Photoelectron Energy Distribution Research Articles

Kinetic energy distribution of photoelectrons in the tunnel ionization process in the case of ultrashort laser pulses

In this paper, we use semiclassical model to discusse the kinetic electron energy distribution in the single tunnel ionization process of an atom with respect on the pulse duration and the spatial–temporal laser beam distribution. Our theoretical study show that the pulse duration is an important parameter in the ionization process, as well as beam shape.

Measurement of solar wind electron density and temperature in the shocked region of Venus and the density and temperature of photoelectrons within the ionosphere of Venus

AbstractPresented herein are measurements of the solar wind electron number density and temperature near and within the bow shock of Venus. The measurements were made by the Pioneer Venus mission Orbiter Retarding Potential Analyzer operating in its suprathermal electron mode. The measurements are essentially point measurements. The spacecraft travels approximately 0.8 km during the 0.1 s time interval required to record a single I‐V curve. The dual measurement of a density and temperature is obtained from one sweep by least squares fitting a mathematical Maxwellian expression to the I‐V curve. The distance between successive measurements is approximately 100 km. In many orbits, when the spacecraft is crossing or traveling within the bow shock, the derived densities and temperatures (high density, high temperature (HDHT)) are large, densities of the order of 100 cm−3 and temperatures of the order of several hundred eV. We interpret these HDHT measurements as measurements in regions where the large, directed kinetic energy of the solar wind ions is being degraded into randomized, more thermal‐like energy distributions of the electrons and ions through wave‐particle interactions. The HDHT values define the electron energy distribution in the limited energy interval 0 to 50 eV. We assume that the underlying electron flux distributions are flat topped like those measured in the Earth's bow shock. We also report densities and temperatures of EUV produced photoelectron energy distributions measured within the ionosphere.

Supercontinuum in ionization by relativistically intense and short laser pulses: Ionization without interference and its time analysis

Ionization by relativistically intense laser pulses of finite duration is considered in the framework of strong-field quantum electrodynamics. Our main focus is on the formation of ionization supercontinua. More specifically, when studying the energy distributions of photoelectrons ionized by circularly polarized pulses, we observe the appearance of broad structures lacking the interference patterns. These supercontinua extend over hundreds of driving photon energies, thus corresponding to high-order nonlinear processes. The corresponding polar-angle distributions show asymmetries which are attributed to the radiation pressure experienced by photoelectrons. Moreover, our time analysis shows that the electrons comprising the supercontinuum can form pulses of short duration. While we present the fully numerical results, their interpretation is based on the saddle-point approximation for the ionization probability amplitude.

Imaging a multidimensional multichannel potential energy surface: Photodetachment of H(-)(NH3) and NH4 (.).

Probes of the Born-Oppenheimer potential energy surfaces governing polyatomic molecules often rely on spectroscopy for the bound regions or collision experiments in the continuum. A combined spectroscopic and half-collision approach to image nuclear dynamics in a multidimensional and multichannel system is reported here. The Rydberg radical NH4 and the double Rydberg anion NH4 (-) represent a polyatomic system for benchmarking electronic structure and nine-dimensional quantum dynamics calculations. Photodetachment of the H(-)(NH3) ion-dipole complex and the NH4 (-) DRA probes different regions on the neutral NH4 PES. Photoelectron energy and angular distributions at photon energies of 1.17, 1.60, and 2.33 eV compare well with quantum dynamics. Photoelectron-photofragment coincidence experiments indicate dissociation of the nascent NH4 Rydberg radical occurs to H + NH3 with a peak kinetic energy of 0.13 eV, showing the ground state of NH4 to be unstable, decaying by tunneling-induced dissociation on a time scale beyond the present scope of multidimensional quantum dynamics.

