Line Shape Measurements Research Articles

Line Shape Measurement and Modelling for Plasma Diagnostics

In this paper we discuss different methods of narrow spectral line shape measurements for a wide spectral range by means of high-resolution spectrometers such as the Fabry-Perot spectrometer, Zeeman spectrometer and Fourier transform spectrometer as well as a theoretical model for spectral line shape modelling and solving of the inverse task based on Tikhonov's regularization method. Special attention is devoted to the line shape measurements for the optically thin light sources filled with Hg, Ar, Xe, Kr for their use in high precision analysers for detection of heavy metals and benzene.

Hydrogen Balmer Series Measurements in Laser-Induced Air Plasma

Time-resolved spectroscopy is employed to analyze micro plasma generated in laboratory air. Stark-broadened emission profiles for hydrogen alpha and beta allow us to determine plasma characteristics for specific time delays after plasma generation. Stark shift, asymmetry, and full width half maximum measurements are used to infer electron density. The measurements of hydrogen alpha and beta Balmer series line shapes are analyzed using various theory results. Our laser-induced breakdown spectroscopy arrangement uses a Q- switched Nd:YAG laser operating at the fundamental wavelength of 1064 nm that is focused for plasma generation. The hydrogen alpha and beta lines emerge from the free electron background radiation for time delays larger than 0.3 ps and 1.4 ps, respectively. Neutral and ionized nitrogen emission lines allow us to infer electron density for time delays from 0.1 to 10 μs. The electron density values are compared with results obtained from hydrogen Balmer series line shapes.

Line shape measurements of rubidium 5S-7S two-photon transition

We report the use of a digital lock to measure the line profile and center frequency of rubidium 5S-7S two-photon transitions with a cw laser referenced to an optical frequency comb. The narrow, two-photon transition, 5S-7S (760 nm), insensitive to first-order in a magnetic field, is a promising candidate for frequency reference.

Precise cavity enhanced absorption spectroscopy

A short review of recent achievements in high-precision cavity enhanced absorption spectroscopy is presented. Actual challenges and development paths in modern line shape study are indicated and discussed. The importance of identification and quantification of systematic instrumental errors affecting the measured line shape is highlighted. New, alternative measurement methods based on cavity enhanced spectroscopy are proposed.

Spectral line-shapes of oxygen B-band transitions measured with cavity ring-down spectroscopy

Results of line-shape measurements of self- and N2-broadened P9 P9 transition of the oxygen B band are presented. Spectra were acquired using the optical frequency comb- assisted Pound-Drever-Hall-locked frequency-stabilized cavity ring-down spectrometer (PDH- locked FS-CRDS). In the line-shape analysis the line narrowing described by Dicke narrowing or/and the speed dependence of collisional broadening were taken into account. The multispectrum fitting technique was used to minimize numerical correlations between line-shape parameters. Collisional broadening and shifting coefficients are reported with sub-percent uncertainties. Influence of the spectral line-shape model used in data analysis on determined line intensities and collisional broadening is discussed.

Line shapes, positions and intensities of water transitions near 1.28 μm

We present measurements of approximately 70 isolated, self-broadened, water vapor lines which are assigned to the (1,0,1)–(0,0,0), (0,0,2)–(0,0,0), (1,2,0)–(0,0,0), and (2,0,0)–(0,0,0) vibrational bands and which occur in the transparency window region from 7710cm−1 to 7920cm−1. We acquired absorption spectra on room-temperature, water samples over the pressure range 150–800Pa with the frequency-stabilized cavity ring-down spectroscopy technique. In order to optimize measurement accuracy, we integrated mK-level temperature control and SI-traceable pressure measurements into our cavity ring-down spectroscopy measurements. This technique yielded relative uncertainties of 0.04% and 0.20% in sample density and measured line intensity, respectively. We also referenced our spectrum frequency axes to a Cs clock, which provided vacuum line positions with a combined standard uncertainty of 3MHz. Comparison of our measured intensities, positions and self-broadening parameters with literature values reveals that the present work substantially reduces uncertainty in these line parameters. For the (1,0,1)–(0,0,0) vibrational band, the measured line intensities agree at the 1%-level with published ab initio calculations. Our spectra exhibited signal-to-noise ratios up to ≈20,000:1 to enable stringent tests of theoretical line profiles through multispectrum least-squares data analysis. We show that the partially correlated, quadratic-speed-dependent Nelkin–Ghatak profile gives a quality of fit that is commensurate with the high spectrum signal-to-noise ratio, and unlike most other profiles considered here, reproduces the measured line shapes without systematic residuals over the entire pressure range. Our results confirm that mechanisms of (1) collisional narrowing, (2) speed-dependent effects and (3) partial correlation between velocity-changing and dephasing collisions, contribute to the self-broadened line shape of isolated water vapor transitions.

