FluoroMatch 2.0-making automated and comprehensive non-targeted PFAS annotation a reality.

Because of the pervasiveness, persistence, and toxicity of per- and polyfluoroalkyl substances (PFAS), there is growing concern over PFAS contamination, exposures, and health effects. The diversity of potential PFAS is astounding, with nearly 10,000 PFAS catalogued in databases to date (and growing). The ability to detect the thousands of known PFAS, and discover previously uncatalogued PFAS, is necessary to understand the scope of PFAS contamination and to identify appropriate remediation and regulatory solutions. Current non-targeted methods for PFAS analysis require manual curation and are time-consuming, prone to error, and not comprehensive. FluoroMatch Flow 2.0 is the first software to cover all steps of data processing for PFAS discovery in liquid chromatography-high-resolution tandem mass spectrometry samples. These steps include feature detection, feature blank filtering, exact mass matching to catalogued PFAS, mass defect filtering, homologous series detection, retention time pattern analysis, class-based MS/MS screening, fragment screening, and predicted MS/MS from SMILES structures. In addition, a comprehensive confidence level criterion is implemented to help users understand annotation certainty and integrate various layers of evidence to reduce overreporting. Applying the software to aqueous film forming foam analysis, we discovered over one thousand likely PFAS including previously unreported species. Furthermore, we were able to filter out 96% of features which were likely not PFAS. FluoroMatch Flow 2 increased coverage of likely PFAS by over tenfold compared to the previous release. This software will enable researchers to better characterize PFAS in the environment and in biological systems.

An Interlaboratory Evaluation of Drift Tube Ion Mobility-Mass Spectrometry Collision Cross Section Measurements.

Collision cross section (CCS) measurements resulting from ion mobility-mass spectrometry (IM-MS) experiments provide a promising orthogonal dimension of structural information in MS-based analytical separations. As with any molecular identifier, interlaboratory standardization must precede broad range integration into analytical workflows. In this study, we present a reference drift tube ion mobility mass spectrometer (DTIM-MS) where improvements on the measurement accuracy of experimental parameters influencing IM separations provide standardized drift tube, nitrogen CCS values (DTCCSN2) for over 120 unique ion species with the lowest measurement uncertainty to date. The reproducibility of these DTCCSN2 values are evaluated across three additional laboratories on a commercially available DTIM-MS instrument. The traditional stepped field CCS method performs with a relative standard deviation (RSD) of 0.29% for all ion species across the three additional laboratories. The calibrated single field CCS method, which is compatible with a wide range of chromatographic inlet systems, performs with an average, absolute bias of 0.54% to the standardized stepped field DTCCSN2 values on the reference system. The low RSD and biases observed in this interlaboratory study illustrate the potential of DTIM-MS for providing a molecular identifier for a broad range of discovery based analyses.

PolyMatch: Novel Libraries, Algorithms, and Visualizations for Discovering Polymers and Chemical Series.

Polymers are integral components of everyday products, ranging from plastics and emulsifiers to lubricants and detergents. Characterization of these materials at the molecular level is essential to understanding their physicochemical properties and potential health impacts, considering factors such as the number of repeating units, chemical moieties, functional groups, and degree of unsaturation. This study introduces a free open-source vendor neutral software, PolyMatch, designed to annotate polysorbates, polysorbides, polyethylene glycols (PEGs), fatty acid esterified species, and related chemical species based on mass spectral and chromatographic patterns inherent in the repeating nature of chemical moieties. PolyMatch facilitates the generation of MS/MS libraries for polymeric chemical species characterization (with over 800 000 structures with associated fragment masses already built in) and covers the entire liquid chromatography-high-resolution mass spectrometry (LC-HRMS/MS) data-processing workflow. PolyMatch covers peak picking, blank filtering, annotation, data visualization, and sharing of interactive data sets via an HTML link to the community. The software was applied to a Tween 80 mixture, using LC-HRMS/MS on an Agilent 6546 Q-TOF instrument with iterative exclusion for comprehensive fragmentation coverage. PolyMatch automatically assigned 86 features with high confidence at the species level, 362 based on PEG containing fragments and accurate mass matching to a simulated polymer database, and over 10 000 based on being a member of a homologous series (three or more) with CH2CH2O repeating units. The ease of use of PolyMatch and comprehensive coverage with species level assignment is expected to contribute to the advancement of materials science, health research, and product development.

