1g Mode Research Articles

Theoretical DFT study of phosphorescence from porphyrins

Geometrical structure of free-base porphin (H 2P) and Mg- and Zn-porphyrins together with their vibrational frequencies and vibronic intensities in phosphorescence are investigated by density functions theory (DFT) with the standard B3LYP functional. These molecules have a closed-shell singlet ground state (S 0) and low-lying triplet (T 1) excited states of ππ* type. The S 0–T 1 transition probability and radiative lifetime of phosphorescence ( τ p) of these molecules are calculated by time-dependent DFT utilizing quadratic response functions for account of spin–orbit coupling (SOC) and electric-dipole transition moments including displacements along active vibrational modes. The infrared and Raman spectra in the ground singlet and first excited triplet states are also studied for proper assignment of vibronic patterns. The long radiative lifetime of free-base porphin phosphorescence ( τ p ∼ 360 s at low temperature limit, 4.2 K) gets considerably shorter for the metalloporphyrins. An order of magnitude reduction of τ p is predicted for Mg-porphyrin but no change of phosphorescence polarization is found. A forty times enhancement of the radiative phosphorescence rate constant is obtained for Zn-porphyrin in comparison with the H 2P molecule which is accompanied by a strong change of polarization and spin-sublevel radiative activity. A strong vibronic activity of free-base porphin phosphorescence is found for the b 2g mode at 430 cm −1, while the 679 and 715 cm −1 vibronic bands of b 3g symmetry are less active. These and other out-of-plane vibrations produce considerable changes in the radiative constants of different spin sublevels of the triplet state; they also promote the S 1 → T 1 intersystem crossing. Among the in-plane vibrations the a g mode at 1614 cm −1 is found very active; it produces a long progression in the phosphorescence spectrum. The time-dependent DFT calculations explain the effects of the transition metal atom on phosphorescence of porphyrins and reproduce differences in their phosphorescence and EPR spectra.

Calcium substituting B-site in relaxor ferroelectrics with perovskite structure probed by chemical ordering

Chemical ordering of Ca 2+ doped 0.9Pb(Mg 1/3Nb 2/3)O 3–0.1PbTiO 3 ceramics were investigated by dielectric spectra, TEM diffraction and A 1g mode in Raman spectra. It is found degree of relaxor behavior increases first, and then decreases. It conflicts the prediction Ca 2+ substitutes for A-site ion Pb 2+ according to crystal chemistry theory. In this letter, a new mechanism that Ca 2+ substitutes for B-site ions has been proposed, which satisfactorily explained change of chemical ordering. It exhibits strong evidence doped ions with larger ionic radius (Ca 2+) are quite possibly substitute much smaller ones (Nb 5+ or Ti 4+) in B-site rather than all substitute larger A-site ion in relaxor ferroelectrics.

Jahn–Teller effects and charge transfer in the positively charged triphenylene and coronene

Vibronic interactions and Jahn–Teller effects in the monocations of triphenylene and coronene are discussed. The E′ mode of 1634 cm −1 and the E 2g mode of 1668 cm −1 afford large electron–phonon coupling constants in the monocations of triphenylene and coronene, respectively. The single charge transfer through the molecules in these positively charged species are discussed. The reorganization energies for elementary charge transfer are estimated to be 0.045 and 0.028 eV for triphenylene and coronene, respectively. The optimized structures of the monocations are discussed in terms of the frontier orbitals and the vibrational modes.

Lattice Vibrations and Electronic Transitions in the Rare-Earth Metals: Praseodymium under Pressure

Praseodymium was investigated by Raman spectroscopy under pressure. A negative pressure shift of the E(2g) mode is observed in the dhcp phase, which indicates that the initial structural sequence hcp-->Sm-type-->dhcp-->fcc as a whole in the regular lanthanides is associated with a softening of this mode. The pressure response of the phonon modes, observed in the monoclinic and alpha-uranium phases, where 4f bonding becomes important, is characteristic for anisotropic bonding properties.

