Near-threshold Photoionization Research Articles

Molecular Association and Reactivity of the Pyridine Dimer Cation.

A recent experimental report has identified the formation of the C-N hemibonded pyridine dimer cation following vacuum ultraviolet near-threshold photoionization [J. Phys. Chem. Lett., 2021, 12, 4936-4943]. Herein, the dynamics and consequent reactivity of the pyridine dimer cation were investigated employing Born-Oppenheimer molecular dynamics (BOMD) simulations. An antiparallel π-stacked pyridine dimer in the neutral ground state is transformed into a noncovalently interacting C-H···N hydrogen-bonded structure which can lead to proton transfer in the cationic state. Additionally, C-N- and N-N-bonded adducts were formed in the cationic state. Further, metastable C-H···H-C-bonded cationic species was observed, which rearranged to an N-N bonded adduct. In contrast to the experimental observation, migration of the proton to the α position was not observed in the C-N bonded adduct owing to a high barrier of about 2 eV. The observed trends in the molecular association, proton transfer, and the formation of C-N and N-N bonded adducts are a consequence of the roaming dynamics of one pyridine moiety over the other in the cationic state.

Glory interference spectroscopy in Sr atom

Slow (meV) photoelectron imaging spectroscopy is employed in the experimental study of near-threshold photoionization of strontium atoms in the presence of an external static electric field. Specifically, the study is devoted to the glory effect, that is, the appearance of an intense peak at the center of the recorded photoelectron images, when dealing with m= 0 final ionized Stark states (m denoting the magnetic quantum number). This critical effect is formally identical to that encountered in classical scattering theory, where, for a nonzero value of the impact parameter, the zero-crossing of the deflection function leads to a divergent classical differential cross section. By recording the magnitude variation of this glory peak as a function of electron excitation energy, we observe that, besides the traces of classical origin, it also exhibits intense quantum interference and beating phenomena, above and below the zero-static-field ionization threshold. We study both, single- and two-photon ionization of Sr, thus enabling a comparison not only between the different excitation schemes, but also with an earlier work devoted to two-photon ionization of Mg atom by Kalaitzis et al (2020 Phys. Rev. A 102 033101). Our recordings are analyzed within the framework of the Harmin–Fano frame transformation Stark effect theory that is applied to both the hydrogen atom and a non-hydrogenic one simulating Sr. We discuss the various aspects of the recorded and calculated glory interference and beating structures and their ‘short time Fourier transforms’ and classify them as either atom-specific or atom independent. In particular, we verify the ‘universal’ connection between the glory oscillations above the zero-field threshold and the differences between the origin-to-detector times of flight corresponding to pairs of classical electron trajectories that end up to the image center.

Ionization of aluminum clusters

Near-threshold photoionization cross sections for several aluminum clusters have been estimated from the ionization efficiency curves. Their magnitude raises the possibility that the edge of a collective electron resonance (surface plasmon) may contribute to the photoemission channel. On the other hand, electron impact ionization efficiently produces multiply charged metal cluster ions and is unaccompanied by noticeable fragmentation. Consequently it does not appear to involve collective excitations.

RF acceleration of ultracold electron bunches.

The ultrafast and ultracold electron source, based on laser cooling and trapping of atomic gas and its subsequent near-threshold two-step photoionization, is capable of generating electron bunches with a high transverse brightness at energies of roughly 10 keV. This paper investigates the possibility of increasing the range of applications of this source by accelerating the bunch using radio frequency electromagnetic fields. Bunch energies up to 35 keV are measured by analyzing the diffraction patterns generated from a mono-crystalline gold sample. It is found that the normalized transverse emittance is largely preserved during acceleration.

Design and optimization of a 100 keV DC/RF ultracold electron source

An ultracold electron source based on near-threshold photoionization of a laser-cooled and trapped atomic gas is presented in this work. Initial DC acceleration to ∼10 keV and subsequent acceleration of the created bunches to 100 keV by RF fields makes the design suitable to serve as injector for accelerator-based light sources, single-shot ultrafast protein crystallography, applications in dielectric laser acceleration schemes, and potentially as an injector for free electron lasers operating in the quantum regime. This paper presents the design and properties of the developed DC/RF structure. It is shown that operation at a repetition frequency of 1 kHz is achievable and detailed particle tracking simulations are presented showing the possibility of achieving a brightness that can exceed conventional RF photosources.

