Direct Photoionization Research Articles

Single photoionization of aluminum-like P2+ and magnesium-like P3+

Absolute single-photoionization cross sections for aluminum-like P2+ have been measured from 26.5 to 66.5eV using photoion yield spectroscopy. In addition, absolute cross section measurements for the single-photoionization of magnesium-like P3+ have been measured from 43 to 60eV. For both ions, the measured spectra are dominated by Rydberg series resonances superimposed on a non-resonant direct photoionization cross section. Analyses of these data have helped identify the initial and final states of the Rydberg series observed in each photoion. In addition, quantum defects are derived for each series.

Dynamics of colliding ultracold plasmas

Formation of a secondary plasma of ${\mathrm{NO}}^{+}$ ions and electrons within an ultracold plasma produces an observable change in the hydrodynamics of the system. Direct photoionization adds energetic electrons, which increases the rate of expansion. The introduction of a secondary Rydberg gas has the opposite effect. In both cases, the added ions create an inertial drag that acts initially to retard the expansion of the electron gas. A cold-ion hydrodynamic shell model, which accounts well for the effect of energy added by photoionization electrons, predicts the formation of collisionless shock waves.

Photoionization of the3s23p43Pand the3s23p41D,1Sstates of sulfur: Experiment and theory

Photoionization of neutral atomic sulfur in the ground and metastable states was studied experimentally at a photon energy resolution of 44 meV FWHM. Relative cross section measurements were recorded by using tunable vacuum ultraviolet (VUV) radiation in the energy range 9 -- 30 eV obtained from a laser-produced plasma and the atomic species were generated by photolysis of molecular precursors. Photoionization of this atom is characterized by multiple Rydberg series of autoionizing resonances superimposed on a direct photoionization continuum. A wealth of resonance features observed in the experimental spectra are spectroscopically assigned and their energies and quantum defects tabulated. The cross-section measurements are compared with state-of-the-art theoretical cross-section calculations obtained from the Dirac Coulomb R-matrix method. Resonances series in the spectra are identified and compared indicating similar features in both the theoretical and experimental spectra.

Application of metabolomics based on direct mass spectrometry analysis for the elucidation of altered metabolic pathways in serum from the APP/PS1 transgenic model of Alzheimer's disease

Metabolomic analysis of brain tissue from transgenic mouse models of Alzheimer's disease has demonstrated a great potential for the study of pathological mechanisms and the development of new therapies and biomarkers for diagnosis. However, in order to translate these investigations to the clinical practice it is necessary to corroborate these findings in peripheral samples. To this end, this work considers the application of a novel metabolomic platform based on the combination of a two-steps extraction procedure with complementary analysis by direct infusion electrospray mass spectrometry and flow infusion atmospheric pressure photoionization mass spectrometry for a holistic investigation of metabolic abnormalities in serum samples from APP/PS1 mice. A number of metabolites were found to be perturbed in this mouse model, including increased levels of di- and tri-acylglycerols, eicosanoids, inosine, choline and glycerophosphoethanolamine; reduced content of cholesteryl esters, free fatty acids, lysophosphocholines, amino acids, energy-related metabolites, phosphoethanolamine and urea, as well as abnormal distribution of phosphocholines depending on the fatty acid linked to the molecular moiety. This allowed the elucidation of possible pathways disturbed underlying to disease (abnormal homeostasis of phospholipids leading to membrane breakdown, energy-related failures, hyperammonemia and hyperlipidemia, among others), thus demonstrating the utility of peripheral samples to investigate pathology in the APP/PS1 model.

Detection of gaseous compounds by needle trap sampling and direct thermal-desorption photoionization mass spectrometry: concept and demonstrative application to breath gas analysis.