Pump laser-induced space-charge effects in HHG-driven time- and angle-resolved photoelectron spectroscopy

With the advent of ultrashort-pulsed extreme ultraviolet sources, such as free-electron lasers or high-harmonic-generation (HHG) sources, a new research field for photoelectron spectroscopy has opened up in terms of femtosecond time-resolved pump-probe experiments. The impact of the high peak brilliance of these novel sources on photoemission spectra, so-called vacuum space-charge effects caused by the Coulomb interaction among the photoemitted probe electrons, has been studied extensively. However, possible distortions of the energy and momentum distributions of the probe photoelectrons caused by the low photon energy pump pulse due to the nonlinear emission of electrons have not been studied in detail yet. Here, we systematically investigate these pump laser-induced space-charge effects in a HHG-based experiment for the test case of highly oriented pyrolytic graphite. Specifically, we determine how the key parameters of the pump pulse—the excitation density, wavelength, spot size, and emitted electron energy distribution—affect the measured time-dependent energy and momentum distributions of the probe photoelectrons. The results are well reproduced by a simple mean-field model, which could open a path for the correction of pump laser-induced space-charge effects and thus toward probing ultrafast electron dynamics in strongly excited materials.

Effect of electronic angular momentum exchange on photoelectron anisotropy following the two-color ionization of krypton atoms

We present photoelectron energy and angular distributions for resonant two-photon ionization via several low-lying Rydberg states of atomic Kr. The experiments were performed by using synchrotron radiation to pump the Rydberg states and a continuous wave laser to probe them. Photoelectron images, recorded with both linear and circular polarized pump and probe light, were obtained in coincidence with mass-analyzed Kr ions. The photoelectron angular distributions and branching ratios for direct ionization into the Kr+ 2P3/2 and 2P1/2 spin-orbit continua show considerable dependence on the intermediate level, as well as on the polarizations of the pump and probe light. Photoelectron angular distributions were also recorded with several polarization combinations following two-colour excitation of the (2P1/2)5f[5/2]2 autoionizing resonance. These results are compared with the results of recent work on the corresponding autoionizing resonance in atomic Xe.

DNA Strand Breaks Induced by 0–1.5 eV UV Photoelectrons under Atmospheric Pressure

Vacuum experiments with monoenergetic low-energy electrons (LEEs) impacting on biomolecules have revealed basic mechanisms of radiobiological damage. To move closer to cellular conditions, we explore the possibility of damaging DNA at standard atmospheric pressure with electrons of 0–1.5 eV. These electrons are produced by UV light incident on a tantalum or n-Si substrate, onto which a five-monolayer film of DNA is deposited. Total conformational damage, cross-links, and double- and single-strand breaks (SSBs) are measured as a function of LEE fluence. The photoelectron energy distribution can be modified by changing the work function of the substrate or the photon energy. In a N2 atmosphere with a tantalum substrate, the yields of SSBs (5 ± 2 × 10–14 electron–1 molecule–1) and their number per energy deposited (46 ± 36 × 10–4/eV) are in good agreement with those obtained, under similar conditions, by monoenergetic 0.5–1.0 eV electron impact in vacuo and 1.5 KeV X-ray photoelectrons at atmospheric pressur...

Stimulated Compton scattering of soft X-ray radiation by hydrogen

We extend our previous studies of stimulated Compton scattering (SCS) of X-ray [H. Bachau, M. Dondera and V. Florescu, Phys. Rev. Lett. 2014, 112, 073001 and M. Dondera and V. Florescu, H. Bachau Phys. Rev. A 2014, 90, 033423] to the case of soft X-ray domain. We consider the ionization of the hydrogen atom interacting with a coherent superposition of two pulses in the frequency range of few hundreds of eV’s. We use the two theoretical approaches developed in previous works, i.e. the resolution of the time dependent Schrödinger equation and an approach based on perturbation theory. After a detailed analysis of the electron spectrum for 15 a.u., 14 a.u., we mainly focus on SCS. We present photoelectron energy distributions, ionization rates and angular distributions at various relative directions of propagation of the pulses. SCS angular distributions are also shown for 10 a.u., 9 a.u. Finally, we present new analytic formula, based on perturbation theory, describing SCS at threshold ( a.u.), i.e. for zero final electron energy.