High-pressure anomaly in the normal state ofMgB2

The E 2g mode phonon frequency anomaly, observed in high-pressure Raman measurements for the high-temperature superconductor MgB 2 and conjectured to arise due to a phonon-assisted electronic topological transition (ETT), has been corroborated by us with the details of electronic structure calculations of this compound. The present studies have been carried out with a B-atom displacement of the frozen-in E 2g -mode phonon that mimics the effects of thermal vibrations. We found a splitting of the doubly degenerate σ band along the Γ-A direction. The splitting of the band reduces with pressure, causing an ETT at 18 GPa. This ETT is expected to cause anomaly in the E 2g phonon mode by influencing the ionic system. Our calculated electron-phonon coupling constant λ , for the distorted lattice with a frozen-in displacement, also depicts an anomalous variation around 18 GPa.

Spectroscopic characterization of chromite from the Moa-Baracoa Ophiolitic Massif, Cuba

The Cuban chromites with a spinel structure, FeCr 2O 4 have been studied using optical absorption and EPR spectroscopy. The spectral features in the electronic spectra are used to map the octahedral and tetrahedral co-ordinated cations. Bands due Cr 3+ and Fe 3+ ions could be distinguished from UV–vis spectrum. Chromite spectrum shows two spin allowed bands at 17390 and 23810 cm −1 due to Cr 3+ in octahedral field and they are assigned to 4A 2g(F) → 4T 2g(F) and 4A 2g(F) → 4T 1g(F) transitions. This is in conformity with the broad resonance of Cr 3+ observed from EPR spectrum at g = 1.903 and a weak signal at g = 3.861 confirms Fe 3+ impurity in the mineral. Bands of Fe 3+ ion in the optical spectrum at 13700, 18870 and 28570 cm −1 are attributed to 6A 1g(S) → 4T 1g(G), 6A 1g(S) → 4T 2g(G) and 6A 1g(S) → 4T 2g(P) transitions, respectively. Near-IR reflectance spectroscopy has been used effectively to show intense absorption bands caused by electronic spin allowed d -d transitions of Fe 2+ in tetrahedral symmetry, in the region 5000–4000 cm −1. The high frequency region (7500–6500 cm −1) is attributed to the overtones of hydroxyl stretching modes. Correlation between Raman spectral features and mineral chemistry are used to interpret the Raman data. The Raman spectrum of chromite shows three bands in the CrO stretching region at 730, 560 and 445 cm −1. The most intense peak at 730 cm −1 is identified as symmetric stretching vibrational mode, A 1g(ν 1) and the other two minor peaks at 560 and 445 cm −1 are assigned to F 2g(ν 4) and E g(ν 2) modes, respectively. Cation substitution in chromite results various changes both in Raman and IR spectra. In the low-wavenumber region of Raman spectrum a significant band at 250 cm −1 with a component at 218 cm −1 is attributed F 2g(ν 3) mode. The minor peaks at 195, 175, 160 cm −1 might be due to E g and F 2g symmetries. Broadening of the peak of A 1g mode and shifting of the peak to higher wavenumber observed as a result of increasing the proportion of Al 3+O 6. The presence of water in the mineral shows bands in the IR spectrum at 3550, 3425, 3295, 1630 and 1455 cm −1. The vibrational spectrum of chromite gives raise to four frequencies at 985, 770, 710 and 650 cm −1. The first two frequencies ν 1 and ν 2 are related to the lattice vibrations of octahedral groups. Due to the influence of tetrahedral bivalent cation, vibrational interactions occur between ν 3 and ν 4 and hence the low frequency bands, ν 3 and ν 4 correspond to complex vibrations involving both octahedral and tetrahedral cations simultaneously. Cr 3+ in Cuban natural chromites has highest CFSE (20,868 cm −1) when compared to other oxide minerals.

