VUV spectroscopy

For wide class characterizations of volatile organic compounds (VOCs), conventional gas chromatography mass spectrometry (GC-MS)-based techniques are utilized. These GC-MS-based chemical identification approaches typically rely on library searches against ion fragmentation patterns of known compounds. Although MS library searches can often provide correct chemical identities, erroneous chemical assignments of structurally similar unknown compounds are also possible. Other detection systems, such as absorption spectrometers, have been used for VOC analysis and can provide complementary absorption data. Here, we demonstrate the analytical advantages of coupling vacuum ultraviolet (VUV) absorption spectroscopy and MS in tandem for the improved characterization of structurally similar VOCs. We also discuss technical considerations and limitations of coupling a VUV spectrometer to a quadrupole mass spectrometer. Moreover, we show that combining the isomer selectivity of VUV spectroscopy,...

Equilibrium geometry and electronic structures of Ca(II), Ni(II) and Zn(II) complexes with hemi-and dicarbahemiporphyrazines were determined by DFT calculations at PBE0/pcseg-2 level followed by natural bond orbital (NBO) analysis of the electron density distribution. Electronic structures of Ni(II) complexes in ground and low-lying excited electronic states were determined by complete active space (CASSCF) method with following accounting dynamic correlation by multiconfigurational quasidegenerate second-order perturbation theory (MCQDPT2). According to data obtained by MCQDPT2 method the complexes of hemi-and dicarbahemiporphyrazine possess the ground states 1 A 1 and 3 B 1 , respectively, and wave functions of the ground states have the form of a single determinant in the case of Ni(II) complex with hemiporphyrazine and the wave function for Ni(II) with dicarbahemiporphyrazine were found to possess a complex composition, therefore this complex could not be treated using single-reference DFT methods. The covalent component of metal-ligand bonding was found to increase significantly in the series: Ca(II) → Zn(II) → Ni(II). Large covalent contribution into Ni-N bonding is explained by additional LP(Np) → 3d x 2-y 2 (Ni) and LP(Ni) → 3d x 2-y 2 (Ni) interactions. The presence of agostic interactions-C-H•••Zn in the dicarbahemiporphyrazine complex was also confirmed.

The relaxation pathways of states in the energy region of the fundamental absorption of NaMgF 3 :Yb 2þ and MgF 2 : Yb 2þ are investigated using time-resolved vacuum ultraviolet spectroscopy. For NaMgF 3 :Yb 2þ , excitation into intrinsic free exciton states or above the band gap results in emission features associated with self-trapped excitons and impurity-trapped excitons. Excitation into host-related states of MgF 2 :Yb 2þ similarly exhibits selftrapped exciton emission as well as emission from Yb 2þ 4f 13 5d states. The excitation features related to Yb 2þ 4f 14 →4f 13 5d transitions are interpreted using semi-empirical crystal field models. For both materials, the interplay between intrinsic distortions and Yb impurity centres, as the excitation wavelength and sample temperature are varied, is presented and analysed.

A large amount of complicated reaction networks of intermediates and radicals taken place in the pyrolysis of 2,5-dimethylfuran have been investigated based on the density function theory (DFT) and the sophisticated wave function theory (WFT) methodologies. The preliminary focus is concentrated on those without the furan-ring opening reaction processes. Calculations are performed for electronic structures, stability and electronic spectra of ground states and excited states for those intermediates and radicals. It is found that both low-lying valence excited states and Rydberg states (3s, 3px, 3py and 3pz) of the pyrolytic species might be involved in pyrolysis and combustion of 2,5-dimethylfuran and influence their chemical reaction kinetics. A generous tendency is also found that the vertical transition energies of the similar transitions become bigger with the removal of methyl in the furan ring.

A theoretical and experimental study of the gas phase and liquid acetic acid based on resonant inelastic x-ray scattering (RIXS) spectroscopy is presented. We combine and compare different levels of theory for an isolated molecule for a comprehensive analysis, including electronic and vibrational degrees of freedom. The excitation energy scan over the oxygen K-edge absorption reveals nuclear dynamic effects in the core-excited and final electronic states. The theoretical simulations for the monomer and two different forms of the dimer are compared against high-resolution experimental data for pure liquid acetic acid. We show that the theoretical model based on a dimer describes the hydrogen bond formation in the liquid phase well and that this bond formation sufficiently alters the RIXS spectra, allowing us to trace these effects directly from the experiment. Multimode vibrational dynamics is accounted for in our simulations by using a hybrid time-dependent stationary approach for t...