Low-energy photoelectron dynamics in Kr3d5/2photoionization followed by Auger decay

The effects induced during the process of inner-shell photoionization followed by Auger decay are investigated for the photoionization of Kr $3{d}_{5/2}$ in an extremely low photoelectron energy regime. Both the photoelectron energy and relative angular distributions are calculated with a recently developed quantum-mechanical method where all the Coulomb interactions between each pair of particles are fully considered. The photoelectron energy distribution is asymmetric with respect to the initial photoelectron energy, and its peak shifts to a lower energy. As the absorbed photon energy is reduced, the peak of the photoelectron energy distribution moves to the negative energy range. As a manifestation of these dynamics, peak structures are observed in the relative angular distribution. We also simulate the process with a classical-trajectory Monte Carlo method, obtaining similar results to the quantum-mechanical ones.

Observation of low-energy electrons in the photoelectron energy distribution from strong-field ionization of naphthalene by circularly polarized pulses

In a joint experimental and theoretical study we reveal the presence of low-energy photoelectrons created by strong circularly polarized laser pulses for both 3D aligned and randomly oriented naphthalene molecules. The analysis within a semiclassical model highlights the essential role of the induced dipole of the molecular cation in the creation of these low-energy electrons. The detailed comparison of experiment and theory points to significant modification of the molecular orbital nodal planes in strong fields.

Application of a time-of-flight spectrometer with delay-line detector for time- and angle-resolved two-photon photoemission

We describe the design and operation of a time-of-flight electron spectrometer which is capable of simultaneously acquiring the energy and momentum distribution of low-energy photoelectrons in two dimensions parallel to the surface. We discuss its capabilities and limitations in particular for time- and angle-resolved two-photon photoemission (2PPE) with pulsed lasers. The performance of the spectrometer is demonstrated by presenting 2PPE data on the momentum-dependent electron dynamics of the first (n=1) image-potential state on Cu(001). The data reveal a weak but systematic dependence of the decay dynamics on sample azimuth with one-fold symmetry which we attribute to a small residual step density on the nominal flat surface.

Photoelectric dust levitation around airless bodies revised using realistic photoelectron velocity distributions

The velocity distribution function of photoelectrons from a surface exposed to solar UV radiation is fundamental to the electrostatic status of the surface. There is one and only one laboratory measurement of photoelectron emission from astronomically relevant material, but the energy distribution function was measured only in the emission angle from the normal to the surface of 0 to about π/4. Therefore, the measured distribution is not directly usable to estimate the vertical structure of a photoelectric sheath above the surface.In this study, we develop a new analytical method to calculate an angle-resolved velocity distribution function of photoelectrons from the laboratory measurement data. We find that the photoelectric current and yield for lunar surface fines measured in a laboratory have been underestimated by a factor of two. We apply our new energy distribution function of photoelectrons to model the formation of photoelectric sheath above the surface of asteroid 433 Eros. Our model shows that a 0.1μm-radius dust grain can librate above the surface of asteroid 433 Eros regardless of its launching velocity. In addition, a 0.5μm grain can hover over the surface if the grain was launched at a velocity slower than 0.4m/s, which is a more stringent condition for levitation than previous studies. However, a lack of high-energy data on the photoelectron energy distribution above 6eV prevents us from firmly placing a constraint on the levitation condition.

Behavior of the relativistic angular and energy distributions of atoms exposed to a strong and low-frequency circularly polarized laser field

In this Letter, we focus on the theoretical analysis of the relativistic energy and angular distributions of the ejected photoelectrons during the relativistic tunnel ionization of atoms by intense, circularly polarized light. We make a small modification of the general analytical expressions for these distributions. The role of the initial momentum, the ponderomotive potential, and the Stark shift are considered. We also present the maximal angle of electron emission.

