Frequencies Of Stretching Modes Research Articles

Effect of oxygen contamination on the properties of the silicon hydrogen alloy materials deposited under conditions near the microcrystalline silicon formation region

Two sets of films of silicon hydrogen alloy (Si:H) were deposited by plasma enhanced chemical vapor deposition (PECVD) method from silane mixed with hydrogen or argon under the deposition conditions suitable for producing microcrystalline silicon (μc-Si:H) growth. The sets were repeated in two different chambers: one with ultra high vacuum capability having a load lock chamber and the other a single chamber unit without any load lock. The samples deposited in the single chamber unit were found to be contaminated with oxygen. It was observed that the incorporation of unintentional oxygen in the film affects the material properties in a different way for hydrogen or argon dilution. In the case of hydrogen dilution the oxygen contamination results in a shift of the stretching mode frequencies of SiH and SiO bonds and the microcrystalline grain formation is also inhibited by the oxygen impurity. With argon dilution the first effect is present but μc-Si:H growth is not much affected by the oxygen contamination.

FTIR study of the oxidation of amorphous carbon by ozone at 300 K — Direct COOH formation

The reaction of O 3 with activated carbon has been investigated with transmission FTIR spectroscopy. A wide range of carbonyl species are observed to form with stretching mode frequencies ranging up to 1900 cm −1, but individual carbonyl species cannot be resolved. A υ(C–O) mode is observed to form near 1190 cm −1, as well as a broad υ(CO–H) mode at 3450 cm −1, indicative of the O 3-induced formation of associated COOH groups. The original surface contains both aromatic and aliphatic υ(C–H) mode absorbances, which are consumed at similar rates by the O 3. A mechanism is proposed for COOH formation through decomposition of an ozonide, where the hydrogen originates from C–H bonds initially present on the carbon surface.

Theoretical model for ultra fast dynamics of vibronic couplings in conjugated polymers

Modulations of the instantaneous frequencies of the stretching modes with an oscillation period around 200 fs have been reported for the non equilibrium vibrational state in the 2 1 A g of PDA. We propose here a model Hamiltonian which allows to study the time evolution of the non equilibrium system towards equilibrium. This includes the electronic, the vibrational, the linear and quadratic electron vibration interaction Hamiltonians of a conjugated polymer. In the off diagonal quadratic interactions the coupling between the high frequency stret ching mode and the low frequency bending mode of the polymers is taken into account. In this framework we study the time evolution of the out of equilibrium system relaxing to the thermalized state and we show that the stretching mode frequencies changes in time in quali tative agreement with the experimental results.

Water structure changes induced by hydrophobic and polar solutes revealed by simulations and infrared spectroscopy

A combination of simulations and Fourier transform infrared spectroscopy was used to examine the effect of three ionic solutes (KCl, NaCl, and KSCN), the polar solute urea, and the osmolyte trimethylamine-N-oxide (TMAO) on a water structure. The ionic solutes increase the mean water–water H-bond angle in their first hydration shell concomitantly shifting the OH stretching mode to higher frequency, and shifting the HOH bending mode to lower frequency. TMAO decreases the mean water–water H-bond angle in its first hydration shell, shifts the OH stretching mode frequency down, and shifting the HOH bending mode frequency up. Urea has no effect on the mean H-bond angle, OH stretch, and HOH bend frequencies. These results can be explained in terms of changes in the relative proportions of two H-bond angle populations: Ionic solutes increase the population of more distorted (larger angle) H bonds relative to the less distorted population, TMAO has the reverse effect, while urea does not affect the H-bond angle probability distribution. The negligible effect of urea on water structure supports the direct binding model for urea-induced protein denaturation.

Equilibrium state of hydrogen in gallium nitride: Theory and experiment

Formation energies and vibration frequencies for H in wurtzite GaN were calculated from density-functional theory and used to predict equilibrium state occupancies and solid solubilities at elevated temperatures for p-type, intrinsic, and n-type material. The solubility of deuterium (D) was measured in p-type, Mg-doped GaN at 600, 700, and 800 °C as a function of D2 pressure and compared with theory. Agreement was obtained by reducing the H formation energies 0.22 eV from ab initio theoretical values. The predicted stretch-mode frequency for H bound to the Mg acceptor lies 5% above an observed infrared absorption attributed to this complex. More limited solubility measurements were carried out for nominally undoped material rendered n-type by donors provisionally identified as O impurities, and results agree well with theory after the aforementioned adjustment of formation energies. It is concluded that currently recognized H states and physical processes can account for the equilibrium, elevated-temperature behavior of H examined in this work.