Novel perfluoroalkyl substances (PFAS) discovered in whole blood using automated non-targeted analysis of dried blood spots.

A small subset of per- and polyfluoroalkyl substances (PFAS) are routinely screened in human blood. These compounds generally explain <50 % of the total PFAS in human blood. The percentage of known PFAS in human blood has been decreasing as replacement PFAS and more complex PFAS chemistries are introduced to the market. Most of these novel PFAS have not been previously identified. Non-targeted methods are required to characterize this "dark matter" PFAS. Our objective was to apply non-targeted PFAS analysis to human blood to gain an understanding about the sources, concentrations, and toxicity of these compounds. A high-resolution tandem mass spectrometry (HRMS) and software workflow for PFAS characterization in dried blood spots is reported. Dried blood spots are a less invasive collection technique compared to venous blood draws, allowing collection from vulnerable populations. Biorepositories of archived dried blood spots are available internationally from newborns and present opportunities to study prenatal exposure to PFAS. In this study, dried blood spot cards were analyzed using iterative MS/MS by liquid chromatography HRMS. Data processing was conducted using FluoroMatch Suite including a visualizer tool that presents homologous series, retention time vs m/z plots, MS/MS spectra, feature tables, annotations, and fragments for fragment screening. The researcher performing data-processing and annotation was blinded to the fact that standards were spiked in, and was able to annotate 95 % of standards spiked on dried blood spot samples, signifying a low false negative rate using FluoroMatch Suite. A total of 28 PFAS (20 standards and 4 exogenous compounds) were detected across five homologous series with Schymanski Level 2 confidence. Of these 4, 3 were perfluoroalkyl ether carboxylic acids (PFECA), a chemical class of PFAS which is increasingly being detected in environmental and biological matrices but is not currently screened in most targeted analysese. A further 86 potential PFAS were detected using fragment screening. PFAS are extremely persistent and widespread yet remain largely unregulated. Our findings will contribute to an improved an understanding of exposures. Application of these methods in environmental epidemiology studies have the potential to inform policy with regards to PFAS monitoring, regulation, and individual-level mitigation strategies.

Expanding Per- and Polyfluoroalkyl Substances Coverage in Nontargeted Analysis Using Data-Independent Analysis and IonDecon.

Per- and polyfluoroalkyl substances (PFAS) are widespread, persistent environmental contaminants that have been linked to various health issues. Comprehensive PFAS analysis often relies on ultra-high-performance liquid chromatography coupled with high-resolution mass spectrometry (UHPLC HRMS) and molecular fragmentation (MS/MS). However, the selection and fragmentation of ions for MS/MS analysis using data-dependent analysis results in only the topmost abundant ions being selected. To overcome these limitations, All Ions fragmentation (AIF) can be used alongside data-dependent analysis. In AIF, ions across the entire m/z range are simultaneously fragmented; hence, precursor-fragment relationships are lost, leading to a high false positive rate. We introduce IonDecon, which filters All Ions data to only those fragments correlating with precursor ions. This software can be used to deconvolute any All Ions files and generates an open source DDA formatted file, which can be used in any downstream nontargeted analysis workflow. In a neat solution, annotation of PFAS standards using IonDecon and All Ions had the exact same false positive rate as when using DDA; this suggests accurate annotation using All Ions and IonDecon. Furthermore, deconvoluted All Ions spectra retained the most abundant peaks also observed in DDA, while filtering out much of the artifact peaks. In complex samples, incorporating AIF and IonDecon into workflows can enhance the MS/MS coverage of PFAS (more than tripling the number of annotations in domestic sewage). Deconvolution in complex samples of All Ions data using IonDecon did retain some false fragments (fragments not observed when using ion selection, which were not isotopes or multimers), and therefore DDA and intelligent acquisition methods should still be acquired when possible alongside All Ions to decrease the false positive rate. Increased coverage of PFAS can inform on the development of regulations to address the entire PFAS problem, including both legacy and newly discovered PFAS.

Long bonds and short barriers: ionization and isomerization of alkyl nitriles.

