Resolvent technique and Padé approximants in configuration interaction calculations

A perturbation approach based on resolvent technique and Padé approximants is proposed. The eigenvalue of interest is part of a solution of two nonlinear algebraic equations. The nonlinear equations are arrived at by considering two different expression of the expectation value of the resolvent of an outer projection of the Hamiltonian. The first expression is based on the spectral resolution of the resolvent, and the second one is obtained by a power series expansion analogous to that applied in the derivation of the energy expression in the Brillouin–Wigner perturbation theory. The truncated power series is extrapolated by Padé approximants of type II. The method is tested on a CI calculation of the energy of the lowest ¹Σ state of the B₂ molecule.     

Molecular analysis of water clusters: Calculation of the cluster structures and vibrational spectrum using density functional theory

Density functional theory (DFT) is applied to obtain absorption spectra at THz frequencies for molecular clusters of H₂O. The vibrational modes of the clusters are calculated. Coupling among molecular vibrational modes explains their spectral features associated with THz excitation. THz excitation is associated with vibrational frequencies which are here calculated within the DFT approximation of electronic states. This is done for both isolated molecules and collections of molecules in a cluster. The principal result of the paper is that a crystal-like cluster of 38 water molecules together with a continuum solvent background is sufficient to replicate well the experimental vibrational frequencies.     

Electron energy loss spectroscopy of van-der-Waals clusters

A method to measure electron energy loss spectra (EELS) of clusters with a high resolution (30 meV) has been developed and has been applied to some van der Waals clusters (Ar _n , Kr _n ). Structures have been found which relate to the excitation of atoms on the surface and inside the cluster. An influence of the cluster size on the spectra has been observed.     

Hyphenation of column liquid chromatography and Raman spectroscopy via a liquid-core waveguide: chemometrical elimination of spectral eluent background

In column liquid chromatography (LC) coupled to conventional Raman spectroscopy (RS) removal of the spectral background of the eluent is often demanding, because of the strong signals of the organic modifier. A new chemometrical method is proposed, called the eluent background subtraction (EBS) method, which can correct for small shape and intensity differences of the eluent spectra. The variations in the eluent spectra are modelled using principal component analysis (PCA). The PCA loading vectors are subsequently used for eluent background correction of the elution spectra of the analyte. The loading vectors are fitted under these spectra by an asymmetric least-squares method. This method was successfully applied under various experimental conditions and performed much better than conventional background correction methods. Analyte detectability was improved by (weighted) averaging of all elution spectra and smoothing via a p-spline function.     

Plasmons in fissioning metal clusters

We study the plasmon response of Na₁₈ ⁺⁺ along its two most favorable (symmetric and asymmetric) fission paths. Strong symmetry breaking and unusually large difference between the ion and electron distribution give rise to strong Landau fragmentation of the resonance spectra which has not been observed in normal configurations near the cluster ground state. This carries forth in the multiplasmon regime to a strong dissipation with large spectral noise. Furthermore, we find a low-lying resonance peak as indicator of the pre-scission stage of the electron cloud.     

A relativistic 4-component general-order multi-reference coupled cluster method: initial implementation and application to HBr

We present the initial implementation of a determinant-based general-order coupled cluster method which fully accounts for relativistic effects within the four-component framework. The method opens the way for the treatment of multi-reference problems through a state-selective expansion of the model space. The evaluation of the coupled cluster vector function is carried out via relativistic configuration interaction expansions. The implementation is based on a large-scale configuration interaction technique, which may efficiently treat long determinant expansions of more than 10⁸ terms. We demonstrate the capabilities of the new method in calculations of complete potential energy curves of the HBr molecule. The inclusion of spin–orbit interaction and higher excitations than coupled cluster double excitations, either by multi-reference model spaces or the inclusion of full iterative triple excitations, lead to highly accurate results for spectral constants of HBr. An erratum to this article can be found at     