A theoretical analysis of the sum frequency generation spectrum of the water surface

The present paper provides a theoretical analysis of the sum frequency generation (SFG) spectrum of the water surface in the OH stretch mode frequency region based on ab initio molecular orbital theory and molecular dynamics simulation. The environmental effects on the normal modes of OH stretching vibration, their frequency shifts and hyperpolarizability are formulated and tested. The simulated SFG spectrum reproduces experimental results quite well. The surface susceptibilities of the dangling bond and hydrogen-bonded bands have opposite signs in their imaginary parts, which indicates opposite OH directions at the surface associated with these bands. The former band turns out to be sensitive only to the top monolayer, and the latter band to a few top monolayers. Further analysis reveals that those two bands arise from quite different types of molecular orientations. The assignment of the SFG spectrum is also analyzed in terms of the symmetric/antisymmetric character of the OH stretching modes and the degree of mode delocalization.

Structure, energetics, and vibrational properties of Si-H bond dissociation in silicon

We investigate hydrogen dissociation from an isolated Si-H bond in bulk silicon, using ab initio density- functional total-energy calculations. From the bonding site, we find that hydrogen needs to overcome a barrier of less than 2.0 eV in order to reach the next lowest local minimum in the energy surface. This minimum occurs at the antibonding site and is 1.2 eV higher in energy than the ground state. In addition, we consider the role of lattice relaxations and free carriers during the dissociation process. We discuss the relevance of our results for Si-H dissociation in several systems, including the Si-SiO 2 interface. @S0163-1829~99!05919-6# passivating the dangling bond, forming a strong Si-H bond. An additional local minimum is found for hydrogen at the antibonding site ~i.e., behind the Si atom, on the other side of the dangling bond!. Compared to hydrogen in the bonding site, hydrogen in the antibonding site has an energy 1.2 eV higher, an Si-H bond length ;3% longer and an Si-H stretch-mode frequency which is 180 cm 21 lower. When hy- drogen is trapped in the antibonding site, electronic levels are introduced into the gap allowing for carrier-enhanced Si-H dissociation. The saddle point for migration from the dangling bond to the antibonding site occurs at the bond- center site, with a migration barrier of 1.75 eV. Large lattice relaxations are required for the H atom to approach the bond- center site; the kinetics of Si-H dissociation through the bond-center site will thus be tied to silicon-silicon vibra- tional modes. An alternative pathway from the ground state to the antibonding site exists, which does not go through the bond-center site, but still requires sizable displacements of the Si atoms; this pathway has a slightly higher barrier ~1.9 eV!. The rest of this paper is organized as follows. In Sec. II, we discuss details of the theoretical approach. Our results are reported in Sec. III. In Sec. IV we discuss the implications of these results for device degradation and for experiments per- formed on hydrogen-passivated Si-SiO2 interfaces. Sec. V summarizes the paper.

The Singlet-State Photophysics and Photochemistry of Polyenes: Application of the Twin-State Model and of the Phase-Change Theorem

The A(21Ag) state of polyenes has been shown by Kohler and co-workers to be of central importance for the understanding of polyene photophysics and photochemistry. The twin-state model is used to provide a physical explanation for the well-known frequency exaltation of the ag symmetric stretch mode frequency upon excitation of the molecule from the ground X(11Ag) state to the A(21Ag) state and for the increased stabilization of the planar form. The smaller members of the polyene series, ethylene and butadiene, are nonfluorescent, while higher members are. It is shown that the direct (singlet) photochemistry of all polyenes can be largely accounted for by assuming that these two lowest lying Ag states are connected by a conical intersection. The nature of the products and the stereochemical characteristics of the photoreactions can be rationalized using the phase-change theorem of Longuet-Higgins (Longuet-Higgins, H. C. Proc. R. Soc. London A 1975, 344, 147). A general procedure for locating the conical intersections and their associated products is suggested.

