Showing papers on "Photoionization published in 2015"
TL;DR: In this article, the rate coefficients for ionization and dissociation in the radiation fields of the quiet and the active Sun at 1 AU heliocentric distance and blackbodies at four selected temperatures in the range from T = 1000 K to 1,000,000 K without factors for radiation dilution with distance from the source were calculated.
173 citations
TL;DR: Single ionization of He by two oppositely circularly polarized, time-delayed attosecond pulses is shown to produce photoelectron momentum distributions in the polarization plane having helical vortex structures sensitive to the time delay between the pulses, their relative phase, and their handedness.
Abstract: Single ionization of He by two oppositely circularly polarized, time-delayed attosecond pulses is shown to produce photoelectron momentum distributions in the polarization plane having helical vortex structures sensitive to the time delay between the pulses, their relative phase, and their handedness Results are obtained by both ab initio numerical solution of the two-electron time-dependent Schrodinger equation and by a lowest-order perturbation theory analysis The energy, bandwidth, and temporal duration of attosecond pulses are ideal for observing these vortex patterns
134 citations
TL;DR: The results suggest that accurate molecular photoionization cross sections can be computed with a modified central potential model that accounts for the nonspherical charge distribution of the core by adjusting the charge in the center of the expansion.
Abstract: The calculation of absolute total cross sections requires accurate wave functions of the photoelectron and of the initial and final states of the system. The essential information contained in the latter two can be condensed into a Dyson orbital. We employ correlated Dyson orbitals and test approximate treatments of the photoelectron wave function, that is, plane and Coulomb waves, by comparing computed and experimental photoionization and photodetachment spectra. We find that in anions, a plane wave treatment of the photoelectron provides a good description of photodetachment spectra. For photoionization of neutral atoms or molecules with one heavy atom, the photoelectron wave function must be treated as a Coulomb wave to account for the interaction of the photoelectron with the +1 charge of the ionized core. For larger molecules, the best agreement with experiment is often achieved by using a Coulomb wave with a partial (effective) charge smaller than unity. This likely derives from the fact that the effective charge at the centroid of the Dyson orbital, which serves as the origin of the spherical wave expansion, is smaller than the total charge of a polyatomic cation. The results suggest that accurate molecular photoionization cross sections can be computed with a modified central potential model that accounts for the nonspherical charge distribution of the core by adjusting the charge in the center of the expansion.
124 citations
TL;DR: In this article, the effects of local damage in serial femtosecond crystallography (SFX), Clostridium ferredoxin was used as a model system, and the electron density maps calculated from high-dose SFX and synchrotron data showed peaks at the iron positions of the clusters, indicative of decrease of atomic scattering factors due to ionization.
Abstract: Proteins that contain metal cofactors are expected to be highly radiation sensitive since the degree of X-ray absorption correlates with the presence of high-atomic-number elements and X-ray energy. To explore the effects of local damage in serial femtosecond crystallography (SFX), Clostridium ferredoxin was used as a model system. The protein contains two [4Fe–4S] clusters that serve as sensitive probes for radiation-induced electronic and structural changes. High-dose room-temperature SFX datasets were collected at the Linac Coherent Light Source of ferredoxin microcrystals. Difference electron density maps calculated from high-dose SFX and synchrotron data show peaks at the iron positions of the clusters, indicative of decrease of atomic scattering factors due to ionization. The electron density of the two [4Fe–4S] clusters differs in the FEL data, but not in the synchrotron data. Since the clusters differ in their detailed architecture, this observation is suggestive of an influence of the molecular bonding and geometry on the atomic displacement dynamics following initial photoionization. The experiments are complemented by plasma code calculations.
108 citations
TL;DR: Tunable attosecond pulses combined with weak infrared radiation in an interferometric setup are used to measure not only the intensity but also the phase variation of the photoionization amplitude across an autoionization resonance in argon, indicating a new route towards monitoring electron correlations in time.
Abstract: We study photoionization of argon atoms close to the 3s(2)3p(6) -> 3s(1)3p(6)4p Fano resonance using an attosecond pulse train and a weak infrared probe field. An interferometric technique combined with tunable attosecond pulses allows us to determine the phase of the photoionization amplitude as a function of photon energy. We interpret the experimental results using an analytical two-photon model based on the Fano formalism and obtain quantitative agreement.
101 citations
TL;DR: Intensity studies reveal dissociative ionization as the origin of the observed PECD effect, whereas ionization of the intermediate resonance is dominating the signal.