Intensities, broadening and narrowing parameters in the ν3 band of methane

The P-branch of methane׳s ν3 band is probed to carry out an extensive study of the 2905–2908cm−1 infrared spectral region. Absolute line intensities as well as N2-, O2-, H2-, He-, Ar- and CO2-broadening coefficients are determined for nine transitions at room temperature. Narrowing parameters due to the Dicke effect have also been investigated. A narrow emission line-width (~0.0001cm−1) difference-frequency-generation (DFG) laser system is used as the tunable light source. To retrieve the CH4 spectroscopic parameters, Voigt and Galatry profiles were used to simulate the measured line shape of the individual transitions.

Observation of coherence oscillations of single ensemble excitations in methanol

We demonstrate coherence measurements of single-photon-level collective excitations of vibrational states using transient coherent spontaneous Raman scattering in liquid methanol. We observe the decay of the 1033 cm−1 mode and coherence oscillations due to simultaneous excitation of the 2834 and 2944 cm−1 modes. The coherence life-times and oscillation frequencies agree with frequency-domain line-shape measurements and femtosecond coherent anti-Stokes Raman scattering measurements. The demonstrated technique is complementary to and, in some cases, simpler than traditional stimulated spectroscopy techniques in that it does not require more than one laser and is free of nonresonant background.

Velocity-changing collisions in pure H2 and H2-Ar mixture.

We show how to effectively introduce a proper description of the velocity-changing collisions into the model of isolated molecular transition for the case of self- and Ar-perturbed H2. We demonstrate that the billiard-ball (BB) approximation of the H2-H2 and H2-Ar potentials gives an accurate description of the velocity-changing collisions. The BB model results are compared with ab initio classical molecular dynamics simulations. It is shown that the BB model correctly reproduces not only the principal properties such as frequencies of velocity-changing collisions or collision kernels, but also other characteristics of H2-H2 and H2-Ar gas kinetics like rate of speed-changing collisions. Finally, we present line-shape measurement of the Q(1) line of the first overtone band of self-perturbed H2. We quantify the systematic errors of line-shape analysis caused by the use of oversimplified description of velocity-changing collisions. These conclusions will have significant impact on recent rapidly developing ultra-accurate metrology based on Doppler-limited spectroscopic measurements such as Doppler-width thermometry, atmosphere monitoring, Boltzmann constant determination, or transition position and intensity determination for fundamental studies.

A study of phenylalanine side-chain dynamics in surface-adsorbed peptides using solid-state deuterium NMR and rotamer library statistics.

Extracellular matrix proteins adsorbed onto mineral surfaces exist in a unique environment where the structure and dynamics of the protein can be altered profoundly. To further elucidate how the mineral surface impacts molecular properties, we perform a comparative study of the dynamics of nonpolar side chains within the mineral-recognition domain of the biomineralization protein salivary statherin adsorbed onto its native hydroxyapatite (HAP) mineral surface versus the dynamics displayed by the native protein in the hydrated solid state. Specifically, the dynamics of phenylalanine side chains (viz., F7 and F14) located in the surface-adsorbed 15-amino acid HAP-recognition fragment (SN15: DpSpSEEKFLRRIGRFG) are studied using deuterium magic angle spinning ((2)H MAS) line shape and spin-lattice relaxation measurements. (2)H NMR MAS spectra and T1 relaxation times obtained from the deuterated phenylalanine side chains in free and HAP-adsorbed SN15 are fitted to models where the side chains are assumed to exchange between rotameric states and where the exchange rates and a priori rotameric state populations are varied iteratively. In condensed proteins, phenylalanine side-chain dynamics are dominated by 180° flips of the phenyl ring, i.e., the "π flip". However, for both F7 and F14, the number of exchanging side-chain rotameric states increases in the HAP-bound complex relative to the unbound solid sample, indicating that increased dynamic freedom accompanies introduction of the protein into the biofilm state. The observed rotameric exchange dynamics in the HAP-bound complex are on the order of 5-6 × 10(6) s(-1), as determined from the deuterium MAS line shapes. The dynamics in the HAP-bound complex are also shown to have some solution-like behavioral characteristics, with some interesting deviations from rotameric library statistics.

