Scissor Mode Research Articles

Chemisorption of chlorine on GaAs(100) surfaces studied by high-resolution electron energy-loss spectroscopy

The adsorption of chlorine on GaAs(100)-c(8 × 2) surfaces was investigated by high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED). Chlorine molecules were released from an electrochemical AgAgCl-graphite cell. For the clean c(8 × 2)-reconstructed surface, besides the well-known Fuchs-Kliewer phonon of GaAs at 35.8 meV, two further surface phonons are observed at 7.9 and 11.5 meV. They can be characterized by comparison with the volume phonon dispersion curves of GaAs in the high-symmetry directions of the Brillouin zone. After chlorine exposure, the XPS measurements show an etching process which removes gallium atoms from the surface. With HREELS three adsorbate-induced loss features at 18, 43 and 46 meV are found. Comparing the loss energies with vibrational energies of AsClBr 2, GaCl and GaCl 2 suggests the attribution of the loss features to the GaCl valence vibration (46 meV) and the GaCl 2 scissor mode (18 meV) for small chlorine exposures, and to the AsCl stretching vibration (43 meV) in the high exposure regime when etching occurs.

Hydrogen desorption from ion-roughened Si(100)

Low energy Kr+ ions were used to create rough Si(100) surfaces. Hydrogen adsorption and desorption on these surfaces were studied using high resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD). A comparison of hydrogen TPD data from smooth and ion-roughened Si(100) surfaces showed no significant shift for the dihydride and monohydride related desorption features, revealing that ion impact roughening does not change the activation barriers for hydrogen desorption. In contrast to smooth Si(100), HREELS spectra from rough Si(100) surfaces with different hydrogen coverages showed a dihydride scissor mode at 0.8 ML and dihydride desorption below monolayer coverage in the corresponding TPD experiment. The vibrational spectra of hydrogen covered rough surfaces generally showed a smaller ratio of the scissor mode/bending mode intensity than the smooth Si(100) surfaces, suggesting a lower dihydride/monohydride coverage ratio. The corresponding TPD results on the other hand showed increased dihydride desorption and higher total coverages up to 2 ML. Although different dihydride units on terrace sites and at edge sites could not be unambiguously identified in the vibrational spectra, they provide a reasonable explanation for this inconsistency.

The adsorption of methyl radicals on oxygen-modified Mo(100) studied by HREELS

The adsorption of methyl radicals on two different oxygen-modified Mo(100) surfaces at room temperature has been studied using high-resolution electron energy-loss spectroscopy (HREELS) and low-energy electron diffraction (LEED). Previous experiments have shown that methyl radicals adsorbed to these surfaces produce CH 4, H 2 and CO as reaction products. Since the data from this earlier study was inconclusive, vibrational spectroscopic evidence was sought in order to obtain a chemical identification of the surface species. This study confirms the previous data, which suggested that methyl radicals do not form surface methoxy but rather a metal alkyl analog when adsorbed at 300 bdK. Methyl groups dehydrogenate at room temperature and reveal an OH stretching vibration as well as the CH 2 scissor mode. This data is compared to the results observed when CH 3OH is adsorbed on the same surface. In the latter case, no OH vibrations are detected in the spectrum and modes corresponding to adsorbed methoxy are seen.

High resolution electron energy loss spectroscopy study of the Si(001) 3 × 1 hydrogenated surface

Hydrogenation of the Si(001) 2 × 1 surface held at 400 K has been performed in a new experimental set-up for a characterization by high resolution electron energy loss spectroscopy (HREELS). Angular scan along the [110] direction recorded at 55 eV primary energy confirms the 3 × 1 surface reconstruction. HREELS performed at 6 eV demonstrates the existence of a dihydride phase, in particular through the observation of the scissor mode around 904 cm −1. The good quality of this surface, as shown by a high count rate on the elastic peak, allowed to obtain a resolution as good as 45 cm −1 at room temperature. Lineshape studies of the vibrational peaks related to hydrogen reveal an abnormal width of the 639 cm −1 mode. In fact, the resolution was improved by cooling down to ∼ 110 K, and a double peak was clearly resolved, separated by 35 cm −1. In view of previous HREELS studies of the Si(001) 1 × 1 and 2 × 1 hydrogenated surfaces, as well as of theoretical calculations of the vibrational modes of SiH 2 species, the assignment of these modes is given. For the first time, a clear monohydride signature within a dihydride phase is reported by HREELS.