Abstract: Photoelectron circular dichroism (PECD) is a CD effect up to the ten-percent regime and shows contributions from higher-order Legendre polynomials when multiphoton ionization is compared to single-photon ionization. We give a full account of our experimental methodology for measuring the multiphoton PECD and derive quantitative measures that we apply on camphor, fenchone and norcamphor. Different modulations and amplitudes of the contributing Legendre polynomials are observed despite the similarity in chemical structure. In addition, we study PECD for elliptically polarized light employing tomographic reconstruction methods. Intensity studies reveal dissociative ionization as the origin of the observed PECD effect, whereas ionization of the intermediate resonance is dominating the signal. As a perspective, we suggest to make use of our tomographic data as an experimental basis for a complete photoionization experiment and give a prospect of PECD as an analytic tool.
99 citations
TL;DR: An experimental and numerical study of the laser-induced damage of the surface of optical material in the femtosecond regime is presented in this paper, where the authors investigate the different processes involved as a function of the ratio of photon to bandgap energies and compare the results to models based on nonlinear ionization processes.
Abstract: An experimental and numerical study of the laser-induced damage of the surface of optical material in the femtosecond regime is presented The objective of this work is to investigate the different processes involved as a function of the ratio of photon to bandgap energies and compare the results to models based on nonlinear ionization processes Experimentally, the laser-induced damage threshold of optical materials has been studied in a range of wavelengths from 1030 nm (12 eV) to 310 nm (4 eV) with pulse durations of 100 fs with the use of an optical parametric amplifier system Semi-conductors and dielectrics materials, in bulk or thin film forms, in a range of bandgap from 1 to 10 eV have been tested in order to investigate the scaling of the femtosecond laser damage threshold with the bandgap and photon energy A model based on the Keldysh photo-ionization theory and the description of impact ionization by a multiple-rate-equation system is used to explain the dependence of laser-breakdown with the
93 citations
TL;DR: The recent upgrade of the SAPHIRS permanent photoionization end-station at the DESIRS vacuum ultraviolet beamline of synchrotron SOLEIL, whose performances have been enhanced by installing an additional double-skimmer differential chamber, is reported here.
Abstract: We report here the recent upgrade of the SAPHIRS permanent photoionization end-station at the DESIRS vacuum ultraviolet beamline of synchrotron SOLEIL, whose performances have been enhanced by installing an additional double-skimmer differential chamber. The smaller molecular beam profile obtained at the interaction region has increased the mass resolution of the double imaging photoelectron photoion coincidence (i(2)PEPICO) spectrometer, DELICIOUS III, installed in the photoionization chamber of the SAPHIRS endstation, by a factor of two, to M/ΔM ∼ 1700 (FWHM). The electron kinetic energy resolution offered by the velocity map imaging (VMI) part of the spectrometer has been improved down to 2.8% (ΔE/E) as we show on the N2 photoionization case in the double skimmer configuration. As a representative example of the overall state-of-the-art i(2)PEPICO performances, experimental results of the dissociation of state-selected O2(+)(B(2)∑(g)(-), v(+) = 0-6) molecular ions performed at the fixed photon energy of hν = 21.1 eV are presented.
87 citations
TL;DR: A novel approach for synthesizing and detecting phosphanes formed via non-classical synthesis exploiting irradiation of phosphine ices with energetic electrons, subliming the newly formed phosphanes via fractionated sublimation, and detecting these species via reflectron time-of-flight mass spectrometry (ReTOF) coupled with vacuum ultraviolet (VUV) single photon ionization.
Abstract: Isovalency rationalizes fundamental chemical properties of elements in the same group, but often fails to account for differences in the molecular structure due to the distinct atomic sizes and electron-pair repulsion of the isovalent atoms. With respect to main group V, saturated hydrides of nitrogen are limited to ammonia (NH3) and hydrazine (N2H4) along with ionic and/or metal-bound triazene (N3H5) and potentially tetrazene (N4H6). Here, we present a novel approach for synthesizing and detecting phosphanes formed via non-classical synthesis exploiting irradiation of phosphine ices with energetic electrons, subliming the newly formed phosphanes via fractionated sublimation, and detecting these species via reflectron time-of-flight mass spectrometry (ReTOF) coupled with vacuum ultraviolet (VUV) single photon ionization. This approach is able to synthesize, to separate, and to detect phosphanes as large as octaphosphane (P8H10), which far out-performs the traditional analytical tools of infrared spectroscopy and residual gas analysis via mass spectrometry coupled with electron impact ionization that could barely detect triphosphane (P3H5) thus providing an unconventional tool to prepare complex inorganic compounds such as a homologues series of phosphanes, which are difficult to synthesize via classical synthetic methods.