Experimental study on n-hexane pyrolysis initiated by 1-nitropropane in a low-pressure flow reactor

In order to investigate the species pool and primary channels of initiated pyrolysis of n-hexane, the low-pressure (30 Torr, 4.0 kPa) flow reactor pyrolysis of n-hexane initiated by 5% 1-nitropropane (1-NP) was studied experimentally in a wide range of temperature from 623 K to 1223 K. Combined with synchrotron vacuum ultraviolet (SVUV) photoionization mass spectra scanned at photon energies ranging from 9.0 to 14.7 eV and near-threshold photoionization efficiency (PIE) spectra, 29 pyrolysis species, covering C1–C6 hydrocarbons and several pyrolysis species containing O or N atoms, were identified. Based on the ion signal intensities of the visible peaks in the mass spectra, the mole fraction profiles of all the experimental species were evaluated. The experimental results suggested that the initial pyrolysis temperature of n-hexane was reduced by approximately 275–325 K. On the basis of the mole fraction profiles and the initial formation temperatures reported in this work and rate coefficients proposed in previous studies, the 1-NP-initiated pyrolysis mechanisms of n-hexane were discussed. It was suggested that n-hexane was dominantly consumed by the H-abstraction reactions by the attack of OH radical in this work. The 1-NP-initiated pyrolysis of n-hexane mainly proceeded via reactions to produce C3H6 + C3H7, or 1-C4H8 + C2H5, or 1-C5H10 + CH3.

Dynamical aspects of photoionization from the 1s22snp1P1o levels belonging to the C iii ion near the first ionization threshold

This paper presents time-dependent Dirac calculations for femtosecond laser-driven single-photon ionization of $1{s}^{2}2snp{\phantom{\rule{0.16em}{0ex}}}^{1}{P}_{1}^{o}$ states belonging to the C iii ion, with $n\ensuremath{\le}5$, near the first ionization threshold. Increasing the field intensity broadens the photoelectron spectral distribution, pushing the lower energetic tail into the forbidden region below threshold. The energy domain cutoff condition leads to the formation of nonlocal temporal terms in the evolution of atomic states. These terms effectively give rise to discrete-continuum Rabi oscillations consisting in cycles of pulsed photoionization and stimulated radiative recombination processes, suppressing the electron yield and modulating the spectral distribution into multipeak structures. Field intensity scaling with photoelectron energy restricts these oscillations to near-threshold photoionization. These effects become relevant as state-of-the-art lasers provide shorter pulses designed to monitor electron dynamics at their natural timescales in various chemical reactions or physical processes.

Experimental investigation on the low-pressure pyrolysis of cyclohexane initiated by 1-nitropropane in a flow tube reactor

An experimental study of 5% 1-nitropropane (1-NP)-initiated cyclohexane pyrolysis was carried out in a flow reactor at 30 Torr (4.0 kPa) over a temperature range of 673–1273 K. The pyrolysis species were detected with synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) technique. Pyrolysis species from m/z 15–106, including C1-C8 hydrocarbons, CO, NO, NO2, CH2O, CH3OH and C3H5NO, were identified with near-threshold photoionization efficiency (PIE) spectra and quantified at different temperatures. The experimental results indicated that the addition of 1-NP obviously reduced the initial pyrolysis temperature of cyclohexane from 1048 to 1098–723 K, and caused the decreases of the initial formation temperatures for the species from cyclohexane pyrolysis. Almost all the high-temperature pyrolysis species in the mechanisms proposed in the literature were identified in the experiments. Based on the experimental results in this work and rate coefficients proposed in the literature, the primary pyrolysis mechanisms of cyclohexane in the presence of 1-NP were discussed. The 1-NP pyrolysis occurs at a lower temperature than the cyclohexane does, and produces HONO, NO2, C3H7, etc. It was demonstrated that cyclohexane was mainly consumed via the H-atom abstractions by H and OH attacks in this work and the cyclohexyl radical formed via H-atom abstractions mainly isomerized into 1-C6H11-6 radical, which further combines with H to form 1-C6H12.