Scaled quantum mechanical force field for glycine in basic solution

We obtain scale factors for three glycinate-nH2O ab initio force fields, using the 4–31G basis set, that can be used in building a scaled quantum mechanical force field for alanine and, subsequently, for peptides in aqueous solutions. Force constants from the fully optimized glycinate-nH2O supermolecules were scaled by using experimentally determined vibrational frequencies of glycine in water at pH 13. Similar calculations were performed for methylamine and acetate. Scale factors for the stretching modes of acetate are within 2% of the related scale factors for glycinate. The scale factor for the NH2 scissor mode in methylamine is also in agreement with that of glycinate. Changes in the scale factors as a function of the number of hydrating water molecules were also similar between glycinate and acetate. Amine groups showed relatively small changes. Scale factors for glycinate with no hydrating molecules were extrapolated from the supermolecule results, since the optimized structure of isolated glycinate obtained with the 4–31G basis set yielded one imaginary frequency. Good agreements between calculated and experimental frequencies for glycinate, acetate, and methyl amine were obtained for each set of scale factors. Scaling appears to compensate for the systematic effects of hydration on force constants, making it possible to obtain reliable frequency predictions for amino acids in water without resorting to expensive super-molecule calculations.

Decomposition of silicon hydrides following disilane adsorption on porous silicon surfaces

Disilane (Si 2H 6) is used for silicon chemical vapor deposition (CVD) and is a potential precursor for atomic layer epitaxy (ALE) on silicon surfaces. In this study, Fourier transform infrared (FTIR) transmission spectroscopy was employed to examine the adsorption and decomposition of disilane on high surface area porous silicon surfaces. The FTIR spectra revealed that disilane dissociatively adsorbs on porous silicon surfaces at 200 K to form a large fraction of SiH 3 surface species and some SiH 2 and SiH surface species. The SiH x stretching modes between 2125–2156 cm −1, the SiH 3 degenerate deformation mode at 862 cm −1 and the SiH 2 scissor mode at 907 cm −1 were then employed to monitor the decomposition of the surface hybride species during thermal annealing. Between 200–600 K, the SiH 3 species decreased and were depleted from the silicon surface. Concurrently, the SiH 2 surface species were observed to increase between 200–440 K and subsequently to decrease between 440–620 K. Only monohydride species remained on the porous silicon surface at 620 K. Above 640 K, the SiH surface species decreased concurrently with H 2 desorption. Adsorption studies were also conducted at various isothermal temperatures. These disilane adsorption and decomposition experiments provide important insights to the surface chemistry during silicon CVD and ALE processing.

Chemisorption of H, H2O and C2H4 on Si(113): implications for the structure

Abstract The chemisorption of H, H 2 O and C 2 H 4 on Si(113) is studied by means of high-resolution electron energy loss spectroscopy (HREELS) and low-energy electron diffraction (LEED). By saturating the dangling bonds of the clean Si(113)3 × 2 surface, H forms a monohydride first. At higher exposures a dihydride phase is formed, clearly distinguishable in HREELS through the appearance of the H-Si-H scissor mode. H 2 O dissociates into H and OH. Its sticking coefficient is of the order of one, indicating that Si dimers may constitute the Si(113)3 × 2 surface to a large part. H, OH and C transfer the surface structure from 3 × 2 into 3 × 1 adsorbate. At least in the case of H chemisorption the change of structure occurs very likely without lateral Si transport. This has strong implications for the structure of the clean Si(113)3 × 2 surface. To account for these findings together with our photoemission results and two scanning tunneling microscopy (STM) studies from the literature we propose a buckling model for the Si(113)3 × 2 surface. Our investigation underlines that only the absence of H, OH and C — the main residues from contamination through background gas — guarantees the occurrence of the 3 × 2 reconstruction for the clean Si(113) surface.

