Molecular Line Shapes Research Articles

A time averaged semiclassical approach to IR spectroscopy.

We propose a new semiclassical approach to the calculation of molecular IR spectra. The method employs the time averaging technique of Kaledin and Miller upon symmetrization of the quantum dipole-dipole autocorrelation function. Spectra at high and low temperatures are investigated. In the first case, we are able to point out the possible presence of hot bands in the molecular absorption line shape. In the second case, we are able to reproduce accurate IR spectra as demonstrated by a calculation of the IR spectrum of the water molecule, which is within 4% of the exact intensity. Our time averaged IR spectra can be directly compared to time averaged semiclassical power spectra as shown in an application to the CO2molecule, which points out the differences between IR and power spectra and demonstrates that our new approach can identify active IR transitions correctly. Overall, the method features excellent accuracy in calculating absorption intensities and provides estimates for the frequencies of vibrations in agreement with the corresponding power spectra. In perspective, this work opens up the possibility to interface the new method with the semiclassical techniques developed for power spectra, such as the divide-and-conquer one, to get accurate IR spectra of complex and high-dimensional molecular systems.

High-precision cavity-enhanced spectroscopy for studying the H2-Ar collisions and interactions.

Information about molecular collisions is encoded in the shapes of collision-perturbed molecular resonances. This connection between molecular interactions and line shapes is most clearly seen in simple systems, such as the molecular hydrogen perturbed by a noble gas atom. We study the H2-Ar system by means of highly accurate absorption spectroscopy and ab initio calculations. On the one hand, we use the cavity-ring-down-spectroscopy technique to record the shapes of the S(1) 3-0 line of molecular hydrogen perturbed by argon. On the other hand, we simulate the shapes of this line using ab initio quantum-scattering calculations performed on our accurate H2-Ar potential energy surface (PES). In order to validate the PES and the methodology of quantum-scattering calculations separately from the model of velocity-changing collisions, we measured the spectra in experimental conditions in which the influence of the latter is relatively minor. In these conditions, our theoretical collision-perturbed line shapes reproduce the raw experimental spectra at the percent level. However, the collisional shift, δ0, differs from the experimental value by 20%. Compared to other line-shape parameters, collisional shift displays much higher sensitivity to various technical aspects of the computational methodology. We identify the contributors to this large error and find the inaccuracies of the PES to be the dominant factor. With regard to the quantum scattering methodology, we demonstrate that treating the centrifugal distortion in a simple, approximate manner is sufficient to obtain the percent-level accuracy of collisional spectra.

Hydroxyl Molecular Line Shapes in Laser-Ignition of Air

This work communicates measurement and analysis of diatomic molecular hydroxyl (OH) spectra after generation of laser-induced plasma. A relative laboratory-air humidity of the order of 25% causes the occurrence of OH recombination radiation that is recorded with optical emission spectroscopy. A Q-switched, 150 mJ, 6 ns pulsed Nd:YAG laser beam at the fundamental wavelength of 1064 nm is focused in air with f/5 optics. Formation of OH is clearly discernible at time delays of several dozen microseconds after plasma initiation. Optical emissions are dispersed by a 0.64-m Czerny-Turner spectrometer and an intensified charge-coupled device records the data along the wavelength and slit dimensions.

Emission spectra of glows produced by ionic liquid ion sources

Electrospray devices, such as ionic liquid ion sources, often exhibit glows during operation in high vacuum facilities. The lack of electron excitation mechanisms during electrospray operation prompts the question: “What causes glow in an electrospray device?” Our optical emission spectroscopy results show that electrospray glow exhibits a broad spectral response between 350 and 800 nm with emission lines corresponding to atomic metal constituents of impinged surfaces, neutral and ionized atomic constituents of the ionic liquid propellant, and molecular line shapes that may also be dissociation products of the ionic liquid. We have previously defined secondary species emission to describe the many complex interfacial phenomena that occur when electrosprayed species impact surfaces downstream of the emitter. Our analysis of the optical emission spectra shows that these glows are only possible in the presence of secondary species emission. Therefore, the answer to the proposed question: high-velocity impacts that generate secondary species are the root cause of glow for electrosprays in high vacuum facilities.

