Near-threshold Photoionization Research Articles

Determination of absolute photoionization cross-sections of oxygenated hydrocarbons

The near-threshold absolute photoionization cross-sections of 12 oxygenated hydrocarbons, including 1-butanol, 2-butanol, iso-butanol, tert-butanol, 1-pentanol, tert-pentanol, iso-pentanol, methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), furan, 2-methylfuran, 2,5-dimethylfuran, were measured in the photon energy range from respective ionization thresholds to 11.5 eV. The experiments were performed with photoionization mass spectrometry and tunable synchrotron vacuum ultraviolet (VUV) light. Binary-liquid-mixtures of the investigated sample and benzene were used in the measurements where benzene acts as a calibration standard, due to its well known photoionization cross-section at the photon energies from its ionization energy (9.24 eV) to 11.5 eV. Photodissociative fragments from the molecules were also observed, and their photodissociation cross-sections are also presented.

Photoionization of ions in arbitrary charge states by synchrotron radiation in an electron beam ion trap

Photoionization of ions in various charge states is studied with an electron beam ion trap at the synchrotron BESSY II. The ion target density achieved by this method, representing an increase of up to four orders of magnitude with respect to conventional techniques, gives unprecedented access to photoionization of highly charged ions at photon energies reaching the keV range. Data on near-threshold photoionization of N3+, Ar12+, Fe12+ combined with measurements on neutral gas targets in the same setup demonstrate the versatility of this technique and show both very good resolution and accuracy.

Phase-resolved attosecond near-threshold photoionization of molecular nitrogen

We photoionize nitrogen molecules with a train of extreme ultraviolet attosecond pulses together with a weak infrared field. We measure the phase of the two-color two-photon ionization transition (molecular phase) for different states of the ion. We observe a 0.76 $\pi$ shift for the electrons produced in the ionization channels leading to the X$^2 \Sigma_g^+$, $v'=1$ and $v'=2$ states. We relate this phase shift to the presence of a complex resonance in the continuum. By providing both a high spectral and temporal resolution, this general approach gives access to the evolution of extremely short lived states, which is hardly accessible otherwise.

Ultracold Electron Source for Single-Shot, Ultrafast Electron Diffraction

Ultrafast electron diffraction (UED) enables studies of structural dynamics at atomic length and timescales, i.e., 0.1 nm and 0.1 ps, in single-shot mode. At present UED experiments are based on femtosecond laser photoemission from solid state cathodes. These photoemission sources perform excellently, but are not sufficiently bright for single-shot studies of, for example, biomolecular samples. We propose a new type of electron source, based on near-threshold photoionization of a laser-cooled and trapped atomic gas. The electron temperature of these sources can be as low as 10 K, implying an increase in brightness by orders of magnitude. We investigate a setup consisting of an ultracold electron source and standard radio-frequency acceleration techniques by GPT tracking simulations. The simulations use realistic fields and include all pairwise Coulomb interactions. We show that in this setup 120 keV, 0.1 pC electron bunches can be produced with a longitudinal emittance sufficiently small for enabling sub-100 fs bunch lengths at 1% relative energy spread. A transverse root-mean-square normalized emittance of epsilon(x) = 10 nm is obtained, significantly better than from photoemission sources. Correlations in transverse phase-space indicate that the transverse emittance can be improved even further, enabling single-shot studies of biomolecular samples.

Experimental Study of Laminar Lean Premixed Methylmethacrylate/Oxygen/Argon Flame at Low Pressure

A fuel-lean laminar premixed methylmethacrylate/oxygen/argon flame at 2.67 kPa with an equivalence ratio (phi) of 0.75 has been investigated with the tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular beam sampling mass spectrometry techniques. Isomers of most observed species in the flame have been identified by measurements of photoionization mass spectra and the near-threshold photoionization efficiency spectra. Mole fraction profiles for about 42 flame species are displayed. Free radicals such as CH3, C2H3, C2H5, C3H3, C3H5, C2H3O, C4H7, C3H5O, C3H7O, C4H3O, C4H9O, C4H5O2, C4H7O2, and C5H7O2, which should be of importance in understanding the formation mechanism of some toxic substances, were detected in the flame. Moreover, no isomers of any PAHs have been detected in the lean flame. Combined with the mole fraction profiles, the formation mechanisms of the free radicals, oxygenated compounds, and other molecular intermediates are proposed and will provide important information on modeling the combustion kinetics of methylmethacrylate (MMA).

