Photoelectron Energy Spectra Research Articles

Profiling N-Type Dopants in Silicon

Variously doped n-type structures (dopant concentration between 1.5*1016 cm−3 and 1.5*1019 cm−3) on a lightly doped p-type silicon substrate (doped to 1.9*1015 cm−3) have been examined by a photoemission electron microscope equipped with a high-pass energy filter and by an ultra-high vacuum scanning low energy electron microscope. High contrast have been observed between the n-type areas and the p-type substrate and its monotone dependency on the doping level of structures has been manifested. The relation between the energy spectra of photoelectrons and the doping level has been studied, too. The scanning electron microscope images obtained with the landing energy of the primary beam in the low keV range exhibit contrasts similar to those appearing in the full threshold photoemission micrographs.

超短强激光场中Ar原子多光子电离实验研究

We present high resolution photoelectron energy spectra from multiphoton ionization (MPI) of Ar subject to laser pulses with wavelength. of 400 mn, pulse duration of 35 fs, and maximum intensity of 5x10(13) W/cm(2) Ionizations into (2)P(3/2) and (2)P(1/2) channels are observed and distinct resonance structures are found in both ionization channels The intensity dependence of the resonance structures is explained in terms of the mechanism of Freeman resonance, i.e transient resonances of alternating current, (AC) Stark-shifted Rydberg states at specific intensities within the laser pulse.

Ionization of a multilevel atom by ultrashort laser pulses

Specific features of ionization of single atoms by laser fields of a near-atomic strength are investigated. Calculations are performed for silver atoms interacting with femtosecond laser pulses with wavelengths λ = 800 nm (Ti:Sapphire) and λ = 1.064 μm (Nd:YAG). The dependences of the probability of ionization and of the form of the photoelectron energy spectra on the field of laser pulses for various values of their duration are considered. It is shown that the behavior of the probability of ionization in the range of subatomic laser pulse fields is in good agreement with the Keldysh formula. However, when the field strength attains values close to the atomic field strength, the discrepancies in these dependences manifested in a decrease in the ionization rate (ionization stabilization effect) or in its increase (accelerated ionization) are observed. These discrepancies are associated with the dependence of the population dynamics of excited discrete energy levels of the atom on the laser pulse field amplitude.

A highly effective method for synthesizing hybrid Pt–CdSe nanocomposite

Hybrid Pt–CdSe nanocomposite was fabricated by a two-step chemical route. Cadmium selenide (CdSe) quantum rods (QRs) were prepared by a one-pot approach with tunable size. After ligand exchange, CdSe QRs were loaded with monodisperse 1.9 nm Pt nanopaticles in aqueous solution. Transmission electron microscopy (TEM) revealed the morphology of the Pt–CdSe nanostructure, and the decreased photoluminescence (PL) intensity demonstrated that electron and hole separation can be enhanced after loading Pt on CdSe QRs. X-ray photoelectron energy spectrum (XPS) was applied to confirm the existence of Pt and detect the Pt mass concentration of 3%.

Classical Aspects in Above-Threshold Ionization with a Midinfrared Strong Laser Field

We present high resolution photoelectron energy spectra of noble gas atoms from high intensity above-threshold ionization (ATI) at midinfrared wavelengths. An unexpected structure at the very low-energy portion of the spectra, in striking contrast to the prediction of the simple-man theory, has been revealed. A semiclassical model calculation is able to reproduce the experimental feature and suggests the prominent role of the Coulomb interaction of the outgoing electron with the parent ion in producing the peculiar structure in long wavelength ATI spectra.