A fast detection method to analyze gaseous organic compounds in complex gas mixtures was developed, using a needle trap device (NTD) in conjunction with thermal-desorption photoionization time-of-flight mass spectrometry (TD-PI-TOFMS). The mass spectrometer was coupled via a deactivated fused silica capillary to an injector of a gas chromatograph. In the hot injector, the analytes collected on the NTD were thermally desorbed and directly transferred to the PI-TOFMS ion source. The molecules are softly ionized either by single photon ionization (SPI, 118 nm) or by resonance enhanced multiphoton ionization (REMPI, 266 nm), and the molecular ion signals are detected in the TOF mass analyzer. Analyte desorption and the subsequent PI-TOFMS detection step only lasts ten seconds. The specific selectivity of REMPI (i.e., aromatic compounds) and universal ionization characteristics render PI-MS as a promising detection system. As a first demonstrative application, the alveolar phase breath gas of healthy, nonsmoking subjects was sampled on NTDs. While smaller organic compounds such as acetone, acetaldehyde, isoprene, or cysteamine can be detected in the breath gas with SPI, REMPI depicts the aromatic substances phenol and indole at 266 nm. In the breath gas of a healthy, smoking male subject, several xenobiotic substances such as benzene, toluene, styrene, and ethylbenzene can be found as well. Furthermore, the NTD-REMPI-TOFMS setup was tested for breath gas taken from a mechanically ventilated pig under continuous intravenous propofol (2,6-diisopropylphenol, narcotic drug) infusion.

Site- and state-selected photofragmentation of 2Br-pyrimidine.

The fragmentation of the 2Br-pyrimidine molecule following direct valence photoionization or inner shell excitation has been studied by electron-ion coincidence experiments. 2Br-pyrimidine has been chosen as a model for the class of pyrimidinic building blocks of three nucleic acids and several radiosensitizers. It is known that the site- and state-localization of energy deposition, typical of inner shell excitation, results in the enhancement of the total ion yield as well as in changes in the relative intensity of the different fragmentation channels. Here we address the question of the origin of this selective fragmentation by using electron-ion coincidence techniques. The results show that the fragmentation is strongly selective in the final singly charged ion state, independently of the process that leads to the population of that state, and the dominant fragmentation patterns correlate with the nearest appearance potential.

Effect of Temperature on the Molecular Weight Distribution in the Different Ranks of Coal during the On-Line Investigation of Coal Pyrolysis Gas Using Direct Photoionization Mass Spectroscopy

Coal pyrolysis gas from different ranks of coal was monitored on real time basis using photoionization mass spectroscopy. The molecular weight distribution of different products as a function of temperature from various coal ranks studied was observed. It was noted that the release of different classes of compounds like phenols, alkenes, alkylated aromatics and aromatic skeletons was temperature dependent. For all the coal ranks at lower temperatures phenols were the main component, with alkenes and alkylated aromatics at slight higher temperatures and aromatic skeletons were released at the highest temperatures studied.

Mass analyzed threshold ionization detected infrared spectroscopy: isomerization activity of the phenol-Ar cluster near the ionization threshold.

The structure of the phenol-argon cluster (PhOH-Ar) in high-n Rydberg states is investigated by the newly developed technique of mass analyzed threshold ionization detected infrared (MATI-IR) spectroscopy. This method selectively measures IR spectra of molecular clusters in very high-n Rydberg states (n > 100) utilized in zero kinetic energy (ZEKE) photoelectron and MATI spectroscopy, whose ionic cores are essentially the same as the corresponding bare cation. The MATI-IR spectrum exhibits only the free OH stretching vibration (ν) when the π-bound cluster of the neutral ground electronic state (S0) is resonantly excited via the S1 origin to Rydberg states converging to its adiabatic ionization energy level, IE0(π). When Rydberg states converging to vibrationally excited levels of the local π-bound minimum are prepared, in addition to ν also the hydrogen-bonded OH stretching vibration (ν) of the H-bonded global minimum is observed in the MATI-IR spectra, even for vibrational excitation of only 14 cm(-1) above IE0(π). These results show that the π→ H site switching reaction of the Ar ligand from the aromatic ring to the OH group proceeds only from vibrationally excited states in the π-bound cation core with a small barrier of less than 14 cm(-1) from IE0(π). On the other hand, directly photoionized PhOH(+)-Ar shows both ν and ν in the IR spectra, even when it is just ionized to IE0(π). This result implies that the ionization-induced π→ H site switching occurs without excess energy in the H-bound or π-bound cations, in contrast to very high-n Rydberg states converging to levels of the π-bound cation. The different efficiencies of the site switching for the Rydberg ion core and the bare ion and the mechanism for the π→ H site switching are interpreted by direct ionization from the π-bound to the H-bound structures in addition to the conventional vertical ionization and transitions to high-n Rydberg states.