光电子能量角度分布和空间电荷效应对变像管条纹相机的影响

光电子能量角度分布和空间电荷效应对变像管条纹相机的影响

Ultrafast Breakdown of dielectrics: Energy absorption mechanisms investigated by double pulse experiments

We investigate the mechanisms involved in the modification of dielectric materials by ultrashort laser pulses. We show that the use of a double pulse (fundamental and second harmonic of a Ti–Sa laser) excitation allows getting new insight in the fundamental processes that occur during the interaction. We first measure the optical breakdown (OB) threshold map (intensity of first pulse versus intensity of second pulse) in various materials (Al2O3, MgO, α-SiO2). Using a simple model that includes multiphoton excitation followed by carrier heating in the conduction band, and assuming that OB occurs when a critical amount of energy is deposited in the material, we can satisfactorily reproduce this evolution of optical breakdown thresholds. The results demonstrate the dominant role of carrier heating in the energy transfer from the laser pulse to the solid. This important phenomenon is also highlighted by the kinetic energy distribution of photoelectrons observed in a photoemission experiment performed under similar conditions of double pulse excitation. Finally we show, in the case of α-SiO2, that the initial electronic excitation plays a key role in the formation of surface ripples and that their characteristics are determined by the first pulse, even at intensities well below OB threshold.

Comparison of different quantum mechanical methods for inner atomic shell photo-ionization followed by Auger decay

A numerical time-dependent quantum mechanical approach was developed previously for simulating the process of photoionization followed by Auger decay for cases where the photoelectron energy is not very large; the method accurately calculates the interaction between the two active electrons, but simplifies their interaction with the core electrons. More established theoretical methods, which take account of postcollision interaction effects, allow an accurate description of this process when the photoelectron energy is not too low. We demonstrate that using the time-dependent method (although with some simplifications that are needed for its numerical implementation) for low energy photoelectrons and more established methods for higher energy allows accurate calculations for nearly all possible combinations of electron energy. This is confirmed by performing calculations of the photoelectron energy and angular distributions for the 1 s photoionization of Ne, with a subsequent KLL Auger transition. By computing the energy and angular distributions for energies where the two groups of methods should agree and where they should disagree, we demonstrate their consistency and range of accuracy. For the regions where the methods disagree, we discuss the reasons for any discrepancies and the trends in the differences. In addition, some of our calculations are compared with existing experimental data for the same system. The agreement found in the comparison confirms the reliability of the theoretical approaches.

Analysis of Energy Distribution of Photoelectrons in Metals: Comparison with Experiment for Molybdenum

The energy distribution of photoelectrons, emitted from the surface of a metal has been analyzed on the basis of four models of absorption of a photon by an electron, including the ones, proposed by Fowler and DuBridge. In contrast to some earlier work the transmission coefficient as a function of normal energy has been accounted for. The experimental energy distribution, of photoelectrons for molybdenum is in better agreement with the theoretical (DuBridge, as modified herein) energy distribution corresponding to the enhancement of total (not normal) energy of an electron by the absorption of a photon, than that corresponding to the enhancement of normal energy. The theoretical results do not depend appreciably on the site of absorption of a photon by an electron (on the surface or inside the metal). The dependence of mean energy on frequency of radiation has also been discussed. The original derivation by DuBridge has been improved by discarding a couple of unjustified assumptions made by him.