Ab initio molecular orbital calculations demonstrate that ionizing alkyl nitriles produces a dramatic geometry change involving lengthening of a C-CH(2)CN bond. The experimental determination of the adiabatic ionization energy of these species is thus very difficult. In addition, there are generally low barriers for 1,2-H shift reactions in the molecular ions leading to RCHCHN(+*) and RCHCNH(+*) isomers, which makes generating pure ionized alkyl nitrile in a mass spectrometer a challenge. Threshold photoelectron spectroscopy and threshold photoelecton photoion coincidence spectroscopy were employed to study the ionization and dissociation of two alkyl nitriles, in particular, pentanenitrile and 2,2-dimethylpropanenitrile. Threshold ionization is shown to result not in the respective molecular ions, but rather in isomeric forms, resulting in dissociation thresholds that lie below the calculated adiabatic ionization energies of the two molecules. Appearance energies for all observed fragment ions are reported and compared to available literature values. Charge separation in the dissociation of doubly ionized 2,2-dimethylpropanenitrile is observed as fragment-ion time-of-flight peak broadening at high photon energies.

Methyl t-butyl ether and methyl trimethylsilyl ether ions dissociate near their ionization thresholds: a TPES, TPEPICO, RRKM, and G3 investigation.

The threshold photoelectron spectra and threshold photoelectron photoion coincidence (TPEPICO) mass spectra of methyl t-butyl ether, (CH(3))(3)COCH(3) (MTBE), and methyl trimethylsilyl ether, (CH(3))(3)SiOCH(3) (MTMSE), have been measured using synchrotron radiation. The effect of silicon substitution on the unimolecular dissociation processes and the threshold photoelectron spectrum has been investigated. Both molecular ions dissociate at low internal energies. For ionized MTBE, the parent ion is no longer observed at an internal energy of only 0.2 eV. For this reason, it was not possible to fit the TPEPICO data to extract reliable thermochemical information. G3 level calculations place the molecular ion 5 kJ mol(-1) above the lowest-energy dissociation products, (CH(3))(2)COCH(3)(+) + (*)CH(3), suggesting the participation of an isomer, potentially the distonic ion (*)CH(2)(CH(3))(2)CO(+)(H)CH(3), in the dissociation. However, the calculations are not considered accurate enough to reliably determine the role this isomer plays, if any. RRKM modeling of the threshold region of the TPEPICO breakdown curves for ionized MTMSE leads to an E(0) for methyl loss of 63 +/- 2 kJ mol(-1), in good agreement with the G3 value of 66 kJ mol(-1). The resulting Delta(f)H(0) for (CH(3))(2)SiOCH(3)(+) of 384 +/- 10 kJ mol(-1) (Delta(f)H(298) = 361 +/- 10 kJ mol(-1)) is 28 kJ mol(-1) lower than the G3 value of 412 kJ mol(-1) due to the G3 Delta(f)H(0) for neutral MTMSE being 16 kJ mol(-1) higher than the previously reported value and the fact that the experimental IE(a) is 6 kJ mol(-1) lower than the G3 estimate. Appearance energy values for higher-energy fragmentation channels up to 36 (for MTBE) and 32 eV (for MTMSE) are reported and compared to literature values. An investigation of fragment ion peak broadening at high internal energy indicated that the two doubly charged molecular ions are not stable on the microsecond time scale. Each was found to dissociate into two singly charged ions along one or more neutral species.

Does tetrahydrofuran ring open upon ionization and dissociation? A TPES and TPEPICO investigation.