Order–disorder behaviour in 0.9Ba([Zn 0.60Co 0.40] 1/3Nb 2/3)O 3–0.1Ba(Ga 0.5Ta 0.5)O 3 microwave dielectric resonators

0.9Ba([Zn 0.60Co 0.40] 1/3Nb 2/3)O 3–0.1Ba(Ga 0.5Ta 0.5)O 3 (BCZN–BGT) ceramic resonators (quality factor, Q=32,000 at the rate of 3.05 GHz, relative permittivity, ε r=35 and temperature coefficient of the resonant, τ f=0) have been fabricated which are suitable in terms of cost and performance for base stations supporting third generation architecture. The new compounds are perovskite structured ( a=4.09 Å) but exhibit no superlattice reflections at any heat treatment temperature according to X-ray diffraction (XRD). However, annealing and quenching of samples followed by transmission electron microscopy and Raman spectroscopy revealed an order–disorder phase transition at ∼1200 °C. Annealing below this temperature (1100 °C) gave rise to discrete ±1/3{ h k l } p and diffuse 1/2{ h k l } p superlattice reflections in the same 〈1 1 0〉 p zone axis electron diffraction patterns and the presence of F 2g and A 1g modes in Raman spectra. It is proposed that ±1/3{ h k l } p reflections result from 1:2 long-range ordered domains of BCZN whereas the diffuse 1/2{ h k l } p reflections arise from short range fcc ordered BGT rich regions at the 1:2 domain boundaries. A short-range ordered fcc superlattice was observed in samples quenched from above the order–disorder phase transition (>1200 °C) which was accompanied by the presence of only the A 1g mode in Raman spectra.

Resonance Raman spectroscopy reveals new insight into the electronic structure of beta-hematin and malaria pigment.

Resonance Raman spectra of beta-hematin and hemin are reported for a range of excitation wavelengths including 406, 488, 514, 568, 633, 780, 830, and 1064 nm. Dramatic enhancement of A(1g) modes (1570, 1371, 795, 677, and 344 cm(-1)), ring breathing modes (850-650 cm(-1)), and out-of-plane modes including iron-ligand modes (400-200 cm(-1)) were observed when irradiating with 780- and 830-nm laser excitation wavelengths for beta-hematin and to a lesser extent hemin. Absorbance spectra recorded during the transformation of hemin to beta-hematin showed a red-shift of the Soret and Q (0-1) bands, which has been interpreted as excitonic coupling resulting from porphyrin aggregation. A small broad electronic transition observed at 867 nm was assigned to a z-polarized charge-transfer transition d(xy) --> e(g)(pi). The extraordinary band enhancement observed when exciting with near-infrared excitation wavelengths in beta-hematin when compared to hemin is explained in terms of an aggregated enhanced Raman scattering hypothesis based on the intermolecular excitonic interactions between porphyrinic units. This study provides new insight into the electronic structure of beta-hematin and therefore hemozoin (malaria pigment). The results have important implications in the design and testing of new anti-malaria drugs that specifically interfere with hemozoin formation.

Ab initiophonon dispersions of single-wall carbon nanotubes

Phonon-dispersion curves for a series of single-wall carbon nanotubes (SWCNT's) have been obtained by ab initio supercell approach. Force constants are calculated using norm-conserving pseudopotential plane-wave method in the framework of the density-functional theory and local-density approximation. Cumulant force-constant method is used instead of interatomic force-constant method to construct the dynamical matrices. We get much better low-frequency dispersions than existing results with no soft phonon mode presented. Good accuracy is shown in the full frequency range. Residue forces in the relaxed SWCNT structures are filtered out by a linear fitting force-constant scheme. Raman and IR active modes are reanalyzed by D-2nd group symmetry. The frequencies are found to agree excellently with the experimental observations. Our results confirm the reported frequency drop of the A(1g) mode. Some issues in the low-temperature specific-heat experiment and the neutron-scattering experiment are well explained by our phonon dispersions. Phonon dispersions of graphite are also calculated using the same method, being in good agreement with experiment. Theoretical analysis is also presented to explain the advantages of cumulant force-constant method to get better low-frequency dispersions.

