Collisional Spectra Research Articles

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.

Cross sections for vibronic excitation of CH+ by low-energy electron impact

A theoretical approach for the electron-impact vibronic excitation of molecular ions with low-lying excited electronic states is described. In this approach, the fixed-nuclear $R$-matrix method is employed to compute electron-ion scattering matrices in the Born-Oppenheimer approximation. A vibronic frame transformation and the closed-channel elimination procedure in a spirit of molecular quantum defect theory are employed to construct an energy-dependent scattering matrix describing interactions between vibronic channels of the target ion induced by the incident electron. The obtained scattering matrix accounts for Rydberg series of vibronic resonances in the collisional spectrum. The approach is applied to the ${\mathrm{CH}}^{+}$ ion of an astrophysical and technological interest. Cross sections for vibronic excitation for different combinations of initial and final vibronic states are computed. A good agreement between electronic-excitation cross sections, obtained using the quantum defect theory and in a direct $R$-matrix calculation, demonstrates that the present approach provides a reliable tool for determination of vibronic (de-)excitation cross sections for targets with low-energy electronic resonances. Such targets were difficult to treat theoretically using earlier methods.

ISR Spectra Simulations With Electron‐Ion Coulomb Collisions

AbstractIncoherent scatter radars (ISR) rely on Thomson scattering of very high frequency or ultrahigh frequency radio waves off electrons in the ionosphere and measure the backscattered power spectra in order to estimate altitude profiles of plasma density, electron temperature, ion temperature, and ion drift speed. These spectra result from the collective behavior of coupled ion and electron dynamics, and, for most cases, existing theories predict these well. However, when the radar points nearly perpendicular to the Earth's magnetic field, the motion of the plasma across the field lines becomes complex and Coulomb collisions between electrons and ions become important in interpreting ISR measurements. This paper presents the first fully kinetic, self‐consistent, particle‐in‐cell simulations of ISR spectra with electron‐ion Coulomb collisions. We implement a grid‐based Coulomb collision algorithm in the Electrostatic Parallel Particle‐in‐Cell simulator and obtain ISR spectra from simulations both with and without collisions. For radar directions greater than 5° away from perpendicular to the magnetic field, both sets of simulations match collisionless ISR theory well. For angles between 3° and 5°, the collisional simulation is well described by a simplified Brownian motion collision process. At angles less than 3° away from perpendicular the Brownian motion model fails, and the collisional simulation qualitatively agrees with previous single particle simulations. For radar directions exactly perpendicular to the magnetic field the simulated collisional spectra match those from the Brownian motion collision theory, in agreement with previous single particle simulations.

Nonlinear properties and collisional spectra in hydrogen-(heavy) noble-gas-atom mixtures

In a series of previously published works the phenomenon of collision induced nonlinear hyper-Rayleigh scattering of light (CIHRS) was studied on the basis of the numerical quantum chemistry (QC) methods confronted with a theoretical approach. A class of super-molecular systems composed of hydrogen (H2) and lighter inert gas atoms (Rg) was considered. In this report a development of this research work is presented with more massive and highly polarizable, Kr and Xe, perturbers involved. The collision-induced hyperpolarizability tensorial values, Δβ obtained by means of the QC ab initio methods are applied in order to produce the CIHRS spectral distributions; the influence of Δβ features on the line shapes is compared with the earlier theoretical predictions. Namely, the validity of the multipole-induced-multipole (MIM) mechanism is assessed. In particular, its relevance to reproduce long-range functional behavior of the so-called symmetry adapted (SA) components of the hyperpolarizability tensor Δβ(R) is discussed. A thorough analysis of the translational CIHRS spectra is then performed to identify the role of the hyperpolarizability spatial distribution in forming particular sections of the line shapes. An extension of the dipole-induced-quadrupole analytical model is suggested and tested with regard to its ability to reproduce more accurate profiles.

Analysis of diacylglycerol molecular species in cellular lipid extracts by normal-phase LC-electrospray mass spectrometry

The quantitative determination of 48 molecular species of regioisomeric diacylglycerols has been made in a single analysis of an extract of bone marrow derived macrophages. The analytical procedure involves solvent extraction of neutral lipids, including diacylglycerols, derivatization of free hydroxyl moieties as 2,4-difluorophenyl urethane, and analysis by normal phase liquid chromatography–tandem mass spectrometry. The derivatization step not only prevents fatty acyl group migration, thus allowing determination of both 1,2- and 1,3-diacylglycerols, but also yields species that are sensitively and uniquely determined by constant neutral loss mass spectrometry. The method also detected monoacylglycerols, which were characterized by unique retention time and collisional spectra, and were present in mouse bone marrow derived macrophage extracts.

