Dependence Of Photoelectron Angular Distributions Research Articles

Alignment dependence of photoelectron angular distributions in the few-photon ionization of molecules by ultraviolet pulses

We probe the time-dependent ionization dynamics of impulsively excited rotational wave packets of ${\mathrm{N}}_{2}, {\mathrm{CO}}_{2}$, and ${\mathrm{C}}_{2}{\mathrm{H}}_{4}$ using broadband ultraviolet pulses centered at 262 nm. Photoelectron momentum distributions recorded by velocity-map imaging show a strong dependence on alignment, on multiphoton order, and on the electronic and vibrational states of the cation. We show that substantial information about the molecular-frame photoelectron angular distribution can be obtained from the high-order laboratory-frame asymmetry parameters without any prior knowledge of the photoionization process. We also compare few-photon ionization with one-photon ionization and strong-field ionization.

Magnetic quantum number dependence of hydrogen photoelectron spectra under circularly polarized pulse in barrier suppression ionization regime

Photoelectron spectra and transition mechanisms of a hydrogen atom in the n = 2 state under circularly polarized pulses in the barrier suppression ionization (BSI) regime, found by solving the time-dependent Schrödinger equation, are presented in detail. The photoelectron peak emerges from low-energy, in contrast to being centered at higher energy in the case of ionization from the ground state. We show that the magnetic quantum number dependence of photoelectron angular distributions on initial states yields 3- and 2-band structures for 2p− and 2p0 in zenith angle, respectively, and 1-band structures for both 2p+ and 2s. The intermediate transition pathways from these initial states to the corresponding final states are also revealed.

Ionization of O2 molecule by n-cycle circularly polarized laser pulses

In this paper we investigate the ionization of O2 molecule exposed to few-cycle pulses with different optical cycle numbers by calculating the photoelectron angular distributions (PADs) of O2 molecule. In order to show the influence of the molecular electronic structure and the laser pulse on the PADs, we also calculate the PADs of H atom, which has an isotropic electronic distribution in its ground state. We find that for one-cycle laser pulse, the PADs of O2 molecule are inversion asymmetric, and the PADs are not dependent on the photoelectron energy. For two-cycle and three-cycle laser pulses, although the PADs are still inversion asymmetric, the PADs have a certain dependence on the photoelectron energy. According to this phenomenon, we can control the emission directions of photoelectrons by choosing a fitted photoelectron energy. At same time, with a fitted photoelectron energy, by changing the carrier-envelope (CE) phase, the split position of the PADs of O2 molecule can be controlled, and then can forecast the main emission direction of photoelectron. While for long pulse (ten-cycle pulse) the dependence of PADs on the photoelectron energy nearly disappears, and the ionization behaviors extend to that in an infinitely monochromatic plane wave.

Orientation Dependence of Photoelectron Angular Distribution in Nitrogen Molecule Irradiated by XUV Laser Field

We theoretically study the dependence of photoelectron angular distribution on laser polarization direction in nitrogen molecules. The approach is based on the time-dependent density functional theory at the level of local density approximation complemented by self-interaction correction. It is found that photoelectron emission in one photon regime could be considered as a probing tool for the main character of different types of molecular orbitals (σ or π). The pattern of emitted photoelectrons strongly depends on the polarized angle of the laser, for σ orbital, the number of photoelectron decreases with increasing the polarized angle, while for π orbital, it has the inverse relation to the polarized angle, which reveals the multi-electron effect in molecules. On the other hand, concerning the total photoelectron emission, one should take into account a few occupied orbitals instead of only the outmost one.

Femtosecond Two-Photon Detachment of Cu− Studied By Photoelectron Imaging

The wavelength dependence of photoelectron angular distributions (PADs) of two-photon detachment of Cu− has been directly studied by using the photoelectron map imaging. Results show that for the laser field intensity of 6.0×1010 W/cm2, PADs exhibit dramatic change with the external field wavelength. Comparison between the experimental observation and the lowest-order perturbation theory prediction indicates that the pattern of PADs can be explained by the interference of the s and d partial waves in the final state. Relative contributions of s and d partial waves in the two-photon detachment at different laser wavelengths are obtained.