73 citations
TL;DR: In this paper, the effect of heating due to ambipolar diffusion and decaying magnetic turbulence on the primordial magnetic field (PMF) was studied and it was shown that changes to the ionization history computed with recfast are significantly overestimated when compared with CosmoRec.
Abstract: Primordial magnetic fields (PMF) damp at scales smaller than the photon diffusion and free-streaming scale. This leads to heating of ordinary matter (electrons and baryons), which affects both the thermal and ionization history of our Universe. Here, we study the effect of heating due to ambipolar diffusion and decaying magnetic turbulence. We find that changes to the ionization history computed with recfast are significantly overestimated when compared with CosmoRec. The main physical reason for the difference is that the photoionization coefficient has to be evaluated using the radiation temperature rather than the matter temperature. A good agreement with CosmoRec is found after changing this aspect. Using Planck 2013 data and considering only the effect of PMF-induced heating, we find an upper limit on the r.m.s. magnetic field amplitude of B0 < 1.1 nG (95% c.l.) for a stochastic background of PMF with a nearly scale-invariant power spectrum. We also discuss uncertainties related to the approximations for the heating rates and differences with respect to previous studies. Our results are important for the derivation of constraints on the PMF power spectrum obtained from measurements of the cosmic microwave background anisotropies with full-mission Planck data. They may also change some of the calculations of PMF-induced effects on the primordial chemistry and 21cm signals.
71 citations
TL;DR: Time-resolved photoelectron spectra at a pump wavelength of 217 nm are presented alongside detailed quantum dynamics calculations that suggest that population of the B1(πσ(∗)) state may occur directly when pyrrole is excited at energies in the near UV part of its electronic spectrum.
Abstract: The dynamics of pyrrole excited at wavelengths in the range 242-217 nm are studied using a combination of time-resolved photoelectron spectroscopy and wavepacket propagations performed using the multi-configurational time-dependent Hartree method. Excitation close to the origin of pyrrole's electronic spectrum, at 242 and 236 nm, is found to result in an ultrafast decay of the system from the ionization window on a single timescale of less than 20 fs. This behaviour is explained fully by assuming the system to be excited to the A2(πσ(∗)) state, in accord with previous experimental and theoretical studies. Excitation at shorter wavelengths has previously been assumed to result predominantly in population of the bright A1(ππ(∗)) and B2(ππ(∗)) states. We here present time-resolved photoelectron spectra at a pump wavelength of 217 nm alongside detailed quantum dynamics calculations that, together with a recent reinterpretation of pyrrole's electronic spectrum [S. P. Neville and G. A. Worth, J. Chem. Phys. 140, 034317 (2014)], suggest that population of the B1(πσ(∗)) state (hitherto assumed to be optically dark) may occur directly when pyrrole is excited at energies in the near UV part of its electronic spectrum. The B1(πσ(∗)) state is found to decay on a timescale of less than 20 fs by both N-H dissociation and internal conversion to the A2(πσ(∗)) state.
TL;DR: The absolute vacuum ultraviolet (VUV) photoionization spectra of the hydroperoxyl radical, hydrogen peroxide, and formaldehyde have been measured from their first ionization thresholds to 12.008 eV.
Abstract: The absolute vacuum ultraviolet (VUV) photoionization spectra of the hydroperoxyl radical (HO2), hydrogen peroxide (H2O2), and formaldehyde (H2CO) have been measured from their first ionization thresholds to 12.008 eV. HO2, H2O2, and H2CO were generated from the oxidation of methanol initiated by pulsed-laser-photolysis of Cl2 in a low-pressure slow flow reactor. Reactants, intermediates, and products were detected by time-resolved multiplexed synchrotron photoionization mass spectrometry. Absolute concentrations were obtained from the time-dependent photoion signals by modeling the kinetics of the methanol oxidation chemistry. Photoionization cross sections were determined at several photon energies relative to the cross section of methanol, which was in turn determined relative to that of propene. These measurements were used to place relative photoionization spectra of HO2, H2O2, and H2CO on an absolute scale, resulting in absolute photoionization spectra.
