Direct Photoionization Research Articles

Recovery of High-Energy Photoelectron Circular Dichroism through Fano Interference.

It is commonly accepted that the magnitude of a photoelectron circular dichroism (PECD) is governed by the ability of an outgoing photoelectron wave packet to probe the chiral asymmetry of a molecule. To be able to accumulate this characteristic asymmetry while escaping the chiral ion, photoelectrons need to have relatively small kinetic energies of up to a few tens of electron volts. Here, we demonstrate a substantial PECD for very fast photoelectrons above 500eV kinetic energy released from methyloxirane by a participator resonant Auger decay of its lowermost O 1s excitation. This effect emerges as a result of the Fano interference between the direct and resonant photoionization pathways, notwithstanding that their individual effects are negligibly small. The resulting dichroic parameter has an anomalous dispersion: It changes its sign across the resonance, which can be considered as an analogue of the Cotton effect in the x-ray regime.

Is methanol a dopant under atmospheric pressure photoionization conditions?

Using a modified atmospheric pressure photoionization (APPI) source coupled to a tunable vacuum ultraviolet source at the SOLEIL synchrotron radiation facility, we determined the appearance energy of protonated methanol monomer and dimers at different source temperatures. The experimental data indicated that protonated monomers can be formed through the direct photoionization of neutral methanol by the second emission band of a krypton discharge UV lamp at 116.49 nm (10.64 eV) explaining why methanol can be considered as a weak dopant under APPI conditions. This point is in contradiction with the previously proposed mechanism where photoionization of methanol dimer was followed by the dissociation of the protonated methanol dimer into protonated methanol monomer.

Solid-Phase Microextraction Coupled to Capillary Atmospheric Pressure Photoionization-Mass Spectrometry for Direct Analysis of Polar and Nonpolar Compounds

A novel capillary ionization source based on atmospheric pressure photoionization (cAPPI) was developed and used for the direct interfacing between solid-phase microextraction (SPME) and mass spectrometry (MS). The efficiency of the source was evaluated for direct and dopant-assisted photoionization, analyzing both polar (e.g., triazines and organophosphorus pesticides) and nonpolar (polycyclic aromatic hydrocarbons, PAHs) compounds. The results show that the range of compound polarity, which can be addressed by direct SPME-MS can be substantially extended by using cAPPI, compared to other sensitive techniques like direct analysis in real time (DART) and dielectric barrier discharge ionization (DBDI). The new source delivers a very high sensitivity, down to sub parts-per-trillion (ppt), making it a viable alternative when compared to previously reported and less comprehensive direct approaches.

Measuring ultrafine aerosols by direct photoionization and charge capture in continuous flow

Direct ultraviolet (UV) photoionization enables electrical charging of aerosol nanoparticles without relying on the collision of particles and ions. In this work, a low-strength electric field is a...

Time–frequency representation of autoionization dynamics in helium

Autoionization, which results from the interference between direct photoionization and photoexcitation to a discrete state decaying to the continuum by configuration interaction, is a well known example of the important role of electron correlation in light–matter interaction. Information on this process can be obtained by studying the spectral, or equivalently, temporal complex amplitude of the ionized electron wave packet. Using an energy-resolved interferometric technique, we measure the spectral amplitude and phase of autoionized wave packets emitted via the sp2+ and sp3+ resonances in helium. These measurements allow us to reconstruct the corresponding temporal profiles by Fourier transform. In addition, applying various time–frequency representations, we observe the build-up of the wave packets in the continuum, monitor the instantaneous frequencies emitted at any time and disentangle the dynamics of the direct and resonant ionization channels.

Effects of collision energy and vibrational excitation of CH3+ cations on its reactivity with hydrocarbons: But-2-yne CH3CCCH3 as reagent partner.