Space charge effect in low-density ultracold ion bunches

We have implemented an approach to investigate the space charge effect (SCE) in the ultracold ion bunch produced through the near-threshold photoionization of laser-cooled rubidium atoms trapped in a magneto-optical trap. The non-linear broadening of spatial profile of the ultracold ion bunch induced by SCE within the initial density range of 3.7 × 106–4.5 × 107/cm3 was explored using a time-of-flight spectrometer coupled with an imaging detector. A charged particle tracing simulation accounting for all pairwise ion–ion Coulomb interactions and an analytical model calculation, which predicts the dependence of the ion bunch density on time evolution and initial density, reproduced the experimental results successfully, indicating that the study could capture the evolution dynamics of ion bunch dominated by SCE. The aim of this work is to extend the investigation on SCE to extreme low-density regions of the order of 106 /cm3 and is expected to be useful in optimizing the performance of ultracold ion/electron sources.

Erratum: Phase-resolved attosecond near-threshold photoionization of molecular nitrogen [Phys. Rev. A 80 , 011404(R) (2009)

Erratum: Phase-resolved attosecond near-threshold photoionization of molecular nitrogen [Phys. Rev. A <b>80</b> , 011404(R) (2009)

Molecular Orbital Insight into the Near-Threshold Photoionization Cross Sections of Monocyclic Substituted Aromatic Compounds

Molecular Orbital Insight into the Near-Threshold Photoionization Cross Sections of Monocyclic Substituted Aromatic Compounds

Experimental study of nitropropane pyrolysis with molecular-beam mass spectrometry and tunable synchrotron VUV photoionization. Part II. The flow reactor pyrolysis of 2-nitropropane

An experimental study of 2-nitropropane (2-NP) pyrolysis was performed in a flow reactor at low pressure (4.0 kPa) and in the temperature range of 523–923 K. The pyrolysis species were detected with the tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry (MBMS) techniques. Species from m/z = 15 to 87, such as radicals of CH3, C2H5, C3H5 and C3H7; isomers of C3H4 (allene and propyne), C5H8 (1,3-pentadiene and cyclopentene), were identified from near-threshold photoionization efficiency (PIE) spectra. Four nitrogenous intermediates never reported in the homogeneous pyrolysis of 2-NP (i.e., 2-propen-1-imine, 2-propen-1-amine, acetamide and 2-nitropropene) were identified in this work. The mole fraction profiles of the pyrolysis species were evaluated at different temperatures. Based on the experimental findings, the pyrolysis pathways of 2-NP were discussed.

Experimental study of nitropropane pyrolysis with molecular-beam mass spectrometry and tunable synchrotron VUV photoionization. Part I. The flow reactor pyrolysis of 1-nitropropane

An experimental study of 1-nitropropane (1-NP) pyrolysis was performed in a flow reactor at low pressure (4.0 kPa) in the temperature range of 623–973 K. The pyrolysis species was detected with the tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry (MBMS). Species from m/z = 15–87 were identified from near-threshold photoionization efficiency (PIE) spectra, such as radicals of CH3, C2H5, C3H5 and C3H7; isomers of C2H4O (ethenol and acetaldehyde), C3H6O (acetone and propanal), C4H10 (butane and isobutane), HNC (hydrogen isocyanide), HCN (hydrogen cyanide)), C3H5N (ethyl isocyanide and species with unidentified structure) and C3H5NO (methoxyacetonitrile and species with unidentified structure). Their mole fraction profiles were evaluated at different temperature. Based on the mole fractions of species measured at the minimum temperature of 1-NP pyrolysis, some new channels of 1-NP primary pyrolysis were speculated, such as the channels involving O-transfer from N to C-atoms in different positions, deoxygenation, dehydrogenation and OH-abstraction. Based on the mechanisms proposed in the literature, the species produced from secondary or subsequent reactions were also discussed. These results are valuable to a further quantum mechanical calculation for a better understanding of 1-NP chemistry.