TL;DR: In this article, the X-shooter spectrograph was used for spectroscopy of seven weak emission line quasars (WLQs) at moderate redshifts (z = 1.4-1.7).
Abstract: Over the past 15 yr, examples of exotic radio-quiet quasars with intrinsically weak or absent broad emission line regions (BELRs) have emerged from large-scale spectroscopic sky surveys. Here, we present spectroscopy of seven such weak emission line quasars (WLQs) at moderate redshifts (z = 1.4–1.7) using the X-shooter spectrograph, which provides simultaneous optical and near-infrared spectroscopy covering the rest-frame ultraviolet (UV) through optical. These new observations effectively double the number of WLQs with spectroscopy in the optical rest-frame, and they allow us to compare the strengths of (weak) high-ionization emission lines (e.g., C iv) to low-ionization lines (e.g., Mg ii, Hβ, Hα) in individual objects. We detect broad Hβ and Hα emission in all objects, and these lines are generally toward the weaker end of the distribution expected for typical quasars (e.g., Hβ has rest-frame equivalent widths ranging from 15–40 Å). However, these low-ionization lines are not exceptionally weak, as is the case for high-ionization lines in WLQs. The X-shooter spectra also display relatively strong optical Fe ii emission, Hβ FWHM ≲ 4000 km s−1, and significant C iv blueshifts (≈1000–5500 km s−1) relative to the systemic redshift; two spectra also show elevated UV Fe ii emission, and an outflowing component to their (weak) Mg ii emission lines. These properties suggest that WLQs are exotic versions of “wind-dominated” quasars. Their BELRs either have unusual high-ionization components, or their BELRs are in an atypical photoionization state because of an unusually soft continuum.
TL;DR: The key finding is that the aqueous medium is remarkably efficient in screening the interactions within DNA such that, unlike in the gas phase, ionization of a base, nucleoside, or nucleotide depends only very weakly on the particular DNA context.
Abstract: Radiation damage to DNA is usually considered in terms of UVA and UVB radiation. These ultraviolet rays, which are part of the solar spectrum, can indeed cause chemical lesions in DNA, triggered by photoexcitation particularly in the UVB range. Damage can, however, be also caused by higher energy radiation, which can ionize directly the DNA or its immediate surroundings, leading to indirect damage. Thanks to absorption in the atmosphere, the intensity of such ionizing radiation is negligible in the solar spectrum at the surface of Earth. Nevertheless, such an ionizing scenario can become dangerously plausible for astronauts or flight personnel, as well as for persons present at nuclear power plant accidents. On the beneficial side, ionizing radiation is employed as means for destroying the DNA of cancer cells during radiation therapy. Quantitative information about ionization of DNA and its components is important not only for DNA radiation damage, but also for understanding redox properties of DNA in redox sensing or labeling, as well as charge migration along the double helix in nanoelectronics applications. Until recently, the vast majority of experimental and computational data on DNA ionization was pertinent to its components in the gas phase, which is far from its native aqueous environment. The situation has, however, changed for the better due to the advent of photoelectron spectroscopy in liquid microjets and its most recent application to photoionization of aqueous nucleosides, nucleotides, and larger DNA fragments. Here, we present a consistent and efficient computational methodology, which allows to accurately evaluate ionization energies and model photoelectron spectra of aqueous DNA and its individual components. After careful benchmarking, the method based on density functional theory and its time-dependent variant with properly chosen hybrid functionals and polarizable continuum solvent model provides ionization energies with accuracy of 0.2-0.3 eV, allowing for faithful modeling and interpretation of DNA photoionization. The key finding is that the aqueous medium is remarkably efficient in screening the interactions within DNA such that, unlike in the gas phase, ionization of a base, nucleoside, or nucleotide depends only very weakly on the particular DNA context. An exception is the electronic interaction between neighboring bases which can lead to sequence-specific effects, such as a partial delocalization of the cationic hole upon ionization enabled by presence of adjacent bases of the same type.
TL;DR: A microwave discharge flow tube coupled with a double imaging electron/ion coincidence device and vacuum ultraviolet (VUV) synchrotron radiation provides a high selectivity and yields the spectra of the pure radicals, removing the ever-present contributions from excess reactants, background, or secondary products, and therefore obviating the need for a prior knowledge of all possible byproducts.