Based on 280 reference vertical transition energies of various excited states (singlet, triplet, valence, Rydberg, n → π*, π → π*, and double excitations) extracted from the QUEST database, we assess the accuracy of complete-active-space third-order perturbation theory (CASPT3), in the context of molecular excited states. When one applies the disputable ionization-potential-electron-affinity (IPEA) shift, we show that CASPT3 provides a similar accuracy as its second-order counterpart, CASPT2, with the same mean absolute error of 0.11 eV. However, as already reported, we also observe that the accuracy of CASPT3 is almost insensitive to the IPEA shift, irrespective of the transition type and system size, with a small reduction in the mean absolute error to 0.09 eV when the IPEA shift is switched off.

The high resolution far-infrared spectrum of trans-butadiene has been re-investigated by Fouriertransform spectroscopy at two synchrotron radiation facilities, SOLEIL and the Canadian Light Source, at temperatures ranging from 50 to 340 K. Beyond the well-studied bands, two new fundamental bands lying below 1100 cm −1 , ν 10 and ν 24 , have been assigned using a combination of cross-correlation (ASAP software) and Loomis-Wood type (LWWa software) diagrams. While the ν 24 analysis was rather straightforward, ν 10 exhibits obvious signs of a strong perturbation, presumably owing to interaction with the dark ν 9 + ν 12 state. Effective rotational constants have been derived for both the v 10 = 1 and v 24 = 1 states. Since only one weak, infrared active fundamental band (ν 23) of trans-butadiene remains to be observed at high resolution in the far-infrared, searches for the elusive gauche conformer can now be undertaken with considerably greater confidence in the dense ro-vibrational spectrum of the trans form.

The luminescence spectra and the excitation spectra of intrinsic (7.6 and 3.77 eV) and extrinsic emissions for a-Al O , 2 3 Al O :Sc and Al O :Ga crystals have been measured by means of synchrotron radiation of 4-40 eV at 8 and 80 K. The 2 3 2 3 luminescence of self-trapped excitons (7.6 eV) can be excited at the direct photocreation of excitons or at the recombination of electrons with unrelaxed holes. Fast intraband luminescence (t,2 ns) has been detected in a spectral region from 2.5 to 6.5 eV. The absorption of one photon of 25-37 eV leads to the formation of two or three electron-hole pairs causing a sharp 31 31 1 increase of the efficiency of extrinsic emission of Sc (5.6 eV), Ga (4.6 eV) and F centers (3.85 eV).

Beryllium oxide (BeO) crystals were investigated by time-resolved low temperature VUV-spectroscopy at the SUPERLUMI station and BW3 beam line of HASYLAB (DESY, Hamburg). Photoluminescence spectra (3-10.5 eV), luminescence decay kinetics upon selective photoexcitation, as well as luminescence excitation (50-650 eV) and reflectivity (9-35 eV) spectra were measured and analyzed for oriented BeO crystals. It was shown that study of oriented crystals makes the traditional time-resolved spectroscopy method essentially more informative. Formation of the self-trapped exciton excited states of different multiplicity was found to sensitively depend on excitation energy and mutual orientation of the crystal's C optical axis and electric vector E of exciting polarized synchrotron radiation.

Fast (τ ∼ 1.7 ns) broadband (full width at half-maximum = 1.1 eV) vacuum ultraviolet (VUV; hν = 8.4 eV) luminescence from MgF 2 crystals has been detected at low temperature (T < 80 K) and analyzed with techniques of cathodoluminescence and time-resolved VUV spectroscopy using synchrotron radiation. The VUV emission discovered has been attributed to the luminescence of spin-singlet self-trapped excitons (STEs). In contrast to the case for alkaline earth fluoride crystals with fluorite structure, the luminescence of the singlet STEs in MgF 2 shows smaller Stokes shift than that of the triplet STEs, which is similar to 'typical' behavior observed for alkali halide crystals.