Singlet exciton fission process in rubrene-doped organic films

In this work, the magnetic field effect of photoluminescence (MPL) from rubrene-doped organic films was recorded at room temperature. The measured line shapes were attributed to the field modulation on the rate constant of singlet exciton fission which occurred between the doped rubrene molecules. It was found that the amplitude of MPL exhibited a non-linear dependence on the averaged intermolecular distance. Such an observation implies that the intermolecular coupling (IMC) which is tuned by changing the intermolecular distance is able to significantly affect the intensity of fission process. Theoretically, the non-linear dependence of MPL as a function of intermolecular distance can be explained by using the Landau-Zener theory for non-adiabatic transition. Experimentally, investigating the variation of singlet fission with different IMC could be an important means to study the microscopic dynamics of fission process. Our work reveals that the importance of IMC should be considered for the design of an efficient singlet fission sensitized organic photovoltaic device.

Line shape parameters of PH3 transitions in the Pentad near 4–5 μm: Self-broadened widths, shifts, line mixing and speed dependence

Accurate knowledge of spectroscopic line parameters of PH3 is important for remote sensing of the outer planets, especially Jupiter and Saturn. In a recent study, line positions and intensities for the Pentad bands of PH3 have been reported from analysis of high-resolution, high signal-to noise room-temperature spectra recorded with two Fourier transform spectrometers (2014) [1]. The results presented in this study were obtained during the analysis of positions and intensities, but here we focus on the measurements of spectral line shapes (e.g. widths, shifts, line mixing) for the 2ν4, ν2+ν4, ν1 and ν3 bands. A multispectrum nonlinear least squares curve fitting technique employing a non-Voigt line shape to include line mixing and speed dependence of the Lorentz width was employed to fit the spectra simultaneously. The least squares fittings were performed on five room-temperature spectra recorded at various PH3 pressures (∼2–50Torr) with the Bruker IFS-125HR Fourier transform spectrometer (FTS) located at the Pacific Northwest National Laboratory (PNNL), in Richland, Washington. Over 840 Lorentz self-broadened half-width coefficients, 620 self-shift coefficients and 185 speed dependence parameters were measured. Line mixing was detected for transitions in the 2ν4, ν1 and ν3 bands, and their values were quantified for 10 A+A− pairs of transitions via off-diagonal relaxation matrix element formalism. The dependences of the measured half-width coefficients on the J and K rotational quanta of the transitions are discussed. The self-width coefficients for the ν1 and ν3 bands from this study are compared to the self-width coefficients for transitions with the same rotational quanta (J, K) reported for the Dyad (ν2 and ν4) bands. The measurements from present study should be useful for the development of a reliable theoretical modeling of pressure-broadened widths, shifts and line mixing in symmetric top molecules with C3v symmetry in general, and of PH3 in particular.

Li Ion Self-Diffusion in Metastable Li15Si4 prepared from Monocrystalline Si — an Ex Situ 7Li NMR Relaxometry Study