Communication: The influence of vibrational parity in chiral photoionization dynamics

A pronounced vibrational state dependence of photoelectron angular distributions observed in chiral photoionization experiments is explored using a simple, yet realistic, theoretical model based upon the transiently chiral molecule H2O2. The adiabatic approximation is used to separate vibrational and electronic wavefunctions. The full ionization matrix elements are obtained as an average of the electronic dipole matrix elements over the vibrational coordinate, weighted by the product of neutral and ion state vibrational wavefunctions. It is found that the parity of the vibrational Hermite polynomials influences not just the amplitude, but also the phase of the transition matrix elements, and the latter is sufficient, even in the absence of resonant enhancements, to account for enhanced vibrational dependencies in the chiral photoionization dynamics.

Frequency dependence of photoelectron angular distributions in small Na clusters

We investigate from a theoretical perspective photoelectron angular distributions (PADs) in small $\mathrm{Na}$ clusters in relation to recently available experimental results. We consider various (increasingly refined) levels of theory in order to better understand relevant physical trends. It is found that PADs are extremely sensitive to all details of the modeling such that a detailed description of the final state and the ionic background is necessary. Finally, we compare the theoretical description with recent experimental data on the cluster anion ${\mathrm{Na}}_{7}^{\ensuremath{-}}$ and find a satisfying agreement for full time-dependent local-density approximation (TDLDA) with ionic background.

Energy dependence of photoelectron angular distributions from two- and four-photon ionization of Mg in the vicinity of the 3p2 1S0 doubly excited state

We report on measurements concerning the variation of photoelectron angular distributions (PADs) from two- and four-photon ionization of ground state Mg, as the laser wavelength is scanned across the 3p2 1S0 resonance. The energy dependence of the asymmetry parameters determined from the PADs and corresponding to two-photon ionization is found to be in good agreement with prior theoretical predictions. For the four-photon PADs there are at present no theoretical results. Nevertheless, the resonant character is found considerably red-shifted in accordance with earlier experimental and theoretical studies of the single and double ionization yields. In either case, the PADs show a dominant s-wave on-resonance contribution, while the d-wave contribution apparently dominates under non-resonant excitation within the scanned wavelength range. The PADs offer detailed wavelength- and channel-dependent information on the dynamics of the autoionization resonances under pulsed laser excitation, corroborating and elucidating the conclusions reached from ionization yield studies.

圆偏振周期量级激光场中负离子的阈上离解

Photoelectron angular distributions (PADs) from two-photon ionization of atoms in linearly polarized strong laser fields are obtained in accordance with the nonperturbative quantum scattering theory. We also study the influence of laser wavelength on PADs. For two-photon ionization very close to the ionization threshold, most of the ionized electrons are vertically ejected to the laser polarization. PADs from two-photon ionization of atoms are determined by the second order generalized phased Bessel function at which the ponderomotive parameter plays a key role. In terms of dependence of PADs on laser wavelength, corresponding variations for the ponderomotive parameter are demonstrated.

Phase-dependent angular distributions of photoelectrons in an infinite sequence of linearly polarized few-cycle pulses

Above-threshold ionization of Xe atoms in an infinite sequence of linearly polarized few-cycle pluses is studied. The dependence of photoelectron angular distributions (PADs) on the carrier-envelope (CE) phase in various few-cycle pulses are obtained. Our study shows that PADs in linearly polarized few-cycle pulses vary with the CE phase. The PADs are inversion asymmetric for most CE phases and inversion symmetric for some special CE phases. The PADs are always symmetric about the polarization vector, but the maximal ionization rates vary with the CE phase. Our study also shows the jet structures in PADs arising from the inherent property of photon ionization and the variation of PADs with the number of optical cycles in a single pulse.

Carrier-Envelope Phase-Dependent Effect on PhotoelectronAngular Distribution in Single-Cycle Laser Pulses

Using a nonperturbative scattering theory, we study the photoelectronangular distributions (PADs) of Kr atoms irradiated by an infinite sequenceof intense single-cycle pulses of circular polarization. We demonstrate theinversion asymmetry of PADs and the dependence of PADs on thecarrier-envelope phase of the single-cycle pulses. The inversion asymmetry iscaused by the interference between transition channels where the differentchannels are characterized by different combinations of absorbed-photonnumbers in the ionization process. Our results provide a possible method todetermine the value of carrier-envelope phase by the detected PADs.

