Photoabsorption in sodium clusters on the basis of time-dependent density-functional theory

The photoabsorption spectra of a continuous series of Na(n) clusters (n相似文献   

Electronic State Spectroscopy of 1,4-Pentadiene As Studied by VUV Photoabsorption Spectroscopy and ab Initio Calculations

We present high resolution VUV photoabsorption spectra of 1,4-pentadiene, C(5)H(8), over the wavelength range 115-247 nm (10.8-5.0 eV). These spectra reveal several new features not previously reported in the literature. These measurements are complemented by the first ab initio calculations for the three most abundant conformational isomers of 1,4-pentadiene, C(5)H(8), which we then use in the assignment of valence and Rydberg transitions. Calculations of the two lowest energy ionic states of 1,4-pentadiene are also presented and compared with the experimental data available in the literature. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of 1,4-pentadiene in the upper stratosphere (20-50 km).     

Hydrogen bonding in methanol clusters probed by inner-shell photoabsorption spectroscopy in the carbon and oxygen K-edge regions

Hydrogen bonding in methanol clusters has been investigated by using inner-shell photoabsorption spectroscopy and density functional theory (DFT) calculations in the carbon and oxygen K-edge regions. The partial-ion-yield (PIY) curves of H(CH(3)OH)(n)(+) were measured as the soft x-ray absorption spectra of methanol clusters. The first resonance peak in the PIY curves, which is assigned to the sigma*(O-H) resonance transition, exhibits a 1.20 eV blueshift relative to the total-ion-yield (TIY) curves of molecular methanol in the oxygen K-edge region, while it exhibits a shift of only 0.25 eV in the carbon K-edge region. Decreased intensities of the transitions to higher Rydberg orbitals were observed in the PIY curves of the clusters. The drastic change in the sigma*(O-H) resonance transition is interpreted by the change in the character of the sigma*(O-H) molecular orbital at the H-donating OH site due to the hydrogen-bonding interaction.     

Inner shell excitation of SO2 by high-energy electron impact: a comparison with multichannel quantum defect theory

High-resolution electron energy loss spectra of the sulfur 2p, 2s and oxygen 1s inner shells of SO₂ have been obtained under conditions of high impact energy (1–3 keV) and zero-degree scattering angle. The results are also compared with photoabsorption spectra in the sulfur 1s and 2p regions. Multichannel quantum defect theory calculations of energies and oscillator strengths for S 2p, 2s, O 1s and S 1s are in excellent quantitative agreement with the measurements and are used to assign the spectra.     

A theoretical and experimental study of the near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectra (XPS) of nucleobases: Thymine and adenine

The core level electron excitation and ionization spectra of thymine and adenine have been investigated by photoabsorption and photoemission spectroscopy, and the results interpreted by means of ab initio calculations using the second-order algebraic–diagrammatic construction (ADC(2)) method for the polarization propagator and the fourth-order ADC method (ADC(4)) for the one-particle Green’s function. The photoabsorption spectra are dominated by transitions from core levels to unoccupied π states, but also show clear structures due to Rydberg transitions. The calculated spectra are in good agreement with the experimental results, and many of the observed structures are assigned.     

Surface and volume collective modes in small clusters and their interaction with single-particle levels

As has been recently established, collective excitations in small metal clusters exhibit some unusual features. A theoretical analysis of cluster photoabsorption spectra is presented. It is shown that small systems can support volume collective modes which, unlike in the case of bulk metals, make a noticeable contribution to photoabsorption. Furthermore, single particle-hole transitions cause splitting of resonance peaks. The theory agrees well with experimental data.     

Optical response of cesium coated C60

Photoabsorption spectra are reported for Cs _n ⁺ and C₆₀Cs_n ⁺ + clusters for n=40, 60, 120 and 310. The spectra were obtained by heating the mass selected clusters in a beam by means of photoabsorption until they evaporated metal atoms. The resulting mass loss was observed in a time-of-flight mass spectometer. The plasmon-like resonance in pure Cs clusters shifts to lower energies with decreasing cluster size. The collective electronic excitations in clusters containing C₆₀ are split in energy as would be expected for fullerene molecules coated with layers of metal.     

Investigation of surface plasma resonances in small sodium clusters

Photoabsorption cross sections of small neutral sodium clusters are measured at wavelengths from 452 to 604 nm. We find that surface plasma oscillations of the valence electrons dominate the photoabsorption spectra. The experimental results are consistent with predictions based on an extended Clemenger-Nilsson model, general sum rules, and the experimental static electric polarizabilities. For Na₈ a single broad resonance is observed, the peak wavelength is in excellent agreement with the model. The data for Na₉ and Na₁₀ support the prediction of double resonances.     

Photoabsorption spectra of H2S, CH3SH and SO2 near the sulfur K edge

Photoabsorption spectra of the isolated sulfur containing molecules H₂S, CH₃SH and SO₂ are measured at high resolution near the sulfur K edge, using synchrotron radiation. Resonances are observed in the discrete and continuum parts of the spectra. The most intense resonances are shown to result from virtual orbital effects on the basis of comparisons with the L edge photoabsorption spectra (H₂S, SO₂), the equivalent core model (SO₂) and low-energy electron scattering measurements (H₂S, SO₂).     