Abstract: We present a microwave discharge flow tube coupled with a double imaging electron/ion coincidence device and vacuum ultraviolet (VUV) synchrotron radiation. The system has been applied to the study of the photoelectron spectroscopy of the well-known radicals OH and OD. The coincidence imaging scheme provides a high selectivity and yields the spectra of the pure radicals, removing the ever-present contributions from excess reactants, background, or secondary products, and therefore obviating the need for a prior knowledge of all possible byproducts. The photoelectron spectra encompassing the X3Σ− ground state of the OH+ and OD+ cations have been extracted and the vibrational constants compared satisfactorily to existing literature values. Future advantages of this approach include measurement of high resolution VUV spectroscopy of radicals, their absolute photoionization cross section, and species/isomer identification in chemical reactions as a function of time.
TL;DR: In this article, a nine-level model of C I-C III with the transitions treated assuming complete frequency redistribution (CRD) suffices to describe the 133.5 nm lines.
Abstract: The 133.5 nm lines are important observables for the NASA/SMEX mission Interface Region Imaging Spectrograph (IRIS). To make 3D non-LTE radiative transfer computationally feasible it is crucial to have a model atom with as few levels as possible while retaining the main physical processes. We here develop such a model atom and we study the general formation properties of the C II lines. We find that a nine-level model atom of C I-C III with the transitions treated assuming complete frequency redistribution (CRD) suffices to describe the 133.5 nm lines. 3D scattering effects are important for the intensity in the core of the line. The lines are formed in the optically thick regime. The core intensity is formed in layers where the temperature is about 10kK at the base of the transition region. The lines are 1.2-4 times wider than the atomic absorption profile due to the formation in the optically thick regime. The smaller opacity broadening happens for single peak intensity profiles where the chromospheric temperature is low with a steep source function increase into the transition region, the larger broadening happens when there is a temperature increase from the photosphere to the low chromosphere leading to a local source function maximum and a double peak intensity profile with a central reversal. Assuming optically thin formation with the standard coronal approximation leads to several errors: Neglecting photoionization severly underestimates the amount of C II at temperatures below 16kK, erroneously shifts the formation from 10kK to 25kK and leads to too low intensities.
TL;DR: It is demonstrated that it is possible to experimentally select distinct molecular-fragmentation pathways in which the core hole can be considered as either localized on one sulfur atom or delocalized between two indistinguishable sulfur atoms.
Abstract: Electronic core levels in molecules are highly localized around one atomic site. However, in single-photon ionization of symmetric molecules, the question of core-hole localization versus delocalization over two equivalent atoms has long been debated as the answer lies at the heart of quantum mechanics. Here, using a joint experimental and theoretical study of core-ionized carbon disulfide (CS2), we demonstrate that it is possible to experimentally select distinct molecular-fragmentation pathways in which the core hole can be considered as either localized on one sulfur atom or delocalized between two indistinguishable sulfur atoms. This feat is accomplished by measuring photoelectron angular distributions within the frame of the molecule, directly probing entanglement or disentanglement of quantum pathways as a function of how the molecule dissociates.
TL;DR: Three-dimensional shapes and their variation with laser intensity can be interpreted in terms of ionization from the highest occupied molecular orbital (HOMO) and lower lying orbitals, and the Dyson orbitals for the ground and excited states of the cations.
Abstract: The angle-dependence of strong field ionization of O2, N2, CO2, and CH2O has been studied theoretically using a time-dependent configuration interaction approach with a complex absorbing potential (TDCIS-CAP). Calculation of the ionization yields as a function of the direction of polarization of the laser pulse produces three-dimensional surfaces of the angle-dependent ionization probability. These three-dimensional shapes and their variation with laser intensity can be interpreted in terms of ionization from the highest occupied molecular orbital (HOMO) and lower lying orbitals, and the Dyson orbitals for the ground and excited states of the cations.
TL;DR: In this paper, the potential of the C60 shell of a C60 atom has been calculated in the framework of the self-consistent spherical jellium model, where the parameters of the model Lorentz-bubble potential (depth and thickness) have been selected so that in the potential well there would be an electronic level corresponding to the experimental electron affinity of the c60 molecule.