Circular dichroism in molecular-frame photoelectron angular distributions in the dissociative photoionization ofH2andD2molecules

The presence of net circular dichroism in the photoionization of nonchiral homonuclear molecules has been put in evidence recently through the measurement of molecular-frame photoelectron angular distributions in dissociative photoionization of ${\text{H}}_{2}$ [Dowek et al., Phys. Rev. Lett. 104, 233003 (2010)]. In this work we present a detailed study of circular dichroism in the photoelectron angular distributions of ${\text{H}}_{2}$ and ${\text{D}}_{2}$ molecules, oriented perpendicularly to the propagation vector of the circularly polarized light, at different photon energies (20, 27, and 32.5 eV). Circular dichroism in the angular distributions at 20 and to a large extent 27 eV exhibits the usual pattern in which inversion symmetry is preserved. In contrast, at 32.5 eV, the inversion symmetry breaks down, which eventually leads to total circular dichroism after integration over the polar emission angle. Time-dependent ab initio calculations support and explain the observed results for ${\text{H}}_{2}$ in terms of quantum interferences between direct photoionization and delayed autoionization from the ${\mathcal{Q}}_{1}$ and ${\mathcal{Q}}_{2}$ doubly excited states into ionic states $(1s{\ensuremath{\sigma}}_{g}$ and $2p{\ensuremath{\sigma}}_{u})$ of different inversion symmetry. Nevertheless, for ${\text{D}}_{2}$ at 32.5 eV, there is a particular case where theory and experiment disagree in the magnitude of the symmetry breaking: when ${\text{D}}^{+}$ ions are produced with an energy of around 5 eV. This reflects the subleties associated to such simple molecules when exposed to this fine scrutiny.

Absolute measurements of chlorine Cl+ cation single photoionization cross section

The photoionization of Cl+ leading to Cl2+ was measured in the photon energy range of 19.5–28.0eV. A spectrum with a photon energy resolution of 15meV normalized to absolute cross-section measurements is presented. The measurements were carried out by merging a Cl+ ion beam with a photon beam of highly monochromatic synchrotron radiation at the Advanced Light Source at Lawrence Berkeley National Laboratory. The measured photoionization of Cl+ consists of several autoionization resonances surperimposed on the direct photoionization signal. Most of the prominent resonances are assigned to members of Rydberg series originating from the singlet ground state and from metastable triplet levels within the ground-state configuration of Cl+. The direct ionization cross section is no larger than 12Mb.

Rotationally resolved state-to-state photoionization and photoelectron study of titanium carbide and its cation (TiC/TiC+)

