Experimental Linewidth Research Articles

Microscopic potential fluctuations in Si-doped AlGaN epitaxial layers with various AlN molar fractions and Si concentrations

Nanoscopic potential fluctuations of Si-doped AlGaN epitaxial layers with the AlN molar fraction varying from 0.42 to 0.95 and Si-doped Al0.61Ga0.39N epitaxial layers with Si concentrations of 3.0–37 × 1017 cm−3 were investigated by cathodoluminescence (CL) imaging combined with scanning electron microscopy. The spot CL linewidths of AlGaN epitaxial layers broadened as the AlN molar fraction was increased to 0.7, and then narrowed at higher AlN molar fractions. The experimental linewidths were compared with the theoretical prediction from the alloy broadening model. The trends displayed by our spot CL linewidths were consistent with calculated results at AlN molar fractions of less than about 0.60, but the spot CL linewidths were markedly broader than the calculated linewidths at higher AlN molar fractions. The dependence of the difference between the spot CL linewidth and calculated line broadening on AlN molar fraction was found to be similar to the dependence of reported S values, indicating that the vacancy clusters acted as the origin of additional line broadening at high AlN molar fractions. The spot CL linewidths of Al0.61Ga0.39N epitaxial layers with the same Al concentration and different Si concentrations were nearly constant in the entire Si concentration range tested. From the comparison of reported S values, the increase of VAl did not contribute to the linewidth broadening, unlike the case of the VAl clusters.

Use of a Continuous Wave Laser and Pockels Cell for Sensitive High-Resolution Collinear Resonance Ionization Spectroscopy.

New technical developments have led to a 2 orders of magnitude improvement of the resolution of the collinear resonance ionization spectroscopy (CRIS) experiment at ISOLDE, CERN, without sacrificing the high efficiency of the CRIS technique. Experimental linewidths of 20(1) MHz were obtained on radioactive beams of francium, allowing us for the first time to determine the electric quadrupole moment of the short lived [t_{1/2}=22.0(5) ms] ^{219}Fr Q_{s}=-1.21(2) eb, which would not have been possible without the advantages offered by the new method. This method relies on a continuous-wave laser and an external Pockels cell to produce narrow-band light pulses, required to reach the high resolution in two-step resonance ionization. Exotic nuclei produced at rates of a few hundred ions/s can now be studied with high resolution, allowing detailed studies of the anchor points for nuclear theories.

Double-resonance-ionization mapping of the hyperfine structure of the stable Cu isotopes using pulsed narrowband Ti:sapphire lasers

We present two approaches to enhance the resolving power for measuring hyperfine structure constants using resonance ionization spectroscopy. The first method employs a 2D-resonance ionization spectroscopy scanning technique with pulsed, narrowband Ti:sapphire lasers (1 GHz linewidth), allowing us to resolve hyperfine components that cannot be separated using the standard 1D-scanning method across only one optical transition. In a second refinement, the resolving power is further enhanced through the use of a ring design of the laser cavity. This layout leads to a reduction of the laser linewidth from 1 GHz to below 50 MHz, resulting in experimental linewidths of about 150 MHz. Motivated by the current nuclear physics interest in radioactive Cu isotopes, the hyperfine structure of the $3{d}^{10}4s{\phantom{\rule{4pt}{0ex}}}^{2}{S}_{1/2}$ ground state and the $3{d}^{9}4s4p{\phantom{\rule{4pt}{0ex}}}^{4}{P}_{3/2}^{o}$ and $3{d}^{10}6s{\phantom{\rule{4pt}{0ex}}}^{2}{S}_{1/2}$ excited states in stable Cu isotopes were studied using both approaches. The hyperfine $A$ parameter of the $3{d}^{10}6s{\phantom{\rule{4pt}{0ex}}}^{2}{S}_{1/2}$ state was measured with values of $296(18)$ MHz for $^{63}\mathrm{Cu}$ and $307(18)$ MHz for $^{65}\mathrm{Cu}$.

Sub-Doppler infrared spectroscopy of propargyl radical (H2CCCH) in a slit supersonic expansion.

