B2 Modes Research Articles

Nonresonant two-photon mass analyzed threshold ionization and zero kinetic energy photoelectron investigation of the X̃ 2B1 ground state of CH2CO+ and CD2CO+

Rotationally resolved nonresonant two-photon mass analyzed threshold ionization (MATI) and zero kinetic energy (ZEKE) photoelectron spectra of CH2CO+ and CD2CO+ are reported. The spectra are dominated by the origin band and totally symmetric a1 vibrations, ν4 (C=C symmetric stretch) and ν2 (C=O asymmetric stretch) for CH2CO+, and ν1 (C=O asymmetric stretch), ν3 (C=C symmetric stretch), and ν4 (CD2 scissor) for CD2CO+. In addition, several weaker bands are observed in the MATI spectra: ν3 (CH2 scissor) in CH2CO+; b1 vibrations ν5 and ν6 (C=C=O linear bend and CH2 wag) in both isotopomers; b2 vibration ν8 (CD2 rock) for CD2CO+; b2 vibration ν9 (C=C=O linear bend) for CH2CO+; as well as overtones and combination bands. Rotational structure of the origin band is dominated by three very strong ΔKa=±1 bands with 2 orders of magnitude weaker ΔKa=+3 bands. A similar ΔKa=±1 three-band pattern is observed for the a1 vibrational modes. For the b1 modes a single-band pattern resulting from the ΔKa=0 selection rule is present. Fundamental wave numbers for six vibrational modes of CH2CO+ and seven modes of CD2CO+ are determined. Rotational analysis of the partially resolved ZEKE spectra yields the ionization potentials (77539.4±2.0 cm−1 for CH2CO and 77534.3±2.0 cm−1 for CD2CO) and rotational constants for the ground states of both ketene cations.

Fluorescence excitation and UV–UV double-resonance spectroscopy of the S0 → S1(Lb) transition of 1,6-methano[10]annulene cooled in a supersonic jetDedicated to Professor F. Dörr on the occasion of his 80th birthday.

The first electronic singlet transition S0→S1 of the 10π-aromatic compound 1,6-methano[10]annulene (MA) cooled in a supersonic jet has been studied up to an excess energy of 4000 cm−1. The strongest line at 25154 cm−1 is assigned as the electronic origin. Analysis of the rotational envelope of this line proved that the transition dipole is parallel to the long axis of the molecule. Optical–optical double resonance was used to identify the lines which share the same ground state with the origin transition. These lines occur all at higher energies. A few weaker lines which are always present but do not lead to double resonances are tentatively attributed to a van-der-Waals dimer of MA. The rich vibrational structure is interpreted in terms of 13 fundamental vibrations of a1 symmetry and 11 of a2 symmetry, based on the analysis of the rotational contours. The fundamental vibrational frequencies of the excited state are in very good agreement with ab initio calculations. Based on these calculations 8 further lines which are not combination bands are tentatively assigned to double quantum transitions in b1 and b2 modes. These results strongly support the assignment of a delocalized structure without bond length alternation to the electronic ground state as well as to the first electronically excited singlet state.

Nanometer spot allocation for Raman spectroscopy on ferroelectrics by polarization and piezoresponse force microscopy

We report the 100% correlation between polarized light microscopy (PLM), piezoresponse force microscopy (PFM), and micro-Raman spectroscopy when investigating domain-rich ferroelectric systems. In order to allocate the desired spot on a submicrometer scale, both PLM and PFM were combined to elucidate the effective three-dimensional ferroelectric domain distribution. With PFM we observe spike-like a and c domains well inside extended a and c-polarized areas, which were not conclusive with PLM. The knowledge on such a domain distribution is essential when addressing quantitative micro-Raman spectroscopy. In addition, we show the unambiguous differentiation between a and c domains on the submicrometer scale using the B2 mode of lattice vibrations.

Polarized Infrared Absorption Spectra of Matrix-Isolated Allyl Radicals

We have measured the polarized infrared absorption spectrum of the allyl radical, CH2CHCH2 (X) 2A2, in an argon matrix at 10 K. The experimental CH2CHCH2 frequencies (cm-1) and polarizations follow: a1 modes, 3109, 3052, 3027, 1478, and 1242; b1 modes, 983, 801, and 510; b2 modes, 3107, 3020, 1464, 1390, and 1182. Two modes (ν6 and ν18) are very weak and could not be detected; the lowest frequency a1 mode (the CH2−CH−CH2 bending mode ν7) is estimated to be beyond the wavelength range of our MCT infrared detector. Infrared absorption spectra of two deuterated isotopomers, CH2CDCH2 and CD2CDCD2, were recorded in order to compare experimental frequency shifts with calculated [UB3LYP/6-311-G(d,p)] harmonic frequencies. Linear dichroism spectra were measured with photooriented samples in order to establish experimental polarizations of most vibrational bands. True gas-phase vibrational frequencies were estimated by considering the gas-to-matrix shifts and matrix inhomogeneous line broadening. The allyl radic...