Glory oscillations in photoionization microscopy: Connection with electron dynamics and Stark spectral structures in the continuum

In classical scattering theory, the term “glory scattering” implies the divergence of the classical differential cross section that occurs as soon as the deflection function goes through zero for a nonzero value of the impact parameter. This critical effect also occurs in slow photoelectron imaging where near-threshold atomic photoionization is performed in the presence of an external static electric field. In this case, glory scattering manifests itself by the appearance of an intense peak at the center of the photoelectron momentum distribution. In the present work we examine the magnitude variation of this central peak as a function of electron energy. We experimentally study near-threshold two-photon ionization of ground state magnesium atoms, below as well as above the field-free ionization limit. It is found that, apart from its behavior of classical origin, the glory signal additionally exhibits strong oscillations and beating effects over the full spectral range of the recordings. Of particular interest are its oscillations above the zero-field limit, many aspects of which are expected to be independent of the atomic target. Our results are analyzed with the help of classical, semiclassical, and quantum mechanical calculations devoted to the hydrogenic Stark effect. It is theoretically found that these continuum glory oscillations are related to the resonantlike Stark structures appearing under certain conditions in the total photoionization cross section and implying energy quantization in the continuum. The striking outcome of the present study, however, is that both theory and experiment clearly support the connection between the energy- and static field-dependent periodicity of glory oscillations with the classical dynamics of electron motion. In particular, it is shown that the Fourier transform of the glory signal provides information on the differences between the origin-to-detector times of flight corresponding to specific pairs of classical electron trajectories.

Conjugate photoelectron recapture peaks in the high resolution Auger electron spectra following near-threshold Ar 2p photoionization

We have observed formation of Rydberg series superimposed on the Ar (L3M2,3M2,3) Auger electron spectral profile with photoexcitation just above the 2p−13/2 photoionization threshold. This is due to the recapture of photoelectrons induced by post-collision interaction between the Auger electron and photoelectron: Ar+ 2p−13/2 + e− → Ar+ 3p−2ml + e−. The intensity distribution of the angular momentum state resolved Rydberg peaks revealed prominent conjugate peaks due to angular momentum exchange between photoelectrons and Auger electrons in the process. In addition, we measured the 2p−13/2nl → 3p−2ml + e− resonant Auger spectra with high resolution below the 2p−13/2 threshold. Our results showed that conjugate shake-up/down 2p−13/2nd → 3p−2mp + e− processes are stronger than the direct 2p−13/2nd → 3p−2ms, (m−1)d + e− processes in the spectator Auger processes, implying a strong angular momentum exchange effect between photoelectron and Auger electron.

Narrow-band pulsed electron source based on near-threshold photoionization of Cs in a magneto-optical trap

The newly developed method of time-of-flight (ToF) momentum microscopy was used to analyse the cold electron emission from a Cs 3D magneto-optical trap (MOT). Three-step resonant photoionization was implemented via two intermediate states (6P3/2 pumped with 852 nm laser and 7S1/2 with 1470 nm) and a tuneable femtosecond Ti:sapphire laser for the final ionization step. The magnetic field of the MOT is switched off during the photoionization step. The natural bandwidth of the fs-laser is reduced to 4 meV using optical spectral filters. Precise tuning of the photon energy makes it possible to observe the transition regime between direct photoemission into the open continuum and field induced ionization of highly-excited Rydberg states. The paths can be identified by their characteristic dependency on the extraction field and on the Ti:sapphire polarization. ToF analysis allowed us to disentangle the ionization paths and the dependence of the spatio-temporal distribution of the cold electrons on the polarization of the ionizing laser.

Two-color XUV+NIR femtosecond photoionization of neon in the near-threshold region

Results of angle-resolved electron spectroscopy of near-threshold photoionization of Ne atoms by combined femtosecond extreme ultraviolet and near infrared fields are presented. The dressed-electron spectra show an energetic distribution into so-called sidebands, being separated by the photon energy of the dressing laser. Surprisingly, for the low kinetic energy (few eV) sidebands, the photoelectron energy varies as a function of the emission angle. Such behavior has not yet been observed in sideband creation and has not been predicted in commonly used theoretical descriptions such as strong field approximation and soft photon approach. Describing the photoionization with a time-dependent Schrödinger equation allows a qualitative description of the observed effect, as well as the prediction of fine structure in the sideband distribution.