Exact Low-Order Classical Moments in Collision-Induced Bands by Linear Rotors:CO2−CO2

Exact and general analytic expressions are reported for the integrated intensity and the width of collision-induced absorption (CIA) and collision-induced scattering (CIS) bands by gases of centrosymmetric linear molecules. These expressions provide significant insight and allow assignment of partial second moments to the degrees of freedom of the colliding molecules. The expressions are applied to ambient CO(2), whose collisional spectra are reputed to be useful probes for terrestrial and planetary atmospheres. Compelling evidence of the substantial role of hitherto missing polarization and polarizability mechanisms is provided and is in remarkable agreement with experimental observation. Our findings allow the long-overdue simple interpretation of CIA and CIS by CO(2)-CO(2) without the need to resort to short-range interactions to offset the discrepancies between theory and experiment.

The Behaviour of M+. and [M+H]+ Ions of Some Oxoisoaporphines and Quinolinone Analogs

The mass spectrometric behaviour of some oxoisoaporphines and quinolinone analogs has been studied by both electrospray and electron ionization methods. By the former approach, information can be obtained on the decomposition pattern of the compounds under investigation in acidic condition, while by the latter the behaviour related to both cationic and radical character of molecular ion can be put in evidence. The collisional spectra of the protonated molecules indicate that protonation has taken place on both oxygen and nitrogen atoms. This can be justified by the fact that even if the most basic site present in the molecule is surely the N atom, in mass spectrometry conditions the protonation reactions are not governed by thermodynamics only, but kinetic effects can also play a fundamental role. Some exception to the even electron rule have been evidenced, and can be well justified by the high stability of the odd electron fragment ion. In electron ionization conditions fragmentation patterns well related to the original structures are present, allowing the characterization of isomeric compounds by the presence of specific fragmentation routes.

The hyper-Rayleigh light scattering spectrum of gaseous Ne–Ar mixture

Hyper-Rayleigh binary collisional spectra for a Ne–Ar gaseous system are computed within the frequency range up to 500 cm−1 with four new ab initio hyperpolarizability models used. The properties of the profiles as well as the depolarization ratio frequency dependence are discussed. The contributions to the spectra given by the dipolar b1(r) and the octopolar b3(r) components of the hyperpolarizability tensor and the multipolar model HR spectrum are compared.

Quantitative gas chromatography/tandem mass spectrometry using resonant and non-resonant collisions in ion traps

In ion trap mass spectrometry, collisional experiments are easily performed by precursor ion selection and software controlled operations. Collisional activation can be achieved by two different approaches, resonant and non-resonant excitation. Different results are usually obtained by the two approaches. Four related compounds of biological interest (lignin monomers), i.e. guaiacol (2-methoxyphenol), syringol (2,6-dimethoxyphenol), 4-allyl syringol (2,6-dimethoxy-4-allyl-phenol) and 4-syringaldehyde (4-hydroxy-3,5-dimethoxybenzaldehyde) have been studied by gas chromatography/tandem mass spectrometry. Sample concentration affects both single ion storage and collisional spectra of these compounds. The use of the two collisional approaches leads to different collision induced dissociation spectra of molecular species. The present work attempts to rationalize the observed behaviour. © 1998 John Wiley & Sons, Ltd.

Stereospecific decompositions induced by low‐energy collision‐induced dissociation of MH+ ions of epimeric alkaloids

AbstractSeveral studies have investigated stereochemical effects observed in the electron ionization and low‐energy collision‐induced dissociation (CID) mass spectra of molecular ions from synthetic and natural alkaloids. More recently, it has been shown that low‐collision‐energy CID spectra of protonated epimeric alkaloids can yield additional information. For instance, three epimeric tetracyclic alkaloids studied in the present work are characterized by different cis/trans ring junction geometries and by different configurations of the ethyl group linked to the C‐ring. Low‐energy collisional spectra of the epimeric MH+ ions show large differences depending both on the ring junction geometry and the stereochemistry of the side chain. Interpretation of certain specific processes invokes ion‐dipole formation, allowing internal electron transfer. Consecutive decompositions have also been scrutinized.

A hybrid model for vibrational energy transfer at the gas–solid interface: Discrete surface atoms plus a continuous elastic bulk

