Using the van der Waals broadening of spectral atomic lines to measure the gas temperature of an argon–helium microwave plasma at atmospheric pressure

The applications of plasmas generated with gas mixtures have become increasingly common in different scientific and technological fields. In order to understand the advantages of these discharges, for instance in chemical analysis, it is necessary to know the gas temperature (T_g, kinetic energy of the heavy particles) since it has a great influence on the atomization reactions of the molecules located in the discharge, along with the dependence of the reaction rate on this parameter. The ro-vibrational emission spectra of the molecular species are usually used to measure the gas temperature of a discharge at atmospheric pressure although under some experimental conditions, these are difficult to detect. In such cases, the gas temperature can be determined from the van der Waals broadening of the emitted atomic spectral lines related to this parameter. The method proposed is based on the van der Waals broadening taking into account two perturbers.     

Laser induced fluorescence spectroscopy of van der Waals complexes of aniline with N2, H2, and CH4 formed in a supersonic free jet

The laser induced fluorescence spectra for the Ã(¹B₂)X̃(¹A₁) transition of van der Waals (vdW) complexes of aniline with N₂, H₂, and CH₄ have been observed. Based on the analysis of the rotational structure of the spectra, it is suggested that two vdW conformers exist for the N₂aniline complex though only one conformer is identified for the other complexes. In the electronically excited state of the CH₄aniline complex, energy level splittings are observed and attributed to the intramolecular rotation of CH₄.     

Inelastic neutron scattering,Raman, vibrational analysis with anharmonic corrections,and scaled quantum mechanical force field for polycrystalline l-alanine

A scaled quantum mechanical harmonic force field (SQMFF) corrected for anharmonicity is obtained for the 23 K l-alanine crystal structure using van der Waals corrected periodic boundary condition density functional theory (DFT) calculations with the PBE functional. Scale factors are obtained with comparisons to inelastic neutron scattering (INS), Raman, and FT-IR spectra of polycrystalline l-alanine at 15–23 K. Calculated frequencies for all 153 normal modes differ from observed frequencies with a standard deviation of 6 wavenumbers. Non-bonded external k = 0 lattice modes are included, but assignments to these modes are presently ambiguous. The extension of SQMFF methodology to lattice modes is new, as are the procedures used here for providing corrections for anharmonicity and van der Waals interactions in DFT calculations on crystals. First principles Born–Oppenheimer molecular dynamics (BOMD) calculations are performed on the l-alanine crystal structure at a series of classical temperatures ranging from 23 K to 600 K. Corrections for zero-point energy (ZPE) are estimated by finding the classical temperature that reproduces the mean square displacements (MSDs) measured from the diffraction data at 23 K. External k = 0 lattice motions are weakly coupled to bonded internal modes.     

A vibrational molecular force field of model compounds with biological interest—III. Harmonic dynamics of α- and β-d-galactose in the crystalline state

The infrared spectra of α- and β-d-galactose were recorded, both in the mid-IR range (4000-500 cm⁻¹) and in the far-IR (500-50 cm⁻¹). The Raman spectra were also obtained. These spectra constitute the basis of a crystalline-state force field established for these two molecules through a normal coordinate analysis. A modified Urey—Bradley—Shimanouchi force field was combined with an intermolecular potential energy function which includes van der Waals interactions, electrostatic terms and an explicit hydrogen bond function. The force constants were varied, so as to obtain an agreement between the observed vibrational frequencies and the calculated ones of α-d-galactose. The force field obtained was then applied to α-d-galactose O-d5 and β-d-galactose, in order to test its transferability. The computed potential energy distribution was found to be compatible with previous assignments for d-glucose, particularly for the modes involving C6 and COH groups. For β-d-galactose the same force field was used with changing the force constants due to the C1 and C6 groups.     