Assessment of the precision, bias and numerical correlation of fitted parameters obtained by multi-spectrum fits of the Hartmann-Tran line profile to simulated absorption spectra

Although the Voigt profile has long been used to analyze absorption spectra, the quest for increased precision, accuracy and generality drives the application of advanced models of atomic and molecular line shapes. To this end, the Hartmann-Tran profile is now recommended as a standard for high-resolution spectroscopy because it parameterizes relevant higher-order physical effects, is computationally efficient, and reduces to other widely used profiles as limiting cases. This work explores the uncertainty with which line shape parameters can be obtained from constrained multi-spectrum fits of spectra simulated with this standard profile, varying uncertainty levels in the spectrum detuning and absorption axes, and spanning a range of sampling density, pressure, and line shape parameter values. The analysis focuses on how noise-limited measurement precision of frequency detuning and absorption drive statistical uncertainties in fitted parameters and numerical correlations between these quantities. Also, we quantify the degree of equivalence between the full Hartmann-Tran profile and those derived from it in terms of fitted peak areas and line shape parameters. Finally, we introduce a new open-source software package named Multi-spectrum Analysis Tool for Spectroscopy (MATS), which allows users to fit the HTP and its derived profiles to experimental or simulated absorption spectra to explore the limits of the HTP under actual experimental or user-defined conditions.

Statistical Characterization of Temperature and Pressure Vertical Profiles for the Analysis of Laser Heterodyne Radiometry Data.

The statistical analysis of historic pressure and temperature profiles from radiosonde launches for use in the fitting of molecular oxygen line shapes is presented. As the O2 mixing ratio is nearly constant throughout the lower atmosphere, only variations in pressure and temperature profiles will affect the fit of observed O2 features in Laser Heterodyne Radiometry (LHR) spectra. Radiosonde temperature and pressure data are extracted from the Integrated Global Radiosonde Archive (IGRA) for a given station, date, and launch time. Data may be extracted for a single launch, for the same date over several years, and/or within a window centered on a target date. The temperature and pressure profiles are further characterized by the statistical variation in coefficients of polynomial fits in altitude. The properties of the probability distributions for each coefficient are used to constrain fits of O2 line shapes through Nelder–Mead optimization. The refined temperature and pressure profiles are then used in the retrieval of vertically resolved mixing ratios for greenhouse gases (GHGs) measured in the same instrument. In continuous collections, each vertical profile determination may be treated as a Bayesian prior to inform subsequent measurements and provide an estimate of uncertainties.

DNA/RNA of gum cancer cells-anti–cancer nano drugs ligands structure determination with the two–dimensional NMR molecular line shape analysis of single, multiple, zero and double quantum correlation experiments

DNA/RNA of gum cancer cells-anti–cancer nano drugs ligands structure determination with the two–dimensional NMR molecular line shape analysis of single, multiple, zero and double quantum correlation experiments

Comb-locked cavity ring-down spectroscopy with variable temperature.

Temperature dependence of molecular absorption line shape is important information for spectroscopic studies and applications. In this work, we report a comb-locked cavity ring-down spectrometer employing a cryogenic cooler to perform absorption spectroscopy measurements at temperatures between 40 K and 300 K. As a demonstration, we recorded the spectrum of the R(0) line in the (2-0) band of HD at 46 K. The temperature was also confirmed by the Doppler width of the HD line. Spectra of CH4 near 1.394 μm were also recorded in a wide temperature range of 70-300 K. Lower-state energies of methane lines were analyzed by fitting these spectra, which can be directly compared with the HITRAN and TheoReTS databases. Considerable deviations were observed, indicating the need to investigate the assignments of the methane lines in this region.

Atomic and molecular spectral line shapes in laboratory and selected astrophysical plasma

This work elaborates on laboratory measurements of hydrogen Balmer series lines and diatomic molecular species in laser-plasma. Comparisons with astrophysical white dwarf spectra, recorded at various observatories and the Hubble space telescope, point out direct applications of experimental results. The recorded general relativity gravitational- or Einstein-shift of the atomic lines allows one to infer the ratio of mass and radius. The Stark-effect redshifts of hydrogen alpha and hydrogen beta lines investigated with time-resolved emission spectroscopy are usually one order of magnitude larger than the gravitational shift. In view of white dwarf atmospheres dominated by hydrogen and the associated absorption spectra, averaging the laser-induced plasma data reveals spectral line shapes that mask the redshift caused by the Stark effect. The available white dwarf data indicate that the collected radiation propagated through regions of different density in the star’s atmosphere. Diatomic spectra are typically recorded as white dwarfs further progress in their transformation.