Chemisorption of H, H 2O and C 2H 4 on Si(113): implications for the structure

The chemisorption of H, H 2O and C 2H 4 on Si(113) is studied by means of high-resolution electron energy loss spectroscopy (HREELS) and low-energy electron diffraction (LEED). By saturating the dangling bonds of the clean Si(113)3 × 2 surface, H forms a monohydride first. At higher exposures a dihydride phase is formed, clearly distinguishable in HREELS through the appearance of the H-Si-H scissor mode. H 2O dissociates into H and OH. Its sticking coefficient is of the order of one, indicating that Si dimers may constitute the Si(113)3 × 2 surface to a large part. H, OH and C transfer the surface structure from 3 × 2 into 3 × 1 adsorbate. At least in the case of H chemisorption the change of structure occurs very likely without lateral Si transport. This has strong implications for the structure of the clean Si(113)3 × 2 surface. To account for these findings together with our photoemission results and two scanning tunneling microscopy (STM) studies from the literature we propose a buckling model for the Si(113)3 × 2 surface. Our investigation underlines that only the absence of H, OH and C — the main residues from contamination through background gas — guarantees the occurrence of the 3 × 2 reconstruction for the clean Si(113) surface.

The adsorption and thermal decomposition of digermane on Ge(111)

We have used multiple internal reflection infrared spectroscopy to investigate the interaction of digermane with Ge(111) at temperatures between 104–600 K. Digermane predominantly adsorbs molecularly on the surface below 120 K, displaying a vibrational spectrum similar to that of condensed digermane. At temperatures between 120–150 K, digermane dissociates via Ge–Ge bond scission to form adsorbed GeH3. Chemisorbed germyl GeH3 has a distinct symmetric deformation vibration at ∼772 cm−1, compared to a value of 721 cm−1 for molecularly adsorbed Ge2H6. At 200 K, Ge2H6 adsorption produces surface GeH3, GeH2, and GeH species with stretching vibrations at 2063, 2023, and 1968 cm−1, respectively. The surface GeH2 species is also identified by a characteristic scissor mode at ∼830 cm−1. Adsorption at 300 and 400 K produces only GeH2 and GeH, with a much lower concentration of GeH2 at 400 K. The surface GeH2 and GeH species are also generated by the successive decomposition of GeH3 upon heating. All surface hydrogen desorbs at ∼600 K.

Para phenylenediamine radical cation structure studied by resonance Raman and molecular orbital methodsa)