The methyl carbocation is ubiquitous in gaseous environments, such as planetary ionospheres, cometary comae, and the interstellar medium, as well as combustion systems and plasma setups for technological applications. Here we report on a joint experimental and theoretical study on the mechanism of the reaction CH3+ + CH3CCCH3 (but-2-yne, also known as dimethylacetylene), by combining guided ion beam mass spectrometry experiments with ab initio calculations of the potential energy hypersurface. Such a reaction is relevant in understanding the chemical evolution of Saturn's largest satellite, Titan. Two complementary setups have been used: in one case, methyl cations are generated via electron ionization, while in the other case, direct vacuum ultraviolet photoionization with synchrotron radiation of methyl radicals is used to study internal energy effects on the reactivity. Absolute reactive cross sections have been measured as a function of collision energy, and product branching ratios have been derived. The two most abundant products result from electron and hydride transfer, occurring via direct and barrierless mechanisms, while other channels are initiated by the electrophilic addition of the methyl cation to the triple bond of but-2-yne. Among the minor channels, special relevance is placed on the formation of C5H7+, stemming from H2 loss from the addition complex. This is the only observed condensation product with the formation of new C-C bonds, and it might represent a viable pathway for the synthesis of complex organic species in astronomical environments and laboratory plasmas.

Quantitative Analysis of Bisphenol A in Recycled Paper with a Novel Direct Inlet Probe-Atmospheric Pressure Photoionization–IonTrap-MS

A new ambient ion source was developed and its application to real samples has been demonstrated. This ion source is based on a previously build ion source using chemical ionization under atmospheric pressure and extractive electrospray ionization (DIP-APCI/EESI). To enhance the range of analytes, we added to this ion source a photoionization lamp to enable also photoionization, which allows the possibility to analyze polar (EESI), middle (APCI) and non-polar (APPI) analytes with the same source and with no or only a small change of the setup. In comparison with DIP-APCI, this ion source reaches similar analytical limits but shows better sensitivity for some aromatic and non-polar analytes. The analysis of bisphenol A from extracts of recycled paper demonstrates the use of this ion source for real samples. Quantification was possible by the use of a deuterated internal standard that was added to the papers before extraction. Here atmospheric pressure chemical ionization leads to bad results due to ion suppression, whereas atmospheric pressure photoionization shows results that fit to literature values.

Direct Analysis of Organic Compounds in Liquid Using a Miniature Photoionization Ion Trap Mass Spectrometer with Pulsed Carrier-Gas Capillary Inlet.

A miniature ion trap mass spectrometer with capillary direct sampling and vacuum ultraviolet photoionization source was developed to conduct trace analysis of organic compounds in liquids. Self-aspiration sampling is available where the samples are drawn into the vacuum chamber through a capillary with an extremely low flow rate (less than 1 μL/min), which minimizes sample consumption in each analysis to tens of micrograms. A pulsed gas-assisted inlet was designed and optimized to promote sample transmission in the tube and facilitate the cooling of ions, thereby improving instrument sensitivity. A limit of detection of 2 ppb could be achieved for 2,4-dimethylaniline in a methanol solution. The sampling system described in the present study is specifically suitable for a miniature photoionization ion trap mass spectrometer that can perform rapid and online analysis for liquid samples. Graphical Abstract ᅟ.

Pulse length of ultracold electron bunches extracted from a laser cooled gas.

We present measurements of the pulse length of ultracold electron bunches generated by near-threshold two-photon photoionization of a laser-cooled gas. The pulse length has been measured using a resonant 3 GHz deflecting cavity in TM110 mode. We have measured the pulse length in three ionization regimes. The first is direct two-photon photoionization using only a 480 nm femtosecond laser pulse, which results in short (∼15 ps) but hot (∼104 K) electron bunches. The second regime is just-above-threshold femtosecond photoionization employing the combination of a continuous-wave 780 nm excitation laser and a tunable 480 nm femtosecond ionization laser which results in both ultracold (∼10 K) and ultrafast (∼25 ps) electron bunches. These pulses typically contain ∼103 electrons and have a root-mean-square normalized transverse beam emittance of 1.5 ± 0.1 nm rad. The measured pulse lengths are limited by the energy spread associated with the longitudinal size of the ionization volume, as expected. The third regime is just-below-threshold ionization which produces Rydberg states which slowly ionize on microsecond time scales.