Measured and calculated O2-broadening coefficients of C2H4 in the 1800–2350 cm-1 spectral region

The goal of this study is to measure and calculate for the first time the O2-broadening coefficients of ethylene (C2H4) in the 5 µm region. High-resolution Fourier transform spectra were recorded at various pressures for the whole 1750–3000 cm-1 spectral range. Using these spectra, a large set of 590 O2-broadening coefficients was measured for the ν7 + ν8, ν4 + ν8, ν6 + ν10, ν6 + ν7, ν3 + ν10 and ν4 + ν6 bands. The broadening coefficients were measured using a mono-spectrum non-linear least squares fitting procedure of Voigt profiles which appeared to properly model the observed molecular line shapes within the noise level. These coefficients are compared to theoretical results calculated with the semi-classical Robert and Bonamy formalism in which the intermolecular potential includes electrostatic and dispersion energy contributions. These results are generally in satisfactory agreement with experimental data. The average discrepancy 〈((γmea−γcal)/γmea)×100〉 and the absolute difference 〈(|γmea−γcal|/γmea)×100〉 between the measured and calculated values for 590 lines are (−2.3 ± 8.9)% and (7.3 ± 5.5)%, respectively. The rotational and vibrational dependences of O2-broadening coefficients are examined. The present results of C2H4O2 and those of C2H4N2 are used to determine the corresponding C2H4-air system. Comparison between obtained results leads to an average value of the absolute difference of (7.2 ± 5.5)%.

A Direct, Quantitative Connection between Molecular Dynamics Simulations and Vibrational Probe Line Shapes.

A quantitative connection between molecular dynamics simulations and vibrational spectroscopy of probe-labeled systems would enable direct translation of experimental data into structural and dynamical information. To constitute this connection, all-atom molecular dynamics (MD) simulations were performed for two SCN probe sites (solvent-exposed and buried) in a calmodulin-target peptide complex. Two frequency calculation approaches with substantial nonelectrostatic components, a quantum mechanics/molecular mechanics (QM/MM)-based technique and a solvatochromic fragment potential (SolEFP) approach, were used to simulate the infrared probe line shapes. While QM/MM results disagreed with experiment, SolEFP results matched experimental frequencies and line shapes and revealed the physical and dynamic bases for the observed spectroscopic behavior. The main determinant of the CN probe frequency is the exchange repulsion between the probe and its local structural neighbors, and there is a clear dynamic explanation for the relatively broad probe line shape observed at the "buried" probe site. This methodology should be widely applicable to vibrational probes in many environments.

Radiation Transfer Calculations and Assessment of Global Warming by CO2

We present detailed line-by-line radiation transfer calculations, which were performed under different atmospheric conditions for the most important greenhouse gases water vapor, carbon dioxide, methane, and ozone. Particularly cloud effects, surface temperature variations, and humidity changes as well as molecular lineshape effects are investigated to examine their specific influence on some basic climatologic parameters like the radiative forcing, the long wave absorptivity, and back-radiation as a function of an increasing CO2 concentration in the atmosphere. These calculations are used to assess the CO2 global warming by means of an advanced two-layer climate model and to disclose some larger discrepancies in calculating the climate sensitivity. Including solar and cloud effects as well as all relevant feedback processes our simulations give an equilibrium climate sensitivity of CS = 0.7°C (temperature increase at doubled CO2) and a solar sensitivity of SS = 0.17°C (at 0.1% increase of the total solar irradiance). Then CO2 contributes 40% and the Sun 60% to global warming over the last century.

Measurements and modeling of 16O12C17O spectroscopic parameters at 2 µm

The lack of spectroscopic measurements for rare CO2 isotopologues was the main motivation of this work. In our present study we report line intensity measurements for 16O12C17O made with a high resolution Fourier transform spectrometer (Bruker IFS-125HR) and a 21 m path cryogenic Herriott cell at Jet Propulsion Laboratory. For this, a 17O-enriched CO2 gas sample was used, which comes as a mixture of primary and several minor CO2 isotopologues. The mole fraction of the 16O12C17O isotopologue in the mixture was determined to be 0.3991 by mass spectrometry from a Stable Isotope Ratio Mass Spectrometer (SIRMS) under stochastic distribution assumption at thermal equilibrium. Since the collisional narrowing effect was observed, the Rautian molecular line shape profile was systematically adopted instead of the Voigt profile. Absolute line positions were also investigated by performing a wavenumber calibration based on CO, HCl and a few well-known 16O12C16O transitions. Finally, around 1000 transitions were studied between 4604 and 5126cm−1 involving 15 bands of the 16O12C17O isotopologue. All the measured line intensities were renormalized to be the values for 100% pure isotopologue sample. Transition dipole moments and Herman-Wallis factors were derived enabling a global comparison with theoretical calculations and predictions for the 15 bands of the 16O12C17O isotopologue. For the measured line positions, the absolute accuracy is around 2×10−4cm−1. The accuracies of retrieved line intensities are 2 – 5% for five cold and two hot bands, and 6–30% for eight other weaker hot bands. Results from this work were in a good agreement with HITRAN 2012 for positions, but showed rather significant discrepancies for line intensities. An extensive line list was generated from new experimental measurements in order to improve and validate spectroscopic knowledge of 12C16O17O isotopologue in support of atmospheric remote sensing for the Earth (e.g., OCO-2 mission), Mars and Venus.