The threshold photoelectron spectrum (TPES) of tetrahydrofuran (THF) is compared to that of the unsaturated furan molecule. In general, there is a similarity in the orbital ionization profile for the two species, though unlike furan, THF exhibits (modest) vibrational detail only in the (9b)(-1) X (2)B band. An adiabatic ionization energy of 9.445 +/- 0.010 eV has been derived from the onset of the TPES spectrum. Threshold photoelectron photoion coincidence spectroscopy was used to explore the loss of a hydrogen atom from ionized THF over the photon energy range of 9.9-10.4 eV. RRKM fitting of the resulting breakdown curves yields an E(0) of 0.85 +/- 0.03 eV (82 +/- 3 kJ mol(-1)) (AE = 10.30 +/- 0.04 eV). If the G3 IE of 9.48 eV is used to convert the experimental data from photon energy to THF ion internal energy, E(0) = 0.81 +/- 0.01 eV (78 +/- 1 kJ mol(-1)). The latter value is closer to the G3 E(0) of 72 kJ mol(-1) for the formation of the cyclic ion 1. A variety of ring-opening reactions were also probed at the B3-LYP/6-31+G(d) and G3 levels of theory. The distonic isomer (*)CH(2)CH(2)CH(2)OCH(2)(+) lies 70 kJ mol(-1) higher than ionized THF, which places it within 1 kJ mol(-1) of the threshold for the dissociation to 1. All of the probed H-loss products from the distonic isomer (which includes singlet and triplet species) lie significantly higher in energy than ion 1, eliminating the possibility that ionized THF dissociates to m/z 71 via a ring-opening reaction in the present experiment. The derived Delta(double dagger)S value for the dissociation, 8 +/- 5 J K(-1) mol(-1), is also consistent with the formation of 1. The experimentally derived E(0) values can be used to derive the Delta(f)H(o)(0) for ion 1. Together with the Delta(f)H(o)(0) values for the THF ion (752.0 +/- 2 kJ mol(-1), derived from the neutral Delta(f)H(o)(0) of -154.9 +/- 0.7 kJ mol(-1) and experimental IE of 9.445 +/- 0.010 eV) and H atom (218.5 kJ mol(-1)) our E(0) of 82 +/- 3 kJ mol(-1) yields a Delta(f)H(o)(0) for ion 1 of 620 +/- 4 kJ mol(-1) (Delta(f)H(o)(298) = 594 +/- 4 kJ mol(-1)), in good agreement with the G3 Delta(f)H(o)(0) of 621 kJ mol(-1). Appearance energies for all fragment ions up to photon energies of 34 eV are also reported and discussed in comparison with the available literature.

Thermochemistry of N-N containing ions: a threshold photoelectron photoion coincidence spectroscopy study of ionized methyl- and tetramethylhydrazine.

The unimolecular dissociation reactions of the methylhydrazine (MH) and tetramethylhydrazine (TMH) radical cations have been investigated using tandem mass spectrometry and threshold photoelectron photoion coincidence spectroscopy in the photon energy ranges 9.60-31.95 eV (for the MH ion) and 7.74-29.94 eV (for the TMH ion). Methylhydrazine ions (CH3NHNH2(+*)) have three low-energy dissociation channels: hydrogen atom loss to form CH2NHNH2(+) (m/z 45), loss of a methyl radical to form NHNH2(+) (m/z 31), and loss of methane to form the fragment ion m/z 30, N2H2(+*). Tetramethylhydrazine ions only exhibit two dissociation reactions near threshold: that of methyl radical loss to form (CH3)2NNCH3(+) (m/z 73) and of methane loss to form the fragment ion m/z 72 with the empirical formula C3H8N2(+*). The experimental breakdown curves were modeled with Rice-Ramsperger-Kassel-Marcus theory, and it was found that, particularly for methyl radical loss, variational transition state theory was needed to obtain satisfactory fits to the data. The 0 K enthalpies of formation (delta(f)H0) for all fragment ions (m/z 73, m/z 72, m/z 45, m/z 31, and m/z 30) have been determined from the 0 K activation energies (E0) obtained from the fitting procedure: delta(f)H0[(CH3)2NNCH3(+)] = 833 +/- 5 kJ mol(-1), delta(f)H0 [C3H8N2(+*)] = 1064 +/- 5 kJ mol(-1), delta(f)H0[CH2NHNH2(+)] = 862 +/- 5 kJ mol(-1), delta(f)H0[NHNH2(+)] = 959 +/- 5 kJ mol(-1), and delta(f)H0[N2H2(+*)] = 1155 +/- 5 kJ mol(-1). The breakdown curves have been measured from threshold up to h nu approximately 32 eV for both hydrazine ions. As the photon energy increases, other dissociation products are observed and their appearance energies are reported.

Entropy effects in the fragmentation of 1,1-dimethylhydrazine ions.