The acetylenic CH stretch mode (ν1) of propargyl (H2CCCH) radical has been studied at sub-Doppler resolution (∼60 MHz) via infrared laser absorption spectroscopy in a supersonic slit-jet discharge expansion, where low rotational temperatures (Trot = 13.5(4) K) and lack of spectral congestion permit improved determination of band origin and rotational constants for the excited state. For the lowest J states primarily populated in the slit jet cooled expansion, fine structure due to the unpaired electron spin is resolved completely, which permits accurate analysis of electron spin-rotation interactions in the vibrationally excited states (εaa = - 518.1(1.8), εbb = - 13.0(3), εcc = - 1.8(3) MHz). In addition, hyperfine broadening in substantial excess of the sub-Doppler experimental linewidths is observed due to nuclear spin-electron spin contributions at the methylenic (-CH2) and acetylenic (-CH) positions, which permits detailed modeling of the fine/hyperfine structure line contours. The results are consistent with a delocalized radical spin density extending over both methylenic and acetylenic C atoms, in excellent agreement with simple resonance structures as well as ab initio theoretical calculations.

N2-broadening coefficients of CH3CN rovibrational lines and their temperature dependence for the Earth and Titan atmospheres

Theoretical nitrogen-broadening coefficients and their temperature exponents for methyl cyanide lines in parallel (ΔK=0) bands are calculated by a semi-empirical approach suitable for CH3X-type absorbers with large dipole moments. Together with the standard four-parameter correction factor determined from fitting on experimental line widths for each fixed K and requiring an extrapolation for higher K inaccessible experimentally, a new form accounting explicitly for the K-dependence and reducing the number of required parameters is proposed and successfully tested. Extensive (0⩽J⩽70, 0⩽K⩽20) R- and P-line lists are provided for the Earth and Titan atmosphere temperature ranges, which could be useful for atmospheric/astrophysical applications and spectroscopic databases.

Molecular beam study of thea3Σ+state of NaK up to the dissociation limit

We provide spectroscopic data for the $a{\phantom{\rule{0.16em}{0ex}}}^{3}{\ensuremath{\Sigma}}^{+}$ state of the $^{23}\mathrm{Na}^{39}\mathrm{K}$ molecule. The experiment is done in an ultrasonic beam apparatus, starting from the ground state $X{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+}$ and driving the population to the $a{\phantom{\rule{0.16em}{0ex}}}^{3}{\ensuremath{\Sigma}}^{+}$ state, using a $\ensuremath{\Lambda}$ scheme with fixed pump and scanning dump laser. The signals are observed as dips of the total fluorescence. The intermediate level is chosen to be strongly perturbed by the $B{\phantom{\rule{0.16em}{0ex}}}^{1}\ensuremath{\Pi}/c{\phantom{\rule{0.16em}{0ex}}}^{3}{\ensuremath{\Sigma}}^{+}$ states mixing to overcome the singlet-triplet transfer prohibition. We observed highly resolved hyperfine spectra of various rovibrational levels of the $a{\phantom{\rule{0.16em}{0ex}}}^{3}{\ensuremath{\Sigma}}^{+}$ state from ${v}_{a}=2$ up to the highest vibrational levels for rotational quantum numbers ${N}_{a}=4,6,8$. By the typical experimental linewidth of 17 MHz, the vibrational dependence of the hyperfine splitting is clearly revealed for NaK. The absolute frequency measurements of the vibrational levels are used for improvement of the $a{\phantom{\rule{0.16em}{0ex}}}^{3}{\ensuremath{\Sigma}}^{+}$ potential curve and of the derived scattering length of all natural isotope combinations. Applying the $\ensuremath{\Lambda}$ scheme in the reverse direction can provide a pathway for efficient transfer of ultracold $^{23}\mathrm{Na}^{39}\mathrm{K}$ molecules from the $\text{Na}(3s)+\text{K}(4s)$ asymptote to the lowest levels of the ground state. We show spectra that couple the absolute ground state ${v}_{X}=0,J=0$ with an appropriate intermediate state for direct realization of the reverse path. The refined theoretical model of the coupled excited states of the $\text{Na}(3s)+\text{K}(4p)$ asymptote allows predictions of efficient paths for $^{23}\mathrm{Na}^{40}\mathrm{K}$; one example is calculated.