Molecular line shape parameters for exoplanetary atmospheric applications

We describe the recent updates to the ExoMol database regarding the molecular spectral line shapes. ExoMol provides comprehensive molecular line lists with a special emphasis on the applications involving characterization of hot atmospheres such as those found in exoplanets and cool stars. Among important requirements of such applications are (i) the broadening parameters for hydrogen and helium dominating atmospheres and (ii) very broad ranges of temperature and pressures. The current status of the available line shape data in the literature, demands from the exoplanetary community and their specific needs are discussed.

A test of H2-He potential energy surfaces

The close-coupling method is used to calculate purely rotational relaxation rates and pressure broadening and shifting coefficients for H2-He collisions, in order to test various potential energy surfaces. Downward rate coefficients k 3→1(T) and k 2→0(T) are compared with experimental data, but the subtle differences in the potential energy surfaces are hardly reflected in these rates. Helium pressure broadening and shifting generalized cross sections for the isotropic Raman Q(1) lines of the fundamental bands of D2 and H2 as well as the purely rotational Stokes S0(1) line of H2 are therefore also considered. While these spectroscopic characteristics are much more sensitive to the precise form of the interaction potential, a proper validation cannot be performed without taking into account the influence of the translational motion on the molecular line shapes. After including this, it is found that the potential energy surface of Bakr, Smith and Patkowski [B.W. Bakr, D.G.A. Smith, K. Patkowski, J. Chem. Phys. 139, 144305 (2013)] allows the best reproduction of the experimental data.

Self- and N2-collisional broadening coefficients of ethylene in the 1800–2350 cm−1 spectral region

Self- and N2-broadening coefficients have been retrieved for 566 lines of C2H4 at room temperature in the 5μm region including the ν7+ν8, ν4+ν8, ν6+ν10, ν6+ν7, ν4+ν6 and ν3+ν10 vibrational bands. Measurements have been performed using Fourier transform infrared spectroscopy. The lines were fitted with a single-spectrum non-linear least squares fitting procedure of Voigt profiles which appeared to properly model the observed molecular line shapes within the noise level. The experimental results are compared with theoretical values calculated using the Robert and Bonamy formalism which reproduces the measured broadening coefficients. For the self- and N2-broadening coefficients, the average discrepancy 〈(γmea−γcal/γmea)×100〉 for 566 lines, is (−1.6±7.8)% and (−2.8±9.9)%, respectively. One standard deviation is given after ±. These coefficients show dependence with both rotational quantum numbers J and Ka. Comparisons with previous measurements taken in the ν7 band of C2H4 show difference range between 7% and 15%. These differences not insignificant can come from inconsistency between experimental measurements.

Self- and N2-broadening of CH3Br ro-vibrational lines in the ν2 band: The J and K dependence

Methyl bromide (CH3Br) is the major source of inorganic bromine in the atmosphere and contributes significantly to ozone depletion. Indeed, CH3Br is dissociated by UV radiation, producing Br radicals that catalyze the destruction of ozone. In this paper, we report measured Lorentz self- and N2-broadening coefficients of CH3Br in the ν2 fundamental band using a mono-spectrum non-linear least squares fitting of Voigt profiles which appeared to properly model the observed molecular line shapes within the noise level. These measurements were made by analyzing 12 laboratory absorption spectra recorded at high resolution (0.005, 0.003 or 0.002cm−1) using the Fourier transform spectrometer Bruker IF125HR located at the LISA facility in Créteil. The spectra were obtained at room temperature using a White-type multipass cell with an optical path of 0.849m and various pressures. We have been able to determine the self- and N2-broadening coefficients of 948 ν2 transitions with quantum numbers as high as J=49 and K=10. The measured self-broadening coefficients range from 0.1542 to 0.4930cm−1atm−1 and the N2-broadening coefficients range from 0.0737 to 0.1284cm−1atm−1 at 295K. The accuracy of the broadening coefficients measured in this work is between 4% and 8%, depending on the studied transition. Comparisons with measurements taken in the ν5 and ν6 bands of CH3Br did not show any clear vibrational dependence. The J and K dependences of the self- and N2-broadening coefficients have been observed and the rotational K dependence has been modeled using empirical polynomial expression. On average, the empirical expression reproduce the measured broadening coefficients to within 6%. The data obtained represent a significant contribution to the determination of broadening coefficients of CH3Br useful for atmospheric remote sensing and applications.