New energy systems became one of the basic needs in modern society. The explosive increase of portable electronic devices, such as cell phones, laptop computers etc., and the storage of electricity from renewable resources calls for reliable batteries with extraordinary capacities. For this purpose, tremendous research effort has been devoted to novel high-capacity electrode materials.Silicon containing electrodes are one of the most promising anode materials, not only due to their extremely large theoretical charge capacity but also owing to the availability of state-of-the-art fabrication methods. To overcome, however, the hurdles of Si-based batteries an in-depth understanding of the fundamental ion transport processes in such systems is expected to be highly useful. In the past, mainly macroscopic electrochemical methods have been used to estimate Li ion diffusivities in amorphous Si and metastable Li15Si4. By comparing the studies published so far, in some cases the resulting diffusivities differ from each other by many orders of magnitude. In the present contribution, we used an atomic-scale technique to probe Li self-diffusion parameters viz time-domain nuclear magnetic resonance (NMR) relaxometry.Here, temperature-variable 7Li NMR relaxometry is known to be a powerful tool for the investigation of Li ion dynamics in crystalline as well as amorphous solids and enables, in the ideal case, direct access to both Li+ jump rates and activation energies [1-3]. For that purpose, half-cells of boron doped, monocrystalline Si (100) on Cu were cycled at 20 mV vs. Li/Li+ (2.5 cycles) with charge cut-off and current limitation. X-ray powder diffraction confirmed the formation of phase pure crystalline, metastable Li15Si4. The samples prepared are also expected to contain amorphous Si. 7Li NMR line shape measurements point to heterogeneous dynamics with relatively fast Li ions at temperatures well below 20 °C. A comprehensive NMR spin-lattice relaxation (SLR) study including both measurements in the laboratory (R 1, 116 MHz) and the rotating frame of reference (R 1ρ, 20 kHz) was used to deduce microscopic Li ion diffusion parameters. Fortunately, the temperature variable R 1ρ rates recorded at locking frequencies in the kHz range reveal a diffusion induced rate peak when the rates are plotted versus inverse temperature 1/T in an Arrhenius diagram. From the low-temperature flank of this peak an activation energy of 0.55(5) eV can be estimated. Moreover, the position of the maximum on the 1/T-axis leads to a Li jump rate of approximately 2.5 × 105 1/s at –30 °C. According to the Einstein-Smoluchowski equation this corresponds to a Li ion self-diffusion coefficient in the order of 10–11 cm2/s which points to relatively fast Li diffusivity in the sample studied, see also ref. [4]. For comparison, ongoing NMR studies also include the investigation of a completely amorphous Si sample prepared by electrochemical reaction with Li as described above. Acknowledgement Financial support by the Federal Ministry of Economy, Family and Youth and the National Foundation for Research, Technology and Development is gratefully acknowledged.

Multi-harmonic measurements of line shape under low absorption conditions

We propose a method that employs the ratios of the 2nd and 4th harmonics at the line center to measure line shape under low absorption conditions. To verify this method, the transition of CO2 at 6,982.0678 cm−1 is selected to measure line shape by using the proposed method and direct absorption spectroscopy in laboratory conditions. The results from both methods have a high degree of consistency. This satisfactory agreement indicates the validity of the proposed method.

Measurement procedure for absolute broadband infrared up-conversion photoluminescent quantum yields: correcting for absorption/re-emission.

The internal photoluminescent quantum yield (iPLQY)--defined as the ratio of emitted photons to those absorbed--is an important parameter in the evaluation and application of luminescent materials. The iPLQY is rarely reported due to the complexities in the calibration of such a measurement. Herein, an experimental method is proposed to correct for re-emission, which leads to an underestimation of the absorption under broadband excitation. Although traditionally the iPLQY is measured using monochromatic sources for linear materials, this advancement is necessary for nonlinear materials with wavelength dependent iPLQY, such as the application of up-conversion to solar energy harvesting. The method requires an additional measurement of the emission line shape that overlaps with the excitation and absorption spectra. Through scaling of the emission spectrum, at the long wavelength edge where an overlap of excitation does not occur, it is possible to better estimate the value of iPLQY. The method has been evaluated for a range of nonlinear material concentrations and under various irradiances to analyze the necessity and boundary conditions that favor the proposed method. Use of this refined method is important for a reliable measurement of iPLQY under a broad illumination source such as the Sun.

Vibrational and rotational excitation of CO2 in the collisional quenching of H2(v = 1)