Empirical Monod-Beuneu relation of spin relaxation revisited for elemental metals

Monod and Beuneu [Monod and Beuneu, Phys. Rev. B 19, 911 (1979)] established the validity of the Elliott-Yafet theory for elemental metals through correlating the experimental electron spin resonance line-width with the so-called spin-orbit admixture coefficients and the momentum-relaxation theory. The spin-orbit admixture coefficients data were based on atomic spin-orbit splitting. We highlight two shortcomings of the previous description: i) the momentum-relaxation involves the Debye temperature and the electron-phonon coupling whose variation among the elemental metals was neglected, ii) the Elliott-Yafet theory involves matrix elements of the spin-orbit coupling (SOC), which are however not identical to the SOC induced energy splitting of the atomic levels, even though the two have similar magnitudes. We obtain the empirical spin-orbit admixture parameters for the alkali metals by considering the proper description of the momentum relaxation theory. In addition, we present a model calculation which highlights the difference between the SOC matrix element and energy splitting.

The cyclic ground state structure of the HF trimer revealed by far infrared jet-cooled Fourier transform spectroscopy

The rovibrationally resolved Fourier transform (FT) far infrared (FIR) spectra of two intermolecular librations of (HF)3, namely the in-plane ν6 and out-of-plane ν4 bending fundamentals centered, respectively, at about 494 cm(-1) and 602 cm(-1), have been recorded for the first time under jet-cooled conditions using the supersonic jet of the Jet-AILES apparatus. The simultaneous rotational analysis of 245 infrared transitions belonging to both bands enabled us to determine the ground state (GS), ν6 and ν4 rotational and centrifugal distortion constants. These results provided definite experimental answers to the structure of such a weakly bound trimer: firstly the vibrationally averaged planarity of cyclic (HF)3, also supported by the very small value of the inertia defect obtained in the GS, secondly the slight weakening of the hydrogen bond in the intermolecular excited states evidenced from the center of mass separations of the HF constituents determined in the ground, ν6 = 1 and ν4 = 1 states of (HF)3 as well as the decrease of the fitted rotational constants upon excitation. Finally, lower bounds of about 2 ns on ν6 and ν4 state lifetimes could be derived from the deconvolution of experimental linewidths. Such long lifetimes highlight the interest in probing low frequency intermolecular motions of molecular complexes to get rid of constraints related to the vibrational dynamics of coupled anharmonic vibrations at higher energy, resulting in loss of rotational information.

Why is the conclusion of the Gerda experiment not justified

The first results of the GERDA double beta experiment in Gran Sasso were recently presented. They are fully consistent with the Heidelberg-Moscow experiment, but because of its low statistics cannot proof anything at this moment. It is no surprise that the statistics is still far from being able to test the signal claimed by the HEIDELBERG-MOSCOW experiment. The energy resolution of the coaxial detectors is a factor of 1.5 worse than in the HEIDELBERG-MOSCOW experiment. The original goal of background reduction to 10^{-2}counts/kgykeV, or by an order of magnitude compared to the HEIDELBERG-MOSCOW experiment, has not been reached. The background is only a factor 2.3 lower if we refer it to the experimental line width, i.e. in units counts/kgy energy resolution. With pulse shape analysis (PSA) the background in the HEIDELBERG-MOSCOW experiment around Q_{\beta\beta} is 4x10^{-3} counts/kgykeV \cite{HVKK-IVK-MPhLA2006}, which is a factor of 4 (5 referring to the line width) lower than that of GERDA with pulse shape analysis. The amount of enriched material used in the GERDA measurement is 14.6kg, only a factor of 1.34 larger than that used in the HM experiment. The background model is oversimplified and not yet adequate. It is not shown that the lines of their background can be identified. GERDA has to continue the measurement further ~5years, until they can responsibly present an understood background. The present half life limit presented by GERDA of T_{1/2}^{0\nu} > 2.1 x 10^{25} y (90% confidence level, i.e. 1.6sigma)) is still lower than the half-life of T_{1/2}^{0\nu} = 2.23^{{+0.44}_{-0.31}} x 10^{25} y \cite{HVKK-IVK-MPhLA2006} determined in the HEIDELBERG-MOSCOW experiment.