Yttrium trifluoride (YF 3) doped by rare-earth ions is capable of producing efficient visible emissions under vacuum UV irradiation [1,2] and may be among good candidates of efficient phosphors. This material provides possibility for doping with rare earth ions due to its wide band gap (>10 eV) and suitable Y 3+ sites where other trivalent rare-earth elements can be easily substituted without any additional charge compensation. However, the radiation resistance and formation processes of radiation defects inYF 3 under ionizing radiation (including VUV) are poorly studied so far. Also the luminescence properties of pure YF 3 need more attention. Luminescence and excitation spectra as well as their temperature dependence were investigated using pulsed synchrotron radiation at the SUPERLUMI station. The excitation spectra were recorded with spectral resolution of 0.3 nm. The excitation spectra were normalized to equal quantum intensities of synchrotron radiation falling onto sample by means of sodium salicylate as reference. Luminescence spectra in the UV and visible range were recorded with a spectrograph SpectraPro-308i (Acton Inc.) equipped with a photomultiplier (Hamamatsu R6358P). The Poueytype VUV monochromator together with a solar-blind photomultiplier (Hamamatsu R6836) were applied for the luminescence spectra detection in the VUV/UV spectral range. The spectral resolution of both analyzing monochromators was typically 11 nm. Emission spectra were not corrected for the spectral response of the detection systems. The excitation and emission spectra were recorded using time windows correlated with the excitation pulses of synchrotron radiation (delayed by δt). Namely time-integrated (TI) spectra as well as in the fast time window (FW length ∆t = 12 ns, δt = 1.8) and the slow one (SW ∆t = 95 ns, δt = 90 ns) were simultaneously recorded. The time resolution of detection systems was about 1 ns.

The luminescence spectra and the excitation spectra of intrinsic (7.6 and 3.77 eV) and extrinsic emissions for a-Al O , 2 3 Al O :Sc and Al O :Ga crystals have been measured by means of synchrotron radiation of 4-40 eV at 8 and 80 K. The 2 3 2 3 luminescence of self-trapped excitons (7.6 eV) can be excited at the direct photocreation of excitons or at the recombination of electrons with unrelaxed holes. Fast intraband luminescence (t,2 ns) has been detected in a spectral region from 2.5 to 6.5 eV. The absorption of one photon of 25-37 eV leads to the formation of two or three electron-hole pairs causing a sharp 31 31 1 increase of the efficiency of extrinsic emission of Sc (5.6 eV), Ga (4.6 eV) and F centers (3.85 eV).

The L 3 absorption edges of 82 Pb and 81 Tl compounds have been measured using DEXAFS at INDUS-2 Synchrotron radiation source at RRCAT, Indore. The energy shift of $(À6 to 2) eV are observed in preedge of 82 Pb compounds while energy shift varies $(À5 to 10) eV in pre-edge and $(2-9) eV in postedge of 81 Tl compounds. The chemical effects in both compounds have been described by calculating effective charge using Suchet and Pauling methods. A linear relation is established between chemical effect and effective charge of different compounds for the first time in literature.

The mechanism of intrinsic luminescence of excitons in crystalline Xe and Ar is treated. The band a t 9.7 eV in Ar is shown to be non-elementary. The height of the exciton self-trapping barrier in Ar is determined experimentally ( E = 3.5 meV). An experimental and theoretical study of hot luminescence of self-trapping excitons in rare gas crystals is carried out. The contribution of' hot luminescence to the total light sum increases in the series Xe-Ar-Ne.

The mechanism of intrinsic luminescence of excitons in crystalline Xe and Ar is treated. The band a t 9.7 eV in Ar is shown to be non-elementary. The height of the exciton self-trapping barrier in Ar is determined experimentally (E = 3.5 meV). An experimental and theoretical study of hot luminescence of self-trapping excitons in rare gas crystals is carried out. The contribution of' hot luminescence to the total light sum increases in the series Xe-Ar-Ne.

Vibrational relaxation of self-trapped excitons in solid xenon has been investigated both experimentally and theoretically. Hot luminescence spectra of the crystal have been measured in a wide temperature range (from liquid helium temperature to 100 K), using x-ray and two-photon excimer laser (ArF and KrF) excitation. The theoretical analysis is based on a recent nonperturbative relaxation theory and on a rigorous quantum mechanical Franck-Condon approach.