Accurate broadband rotational BWO-based spectroscopy

The capability, accuracy and precision of broadband spectrometers developed in the Microwave Spectroscopy Lab in Nizhniy Novgorod on a basis of backward wave oscillators (BWOs) are presented. Examples of high accuracy laboratory measurements of molecular transition frequencies, shifts, and line shapes spanning many orders of magnitude in pressure are given. The importance of sensitive broadband line measurements in the millimeter and submillimeter wavelength range to both atmospheric and astronomical observations is discussed in the context of the laboratory precision and accuracy required for their interpretation. The demands on the quality of laboratory measurements have become very stringent. The present state of broadband BWO-based spectroscopy is shown to satisfy these demands. Accurate measurements are also needed for further studies of the physics of the molecules and their interactions. The capabilities and prospects for further research and development are discussed.

Speed-dependent effects in the near-infrared spectrum of self-collidingH2O18molecules

The absorption line shape of a given ${\mathrm{H}}_{2}$$^{18}\mathrm{O}$ vibration-rotation line at 1.38 $\ensuremath{\mu}$m was deeply investigated using a spectroscopic approach based upon the use of a pair of offset-frequency locked extended-cavity diode lasers. This dual-laser apparatus ensures extreme levels of accuracy in controlling and measuring any variation of the laser frequency around a given absolute reference. As a result, high levels of precision and accuracy were reached in the observation of a molecular absorption line shape in the near infrared. A variety of semiclassical models, accounting for Dicke narrowing and speed-dependent effects, were implemented and tested in order to describe the physical situation of self-colliding ${\mathrm{H}}_{2}$$^{18}\mathrm{O}$ molecules. Our study demonstrates that the molecular confinement alone is unable to explain entirely the departures from the Voigt profile and that the speed dependence of pressure-induced broadening and shift cannot be ignored, even in the case of pure water samples at relatively small pressures. The absorption spectrum was successfully interpolated using the uncorrelated version of the speed-dependent Galatry profile, with a hypergeometric dependence on the absorber speed for both pressure broadening and pressure shift parameters, thus reaching an agreement between theory and experiment at the level of $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$.

3D simulations of M star atmosphere velocities and their influence on molecular FeH lines

<i>Context. <i/>The measurement of line broadening in cool stars is in general a difficult task. In order to detect slow rotation or weak magnetic fields, an accuracy of 1 km s<sup>-1<sup/> is needed. In this regime the broadening from convective motion becomes important. We present an investigation of the velocity fields in early to late M-type star hydrodynamic models, and we simulate their influence on molecular line shapes. The M star model parameters range between of and effective temperatures from 2500 K to 4000 K.<i>Aims. <i/>Our aim is to characterize the - and -dependence of the velocity fields and express them in terms of micro- and macro-turbulent velocities in the one dimensional sense. We present a direct comparison between 3D hydrodynamical velocity fields and 1D turbulent velocities. The velocity fields strongly affect the line shapes of , and it is our goal to give a rough estimate of the and parameter range in which 3D spectral synthesis is necessary and where 1D synthesis suffices. We want to distinguish between the velocity-broadening from convective motion and the rotational- or Zeeman-broadening in M-type stars we are planning to measure. For the latter, lines are an important indicator.<i>Methods. <i/>In order to calculate M-star structure models, we employ the 3D radiative-hydrodynamics (RHD) code CO<sup>5<sup/>BOLD. The spectral synthesis in these models is performed with the line synthesis code LINFOR3D. We describe the 3D velocity fields in terms of a Gaussian standard deviations and project them onto the line of sight to include geometrical and limb-darkening effects. The micro- and macro-turbulent velocities are determined with the “curve of growth” method and convolution with a Gaussian velocity profile, respectively. To characterize the and dependence of lines, the equivalent width, line width, and line depth are examined.<i>Results. <i/>The velocity fields in M-stars strongly depend on and . They become stronger with decreasing and increasing . The projected velocities from the 3D models agree within ~100 m s<sup>-1<sup/> with the 1D micro- and macro-turbulent velocities. The line quantities systematically depend on and .<i>Conclusions. <i/>The influence of hydrodynamic velocity fields on line shapes of M-type stars can well be reproduced with 1D broadening methods. lines turn out to provide a means to measure and in M-type stars. Since different lines all behave in a similar manner, they provide an ideal measure for rotational and magnetic broadening.