Temperature Dependence Of Line Shape Research Articles

Low frequency vibrational anharmonicity and nuclear spin effects of Cl-(H2) and Cl-(D2).

Low frequency combination bands of 35Cl-(H2) and 35Cl-(D2) have been measured in the region between 600 and 1100 cm-1 by infrared predissociation spectroscopy in a cryogenic 22-pole ion trap using a free electron laser at the FELIX Laboratory as a tunable light source. The 35Cl-(H2) (35Cl-(D2)) spectrum contains three bands at 773 cm-1 (620 cm-1), 889 cm-1 (692 cm-1), and 978 cm-1 (750 cm-1) with decreasing intensity toward higher photon energies. Comparison of the experimentally determined transition frequencies with anharmonic vibrational self-consistent field and vibrational configuration interaction calculations suggests the assignment of the combination bands v1 + v2, 2v1 + v2, and 3v1 + v2 for 35Cl-(H2) and 2v1 + v2, 3v1 + v2, and 4v1 + v2 for 35Cl-(D2), where v1 is the 35Cl-⋯H2 stretching fundamental and v2 is the Cl-(H2) bend. The observed asymmetric temperature dependent line shape of the v1 + v2 transition can be modeled by a series of ∑+-∏ ro-vibrational transitions, when substantially decreasing the rotational constant in the vibrationally excited state by 35%. The spectrum of 35Cl-(D2) shows a splitting of 7 cm-1 for the strongest band which can be attributed to the tunneling of the ortho/para states of D2.

Temperature-dependent line parameter study of acetylene transitions near 3 µm

Knowledge of the acetylene spectrum is important for astrophysics, atmospheric, and combustion chemistry research. Twelve acetylene transitions across eleven bands have been studied between 3335.0 and 3336.5 cm−1 with a tunable interband cascade laser. Linestrengths and temperature-dependent lineshape parameters, including collisional broadening and narrowing, are reported through Voigt, Rautian, and Galatry lineshape fits to two strong cold band transitions. Measurements were made in pure acetylene, as well as with diluents argon, nitrogen, and methane, over a temperature range of 296–637 K and pressures of 5–90 Torr. Linestrengths and temperature-dependent self-broadening coefficients are also reported for ten weak hot band transitions through Voigt lineshape fits, measured between 296–513 K and 5–20 Torr. Line parameter measurements are found to be in good agreement with the literature and extend experimental lineshape studies of acetylene into new rovibrational bands, a sparsely-studied temperature regime, and a new perturber: methane.

Stochastic Approach to Phonon-Assisted Optical Absorption.

We develop a first-principles theory of phonon-assisted optical absorption in semiconductors and insulators which incorporates the temperature dependence of the electronic structure. We show that the Hall-Bardeen-Blatt theory of indirect optical absorption and the Allen-Heine theory of temperature-dependent band structures can be derived from the present formalism by retaining only one-phonon processes. We demonstrate this method by calculating the optical absorption coefficient of silicon using an importance sampling MonteCarlo scheme, and we obtain temperature-dependent line shapes and band gaps in good agreement with experiment. The present approach opens the way to predictive calculations of the optical properties of solids at finite temperature.

Ferrimagnetism and magnetic phase separation in Nd1−xYxMnO3 studied by magnetization and high frequency electron paramagnetic resonance

Ferrimagnetism and metamagnetic features tunable by composition are observed in the magnetic response of Nd1−xYxMnO3, for x=0.1–0.5. For all values of x in the series, the compound crystallizes in orthorhombic Pbnm space group similar to NdMnO3. Magnetization studies reveal a phase transition of the Mn-sublattice below TNMn≈80K for all compositions, which, decreases up on diluting the Nd-site with Yttrium. For x=0.35, ferrimagnetism is observed. At 5K, metamagnetic transition is observed for all compositions x<0.4. The evolution of magnetic ground states and appearance of ferrimagnetism in Nd1−xYxMnO3 can be accounted for by invoking the scenario of magnetic phase separation. The high frequency electron paramagnetic resonance measurements on x=0.4 sample, which is close to the critical composition for phase separation, revealed complex temperature dependent lineshapes clearly supporting the assumption of magnetic phase separation.