The gas phase “matrix isolation” spectroscopy of CH3F

We report infrared photodissociation spectra for Ne, Ar, Kr, N₂ and CH₄ clusters which contain CH₃F chromophores. The CH₃Fv ₃ mode is excited with a line tunable CO₂ laser. Mass spectrometer detection of changes in the cluster beam intensity serve to partially distinguish the spectra of different size neutral clusters. Many spectra consist of rather broad, inhomogeneous profiles. For intermediate size Ar_nCH₃F clusters a sharp, narrow peak is observed in the spectrum. We assign this peak as due to a cluster in which a central CH₃F molecule is surrounded by at least a full shell of Ar atoms packed in a contracted icosohedral geometry. Because the Ar atoms in a gas phase cluster are unconstrained by an extended crystalline structure, the CH₃F dipole is more fully stabilized (and thus red-shifted) than in a solid matrix. The dependence of the observed spectrum on cluster size is discussed. For comparison, no comparable narrow spectral features are observed in Ar_nC₂H₄ cluster spectra. Clear evidence is also presented that the fragmentation of the neutral clusters upon electron impact ionization is fairly specific. Finally, we note that ionization of Ar_nCH₃F clusters sometimes produces Ar_nF⁺ ions. This is a fragmentation process which does not occur in free CH₃F.     

Generation of Supraclusters and Nanoclusters Using Laser Desorption/Ionisation Mass Spectrometry

Laser desorption/ionisation of discrete molecular clusters combined with time-of-flight (TOF) or Fourier-transform ion cyclotron resonance (FTICR) mass spectrometry affords spectra in which extensive higher mass clusters are observed. The size of the largest cluster aggregates (or supraclusters) is of the same order of magnitude as nanoclusters. The spectra obtained using TOF mass spectrometry sometimes exhibit post-source decay fragmentation, depending upon the operational conditions employed during data acquisition, which, although providing useful data on the ligand dissociation dynamics, complicate spectral interpretation. Complementary FTICR mass spectra are free of such features. The identities of the supra/nanoclusters generated from the molecular cluster precursors have not been conclusively established but are mostly coordinatively unsaturated. Density functional molecular orbital calculations have identified the possible structures of the comparatively simple electronically unsaturated system, [Ru₃(CO)₆]^–, that provides a clue to the aggregation mechanism.     

Geometries and stabilities of Ag n v (v?=?±1, 0; n?=?21–29) clusters

We performed a systematical study on the lowest-energy structures of the medium-sized silver clusters Ag _n (n?=?21?C29) by using a genetic algorithm coupled with a tight-binding method, and the DFT calculations with Perdew?CWang generalized-gradient approximation. The corresponding cluster ions were also searched based on the neutral cluster structures. It is found that the Ag_21?C23 prefer icosahedron or double-icosahedron as core structures. Ag _n (n?=?24?C27) favor a bulk-like fcc stacking motif. Ag₂₈ and Ag₂₉ tend to high symmetrical structures. The relative stabilities, the ionization potentials and electronic affinities of silver clusters analyzed in the paper are consistent with the experimental data. It is interesting to find that the experimental spectra fit reasonable well the optical absorption spectra obtained with the structures calculated by us.     

自适应拉曼光谱成像数据去噪及其在植物细胞壁光谱分析中的应用

拉曼光谱成像数据存在基线漂移与宇宙射线干扰峰两类噪声信号,无法直接用于光谱分析研究,必须去除。现有单光谱去噪方法处理结果不稳定、可重复性差。针对这一问题,本研究提出了一种自适应拉曼光谱成像数据新型去噪法,采用优化的自适应迭代惩罚最小二乘法( Adaptive iteratively reweighted penalized least-squares,airPLS)和基于主成分分析( PCA)的干扰峰消除算法修正光谱基线漂移和宇宙射线干扰峰,具有输入参数少、光谱失真小、处理速度快、去噪结果稳定等优点。利用本方法去除了芒草( Miscanthus sinensis)细胞壁拉曼光谱成像数据(9010条光谱)中的噪声信号,并对去噪后数据进行PCA和聚类分析(CA),成功区分非植物光谱与植物光谱,分类结果优于未去噪数据。预期本方法可应用于其它光谱成像数据去噪,为光谱的解译和定量分析提供可靠的研究基础。     

Thermodynamic considerations and optical emission diagnostics of a N2/O2 mixture in an inductively coupled air plasma

Composition, monatomic spectral line intensities (Cu, O, N) and enthalpy are calculated for thermal and non-thermal equilibrium conditions in a plasma composed of three plasma gas mixtures of N₂, O₂ and an aerosol of copper sulfate in water. With these mixtures, the excitation and rotational temperatures are measured in an inductively coupled plasma (ICP) torch using monatomic Cu spectral lines and CuO molecular spectra, respectively. It is shown that with the enthalpy it is possible to deduce the electronic translation temperature for other mixtures and that this ICP torch allows us to check the diatomic simulated spectra.     