The 1,1-dimethylhydrazine ion ((CH3)2NNH2+*) has two low-energy dissociation channels, the loss of a hydrogen atom to form the fragment ion m/z 59, (CH3)(CH2)NNH2+, and the loss of a methyl radical to form the fragment ion m/z 45, the methylhydrazyl cation, CH3NNH2+. The dissociation of the 1,1-dimethylhydrazine ion has been investigated using threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy, in the photon energy range 8.25-31 eV, and tandem mass spectrometry. Theoretical breakdown curves have been obtained from a variational transition state theory (VTST) modeling of the two reaction channels and compared to those obtained from experiment. Seven transition states have been found at the B3-LYP/6-31+G(d) level of theory for the methyl radical loss channel in the internal energy range of 2.32-3.56 eV. The methyl loss channel transition states are found at R(N-C) = 4.265, 4.065, 3.965, 3.165, 2.765, 2.665, and 2.565 A over this internal energy range. Three transition states have been found for the hydrogen atom loss channel: R(H-C) = 2.298, 2.198, and 2.098 A. The DeltaS++(45) value, at an internal energy of 2.32 eV and a bond distance of R(N-C) = 4.265 A, is 65 J K-1 mol-1. As the internal energy increases to 3.56 eV the variational transition state moves to lower R value so that at R(N-C) = 2.565 A, the DeltaS++ decreases to 29 J K-1 mol-1. For the hydrogen atom loss channel the variation in DeltaS++ is less than that for the methyl loss channel. To obtain agreement with the experimental breakdown curves, DeltaS++(59) = 26-16 J K-1 mol-1 over the studied internal energy range. The 0 K enthalpies of formation (DeltafH0) for the two fragment ions m/z 45 and m/z 59 have been calculated from the 0 K activation energies (E0) obtained from the fitting procedure: DeltafH0[CH3NNH2+] = 906 +/- 6 kJ mol-1 and DeltafH0[(CH3)(CH2)NNH2+] = 822 +/- 7 kJ mol-1. The calculated G3 values are DeltafH0[CH3NNH2+] = 911 kJ mol-1 and DeltafH0[(CH3)(CH2)NNH2+] = 825 kJ mol-1. In addition to the two low-energy dissociation products, 21 other fragment ions have been observed in the dissociation of the 1,1-dimethylhydrazine ion as the photon energy was increased. Their appearance energies are reported.

Threshold-photoelectron spectroscopic study of methyl-substituted hydrazine compounds.

The valence shell electronic structures of methylhydrazine (CH(3)NHNH(2)), 1,1-dimethylhydrazine ((CH(3))(2)NNH(2)) and tetramethylhydrazine ((CH(3))(4)N(2)) have been studied by recording threshold and conventional (kinetic energy resolved) photoelectron spectra. Ab initio calculations have been performed on ammonia and the three methyl substituted hydrazines, with the structures being optimized at the B3-LYP/6-31+G(d) level of theory. The ionization energies of the valence molecular orbitals were calculated using the Green's function method, allowing the photoelectron bands to be assigned to specific molecular orbitals. The ground-state adiabatic and vertical ionization energies, as determined from the threshold photoelectron spectra, were IE(a) = 8.02 +/- 0.16 eV and IE(v) = 9.36 +/- 0.02 eV for methylhydrazine, IE(a) = 7.78 +/- 0.16 eV and IE(v) = 8.86 +/- 0.01 eV for 1,1-dimethylhydrazine and IE(a) = 7.26 +/- 0.16 eV and IE(v) = 8.38 +/- 0.01 eV for tetramethylhydrazine. Due to the large geometry change that occurs upon ionization, these IE(a) values are all higher than the true thresholds. New features have been observed in the inner valence region and these have been compared with similar structure in the spectrum of hydrazine. The effect of resonant autoionization on the threshold photoelectron yield is discussed. New heats of formation (Delta(f)H) are proposed for the three hydrazines on the basis of G3 calculations: 107, 94, and 95 kJ/mol for methylhydrazine, 1,1-dimethyhydrazine and tetramethylhydrazine, respectively. The previously reported Delta(f)H for tetramethylhydrazine is shown to be erroneous.

A photoelectron and TPEPICO investigation of the acetone radical cation.

