Line Shapes Of Transitions Research Articles

Study of the chemical bonding of titanium with silicon and oxygen by AES and SEELS

Auger electron Spectroscopy (AES) and slow electron energy loss Spectroscopy (SEELS) have been employed to study the electronic structure of Ti, TiSi 2 and TiO 2. The changes in the Auger and loss spectra when Ti chemically binds with silicon to form TiSi 2 and with oxygen to form TiO 2 have been understood as manifestations of changes in electronic participation. AES spectra show distinct changes in line shapes of transitions involving the Ti valence electrons. The SEELS spectra provide information regarding shallow core levels, valence band and the collective excitation energies of the volume and surface plasmons. By monitoring the changes in the Auger peak at 387 eV and the 3 p→ 3 d quasiatomic transition (at about 45 eV), the role of d-orbital occupancies are studied in Ti and its compounds. The SEELS studies in the 0-80 eV range have enabled the authors to observe the behaviour of the 3 p → 3 d quasiatomic transition in Ti, which persists after oxidation but almost disappears during TiSi 2 formation. The values of the plasmon losses are related to the collective behaviour of conduction electrons.

Field dependence of the nuclear magnetic resonance static powder line shape: Studies of the mutual orientation of interaction tensors from the field dependence of the critical frequencies

A procedure for determining the interaction tensor orientations and the interaction parameters for mutually oriented electric field gradient (efg) or dipolar and shielding tensors has been developed based upon the magnetic field dependence of the critical frequencies from the polycrystalline nuclear magnetic response (NMR) spectrum. The central transition line shape of half-integer quadrupolar nuclei in the presence of mutually oriented shift anisotropy and second order quadrupolar interaction is discussed for the first time. Analytical expressions for the field dependent critical frequencies have been determined for special orientations when the shift principal Z axis lies on the XZ, YZ, or XY plane of the efg (or dipolar) tensor for the first order as well as the second order interactions. The plots resulting from the analytical expressions provide a convenient graphical approach in determining approximate tensorial orientations and interaction parameters through pattern recognition. For general orientations, a numerical procedure has been developed to determine these parameters by iteratively minimizing the squares of the differences of the calculated and the experimental critical frequencies. Higher order perturbation terms can be incorporated in the present treatment. The method is demonstrated by variable field static proton spectra of trichloro–acetic acid at three different fields (1.3, 2.3, 5.2 T). The near-orthogonal orientation between the dipolar and shielding tensors and the interaction parameters obtained from this approach are consistent with those obtained previously from single crystal studies.

M4,5N4,5X Auger line shapes of La compounds from one-step model calculations.

The x-ray-excited M/sub 4,5/N/sub 4,5/X (X = O/sub 1/, O/sub 2,3/, and N/sub 6,7/) Auger spectra of La metal, La/sub 2/O/sub 3/, and LaF/sub 3/ are calculated by use of the one-step model for which the decoupling of Auger decay and photoemission processes are not assumed. It is shown that the line shape of the individual Auger transition for La metal is quite different from those of LaF/sub 3/ and La/sub 2/O/sub 3/ because of the interference effect between photoemission and the Auger decay arising from the N/sub 67/VV extra relaxation mechanism, although this difference is mostly smeared out in the experimental Auger spectra due to multiplets and other broadening effects. The well screened and poorly screened Auger final-state peaks are separated by the energy difference ..delta..U = U/sub f//sub ..cap alpha../+U/sub f//sub ..beta../-U/sub fc/, where U/sub f//sub c/, U/sub f//sub ..cap alpha../, and U/sub f//sub ..beta../ are Coulomb attraction energies between a 4f electron and core holes c, ..cap alpha.., and ..beta.. involved in the c..cap alpha beta.. Auger process. From the comparison of the calculated total Auger line shape with the experimental one, we can estimate ..delta..U 5 eV for La compounds, and this makes the two-step modelmore » a good approximation for calculating relative intensities of various Auger peaks.« less

Dopper shift attenuation lifetime measurement in 54Cr following thermal neutron capture

The double crystal spectrometer GAMS4 in combination with the ILL high flux reactor has been used to determine the lifetimes of the 2620 KeV 2 + 2, 3074 KeV 2 + 3 and 3720 KeV (1, 2) + states in 54Cr. The initial recoil energy of about 0.5 KeV imparted by the primary γ-radiation after thermal neutron capture in 53Cr produces Doppler broadened line shapes of the secondary transitions. The large 2 + 3→2 + 1 M1 strength of B(M1)=0.39(6) μ 2 N suggests the 2 + 3 state to be mixed symmetry character within the interacting boson model IBM-2.