Diode laser measurements of linestrength and temperature-dependent lineshape parameters of H2O-, CO2-, and N2-perturbed H2O transitions near 2474 and 2482 nm

Absorption lineshapes for two unresolved H2O doublets near 4029.52 and 4041.92cm−1 were measured at high-resolution in a heated static cell using two distributed-feedback diode lasers. Measurements were acquired for H2O, CO2, and N2 perturbers over a temperature and pressure range of 650–1325K and 2–760Torr, respectively. Strong collisional narrowing effects were observed in CO2 and N2, but not in pure H2O. The Galatry profile was used to infer collisional-broadening and -narrowing coefficients and their respective temperature dependence for CO2 and N2 perturbers. The collisional-broadening and -narrowing coefficients for CO2 perturbers were found to decrease with increasing temperature in a similar manner. For N2 perturbers, the collisional-broadening coefficients increased with temperature while the collisional-narrowing coefficients decreased with increasing temperature. Self-broadening coefficients were inferred from Voigt profile fits and are compared with HITEMP 2010. The linestrengths of 17H2O transitions are also reported.

The interplay between the solid effect and the cross effect mechanisms in solid state 13C DNP at 95 GHz using trityl radicals

The 13C solid state Dynamic Nuclear Polarization (DNP) mechanism using trityl radicals (OX63) as polarizers was investigated in the temperature range of 10–60K. The solutions used were 6M 13C urea in DMSO/H2O (50% v/v) with 15mM and 30mM OX63. The measurements were carried out at ∼3.5T, which corresponds to Larmor frequencies of 95GHz and 36MHz for the OX63 and the 13C nuclei, respectively. Measurements of the 13C signal intensity as a function of the microwave (MW) irradiation frequency yielded 13C DNP spectra with temperature dependent lineshapes for both samples. The maximum enhancement for the 30mM sample was reached at 40K, while that of the 15mM sample at 20–30K. Furthermore, the lineshapes observed showed that both the cross effect (CE) and the solid effect (SE) DNP mechanisms are active in this temperature range and that their relative contribution is temperature dependent. Simulations of the spectra with the relative contributions of the CE and SE mechanisms as a fit parameter revealed that for both samples the CE contribution decreases with decreasing temperature while the SE contribution increases. In addition, for the 15mM sample the contributions of the two mechanisms are comparable from 20K to 60K while for the 30mM the CE dominates in this range, as expected from the higher concentration. The steep decrease of the CE contribution towards low temperatures is however unexpected. The temperature dependence of the OX63 longitudinal relaxation, DNP buildup times and 13C spin lattice relaxation times did not reveal any obvious correlation with the DNP temperature dependence. A similar behavior of the CE and SE mechanism was observed for 1H DNP with the nitroxide radical TEMPOL as a polarizer. This suggests that this effect is a general phenomenon involving a temperature dependent competition between the CE and SE mechanisms, the source of which is, however, still unknown.

Quantitative high-speed burned gas temperature measurements in internal combustion engines using sodium and potassium fluorescence

When sodium- and potassium-containing fuel additives are used in internal combustion engines, the bright fluorescence that sodium and potassium atoms emit in the burned gas zone offers a large potential for spectroscopic combustion analysis. To utilize this potential quantitatively, it is crucial to fully understand all physical and chemical processes involved. This includes (1) the temperature dependence of the fluorescence intensity due to gas-phase collisions, (2) the pressure, temperature and equivalence ratio effects on thermodynamic equilibria in the burned gas zone and (3) pressure and temperature-dependent line shapes for quantitative correction of fluorescence reabsorption. High-speed imaging of sodium and potassium fluorescence in a spark-ignited, direct injection, single-cylinder research engine was conducted under well-controlled homogeneous operating conditions at equivalence ratios ranging from 0.71 to 1.43, cylinder pressure from 3 to 15 bar and burned gas temperatures from 1,700 to 2,600 K. This study demonstrates that the influence of pressure, temperature and equivalence ratio on the fluorescence signals of sodium and potassium is understood quantitatively and establishes the potentials and limitations of this tool for burned gas temperature measurements with high temporal and two-dimensional spatial resolution in a homogeneously operated internal combustion engine.