Spectroscopic and theoretical investigations of vibrational frequencies in binary unsaturated transition-metal carbonyl cations, neutrals, and anions.

Spectroscopic and theoretical investigations of vibrational frequencies in binary unsaturated transition-metal carbonyl cations, neutrals, and anions.

Vibrational analysis of Ni(II)‐ and Cu(II)‐octamethylchlorin by polarized resonance Raman and Fourier transform infrared spectroscopy

AbstractWe measured the polarized resonance Raman spectra of Cu(II)‐2,2,7,8,12,13,17,18‐octamethylchlorin in CS2 at various excitation wavenumbers in a spectral region covering the Qy, Qx and Bx optical absorption bands. Additionally, we measured the FTIR‐Raman spectrum of the highly overcrowded spectral region between 1300 and 1450 cm−1. The spectral decomposition was carried out by a self‐consistent global fit to all spectra obtained. The thus identified Raman and IR lines were assigned by comparison with the resonance Raman spectra of Cu(II)‐octaethylporphyrin, by utilizing their depolarization ratio dispersions and by a normal mode analysis. The latter was based on a modified transferable molecular mechanics force field of Ni(II)‐octaethylporphyrin [E. Unger, M. Beck, R.J. Lipski, W. Dreybrodt, C.J. Medforth, K.M. Smith and R. Schweitzer‐Stenner, J. Phys. Chem. B 103, 10229 (1999)]. A comparison of normal mode patterns obtained for Cu(II)‐octamethylchlorin and Cu(II)‐octaethylporphyrin revealed that some modes are significantly distorted by the reduction of the pyrrole ring, in accordance with results which Boldt et al. reported earlier for Ni(II)‐octaethylchlorin [N.J. Boldt, F.J. Donohoe, R.R. Birge and D.F. Bocian, J. Am. Chem. Soc. 109, 2284 (1987)]. In contrast to conclusions drawn from this study, however, the results of our vibrational analysis and several further lines of evidence suggest that the normal modes of corresponding chlorines and porphyrins are still comparable, because they display contributions from the same local coordinates. Thus, the classical normal mode classification developed for metalloporphyrins is also applicable to metallochlorins. Finally, we performed a preliminary analysis of the absorption spectrum and the resonance excitation profiles and depolarization ratio dispersions of some Raman lines. The results show that the electronic properties of Cu(II)‐octamethylchlorin can still be described in terms of Gouterman's four orbital model [M. Gouterman, J. Chem. Phys. 30, 1139 (1959)]. In regions of the Q bands, Raman scattering of A1 modes is determined by interferences between Franck– Condon coupling and interstate Herzberg– Teller coupling between Qx(Qy) and Bx(By) states. The B2 modes are resonance enhanced by Herzberg– Teller coupling between Qx and Qy and between Qx(Qy) and By(Bx). Franck– Condon coupling of A1 modes with large contributions from CαCm stretching vibrations is comparatively strong for Qx. This is interpreted as reflecting the expansion of the chlorin macrocycle by an electronic transition into this excited state. Copyright © 2001 John Wiley & Sons, Ltd.

Raman spectra of the chalcopyrite compound CuGaTe2

Raman spectra of the chalcopyrite compound CuGaTe2 were obtained between 80 and 300K. Lines observed at 54, 76, 155, 170, 211 and 220cm−1, and at 70, 204 and 224cm−1 were assigned to E and B2 modes, respectively. The most intense line at 135cm−1, and others at 64, 148, and 190cm−1, are assigned to A1 mode and to B1 symmetry, respectively. The highest-frequency phonon mode at 272cm−1 is attributed to a combination of E and B2 modes. It is suggested that the line at 117cm−1 is due to the chalcopyrite-forbidden A2 mode activated by structural disorder. A broad, but weak, band observed between 80 and 110cm−1 is tentatively attributed to scattering, due to free holes or collective excitations. A shift to higher frequency with increasing temperature, of A1, B2 and E modes below 120K, is explained due to the contribution of the negative value of the thermal expansion coefficient of chalcopyrites at low temperatures. The temperature variation of the full width at half maximum (FWHM) of the A1 mode could be explained by considering the decay of the Raman phonon in a combination of two phonons, in addition to the contribution, at low temperatures, of a thermal expansion term.