Modelling of direct ultraviolet photoionization and charge recombination of aerosol nanoparticles in continuous flow

Ultrafine aerosol particles are electrically charged in a range of devices to enable their detection, capture, and control. Direct ultraviolet (UV) photoionization enables increased charging of some nanoparticle materials over alternative charging mechanisms such as diffusion charging, particularly in size ranges below 50 nm diameter. The aim of this work is to provide modelling and simulation of ion and particle charge and discharge processes and transport and collection in a continuous flow. A non-dimensional analysis indicates regimes under which the photocharging process is dominated by diffusion, electric field transport, convection, photoionization, or recombination. The computational fluid dynamics (CFD) model developed in this work is the first to include UV photoionization and detailed ion and particle recombination theory. The validity of assumptions made for diffusional wall losses and external electric field action is evaluated by comparison with 0D Numerical and 3D CFD models. Regimes are identified to distinguish the level of details required for aerosol transport and charging models.

A HIGH-RESOLUTION VACUUM ULTRAVIOLET LASER PHOTOIONIZATION AND PHOTOELECTRON STUDY OF THE CO ATOM

ABSTRACT We have measured the vacuum ultraviolet–photoionization efficiency (VUV–PIE) spectrum of Co in the energy range of 63,500–67,000 cm−1, which covers the photoionization transitions of Co(3d74s2 4F9/2) Co+(3d8 3F4), Co(3d74s2 4F7/2) Co+(3d8 3F3), Co(3d74s2 4F9/2) Co+(3d8 3F3), Co(3d74s2 4F9/2) Co+(3d8 3F2), and Co(3d74s2 4F9/2) Co+(3d74s1 5F5). We have also recorded the pulsed field ionization photoelectron spectrum of Co in the same energy range, allowing accurate determinations of ionization energies (IEs) for the photoionization transitions from the Co(3d74s2 4F9/2) ground neutral state to the Co+(3F J ) (J = 4 and 3) and Co+(5F5) ionic states, as well as from the Co(3d74s2 4F7/2) excited neural state to the Co+(3d8 3F3) ionic state. The high-resolution nature of the VUV laser used has allowed the observation of many well-resolved autoionizing resonances in the VUV–PIE spectrum, among which an autoionizing Rydberg series, 3d74s1(5F5)np (n = 19–38), converging to the Co+(3d74s1 5F5) ionic state from the Co(3d74s2 4F9/2) ground neutral state is identified. The fact that no discernible step-like structures are present at these ionization thresholds in the VUV–PIE spectrum indicates that direct photoionization of Co is minor compared to autoionization in this energy range. The IE values, the autoionizing Rydberg series, and the photoionization cross sections obtained in this experiment are valuable for understanding the VUV opacity and abundance measurement of the Co atom in stars and solar atmospheres, as well as for benchmarking the theoretical results calculated in the Opacity Project and the IRON Project, and thus are of relevance to astrophysics.

Photoionization of Cl+from the 3s23p4 3P2,1,0and the 3s23p4 1D2,1S0states in the energy range 19–28 eV

Absolute photoionisation cross sections for the Cl$^+$ ion in its ground and the metastable states; $3s^2 3p^4\; ^3P_{2,1,0}$, and $3s^2 3p^4\; ^1D_2,\; ^1S_0$, were measured recently at the Advanced Light Source ALS) at Lawrence Berkeley National Laboratory using the merged beams photon-ion technique at an photon energy resolution of 15 meV in the energy range 19 -- 28 eV. These measurements are compared with large-scale Dirac Coulomb {\it R}-matrix calculations in the same energy range. Photoionisation of this sulphur-like chlorine ion is characterized by multiple Rydberg series of autoionizing resonances superimposed on a direct photoionisation continuum. A wealth of resonance features observed in the experimental spectra are spectroscopically assigned and their resonance parameters tabulated and compared with the recent measurements. Metastable fractions in the parent ion beam are determined from the present study. Theoretical resonance energies and quantum defects of the prominent Rydberg series $3s^2 3p^3 nd$, identified in the spectra as $3p\rightarrow nd$ transitions are compared with the available measurements made on this element. Weaker Rydberg series $3s^2 3p^3 ns$, identified as $3p \rightarrow ns$ transitions and window resonances $3s3p^4 (^4P)np$ features, due to $3s \rightarrow np$ transitions are also found in the spectra.