Temperature dependence of electron magnetic resonance spectra of iron oxide nanoparticles mineralized in Listeria innocua protein cages

Electron magnetic resonance (EMR) spectroscopy was used to determine the magnetic properties of maghemite (γ-Fe(2)O(3)) nanoparticles formed within size-constraining Listeria innocua (LDps)-(DNA-binding protein from starved cells) protein cages that have an inner diameter of 5 nm. Variable-temperature X-band EMR spectra exhibited broad asymmetric resonances with a superimposed narrow peak at a gyromagnetic factor of g ≈ 2. The resonance structure, which depends on both superparamagnetic fluctuations and inhomogeneous broadening, changes dramatically as a function of temperature, and the overall linewidth becomes narrower with increasing temperature. Here, we compare two different models to simulate temperature-dependent lineshape trends. The temperature dependence for both models is derived from a Langevin behavior of the linewidth resulting from "anisotropy melting." The first uses either a truncated log-normal distribution of particle sizes or a bi-modal distribution and then a Landau-Liftshitz lineshape to describe the nanoparticle resonances. The essential feature of this model is that small particles have narrow linewidths and account for the g ≈ 2 feature with a constant resonance field, whereas larger particles have broad linewidths and undergo a shift in resonance field. The second model assumes uniform particles with a diameter around 4 nm and a random distribution of uniaxial anisotropy axes. This model uses a more precise calculation of the linewidth due to superparamagnetic fluctuations and a random distribution of anisotropies. Sharp features in the spectrum near g ≈ 2 are qualitatively predicted at high temperatures. Both models can account for many features of the observed spectra, although each has deficiencies. The first model leads to a nonphysical increase in magnetic moment as the temperature is increased if a log normal distribution of particles sizes is used. Introducing a bi-modal distribution of particle sizes resolves the unphysical increase in moment with temperature. The second model predicts low-temperature spectra that differ significantly from the observed spectra. The anisotropy energy density K(1), determined by fitting the temperature-dependent linewidths, was ∼50 kJ/m(3), which is considerably larger than that of bulk maghemite. The work presented here indicates that the magnetic properties of these size-constrained nanoparticles and more generally metal oxide nanoparticles with diameters d < 5 nm are complex and that currently existing models are not sufficient for determining their magnetic resonance signatures.

Transverse susceptibilities of ferromagnetic spin wave interacting with phonon reservoir, II: Temperature dependence of line shapes

A form of the transverse magnetic susceptibility of a ferromagnetic spin system with a uniaxial anisotropy energy and an anisotropic exchange interaction, interacting with a phonon reservoir, is derived in the spin-wave approximation using the TCLE method, where the phonon reservoir interacts with not only the x and y components of each spin but also its z component. The transverse magnetic susceptibility is numerically and analytically studied for the system of one-dimensional infinite spins in the lowest spin-wave approximation, by assuming a damped oscillator model of the phonon reservoir. The temperature dependence and wave number dependence of the susceptibility are numerically investigated for the half-widths and peak-heights of the line shapes in the resonance region. It is shown that as the temperature increases, the half-widths of the line shapes increase and the peak heights decrease in the resonance region, and that as the wave number increases, the half-widths of the line shapes decrease and the peak heights increase in the resonance region. It is also shown that as the uniaxial anisotropy energy of the z direction increases or as the exchange interaction between the z components of spins increases, the half-widths of the line shapes decrease and the peak heights increase in the resonance region. It is besides shown that as the characteristic frequency of the phonon increases, the line shapes show ‘motional broadening’ at the low temperature and show ‘motional narrowing’ at the high temperature. Furthermore, the resonance frequency is shown to increase as the wave number increases or as the temperature increases. The numerical results are examined analytically.