An ab Initio and Raman Investigation of Magnesium(II) Hydration

The weak polarized Raman band assigned to the ν1-MgO6 mode of the hexaaquo Mg(II) ion has been studied over the temperature range 25 to 125 °C. The 356 cm-1 stretching mode frequency decreases by about 3 cm-1 but broadens by 13 cm-1 over a 100 °C temperature range. A depolarized mode at 235 cm-1 could be assigned to ν2. These data suggest that the hexaaquo Mg(II) ion is thermodynamically stable in perchlorate and chloride solutions. In sulfate solutions, an equilibrium exists between the hexaaquo ion and an inner-sphere sulfato complex. Ab initio geometry optimizations of Mg(H2O)62+ were carried out at the Hartree−Fock and Moller−Plesset levels of theory, using various basis sets up to 6-31+G*. Frequency calculations confirm that the Th structure is a minimum. The unscaled frequencies of the MgO6 unit are lower than the experimental frequencies, and scaling only marginally improves the agreement. The theoretical binding enthalpy for the hexaaquo Mg(II) ion accounts for about 70% of the experimental hydrat...

Dimerization of 1,3-Butadiene on Highly Characterized Hydroxylated Surfaces of Ultrathin Films of γ-Al2O3

Dimerization of 1,3-Butadiene on Highly Characterized Hydroxylated Surfaces of Ultrathin Films of γ-Al₂O₃

Molecular bending mode frequencies of the surface and interior of crystalline ice

The Fourier transform infrared (FT-IR) spectra of the bending modes of the three categories of molecules at the surface of ice nanocrystals have been determined for both H2O and D2O samples and the response of the bending mode of the dangling hydrogen surface molecules to association with adsorbates of varying acceptor strengths has been examined. From these combined data for water molecules in a wide range of environments at the ice surface, a clear picture of the dependence of the bending mode frequency on the extent of H-bonding is revealed for ice-related conditions. These frequencies have been examined in terms of the linear equation developed by Falk that relates the water decoupled bend mode frequency to H-bond strength through the average of the symmetric and asymmetric stretch mode frequencies. The published value of 1735 cm−1, for the decoupled bend mode frequency of bulk H2O ice, is consistent with this presentation of the new data, but a new value, higher by at least 40 cm−1 than the accepted value of ∼1220 cm−1, is indicated for D2O ice. A value of 1265 cm−1 is offered as the best estimate of the bend mode frequency of D2O ice.

Gateway Modes in the Collisional Energy Transfer from Highly Vibrationally Excited CS2

The role of different vibrational modes in the energy transfer from highly vibrationally excited CS2 is investigated in model classical trajectory calculations. The primary tool for this work involves determining the dependence of the average energy transfer on the vibrational frequency of each mode. These calculations show that the energy transfer is highly sensitive to the frequency of the bend and the symmetric stretch mode. By increasing or decreasing the bend frequency the efficiency of the energy transfer decreases or increases, respectively. Variation of the symmetric stretch frequency causes a comparable effect. Variation of the asymmetric stretch mode frequency can lead to activation of energy transfer only if the frequency of the other two modes is made so high that the energy transfer through those modes becomes negligible. If the vibrational frequencies of highly excited CS2 are set to their “natural” values, the bending mode is the conduit through which the energy “leaks out” of the molecule, similarly to the slightly excited molecule. Bending is the gateway mode not only because it is the lowest frequency mode but also because of the shape of the molecule: a sideways attack, which is much more probable than an axial attack, always interacts with the bend mode but only to a small extent with the asymmetric stretch mode, independently of the actual frequencies. The activity of a mode in energy transfer is determined mostly by its frequency, but the nature of the motion of the atoms involved in the mode also play a role.