Abstract: Approximating the C60 shell as a collection of carbon atoms, the potential experienced by a confined atom has been calculated within the framework of the self-consistent spherical jellium model. It has been found that the potential well in this model has a cusp-shaped Lorentz-like profile. The parameters of the model Lorentz-bubble potential (depth and thickness) have been selected so that in the potential well there would be an electronic level corresponding to the experimental electron affinity of the C60 molecule. The spatial distribution of the positive charge of the C-atomic nuclei and the negative charge of the electron clouds forming the electrostatic potential of C60, as a whole, has been analyzed using the Poisson equation. It is demonstrated that the often used radial square-well potential to approximate the C60 corresponds to a non-physical charge density for the C60 molecule. This analysis demonstrates that the phenomenological potentials simulating the C60 shell potential should belong to a family of potentials with a non-flat bottom and non-parallel potential walls similar to the Lorentz-bubble potential. The photoionization cross-sections of a hydrogen atom localized at the center of the C60 shell have been calculated as well. It is found that confinement oscillations in the cross-sections are exhibited within the framework of the cusp-shaped potential model and these oscillations are essentially the same as those in the case of the potential wells with well-defined borders (parallel walls), thereby demonstrating that the inherent characteristic distances of the potential, e.g., radii of the potential walls, or the distances between potential walls, are not necessary to produce confinement resonances; this should be a general result for atoms or molecules confined in near-spherical fullerenes.
TL;DR: Single, double, and triple ionization of C(1+) ions by single photons is investigated in the energy range of 286-326 eV, i.e., in the range from the lowest-energy K-vacancy resonances to well beyond the K-shell ionization threshold.
Abstract: Using a new photon-ion merged-beam setup at PETRA III, DESY, resonant Auger decay in which three electrons are emitted simultaneously is observed in carbon ions.
01 Jan 2015
TL;DR: In this article, the authors combined four different MBMS spectrometers (in Bielefeld, Germany, the Advanced Light Source in Berkeley, USA, the Swiss light source in Villigen, Switzerland, and the SOLEIL synchrotron in St. Aubin, France) to study a rich premixed argon-diluted low-pressure (40mbar) ethylene-oxygen flame under comparable conditions.
Abstract: Quantitative species data for the development and critical examination of combustion mechanisms are in high demand regarding the need for predictive combustion models that may assess the emission potential of current and emerging fuels. Mass spectrometric investigation is one of the often-used techniques to provide mole fractions of stable and reactive intermediates including radicals from specifically designed laboratory experiments. Molecular-beam mass spectrometry (MBMS) has been coupled with electron ionization (EI) and photoionization (PI) to determine the species compositions, and combinations of these techniques have been successful in the investigation of the combustion pathways in flames of numerous hydrocarbon, oxygenated and nitrogenated fuels. Photoelectron/photoion coincidence spectroscopy (PEPICO) has recently emerged as a novel diagnostics to be combined with flame-sampling mass spectrometry, and its potential as a complement of existing techniques is just about being explored. In a multi-laboratory investigation, the present study has thus combined four different MBMS spectrometers (in Bielefeld, Germany, the Advanced Light Source in Berkeley, USA, the Swiss Light Source in Villigen, Switzerland, and the SOLEIL synchrotron in St. Aubin, France) to study a rich premixed argon-diluted low-pressure (40 mbar) ethylene–oxygen flame under comparable conditions. This was done with the aim of illustrating the respective properties and capabilities of the methods under these conditions, with an emphasis on the power offered by the synchrotron-based techniques, including PEPICO, for combustion chemistry studies. Examples include comparisons of selected species quantification as well as PEPICO spectra measured at different instruments.
01 Jan 2015
TL;DR: In this article, the authors combined experimental and theoretical approaches to gain new insight into the mechanisms of PAH growth and soot formation in a C 2 H 2 /O 2 counterflow diffusion flame at nearly atmospheric pressure.
Abstract: We have combined experimental and theoretical approaches to gain new insight into the mechanisms of PAH growth and soot formation. The experimental approach involves aerosol-mass spectrometry in conjunction with vacuum-ultraviolet photoionization of volatile species vaporizing from particles sampled from an Ar-diluted C 2 H 2 /O 2 counter-flow diffusion flame at nearly atmospheric pressure (700 Torr). We recorded aerosol mass spectra at different distances from the fuel outlet for fixed ionization energies and in a fixed position while tuning the photoionization energy. The mass spectra contain a large distribution of peaks, highlighting the importance of small building blocks and showing a variety of chemical species that extends beyond the traditional classification of PAHs based on thermodynamic stability. In addition, we performed stochastic simulations of PAH growth in the flame in order to provide better insight into the chemical composition of species associated with peaks in the measured mass spectra. These simulations were conducted using a stochastic nanoparticle simulator (SNAPS). Synthesis of experimental and simulated results showed that peaks in the observed mass spectra generally consisted of a mixture of PAH isomers. At m / z = 154 and 202, for example, experiments and simulations suggested that additional isomers than biphenyl and pyrene are important. Furthermore, the results highlight the importance of odd-carbon numbered species and complex growth paths. The experimental results suggest that species of higher masses can build up concentration ahead of species of lower masses. Our experimental results show, for example, that the peak at m / z = 278 appears closer to the burner outlet than the peak at m / z = 202, i.e., suggesting that a single monotonic growth mechanism is not enough.