The vibrational and electronic structures of p-phenylenediamine radical cation have been studied using time-resolved resonance Raman spectroscopy and molecular orbital calculations. The Raman spectra in aqueous solution show striking variations in intensity profile when excitation is varied from 340 to 480 nm. Excitation in resonance with the 460–480 nm absorption shows enhancement of only totally symmetric vibrations. Most prominent are the δNH2 scissor mode at 1658 cm−1 and the Wilson modes ν8a (CC stretch) at 1644 cm−1, ν7a (CN stretch) at 1423 cm−1, and ν9a (CH bend) at 1184 cm−1. Also observed are ν1 (ring breathe) at 840 cm−1 and ν6a (CCC bend) at 467 cm−1. With excitation in the 340–410 nm region the ν1 band becomes relatively stronger and an additional band at 1524 cm−1 appears. This latter band, which dominates the spectrum at 360 nm, is assigned to the nontotally symmetric vibration ν8b (CC stretch) having b3g symmetry that gains intensity through vibronic coupling. Raman spectra of ring- and amine-deuterated radicals allow distinction of overtone and combination bands enhanced by Fermi resonance from fundamentals due to δNH2 and ν8a (CC stretch) modes in the 1600–1700 cm−1 region. The theoretical calculations show reasonable agreement with the observed vibrational frequencies and lead to detailed information on the bond structure and normal modes of the radical. The calculated CN bond lengths of 1.33 Å are intermediate between lengths typical of single and double bonds in aromatic amines. The calculations also allow interpretation of the experimental absorption spectrum. The broad medium intensity absorption peaked at 460 nm with a shoulder at 480 nm is assigned to a 2B3u ←2B2g transition, the very weak 360 nm absorption to 2Au ← 2B2g and the very strong ∼325 nm absorption to a higher 2B3u ← 2B2g. Calculations with the simple relation Ik∝ (V′k)2/ωk, which requires evaluation only of vertical excited state gradients, are found to satisfactorily describe the dramatic changes in Raman intensity profile that are observed with different excitation wavelengths. The 460 and 480 nm Raman spectra are governed by scattering from the lowest 2B3u state. Spectra taken in the 410–340 nm range are governed mainly by preresonance scattering from the higher 2B3u state, which is vibronically coupled to the weak 2Au state via the b3g symmetry ν8b (CC stretch) vibration. The relative intensities of totally symmetric modes calculated for excitation into the two 2B3u excited states agree well with those in the two distinct patterns observed experimentally.

Time evolution of the localized vibrational mode infrared absorption of porous silicon in air

We have fabricated porous silicon layers (PSLs), referred to as PSL1 and PSL2, using two kinds of anodizing solutions which have been extensively used in the literature, i.e. HF(48 wt. %) : H 2O = 1:1 and HF(48 wt. %) : C 2H 5OH = 1:1, respectively. We observed infrared absorption of the localized vibrational modes on the inner walls of the samples after exposure to air for 2h, 6 days, 1 month and 4 months after manufacture by Fourier-transform infrared (FTIR) spectroscopy. For both PSL1 and PSL2, there is a change in intensity for the IR bands with time. The 627 cm −1 (SiH 2 deformation mode), 667 cm −1 (SiH deformation mode), 907 cm −1 (SiH scissor mode), 914 cm −1 (SiF stretching mode), 2092 cm −1 (SiH stretching mode), 2116 cm −1 (SiH 2 stretching mode), and 2140 cm −1 (possibly SiH 3 stretching mode) band all decrease. The intensity of the 1056 cm −1 (SiOSi stretching mode) band instead increases with the time of exposure. For PSL2, the time evolution of the IR band intensity is slower than that for PSL1. Two bands, located at 2199 and 2250 cm −1, appeared after 6 days exposure to air and rose in intensity with time, showing that they are related to oxygen.

High-resolution FTIR spectrum of the ν5 band of 1,1-difluoroethylene around 550 cm −1

The infrared spectrum of CF 2CH 2 recorded in the range 400–4000 cm −1 with a Fourier transform infrared spectrometer has been studied in the ν 5 region. This fundamental approximately described as a CF 2 scissor mode gives rise to an a-type band showing in most regions of the spectrum distinct patterns characteristic of molecules approaching the oblate symmetric top limit. The rovibrational analysis extended to the P, Q, and R branches led to the identification of about 3000 transitions covering J and K a values up to 70 and 46, respectively. Using Watson's A-reduction Hamiltonian the assigned transitions provided a set of accurate rotational and centrifugal distortion constants for the first excited vibrational state of the ν 5 band of CF 2CH 2.