Photomodulated absorption spectroscopy on AlGaAs-GaAs heterostructures

Optical transitions in quantum-well heterostructures are very well revealed by photomodulated absorption spectroscopy. With respect to nonmodulated absorption spectroscopy, a strong increase in both room-temperature resolution and signal-to-noise ratio is observed. This new technique is most attractive for the investigation of multilayers grown on transparent substrates. The evolution of the spectral lineshapes of bulk and excitonic transitions as a function of temperature is shown.

Investigation of collisional lineshapes of neon transitions in noble gases' mixtures

We have investigated sub-Doppler lineshapes of neon transitions by means of intermodulated optogalvanic technique in a direct current hollow cathode and a radio frequency glow discharge. The intermodulated Doppler-free signal, as usual, is superimposed on a broad Gaussian background due to velocity-changing collisions. The ratio between the collisional and the homogeneous signal has been systematically investigated as a function of gas pressure and discharge current. By detecting neon as a trace in other buffer gases, the lineshapes have been also carefully investigated as a function of the mass of the perturbers (helium and argon). Comparison between theoretical predictions and experimental data leads to quantitative evaluations of collisional parameters.

Analysis of d-core excitons and interband transitions in synchrotron-radiation reflectance spectra of Cd1-xMnxTe and Cd1-xZnxTe within the energy range from 11 to 20 eV.

Both Te 5s\char21{}derived interband transitions and the excitations of Cd 4d and Zn 3d upper core states were studied in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te and ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Zn}}_{\mathrm{x}}$Te, respectively. Reflection experiments using synchrotron radiation were performed at T=115 K and at room temperature. In ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te we found new assignments of some spectral lines to interband transitions and core excitons, respectively. In ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Zn}}_{\mathrm{x}}$Te we have observed a distinct asymmetry in the line shapes of transitions originating from the zinc ions. Based on the model of Fano resonances, these lines are interpreted as resonances of the Zn 3d core excitons which decay into the quasicontinuum of the conduction band near the \ensuremath{\Gamma} point.

Optical absorption line shape at a CDW-gap: Application to BaBiO3

BaBiO 3 is a superconductor if doped with Pb at the Bi sites (BaPb 1− x ,Bi x O 3 , x 3 was doped with K at the Ba sites (Ba 0.9 K x Bi03, x ≈ 0.2). BaBiO 3 , the reference compound for both of these superconductors is semi conducting because of a CDW distortion that produces two non-equivalent Bi sites. I study the absorption line shape of transitions across a model CDW-gap theoretically and compare it with recent optical experiments.

One-dimensional vibrational excitons in 1,2,4,5-tetrachlorobenzene

We have studied the line shapes of vibronic transitions in the phosphorescence spectrum of the one-dimensional triplet exciton of TCB between 4.2 and 0.4 K. It is shown that also the vibrational excitons are highly one dimensional. This finding is confirmed by time-resolved emission spectra. Observed vibron bandwidths vary up to 15 cm−1 with k=0 either at the top or at the bottom of the band. In contrast to the fundamental vibrations, no overtones or combination bands give evidence of a well-defined one-dimensional quasimomentum.

Stark broadening of C IV and N V lines in the vacuum-uv spectral range.

Spectral line shapes of 3p-4d, 3d-4f, 4d-5f, and 4f-5g transitions in the lithiumlike ions C iv and N v have been measured in a gas-liner pinch discharge at a density of 7.7\ifmmode\times\else\texttimes\fi{}${10}^{23}$ ${\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$. The full widths at half maximum are larger than theoretical values obtained by the impact approximation by factors between 3 and 4 and smaller than theoretical full widths at half maximum calculated in the quasistatic, linear Stark effect approximation by factors between 1.2 and 3.