Two-Photon Fluorescence Excitation Spectrum of 1,6-Methano-[10]annulene

The two-photon fluorescence excitation spectrum of 1,6-methano-[10]annulene has been measured in fluid solution at room temperature between 25 000 and 40 000 cm-1 and in rigid solution at 77 and 15 K between 25 000 and 28 000 cm-1. The two-photon polarization ratios of both spectra have been determined. Spectral assignments have been made on the basis of two-photon polarization data, ab initio calculations, and comparison with one-photon data. The four excited states below 5 eV have been identified. Calculated one- and two-photon allowed intensities are in good agreement with results derived from the alternant hydrocarbon theory. The S0 → S1 two-photon intensity is mainly vibronically induced by b1 modes. The most active b1 vibration, the “Kekule”-type mode responsible for double bond localization in 1,6-methano-[10]annulene, shifts to ≈1540 cm-1 in S1 from the ground state 1355 cm-1 value. Applying two-photon polarization results to 1,6-methano-[10]annulene (C2v symmetry), the tensor elements of the strongest totally symmetric S0 → S1 (B1 × b1 × a1) transition have been estimated.

Photoluminescence, infrared reflectivity, and Raman spectra of the ordered vacancy compound CuGa3Se5

The photoluminescence (PL), infrared reflectivity (IR), and Raman spectra of the ordered vacancy compound CuGa3Se5 have been studied. Analysis of the data indicates that the main PL emission is due to a donor-acceptor pair recombination. Donor and acceptor levels with activation energy of 15 and 300 meV, respectively, were estimated. The lattice vibrational modes of this compound have also been obtained at 300 K by a combined analysis of IR and Raman spectra. Lines common to both spectra, observed at 48, 64, 105, 142, 187, 200, 218, 250, and 274 cm−1, were identified with E or B2 modes. The most intense peak at 166 cm−1 and those observed at 72 and 90 cm−1, which are only Raman active, are assigned to the A1 and B1 modes, respectively.

Acetone n-radical cation internal rotation spectrum: The torsional potential surface

The one color REMPI and two color ZEKE-PFI spectra of acetone-d3 have been recorded. The 3px Rydberg state of acetone-d3 lies at 59 362.3 cm−1 and both of the torsional modes are visible in this spectrum. The antigearing Rydberg (a2) mode, v12*, has a frequency of 62.5 cm−1, while the previously unobserved gearing (b1) mode, v17*, is found at 119.1 cm−1. An ionization potential of 78 299.6 cm−1 for acetone-d3 has been measured. In acetone-d3 n-radical cation ground state, the fundamentals of both of the torsional modes have been observed, v12+ at 51.0 cm−1 and v17+ at 110.4 cm−1, while the first overtone of v12+ has been measured at 122.4 cm−1. Deuterium shifts show that v12+ behaves like a local C3v rotor, but that v17+ is canonical. Combining this data with that for acetone-d0 and aacetone-d6 has allowed us to fit the observed frequencies to a torsional potential energy surface based on an ab initio C2v cation ground state geometry. This potential energy surface allows for prediction of the v17 vibration in acetone-d0 and acetone-d6. The barrier to synchronous rotation is higher in the cation ground state than in the neutral ground state, but significantly lower than in the 3s Rydberg state. The 3px Rydberg and cation ground state potential energy surfaces are found to be very similar to each other, strongly supporting the contention that the 3px Rydberg state has C2v geometry and is a good model for the ion core. The altered 3s Rydberg state potential surface suggests this state has significant valence character.

Coherent excitation of ferroelectric modes of KDP and LiNbO3

The ferroelectric modes in KDP and LiNbO3 are coherently excited by the femtosecond impulsive stimulated Raman scattering (ISRS). The time dependence of the mode excited artificially shows a characteristic behavior in real time; the soft B2 modes of KDP and its deuterated isomorphous DKDP show a single-exponential time decay in contrast to the A1 phonon of LiNbO3 which behaves like a single damped harmonic oscillator. A critical-slowing down of the B2 modes has been observed above Tc for both KDP and DKDP. The isotope effect appears as the difference between the characteristic relaxation time τ0equals;1·1×10−11 s of KDP and 8·2×10−11s of DKDP. The dispersion relation of the A1phonon-polariton in LiNbO3 is studied by an improved ISRS method and confirmed to be a simple one without any resonance point in contrast to the result reported previously.