Near-threshold absolute photoionization cross-sections of some reaction intermediates in combustion

The use of photoionization mass spectrometry for the development of quantitative kinetic models for the complex combustion chemistry of both conventional hydrocarbon fuels and oxygenated biofuels requires near-threshold measurements of absolute photoionization cross-sections for numerous reaction intermediates. Near-threshold absolute cross-sections for molecular and dissociative photoionization for 20 stable reaction intermediates (methane, ethane, propane, n-butane, cyclopropane, methylcyclopentane, 1-butene, cis-2-butene, isobutene, 1-pentene, cyclohexene, 3,3-dimethyl-1-butene, 1,3-hexadiene, 1,3-cyclohexadiene, methyl acetate, ethyl acetate, tetrahydrofuran, propanal, 1-butyne, 2-butyne) are presented. Previously measured total photoionization cross-sections for 9 of these molecules are in good agreement with the present results. The measurements are performed with photoionization mass spectrometry (PIMS) using a monochromated VUV synchrotron light source with an energy resolution of 40 meV (fwhm) comparable to that used for flame-sampling molecular beam PIMS studies of flame chemistry and reaction kinetics.

Simulated performance of an ultracold ion source

At present, the smallest spot size which can be achieved with state-of-the-art focused ion beam (FIB) technology is mainly limited by the chromatic aberrations associated with the 4.5 eV energy spread of the liquid-metal ion source. Here we numerically investigate the performance of an ultracold ion source which has the potential for generating ion beams which combine high brightness with small energy spread. The source is based on creating very cold ion beams by near-threshold photoionization of a laser-cooled and trapped atomic gas. We present ab initio numerical calculations of the generation of ultracold beams in a realistic acceleration field and including all Coulomb interactions, i.e., both space charge effects and statistical Coulomb effects. These simulations demonstrate that with existing technology reduced brightness values exceeding 105 A m−2 sr−1 V−1 are feasible at an energy spread as low as 0.1 eV. The estimated spot size of the ultracold ion source in a FIB instrument ranges from 10 nm at a current of 100 pA to 0.8 nm at 1 pA.

Relativistic R-matrix close-coupling calculations for photoionization of Ne-like Al IV

We present relativistic close-coupling photoionization calculations of Al IV using the Breit-Pauli R-matrix method to obtain photoionization cross-section of Al IV from the ground state and the lowest two J=0 (even) excited states. A multi-configuration eigenfunctions expansion of the core Al V is employed with spectroscopic configurations 2s22p5, 2s2p6, 2s22p43s, 2s22p43p, 2s22p43d and 2s22p44s. We have included, for the first time, the lowest 68 level target states of Al V in the photoionization calculations of Al IV. Extensive configuration interaction wavefunctions are used to describe both the initial Al IV states and the final Al V states. Cross-sections are compared from three level calculations including only 2s2 2p5 2Po 3/2, 1/2 and 2s 2p6 2S1/2 levels of Al V. The present calculation using the lowest 68 target levels of Al V are presented for the first time and should provide reasonably complete database for practical application for photoionization cross-section for Al IV, where high-energy cross-sections along with near-threshold photoionization cross-section is required.

Near-threshold photoionization of hot isopropyl radicals

A combination of ion imaging and vacuum ultraviolet, single-photon ionization is used to study the internal energy dependence of the photoionization cross section of isopropyl radicals produced by the 266 nm photodissociation of isopropyl iodide. The isopropyl radicals so produced have internal energies of approximately 0.3-2.0 eV. Images recorded for photoionization energies from just below the adiabatic ionization threshold at 7.37+/-0.02 and 8.04 eV are essentially identical both to each other and to that recorded at 9.67 eV. These results imply that the photoionization cross section is only weakly dependent on internal energy. Several factors contributing to this observation are discussed, as are the implications for the photoionization of other systems with significant internal excitation.