ChemInform Abstract: Variable Temperature 1H MAS NMR: A Powerful Tool for the Investigation of Dynamic Properties of Acidic Protons in Zeolites and Heteropoly Compounds

The dynamic nature of the protons in zeolites such as H-ZSM-5, metallosilicates molecular sieves and heteropoly compounds was examined by temperature dependence of 1 H MAS NMR in the range of 298 and 473K. The temperature-dependent lineshape of 1 H MAS NMR spectra of acidic protons in zeolites showed the first and unambiguous evidence for proton migration in zeolite structure. The dynamic property of protons was a more sensitive measure for the activity of protons than the static parameters such as NMR chemical shifi and I R band position of OH bonds. The temperature-dependent 1 H MAS NMR also revealed the mobility of protons in heteropoly compounds (H 3 PW I2 O 40 Ag 3 PW 12 O 40 and palladium metal loaded on H 3 PW 12 O 40 ). This method gave the information of the dynamical process between protons and hydrogen molecules in the gas phase. The variable temperature 1 H MAS NMR is a very useful technique for understanding dynamic properties of protons in solid acids The dynamic property is an important factor infiuencing the catalytic activity of a solid acid catalyst. The various experimental facts described above show that the temperature variable 1 H MAS NMR could reveal the dynamics of protons in solid acids. Information obtained by this technique offers new insights, which could not be obtained by other means, on the acidic properties of solid acids and their catalysis. The significance of proton mobility in catalysts will be able to be discussed in the future.

Inhomogeneously broadening effect on temperature-dependent lineshape in single quantumdot luminescence

We study, in single quantum dot (QD) luminescence, the temperature-dependentbroadening mechanism due to the coupling between localized excitons and acousticphonons as well as the environment surrounding the QD system. As a treatment of theinteraction between the QD system and the environment, a configuration of excitoniceigenenergies is introduced. Using the Green’s function method, analytical formulae of thelineshape of zero-phonon lines (ZPL) are given. The numerical results show that phononsinducing initial fast decay contributes to the background wings of the spectra leading to awings-asymmetry photoluminescence (PL) spectra. Our model gives a semiquantitativedescription of the underlying broadening mechanism and is helpful in understanding theorigin of the experimental results for the temperature dependence of the ZPL inself-assembled QDs.

Energy-Transfer Dynamics in the LHCII Complex of Higher Plants: Modified Redfield Approach

We have modeled energy-transfer dynamics in the peripheral plant light-harvesting complex LHCII using both standard Redfield theory and its modification for the case of strong exciton−phonon coupling (Zhang, W. M.; Meier, T.; Chernyak, V.; Mukamel, S. J. Chem. Phys. 1998, 108, 7763). A quantitative simultaneous fit of the absorption (OD), linear dichroism (LD), steady-state fluorescence (FL) spectra at 7−293 K, and transient absorption (TA) kinetics at 77 and 293 K has been obtained using the experimental exciton−phonon spectral density to model the temperature-dependent line shape. We use configurations of the antenna (i.e., chlorophyll (Chl) a/b identities, orientations, and site energies) close to those proposed in our previous study (Novoderezhkin, V.; Salverda, J. M.; van Amerongen, H.; van Grondelle, R. J. Phys. Chem. B 2003, 107, 1893). These configurations have been further adjusted from the fit with the modified Redfield approach. The new (adjusted) models allow a better quantitative explanation of the spectral shapes. A combination of fast (femtosecond) interband energy transfer and slow (picosecond) intraband equilibration can be better reproduced as well. We paid special attention to unravel the origins of the slow components preliminarily assigned to localized states in the previous work. These “bottleneck” states have been directly visualized in this study via selective femtosecond excitation and probing at different wavelengths. In our modeling, these states are determined by two or three (depending on the model) monomeric Chls a or b shifted to the spectral region of 655−670 nm between the main absorption peaks of Chl b (650 nm) and Chl a (675 nm). In all configurations we have found these energy-shifted Chls to be bound at mixed sites (i.e., A3, A6, A7, or B3). Experiments and self-consistent modeling using the modified Redfield theory allow us to explore the participation of these states in the overall energy-transfer dynamics. This has led to a more complete and physically adequate model for the energy-transfer dynamics in LHCII.