The impact theory of spectral line shapes: a paradigm shift

An overview of the binary collision impact theory of spectral line shapes has been given to provide a unified statistical mechanical approach to line-shape theory, laser theory, nonlinear optics, and transport phenomena in dilute gases. The computation of spectral line profiles corresponding to those obtained from ultra-high-resolution spectral line-shape measurements requires numerical ab initio calculation of scattering amplitudes directly from the underlying dynamics of collisions between radiatively active molecules and their perturbers. The Wigner distribution function–density matrix is utilized to describe the kinetic theory of spectral line shapes and to discuss the various collisional processes that contribute to the kernel of kinetic equations. The influence of features of the potential energy surface on spectral parameters is also discussed, and the importance of comparing experimental line profiles directly with numerically computed line shapes obtained from reliable interaction potentials is emphasized. This contrasts sharply with the universal practice of comparing experimental line widths and shifts using some average or approximate theoretical scattering cross-sections and it contrasts sharply with fitting experimental profiles to some convenient analytical line-shape model; hence the phrase “a paradigm shift” in the title of this work.

Proton Disorder in Ice Ih and Inhomogeneous Broadening in Two-Dimensional Infrared Spectroscopy

It is well-known that in ice Ih the oxygen atoms form a regular hexagonal lattice while the positions of the hydrogen atoms are disordered, called proton disorder in the literature. Various OH (OD) stretch vibrational spectroscopies (e.g., IR, Raman, two-dimensional IR (2DIR), and hole burning) have been used to probe this proton disorder in the past several decades. However, the presence and the magnitude of the inhomogeneous broadening due to this proton disorder in the vibrational spectroscopy is still controversial. In this work, we calculate 2DIR spectroscopy for HOD in D2O ice Ih at 80 K with a mixed quantum/classical approach, and make comparison to a recent 2DIR experiment on the same system. Fair agreement is achieved between theory and experiment, although the calculated 2DIR line shape shows inhomogeneous broadening that was not observed in the experiment. However, the theory reproduces the linear IR for the same system fairly well, and the inhomogeneous broadening from the calculation is consistent with the extrapolation of the experimental IR line-widths in the literature. The effect of this proton disorder on the 2DIR line shape is explored in detail. We also calculate the vibrational three-pulse photon echo peak shift signal, which shows signatures of both low-frequency dynamics and inhomogeneous broadening.

An advanced approach to the evaluation of the spin-rotational term for a nitronyl nitroxide in fluid solution

In this work we focus on the spin-rotational contribution to transverse relaxation rate 1/T2 for a nitronyl nitroxide radical (PTIO) in fluid solution of toluene. We recorded the X-band continuous-wave electron paramagnetic resonance spectra of the radical in a wide temperature range and compared them with the calculated spectra. The calculation was afforded in two steps: first, we calculated the spectral profiles in the same range of temperatures taking into account the hyperfine interaction with the two 14N nuclei, by integration of the stochastic Liouville equation with the E-SpiReS package in the presence of the tumbling motion of the molecule, which modulates the hyperfine and the g tensors. In the second step, we included the proton hyperfine structure by convoluting the spectrum with the pattern due to the 17 PTIO-coupled protons. A further Lorentzian broadening was added by a best fitting procedure to reproduce the experimental linewidths. The additional broadening is associated with a relaxation rate, Wγ, which varies linearly on kB; this trend is expected for the spin-rotational relaxation term modelled by the well-known Atkins–Kivelson expression. We found a discrepancy between the two parameters associated with the radii of the radical, obtained either from the rotational diffusion tensor in the framework of the Debye–Stokes–Einstein model or from the spin-rotational contribution. We discuss this issue in relation to the intrinsic approximations of the spin-rotational model and, in particular, the isotropic Brownian rotation.