Radiation properties of high-order harmonic generation for the measurements of femto- and attosecond X-ray pulses

Radiation properties of high-order harmonic generation (HHG) are calculated for atoms in a strong laser field. The laser-duration dependence and the carrier-envelope-phase (CEP) dependence of HHG radiation properties are presented. The CEP dependence of the pure single distribution pulse of HHG radiation properties shows interesting 180° periodic structures. The quantum enhancement of the laser-assisted photo-ionization by femtosecond (1 fs=10−15 s) and attosecond (1 as=10−18 s) X-ray pulses and the interference patterns of photo-electron energy spectra are theoretically investigated. Transfer equations are presented for pulse reconstructions. The theoretical root-mean-square time (energy) differences of attosecond pulse reconstructions with different durations are less than 2 as (0.8 eV). These methods may be developed as basic techniques to access ultra-fast measurements and molecular movie.

Quantum-mechanical analysis of pulse reconstruction for a narrow bandwidth attosecond x-ray pulse

The photoelectron energy spectra (PESs) excited by narrow bandwidth attosecond x-ray pulses in the presence of a few-cycle laser are quantum-mechanically calculated. Transfer equations are used to reconstruct the detailed temporal structure of an attosecond x-ray pulse directly from a measured PES. Theoretical analysis shows that the temporal uncertainties of the pulse reconstruction depend on the x-ray bandwidth. The procedure of pulse reconstruction is direct and simple without making any previous pulse assumption, data fitting analysis and time-resolved measurement of PESs. The temporal measurement range is half of a laser optical cycle.

Freeman resonances in high-order above-threshold ionization

Photoelectron energy spectra for ionization in a strong laser field are calculated within the Floquet framework, for a one-electron model of argon and for other model potentials. The results indicate that, at least for linear polarization and not too short pulse durations, the plateau enhancements dominating high-order above-threshold ionization in rare gases originate from Stark-shift-induced resonances with dressed excited states similar in spatial extension to low-excited field-free states.

Comparative analysis of the strong-field ionization of a quantum system with the Coulomb and short-range potentials

The dynamics of a hydrogen atom and a 3D model quantum system with a short-range potential is investigated using the direct numerical integration of the nonstationary Schrodinger equation in a wide range of laser intensities and frequencies. The simulation data are compared with the predictions of variants of the Keldysh-type theories. It is demonstrated that, in the low-frequency (tunnel) limit, the ionization rates of the systems with the Coulomb and short-range potentials and the same values of the ionization potential significantly differ from each other whereas, in the high-frequency (single-photon) limit, we do not observe a substantial difference between the ionization rates. Specific features of the angular distribution of the photoelectron emission and the photoelectron energy spectra are investigated in detail. In addition, the ionization suppression is studied for both Coulomb- and short-range-potential atoms. The stabilization is due to the dramatic reconstruction of the atom in the presence of a strong laser field and the formation of a new system (Kramers-Henneberger atom) that exhibits an increasing resistance to the ionization upon an increase in the laser intensity. In the two-photon ionization regime, the stabilization phenomenon is substantially more pronounced for the system with the Coulomb potential. This results from the effective excitation of the Rydberg states of the dressed atom in the strong-field limit.

Multiphoton ionization through the triplet states of Mg by linearly and circularly polarized laser pulses

We theoretically study multiphoton ionization through the triplet states of Mg by linearly polarized and circularly polarized fs laser pulses. After the construction of the atomic basis using the frozen-core Hartree-Fock potential as well as the model potential approaches for both singlet and triplet series which show rather good agreements with the existing data in terms of state energies and dipole matrix elements, we solve time-dependent Schr\"odinger equations with $3s3p$ $^{3}P_{1}$ as an initial state, and calculate the total ionization yield and photoelectron energy spectra.

Quantum interference in laser-assisted photo-ionization excited by a femtosecond x-ray pulse

The photoelectron energy spectra (PESs) excited by monochromatic femtosecond x-ray pulses in the presence of a femtosecond laser are investigated. A PES is composed of a set of separate peaks, showing interesting comb-like structures. These structures result from the quantum interferences between photoelectron wave packets generated at different times. The width and the localization of each peak as well as the number of peaks are determined by all the laser and x-ray parameters. Most of peak heights of the PES are higher than the classical predictions.