Resonance Raman depolarization ratios as a probe of excited state dephasing times

The effect of the finite time for resonance Raman scattering on Raman depolarization ratios is evaluated. Partial depolarization of the resonance Raman emission will occur if the scatterers can rotate during the time interval between the absorption and emission steps. Equations are derived which express the depolarization ratio in terms of Fourier transforms of the usual time-dependent vibrational overlap multiplied by a rotational correlation function. Particularly simple results are obtained in the limit of severe inhomogeneous broadening of the electronic transition. The effect of the finite resonance scattering time on the line shape of the resonance Raman transition is also discussed.

On the spectral profiles of optical transitions to the autoionizing 7d′ and 9s′ levels and configuration interaction in 7d′ levels of Kr

Spectral line shapes of the optical transitions from the 5p′ to the autoionizing 7d′ or 9s′ levels of Kr are found by optogalvanic spectroscopy to be symmetric, in contrast to the asymmetric profiles of the ordinary autoionization lines and the widths of some of the observed lines which are varied by changing the pumped level in an optogalvanic double-resonance spectroscopy. We conclude from the analysis of this behavior that the direct photoionization which introduces the asymmetric profile by interference with the autoionizing transition is negligibly weak and that the autoionizing levels which are close to each other are mixed by a second-order configuration interaction through the continuous levels. Based on this analysis, the accurate term values of the autoionizing 9s′ and 7d′ levels are determined.

Analysis of the J = 1 odd-parity spectrum of germanium by the use of multichannel quantum-defect theory

Eigen quantum defects, a transformation matrix, and ratios of eigenchannel dipole moments are obtained from the analysis of J = 1° discrete energy levels of Ge with ν3/2 > 6.9 and an intensity rule in pure LS coupling. These parameters are used to calculate the quantum defects, width parameters, and asymmetry parameters that describe photoabsorption line shapes for transitions from 4p2 3P0 to the three J = 1° autoionizing Rydberg series above the first ionization limit. These line-shape parameters are shown to correlate with the parameters previously obtained by fitting the autoionization spectrum.

Asymmetric line shapes for weak transitions in strong standing-wave fields.

We have observed the resonance line shape for a very weak atomic transition excited when an atomic beam intersects a strong standing-wave laser field. The line shape has a dramatic intensity-dependent distortion which is Doppler free and independent of the excitation rate. We have calculated the line shape predicted by optical Bloch equations that include a spatially varying ac Stark shift, and find good agreement with our experimental results.

Lf and (simultaneous) rf excitation of Cs atoms in an atomic beam

The experimental frequency standard CSX of the PTB has been used to perform experiments in which a Cs atomic beam is subject to the usual rf excitation coupling the hyperfine levels of the ground-state, and to a low frequency (lf) excitation which couples the Zeeman sublevels. As the lf excitation is done within the drift region between the two rf interaction zones of the Ramsey-type microwave cavity, this is equivalent to a double resonance experiment performed on the Cs atoms. Experimental and theoretical results concerning line shape and centre frequency of the rf transitions are given. The If resonance and its use to determine the magnetic field correction of the clock transition frequency in a Cs atomic clock is also discussed.

N 2 and air broadening in the fundamental bands of HF and HCl

N 2- and air-broadened lineshapes of HF and HCl transitions in the v = 1 ← 0 bands have been measured at T = 295 and 202 K with a high-resolution difference-frequency laser spectrometer. Pressure broadening, shift and collisional narrowing parameters have been extracted by leastsquares fitting of several collisional profiles to the spectra. At low pressures, the collisional, or Dicke, narrowing effect causes deviations from the Voigt profile having a Doppler-fixed Gaussian component and yields a measure of the diffusion constants for the hydrogen halides in the buffer gases. At high J, where the pressure shifts are comparable to or larger than the broadenings, a slight asymmetry is observed in the lineshapes which is attributed to statistical correlation between velocity- and state-changing collisions. The ratio of air-to-N 2 broadening is roughly given by the relative quadrupolar contributions of O 2 and N 2, though it varies systematically with rotational level. The temperature variations in the broadening coefficients are also J dependent, deviating significantly from T − 1 2 and exhibiting opposite behavior for HF and HCl.