A 57Fe Mössbauer study of charge ordering and phase separation in the rare-earth manganates, Nd 0.5Ca 0.5MnO 3 and Nd 0.5Sr 0.5MnO 3

Mössbauer studies of 2% 57Fe-doped Nd 0.5Ca 0.5MnO 3 and Nd 0.5Sr 0.5MnO 3 have been carried out over the 4.2–300 K range after ensuring that such doping does not change their basic properties. The charge-ordering transition in these manganates is marked by abrupt changes in the quadrupole splitting. In the case of Nd 0.5Ca 0.5MnO 3, two phases manifest themselves on cooling below the charge-ordering transition temperature. The evolution of the spectra as a function of temperature shows that long-range magnetic order does not occur sharply. The observed evolution with temperature is different in the two materials studied. In Nd 0.5Ca 0.5Mn 0.98 57Fe 0.02O 3, it resembles that of a disordered magnetic material, whereas the temperature dependence of line shape of Nd 0.5Sr 0.5Mn 0.98 57Fe 0.02O 3 is typical of a superparamagnetically relaxed magnetic system. Although both the manganates show well-resolved magnetic hyperfine spectra at 4.2 K, the lines are slightly broad indicating possible coexistence of phases at low temperatures. A weak paramagnetic signal is also seen in the spectra of both the manganates at 4.2 K.

Vibrational dynamics of bond-center hydrogen in crystalline silicon

The absorption line shape associated with the fundamental transition of the H-related stretch mode of bond-center hydrogen in crystalline silicon is measured as a function of temperature with infrared spectroscopy. In addition to the shift in frequency and increase in linewidth usually observed for local vibrational modes in solids, the absorption line becomes asymmetric at elevated temperatures. A theoretical model is developed, which describes the temperature-dependent line shape in terms of thermal fluctuations in the occupation number of a low-frequency mode coupled anharmonically to the stretch mode. The model successfully describes the shift, broadening, and asymmetry of the absorption line and gives new insight into the nature of the low-frequency mode. The mode has a frequency of 114cm{sup -1}, is twofold degenerate, and exhibits no isotopic shift when deuterium is substituted for hydrogen. It is assigned to a pseudolocalized, Si-related mode of bond-center hydrogen.

The methyl group as a built-in probe of the glassy dynamics in propylene carbonate

Deuteron nuclear magnetic resonance was employed to study primarily the glassy phase of the fragile glass-former propylene carbonate, selectively deuterated at the methyl group. The analysis of temperature dependent line shapes allowed us to map out the energy barrier distribution below the glass transition temperature. The results were compared to measurements of spin-relaxation times. We determined the relative width of the barrier distribution and the extent of the decoupling between the methyl group rotation time and the structural relaxation time near the glass transition. These findings are discussed in the context of what has been reported for secondary relaxation processes in other glass-formers. Stimulated echo experiments on propylene carbonate demonstrate that, while in the supercooled liquid small angle reorientations dominate, in the glass the methyl group performs 120° (or 60°) jumps.

Temperature dependence of solid state 1H NMR line shapes and M2 in polycrystalline BF4− salts of 1-propyltetrazole complexes of iron(II) and zinc(II)

1H NMR spectra of the spin-crossover compound [Fe(ptz)6](BF4)2 and its zinc(II) analogue were recorded on polycrystalline samples as the function of decreasing temperature in order to study the connection of the change in the electronic state and the molecular motions present in both crystals. The temperature range for the zinc compound was 295–30K, with a cooling rate of ∼1Kmin−1, while in the case of the iron(II) compound this temperature range was 298–10K with the same cooling rate as in the case of [Zn(ptz)6](BF4)2. The analysis of the temperature dependence of the line shape parameters and calculations of 1H second moments showed the following. The temperature of the spin transition in the iron(II) compound is 132K on cooling. On the basis of the similarity of the temperatures at which the different molecular motions appear, the same motions are present in both compounds in the whole temperature range. The [Zn(ptz)6](BF4)2 probably goes through a structural transition between 90 and 60K, this transition manifests in the virtual increase of the average proton–proton distance, and the sign of a similar transition is also observed in [Fe(ptz)6](BF4)2.

Structure and dynamics of [(CH3)3N–CH2]+• radical generated in γ-irradiated Al-offretite