A DFT Investigation of Surface‐Enhanced Raman Scattering of Adenine and 2′‐Deoxyadenosine 5′‐Monophosphate on Ag20 Nanoclusters

We present a detailed analysis of the surface‐enhanced Raman scattering (SERS) of adenine and 2′‐deoxyadenosine 5′‐monophosphate (dAMP) adsorbed on an Ag₂₀ cluster by using density functional theory. Calculated Raman spectra show that spectral features of all complexes depend greatly on adsorption sites of adenine and dAMP. The complexes consisting of adenine adsorbed on the Ag₂₀ cluster through N3 reproduce the measured SERS spectra in silver colloids, and thus demonstrated that adenine interacts with the silver surface via N3. We also investigate the SERS spectrum of adenine at the junction between two Ag₂₀ clusters and demonstrate that adenine can bind to the clusters through N3 and the external amino group, while dAMP can be adsorbed on the cluster in an end‐on orientation with the ribose and phosphate groups near to or away from the silver cluster. In contrast to the adenine–Ag₂₀ complexes, the dAMP–Ag₂₀ complexes produce new and strong bands in the low‐ or high‐wavenumber region of the Raman spectra, due to vibrations of the ribose and phosphate groups. Furthermore, the spectrum of dAMP bound to the Ag₂₀ cluster via N7 approaches the experimental SERS spectra on silver colloids.     

Diffraction and vibrational dynamics of large clusters from He atom scattering

The vibrational dynamics of large Ar _n clusters from n=30 to n=4500 is investigated by measuring the energy loss of He atoms in a high resolution scattering experiment. The clusters are generated by adiabatic expansion through conical nozzles and contain a distribution of cluster sizes. The He supersonic nozzle beam provides a resolution of better than 1 meV. The results are compared with calculated spectral density functions for single cluster sizes and bulk phonon spectra.     

A resolvent expansion theorem for the Fourier transform of the retarded Green function and its application to the absorption bandshape of chromophore aggregates

A resolvent expansion theorem is developed for the Fourier transform of the retarded Green function. The resolvent expansion theorem coupled with a decorrelation approximation for a statistical average over the vibrational sublevels of the electronic ground state is applied to a theoretical formulation of the averaged Green function of chromophore aggregates. Consequently, thermal and static effects upon the aggregate's absorption bandshape are incorporated in terms of the correction shift of the individual chromophore's bandshapes. The utility of the theoretical formulation is shown by an illustrative calculation of the absorption bandshape of a model dimer.     

Vibrational spectroscopy of gas-phase neutral and cationic phenanthrene in their electronic groundstates

Various experimental methods are applied to retrieve the vibrational structure of phenanthrene in its neutral and cationic groundstates. The linear infrared (IR) absorption spectra in the 400-1650 cm(-1) range of jet-cooled phenanthrene and its cation, both clustered with either an argon or a neon atom, are obtained via photo-induced cluster dissociation spectroscopy. The spectra observed are in good agreement with calculated spectra of the bare species. However, the observed spectrum of cationic phenanthrene shows more lines and lines with different intensities in the 900-1400 cm(-1) range than expected from calculations. Additional spectra of the perdeuterated phenanthrene Ar cation, and the warm (T approximately > room temperature) bare phenanthrene cation are recorded. Also the mass-analyzed threshold ionization spectra of bare phenanthrene and phenanthrene-Ar are recorded and compared with each other. Comparison of the spectral data recorded to calculated spectra of bare neutral, cationic and cationic perdeuterated phenanthrene, as well as to IR spectra recorded in matrix-isolation experiments, explicitly demonstrates that cluster dissociation spectroscopy is a valid and powerful method to obtain IR spectroscopic information of bare neutral and cationic jet-cooled poly-aromatic hydrocarbons.     