Intermolecular interactions and geometry of non-rigid molecules in weak crystalline complexes: Vibrational spectra of 4,4′-dinitrobiphenyl complexes with biphenyl,biphenyl derivatives and p-terphenyl

Raman spectroscopy is used to study the complexes of 4,4′-dinitrobiphenyl with biphenyl, 4-hydroxybiphenyl, 4-bromobiphenyl and p-terphenyl, which crystallize in a highly unusual geometry. Their phonon spectra at 125 K and 18 K are compared and the effect of isotopic substitution of biphenyl on the phonon spectra of its complex is examined. Internal vibrations of the components in the crystalline complex are compared with those observed in the pure crystals of the components. The results from both phonon and intramolecular vibration studies show that these complexes form in fixed stoichiometries, are governed by geometrical factors, and are stabilized primarily by van der Waals interaction, although other kinds of interactions may provide additional stabilization. The 4,4′-dinitrobiphenyl molecule as well as biphenyl and p-terphenyl are centrosymmetric and remain so when the complexes are cooled from room temperature to 18 K. For biphenyl complex, this conclusion is supported by the observed IR spectra which show mutual exclusion between IR-active and Raman active vibrations. Crystal splitting is observed on the 410 cm^?1 vibration of 4,4′-dinitrobiphenyl. This splitting is attributed to the presence of more than one 4,4′-dinitrobiphenyl molecules in the complex unit.     

4-N,N-dimethylaminobenzonitrile: the absence of a1 fluorescence under jet-cooled conditions

Excitation and dispersed fluorescence spectra and fluorescence lifetimes of jet-cooled 4-N,N-dimethylaminobenzonitrile (DMABN) and its van der Waals complex with methanol are reported. Neither the bare molecule nor the monosolvated complex exhibits the anomalous a fluorescence seen in polar solution. Multiple solvation of DMABN with methanol resulted in a dramatic reduction in fluorescence quantum yield which can be interpreted in terms of formation of an a state which is non-emissive under jet-cooled conditions.     

Using the van der Waals broadening of the spectral atomic lines to measure the gas temperature of an argon microwave plasma at atmospheric pressure

The ro-vibrational emission spectra of the molecular species are usually used to measure the gas temperature of a discharge at atmospheric pressure. However, under some experimental conditions, it is difficult to detect them. In order to overcome this difficulty and obtain the temperature, there are methods based on the relation between the gas temperature and the van der Waals broadening of argon atomic spectral lines with a Stark contribution negligible. In this work, we propose a method based on this relation but for lines with a Stark broadening comparable with the van der Waals one.     

Molecular structure of van der Waals complex of o- and m-methylaniline with CF3Cl, CF3H, CF4 and CH4 studied by laser induced fluorescence spectroscopy and ab initio calculations

The excitation LIF spectra of van der Waals complexes of o- and p-methylaniline and CF₃Cl, CF₃H, CH₄ and CF₄ in a supersonic free jet are reported. The spectra show a resolved structure characteristic due to the internal rotational transitions of the methyl group. The equilibrium geometries in the ground state of the complexes have been calculated at MP2/6-31+G^∗ level of calculation and the intermolecular interaction have been discussed.     

Fluorescence of glyoxal in supersonic jets

Fluorescence excitation spectra have been measured in helium supersonic jets for S₁—S₀ O⁰₀ and 8¹₀ transitions of glyoxal. The polarization of the 8¹₀ transition was determined. The corresponding bands of the glyoxal—He van der Waals complex are absent.     

Author index to volume 59

Expansion of tetracene vapour with excess argon carrier gas through a nozzle of a supersonic jet apparatus permits the controlled formation of (C₁₈H₁₂)Ar_n molecules. These have been studied by laser fluorescence techniques in the 450 nm region of the spectrum corresponding to excitation of the parent tetracene molecules to the ¹B_1u electronic state. We have been able to observe up to the n = 10 complex and have identified three different binding sites on the tetracene framework. From the resolved fluorescence spectra of these species one obtains an upper limit of the van der Waals binding energy of 314 cm^?1 in the upper electronic states and 274 cm^?1 in the ground state. These resolved spectra also exhibit features identified as due to the tetracene-argon stretching vibration giving a value of 36.5 ± 2 cm^?1 for the lowest transition in the ground state of (C₁₈H₁₂)Ar.     

How Van der Waals Interactions Influence the Absorption Spectra of Pheophorbide a Complexes: A Mixed Quantum–Classical Study

The computation of dispersive site energy shifts due to van der Waals interaction (London dispersion forces) was combined with mixed quantum–classical methodology to calculate the linear optical absorption spectra of large pheophorbide a (Pheo) dendrimers. The computed spectra agreed very well with the measurements considering three characteristic optical features occurring with increasing aggregate size: a strong line broadening, a redshift, and a low‐energy shoulder. The improved mixed quantum–classical methodology is considered a powerful tool in investigating molecular aggregates.     