Nondipolar electron angular distributions from fixed-in-space molecules.

The first indication of nondipole effects in the azimuthal dependence of photoelectron angular distributions emitted from fixed-in-space molecules is demonstrated in N (2). Comparison of the results with angular distributions observed for randomly oriented molecules and theoretical derivations for the nondipole correction first order in photon momentum suggests that higher orders will be needed to describe distributions measured in the molecular frame.

Multiphoton angular distributions with elliptically polarized light. I. Analytic ellipticity dependence of photoelectron distributions in the polarization plane

Multiphoton excitation with elliptically polarized light brings out more information in the photodetachment or photoionization processes than linearly polarized light. This information can only be reached, however, by recording photoelectron angular distributions for several values of the ellipticity angle, and fitting the data with a common formula. But the analytic dependence of photoelectron angular distributions on the ellipticity has been unknown up to now, except for the one- and two-photon excitation cases. Here we give a demonstration of a general formula, valid for any number N of exciting photons, with the assumptions of (i) an initially even and momentumless (J=0) atom or ion and (ii) that after excitation of an electron, the residual core is left in an odd state. Slight modifications in the demonstration would make it possible to extend the general formula to any J=0 initial atomic state.

Multiplet-specific shape resonance and autoionization effects in (2+1) resonance enhanced multiphoton ionization of O2 via the d 1Πg state

In this paper we discuss the single-photon ionization dynamics of the d 1Πg Rydberg state of O2. Comparision is made with vibrationally resolved measurements of photoelectron spectra which employ (2+1) resonance enhanced multiphoton ionization (REMPI) through the d 1Πg state. A σu shape resonance near the ionization threshold leads to non-Franck–Condon vibrational branching ratios and a substantial dependence of photoelectron angular distributions on the vibrational state of the X 2Πg ion. Significant differences exist between our one-electron predictions and experiment. These are mainly attributed to electronic autoionization of repulsive 1∑−u, 1∑+u, and 1Δu states associated with the 1π3u1π3g configuration. A proposed singlet ‘‘K’’ 1Πu Rydberg state converging to the A 2Πu ion probably also contributes to autoionization in the d 1Πg state spectrum. We also show that autoionizing H and J 3Πu Rydberg states of O2 converging to the a 4Πu and A 2Πu ionic thresholds, respectively, may play a previously unsuspected role in the C 3Πg state one-color REMPI spectra. We discuss multiplet-specific (spin-dependent) effects via comparision of these results with recent experimental and theoretical studies of O2C 3Πg photoionization.

Theoretical analysis of photoelectron angular distributions of linear molecules

A partitioning scheme for the photoelectron asymmetry parameter of linear molecules is proposed and applied in multiple-scattering calculations for thirteen valence ionizations of five molecules. For several limiting cases rigorous bounds for the asymmetry parameter are established. The energy dependence of photoelectron angular distributions is discussed in terms of relative channel cross sections, transition moment phases, and Coulomb phase shifts. Rapid energy-dependent variations of the asymmetry parameter can be rationalized in this manner, e.g. for low-energy Π ionizations and for σ-type resonances in Σ ionizations. The proposed scheme is also used to identify the origin of errors in approximate calculations.

Wavelength and vibrational-state dependence of photoelectron angular distributions. Resonance effects in 5 σ photoionization of CO

Photoelectron angular distribution were studied for the photoionization of CO for v=0−3 in the range 16–26 eV. (AIP)

Shape-Resonance-Enhanced Nuclear-Motion Effects in Molecular Photoionization

Shape resonances in molecular photoionization are shown to induce strong coupling between vibrational and electronic motion over a spectral range several times broader than the resonances half-width. This coupling is manifested by large deviations from Franck-Condon intensity distributions and strong dependence of photoelectron angular distributions on the vibrational state of the residual ion. These effects are illustrated for the $3{\ensuremath{\sigma}}_{g}$ photoionization channel of ${\mathrm{N}}_{2}$.