Ionization suppression of diatomic molecules in strong laser fields

The ionization rate of molecules in intense laser fields may be much lower than that of atoms with similar binding energy. This phenomenon is termed the ionization suppression of molecules and is caused by the molecular inner structure. In this paper, we perform a comprehensive study of the ionization suppression of homonuclear diatomic molecules in intense laser fields of linear and circular polarizations. We find that for linear polarization the total ionization rate and the ionization suppression depend greatly on the molecular alignment, and that for circular polarization the ionization suppression of molecules in the antibonding (bonding) shells disappears (appears) for laser intensities around 10(15) W/cm(2). We also find that the molecular photoelectron energy spectra are greatly changed by the interference effect, even though the total ionization rate of molecules remains almost the same as that of their companion atoms.

Comparison of two forms of the S-matrix element of strong-field photoionization

We numerically compare ionization rates and photoelectron energy spectra resulting from the so-called length gauge (LG) and velocity gauge (VG) forms of the Keldysh–Faisal–Reiss (KFR) theory for a circularly polarized laser field. To this end, we use the well-known analytical formulae for the ground state of a hydrogen atom and we derive analogous formulae for its first excited (degenerate) state. It appears that both forms of the KFR theories show qualitatively different behaviour of ionization rate as a function of frequency and intensity of the laser field. In the LG KFR theory, one obtains much smaller stabilization of the hydrogen atom than in the VG KFR theory. Unlike the latter one, the LG KFR theory shows that for sufficiently strong laser field (in the non-relativistic and dipole approximations) ionization rate from a given initial state is only a function of intensity, but not of frequency of the laser field. We also find substantial differences in the shape of photoelectron energy spectra for both theories. In contrast to the VG KFR theory, the LG one shows a minimum in photoelectron energy spectra for some initial states of the hydrogen atom. We discuss our results by comparing them, where possible, to other theoretical calculations.

Study on the long‐term thermal‐oxidative aging behavior of polyamide 6

AbstractThe long‐term thermal‐oxidative aging behavior of polyamide 6 (PA6) was studied by comparison with the stabilized sample in this work. The variation of mechanical properties of the pure and the stabilized samples of PA6 with aging time at 110°C, 130°C, and 150°C were investigated, respectively. The aging mechanism of PA6 under heat and oxygen was studied in terms of the reduced viscosity, crystallization behavior, dynamic mechanical behavior, and chemical composition through the methods of polarized light microscopy (PLM), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), X‐ray photoelectron energy spectrum (XPS), and so on. The results indicated that at the initial stage of aging, the molecular crosslinking reaction of PA6 dominated resulting in the increase of the mechanical strength, reduced viscosity, and the glass transition temperature of the sample. And the molecular degradation dominated in the subsequent aging process resulting in the decrease of the melting temperature, the increase of the crystallinity, and the formation of the oxides and peroxides products. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Radiation controlled energy of photoelectrons produced by two-color short pulses

We report on numerical results of energy spectra of photoelectrons emitted by irradiating a hydrogen atom with the superposition of two pulses. The spectra have been obtained by numerical integration of the time dependent Schrodinger equation. The highest frequency component of the pulse has been assumed to have low intensity and such a frequency that a single photon may ionize the atom. Its duration has been assumed to lie in the range of subfemtoseconds. The lowest frequency component that redistribute the energy of the ionized electrons has an higher intensity and duration of few femtoseconds. We find that when the field are aligned, the electron energy spectra strongly depend on the time lag of the fields, on the carrier-envelope relative phase of the lowest frequency pulse, and on the duration of the X-ray pulse. Moreover it has been found that by changing the angle between the direction of the low and high frequency fields significant modifications to the spectra occur that could be used to measure the relative orientation of the fields.

