1,720,974 research outputs found
Infrared spectroscopy of particulate matter: between molecular clusters and bulk
No full textThe present contribution focuses on the vibrational dynamics of large molecular aggregates with sizes ranging from the subnanometre to the micrometre region. These aggregates or particles bridge the gap between molecular clusters and bulk matter. Depending on the kind of intermolecular interactions, the vibrational dynamics of the particles can be strongly influenced by intrinsic particle properties such as size, shape, or surface area. Such phenomena are discussed for several examples. In contrast, for some substances the vibrational spectra are mainly governed by local effects, i.e. by the interaction of neighboring molecules. The comparison of the experimental results with different model calculations leads to a deeper insight into the microscopic origin of the characteristic patterns observed. The vibrational dynamics are studied by Fourier transform infrared spectroscopy. In addition, the particle properties are characterized by microscopy and measurements of the size distribution. Four different particle generation methods are discussed. It is shown that the methods of generation can even influence the chemical composition of the particles
Aerosol spectroscopy of dihydroxyacetone: Gas phase and nanoparticles
No full textDihydroxyacetone aerosols with particle diameters around 50 nm were produced in an electrospray at ambient temperature and pressure. The vibrational spectra of the aerosols between 800 and 4000 cm(-1) were recorded with a Fourier transform infrared spectrometer. Depending on the experimental conditions, an appreciable amount of gas-phase 1,3-dihydroxy-2-propanone appeared in the infrared spectra of the particulate phase. In contrast to the gas phase, the particulate phase was found to consist of the chemically bound dimer 2,5-dihydroxymethyl-2,5-dihydroxy-1,4-dioxane. Temperature-dependent gas-phase spectra of dihydroxyacetone monomer between 310 and 440 K reveal interesting torsional dynamics, which are analyzed with the help of a detailed four-dimensional model. Experimental spectra of the nanoparticles were simulated using a Kramers-Heisenberg dielectric function. The comparison with ab initio calculations of the free molecule and small clusters leads to a first microscopic picture of the intermolecular interactions in the condensed phase
Isotope effects on vibrational excitons in carbon dioxide particles
No full textThe vibrational exciton approach is used to answer a long-standing question concerning the infrared spectra Of CO2 particles: What causes the characteristic structure of the infrared absorption bands? The CO2 particles which are generated in a collisional cooling cell at 78 K consist of many thousands of molecules. Isotopically pure ((CO2)-C-12 or (CO2)-C-13) particles as well as isotopically mixed ((CO2)-C-12/(CO2)-C-13) particles are investigated. Combining the experimental observations with exciton calculations, the band structure in the infrared spectra can be traced back to the shape of the particles. (C) 2003 Elsevier Science B.V. All rights reserved
Verification of the vibrational exciton approach for CO[sub 2] and N[sub 2]O nanoparticles
No full textISSN:0021-9606ISSN:1089-7690ISSN:1089-769
Optical properties of molecular ice particles from a microscopic model
No full textThe vibrational exciton approach is used for the first time to calculate indices of refraction for molecular ice particles (NH3 (CO2)-C-12, (CO2)-C-13, and different mixtures) in the region of strong infrared absorption bands. This model provides a microscopic explanation for the characteristic patterns found in the optical data and thus leads to propensity rules for the occurrence of pronounced shape effects in infrared spectra of molecular particles. Compared with experimental data, the calculated indices of refraction do not suffer from deficiencies due to the lack of experimental information. (C) 2003 Elsevier B.V. All rights reserved
Fermi resonance and conformation in glycolaldehyde particles