N–H stretching frequencies and the conformation of substituted ureas: an ab initio MO study

The dependence of N–H stretching-mode frequencies in representative di- and trialkyl ureas on the conformational state of the ureido group has been studied by ab initio MO calculations using HF/3-21G and HF/6-31G** basis sets. The molecules studied were 1,3-dimethylurea, 1-methyl-3,3-dimethylurea and 1-methyl-3,3-di- iso-propylurea. The principal conclusions from the ab initio results are: 1. the trans– trans conformer (N–H bonds trans to the CO bond) has N–H stretching bands with about 20–30 cm −1 higher frequency than the respective cis– cis structure, in accord with earlier literature assignments based on experimental data; 2. the N–H stretching frequency interval in tri-substituted ureas is 15–20 cm −1 higher than the N–H band position in the 1,3-disubstituted molecule studied, the effect being determined mostly by the higher N–H stretching force constant; 3. in the absence of the steric hindrance the stable rotameric forms of the ureido grouping are almost planar at HF/3-21G level of calculations, while HF/6-31G** calculations predict a slightly pyramidal structure at the nitrogen atoms in the trans– trans conformer; 4. in 1-methyl-3,3-di- iso-propylurea the steric influence of the two bulky iso-propyl groups cause a deviation from planarity of the N–H bond. The non-planar conformation is accompanied by a shift of the N–H stretching mode frequency towards higher values; and 5. the variations of the theoretically estimated N–H stretching-mode frequencies appear to be principally determined by changes in the N–H stretching force constants in the different molecules. © 1997 Elsevier Science B.V.

Interaction of Cs and CO on Ru(0001) for Cs submonolayer coverages

The interaction of Cs and CO was studied for submonolayer coverages of Cs on Ru(0001) by means of high-resolution electron energy-loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and low-energy electron diffraction (LEED). From the HREEL spectra and the 2×2 LEED pattern it is concluded that two-dimensional islands of Cs+2CO stoichiometry are formed first. For high CO doses CO adsorbs on the bare Ru surface between the islands as concluded from the ν(C–O) stretch-mode frequency and the (∛×∛)R30° LEED pattern. Besides some local interaction between CO and Cs, there is strong evidence found for a long range, through substrate interaction. Our data can be understood in terms of a charge redistribution model; Cs donates a given amount of charge to the surface which is then backdonated to and shared between the adsorbed CO molecules within the (Cs+2CO)-islands and outside of them on the bare surface. The desorption temperature of CO with θCO→0 increases nearly linearly with θCs indicating CO desorption out of a two dimensional Cs gas in which the lateral interaction increases with θCs.

Vibrational study of C 60 overlayers on [formula omitted

We have studied the vibrational properties of thin C 60 overlayers deposited on ideally H-terminated Si(111) surfaces, by multiple infrared reflection and high-resolution electron energy-loss spectroscopies. We find that the SiH interface remains intact upon C 60 deposition: the SiH stretching mode frequency, initially at 2083.8 cm −1, shifts downward to 2062 cm −1, and broadens while keeping the same integrated intensity. Due to the symmetry-breaking effect at the surface on the adsorbed C 60 molecules, a new IR band is observed at 1445 cm −1 which is mainly s-polarised. No evidence for chemical interaction between C 60 and H Si(111)-(1 × 1) is found, pointing to van der Waals-type bonding. From the relative intensities of the dipole-active and -inactive modes in HREELS in the specular direction, and from the angular width of the scattered elastic peak, the growth of an ordered film is inferred.

Ultraviolet absorption spectrum of gaseous IrF 6 in the 200–500 nm region

The theory of the electronic transitions of iridium hexafluoride is briefly reviewed. Then, we present the ultraviolet absorption spectrum of this molecule in the gas phase over the 20 000–50 000 cm −1 region. Some new electronic transitions are observed. Their physical origin is discussed and some values of the v 1( A 1 g ) totally symmetric stretching mode frequency in these highly excited electronic states are given.

Semiclassical molecular dynamics simulations of low-temperature clusters: Applications to (Ar)13; (Ne)13; (H2O)n, n=2,3,5

Semiclassical molecular dynamics simulations are developed as a tool for studying anharmonic clusters and solids at energies near the zero point. The method employs the time-dependent self-consistent-field approximation, that describes each mode as moving in the mean dynamical field of all other modes. The method further describes each mode by a semiclassical Gaussian wave packet. The scheme is carried out in normal modes. The method is restricted to systems of moderate anharmonicity at low temperatures. It is, however, computationally efficient and practically applicable to large systems. It can be used for the dynamics of nonstationary states as well as for stationary ones. Structural, dynamical and a variety of spectroscopic properties can easily be evaluated. The method is tested for thermal equilibrium states of (Ne)13, (Ar)13 against ‘‘numerically exact’’ quantum Feynman path integral simulations. Excellent quantitative agreement is found for the atom–atom pair distribution functions. The method is also applied to (H2O)n clusters. Good agreement is found with experimentally available fundamental stretch-mode frequencies.