Use of photoelectron laser phase determination method for attosecond measurements with quantum-mechanical calculations

This paper calculates quantum-mechanically the photoelectron energy spectra excited by attosecond x-rays in the presence of a few-cycle laser. A photoelectron laser phase determination method is used for precise measurements of the pulse natural properties of x-ray intensity and the instantaneous frequency profiles. As a direct procedure without any previous pulse profile assumptions and time-resolved measurements as well as data fitting analysis, this method can be used to improve the time resolutions of attosecond timing and measurements with metrological precision. The measurement range is half of a laser optical cycle.

Retrieval of electron-atom scattering cross sections from laser-induced electron rescattering of atomic negative ions in intense laser fields

We investigated the two-dimensional electron momentum distributions of atomic negative ions in an intense laser field by solving the time-dependent Schr\odinger equation (TDSE) and using the first- and second-order strong-field approximations (SFAs). We showed that photoelectron energy spectra and low-energy photoelectron momentum distributions predicted from SFAs are in reasonable agreement with the solutions from the TDSE. More importantly, we showed that accurate electron-atom elastic scattering cross sections can be retrieved directly from high-energy electron momentum spectra of atomic negative ions in the laser field. This opens up the possibility of measuring electron-atom and electron-molecule scattering cross sections from the photodetachment of atomic and molecular negative ions by intense short lasers, respectively, with temporal resolutions in the order of femtoseconds.

Influence of Coulomb potential for photoionization of H atoms in an elliptically polarized laser field: Velocity gauge versus length gauge

We theoretically study the influence of Coulomb potential for photoionization of hydrogen atoms in an intense laser field with elliptical polarization. The total ionization rates, photoelectron energy spectra, and photoelectron angular distributions are calculated with the Coulomb-Volkov wave functions in the velocity gauge and compared with those calculated in the length gauge as well as those calculated with the Volkov wave functions. By comparing the results obtained by the Coulomb-Volkov and Volkov wave functions, we find that for linear polarization the influence of Coulomb potential is obvious for low-energy photoelectrons, and as the photoelectron energy and/or the laser intensity increase, its influence becomes smaller. This trend, however, is not so clear for the case of elliptical polarization. We also find that the twofold symmetry in the photoelectron angular distributions for elliptical polarization is caused by the cooperation of Coulomb potential and interference of multiple transition channels. About the gauge issue, we show that the difference in the photoelectron angular distributions obtained by the velocity and length gauges becomes rather obvious for elliptical polarization, while the difference is generally smaller for linear polarization.

Quantum enhancement in laser-assisted photoionization and a method for the measurement of an attosecond xuv pulse

Quantitative investigations are made into the quantum enhancements achieved in laser-assisted photoionizations for attosecond extreme-ultraviolet (xuv) pulses with different chip properties. An improved pulse reconstruction scheme is presented based on the analysis of two photoelectron energy spectra (PESs) measured with different laser intensities. A direct procedure is developed to completely reconstruct the intensity and chirp of an isolated pulse (within one-half the laser period) with high precision. Results show root-mean-square frequency (time) differences less than 0.8 eV (2 as, except for a few data points near the start and/or end of the pulse). A stereo measurement (recording PESs from two opposite directions along the direction of laser polarization) is needed to reconstruct a broad bandwidth xuv pulse with a nonmonotonous instantaneous frequency.

Atom under an intense laser pulse: Stabilization effect and strong-field approximation

Direct numerical calculations of the single-photon ionization dynamics of the hydrogen atom were compared with the data obtained within the strong-field approximation (SFA). An analysis showed that the SFA model accurately determines the range of electromagnetic field intensities, upon reaching of which the ionization mode deviates from that described within perturbation theory; in particular, the ionization rate decreases with increasing intensity. It was demonstrated that the actual ionization mechanism under an intense pulse differs significantly from the SFA predictions. For example, an analysis of photoelectron angular distributions and energy spectra showed that the strong-field ionization features within the SFA model are primarily controlled by the ionization channel closing effect associated with the ponderomotive shift of the continuum boundary. At the same time, the results of direct numerical calculations of the ionization dynamics suggest that the Kramers-Henneberger atom is formed in a strong field, which is characterized by increased stability to strong-field ionization.