Core level study of alanine and threonine

Core level X-ray photoemission spectra (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectra of alanine and threonine in the gas phase have been measured at the carbon, nitrogen, and oxygen K edges and interpreted in the light of theoretical calculations. For the computations, a set of approximations is made which allows sufficiently accurate calculations of several conformers to be performed in reasonable computing time. The accuracy has been checked by comparing results obtained for proline to our previous, higher level calculations. The photoemission spectra at the carbon and oxygen edges are assigned and compared. The nitrogen 1s photoemission peaks show anomalous broadening which we relate to the populations and types of conformers. The carbon K-edge NEXAFS spectra of alanine and threonine are compared with our previous data on glycine and resonances assigned accordingly. The nitrogen K-edge NEXAFS spectra of alanine and threonine do not show measurable effects due to the population of conformers, in contrast to the photoemission results. At the oxygen K edge, the spectra of these amino acids are similar with two prominent peaks assigned to transitions of O 1s electrons from the oxo and hydroxyl groups to vacant pi* and sigma* orbitals and additional intensity for threonine due to the second OH group. Conformer effects are observable in photoemission but appear to be more difficult to resolve in photoabsorption. We explain this by energetic shifts of opposite sign for the core hole states and unoccupied orbitals, which causes partial cancelation in NEXAFS but not in photoemission.     

Study of NH stretching vibrations in small ammonia clusters by infrared spectroscopy in He droplets and ab initio calculations

Infrared spectra of the NH stretching vibrations of (NH3)n clusters (n = 2-4) have been obtained using the helium droplet isolation technique and first principles electronic structure anharmonic calculations. The measured spectra exhibit well-resolved bands, which have been assigned to the nu1, nu3, and 2nu4 modes of the ammonia fragments in the clusters. The formation of a hydrogen bond in ammonia dimers leads to an increase of the infrared intensity by about a factor of 4. In the larger clusters the infrared intensity per hydrogen bond is close to that found in dimers and approaches the value in the NH3 crystal. The intensity of the 2nu4 overtone band in the trimer and tetramer increases by a factor of 10 relative to that in the monomer and dimer, and is comparable to the intensity of the nu1 and nu3 fundamental bands in larger clusters. This indicates the onset of the strong anharmonic coupling of the 2nu4 and nu1 modes in larger clusters. The experimental assignments are compared to the ones obtained from first principles electronic structure anharmonic calculations for the dimer and trimer clusters. The anharmonic calculations were performed at the M?ller-Plesset (MP2) level of electronic structure theory and were based on a second-order perturbative evaluation of rovibrational parameters and their effects on the vibrational spectra and average structures. In general, there is excellent (<20 cm(-1)) agreement between the experimentally measured band origins for the N-H stretching frequencies and the calculated anharmonic vibrational frequencies. However, the calculations were found to overestimate the infrared intensities in clusters by about a factor of 4.     

Infrared spectroscopy of niobium oxide cluster cations in a molecular beam: identifying the cluster structures

Infrared spectra of niobium oxide cluster cations are measured via IR multiple photon dissociation spectroscopy in the 400-1650 cm(-1) region. The cluster cations are obtained directly from a laser vaporization source and irradiated with the infrared light emitted by a free electron laser. For those oxide clusters that fragment after excitation, the IR spectra are recorded by measuring the cluster intensity changes as a function of the IR wavelength. The spectra of all examined oxide clusters exhibit two main absorption features that can be assigned to vibrations of terminal (Nb=O) or bridging (Nb-O-Nb) oxide groups. For selected clusters DFT calculations at the B3LYP/LACVP* level have been performed and the calculated vibrational spectra are compared to the experimental data to identify the gas phase structures of the clusters.     

Ionization thresholds of small carbon clusters: tunable VUV experiments and theory

Small carbon clusters (Cn, n = 2-15) are produced in a molecular beam by pulsed laser vaporization and studied with vacuum ultraviolet (VUV) photoionization mass spectrometry. The required VUV radiation in the 8-12 eV range is provided by the Advanced Light Source (ALS) at the Lawrence Berkeley National Laboratory. Mass spectra at various ionization energies reveal the qualitative relative abundances of the neutral carbon clusters produced. By far the most abundant species is C3. Using the tunability of the ALS, ionization threshold spectra are recorded for the clusters up to 15 atoms in size. The ionization thresholds are compared to those measured previously with charge-transfer bracketing methods. To interpret the ionization thresholds for different cluster sizes, new ab initio calculations are carried out on the clusters for n = 4-10. Geometric structures are optimized at the CCSD(T) level with cc-pVTZ (or cc-pVDZ) basis sets, and focal point extrapolations are applied to both neutral and cation species to determine adiabatic and vertical ionization potentials. The comparison of computed and measured ionization potentials makes it possible to investigate the isomeric structures of the neutral clusters produced in this experiment. The measurements are inconclusive for the n = 4-6 species because of unquenched excited electronic states. However, the data provide evidence for the prominence of linear structures for the n = 7, 9, 11, 13 species and the presence of cyclic C10.     