Vibrational relaxation of self-trapped excitons in solid xenon has been investigated both experimentally and theoretically. Hot luminescence spectra of the crystal have been measured in a wide temperature range (from liquid helium temperature to 100 K), using x-ray and two-photon excimer laser (ArF and KrF) excitation. The theoretical analysis is based on a recent nonperturbative relaxation theory and on a rigorous quantum mechanical Franck-Condon approach.

A uniform picture of the relaxation of the exciton states in a Xe crystal is described. Polariton theory is applied to the calculation of reflectivity, absorption, and resonant luminescence spectra in the region of p (3/2) exciton band. The value of an essential exciton-phonon coupling parameter -the deformation potential -is determined (C=1.3 eV). The nature of a new-type hot luminescence, appearing in the course of relaxation of a quasi-molecule Xe$, is discussed.

A uniform picture of the relaxation of the exciton states in a Xe crystal is described. Polariton theory is applied to the calculation of reflectivity, absorption, and resonant luminescence spectra in the region of p (3/2) exciton band. The value of an essential exciton-phonon coupling parameter-the deformation potential-is determined (C=1.3 eV). The nature of a new-type hot luminescence, appearing in the course of relaxation of a quasi-molecule Xe$, is discussed.

Experiments on permanent lattice defect formation induced by electronic excitation of Rare Gas Solids are presented. The creation of triplet excitons Γ(3/2) is found to be the primary process, followed by the exciton self-trapping. Probable models of defect formation are discussed.

A new synchrotron-based study of the vacuum ultraviolet (VUV) absorption spectrum for cyclooctatetraene (COT) shows a series of broad peaks. A significant sharp structure was extracted from the strongest band between 5.9 and 6.3 eV by fitting this range of the spectrum to a polynomial; the regular residuals show a set of sharp peaks. Comparison of this region of the VUV with the photoelectron spectrum demonstrates the presence of several Rydberg states, all based on the lowest observed ionization energy ionic state. The UV onset contains a broad band in the range 4.0 eV-5.3 eV. Theoretical vertical excitation energies, determined by configuration interaction (CI) studies at the multireference multiroot singles and doubles CI level, enabled interpretation of the principal absorption bands of the VUV spectrum. Adiabatic excitation energies (AEEs) for several singlet and triplet valence states (V) were evaluated by multiconfiguration self-consistent field methods. Theoretical Rydberg series AEEs were obtained by use of extremely diffuse Gaussian orbitals in highly correlated wave-functions. The second moments of the charge distribution identify which roots are valence or Rydberg states. A contrast was found between some density functional methods and Hartree-Fock (HF) wave-functions during single-excitation CI, when degenerate orbitals were involved in the leading configurations. The 7a 1 6e * state contained the expected 8-membered ring in the density functional theory calculations. The HF wave-functions led to a 1,5-cross-ring interaction which converged on a singlet excited state of a bicyclo[3,3,0]octatriene; this is reminiscent of the photochemical conversion of COT to semibullvalene.

We report experimental results of proton-and electron-beam-induced near-infrared (NIR) fluorescence in high-pressure Xe gas at room temperature. The investigated wavelength band spans the range 0.7 ≤ λ ≤ 1.8 µm. In the previously unexplored range for λ > 1.05 µm we have detected a broad continuum NIR fluorescence at λ ≈ 1.3 µm that shifts towards longer wavelengths as pressure is increased up to 1.5 MPa. We believe that this continuum is produced in a way similar to the VUV continua of noble gas excimers and that the pressure-dependent shift can be explained by the interaction of the outer electron of the excimer with the gas.

Absorption, emission and decay kinetics characteristics of Cs PbBr bulk and thin film samples were measured in the 4 6 UV-visible spectral range over 4.2-250 K. An emission band at 375 nm was ascribed to the Cs PbBr structure itself with 4 6 two excitation maxima round 310 and 240 nm coinciding with absorption peaks in the thin film absorption spectrum. The coexistence of Cs PbBr and CsPbBr phases was evidenced in both the bulk and thin film systems. q 1999 Elsevier 4 6 3 Science B.V. All rights reserved.

