Near-threshold Photoelectron Research Articles

Angular distribution of near-threshold photoelectrons from resonance-enhanced multiphoton ionization of argon

We experimentally study the angular distribution of near-threshold photoelectrons of argon in the multiphoton-ionization regime. Those near-threshold photoelectrons are released from atoms by a weak 800-nm laser field and a strong 400-nm laser field with parallel polarizations. In the laser polarization direction, the yield of the near-threshold photoelectron oscillates with twice the 800-nm laser frequency, while higher-frequency components for the yield oscillations are found for the electrons emitted perpendicular to the laser polarization direction. By decomposing the photoelectron angular distribution into different partial waves, we found that those near-threshold photoelectrons are generated by the interference of different photon channels via resonant states. Moreover, we observed a large time delay between the near-threshold photoelectrons emitted along the laser polarization direction via the $5p$ and $4f$ resonant states.

Near-threshold photoelectron holography beyond the strong-field approximation

We study photoelectron angular distributions (PADs) near the ionization threshold with a newly developed Coulomb quantum-orbit strong-field approximation (CQSFA) theory. The CQSFA simulations present an excellent agreement with the result from time-dependent Schr\"odinger equation method. We show that the low-energy fan-shaped structure in the PADs corresponds to a subcycle time-resolved holographic structure and stems from the significant influence of the Coulomb potential on the phase of the forward-scattering electron trajectories, which affects different momenta and scattering angles unequally. For the first time, our work provides a direct explanation of how the fan-shaped structure is formed, based on the quantum interference of direct and forward-scattered orbits.

Design Near-threshold Photoelectron Imaging Spectrometer Based on UV Laser Induced Photoelectron Emission Anion Source

We have developed a compact photoelectron imaging facility, including an anion source with dissociative photoelectron attachment to molecules, a linear time-of-flight mass spectrometry (TOFMS), and an orthogonal high-resolution threshold photoelectron velocity map imaging spectrometer (VMI). Intense and cold cluster anions were prepared in photoelectron-attachment processes upon pulsed UV laser ablation of metal target. Combining this anion source with TOFMS-VMI, the achieved mass resolution is about 200, and the electron kinetic energy resolution is better than 3%, i.e., 30 meV for 1 eV electrons. More importantly, low-energy photoelectron imaging spectra for CH3S− and S2− at 611.46 nm are obtained. In both cases, the refined electron affinities are determined to be 1.8626±0.0020 eV for CH3S and 1.6744±0.0035 eV for S2, respectively. Preliminary results suggest that the apparatus is a powerful tool for estimating precise electron affinities values from threshold photoelectron imaging spectroscopy.

Near-threshold photoelectron angular distributions from two-photon resonant photoionization of He

Two-photon resonant photoionization of helium is investigated both experimentally and theoretically. Ground state helium atoms are excited to the 1s4p, 1s5p and 1s6p 1P states by synchrotron radiation and ionized by a synchronized infrared pulsed picosecond laser. The photoelectron angular distributions of the emitted electrons are measured using a velocity map imaging (VMI) spectrometer. The measured asymmetry parameters of the angular distribution allow the phase differences and the ratios of the dipole matrix elements of the 1sϵs and 1sϵd channels to be determined. The experimental results agree with the calculated values obtained in a configuration–interaction calculation with a Coulomb–Sturmian basis set. The effects of the radiative decay of the intermediate state and the static electric field of the VMI spectrometer on the measurements are discussed.

Near-threshold photoelectron angular distributions from two-photon resonant ionisation of He and Ne atoms

We present an overview of two recent experiments in which photoelectron angular distributions from light-polarised He and Ne atoms are studied, with the emphasis on theoretical description of photoionisation. Polarised He (Ne) atoms are prepared in the 1s4p, 1s5p, or 1s6p (2p53d) states with linearly polarised synchrotron light and ionised with a tunable infrared laser. The asymmetry parameters extracted from the measured velocity map imaging spectra agree well with the calculated asymmetry parameters.