Isotopic substitution evidence against the formation of (H 3O +) ad from coadsorption of hydrogen and water on Cu(110)

We present both high resolution electron energy loss spectra (HREELS) and thermal desorption spectra (TDS) for hydrogen plus water coadsorption on Cu(110). Hydrogen adsorbed at 100 K. increases the desorption temperature of water by ∽ 10 K., indicating a fairly weak interaction. This stabilised water is associated with dramatic changes in the HREELS spectra: the scissor mode at 1600 cm −1 is attenuated and a new vibration is observed at 1150 cm −1. The assignment of this mode to the symmetric bend of an (H 3O +) ad species as recently reported for Pt(111), therefore seemed plausible also for Cu(110). However, TDS showed no isotopic exchange which indicates that a reaction between (H) ad and (H 2O) ad does not occur on Cu(110). The different origin of the 1150 cm −1 mode on Cu(110) is briefly discussed.

Scissor mode applied to the yrast band of the rare-earth nuclei.

The scissor mode was applied to the yrast rotational band of the rare-earth even-even nuclei from $^{152}\mathrm{Gd}$ to $^{192}\mathrm{Os}$. The quadrupole deformations and backbending and forebending effects are well explained. This approach shows that the uppermost angular momentum of the G band exists and the superband and G band are made on the basis of the scissor mode. It is thought that this classical approach of the interacting boson model II can be reasonable in order to explain the low-lying energy levels of the rare-earth nuclei.

An extended version of the rotation-vibration model with different proton and neutron deformations

An extended version of the rotation-vibration model (RVM) including different proton and neutron deformations is presented. Due to this extension in which the so-called scissor mode is given as a relative vibration of proton against neutron surface, it is possible to calculate the collective magnetic properties of well-deformed even-even nuclei with satisfactory agreement to experimental data (examples are given for 152Sm, 166Er and 168Er). It is found that a ground-state relative deformation is important for the calculation of magnetic transition probabilities between low-energy collective states. The interaction between proton and neutron surface is treated within a strong-coupling scheme.

Rotational model and shell model pictures of magnetic dipole excitations.

A comparison is made of the rotational model and the large shell model for the calculation of the magnetic dipole excitations in selected nuclei in the s-d and f-p shells. Focus is given on both the relatively low lying strong states and higher ``spin flip'' excitations. It is found that the two models agree fairly well for summed B(M1) strength, but for the deformations that were used give different results for orbital and spin decomposition. The rotational model yields too much low lying strength as compared with the shell model. A technique for eliminating spurious contributions to the summed strength in the rotational model is developed. It is shown that in the asymptotic limit in some nuclei the magnetic dipole excitations are purely orbital. The implications of these calculations to results in heavier deformed nuclei for ``scissor mode'' excitations is discussed.

Adsorption of H, O, and H2O at Si(100) and Si(111) surfaces in the monolayer range: A combined EELS, LEED, and XPS study

This paper is a summary of a series of experiments studying the exposure of hydrogen, oxygen, and water, on the (2×1) surfaces of Si(100) and Si(111). While the primary focus has been on high resolution electron energy loss (EELS) results, low energy electron diffraction (LEED) and x-ray photoelectron spectroscopy (XPS) are also used in the studies. Both the (100) and cleavage (111) surfaces form a monohydride and a dihydride exhibiting a (2×1) and a (1×1) LEED pattern, respectively. These systems exhibit saturation, which is consistent with the model of hydrogen saturation of the dangling bonds. Upon water adsorption the Si–H and Si–OH vibronic modes are observed, indicating that water is decomposed. On the cleavage surface only, there is evidence of a very weak scissor mode, allowing for the possibility of a few percent of molecular water adsorption. Oxygen adsorption is complex. For samples formed at high temperatures (∼1000 K) the observed vibronic features are similar to those known for the Si–O–Si complexes in vitreous glasses. For thin oxide layers (0.5<θ<1.3 monolayers) a linear relationship is observed between oxygen coverage and asymmetric mode frequency. These data are fit with models developed for glasses which, for the monolayer regime, yield an average bond angle of about 130° and a bond distance of 1.65 Å. The results support the model in which the oxygens are envisioned as being inserted into the Si–Si back bonds.