The dynamics of laser-excited sodium atoms at the surface of helium nanodroplets has been investigated as a function of quantum state. For all cases, excitation of the system leads to desorption of the sodium atom from the droplet surface. The mean kinetic energy of the desorbed atoms scales linearly with excitation frequency, indicative of an impulsive desorption process. The energy partitioning between the helium and the desorbing sodium atom depends on the quantum state and appears to be related to the size and shape of the electron orbital. The speed distributions of desorbed NaHe exciplexes point toward a direct formation process of an exciplex with no internal energy. Photoelectron spectroscopy reveals an increasing importance of helium-induced relaxation with increasing quantum state, which is tentatively attributed to curve crossing between different NaHe N interaction potentials during the desorption process.

A number of fundamental phenomena related to free and self-trapped excitons and their dynamics have been investigated in detail [1-3] using luminescence spectroscopy methods applied to Rare Gas Solids (RGS). Charge carriers, their behavior and relaxation became to the focus of studies in last years . Holes in RGS are self-trapped within 10 -12 s into a configuration resembling the configuration of ionic dimers Xe 2 + as it was predicted theoretically [5] and proved experimentally [6]. Electrons are not self-trapped in RGS, but in view of positive electron affinity of solid Xe (E a =0.5 eV) it is easy to find shallow electron traps in the lattice of nominally pure solid. Spectra of radiation recombination yield information on recombination centers (for nominally pure solid it means self-trapped holes STH), however there are only few measurements performed with nominally pure solid Xe . We carried out the spectral study of thermally stimulated recombination luminescence of the intrinsic ionic centers generated in high quality samples.

Computational studies of electronically excited states of the acetone molecule [(CH 3) 2 CO] and its fully deuterated isotopologue [(CD 3) 2 CO] are performed using the time dependent density functional (TDDFT) methodology. In addition to vertical excitation energies for singlet and triplet states, equilibrium geometries and vibrational frequencies of the n ¼ 3 Rydberg states (3s, 3p and 3d) are obtained. This is the first report of geometry optimization and frequency calculations for the 3p x , 3p z , 3d yz , 3d xy , 3d xz , 3d x2–y2 and 3d z2 Rydberg states. Results of the geometry optimization indicate that the molecule retains approximate C 2V geometry in most of these excited Rydberg states, with the most significant structural change seen in the CCO bond angle which is found to be reduced from the ground state value. Detailed comparison of the computationally predicted vibrational wavenumbers with experimental studies helps to confirm several of the earlier vibronic assignments while leading to revised/new assignments for some of the bands. The important role of hot bands in analysis of the room temperature photoabsorption spectra of acetone is corroborated by this study. While the vibrational frequencies in excited Rydberg states are overall found to be close to those of the ionic ground state, geometry optimization and vibrational frequency computation for each excited state proves to be very useful to arrive at a consistent set of vibronic assignments. Isotopic substitution helps in consolidating and confirming assignments. An offshoot of this study is the interpretation of the band at $ 8.47 eV as the π–3s Rydberg transition converging to the second ionization potential.

We report on the first measurements of the electron impact electronic excitation cross sections for C 4 F 6 isomers, hexafluoro-1,3butadiene (1,3-C 4 F 6 ), hexafluorocyclobutene (c-C 4 F 6 ), and hexafluoro-2butyne (2-C 4 F 6 ), measured at 100 eV, 3°scattering angle, while sweeping the energy loss over the range 2.0−15.0 eV. Under these experimental conditions, optically allowed transitions are favored. The electronic state spectroscopy has been investigated and the assignments supported by quantum chemical calculations. The n = 3 members of the Rydberg series have been assigned converging to the lowest ionization energy limits of the C 4 F 6 isomers and classified according to the magnitude of the quantum defects (δ).

Vibrational relaxation and hot luminescence of self-trapped excitons in Xe and Kr crystals have been studied experimentally and theoretically. Crystals were grown from the liquid in a He cryostat near the triple point under near-equilibrium conditions. Two-photon excimer laser excitation was used in luminescence experiments. The emission spectra have been recorded with the help of a double vacuum monochromator. Theoretical treatment was based on a new nonperturbative approach, which predicts abrupt acceleration of relaxation near characteristic amplitudes of the local vibration. Fully quantum-mechanical calculation of the hot luminescence spectrum of the quasi-molecular centre XeH in solid Xe has been performed and good "t with the experimental spectrum achieved.