The valence shell photoelectron spectrum, threshold photoelectron spectrum, and threshold photoelectron photoion coincidence (TPEPICO) mass spectra of acetone have been measured using synchrotron radiation. New vibrational progressions have been observed and assigned in the X 2B2 state photoelectron bands of acetone-h6 and acetone-d6, and the influence of resonant autoionization on the threshold electron yield has been investigated. The dissociation thresholds for fragment ions up to 31 eV have been measured and compared to previous values. In addition, kinetic modeling of the threshold region for CH3* and CH4 loss leads to new values of 78 +/- 2 kJ mol(-1) and 75 +/- 2 kJ mol(-1), respectively, for the 0 K activation energies for these two processes. The result for the methyl loss channel is in reasonable agreement with, but slightly lower than, that of 83 +/- 1 kJ mol(-1) derived in a recent TPEPICO study by Fogleman et al. The modeling accounts for both low-energy dissociation channels at two different ion residence times in the mass spectrometer. Moreover, the effects of the ro-vibrational population distribution, the electron transmission efficiency, and the monochromator band-pass are included. The present activation energies yield a Delta(f)H298 for CH3CO+ of 655 +/- 3 kJ mol(-1), which is 4 kJ mol(-1) lower than that reported by Fogleman et al. The present Delta(f)H298 for CH3CO+ can be combined with the Delta(f)H298 for CH2CO (-47.5 +/- 1.6 kJ mol(-1)) and H+ (1530 kJ mol(-1)) to yield a 298 K proton affinity for ketene of 828 +/- 4 kJ mol(-1), in good agreement with the value (825 kJ mol(-1)) calculated at the G2 level of theory. The measured activation energy for CH4 loss leads to a Delta(f)H298 (CH2CO+*) of 873 +/- 3 kJ mol(-1).

Confirmation of the "long-lived" tetra-nitrogen (N4) molecule using neutralization-reionization mass spectrometry and ab initio calculations.

Tetra-nitrogen (N(4)), which has been the subject of recent controversy [Cacace, d. Petris, and Troiani, Science 295, 480 (2002); Cacace, Chem. Eur. J. 8, 3839 (2002); Nguyen et al., J. Phys. Chem. A 107, 5452 (2003); Nguyen, Coord. Chem. Rev. 244, 93 (2003)] as well as of great theoretical interest, has been prepared from the N(4) (+) cation and then detected as a reionized gaseous metastable molecule with a lifetime exceeding 0.8 micros in experiments based on neutralization-reionization mass spectrometry. Moreover, we have used the nature of the charge-transfer reaction which occurs between a beam of fast N(4) (+) ions (8 keV translational energy) and various stationary gas targets to identify the vertical neutralization energy of the N(4) (+) ion. The measured value, 10.3+/-0.5, most closely matches that of the lowest energy azidonitrene (4)N(4) (+)C(s)((4)A(')) ion, resulting in the formation of the neutral bound azidonitrene (3)N(4)C(s)((3)A(")). Neutralization of the global minimum (2)N(4) (+)D( infinity h)((2)Sigma(u) (+)) ion leads to a structure 166 kJ mol(-1) above the dissociation products [N(2)((1)Sigma(g) (+))+N(2)((1)Sigma(g) (+))]; moreover, it was not possible to find a minimum on the (1)N(4) neutral potential energy surface for a covalently bonded structure. Ab initio calculations at the G3, QCISD/6-31G(d), and MP2/AUG-cc-pVTZ levels of theory have been used to determine geometries and both vertical neutralization energies of ions (doublet and quartet) and ionization energies of neutrals (singlet and triplet). In addition, we have also described in detail the EI ion source for the Ottawa VG ZAB mass spectrometer [Holmes and Mayer, J. Phys. Chem. A 99, 1366 (1995)] which was modified for high-pressure use, i.e., for the production of dimer and higher number cluster ions.

Photoelectron circular dichroism in core level ionization of randomly oriented pure enantiomers of the chiral molecule camphor.

The inner-shell photoionization of unoriented camphor molecules by circularly polarized light has been investigated from threshold to a photoelectron kinetic energy of approximately 65 eV. Photoelectron spectra of the carbonyl C 1s orbital, recorded at the magic angle of 54.7 degrees with respect to the light propagation direction, show an asymmetry of up to 6% on change of either the photon helicity or molecular enantiomer. These observations reveal a circular dichroism in the angle resolved emission with an asymmetry between forward and backward scattering (i.e., 0 degrees and 180 degrees to the light beam) which can exceed 12%. Since the initial state is an atomiclike spherically symmetric orbital, this strongly suggests that the asymmetry is caused by final-state effects dependent on the chiral geometry of the molecule. These findings are confirmed by electron multiple scattering calculations of the photoionization dynamics in the electric-dipole approximation.