Two-color ionization of hydrogen close to threshold with keV photons

In a recent Letter [H. Bachau, M. Dondera, and V. Florescu, Phys. Rev. Lett. 112, 073001 (2014)] we considered the hydrogen atom in interaction with an electromagnetic field consisting in the coherent superposition of two keV pulses centered around two frequencies ${\ensuremath{\omega}}_{1}$ and ${\ensuremath{\omega}}_{2}$ that differ by a few atomic units. The analysis of the results obtained from the resolution of the time-dependent Schr\odinger equation focused on stimulated Compton scattering (SCS). We have developed in parallel an approach based on perturbation theory and it proved to be an appropriate and useful tool in complement to the nonperturbative approach. In this paper we present a detailed analysis of the electron spectra obtained for $\ensuremath{\hbar}{\ensuremath{\omega}}_{1}=$ 55 a.u. and two values of $\ensuremath{\hbar}{\ensuremath{\omega}}_{2}$, 50 and 54 a.u. Emphasis is put on the case $\ensuremath{\hbar}{\ensuremath{\omega}}_{2}=54$ a.u., where the electron emitted through SCS has the lowest energy, showing in particular that in the vicinity of the ionization threshold the cross section increases as $\ensuremath{\hbar}{\ensuremath{\omega}}_{1}\ensuremath{-}\ensuremath{\hbar}{\ensuremath{\omega}}_{2}$ decreases. We calculate photoelectron energy and angular distributions at various relative directions of propagation of the pulses. We discuss the limitations of the approximations underlying the numerical and analytical approaches used in this work.

Diffraction using laser-driven broadband electron wave packets.

Directly monitoring atomic motion during a molecular transformation with atomic-scale spatio-temporal resolution is a frontier of ultrafast optical science and physical chemistry. Here we provide the foundation for a new imaging method, fixed-angle broadband laser-induced electron scattering, based on structural retrieval by direct one-dimensional Fourier transform of a photoelectron energy distribution observed along the polarization direction of an intense ultrafast light pulse. The approach exploits the scattering of a broadband wave packet created by strong-field tunnel ionization to self-interrogate the molecular structure with picometre spatial resolution and bond specificity. With its inherent femtosecond resolution, combining our technique with molecular alignment can, in principle, provide the basis for time-resolved tomography for multi-dimensional transient structural determination.

Ion and electron spectroscopy of strontium in the vicinity of the two-photon-excited5p2 1S0state

Two-photon ionization of ground-state strontium is investigated experimentally in the 360--370-nm spectral range with dye laser pulses of long (\ensuremath{\sim}ns) duration and low $(\ensuremath{\sim}10{}^{10}W\mathrm{ cm}{}^{\ensuremath{-}2}$) intensity. The ${\mathrm{Sr}}^{+}$ spectra recorded with linear laser polarization are dominated by the presence of the highly correlated $5{p}^{2}$ ${}^{1}{S}_{0}$ state and by the even parity $[4d$$6d]$]${}_{J}$${}_{=0,2}$ autoionizing states. The partial $J=0$ and $J=2$ two-photon ionization cross sections are recovered from these data along with the spectra obtained with circular polarization. For linear polarization, the observed wavelength dependence of ${\mathrm{Sr}}^{+}$ production is compared with earlier theoretical work based on the $R$-matrix--multichannel-quantum-defect-theory approach and the agreement is generally found to be quite satisfactory. There are, however, a few, albeit noticeable, discrepancies. The latter motivated us to proceed to a more comprehensive experimental characterization of the examined energy range, by also measuring the kinetic energy and angular distribution of the ejected photoelectrons under linear laser polarization. The shapes of the angular distributions vary rapidly in the neighborhood of each resonance, while these distributions are found to be almost spherically symmetric on the maxima of the sharp $[4d$$6d]$]${}_{J}$${}_{=0}$ lines. On these, and only these, maxima, electron energy spectra reveal the absorption of a third photon, resulting in production of ${\mathrm{Sr}}^{+}$ into its excited $4{d}_{j}$ and $5{p}_{j}$ states. The non-negligible $5{p}_{j}$ branching ratio reflects the mixing between the $[4d$$6d]$]${}_{J}$${}_{=0}$ states and the $5{p}^{2}$ ${}^{1}{S}_{0}$ perturber, found to be rather underestimated by the aforementioned theoretical work. Thus, the presently acquired ion and electron spectra as well as the asymmetry parameters obtained by fitting the photoelectron angular distributions provide a stringent test of future theoretical efforts, addressing structure as well as excitation and ionization issues in an improved manner.