A systematic spectroscopic study of oriented single ZnWO 4 crystals was performed using UV and synchrotron excitation. Emission and phosphorescence spectra in the energy region 1-4 eV as well as reflection spectra and excitation spectra of various emissions and phosphorescence in the energy region 3.5-32 eV were studied at 4.2-300 K using synchrotron radiation. The results are compared to those available for CdWO 4 . The peculiarities of the band structure and those of excitonic and electron-hole processes in the systems investigated are discussed. r 0168-9002/02/$ -see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 -9 0 0 2 ( 0 2 ) 0 0 7 4 0 -4

Self-trapping of excitons in rare gas solids concentrates the energy package of several electron volts within a volume of the order of a unit cell and results in a variety of atomic motions including permanent lattice defects creation and desorption of atoms and molecules from the surface . Theoretical studies of lattice defect formation induced by exciton self-trapping in solid Ne revealed that the formation of a cavity (bubble) around an atomic type self-trapped exciton or an excited impurity creates a Frenkel defects due to the Pauli repulsion of the surrounding closed-shell atoms by extended wavefunction of Rydberg electron . Subpicosecond study of bubble formation upon Rydberg state excitation in condensed rare gases [3] and theoretical and experimental investigations of structural relaxation dynamics of electronically excited XeAr N clusters showed the sophisticated picture of interaction of Rydberg electronic cloud with surrounding solvent atoms. The pressure of this cloud would be expected to induce the lattice rearrangement with plastic deformation containing one or more interstitial-vacancy pairs.

The self-trapping of excitons in Rare Gas Solids induces a variety of atomic processes including mass diffusion, desorption and point defect formation, accumulation and aggregation . Defect formation induced by electronic transitions (DFIET) was found in solid Kr at subthreshold excitation by low energy electron beam . The basis for the DFIET is a concentration of the electronic excitation energy within a volume about that a unit cell. Release of the energy can stimulate different kinds of lattice rearrangement. It can induce the displacement of the atom into the defect position in crystal lattice. On the other hand the energy release during relaxation of the molecular self-trapped excitons by relaxation jumps and an explosion-like emission of phonons induces the lability of the lattice and preirradiated defects can be annealed. But cathodoluminescence study shows that at non-selective excitation of Rare Gas Solids relatively weak processes of defect annealing were hidden by DFIET. The aim of the present study is to separate the defect annealing process from DFIET using selective excitation of molecular trapped centers by synchrotron radiation. The experiments were performed at the SUPERLUMI experimental station at HASYLAB, DESY, Hamburg.

The emission of free and self-trapped excitons in solid Kr was investigated using luminescence spectroscopy in the vacuum ultraviolet region. The influence of the sample crystal structure and Xe impurity on the shape and location of the self-trapped exciton band was observed directly using samples grown under different conditions from gases with various degrees of purity. For the first time, the luminescence spectra were measured for practically Xe-free solid Kr samples of high structural quality. New high-resolution reflection spectra of solid Kr were measured as well. Due to the good quality of the samples and excellent vacuum conditions, a new member of the exciton series (n = 5) was revealed. On the basis of high-resolution reflection measurements, more accurate values of several parameters concerning excitons in solid Kr were determined.

Valence-exciton luminescence under inner-shell excitation of the rare gas solids Xe, Kr, and Ar has been measured using time-resolved photoluminescence. Two different processes for exciton creation can be distinguished: creation of " prompt " excitons immediately after excitation (within the experimental time resolution), and creation of " delayed " excitons through electron–hole recombination. The decay structure of the exciton emission in the range of inner-shell excitation is characterized by the coexistence of the two processes. Time-resolved excitation spectra near the 2p edge in Ar, the 3d edge in Kr, and the 4d edge in Xe are discussed. The process of prompt exciton creation is strongly enhanced above an excitation threshold at the energy position of the ionization limit of the core state plus the energy of the valence free exciton.

Impurity core-valence luminescence (CVL) spectra arising under selective VUV excitation of highest-lying 5pCs core states in K 1-xCsxCl and Rb1-xCsxCl mixed crystals has been studied in a wide temperature range (7 to 330 K). The single-peaked emission band at 4.5 eV is revealed to demonstrate significant thermal broadening caused by phonon-assisted lattice-relaxation effects in vicinity of impurity core holes. A model of localized emission centres is shown to account reasonably well for the experimental observations. The influence of both concentration and lattice-relaxation effect on the emission spectrum shape in the high-and low-energy regions is shortly discussed.