Fine- and hyperfine-structure effects in molecular photoionization. I. General theory and direct photoionization.

We develop a model for predicting fine- and hyperfine intensities in the direct photoionization of molecules based on the separability of electron and nuclear spin states from vibrational-electronic states. Using spherical tensor algebra, we derive highly symmetrized forms of the squared photoionization dipole matrix elements from which we derive the salient selection and propensity rules for fine- and hyperfine resolved photoionizing transitions. Our theoretical results are validated by the analysis of the fine-structure resolved photoelectron spectrum of O2 reported by Palm and Merkt [Phys. Rev. Lett. 81, 1385 (1998)] and are used for predicting hyperfine populations of molecular ions produced by photoionization.

Characterization of a bipolar near-infrared laser desorption/ionization aerosol mass spectrometer

A novel method of soft ionization aerosol mass spectrometry (AMS), bipolar near-infrared laser desorption/ionization AMS (BP-NIR-LDI-AMS), has been developed for the on-line, real-time analysis of organic aerosols. Use of a single NIR laser pulse to desorb/ionize aerosols deposited onto an aluminum probe results in minimal analyte fragmentation to produce exclusively intact pseudomolecular ions at [M–H]− for acidic organic analytes and [M+H]+ for basic organic analytes. Incorporation of a bipolar mass spectrometer with the NIR-LDI source enables simultaneous detection of acidic and basic species in organic particles. Limits of detection (total particulate mass sampled) for amino acids common to the organic fraction of atmospheric aerosols ranged from 69.1 pg for ornithine to 197 pg for serine on the positive channel, and from 17.0 pg for glycine to 100 pg for ornithine on the negative channel. From studies of the laser energy dependence of the NIR-LDI mechanism, it was found that [M–H]− formation for oleic acid proceeds through simultaneous action of two 1064 nm photons, suggesting a surface-assisted process rather than direct photoionization, for which photon energy is insufficient. For acidic aerosol species, sensitivity was found to increase as a function of analyte acidity, while for basic species, [M+H]+ ion signals were detected only in the presence of a labile proton source, with the intensity of the ion signals scaling with the acidity of the proton source. The sensitivity of BP-NIR-LDI-AMS to the amino acids was rationalized in terms of their acidic/basic character, as measured by isoelectric point (pI), with the cationic sensitivity scaling proportionally with pI and the anionic sensitivity scaling inversely with pI.© 2016 American Association for Aerosol Research

Ion microscopy based on laser-cooled cesium atoms

We demonstrate a prototype of a Focused Ion Beam machine based on the ionization of a laser-cooled cesium beam and adapted for imaging and modifying different surfaces in the few-tens nanometer range. Efficient atomic ionization is obtained by laser promoting ground-state atoms into a target excited Rydberg state, then field-ionizing them in an electric field gradient. The method allows obtaining ion currents up to 130pA. Comparison with the standard direct photo-ionization of the atomic beam shows, in our conditions, a 40-times larger ion yield. Preliminary imaging results at ion energies in the 1–5keV range are obtained with a resolution around 40nm, in the present version of the prototype. Our ion beam is expected to be extremely monochromatic, with an energy spread of the order of the eV, offering great prospects for lithography, imaging and surface analysis.

Time-discretized extreme and vacuum ultraviolet spectroscopy of spark discharges in air, N2 and O2

In this paper we present time-discretized spectra of spark discharges in air, N2 and O2. In previous work, a system for temporally resolved spectral analysis of extreme ultraviolet (EUV) and vacuum ultraviolet (VUV) emission from spark discharges was presented, along with some initial results. As was noted in this paper, statistical variances and the lacking of an apparatus sensitivity profile limited the usability of the data obtained. We have investigated the cause of these variances and improved the setup to reduce their effect. We also investigated the apparatus sensitivity profile to correct the intensity of measured lines. Newly obtained spectra in dry air, N2 and O2 are presented. Air and N2 show high emission in the vicinity of 100 nm, where direct photoionization of molecular oxygen is possible, in the first 250 ns of the discharge. We conclude this emission originates from nitrogen, which has several intense molecular transitions in this region. This finding is confirmed by our experimental results which show the emission in this region is much lower in oxygen.