TL;DR: The PLUTO-CLOUDY Interface (TPCI) as discussed by the authors is an interface between the (magneto-) hydrodynamics code (PLUTO) and the spectral synthesis code (SCS) to simulate photoevaporative flows under strong irradiation.
Abstract: We present an interface between the (magneto-) hydrodynamics code PLUTO and the plasma simulation and spectral synthesis code CLOUDY. By combining these codes, we constructed a new photoionization hydrodynamics solver: The PLUTO-CLOUDY Interface (TPCI), which is well suited to simulate photoevaporative flows under strong irradiation. The code includes the electromagnetic spectrum from X-rays to the radio range and solves the photoionization and chemical network of the 30 lightest elements. TPCI follows an iterative numerical scheme: First, the equilibrium state of the medium is solved for a given radiation field by CLOUDY, resulting in a net radiative heating or cooling. In the second step, the latter influences the (magneto-) hydrodynamic evolution calculated by PLUTO. Here, we validated the one-dimensional version of the code on the basis of four test problems: Photoevaporation of a cool hydrogen cloud, cooling of coronal plasma, formation of a Stroemgren sphere, and the evaporating atmosphere of a hot Jupiter. This combination of an equilibrium photoionization solver with a general MHD code provides an advanced simulation tool applicable to a variety of astrophysical problems.
TL;DR: In this article, a hybrid anti-symmetrized coupled channels method for the calculation of fully differential photoelectron spectra of multi-electron atoms and small molecules interacting with strong laser fields is presented.
Abstract: We present the hybrid anti-symmetrized coupled channels method for the calculation of fully differential photo-electron spectra of multi-electron atoms and small molecules interacting with strong laser fields. The method unites quantum chemical few-body electronic structure with strong-field dynamics by solving the time dependent Schrodinger equation in a fully anti-symmetrized basis composed of multi-electron states from quantum chemistry and a one-electron numerical basis. Photoelectron spectra are obtained via the time dependent surface flux (tSURFF) method. Performance and accuracy of the approach are demonstrated for spectra from the helium and beryllium atoms and the hydrogen molecule in linearly polarized laser fields at wavelengths from 21 to 400 nm. At long wavelengths, helium and the hydrogen molecule at equilibrium inter-nuclear distance can be approximated as single channel systems whereas beryllium needs a multi-channel description.
TL;DR: Two nitrogen-containing polycyclic aromatic hydrocarbon isomers of C9H7N composition, quinoline, and isoquinoline have been studied by imaging photoelectron photoion coincidence spectroscopy at the VUV beamline of the Swiss Light Source and implications for the formation and destruction of nitrogenated PAHs in the interstellar medium and in Titan's atmosphere are highlighted.
Abstract: Two nitrogen-containing polycyclic aromatic hydrocarbon isomers of C9H7N composition, quinoline, and isoquinoline have been studied by imaging photoelectron photoion coincidence spectroscopy at the VUV beamline of the Swiss Light Source. High resolution threshold photoelectron spectra have been recorded and are interpreted applying a Franck–Condon model. Dissociative ionization mass spectra as a function of the parent ion internal energy are analyzed with the use of breakdown diagrams. HCN loss and H loss are the dominant dissociation paths for both C9H7N•+ isomers at photon energies below 15.5 eV. Computed C9H7N•+ potential energy surfaces suggest that the lowest energy path leading to HCN-loss yields the benzocyclobutadiene cation. A statistical model is used to fit the breakdown diagram and—to account for the kinetic shift—the time-of-flight mass spectra that reveal the dissociation rates. We have derived appearance energies of 11.9 ± 0.1 (HCN loss) and 12.0 ± 0.1 (H loss), as well as 11.6 ± 0.2 (HCN l...
TL;DR: The restricted active space self-consistent field method including spin-orbit coupling is used to cope with this challenge and to calculate valence- and core-level photoelectron spectra and shows good agreement with the experimental data obtained.
