaction proceeding at a rate which is ten times slower than the rate of overlayer formation. The chromium dibromide layer also evaporates as CrBr₂(g). Structural relationships with related metal-halogen systems are discussed.