Dispersion energy in rare gas dimers from an ab initio perturbative procedure: Ne + Ne,Ar + Ar

The dispersion energy between two neon and two argon atoms is computed from an ab initio perturbative procedure not based on the multipole expansion. A comparison with the multipole expansion provides C₆ = 5.36 for Ne and 76.6 for Ar. It is seen that one d polarization function provides the main part of the C₆R^?6 contribution, the exponent of this function probably being related to the polarizability of the molecule. The multipole expansion seems acceptable up to the van der Waals minimum but quite invalid for smaller distances, and doubtful at the van der Waals minimum itself.     

Application of significant structure theory of liquids to critical phenomena. Calculation of critical-point exponents

Three critical-point exponents β, δ, γ and the corresponding coefficients B, D and G are calculated using significant structure theory. The results are compared with those obtained from the van der Waals equation, Berthelot's equation and the Dieterici equation. The significant structure theory and the van der Waals equation behave similarly in the critical region. All these equations of state predict a finite C_ν at the critical point.     

Intermolecular vibronic spectroscopy of small Van Der Waals clusters: phenol- and aniline-(argon)2 complexes

We report the clear observation and assignment of the symmetric stretching and bending van der Waals modes in two three-bodyC _2v complexes, phenol- and aniline-(Ar)₂, using resonant two-photon ionization.     

The non-empirical calculation of second-order molecular properties by means of effective states. II. Effective TDCHF spectra for NO+ CO,CO2, and C2H2

Time-dependent coupled Hartree-Fock calculations have been performed in the large bases molecules NO⁺, CO. CO₂ and C₂H₂. Some first- and second-order properties are presented, in particular the isotropic dispersion interaction coefficients C₆⁰⁰⁰, C₈⁰⁰⁰ and C₁₀⁰⁰⁰ for all possible van der Waals dimers consisting of these monometers. These coefficients, and also the corresponding long-range anisotropic interaction coefficients, can be calculated easily for any of these dimers using the effective TDCHF multipole spectra presented in this paper. Formulas to this end are given.     

The rotational spectrum,nuclear spin—spin coupling,nuclear quadrupole coupling,and molecular structure of KrHF

Rotational spectra have been assigned for the ⁸²Kr, ⁸³Kr, ⁸⁴Kr, and ⁸⁶Kr isotopic species of the KrHF and KrDF van der Waals molecules by using pulsed microwave Fourier transform spectroscopy in a Fabry—Perot cavity with a pulsed supersonic nozzle molecular source. The rotational, centrifugal distortion, nuclear spin—spin, and nuclear quadrupole coupling constants are used to determine the structure and obtain intramolecular potential binding information. The ⁸³Kr nuclear quadrupole coupling constants are 10.28 ± 0.08 MHz and 13.83 ± 0.13 MHz for KrHF and KrDF respectively. The electric field gradient at the krypton nucleus is calculated from the coupling constant and the known nuclear quadrupole moment and explained by Sternheimer shielding and formation of the van der Waals bond. There is a negligible charge transfer in the KrHF bond.     

Measuring molecular force fields: Terahertz,inelastic neutron scattering,Raman, FTIR,DFT, and BOMD molecular dynamics of solid l-serine

A vibrational analysis of polycrystalline l-serine is provided using experimental terahertz, FTIR, Raman and inelastic neutron scattering (INS) spectra, calculated INS spectra – and Born–Oppenheimer molecular dynamics (BOMD) simulations from which the power spectra for the electronegative elements are compared to the THz spectra. Corrections are made to density functional theory (DFT) calculations for van der Waals interactions. Assignments and potential energy distributions are included for all 3N = 336 normal modes of an eight molecule supercell, including those for 48 non-bonded whole molecule translating and rotating vibrations, of which three are acoustic modes, usually not considered. Calculated and observed frequencies differ by an average 3 cm⁻¹ (s = 4). The INS spectrum of these modes below 100 cm⁻¹, calculated from energy second derivatives, show a remarkable similarity to the experimental 10 K spectra. The calculated low frequency modes are insensitive to small changes in cell parameters and geometry. THz intensities are represented by power spectra and not calculated explicitly. Nevertheless, power spectra of 13 ps BOMD trajectories at classical temperatures of 20 K, 400 K, and 500 K are markedly similar to the experimental terahertz spectra at 77 K and 298 K. Calculations on a serine crystal supercell 2 × 2 × 2 molecules deep appear to include, in a crude but fortuitously accurate way, enough of the principle out of phase dispersion to yield a match with experimental frequencies and intensities.     