Experimental characterization and numerical modeling of a micro-syringe deposition system for dispensing sacrificial photopolymers on particulate ceramic substrates

This work addresses the characterization of a UV-based micro-syringe deposition (μSD) system utilized in the micro-dispensing of photopolymers on particulate ceramic substrates. This methodology is used in embedding functionally graded and interconnected micro-features within constructs produced by a novel combined powder-based additive manufacturing (AM) and UV-based micro-syringe deposition (μSD) technique. The process is experimentally characterized using SEM and optical microscopy to study the effect of a wide range of process parameters on the geometrical quality of deposited tracks. Experimental data show that the system can produce features ranging from 200 to 575μm in width and from 20 to 200μm in height on particulate ceramic surfaces. To gain insight into the proposed micro-deposition process, a two-tier model is also developed. The first framework describes an analytical model for predicting the flow rate of the dispensed photopolymer fluid based on the piston displacement. The second model is a stochastic framework for predicting the line width of the features deposited on the substrate using a Monte Carlo probabilistic simulation to compensate for uncertainty in the system input parameters. A comparison between experimental and modeling line width predictions shows that the modeling results are 14–38% higher than the experimental results, depending on the system input variables. The proposed model is enhanced by introducing adjustment factors to compensate for UV exposure delay, fluid migration, and imbibition.

Line-mixing in absorption bands of linear molecules diluted in high-density rare gases: Measurements and modeling for OCS-He

Absorption coefficients in the bending ν2 and stretching 2ν3 bands of OCS perturbed by He at high pressures up to 300 atm have been measured and analyzed in the impact-approximation region by two theoretical models accounting for the line mixing. In the first approach, the frequency-independent relaxation operator is treated semi-classically [N. N. Filippov and M. V. Tonkov, J. Quant. Spectrosc. Radiat. Transf. 50, 111 (1993)] with adiabatic corrections. To characterize the strength of collisions and the line mixing efficiency a single parameter G is used and its value is estimated from the intermolecular interaction potential. The adiabaticity of collisions and the corresponding correction to the collisional cross sections amplitude are taken into account by two other parameters ν(corr) and σ deduced from fitting to experimental line widths. In the second approach, the symmetrized non-Markovian relaxation operator of Energy-Corrected Sudden type developed for rototranslational Raman spectra of linear rotators [J. Buldyreva and L. Bonamy, Phys. Rev. A 60, 370 (1999)] and satisfying all the basic properties (in particular, the detailed balance relation and the double-sided sum rules) is adapted to absorption spectra for the first time. Its off-diagonal elements are determined via the common adiabaticity factor and the basic transition rates whose parameters are adjusted on experimental values of isolated line widths. Both models provide a very consistent description of OCS-He 2ν3 and ν2 band intensities up to the maximal experimentally studied pressure, with a slightly more realistic picturing of the inter-branch exchange by the second approach.

Surface spin waves of fcc cobalt films on Cu(100): High-resolution spectra and comparison to theory

With the help of a recently developed electron energy-loss spectrometer we have studied the surface spin waves on an eight-monolayer cobalt film deposited on Cu(100) surfaces with unprecedented energy resolution. Standing waves of the bulk of the film are discovered in the region of small wave vectors ${q}_{\ensuremath{\parallel}}l0.35$ \AA{}${}^{\ensuremath{-}1}$. The dispersion of surface spin waves is isotropic even for large ${q}_{\ensuremath{\parallel}}$. Because of the high-energy resolution and the complete characterization of the electron optical properties of the spectrometer reliable data for the linewidth of the surface spin waves are obtained. As a byproduct the dispersion of the Rayleigh surface phonon was measured. Data are compared to theoretical spin wave spectra extracted from calculations of the transverse spin susceptibility based on an ab initio electronic structure that incorporates both the metallic substrate and the magnetic film. The calculation takes fully into account the itinerant nature of the electrons responsible for the magnetic moments. The agreement between theoretical and experimental spin wave energies and linewidths is remarkably good.

Electron-Phonon Coupling in the Conventional SuperconductorYNi2B2Cat High Phonon Energies Studied by Time-of-Flight Neutron Spectroscopy

We report an inelastic neutron scattering investigation of phonons with energies up to 159 meV in the conventional superconductor YNi(2)B(2)C. Using the sweep mode, a newly developed time-of-flight technique involving the continuous rotation of a single crystal specimen, allowed us to measure a four-dimensional volume in (Q, E) space and, thus, determine the dispersion surface and linewidths of the A(1g) (≈102 meV) and A(u) (≈159 meV) type phonon modes over the whole Brillouin zone. Despite of having linewidths of Γ=10 meV, A(1g) modes do not strongly contribute to the total electron-phonon coupling constant λ. However, experimental linewidths show a remarkable agreement with ab initio calculations over the complete phonon energy range, demonstrating the accuracy of such calculations in a rare comparison to a comprehensive experimental data set.