No full textThe infrared spectra of gas-phase glycolaldehyde and small glycolaldehyde particles both exhibit a split carbonyl band around 1730 cm(-1). Ab initio calculations show that this splitting can be traced back to a Fermi resonance involving the C=O-stretching band and the first overtone of the C-C-stretching band. The analysis of this resonance leads to detailed information about the conformation of glycolaldehyde in the particles. The results can be summarized as follows. (i) In the particles, monomers with OCCO dihedral angles of 0degrees and CCOH dihedral angles around 50degrees are strongly preferred. (ii) The intramolecular hydrogen bond observed in the gas phase is partly replaced by intermolecular hydrogen bonds. (iii) The calculated infrared spectrum and the results from the Fermi resonance analysis are in good agreement with the formation of long chains of monomeric glycolaldehyde. This is in contrast to the solid bulk, which consists of chemically bound dimers
IR signature of (CO2)(N) clusters: size, shape and structural effects
No full textThe structure of carbon dioxide aggregates is investigated by means of direct absorption IR specroscopy in the region of the antisymmetric stretching vibration v(3). The (CO2)(N) particles are generated under dynamic ( supersonic cooling in Laval nozzles) and static (collisional cooling cells) conditions over a broad mean size range (20 approximate to 10(5)), thus highlighting the absence of agglomeration between primary clusters under our jet conditions. This is in contrast to collisional cooling where the primary particles strongly agglomerate after a few seconds. The spectra for the larger particles ((N) over bar > 10(4)) are well reproduced by the simulations if cuboctahedral or octahedral rather than spherical aggregate shapes are assumed
The competition between hydrogen bonding and chemical change in carbohydrate nanoparticles
No full textISSN:0021-9606ISSN:1089-7690ISSN:1089-769
High-resolution photoelectron spectroscopic study of the first electronic states of Kr2+
No full textThe pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectrum of Kr-2 has been recorded between 103 500 cm(-1) and 118 000 cm(-1). Photoelectronic transitions to four [the I(1/2u), I(3/2u), II(1/2u), and II(1/2g) states] of the first six electronic states of Kr-2(+) have been observed. The photoelectronic transition to the ground I(1/2u) state consists of a long progression of vibrational bands, starting at v(+)=0. From the resolved isotopic substructure of vibrational levels with v(+)greater than or equal to 15, the absolute numbering of the vibrational quantum number could be determined. The analysis of the spectrum has led to improved values of the adiabatic ionization potential [IP(I(1/2u))=(103 773.6 +/-2.0) cm(-1)], the dissociation energy [D-0(+)(I(1/2u))=(9267.8 +/-2.8) cm(-1)] and to the determination of an analytical potential energy curve that reproduces the experimental data from v(+)=0 to beyond 81% of the dissociation energy. The transitions to vibrational levels of the I(1/2u) state with v(+)less than or equal to 30 and v(+)greater than or equal to 65 have vanishing Franck-Condon factors for direct ionization from the ground neutral state and gain intensity from transitions to low Rydberg states that belong to series converging on excited electronic states of Kr-2(+). In the region immediately below the first dissociation limit of Kr-2(+), a second progression was observed and assigned to a photoelectronic transition to the I(3/2u) state. The adiabatic ionization potential [IP(I(3/2u))=(112 672.4 +/-2.0) cm(-1)], the dissociation energy [D-0(+)(I(3/2u))=(369.1 +/-2.8) cm(-1)] and vibrational constants could be extracted for this state. Two further progressions were observed below the second dissociation limit of Kr-2(+) and assigned to transitions to the II(1/2u) and II(1/2g) states. The adiabatic ionization potentials [IP(II(1/2u))=(117 339.7 +/-2.0) cm(-1), IP(II(1/2g))=(117 802.6 +/-2.0) cm(-1)] and the dissociation energies [D-0(+)(II(1/2u))=(1071.7 +/-2.8) cm(-1), D-0(+)(II(1/2g))=(608.8 +/-2.8) cm(-1)] were determined for these two ionic states. In the region just below the ionic dissociation limits, artifact lines are observed in the PFI-ZEKE photoelectron spectra at the position of transitions to Rydberg states of the krypton monomer. At the lowest threshold, collisional and associative ionization of the long lived atomic Rydberg states leads to the formation of ZEKE electrons; at the upper threshold, the rapid autoionization of the atomic Rydberg states forms high ion concentrations, and the electrons that remain trapped in the ion cloud are released by the delayed pulsed field used to produce and extract the PFI-ZEKE electrons. (C) 2001 American Institute of Physics
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