The vibrational structure of benzene adsorbed on Si(001)

High resolution electron energy loss spectroscopy (HREELS) measurements and density functional model cluster calculations are presented to clarify the vibrational structure of the adsorption system C6H6/Si(001). All vibrational modes of the adsorption complex, which previously was identified to exhibit a cyclohexadiene-like structure, have been calculated and characterized according to the motion of the different atoms of the adsorption complex. Special emphasis is placed on the low-frequency modes. The coupling between the adsorbate and the substrate modes is analyzed with the help of a model that represents various limiting situations. Different coupling variants are found to apply to different collective modes of the adsorbate. The A1 and B1 modes can be described rather well by a model that only encompasses the adsorbate and the Si dimer underneath; for the A2 and B2 modes a frozen substrate description of the adsorption complex is more appropriate.

Partial depolarization effect of high-numerical-aperture objectives on polarized microfocus Raman spectra of orthorhombic poly(oxymethylene) single crystal

In microfocus Raman spectroscopic measurement of micron-sized, moth-shaped single crystal of orthorhombic pohombic poly(oxymethylene) (metastable phase), the partial depolarization caused by a sharp focusing with objectives of a high magnification was investigated in order to obtain the information about the vibrational assignments. Polarized microfocus Raman spectra in the frequency range covering molecular vibrations were compared before and after the introduction of an aperture in the collimating system. The spectral difference in the (YX) polarization was observed for the bands of A–B1 and B2–B3 split-pairs, which indicated clearly that the scattering intensities of the B2 modes as well as the A modes were decreased selectively by the introduction of the aperture. In the (XX), (YY), and (XY) spectra, the B2 modes were observed as partial depolarization components of light for electric vector component along the depth direction. Utilizing these partial depolarization effects, it is possible to assign Raman-active fundamentals and to decide the unit cell orientation of the micron-sized, moth-shaped single crystal of orthorhombic poly(oxymethylene). Copyright © 2000 John Wiley & Sons, Ltd.

Optical Characterization of the Ordered Vacancy Compound CuGa3Se5

The lattice vibrational modes of the Ordered Vacancy Compound CuGa3Se5 have been obtained at 300 K by a combined analysis of infrared reflectivity and Raman spectra. Lines common to both spectra, observed at 48, 64, 105, 142, 187, 200, 218, 250, and 274 cm-1, were identified with E or B2 modes. The most intense peak at 166 cm-1 and those observed at 72 and 90 cm-1, which are only Raman active, are assigned to the AI and BI modes, respectively.

Vibrational absorption from vacancy-oxygen-related complexes (VO, V2O, VO2) in irradiated silicon

Infrared absorption from oxygen-related defects in Si crystals irradiated with electrons (2.5MeV) at room temperature (RT) and in the range 300–600°C has been investigated. Two new vibrational bands positioned at 10K at about 1370 and 1430cm−1 were observed in samples irradiated at RT. A good correlation is found between these lines and the bands at 836 and 885cm−1 known to originate from asymmetrical stretching vibrations (B1 mode) of an oxygen atom in the neutral and negative VO complex. An attribution of the 1370 and 1430cm−1 bands to a combination of the B1 mode with the symmetrical stretching A1 mode (weakly IR active) for different charge states of VO is argued to be the most probable. A band at 833.4cm−1 is found to increase in strength upon annihilation of divacancies at 250–300°C. The V2O complex is suggested to give rise to this band. New experimental data confirming an attribution of the 895cm−1 band to the VO2 complex are presented as well.

Disorder-Activated Scattering and Two-Mode Behavior in Raman Spectra of Isotopic GaN and AlGaN

Micro-Raman spectroscopy has been applied to isotopically pure GaiN (i = 14, 15) epitaxial films in which structural disorder was introduced by ion implantation or mechanical damage. A pair of Raman peaks appearing at about 300 (300) and 667 (646) cm—1 is consistent with the anion mass 14 (15) suggesting disorder activation of the silent B1 modes, where the low-frequency mode is related to the Ga vibration (no N isotope shift) and the high-frequency mode to the N vibration (full N isotope shift). Isotopically pure Al0.5Ga0.5iN indicates respective activation of the silent B1 modes due to alloying disorder and confirms a two-mode behavior of the E2 mode. In agreement with recent IR measurements and theoretical predictions, we additionally provide evidence for an E1(TO) two-mode behavior in Raman scattering as obtained from polarisation dependent measurements on AlxGa1—xN epilayers.