Titanium carbide and its cation (TiC/TiC(+)) have been investigated by the two-color visible (VIS)-ultraviolet (UV) resonance-enhanced photoionization and pulsed field ionization-photoelectron (PFI-PE) methods. Two visible excitation bands for neutral TiC are observed at 16,446 and 16,930 cm(-1). Based on rotational analyses, these bands are assigned as the respective TiC((3)Π1) ← TiC(X(3)Σ(+)) and TiC((3)Σ(+)) ← TiC(X(3)Σ(+)) transition bands. This assignment supports that the electronic configuration and term symmetry for the neutral TiC ground state are …7σ(2)8σ(1)9σ(1)3π(4) (X(3)Σ(+)). The rotational constant and the corresponding bond distance of TiC(X(3)Σ(+); v″ = 0) are determined to be B0″ = 0.6112(10) cm(-1) and r0″ = 1.695(2) Å, respectively. The rotational analyses of the VIS-UV-PFI-PE spectra for the TiC(+)(X; v(+) = 0 and 1) vibrational bands show that the electronic configuration and term symmetry for the ionic TiC(+) ground state are …7σ(2)8σ(1)3π(4) (X(2)Σ(+)) with the v(+) = 0 → 1 vibrational spacing of 870.0(8) cm(-1) and the rotational constants of B(e)(+) = 0.6322(28) cm(-1), and α(e)(+) = 0.0085(28) cm(-1). The latter rotational constants yield the equilibrium bond distance of r(e)(+) = 1.667(4) Å for TiC(+)(X(2)Σ(+)). The cleanly rotationally resolved VIS-UV-PFI-PE spectra have also provided a highly precise value of 53 200.2(8) cm(-1) [6.5960(1) eV] for the adiabatic ionization energy (IE) of TiC. This IE(TiC) value along with the known IE(Ti) has made possible the determination of the difference between the 0 K bond dissociation energy (D0) of TiC(+)(X(2)Σ(+)) and that of TiC(X(3)Σ(+)) to be D0(Ti(+)-C) - D0(Ti-C) = 0.2322(2) eV. Similar to previous experimental observations, the present state-to-state PFI-PE study of the photoionization transitions, TiC(+)(X(2)Σ(+); v(+) = 0 and 1, N(+)) ← TiC((3)Π1; v', J'), reveals a strong decreasing trend for the photoionization cross section as |ΔN(+)| = |N(+) - J'| is increased. The maximum |ΔN(+)| change of 7 observed here is also consistent with the previous experimental results for the 3d transition-metal carbides, oxides, and nitrides. However, the VIS-UV-PFI-PE spectra for TiC(+)(X(2)Σ(+); v(+) = 0 and 1, N(+)) are found to display only the negative ΔN(+) (N(+)-J'≤ 0) transitions, indicating that the cross sections for the formation of positive ΔN(+) (N(+)-J' > 0) transitions by both the channel coupling mechanism and direct photoionization are negligibly small.

Distinction of direct and sequential multiple ionization of neon by intense soft XUV femtosecond pulses

In the situation where the slopes of atomic ionization yields as a function of the laser intensity do not permit to distinguish between direct and sequential multiple photoionization processes, an analysis of the magnitude of those ionization yields enable such distinction. Using an analytical rate equation model we show that comparing these magnitudes by introducing reduced ionization yields leads to clustering curves in the sequential case and spreading in the direct one when laser focus averaging is taken into account. Applying this insight to the experimental data, we are able to assess that the sequential process is responsible for ionization of neon up to Ne5+ induced by short, intense and coherent soft XUV light provided by free-electron laser sources.

System for time-discretized vacuum ultraviolet spectroscopy of spark breakdown in air.

A system for time-discretized spectroscopic measurements of the vacuum ultraviolet (VUV) emission from spark discharges in the 60-160 nm range has been developed for the study of early plasma-forming phenomena. The system induces a spark discharge in an environment close to atmospheric conditions created using a high speed puff value, but is otherwise kept at high vacuum to allow for the propagation of VUV light. Using a vertical slit placed 1.5 mm from the discharge the emission from a small cross section of the discharge is allowed to pass into the selection chamber consisting of a spherical grating, with 1200 grooves/mm, and an exit slit set to 100 μm. Following the exit slit is a photomultiplier tube with a sodium salicylate scintillator that is used for the time discretized measurement of the VUV signal with a temporal resolution limit of 10 ns. Results from discharges studied in dry air, Nitrogen, SF6, and Argon indicate the emission of light with wavelengths shorter than 120 nm where the photon energy begins to approach the regime of direct photoionization.

The ionization mechanisms in direct and dopant-assisted atmospheric pressure photoionization and atmospheric pressure laser ionization.