Secondary exciton formation in Ar polycrystals was investigated. Using time-resolved luminescence methods the 'prompt' creation of secondary excitons was experimentally separated from delayed electron}hole recombination. The experimentally observed threshold of secondary exciton production is compared to the values provided by various theoretical approaches. Furthermore luminescence excitation spectra of Ar crystals and small Ar clusters in the threshold region of secondary excitations are compared.

Host luminescence excitation spectra and reflection spectra of PbCl2 and PbBr2 crystals at low temperatures have been measured in the energy region of 4–30 eV using synchrotron radiation. High-efficiency host luminescence is connected to radiative decay of self-trapped cation excitons. It is shown that also in the region of excitation multiplication, e.g. if the excitation energy is larger than twice the band gap energy (E > 2E g), and in the relaxation process of core excitons the radiative self-trapped cation excitons are formed. However, the recombination of hole and self-trapped electrons (band–band excitation) does not give rise to host luminescence.

Many details concerning the mechanism associated with the liberation of molecules from a liquid surface remain to be elucidated. We use the liquid microjet technique coupled with laser spectroscopy to measure the rotational and vibrational energy content of benzene spontaneously evaporating from a water-ethanol solution. These measurements provide molecular level insight into the mass and energy transfer processes associated with evaporation.

The helium atom is the simplest example of a many-electron atom. In light of the simplicity and model character of helium atoms one would naturally be interested to learn how the electronically excited states become modified when an excited helium atom is placed near one, two, or more helium atoms. A straightforward experiment to investigate this problem is to produce a beam of helium clusters of variable size and to photoexcite these clusters in the vacuum ultraviolet (VUV) spectral range using synchrotron radiation. Such an experiment probes a disordered arrangement of atoms because helium clusters and droplets in a free beam are always liquid. A molecular beam of clusters is optically thin and absorbs very little light, but helium clusters have a large fluorescence yield. Therefore, photofluorescence yield detection is the method of choice. The first experiments of 4 He clusters of variable size using a VUV fluorescence light detector identified a number of bands and it was found that the energies of these excitations, unlike the electronic excitations of heavy rare gas clusters, 1,2 did not fit to a Wannier-type excitonic series. 3 Subsequent experiments investigated fluorescence decay channels in different wavelength regions, 4,5 the effect of helium particle density, 6 fluorescence in cavities within large helium droplets, 7 and the possible Rydberg nature of the excited states of small helium clusters. 8 First quantum chemical calculations of an octahedral model cluster reported the energies of the n = 2, 3, and 4 states of the central atom as a function of the internuclear separation in the octahedron. 9 Electronic spectra simulations of a perturbed octahedron as well as an N = 25 atom-large cluster 10 were able to reproduce previously reported experimental data of small clusters. Despite these efforts, we still have an incomplete picture of the electronically excited states of large helium clusters. The number of states increases dramatically with size making the computation and interpretation of the results laborious. The availability of experimental data that cover the entire size range of helium clusters and droplets is prerequisite as a benchmark for testing theoretical interpretation and likewise prerequisite in providing evidence for empirical interpretation. Furthermore, the comprehensive data set presented in this paper shows that each helium cluster size has a specific spectral fingerprint. A unique feature of helium clusters and droplets is the 6À7 Å thick surface region where the density drops smoothly from the bulk value to zero. 11,11,12 With regard to electronically excited states, such a surface layer represents a ABSTRACT: We report a comprehensive investigation of the electronically excited states of helium clusters and droplets of sizes ranging from a few to several 10 7 atoms using time-resolved fluorescence excitation spectroscopy and quantum chemical ab initio calculations. We employ various approaches for our analysis considering the lifetime-dependence of the fluorescence intensity, spectral shifts, intensity scaling with cluster size, isotopic dependence, and density-dependence of the calculated electron wave function radii. A unique feature of helium clusters and droplets is their radially varying particle density. Our results show that short-lived fluorescence is sensitive to regions of increased density and probes excitations located in the bulk volume, whereas long-lived fluorescence is sensitive to regions of reduced density such as for small clusters or for the surface of large droplets. Spectra of 3 He droplets serve as a reference for low density, but are free from contributions of small clusters. This allows us to distinguish regions of reduced density as these can be due to both surface states or small clusters. Our analysis reveals a picture where spectral features are related to regions of different density due to isotopic composition, cluster size, and surface or bulk volume location of the excitations. The 2s and 2p related excitations appear as blueshifted wings for small clusters or for excited atoms within the surface layer, whereas in the bulk-volume of large droplets, they appear as distinct bands with large intensities, dominating the entire spectrum. Excitations at energies higher than 23 eV are unambiguously assigned to regions of low and medium density location within the deeper parts of the surface layer but show no relation to the bulk volume. Our findings support the idea that in liquid helium high-lying states and, in particular, Rydberg states are quenched in favor of the 2s and 2p excitations.