Calculation of the absolute photoionization cross-sections for C1–C4 Criegee intermediates and vinyl hydroperoxides

Criegee Intermediates (CIs) and their isomer Vinyl Hydroperoxides (VHPs) are crucial intermediates in the ozonolysis of alkenes. To better understand the underlying chemistry of CIs and VHPs, progress has been made to detect and identify them by photoionization mass spectrometric experiments. Further reliable quantitative information about these elusive intermediates requires their photoionization cross sections. The present work systematically investigated the near-threshold absolute photoionization cross-sections for ten C1-C4 CIs and VHPs, i.e., formaldehyde oxide (CH2OO), acetaldehyde oxide (syn-/anti-CH3CHOO), acetone oxide ((CH3)2COO), syn-CH3-anti-(cis-CH=CH2)COO, syn-CH3-anti-(trans-CH=CH2)COO and vinyl hydroperoxide (CH2CHOOH), 2-hydroperoxypropene (CH2=C(CH3)OOH), syn-CH2 = anti-(cis-CH=CH2)-COOH, syn-CH2 = anti-(trans-CH=CH2)COOH. The adiabatic ionization energies (AIEs) were calculated at the DLPNO-CCSD(T)/CBS level with uncertainties of less than 0.05 eV. The calculated AIEs for C1-C4 CIs and VHPs vary from 8.75 to 10.0 eV with the AIEs decreasing as the substitutions increase. Franck-Condon factors were calculated with the double Duschinsky approximation and the ionization spectra were obtained based on the calculated ionization energies. Pure electronic photoionization cross sections are calculated by the frozen-core Hartree-Fock (FCHF) approximation. The final determined absolute cross sections are around 4.5-6 Mb for the first and second ionization of CIs and 15-25 Mb for VHPs. It is found that the addition of a methyl group or an unsaturated vinyl substitution for the CIs does not substantially change the absolute value of their cross sections.

Compact ultracold electron source based on a grating magneto-optical trap

The ultrafast and ultracold electron source, based on near-threshold photoionisation of a laser-cooled and trapped atomic gas, offers a unique combination of low transverse beam emittance and high bunch charge. Its use is however still limited because of the required cold-atom laser-cooling techniques. Here we present a compact ultracold electron source based on a grating magneto-optical trap (GMOT), which only requires one trapping laser beam that passes through a transparent accelerator module. This makes the technique more widely accessible and increases its applicability. We show the GMOT can be operated with a hole in the center of the grating and with large electric fields applied across the trapping region, which is required for extracting electron bunches. The calculated values of the applied electric field were found to agree well with measured Stark shifts of the laser cooling transition. The electron beams extracted from the GMOT have been characterised. Beam energies up to 10 keV were measured using a time-of-flight method. The normalised root-mean-squared transverse beam emittance was determined using a waist scan method, resulting in $\epsilon = 1.9 \rm{nm}$. The root-mean-squared transverse size of the ionisation volume is $30 \mu\rm{m}$ or larger, implying an electron source temperature in the few-10K range, $2-3$ orders of magnitude lower than conventional electron sources, based on photoemission or thermionic emission from solid state surfaces.

Observation of Rydberg Blockade Induced by a Single Ion.

We study the long-range interaction of a single ion with a highly excited ultracold Rydberg atom and report on the direct observation of an ion-induced Rydberg excitation blockade mediated over tens of micrometer distances. Our hybrid ion-atom system is directly produced from an ultracold atomic ensemble via near-threshold photoionization of a single Rydberg excitation, employing a two-photon scheme that is specifically suited for generating a very low-energy ion. The ion's motion is precisely controlled by small electric fields, which allows us to analyze the blockade mechanism for a range of principal quantum numbers. Finally, we explore the capability of the ion as a high-sensitivity, single-atom-based electric field sensor. The observed ion-Rydberg-atom interaction is of current interest for entanglement generation or studies of ultracold chemistry in hybrid ion-atom systems.