A novel, gas-tight API interface for gas chromatography-mass spectrometry was used to study the ionization mechanism in direct and dopant-assisted atmospheric pressure photoionization (APPI) and atmospheric pressure laser ionization (APLI). Eight analytes (ethylbenzene, bromobenzene, naphthalene, anthracene, benzaldehyde, pyridine, quinolone, and acridine) with varying ionization energies (IEs) and proton affinities (PAs), and four common APPI dopants (toluene, acetone, anisole, and chlorobenzene) were chosen. All the studied compounds were ionized by direct APPI, forming mainly molecular ions. Addition of dopants suppressed the signal of the analytes with IEs above the IE of the dopant. For compounds with suitable IEs or Pas, the dopants increased the ionization efficiency as the analytes could be ionized through dopant-mediated gas-phase reactions, such as charge exchange, proton transfer, and other rather unexpected reactions, such as formation of [M + 77](+) in the presence of chlorobenzene. Experiments with deuterated toluene as the dopant verified that in case of proton transfer, the proton originated from the dopant instead of proton-bound solvent clusters, as in conventional open or non-tight APPI sources. In direct APLI using a 266 nm laser, a narrower range of compounds was ionized than in direct APPI, because of exceedingly high IEs or unfavorable two-photon absorption cross-sections. Introduction of dopants in the APLI system changed the ionization mechanism to similar dopant-mediated gas-phase reactions with the dopant as in APPI, which produced mainly ions of the same form as in APPI, and ionized a wider range of analytes than direct APLI.

Metastable fragment production at the C 1s and O 1s edges of the CO2 molecule

We have studied the dissociation of the CO2 molecule into neutral fragments by observing coincidences between neutral metastable fragments and photoions at photon energies between 250 eV and 600 eV. We attribute the metastable signal to oxygen atoms in the 2s22p3(4S)3s(5S) state and to fragments in high Rydberg states. Neutral metastable fragments are produced by the dissociation of valence-excited states of that are populated by direct photoionization or, more efficiently, by core excitation and ionization followed by Auger decay. Metastable fragments were detected at all C 1s and O 1s single excitations, and their intensity was enhanced particularly just above the O 1s and C 1s ionization thresholds, where the recapture of slow photoelectrons is possible. They were also abundantly created at core-valence double excitations, at the thresholds of O 1s and C 1s in shake-up satellites, and to a lesser extent, also in regions of the photoionization continua where there are no resonances.

Influence of VUV illumination on breakdown mechanics: pre-ionization, direct photoionization, and discharge initiation

A microdischarge (MD) vacuum ultraviolet (VUV) light source is fired onto a N2–NO (99.92 : 0.08%) target gas. The minor gas constituent, NO, was chosen for its ionization potential (9.23 eV) and photoionization cross-section (1.4 × 10−18 cm2) at the wavelength of interest (121.6 nm, 10.2 eV). The result is a plasma generated entirely by volume photoionization in a N2–NO background. Using a very low electric field amplitude, charge carriers are drifted though the photoplasma at picoampere levels, serving as a non-invasive diagnostic. Using a simple one-dimensional fluid approximation for the low electric field condition, theoretical predictions of photoplasma current were found to be in meaningful agreement with experimental data. The impact of direct photoionization and pre-ionization on nanosecond timescale high voltage breakdown yielded two primary observations: (1) a significant reduction in the formative delay time necessary for spark formation, and (2) almost complete elimination of the statistical delay time. Again utilizing one-dimensional fluid approximations, reasonable agreement between experimental and simulated breakdown voltage was observed. Utilizing the same VUV source to illuminate a HV spark gap biased to about 95% self-breakdown voltage revealed that direct volume photoionization alone was insufficient to trigger breakdown of the high voltage gap. However, permitting electrode illumination, the same source was found to be capable of triggering breakdown in the undervoltaged gap, albeit with a large temporal jitter.

