Interfering one-photon and two-photon ionization by femtosecond VUV pulses in the region of an intermediate resonance

The electron angular distribution after atomic photoionization by the fundamental frequency and its second harmonic is analyzed for a case when the frequency of the fundamental scans the region of an intermediate atomic state. The angular distribution and its left-right asymmetry, due to the two-pathway interference between nonresonant one-photon and resonant two-photon ionization, sharply change as a function of the photon energy. The phenomenon is exemplified by both solving the time-dependent Schr\"odinger equation on a numerical space-time grid and by applying perturbation theory for ionization of the hydrogen atom in the region of the 1s\text{\ensuremath{-}}2p transition for femtosecond pulses as well as an infinitely long exposure to the radiation. Parametrizations for the asymmetry and the anisotropy coefficients, obtained within perturbation theory, reveal general characteristics of observable quantities as functions of the parameters of the radiation beam

Photoelectron angular distribution in two-pathway ionization of neon with femtosecond XUV pulses

We analyze the photoelectron angular distribution in two-pathway interference between non\-resonant one-photon and resonant two-photon ionization of neon. We consider a bichromatic femtosecond XUV pulse whose fundamental frequency is tuned near the $2p^5 3s$ atomic states of neon. The time-dependent Schr\"odinger equation is solved and the results are employed to compute the angular distribution and the associated anisotropy parameters at the main photoelectron line. We also employ a time-dependent perturbative approach, which allows obtaining information on the process for a large range of pulse parameters, including the steady-state case of continuous radiation, i.e., an infinitely long pulse. The results from the two methods are in relatively good agreement over the domain of applicability of perturbation theory

Displacement effect in strong-field atomic ionization by an XUV pulse

We study strong-field atomic ionization driven by an XUV pulse with a non\-zero displacement, the quantity defined as the integral of the pulse vector potential taken over the pulse duration. We demonstrate that the use of such pulses may lead to an extreme sensitivity of the ionization process to subtle changes of the parameters of a driving XUV pulse, in particular, the ramp-on/off profile and the carrier envelope phase. We illustrate this sensitivity for atomic hydrogen and lithium driven by few-femto\-second XUV pulses with intensity in the $\rm 10^{14}~W/cm^2$ range. We argue that the observed effect is general and should modify strong-field ionization of any atom, provided the ionization rate is sufficiently high.Comment: 5 pages, 7 figure

Electron-Impact Excitation from the (4p⁵5s) Metastable States of Krypton

Theoretical results from multistate semirelativistic Breit-Pauli R-matrix calculations and two first-order distorted-wave calculations are presented for electron-impact excitation of krypton from the (4p55s) J = 0,2 metastable states to the (4p55s) and (4p55p) manifolds. Except for a few cases, in which the method to account for relativistic effects becomes surprisingly critical, fair overall agreement between the predictions from the various theoretical models is achieved for intermediate and high energies. However, significant discrepancies remain with the few available experimental data

Electron-Impact Excitation to the 4p⁵5s and 4p⁵5p Levels of Kr | Using Different Distorted-Wave and Close-Coupling Methods

Electron-impact excitation of the 4p55s and 4p55p levels of Kr I has been investigated in detail by calculating cross sections using distorted-wave and close-coupling approaches. The results are presented from the excitation thresholds up to 50 eV incident energy. They are contrasted among the different calculations and compared with other theoretical predictions and experimental data. Significant disagreement is found with many of the recent experimental data of Chilton et al. [Phys. Rev. A 62, 032714 (2000)]

Measurement of laser intensities approaching 10 15 W/cm 2 with an accuracy of 1%

Accurate knowledge of the intensity of focused ultrashort laser pulses is crucial to the correct interpretation of experimental results in strong-field physics. We have developed a technique to measure laser intensities approaching 1015W/cm2 with an accu

A new method for measuring angle-resolved phases in photoemission

Quantum mechanically, photoionization can be fully described by the complex photoionization amplitudes that describe the transition between the ground state and the continuum state. Knowledge of the value of the phase of these amplitudes has been a central interest in photoionization studies and newly developing attosecond science, since the phase can reveal important information about phenomena such as electron correlation. We present a new attosecond-precision interferometric method of angle-resolved measurement for the phase of the photoionization amplitudes, using two phase-locked Extreme Ultraviolet pulses of frequency $\omega$ and $2\omega$, from a Free-Electron Laser. Phase differences $\Delta \tilde \eta$ between one- and two-photon ionization channels, averaged over multiple wave packets, are extracted for neon $2p$ electrons as a function of emission angle at photoelectron energies 7.9, 10.2, and 16.6 eV. $\Delta \tilde \eta$ is nearly constant for emission parallel to the electric vector but increases at 10.2 eV for emission perpendicular to the electric vector. We model our observations with both perturbation and \textit{ab initio} theory, and find excellent agreement. In the existing method for attosecond measurement, Reconstruction of Attosecond Beating By Interference of Two-photon Transitions (RABBITT), a phase difference between two-photon pathways involving absorption and emission of an infrared photon is extracted. Our method can be used for extraction of a phase difference between single-photon and two-photon pathways and provides a new tool for attosecond science, which is complementary to RABBITT

Complex attosecond waveform synthesis at fel fermi

Free-electron lasers (FELs) can produce radiation in the short wavelength range extending from the extreme ultraviolet (XUV) to the X-rays with a few to a few tens of femtoseconds pulse duration. These facilities have enabled significant breakthroughs in the field of atomic, molecular, and optical physics, implementing different schemes based on two-color photoionization mechanisms. In this article, we present the generation of attosecond pulse trains (APTs) at the seeded FEL FERMI using the beating of multiple phase-locked harmonics. We demonstrate the complex attosecond waveform shaping of the generated APTs, exploiting the ability to manipulate independently the amplitudes and the phases of the harmonics. The described generalized attosecond waveform synthesis technique with an arbitrary number of phase-locked harmonics will allow the generation of sub-100 as pulses with programmable electric fields