Population of individual Ne1s−1npRydberg states measured across the1sionization threshold and the evolution of shake-up into shake-down

New high-resolution experimental results on Ne $1s$ near-threshold photoionization with subsequent decay into a $2{p}^{4}(^{1}D_{2}){n}^{\ensuremath{'}}p$ or $2{p}^{4}(^{1}D_{2}){\ensuremath{\epsilon}}^{\ensuremath{'}}p$ state are presented. The population of Rydberg states up to ${n}^{\ensuremath{'}}=12$ is determined as a function of excess energy. These data allow one to track in detail the transition from resonant Auger decay going along with shake-up of the spectator electron to shake-down and photoionization with recapture of the photoelectron. Results are in good agreement with a time-dependent quantum mechanical theory of Auger decay in the presence of a slow photoelectron.

Nonstationary theory for short-pulse near-threshold photoionization of inner atomic shells

A theoretical description of near-threshold ionization of inner atomic shells by short electromagnetic pulses is developed. It is based on a nonstationary approach to the Fano-Feshbach problem of photoexcitation with simultaneous Auger decay of the produced vacancy. We suggest a method of solving the system of time-dependent Schr\odinger equations describing a resonant state embedded into continuum. The method gives a nonstationary description of the post-collision interaction in near-threshold photoionization including the recapture of the slow photoelectron in high Rydberg states. As an example, $1s$ photoionization in Ne with the following $\mathit{KLL}$ Auger decay is considered. The influence of the duration of the pulse on the recapture probability and shape of the emitted electron peak is studied.

Three-photon near-threshold photoionization dynamics of isooctane

The electron survival probability following three-photon (9.3 eV total) near-threshold photoionization of neat isooctane is measured with sub-50 fs time resolution. The measured dynamics are nonexponential in time and are well described by a diffusion-controlled electron-cation recombination model. Excitation-power-dependent studies indicate that the unperturbed three-photon threshold ionization is only observed for pump irradiance below 0.5 TW cm2. At excitation fields above this level, the signal is no longer cubic in the excitation irradiance, and the observed electron survival probability dramatically changes, decaying as a single exponential in time.

Two photoionization thresholds of N3 produced by ClN3 photodissociation at 248 nm: Further evidence for cyclic N3

We present results of near-threshold photoionization of N3 photofragments produced by laser photodissociation of ClN3 at 248 nm. The time of flight of recoiling N3 is used to resolve two photochemical channels producing N3, which exhibit different translational energy release. The two forms of N3 resolved in this way exhibit different photoionization thresholds, consistent with their assignment to linear (X 2pi(g)) and cyclic N3. This result agrees with the existing theoretical calculations of excited and ionic states of N3 and strengthens previous experimental results which suggested that the ClN3 photolysis produces a cyclic form of N3.

A study of photoelectron recapture due to post-collision interaction in Ne at the 1s photoionization threshold

Electron spectra from radiationless decay of a Ne 1s vacancy are measured with excitation energies immediately at the 1s ionization threshold. Special attention is given to the excess energy region where Γ is the lifetime width of the 1s vacancy. Spectra are dominated by recapture of photoelectrons into Rydberg states of Ne+ above the threshold, and by a smooth transition into the spectator resonant Auger spectrum below the threshold. High resolution of the present experiment allowed us to clearly observe Rydberg states up to n = 8 and to determine the individual probabilities for populating of these states by photoelectron recapture. Results are compared with calculations based on the semiclassical theory of post-collision interaction extended to high Rydberg excitations of the primary photoexcited electron, as well as with a non-stationary quantum mechanical theory of near-threshold photoionization with subsequent Auger decay.

An unusual π* shape resonance in the near-threshold photoionization of S1 para-difluorobenzene

Previously reported dramatic changes in photoelectron angular distributions (PADs) as a function of photoelectron kinetic energy following the ionization of S1 p-difluorobenzene are shown to be explained by a shape resonance in the b(2g) symmetry continuum. The characteristics of this resonance are clearly demonstrated by a theoretical multiple-scattering treatment of the photoionization dynamics. New experimental data are presented which demonstrate an apparent insensitivity of the PADs to both vibrational motion and prepared molecular alignment, however, the calculations suggest that strong alignment effects may nevertheless be recognized in the detail of the comparison with experimental data. The apparent, but unexpected, indifference to vibrational excitation is rationalized by considering the nature of the resonance. The correlation of this shape resonance in the continuum with a virtual pi* antibonding orbital is considered. Because this orbital is characteristic of the benzene ring, the existence of similar resonances in related substituted benzenes is discussed.