CEPA calculations of potential energy surfaces for open-shell systems.: III. Van der Waals interaction between O(3P) and He(1S)

Quantum-chemical ab initio calculations have been performed for the van der Waals interaction between helium and oxygen atoms in their respective ground states: He(¹S)+ O(³P). As long as fine-structure effects are neglected, there are two low-lying electronic states, ³Σ^? and ³Π resulting from the degeneracy of the O(³P) ground state. Both states are purely repulsive at the SCF level, after inclusion of electronic correlation by the CEPA method they exhibit shallow van der Waals (dispersion) minima at large interatomic separation: R_? = 3.61 Å, ? = 1.0 meV (³Σ^?) and R_? = 3.05 Å, ? = 2.3 meV (³Π). The analysis of the results shows the very slow convergence of the dispersion interaction with increasing basis size, while SCF repulsion and the repulsion due to the change of the intra-atomic correlation are obtained reasonably accurately with moderate basis stes. Van der Waals coefficients C₆, C₈, C₁₀, potential curves of the type HFD (i.e. Hartree-Fock plus damped dispersion) and the influence of fine-structure effects (mainly spin-orbit coupling) on the shape of the adiabatic potential curves are discussed as well.     

Many-body response of benzene at monolayer MoS2: Van der Waals interactions and spectral broadening

Models of surface enhancement of molecular electronic response properties are challenging for two reasons: (a) molecule-surface interactions require a simultaneous solution of the molecular and the surface dynamic response (a daunting task), and (b) when solving for the electronic structure of the combined molecule + surface system, it is not trivial to single out the particular physical effects responsible for enhancement. To tackle this problem, in this work, we apply a formally exact decomposition of the system's response function into subsystem contributions by using subsystem density functional theory (DFT), which grants access to dynamic polarizabilities and optical spectra. In order to access information about the interactions between the subsystems, we extend a previously developed subsystem-based adiabatic connection fluctuation-dissipation theorem of DFT to separate the additive from the nonadditive correlation energy and identify the nonadditive correlation as the van der Waals interactions. As an example, we choose benzene adsorbed on monolayer MoS₂. We isolate the contributions to benzene's dynamic response arising from the interaction with the surface, and for the first time, we evaluate the enhancements to the effectiveness of C₆ coefficients as a function of benzene-MoS₂ distance and adsorption site. We also quantify the spectral broadening of the benzene's electronic excited states due to their interaction with the surface. We find that the broadening has a similar decay law with the molecule-surface distance as the leading van der Waals interactions (ie, R⁻⁶ ) and that the surface enhancement of dispersion interactions between benzene molecules is less than 5% but is still large enough (0.5 kcal/mol) to likely play a role in the prediction of interface morphologies.     

Perfluorination of Aromatic Compounds Reinforce Their van der Waals Interactions with Rare Gases: The Rotational Spectrum of Pentafluoropyridine-Ne

The rotational spectrum of the pentafluoropyridine-Ne complex, generated in a supersonic jet, has been investigated using chirped-pulse microwave Fourier transform spectroscopy in the 2–8 GHz range. The spectra of the ²⁰Ne and ²²Ne species have been observed, and the rotational constants have been used to determine the structure of the complex. This structure, and those of the previously experimentally studied complexes benzene-Ne and pyridine-Ne, are an excellent benchmark for the theoretical calculations on these adducts. These complexes and hexafluorobenzene-Ne have been investigated at the CCSD/6-311++G(2d,p) level. The calculations reproduce the experimental structures well and show how the van der Waals complexes are stronger for the perfluorinated compound.