Fully differential measurement on above-threshold ionization of CO and CO_2 molecules in strong laser fields

We present a fully differential measurement on above-threshold ionization of diatomic (CO) and linear triatomic (CO2) molecules in strong femtosecond laser fields (25fs, 795nm) at a laser intensity of ∼8×1013W/cm2. The photoelectron longitudinal momentum distribution of CO in the laser polarization direction shows a broader distribution near zero momentum in the laser polarization plane with a shoulder around 0.38a.u., whereas it shows a clear maximum for CO2. The relative photoelectron yields near zero longitudinal momentum are less dependent on the laser intensity. The difference may result from the molecular structure. Even though molecular targets have different binding energy and outer orbitals, the near-threshold photoelectrons have similar angular distribution, and the dominant angular momentum of the near-threshold photoelectron is L=4 (ng Rydberg state) for both targets.

Dynamics of the Acetyloxyl Radical Studied by Dissociative Photodetachment of the Acetate Anion

Photodetachment of the acetate anion, CH3CO2 - , and the subsequent dissociation dynamics of the CH3CO2 ¥ radical were studied using photoelectron-photofragment coincidence spectroscopy at 355 and 257 nm. An upper limit to the adiabatic electron affinity (EA) of CH 3CO2 ¥ ,E A) 3.47 ( 0.01 eV was determined. Evidence for several low-lying electronic states of the CH 3CO2 ¥ radical were observed in the photoelectron spectra. Most CH3CO2 ¥ radicals dissociated to CH3 ¥ + CO2 products with a large kinetic energy release (〈ET〉/Eavl ) 0.72 for 355-nm excitation) and an anisotropic angular distribution ( ‚ 1.2), while 10% of CH3CO2 ¥ radicals were stable on the microsecond time scale at both wavelengths. Structured near-threshold photoelectron spectra at 355 nm were similar when photoelectrons were recorded in coincidence with either stable radicals or dissociation products, indicating a sequential dissociative photodetachment. Experiments were also carried out on CD3CO2 - to aid in the interpretation of the photoelectron spectra and deduction of the dissociation mechanism.

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.

Photoionization threshold shapes of metal clusters

We demonstrate the efficacy of the Fowler equation, derived for the near-threshold photoelectron yield of bulk metal surfaces, for fitting the ionization spectra of finite metal clusters. The known photoionization curves of KN=30–101 are matched to a Fowler plot. The fitted ionization potentials agree well with the previously determined values and extrapolate even more accurately to the bulk work function; the fitted temperature closely matches the prediction of evaporation ensemble theory.

A photoelectron spectrometer for studying reactive intermediates using synchrotron radiation

The construction of a photoelectron spectrometer, designed for studying reactive intermediates with synchrotron radiation, is described. The principal features are the inclusion of several stages of sample pumping, necessary to provide the required gas throughput and to protect the electron optics of the storage ring and spectrometer. The facility to rotate the whole spectrometer is also included in the design. Some data are presented which demonstrate the use of the instrument for obtaining photoelectron spectra, constant-ionic-state (CIS) spectra, and near-threshold photoelectron spectra of reactive species.

Postcollision-interaction recapture of photoelectrons and zero-kinetic-energy electron emission: A quantum model

Near-threshold photoelectron recapture due to postcollision interaction with a single Auger electron is examined within the quantum-mechanical framework of radiationless resonant Raman scattering. Model calculations using a hydrogenic approximation are performed, revealing general features of the escape probability as a function of inner-shell decay width \ensuremath{\Gamma}. The escape probability is useful in estimating diagram and spectator Auger intensities near threshold. The process of recapture produces a delay in the onset of double ionization to energies above threshold, which scales as ${\ensuremath{\Gamma}}^{2/3}.$ The present model is also applied to the cross section for ejection of photoelectrons with zero kinetic energy (ZKE), assuming a stable final state. A comparison with semiclassical results at small \ensuremath{\Gamma} suggests that existing semiclassical distributions are formally correct, with further convolution suggested by Thomas et al. [J. Phys. B 29, 3245 (1996)] not required. Results of the present quantum ZKE model, which does not include additional cascade effects, are in agreement with existing data. Quantities that describe the escape and ZKE-emission distributions are identified and their variation with \ensuremath{\Gamma} is examined.