Temperature dependence of photoreflectance line shapes in [formula omitted] multiple quantum wells

The temperature dependence of the photoreflectance (PR) line shape of the excitonic transitions in GaAs AlGaAs multiple quantum wells has been studied. At low temperatures, near 6K, we find that the PR lines can be described by a dielectric function of a Lorentzian oscillator. At higher temperatures, we have observed the evolution of the dielectric function from one with a Lorentzian absorption profile to one with a Gaussian absorption profile. The dielectric function of excitonic absorptions is expected to exhibit this behavior in cases of weak exciton-phonon coupling.

Multipulse dynamic nuclear magnetic resonance of phospholipid membranes

Multipulse dynamic NMR has been employed to study molecular order and dynamics of deuteron (2H) labeled phospholipid membranes. Variation of pulse sequence and pulse separation provides the large number of independent experiments necessary for a proper molecular characterization of the systems. Analysis of these experiments is achieved by employing a density matrix formalism, based on the stochastic Liouville equation. Arbitrary relaxation rates and line shapes of single and multiple quantum transitions are considered. The various 2H NMR experiments of macroscopically unoriented bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), specifically deuterated at the 6- and 14-position of the 2-chain, are faithfully reproduced by the model. Computer simulations provide the orientational distributions and conformations of the hydrocarbon chains and the correlation times of the various motions. In the Lα phase the correlation times τR∥ and τR⊥ for chain rotation and chain fluctuation are of the order of 10−8 s, while trans–gauche isomerization occurs significantly faster (τJ∼10−10 s). At the main transition all chain motions slow down abruptly. Further cooling in the Pβ′ phase first continuously decreases the motions. However, 10 K below the pretransition (hysteresis), there is another abrupt slow down of the chain dynamics. In the Lβ′ phase at T=265 K all three motions occur with correlation times of 10−6 to 10−5 s. Because of higher activation energies, however, intermolecular chain motions freeze out first on the time scale of a particular NMR experiment. Thus, at temperatures T<210 K, trans–gauche isomerization becomes the dominant process. Detection of this motion is possible even at T=168 K, where τJ is of the order of 10−4 s. Arrhenius plots of the various correlation times provide the motional activation energies. Values of 9<EJ<14 kJ/mol for trans–gauche isomerization correspond to the local character of this process. As expected, the activation energies for chain rotation (50<ER∥ <69 kJ/mol) and chain fluctuation (53<ER⊥ <79 kJ/mol) are substantially higher. The correlation times for methyl group rotation form a continuous straight line on the Arrhenius plot throughout the three phases studied, yielding an activation energy of EJ(CD3) =9.9 kJ/mol. Molecular order of the chains is discussed in terms of two parameters SZZ and SZ′Z′, characterizing the orientational order of the chains as a whole and the conformational order at a particular segment. In the Lα phase the hydrocarbon chains are partially disordered (0.44<SZZ <0.6) and melted, exhibiting segmental order parameters of SZ′Z′ (C-6)∼0.75 and SZ′Z′ (C-13)∼0.35, respectively. As expected, conformational order decreases from the central unit to the terminal one (order gradient). The Pβ′ phase exhibits two different chain order parameters of SZZ ∼0.6 and SZZ ∼0.9, indicating heterogeneous chain packing. A unique structural interpretation of this result is not yet possible since the microscopic heterogeneity is compatible with most proposed models. In the Lβ′ phase we find SZZ >0.95, SZ′Z′ (C-6)>0.95, and SZ′Z′ (C-13)>0.9, consistent with highly ordered, fully extended hydrocarbon chains.

Intersubband cyclotron combined resonances in 2D systems with anisotropic and tilted valleys

Lineshape of intersubband transitions is calculated in a surface space-charge layer on semiconductor whose constant-energy surfaces are ellipsoids arbitrarily oriented with respect to the surface. The case when magnetic field is normal to the surface is considered. Combined intersubband cyclotron transitions are allowed when ellipsoids are tilted with respect to the surface. The depolarization effect affects the combined transition differently from the main transition.

Photoconductivity of confined donors in Al0.3Ga0.7As/GaAs quantum wells

Capacitive coupling is used as a contactless method to study photoconductivity of confined donors in lightly doped AlGaAs/GaAs multi-quantum wells. The high sensitivity allows a clear observation of the 1s→2p+ intra-impurity transition line shape that can be compared with recent calculations. Evidence of a higher frequency line is shown, which is attributed to the 1s→3p+ transition.