Energy transfer in H2 (1,1) +CO2 collisions was investigated using high resolution transient laser spectroscopy. Rotational state selective excitation of v = 1 for rotational level J = 1 was achieved by stimulated Raman pumping. Energy gain into CO2 resulting from collisions with H2 (1,1) was probed using transient absorption techniques, Distributions of nascent CO2 rotational populations in both the ground (00 degrees 0) state and the vibrationally excited (00 degrees 1) state were determined from overtone absorption measurements. Translational energy distributions of the recoiling CO2 in individual rovibrational states were determined through measurement of Doppler-broadened transient line shapes. A kinetic model was developed to describe rates for appearance of CO2 states resulting from collisions with H2(1,1). From scanned CARS (coherent anti-stokes Raman scattering) the spectral peaks population ratio n0/n1 was obtained, where n0 and n1 represent the number densities of H2 at the levels (0,1) and (1,1), respectively. Using rotational Boltzmann distribution of H2 (v = 0) at 300 K, n1 was yielded. Values for rate coefficients were obtained using data for CO2 (00 degrees 0) J = 48 to 76 and CO2 (00 degrees 1) J = 5 to 33. The rate coefficients derived from appearance of the (00 degrees 0) state have values of K(tr) = (3.9 ± 0.8) x 10(-11) cm3 x molecule(-1) x s(-1) for J = 48 and k(tr) = (1.4 ± 0.3) x 10(-10) cm3 x molecule(-1) x s(-1) for J = 76, with a monotonic increase for the higher J states. For the (00 degrees 1) state, values of k(tr) remain fairly constant at k(tr) = (4.3 ± 0.9) x 10(-12) cm3 x molecule(-1) x s(-1). Rotational populations for the nascent CO2 states were measured at 0. 5 μs following excitation of H2. The transient population for each state was fit using a Boltzmann rotational distribution. The CO2 (00 degrees 0) J = 48-76 rotational states were populated substantially relative to the initial 300 K CO2 distributions, and the distribution is described by Trot. The excited (00 degrees 1) state has T(rot), 310 K. The center-of-mass translational temperatures for the (00 degrees 0) state are all much greater than 300 K, with T(rel) = 1 532 K for J = 76. In contrast, transient line profiles for the J = 5 - 33 levels of excited (00 degrees 1) state do not show any broadening above the initial 300 K distributions, indicating that excitation to the (00 degrees 1) state is not accompanied by translational energy change.

Diode laser measurements of the first overtone HF lineshape broadened by Ar, Xe, Kr and N2

A high-resolution spectroscopy study of the HF first overtone vibrational–rotational 0–2 R(0) line profile was carried out in Ar, Xe, Kr and N2 gas mixtures using near-IR tunable diode laser. The HF 0–2 R(0) analytical line is known as the most appropriate for laser detection of HF molecules in the atmosphere. Highly accurate collisional broadening, shift and “narrowing” coefficients were determined from the direct spectroscopic fit employing traditional Voigt, Rautian and Galatry symmetric profiles and using an asymmetric speed-dependent Voigt profile. These data make the information of modern spectroscopic databases, e.g., HITRAN2012, much more exact. A noticeable line shape asymmetry was investigated in detail. In this paper, the validity of the use of model profiles in spectroscopic fitting is severely discussed. For the first time, we draw attention to the need for physically realistic line shape models to avoid systematic errors in retrieved gas concentrations. We stress that in the case of HF, all considered model profiles serve only as ad hoc models; they may conceal the true physics of spectral line formation.

Rayleigh-Brillouin scattering of carbon dioxide.

The spectral line shape of spontaneous Rayleigh-Brillouin scattering in CO2 is studied in a range of pressures. The spectrum is influenced by the bulk viscosity η(b), which is a relaxation phenomenon involving the internal degrees of freedom of the molecule. The associated relaxation rates can be compared to the frequency shift of the scattered light, which demands precise measurements of the spectral line shape. We find η(b)=(5.7±0.6)×10(-6) kg m(-1) s(-1) for the range of pressures p=2-4 bar and for room temperature conditions.

Measuring vibrational coherence lifetimes in liquid methanol using transient coherent Raman scattering

We demonstrate the measurement of vibrational coherence in liquids using transient coherent Raman scattering. This technique measures the coherence lifetime of vibrational states through the interference of time-delayed coherent Raman-scattered photons using low-power, non-resonant optical pulses. We measure the vibrational lifetime of the 1033 cm−1 mode in liquid methanol. The resulting lifetime agrees with frequency-domain lineshape measurements. This technique is a complementary and in some cases simpler alternative to standard nonlinear spectroscopy techniques.

H2 line mixing coefficients in the ν2 and ν4 bands of PH3

Using a tunable diode-laser spectrometer, we measured the H2 line mixing coefficients for 32 lines in the ν2 and ν4 bands of phosphine (PH3) at room temperature. These lines are located in the spectral range from 1016 to 1106cm−1. The pressure line mixing parameters have been obtained by fitting the measured line shapes using a multi-pressure fitting procedure that accounts for the apparatus function as well as the Doppler and collisional effects. The variation of these parameters with the rotational and vibrational quantum numbers is discussed.