We introduce a discrete-continuum hybrid treatment of solid vibrations in order to describe the collisional excitation of adsorbate and defect modes by atom impacts. The inhomogeneous surface is represented by: (a) one or more atom clusters corresponding to the defect sites and their immediate neighbors, which are harmonically coupled to (b) an elastic continuous bulk. The model thus aims at reproducing the long-wavelength spectrum of the lattice as well as the high-frequency localized modes contributed by adsorbates and surface defects. The hybrid model is tested against lattice results in one-dimensional simulations that allow for analytic solution of the surface motion (which would be unfeasible for three-dimensional imperfect lattices); hybrid and lattice results are thus compared in detail under identical conditions. The model is also evaluated under the worst possible conditions for the continuum approximation, since collinear collisions correspond to three-dimensional situations in which the transferred momentum and, therefore, short-wavelength excitations are maximal. Comprehensive tests are presented for He atoms scattering from CO chemisorbed on Pt and on Ni substrates, and from N2 adsorbed on W. The scattering dynamics is treated by time-correlation functions of the transition operator previously developed for polyatomic targets [J. Chem. Phys. 84, 3162 (1987); 85, 2300 (1987); 86, 750 (1987)]. All the energetically open states are thus incorporated, as well as the thermal average over initial states, without need of basis-set expansions. Distributions of transferred energy are presented as would be observed in a one-dimensional scattering ‘‘experiment’’ using impact energies between 0.1 and 1 eV and with the solid at nonzero temperature. The collisional spectra obtained from the hybrid models are found to be in excellent agreement with those of the corresponding inhomogeneous lattices throughout wide ranges of impact velocity, surface initial temperature (including 0 K) and transferred energy. The results indicate that discrete-continuum treatments provide a powerful tool for analyzing the transfer of energy at the gas–adsorbate–solid interface.

Interaction-induced translational Raman scattering in dense krypton gas: Evidence of irreducible many-body effects.

Depolarized interaction-induced light-scattering (DILS) spectra of dense gaseous krypton have been measured at room temperature (297 K) in the pressure range 10--270 bars. The absolute values of the integrated intensity and the second spectral moment have been evaluated, at each density, from the measured spectra, by means of a calibration procedure which makes use of a mixture containing a small fraction of molecular hydrogen, whose rotational Raman lines are used as an intensity standard. Monte Carlo computer simulation has been used to evaluate the first two even spectral moments at various densities and room temperature. The simulation was performed by use of the pairwise-additive potential and polarizability. The pair potential was the one given by R. A. Aziz [Mol. Phys. 38, 177 (1979)] while models for the pair-polarizability anisotropy were derived from the low-density collisional spectrum. The comparison between theory and experiment suggests that the second spectral moment is affected by irreducible many-body terms. It is suggested that, in addition to neutron scattering, DILS can be used as an experimental tool to probe irreducible many-body properties.

The gas-phase ion-molecule reaction of η 3-allyl-PtP(C 6H 5) 3Cl with SnCl 2+.

The electron impact (EI) mass spectral behaviour of η 3-C 3H 5PtP(C 6H 5) 3Cl ( 1) and η 3-C 3H 5PtP(C 6H 5) 3SnCl 3 ( 2) is described. The collisional spectra of 2 are compared with that of addition product obtained by the gas-phase ion-molecule reaction of 1 with SnCl 2 +.. The fragmentation pathway observed for both 2 and ( 1 + SnCl 2) suggests the existence of the pentacoordinate intermediate η3-C 3H 5PtP(C 6H 5) 3SnCl 2Cl.

High-energy collisional spectroscopy in qualitative and quantitative analysis of diesel particulates

High-energy collisional spectroscopy provides a powerful analytical method for the characterization of polynuclear aromatic hydrocarbons present in diesel particulates. The absence of any need for extraction procedures and the large amount of information available from collisional spectra makes this technique of great interest. In particular, 3-methylcholanthrene can be determined from the daughter spectra of its molecular ion, with methylene iodide as internal standard.

Interaction-induced translational Raman scattering of liquid argon: The spectral moments.

The depolarized interaction-induced light scattering spectra of liquid argon have been measured, along the coexistence curve, at four different temperatures in the range 84.1--110.7 K. The experimental spectral moments have been evaluated, on an absolute scale, by means of a calibration procedure which makes use of a solution, in liquid argon, of about 1.4% hydrogen, whose rotational Raman lines are used as an intensity standard. The reproducibility of the calibration technique is very good and the present measurement fully confirms a previous experiment [D. Varshneya et al., Phys. Rev. A 23, 77 (1981)]. Molecular-dynamics computer simulation has been used to evaluate the first two even spectral moments in similar thermodynamic conditions. We used the pairwise additive potential of Barker, Fisher, and Watts and a model of the pair polarizability anisotropy which was derived from the low-density-gas collisional spectrum at room temperature. The overall agreement is good even if some refinements seem necessary. Since the effect of irreducible triple-dipole terms of both polarizability and potential is found to be smaller than the present experimental uncertainty, the small observed inconsistencies between measurement and computer simulation might be attributable to minor defects of the pair functions. More experimental data taken at different temperatures would help in refining the models.

A combined field ionization-ion kinetic energy spectrometer for multicomponent mixture analysis

A combined field ionization-collision-induced dissociation instrument has been constructed that will allow the determination of structure of individual components in a complex mixture. A high transmission mass analyzer, differentially pumped collision cell and a spherical electrostatic analyzer are employed to allow collisional spectra to be recorded using samples containing approximately 1 μg per component.