Two-color resonance-enhanced multiphoton ionization study of the lowest Rydberg p state of bis(eta6-benzene)chromium and its deuterated derivatives.

Two-color resonance-enhanced multiphoton ionization (REMPI) spectra of jet-cooled (eta(6)-C(6)H(6))(2)Cr(1), (eta(6)-C(6)D(6))(2)Cr(2), and (eta(6)-C(6)D(6))(eta(6)-C(6)D(5)H)Cr(3) have been measured with use of the 3d(z)2-->R4p(x,y) Rydberg transition as the first step of the electronic excitation. The 0(0) (0) Rydberg component shifts by 59 and 54 cm(-1) to red when one goes from 1 to 2 and 3, respectively. Surprisingly, the REMPI spectra of 1-3 show very rich vibronic structures revealing both totally symmetric vibrations and degenerate vibrational modes. Presence of intense peaks corresponding to the e(2g) modes in the spectra of 1 and 2 is indicative of Jahn-Teller coupling in the R4p(x,y) Rydberg state. Additional REMPI resonances appear on going from 1 and 2 to 3 as a result of the symmetry reduction. The vibronic components in the spectra of 1-3 were assigned on the basis of the selection rules and comparison with the vibrational frequencies of the 1 and 2 ground-state molecules. The frequencies of over 10 normal vibrations have been determined for the gas-phase 1-3 Rydberg-state molecules from the REMPI experiment. The wavenumber corresponding to the lowest-energy mode (the ring torsion vibration) appears to be 40 cm(-1) in 1 and 35 cm(-1) in the deuterated complexes. The REMPI peaks are homogeneously broadened. The lower lifetime limits for the upper-state components increase on going from the vibrationless level to higher-lying vibronic states and on going from 1 to the deuterated derivatives.

Vacuum ultraviolet mass-analyzed threshold ionization spectroscopy of hexafluorobenzene: the Jahn-Teller effect and vibrational analysis.

One-photon mass-analyzed threshold ionization (MATI) spectrum of hexafluorobenzene was obtained by using vacuum ultraviolet radiation generated by four-wave difference frequency mixing in Kr. The ionization energy of hexafluorobenzene determined from the position of the 0-0 band was 9.9108+/-0.0006 eV. To aid the spectral analysis, the Jahn-Teller coupling parameters for four e(2g) modes of C(6)F(6) (+) in the ground electronic state were calculated from the topographical data of the potential energy surface obtained at the density functional theory (DFT) level. These were used in the initial calculation of the energies of the Jahn-Teller states and upgraded through the multimode fit to the experimental data. Excellent agreement between the experimental and calculated frequencies was achieved. The vibrations which are not linear Jahn-Teller active were observed and could be assigned by referring to the frequencies obtained at the DFT level.

The Jahn-Teller effect in the lower electronic states of benzene cation. III. The ground-state vibrations of C6H6+ and C6D6+.

New mass analyzed threshold ionization (MATI) spectra of the molecules C(6)H(6) (+) and C(6)D(6) (+) have been collected using tunable vacuum ultraviolet (VUV) single photon excitation from the neutral ground state and also using two-photon excitation through the 6(1) vibration of the (1)B(2u) S(1) state. Emphasis was placed on obtaining accurate relative intensities of the vibrational lines in order to use this information in the vibronic analysis. The MATI spectra collected from VUV (S(0) originating state), triplet (T(1)), and resonant two photon (S(1)) excitation schemes were compared with Jahn-Teller calculations employing the classical model of Longuet-Higgins and Moffitt to obtain the Jahn-Teller coupling parameters of 3 of the 4 linearly active modes (e(2g) modes 6-9 in Wilson's notation). Franck-Condon factors, including the effects of geometry changes, were calculated from the vibronic wave functions and used to identify the lines in the various spectra. It is found that most of the lines with substantial intensity can be understood using only the modes 1, 6, 8, and 9. Weaker peaks are due to various non-e(2g) modes, but these do not derive intensity through Jahn-Teller coupling. When the effects of geometry change were included, simulations of the spectra from the calculated vibrational energies and intensities were close to the experimental spectra. This verifies the applicability of the model to the understanding of the vibrational structure of this type of molecule, but some variations indicate directions for further improvement of the model.