Preparation of 57Co Mössbauer sources in a chromium matrix: Methodological aspects

A cycle of studies resulted in the development of a simple technology for preparing 57Co Mossbauer sources in a chromium matrix. X-ray diffraction studies and SEM examination of the surface morphology of specimens with different chemical histories (slides of 99.98% electrolytic Cr; thin films prepared by cold rolling; films of Cr metal on BeO supports, prepared by magnetron sputtering) showed that only electrolytic chromium specimens can serve as matrices for Mossbauer sources, because they have not only undistorted bcc structure, but also sufficiently even and smooth surface. The parameters of the 57Co diffusion in Cr were determined. They fully agree with the known experimental data on migration of transition elements in this metal. Experimental 57Co(Cr) Mossbauer sources were prepared. The yield of the Mossbauer radiation from their face surface is no less than 90% of the theoretically possible yield. The magnitudes of the effects and the experimental line widths were determined using a set of α-Fe reference absorbers. These parameters confirmed the high quality of the sources.

Direct experimental determination of the K2pand Cl2pcore-level binding energy shifts between molecular and solid KCl: Line broadening effects

Using a technique used previously for metals, the K $2p$ and Cl $2p$ photoelectron spectra of KCl have been measured from the molecular vapor and nonconductor solid simultaneously---a direct measurement for a nonconductor. The Cl $2p$ core level in bulk-solid KCl has a 0.79-eV larger binding energy than its molecular KCl counterpart, but the K $2p$ core level in bulk-solid KCl has a 3.75-eV smaller binding energy than in molecular KCl. After correction for the instrumental resolution, the Cl $2p$ experimental linewidths (FWHM) for both molecular (0.40 eV) and solid spectra (0.56 eV) are much smaller than the corresponding K $2p$ molecular and solid linewidths (0.88 and 0.93 eV, respectively). It is concluded that final-state vibrational splitting is the dominant factor in determining both molecular and solid-state linewidths, with phonon broadening being a smaller effect for the solid.

Organic Peracids: A Structural Puzzle for 17O NMR and Ab Initio Chemical Shift Calculations

We have applied (17)O NMR spectroscopy to investigate the structure of the organic peracids formed by reaction of acetic acid (AA) or lactic acid (LA) with aqueous hydrogen peroxide (HP), which are used in several "green chemistry" applications. The interpretation of the experimental spectra has been supported by ab initio calculations of the (17)O chemical shifts for several possible species, using a continuum representation of the solvent. The combined use of these tools has also allowed us to discuss the decomposition mechanism of LA/HP solutions. The calculated electric field gradients for water, HP, and CO(2) (a decomposition product of LA) correlate well with the experimental (17)O line widths.

2DIR spectroscopy of human amylin fibrils reflects stable β-sheet structure.

The aggregation of human amylin to form amyloid contributes to islet β-cell dysfunction in type 2 diabetes. Studies of amyloid formation have been hindered by the low structural resolution or relatively modest time resolution of standard methods. Two-dimensional infrared (2DIR) spectroscopy, with its sensitivity to protein secondary structures and its intrinsic fast time resolution, is capable of capturing structural changes during the aggregation process. Moreover, isotope labeling enables the measurement of residue-specific information. The diagonal line widths of 2DIR spectra contain information about dynamics and structural heterogeneity of the system. We illustrate the power of a combined atomistic molecular dynamics simulation and theoretical and experimental 2DIR approach by analyzing the variation in diagonal line widths of individual amide I modes in a series of labeled samples of amylin amyloid fibrils. The theoretical and experimental 2DIR line widths suggest a "W" pattern, as a function of residue number. We show that large line widths result from substantial structural disorder and that this pattern is indicative of the stable secondary structure of the two β-sheet regions. This work provides a protocol for bridging MD simulation and 2DIR experiments for future aggregation studies.