Selectivity in the photofragmentation of halo-pyrimidines

SynopsisThe fragmentation of 2Br-, 2Cl- and 5Br-pyrimidine following direct valence photoionization or inner shell excitation and decay has been studied by electron-ion coincidence experiments. The results show that the fragmentation is strongly selective on the final singly charged ion state and the dominant fragmentation patterns correlate to the nearest appearance potential.

Generation of warm dense matter using an argon based capillary discharge laser

Argon based capillary discharge lasers operating in the extreme ultra violet (EUV) at 46.9 nm with output up to 0.5 mJ energy per pulse and repetition rates up to 10 Hz are capable of focused irradiances of 109–1012 W cm−2 and can be used to generate plasma in the warm dense matter regime by irradiating solid material. To model the interaction between such an EUV laser and solid material, the 2D radiative-hydrodynamic code POLLUX has been modified to include absorption via direct photo-ionisation, a super-configuration model to describe the ionization-dependent electronic configurations and a calculation of plasma refractive indices for ray tracing of the incident EUV laser radiation. A simulation study is presented, demonstrating how capillary discharge lasers of 1200 ps pulse duration can be used to generate warm dense matter at close to solid densities with temperatures of a few eV and energy densities up to 1 × 105 J cm−3. Plasmas produced by EUV laser irradiation are shown to be useful for examining the properties of warm dense matter as, for example, plasma emission is not masked by hotter, less dense plasma emission that occurs with visible/infra-red laser target irradiation.

Ionization of EPA contaminants in direct and dopant-assisted atmospheric pressure photoionization and atmospheric pressure laser ionization.

Seventy-seven EPA priority environmental pollutants were analyzed using gas chromatography-mass spectrometry (GC-MS) equipped with an optimized atmospheric pressure photoionization (APPI) and an atmospheric pressure laser ionization (APLI) interface with and without dopants. The analyzed compounds included e.g., polycyclic aromatic hydrocarbons (PAHs), nitro compounds, halogenated compounds, aromatic compounds with phenolic, acidic, alcohol, and amino groups, phthalate and adipatic esters, and aliphatic ethers. Toluene, anisole, chlorobenzene, and acetone were tested as dopants. The widest range of analytes was ionized using direct APPI (66/77 compounds). The introduction of dopants decreased the amount of compounds ionized in APPI (e.g., 54/77 with toluene), but in many cases the ionization efficiency increased. While in direct APPI the formation of molecular ions via photoionization was the main ionization reaction, dopant-assisted (DA) APPI promoted ionization reactions, such as charge exchange and proton transfer. Direct APLI ionized a much smaller amount of compounds than APPI (41/77 compounds), showing selectivity towards compounds with low ionization energies (IEs) and long-lived resonantly excited intermediate states. DA-APLI, however, was able to ionize a higher amount of compounds (e.g. 51/77 with toluene), as the ionization took place entirely through dopant-assisted ion/molecule reactions similar to those in DA-APPI. Best ionization efficiency in APPI and APLI (both direct and DA) was obtained for PAHs and aromatics with O- and N-functionalities, whereas nitro compounds and aliphatic ethers were the most difficult to ionize. Halogenated aromatics and esters were (mainly) ionized in APPI, but not in APLI.

Resonant Auger decay of the 4d → 6p excitation in Xe driven by short intense coherent soft x-ray pulses

The dynamics of the resonant Auger decay of the Xe excited state induced by a short coherent and intense soft x-ray laser pulse is investigated theoretically. The present approach includes (i) the non-Hermitian coupling between the ground state and the resonance caused by the driving pulse, (ii) the interference between the coherent populations of the final ionic states by the decay of the resonance and by the direct photoionization of the ground state, and (iii) the direct ionization of the resonance itself. The individual influence of the different competing physical processes on the total ion yield and on the electron spectrum of the most intense Xe spectator Auger decay line is examined. The present numerical spectra are interpreted analytically in terms of the dynamic interference of the electron waves emitted on the rising and falling sides of the driving pulse. Our results provide a theoretical basis for experiments on the verification of the dynamic interference at currently available sources of intense high-frequency laser pulses.