Magnetic field dependent vibrational modes in κ-(ET)2Cu(SCN)2organic superconductor

The infrared reflectance of the 10.4 K organic superconductor κ-(ET) 2 Cu(SCN) 2 has been measured as a function of applied magnetic field at 4.2 K. We investigate changes in intramolecular vibrational modes between the superconducting (low field) and normal (high field) states, of interest for mechanistic reasons. It is shown that the v3 (Ag), ν 60 (B 3g ), and ν 21 (B 1g ) modes display field dependence. These results suggest that intramolecular vibrational modes are involved in the superconducting to normal state transition in κ-(ET) 2 Cu(SCN) 2 below T c .

Evidence of Electron–Phonon Interaction in Al-Substituted Mg1-xAlxB2

Phonon density of states in Al substituted Mg 1-x Al x B 2 has been studied by means of inelastic neutron scattering experiments with pulsed neutron time-of-flight technique. Phonon dynamics have a significant role for realizing superconductivity in the recent discovered superconductor MgB 2 . We present a systematic investigation on generalized phonon density of states (PDOS) up to E = 120 meV on Al content of x = 0.1, 0.25, and 0.5. We have clearly observed a large effect on the PDOS by Al substitution; a large energy shift occurs around at 70 meV and 89 meV. We have also observed a visible increase of PDOS at a particular energy range below T c . Those phonon states are related with E 2g mode to be expected to have a strong coupling with the electron states at the r-A region.

An analytical expression for E g and A g modes of pyrochlores

A general analytical expression has been derived for E g and A 1g modes in terms of stretching and bending force constant for cubic pyrochlore structure compounds. The expression has been applied to different pyrochlore oxides accounting six stretching and four bending force constants. The calculation of E g and A 1g mode has been carried out by using the earlier reported values of force constants for titanates, hafnates and stannates. The analytical expression can be applied to any pyrochlore structure compounds for evaluating and assigning E g and A 1g mode if the interatomic force constants are already known.

Symmetry switching of the fluorescent excited state in alpha,omega-diphenylpolyynes.

By using MO calculations based on DFT, absorption, and fluorescence spectroscopy, we have comprehensively studied the low-lying excited singlet states of alpha,omega-diphenylpolyynes (DPY) having 1-6 triple bonds. The a(g) vibrational modes of the C(triple bond)C stretching and of the phenyl ring motion were observed in the fluorescence spectra of diphenylacetylene and 1,4-diphenylbutadiyne. On the other hand, in the fluorescence spectra of the long DPY with the triple-bond number (N) more than two, the phenyl ring motion with a(g) symmetry disappeared and the b(1g) modes of the phenyl ring twisting (approximately 400 cm(-1)) and of the C-H bending (approximately 900 cm(-1)) were detected. The observed fluorescent states of DPY with N < or = 2 and N > or = 3 are assigned to the 1(1)B(1u) (pi(x)pi(x*)) and 1(1)A(u) (pi(x)pi(y*) and/or pi(y)pi(x*)) states, respectively, based on the vibronic structures, the relatively short lifetimes, and the solvatochromic shifts of the fluorescence spectra. Not only the allowed transition of 1(1)B(1u) <-- S(0) but also the forbidden transition of 1(1)A(u) <-- S(0) was detected in the fluorescence excitation spectra of the long DPY with N > or = 3. The low-lying excited state with A(u) symmetry is characteristic in polyyne, which does not exist in polyene. The oscillator strength (f) of the first absorption band in DPY decreases with an increase in N, which is the opposite behavior of the all-trans-alpha,omega-diphenylpolyenes. The N-dependence of the f value is understood by the configuration interaction between the 1(1)B(1u) and 2(1)B(1u) (pi(y)pi(y*)) states, which is consistent with the reduction of the nonlinear optical response of polyyne.