We study the adsorption of hydrogen molecules on a titanium atom supported by a benzene molecule using generalized gradient corrected Density Functional Theory (DFT). This simple system is found to bear important analogies with titanium adsorption sites in (8, 0) titanium-coated single-walled carbon nanotubes (SWNTs) (T. In particular, we show that up to four H 2 molecules can coordinate to the metal ion center, with adsorption patterns similar to those observed in Ti-SWNTs and no more than one molecule dissociating in the process. We analyze in detail the orbital interactions responsible for Ti-benzene binding and for the electron transfer responsible for the H 2 dissociation. We find the latter to involve a transition from a triplet to a singlet ground state as the hydrogen molecule approaches the adsorption site, similar to what has been observed in Ti-SWNTs. The total Ti-H 2 -binding energy for the first dissociative addition is somewhat inferior (∼0.4 eV) to the value estimated for adsorption on Ti-SWNTs. We analyze in detail the orbital interactions responsible for the H 2 binding.

The ground state properties of isodiazene have been determined by standard ab initio approaches. The vertical excitation energies of isodiazene for the low-lying valence and Rydberg excited states have been calculated by the CASSCF and related MR-CISD and MCQDPT methods. The assignment of the experimental value of 1.95 eV to the transition to the à 1A 2 (2b 2 n, 2b 1 π*) state has been confirmed. The experimentally observed absorption occuring in the range 3.96-4.88 eV, possibly at 4.77 eV, is assigned to the transition to the lowest-lying singlet Rydberg state B˜ 1B 2 (2b 2 n, 6a 1 3s). The equilibrium structures of isodiazene in the lowest-lying singlet and triplet excited states 1,3(n,π∗) are found to be twisted with the C s symmetry. It can be expected that isodiazene, as well as 1,2-diazene, has no well-defined equilibrium structures on the 1,3A ''(n,π∗) potential energy surfaces due to the low inversion barriers. The relative stabilities of isodiazene and 1,2-diazene are found to be quite different in the 1,3(n,π∗) states than in the ground state. For the vertical spectra of the two isoelectronic molecules, isodiazene and formaldehyde, it was found that the orders of singlet electronic states are similar, but the relative stabilities of triplet states are different and the vertical excitation energies of isodiazene are generally lower. The 1(n,π*)- 3(n,π∗) gap is much greater for isodiazene (1.30 eV) than for formaldehyde (0.38 eV) and hence intersystem crossing of these states of isodiazene is highly improbable.

Electronic energy bands of CsCl crystal have been calculated within the mixed basis approach with using the core Bloch states and plane waves. The calculated energy parameters of the crystal are in the satisfactory agreement with the experimental data obtained from the analysis of the core-valence luminescence spectra. The obtained results form a base for calculation of CVL spectra parameters.

The purpose of this work is to (re)interprete some spectral and thermochemical properties of 1H-phospholes. The first topic covers the assignment of absorption bands in their photoelectron and UV-vis electronic spectra. Our goal is to unravel the electronic origin of the observed bands. The second topic deals with their basicity. We investigated the site of protonation and the basicity of 1H-phospholes in both the gas phase and acidic solution. To elucidate structure-property relationships, we studied in a systematic way the influence of common substituents on the electronic structure and emphasized the corresponding consequences for the spectral and thermochemical properties. The geometric and electronic structures were investigated using adequate quantumchemical techniques including multiconfiguration SCF and density functional theory based methods.