Ab initio study of the n-π* electronic transition in acetone: Symmetry-forbidden vibronic spectra

Ab initio calculations of geometry and vibrational frequencies of the first singlet excited A21(1A″) state of acetone corresponding to the n-π* electronic transition have been carried out at the CASSCF/6-311G** level. The major geometry changes in this state as compared to the ground state involve CO out-of-plane wagging, CO stretch and torsion of the methyl groups, and the molecular symmetry changes from C2v to Cs. The most pronounced frequency changes in the A″1 state are the decrease of the CO stretch frequency v3 by almost 500 cm−1 and the increase of the CH3 torsion frequency v12 from 22 to 170 cm−1. The optimized geometries and normal modes are used to compute the normal mode displacements which are applied for calculations of Franck–Condon factors. Transition matrix elements over the one-electron electric field operator at various atomic centers calculated at the state-average CASSCF/6-311+G** level are used to compute vibronic couplings between the ground A11, A21, and Rydberg B21(n-3s), 2 A11(n-3py), 2 A21(n-3px), 2 B21(n-3pz), and B11(n-3dxy) electronic states, and the Herzberg–Teller expansion of the electronic wave function is applied to derive the transition dipole moment for A11→A21 as a function of normal coordinates. The results show that the intensity for this transition is mostly borrowed from the allowed A11-B21(n-3s) transition due to vibronic coupling between A21 and B21 through normal modes Q20, Q22, and Q23 and, to some extent, from the A11-B11 transition due to Q19 (CO in-plane bend) which couples A21 with B11(n-3dxy). The calculated total oscillator strength for the n-π* transition through the intensity-borrowing mechanism, 3.62×10−4, is in close agreement with the experimental value of 4.14×10−4. Ninety-four percent of the oscillator strength comes from the perpendicular component (b1 inducing modes) and 6% from the parallel component (b2 modes). Calculated spectral origin, 30 115 cm−1 at the MRCI/6-311G** level, underestimates the experimental value by ∼300 cm−1. Calculated positions of the most intense peaks in the spectra also reasonably agree with the experimental band maximum. The presence of numerous weak vibronic peaks densely covering a broad energy range (∼12 000 cm−1) explains the diffuse character of the experimental n-π* band. Most of the bands observed in fluorescence excitation spectra [Baba and Hanazaki, Chem. Phys. Lett. 103, 93 (1983); Baba, Hanazaki, and Nagashima, J. Chem. Phys. 82, 3938 (1985)] can be assigned based on the computed spectrum.

A Further Study of the Products of Scandium and Dioxygen Reactions

The products of the reaction of Sc and dioxygen have been reinvestigated. By adding the electron-trapping molecule CCl4, additional information about the IR spectra has been obtained, as well as the observation of new bands. New ab initio calculations are also performed on possible products of the Sc plus O2 reaction. The previously observed band at 722.5 cm-1 is assigned as the b2 mode of ScO2-. Bands arising from ScO+, Sc(O2)+, and(O2)ScO are also assigned. We are still unable to assign any bands to OScO. The problems associated with the computational study of ScO2 are discussed.

Raman spectra of the chalcopyrite compound CuInTe2

The lattice vibrational modes of the chalcopyrite compound CuInTe2 have been obtained at several temperatures by the unpolarized Raman spectra. Lines observed at 48, 74, 129, 138, 155, and 185 cm−1, identified with E modes, and at 60, 106, and 170 cm−1, assigned to B2 modes, agree quite well with the peaks reported from infrared reflectivity data. On the other hand, using the available theoretical models, the peaks that appear at 123 cm−1 and those observed around 65, 116, and 223 cm−1, which are only Raman active, are assigned to the A1 and B1 modes, respectively.

Orientational Phase Transition in a Pyridine Adlayer on Gold(111) in Aqueous Solution Studied by in Situ Infrared Spectroscopy and Scanning Tunneling Microscopy

The orientation and structure of pyridine adsorbed on a highly ordered Au(111) surface from 0.1 M NaClO4 + x M (10-6 ≤ x ≤ 10-3) pyridine aqueous solutions have been investigated as a function of applied potential by in situ surface-enhanced infrared absorption spectroscopy (SEIRAS) and scanning tunneling microscopy (STM). Symmetric in-plane pyridine ring vibrations (A1 modes) were observed in the SEIRA spectra at potentials positive of about −0.3 V versus SCE, while asymmetric in-plane ring vibrations (B1 modes) were hardly detected. The symmetric ring-breathing mode showed a blue shift upon adsorption, indicating the adsorption via the N atom. The band intensities were found not to be proportional to the surface concentration (the relative Gibbs surface excess) reported in the literature. On the basis of the surface selection rule in SEIRAS, the results are explained in terms of the potential-dependent reorientation of pyridine. The molecule is flatly adsorbed on the surface at negative potentials, and its molecular plane rises up as the applied potential increases and the surface concentration increases. Flat-lying, tilted, and vertically standing pyridine molecules were observed at different potentials also by STM for the first time.