Threshold and near-threshold photoelectron spectroscopy around the Ar K edge

Threshold and near-threshold photoelectron spectra were taken around the Ar K edge and up to 150 eV above threshold in order to study the behaviour of the (1s3p)4p shake-up satellite in the energy region where the transition between the sudden limit and threshold takes place. The threshold spectrum is compared to photoelectron and absorption data exhibiting features of both. The satellite intensity at zero kinetic energy is 3.8% of the 1s main line. The whole data set indicates a smooth increase to the higher kinetic energy values of about 9% around 50 eV above threshold. This behaviour can be well described qualitatively by the adiabatic transition model of Thomas, although the sudden limit value of this model seems to be reached too slowly and is, as a consequence of this, too high. Nevertheless, this strongly supports the idea that the strongest K-shell satellites are predominantly 'shake-up' in character, exhibiting a smooth transition to the sudden-limit intensity in the near-threshold region. Furthermore, other related phenomena, such as post-collision interaction and enhanced intensity of zero-kinetic-energy electrons above threshold, are observed and briefly discussed.

Studies of photoionization dynamics of methylidyne, imidogen, and hydroxyl radicals at near-threshold photoelectron kinetic energies

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTStudies of photoionization dynamics of methylidyne, imidogen, and hydroxyl radicals at near-threshold photoelectron kinetic energiesKwanghsi Wang, J. A. Stephens, and V. McKoyCite this: J. Phys. Chem. 1994, 98, 2, 460–466Publication Date (Print):January 1, 1994Publication History Published online1 May 2002Published inissue 1 January 1994https://doi.org/10.1021/j100053a019RIGHTS & PERMISSIONSArticle Views99Altmetric-Citations9LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (772 KB) Get e-Alerts

Rotationally state-selective photoionization dynamics of molecules at near-threshold photoelectron energies

Rotationally resolved photoelectron spectra can provide significant insight into the underlying dynamics of molecular photoionization. Recent advances in experimental techniques now make it possible to readily achieve rotational resolution in molecular photoelectron spectra. Here we discuss results of our recent theoretical studies of rotationally resolved photoelectron spectra at low and very near-threshold energies for molecules such as HBr, NH, OH, NO, CO, H2_O, and H_2S. These studies serve to reveal the rich dynamics of quantum-state specific studies of molecular photoionization and, where possible, provide a robust description of key spectral features of interest in related experimental studies.

Single-photon threshold photoionization of NO

Single-photon threshold photoionization spectra for jet-cooled NO have been measured for the v+=0 and 1 vibrational levels of the X 1Σ+ ground state of NO+. The NO+ rotational state distribution for the v+=0 level is shown to be perturbed by nearby autoionizing levels, whereas the v+=1 level exhibits a cation rotational distribution which is in near quantitative agreement with calculated spectra near threshold. Only small changes in total angular momentum are observed (‖ΔJ‖=‖J+−J″‖≤5/2) even though a wide range of photoelectron angular momenta (l=0–3) are predicted to contribute to the near-threshold photoelectron continua. The present results are also discussed in light of recently published two-photon threshold photoionization spectra of NO which exhibit nearly identical NO+ rotational state distributions.

Rotational branching ratios and photoelectron angular distributions in resonance enhanced multiphoton ionization of HBr via the F 1Δ2 Rydberg state

Results of theoretical studies of rotational ion distributions in the X 2Π1/2 ground state of HBr+ resulting from (2+1) resonance enhanced multiphoton ionization (REMPI) via the S(2) branch of the F 1Δ2 Rydberg state are reported. These results show a strongly parity-favored ion distribution with about 80% population in the (−) component of the Λ doublet of J+ rotational levels. The 20% population in the other parity component of the Λ doublet can be seen to be due to odd partial wave contributions to the photoelectron matrix elements which arise primarily from non-atomic-like behavior of the electronic continuum. This, in turn, is due to angular momentum coupling in the photoelectron orbital brought about by the torques of the nonspherical molecular ion potential. We demonstrate that the effect of alignment on these ion distributions, although not large, is important. Photoelectron angular distributions and alignment of the J levels of the HBr+ ions are also presented. Rotational branching ratios and photoelectron angular distributions resulting from (2+1′) REMPI of HBr via several S branches of the F 1Δ2 state are also shown for near-threshold photoelectron energies.

Near-threshold photoelectron spectrum in resonant two-photon ionization of atoms.

The near-threshold photoelectron spectrum in a resonant two-photon ionization process is investigated using a nonperturbative method. The hydrogen atom is represented by a realistic model including an infinite number of Rydberg states converging at the threshold. When the threshold is crossed a typical two-peak structure of the spectrum is modified by cutting off part of the spectrum which may include one or even two peaks.