An EPR study was carried out to investigate the structure and motional dynamics of the [(CH3)3N–CH2]+• radical formed in a γ-irradiated Al-offretite. Strongly temperature-dependent EPR spectra were observed in the temperature range between 4 and 300 K. The observed 1H hyperfine (hf) splitting was a1=0.46 mT for three magnetically equivalent hydrogens, one of each methyl group at 110 K and a1′=0.15 mT for three of each methyl group at 300 K; the hf splittings due to two equivalent CH2 hydrogens and the central 14N were a2=2.24 and a3=0.35 mT, respectively, independent of the temperature. This result suggests that rotation of the methyl group around the N–CH3 bond is responsible for the temperature dependent line shapes. Assuming a three-site jump model in which three methyl protons interchange their positions with each other, the EPR line shapes were successfully analyzed by a simulation method using the jump rate as a variable parameter in the temperature range from 140 to 270 K. The rate was found to increase from 7.0×106 s−1 (140 K) to 4.1×108 s−1 (270 K) with temperature. From an Arrhenius plot of the rate constants, an activation energy of 8.1 kJ mol−1 was evaluated for the methyl group rotation of [(CH3)3N–CH2]+• in Al-offretite.

Triple proton transfer in crystalline 3,5-dibromo-1H-1,2,4-triazole and 3,5-dichloro-1H-1,2,4-triazole studied by variable-temperature15N NMR andab initio calculations

Results are reported of variable-temperature 15N cross-polarization magic angle spinning (CP/MAS) NMR experiments performed on [15N2]-labelled polycrystalline 3,5-dibromo-1H-1,2,4-triazole (2b) and 3,5-dichloro-1H-1,2,4-triazole (3b), the synthesis of which is also described. According to the crystal structures these compounds form cyclic trimers in the solid state. The molecular and hydrogen bond structures were compared with those derived from ab initio calculations. The 15N CP/MAS NMR spectra show temperature-dependent lineshapes which were analysed in terms of near-degenerate triple proton transfer processes. The equilibrium constants are slightly different from unity as observed by high-temperature line splittings. The populations of the two quasi-degenerate tautomers were calculated from the internal angles at N1 and N2 and compared with those obtained from solid-state NMR; both methods agree fairly well. By lineshape analysis rate constants of the triple proton transfer processes were obtained at different temperatures. The proton transfer kinetics of 2b and 3b were compared with those of 3,5-dimethylpyrazole, 4-nitropyrazole and 4-bromopyrazole, studied previously, which also form cyclic trimers in the solid state, exhibiting more or less concerted, degenerate triple proton transfer processes proceeding by tunnelling at low temperatures. It is shown that the triazoles behave in a similar way to the pyrazoles. The barrier heights of the triple proton transfer increase monotonically with the distance between the two nitrogen atoms involved in the proton transfer. It is shown that the intrinsic nitrogen chemical shifts are related to the N...H distances. Copyright © 2000 John Wiley & Sons, Ltd.

Temperature dependence of the line shapes of two-photon excited transitions in KMgF3:Eu2+

The two-photon excitation line shapes of f-f transitions of Eu2+ in KMgF3 have been investigated as a function of temperature. Results for several transition lines of the 6P J and 6D J multiplets are presented in this work. The two-photon excited transitions appear as zero-phonon lines with a Lorentzian profile whose peak energy is shifted from the zero temperature position. The observed temperature dependence of the zero-phonon line shapes is ascribed to the elastic scattering of the electronic transitions among vibrational states via anharmonic coupling to a phonon mode of the lattice heat-bath. In those components of 4f7 multiplets whose energies are not resonant with the 4f65d broad excited state, a direct proportionality to the phonon occupation number of the coupled mode is found for the line-shapes and peak-positions temperature dependences.

Substituted cyclohexanes in liquid crystalline solution: the molecular behaviour of bromocyclohexane

Selectively and perdeuteriated samples of bromocyclohexane, dissolved in a liquid crystalline solution, are studied by dynamic 2H NMR spectroscopy. The analysis of variable temperature 2H NMR spectra between 238 and 333 K, as well as 2D exchange NMR experiments provide the assignment of the various NMR lines in terms of the equatorial and axial bromocyclohexane conformers that exist in a dynamic equilibrium in relative amounts of about pe=0.7 and pa= 0.3, respectively. By taking into account the molecular geometries of the two conformers (as obtained from molecular mechanics calculations) it is possible to derive the molecular order parameters of both species as a function of temperature. The observed temperature dependent lineshape changes can be attributed to the ring inversion process of the solutes, i.e. a mutual exchange of both conformers. The lineshape analysis yields an activation enthalpy for this internal process of DeltaH=44.5 +/- 4.5 kJ mol-1, in close agreement with earlier findings for isotropic solution.