Abstract: X-ray photoelectron spectra provide a wealth of information on the electronic structure. The extraction of molecular details requires adequate theoretical methods, which in case of transition metal complexes has to account for effects due to the multi-configurational and spin-mixed nature of the many-electron wave function. Here, the restricted active space self-consistent field method including spin-orbit coupling is used to cope with this challenge and to calculate valence- and core-level photoelectron spectra. The intensities are estimated within the frameworks of the Dyson orbital formalism and the sudden approximation. Thereby, we utilize an efficient computational algorithm that is based on a biorthonormal basis transformation. The approach is applied to the valence photoionization of the gas phase water molecule and to the core ionization spectrum of the [Fe(H2O)6]2+ complex. The results show good agreement with the experimental data obtained in this work, whereas the sudden approximation demonstrates distinct deviations from experiments.
TL;DR: An automated, miniature, high-repetition-rate shock tube was developed and can be used to study high-pressure reactive systems behind reflected shock waves, and it is anticipated that this high pressure technique will greatly complement those lower pressure techniques.
Abstract: Tunable synchrotron-sourced photoionization time-of-flight mass spectrometry (PI-TOF-MS) is an important technique in combustion chemistry, complementing lab-scale electron impact and laser photoionization studies for a wide variety of reactors, typically at low pressure. For high-temperature and high-pressure chemical kinetics studies, the shock tube is the reactor of choice. Extending the benefits of shock tube/TOF-MS research to include synchrotron sourced PI-TOF-MS required a radical reconception of the shock tube. An automated, miniature, high-repetition-rate shock tube was developed and can be used to study high-pressure reactive systems (T > 600 K, P < 100 bar) behind reflected shock waves. In this paper, we present results of a PI-TOF-MS study at the Advanced Light Source at Lawrence Berkeley National Laboratory. Dimethyl ether pyrolysis (2% CH3OCH3/Ar) was observed behind the reflected shock (1400 < T5 < 1700 K, 3 < P5 < 16 bar) with ionization energies between 10 and 13 eV. Individual experiment...
01 Jan 2015
TL;DR: In this article, a kinetic model was developed from previous models of aromatic fuels to simulate the decomposition of benzene and formation of large aromatics, and was validated against the measured mole fraction profiles of flame species.
Abstract: Chemical structures of six laminar premixed benzene/oxygen/argon flames with equivalence ratios from 0.75 to 2.0 were investigated at 30 Torr. Dozens of flame species, especially radicals and large aromatics, were qualitatively identified and quantitatively measured using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). A kinetic model was developed from our previous models of aromatic fuels to simulate the decomposition of benzene and formation of large aromatics, and was validated against the measured mole fraction profiles of flame species. Based on the rate of production analysis in the leanest and richest flames, the decomposition processes of benzene and the fate of C6 and smaller products were discussed in detail. Benzene, phenyl radical and cyclopentadienyl radical are concluded as key precursors of large aromatics, and the low formation of PAHs in rich benzene flames compared with rich alkylbenzene flames mainly derives from the low production of benzyl radical, which is consistent with the observed trends of the sooting tendencies of benzene and alkylbenzenes in previous studies.
TL;DR: In this article, a project has been initiated in which electron-impact and photon-induced ionization as well as photorecombination of Wq+ ions are studied, and cross sections and rate coefficients were determined for charge states q ranging from q = 1 to q = 5 for photoionization, for q = 0 up to q ≥ 19 for electron-contact ionization and for q ≥ 18 to q ≤ 21 for electron ion recombination.
Abstract: Collisional processes and details of atomic structure of heavy many-electron atoms and ions are not yet understood in a fully satisfying manner. Experimental studies are required for guiding new theoretical approaches. In response to fusion-related needs for collisional and spectroscopic data on tungsten atoms in all charge states, a project has been initiated in which electron-impact and photon-induced ionization as well as photorecombination of Wq+ ions are studied. Cross sections and rate coefficients were determined for charge states q ranging from q = 1 to q = 5 for photoionization, for q = 1 up to q = 19 for electron-impact ionization and for q = 18 to q = 21 for electron-ion recombination. An overview, together with a critical assessment of the methods and results is provided.
TL;DR: This study describes the first usage of atmospheric pressure photoionization Fourier transform ion cyclotron resonance mass spectrometry (in both positive-ion and negative-ion modes) to characterize and compare extracts of oil sands process water, river water, and groundwater samples from areas associated with oil sands mining activities.