A resonance Raman study of the C=N configurations of octopus rhodopsin, bathorhodopsin, and isorhodopsin.

The resonance Raman spectra of octopus rhodopsin, bathorhodopsin, and isorhodopsin at 120 K have been obtained as well as those of pigments regenerated with isotopically labeled retinals near the C14-C15 bond. Deuteration of the Schiff base nitrogen induces relatively large changes in the C-C stretch region between 1100 and 1300 cm-1, including a large frequency shift of the C14-C15 stretch mode located at 1206-1227 cm-1 in the three octopus species, as revealed by the Raman spectra of their 14,15-(13)C2 derivatives. Such results are different compared to those of the bovine pigments, in which no significant frequency shift of the C14-C15 stretch mode was observed upon Schiff base N deuteration. In an earlier Raman study of a Schiff base model compound which contained only one single bond adjacent to two double bonds, we have found that the stretch mode of this C-C single bond at 1232 cm-1 shifts up by 15 cm-1 and its intensity is also greatly reduced upon Schiff base N deuteration when the C=N configuration is anti [Deng et al., (1994) J. Phys. Chem. 98, 4776-4779]. The same study has also shown that when the C=N configuration is syn, the C-C stretch mode should be at about 1150 cm-1. Since the C14-C15 stretch mode frequency is relatively high in the spectra of octopus rhodopsin and bathorhodopsin (> 1200 cm-1) and since the normal mode pattern near the Schiff base is similar to the model, we suggest that the C=N configuration in these two species is anti. The different responses of the C14-C15 stretch mode to the Schiff base nitrogen deuteration in bovine and octopus pigments are due to the fact that the coupled C14-C15 stretch and the C12-C13 stretch motions in the model compound or in bovine rhodopsin are altered in octopus rhodopsin so that the stretch motion of the C14-15 bond is more localized, similar to the C-C stretch motion in the small Schiff base model compound. In clear contrast with the bovine rhodopsin Raman spectrum, which is very similar to that for the 11-cis-retinal Schiff base, the drastically different octopus rhodopsin spectrum indicates large protein perturbations on the C11=C12-C13 moiety, either by steric or by electrostatic interactions. Further studies are required to determine if such spectral differences indicate a difference of the energy conversion mechanism in the primary photochemical event of these two pigments.

Theoretical electronic and vibrational study of AlCO and Al(CO) 2 using density functional theory

The AlCO molecule has been experimentally characterized as a strong complex with a large red-shift of the ν CO stretching mode. Post Hartree-Fock ab initio methods including the treatment of correlation effects have predicted this molecule to be a weakly bound van der Waals complex, and were not able to reproduce its vibrational spectrum. Electronic and vibrational calculations at the harmonic approximation of this complex have been accomplished using density functional theory (DFT) and close agreement with experiment is now obtained. The credibility of the calculations is also assessed by the good agreement between the DFT-calculated and experimental vibrational frequencies of the Al(CO) 2 molecule. It is to be noticed that for this species the MP2 method reverses the order of the symmetric and antisymmetric ν CO stretching mode frequencies with respect to the experimental ones.

Picosecond isomerization of a linear triatomic molecule with two intense infrared laser pulses

A two-pulse method of rapid laser induced isomerization of a triatomic molecule is proposed. One laser pulse is resonant with the bending vibration while the second pulse has its frequency close to the stretch mode frequency. Alternatively the second frequency can be set equal to the second harmonic of the first laser. Both lasers are linearly polarized and their polarization vectors are perpendicular. We show from numerical solutions of a two-dimensional, time-dependent Schrödinger equation, that such perpendicular combination of intense picosecond laser pulses allows the system to follow the reaction path and thus leads to higher isomerization rates.