Studies of a fuel-rich propane flame with photoionization mass spectrometry

Flame-sampling photoionization mass spectrometry, using continuously tunable synchrotron radiation, offers important advantages for studies of flame chemistry. Mole fraction profile measurements for 24 flame species are presented for a fuel-rich low-pressure premixed laminar C 3H 8/O 2/Ar flame. Near-threshold photoionization efficiency measurements provide selective detection of individual isomers and unambiguous identifications of other flame species of near-equal mass. The absolute molar composition of the allene and propyne isomers of C 3H 4 was determined. Absolute cross-sections for photoionization of C 2H 2, C 2H 4, CH 3OH, C 3H 4 (propyne), CH 3CHO, (CH 3) 2CO, and C 6H 6 were used to determine molecular beam mass discrimination factors valid for ion masses ranging from 26 to 78 amu.

Capture and re-emission of slow photoelectrons in Ar2p6-subshellphotoionization processes

The process of near-threshold Ar2p6 photoionization is studied, with a focus on the phenomena of slow-photoelectron captureinto the discrete spectrum, and the re-emission of these captured electrons into thecontinuum. Using the quantum mechanical model of post-collision interaction, we havecalculated the probabilities for slow-photoelectron capture and re-emission of the capturedelectrons, as well as the combined influence of these effects on the slow-electron yield. Boththe capture and the emission of electrons have an essential influence on the electron and ionyields. The results of the calculations are in good agreement with the availableexperimental data.

Trends in near-threshold photoionization of off-the-center endohedral atoms

Calculations of the photoelectron angular distribution from atoms confined in a fullerene cage have been performed using a recently developed theory of inner-shell near-threshold molecular photoionization. Trends uncovered in the modification of near-threshold photoelectron angular distribution of the A-C{sub 60} molecules as a function of the displacement of the confined atom from the center of the C{sub 60} cage are demonstrated. Specific results for Li 1s from Li-C{sub 60}, and Ar 1s, 2s, and 3s from Ar-C{sub 60} molecules are presented. In addition, the situation where the encapsulated atom lies in the vicinity of the center of one of the carbon hexagons of C{sub 60} and also when it is displaced beyond the fullerene cage, has been studied. It has been found that the photoelectron spectra are extremely sensitive, both qualitatively and quantitatively, to even a small displacement of the confined atom from the center of the cage. The existence of the C{sub 60} molecule even affects the atom's photoelectron angular distribution when it is a significant distance away.

Selective detection of isomers with photoionization mass spectrometry for studies of hydrocarbon flame chemistry

We report the first use of synchrotron radiation, continuously tunable from 8 to 15 eV, for flame-sampling photoionization mass spectrometry (PIMS). Synchrotron radiation offers important advantages over the use of pulsed vacuum ultraviolet lasers for PIMS; these include superior signal-to-noise, soft ionization, and access to photon energies outside the limited tuning ranges of current VUV laser sources. Near-threshold photoionization efficiency measurements were used to determine the absolute concentrations of the allene and propyne isomers of C3H4 in low-pressure laminar ethylene–oxygen and benzene–oxygen flames. Similar measurements of the isomeric composition of C2H4O species in a fuel-rich ethylene–oxygen flame revealed the presence of substantial concentrations of ethenol (vinyl alcohol) and acetaldehyde. Ethenol has not been previously detected in hydrocarbon flames. Absolute photoionization cross sections were measured for ethylene, allene, propyne, and acetaldehyde, using propene as a calibration standard. PIE curves are presented for several additional reaction intermediates prominent in hydrocarbon flames.

Diffraction of slow photoelectrons by C 60 fullerene cage

Diffraction of slow electrons emitted in the near-threshold 1s photoionization of a Li atom located outside a C 60 molecule has been investigated as a function of the distance of the Li atom from the C 60. It is found that, as in the case of the atom located inside the C 60, the photoelectron angular distribution is significantly distorted by the C 60 and is especially pronounced when the polarization vector of the radiation is along the line connecting the nucleus of the ionized atom with the geometrical center of the C 60. The effects of the C 60 extend to surprisingly large distances.