Blind image analysis for the compositional and structural characterization of plant cell walls

A new image analysis strategy is introduced to determine the composition and the structural characteristics of plant cell walls by combining Raman microspectroscopy and unsupervised data mining methods. The proposed method consists of three main steps: spectral preprocessing, spatial clustering of the image and finally estimation of spectral profiles of pure components and their weights. Point spectra of Raman maps of cell walls were preprocessed to remove noise and fluorescence contributions and compressed with PCA. Processed spectra were then subjected to k-means clustering to identify spatial segregations in the images. Cell wall images were reconstructed with cluster identities and each cluster was represented by the average spectrum of all the pixels in the cluster. Pure components spectra were estimated by spectral entropy minimization criteria with simulated annealing optimization. Two pure spectral estimates that represent lignin and carbohydrates were recovered and their spatial distributions were calculated. Our approach partitioned the cell walls into many sublayers, based on their composition, thus enabling composition analysis at subcellular levels. It also overcame the well known problem that native lignin spectra in lignocellulosics have high spectral overlap with contributions from cellulose and hemicelluloses, thus opening up new avenues for microanalyses of monolignol composition of native lignin and carbohydrates without chemical or mechanical extraction of the cell wall materials.     

Accurate adsorption energies for small molecules on oxide surfaces: CH4/MgO(001) and C2H6/MgO(001)

A hybrid method is applied that combines second order Møller–Plesset perturbation theory (MP2) for cluster models with density functional theory for periodic (slab) models to obtain structures and energies for methane and ethane molecules adsorbed on the MgO(001) surface. Single point calculations are performed to estimate the effect of increasing the cluster size on the MP2 energies and to evaluate the difference between coupled cluster (CCSD(T)) and MP2 energies. The final estimates of the adsorption energies are 12.9 ± 1.3 and 18.9 ± 1.8 kJ/mol for CH₄ and C₂H₆, respectively. © 2016 Wiley Periodicals, Inc.     

Development of variance‐filtered instrumental transfer methods for high‐resolution NMR spectroscopy

The introduction of a variance‐filter to both direct standardization (DS) and piece‐wise direct standardization (PDS) instrumental transfer methods for the analysis of NMR spectral data is described. The variance‐filter modification allows for the identification of regions in the NMR spectra that are not adequately represented by the limited number of transfer calibration samples used during the calculation of the instrument‐to‐instrument transfer matrix. For these spectral frequencies, the corresponding portion of the transfer matrix is replaced by identity (or scaled identity) prior to the secondary instrumental data sets being transferred to the target instrument response. The spectral matching performance of the variance‐filtered instrumental transfer method as applied to high‐resolution ¹H NMR spectra is presented along with possible uses and limitations. Copyright © 2010 John Wiley & Sons, Ltd.     

Coupled perturbation theory within the antisymmetrized product of separated geminals (APSG) framework

A coupled perturbation theory for the antisymmetrized product of separated geminals (APSG ) approximation is derived. The variational principle for the APSG wave function in an external oscillating field is employed and a set of equations of the form analogous to the normal RPA is obtained. At this level the reduced resolvent in the form of a spectral expansion is written and it is used for the evaluation of the second-order properties.     

Intramolecular Electronic Energy Transfer in a Supersonic Jet Expansion

Intramolecular electronic energy transfer (intra-EET) was investigated in an isolated bichromophoric naphthalene (N) and anthracene (A) 1:1 molecular cluster. Investigation of the spectroscopic properties of these chromophores, separately and loosely bound in a van der Waals complex, helps to understand the dependence of the EET rate on the initially excited vibronic level and on the cluster's interchromophoric orientation. Measurement of fluorescence excitation spectra of anthracene, at different anthracene pressures shows bands that can be assigned to dimers of anthracene. From measurement of the anthracene excitation spectrum at increasing naphthalene pressures one can identify other spectral features, characterized by different spectral shifts from excitations of the bare molecule. Some transitions are probably due to a 13.5 cm^?1 progression associated with an interchromophore cluster bond. Pressure dependence of fluorescence intensity gives evidence for 1:1 cluster composition, and for a slow intra-EET rate that is associated with an unfavoured orientation of the two chromophores in one of the two possible conformers of the A-N cluster, as supported by a calculation of the cluster geometry and by comparison with a recent study of intra-EET in the A-(CH₂)_n-N bichromophoric molecules.