Absolute dipole oscillator strengths for photoabsorption,photoionization and ionic photofragmentation processes in HBr

Absolute dipole oscillator strengths (cross section) have been obtained for valence shell photoabsorption (7–100eV) and a variety of partial photoionization (11–40 eV) processes in gaseous HBr. Partial dipole oscillator strengths are reported for the formation of the X²Π, A²Σ⁺ and B²Σ electronic state of HBr⁺ as well as the respective photoelectron branching ratios. The photoelectron binding energy spectra show clear evidence of many-body effects in photoionization to the B²Σ state of HBr⁺ with the ionization oscillator strength divided over many bands as predicted by many-body Green's function calculations. Partial dipole oscillator strengths are also reported for molecular ion formation as well as for all dissociative ionization processes. The measurements have been made by the dipole (e,e) (e,2e) and (e,e + ion) methods, which respectively provide quantitative measurements of photoabsorption, photoelectron spectroscopy and photoionization mass spectrometry at continuously tuneable energies. The measurements of dipole oscillator strengths for production of electronic states of HBr⁺ are combined with those for molecular and dissociative photoionization. These, considered together with the ionization and appearance potentials, provide a quantitative dipole breakdown picture for the ionic photofragmentation pathways of HBr in the energy region up to 40 eV.     

Ab-initio-calculations of low-lying verticalK-shell excitations in silane and mono-fluoro-silane

Multi-reference configuration interaction calculations of low-lying resonantK-shell excitations in the energy range below theK-shell ionization threshold have been performed for silane and mono-fluoro-silane. The calculated vertical transition energies are compared to experimentalK-shell photoabsorption spectra recorded by use of synchrotron radiation. The general structure and the intensity ratios of the experimental spectra are reproduced quite well by the present calculations; an assignment of the most prominent spectral features is undertaken. Correlations between the lowest dipole-allowedK-shell excitations of silane and mono-fluorosilane can be recognized.     

Microsolvation effects on the optical properties of crystal violet

We present a joint experimental and theoretical study of the photoabsorption and photodissociation behavior of crystal violet, that is, the tris[p-(dimethylamino)phenyl]methyl cation. The photodissociation spectra of isolated and microsolvated crystal violet have been measured. A single band is observed for the bare cation. This is in good agreement with the calculated vibronic absorption spectrum based on time-dependent density functional theory calculations. The interaction of crystal violet with a single water molecule shifts and broadens the photodissociation spectrum, so that it approaches the spectrum obtained in solution. Theoretical calculations of the structure of the complex suggest that the shift in the absorption spectrum originates from a water molecule bonding with the central carbon atom of crystal violet.     

Valence-shell photoabsorption by CO2 and its connections with electron-CO2 scattering

Photoabsorption cross sections and photoelectron asymmetry parameters, calculated with the multiple-scattering model (MSM), are reported for the 4σ_g, 3σ_u, 1π_u and 1π_g valence levels of CO₂. The results are discussed in the context of photoabsorption and electron energy-loss measurements and other theoretical calculations. Further comparisons are made with previously reported MSM calculations of elastic electron-CO₂ scattering. The close connection between the sets of shape resonances in the electron-scattering and photoabsorption by CO₂ is emphasized with plots of continuum eigenchannel wavefunctions for shape-resonant and non-resonant eigenchannels of σ_u symmetry.     

Structure determination of small vanadium clusters by density-functional theory in comparison with experimental far-infrared spectra

The far-infrared vibrational spectra for charged vanadium clusters with sizes of 3-15 atoms have been measured using infrared multiple photon dissociation of Vn+Ar-->Vn(+)+Ar. Using density-functional theory calculations, we calculated the ground state energy and vibrational spectra for a large number of stable and metastable geometries of such clusters. Comparison of the calculated vibrational spectra with those obtained in the experiment allows us to deduce the cluster size specific atomic structures. In several cases, a unique atomic structure can be identified, while in other cases our calculations suggest the presence of multiple isomers.     

Impurity-stimulated heterogeneous nucleation of supercooled H2 clusters

The sizes and mass spectra of large (N=1900-13,700 molecules) cold (approximately 3.1 K) H2 clusters have been measured after scattering from CO molecules. Cluster-size measurements after between 2 and 8 collisions indicate that 7% of the H2 molecules are evaporated. This loss agrees with calculations for the number of H2 molecules evaporated by the heat released in the transition from an initial liquid state to a final solid state. Even though heterogeneous nucleation is initiated after only a few collisions with CO molecules, the mass spectra show that additional captured CO molecules coagulate to form large CO clusters with up to n=11 molecules, suggesting that the outer layer is sufficiently liquidlike to facilitate mobility of the CO molecules. Since the calculated H2 cluster temperature (approximately 3.1 K) is below the superfluid transition temperature predicted for pH2 with density between 40% and 80% of the triple-point density, a shell-like region of low density near the cluster surface can be expected to be superfluid.