Raman imaging of hemozoin within the food vacuole of Plasmodium falciparum trophozoites

Micro-Raman spectra of hemozoin encapsulated within the food vacuole of a Plasmodium falciparum-infected erythrocyte are presented. The spectrum of hemozoin is identical to the spectrum of β-hematin at all applied excitation wavelengths. The unexpected observation of dramatic band enhancement of A 1g modes including ν 4 (1374 cm −1) observed when applying 780 nm excitation enabled Raman imaging of hemozoin in the food vacuole. This unusual enhancement, resulting from excitonic coupling between linked porphyrin moieties in the extended porphyrin array, enables the investigation of hemozoin within its natural environment for the first time.

Raman spectroscopic study of carbon substitution in MgB 2

We study the effect of carbon doping on the Raman spectrum of the MgB 2 superconductor. Out data show that significant changes in the Raman spectra of the MgB 2− x C x compounds occur for carbon concentrations x>0.04. The E 2g mode at ∼580 cm −1 hardens only moderately upon increased doping despite direct carbon substitution in the boron layers, while the dependence of its full width at half maximum (FWHM) on x reveals the competing effects of reduced electron–phonon coupling and increased disorder. The results are discussed in the framework of anisotropic lattice contraction and the position of the σ sub-bands with respect to the Fermi energy level. The relative intensity of the phonon peak at ∼770 cm −1, associated with a peak in the phonon density of states, increases considerably and dominates for x=0.08. Additionally, it hardens and broadens with increasing x in the range 0.04–0.08. These changes can be associated with carbon substitution-induced disorder in the investigated samples.

Raman spectroscopy analysis of magnetron sputtered RuO 2 thin films

Ruthenium dioxide films have been prepared by reactive r.f. magnetron sputtering at different oxygen partial pressures, different total sputtering pressures and different substrate temperatures (RT-500 °C). Raman measurements have been done for all the films. The three major Raman features, namely the E g, A 1g and B 2g modes can be observed in these films. The films prepared at high oxygen partial pressure and low total pressure show weak A 1g peak intensity. The films prepared at low temperature show high B 2g peak intensity. As the temperature is increased, the intensity of the E g Raman peak increases. This indicates a variation of the structure. By fitting the Raman peak, it has been found that the peak position of the RuO 2 films has a red shift compared to that of the single crystal RuO 2. This peak shift may be related with the residual stress in the films. In this work, these phenomena have been discussed.

Raman spectroscopy on ternary transition metal chalcogenide Rb 2Ni 3S 4

Raman scattering measurements were performed for the first time on the ternary alkali nickel sulfide Rb 2Ni 3S 4 single crystal, in which nickel ions form a Kagome lattice. Factor-group analyses were made to determine the zone center vibrational modes for the crystal structure of the Cs 2Pt 3S 4-type with the space group of Fmmm, to which Rb 2Ni 3S 4 belongs, and also for a unit stacked package layer with the point symmetry of 6/ mmm piling up to the present layered sulfides. Raman active A g(2) and A g(1)+B 2g modes were clearly observed as the subspecies modes which correlate to A 1g and E 2g of the stacked package. The Raman shift and line width of the observed modes, A gs and B 2g, exhibited a monotonic temperature dependence in the range between 4 K and 390 K. These thermal properties were discussed in terms of cubic and quartic lattice anharmonicity. Furthermore, observed decreases in their scattering intensities with increasing temperature were discussed heuristically by taking account of the thermal enhancement of extrinsic carriers with activation energy, which was found to be commensurate with the magnitude obtained from a previous resistivity measurement.