High-resolution threshold photoelectron and photoion spectroscopy of molecular nitrogen in the 15.0–52.7 eV photon energy range

We have performed an extensive study of the threshold photoelectron spectrum of N2 over the valence ionization region 15.0–28.0eV under high-resolution conditions. In addition, we measured the high-resolution photoion spectrum of N2 over the same photon energy range. These complimentary views of ionization in molecular systems provide different perspectives of the mechanism for the production of both ions and threshold electrons. The studies are presented in the context of our current understanding of the relationship between: (i) the autoionization of neutral Rydberg states and direct photoionization to (ii) the production of electrons and the ionic states of nitrogen at threshold. In both cases (ions and threshold electrons) we have found a considerable amount of new information on the vibrational states of a number of neutral and ionic electronic states. The inner valence ionization region of nitrogen was also investigated using threshold photoelectron spectroscopy over the photon energy range (27.7–52.7eV) and is presented from the perspective of the formation of mixed-configuration ionic satellite states and the possibility of the involvement of ionic Rydberg states converging on the double ionization of nitrogen.

An electrospray aerosol generator with X-ray photoionizer for particle charge reduction

A prototype single-capillary electrospray (ES) aerosol generator having a soft X-ray photoionizer for charge reduction has been constructed and its performance under various configurations has been investigated in this study. The prototype essentially consists of spray and charge reduction chambers having an orifice disk plate as the partition. Two soft X-ray irradiation directions (i.e., 90° and 180° relative to the spray direction) and two disk plates (with the orifice diameters of 0.25in. and 1.25in.) were tested for the prototype configuration. In the investigation, the spray current as a function of ES voltage was recorded. It is shown that the presence of soft X-ray irradiation in the close proximity of the electrospray process resulted in the increase of applied voltage needed to operate the spray while the spray current at the cone-jet mode remained unchanged. The fluorescence analysis was applied in this study to quantify the transmission efficiency and the charge fractions of particles exiting from the prototype generator. A maximal 47% transmission efficiency could be achieved by the prototype having the setup of 180° irradiation and use of 0.25in. D-orifice plate, and operated at carrier gas flow rate of 8.0lpm. Under the above setup and operation the charge fraction of particles with the sizes ranging from 30 to 300nm was further characterized. It is found that the measured charge fractions of particles exiting from the generator is in good agreement with the calculated Fuchs bipolar charge factions for the cases having the particle sizes less than 110nm. The deviation from the Fuchs charge fraction increases as the particle size increases. It may be because (1) more ions were needed to reduce the charge level on particles of large sizes; (2) spatial non-uniformity of aerosol concentration and ions in charge reduction chamber; and (3) because of the direct photoionization of particles under the X-ray irradiation. At last, the size measurement of ES particles via a scanning mobility particle sizer (SMPS) shows the good quality of monodispersity for particles generated by the prototype when operated at the cone-jet mode.

Photoionization of metastable state 1s2s 1S0 of helium

The interference effects between the direct and indirect photoionization processes from the metastable state 1s2s 1S0 of helium are studied by employing the multi-configuration Dirac-Fock (MCDF) method combining with the Fano parameterization technique. Good agreement is found between the present calculations and the available experimental results. It is shown that the interference effects, which further modified the direct photoionization cross sections even far away from the resonances, play an important role in reproducing the resultant nonresonant background beside the configuration interaction in bound ionic states.

Double photoionization in ring molecules: search of the cooper pair formation.

We provide a final state selective experimental study on the direct double photoionization of the valence states of benzene and pyrrole. The experiment is carried out using a magnetic-bottle electron time-of-flight coincidence setup at the incident photon energy region of 25-120 eV. We discuss on the recently discovered phenomenon of so-called Cooper pair formation [R. Wehlitz et al., Phys. Rev. Lett. 109, 193001 (2012)] and show that our experiment provides contradicting evidence on its existence in the proposed form. We confirm the finding of a new two-electron continuum resonance structure observed at about 30–70 eV above the double ionization threshold in benzene, provide further information from it, and suggest an alternative explanation.