Abstract
We theoretically study the photoelectron angular distribution (PAD) from the two-photon single ionization of H and He by femtosecond and attosecond extreme-ultraviolet pulses, based on the time-dependent perturbation theory and simulations with the full time-dependent Schrodinger equation. The PAD is formed by the interference of the s and d continuum wave packets, and, thus, contains the information on the relative phase and amplitude ratio between them. We find that, when a spectrally broadened femtosecond pulse is resonant with an excited level, the PAD substantially changes with pulse width, since the competition between resonant and nonresonant ionization paths, leading to distinct from the scattering phase shift difference, changes with it. In contrast, when the Rydberg manifold is excited, and for the case of above-threshold two-photon ionization, and the PAD do not depend much on pulse width, except for the attosecond region. Thus, the Rydberg manifold and the continuum behave similarly in this respect. For a high-harmonic pulse composed of multiple harmonic orders, while the value is different from that for a single-component pulse, the PAD still rapidly varies with pulse width. The present results illustrate a new way to tailor the continuum wave packet.
Highlights
Two-photon ionization (2PI), or multi-photon ionization more generally, has consistently been receiving a great deal of attention for decades
The change in photoelectron angular distributions (PAD) corresponding to the wave packet between 20 and 23 eV, shown in Figure 12, is not so large as one might expect from the variation in δ, again probably due to the relatively small variation with pulse width below 10 fs even for the case of Gaussian pulses
We have numerically studied the two-photon ionization of a hydrogen and helium atom by an ultrashort extreme ultraviolet (EUV) pulse
Summary
Two-photon ionization (2PI), or multi-photon ionization more generally, has consistently been receiving a great deal of attention for decades (see e.g., [1,2,3,4,5,6,7,8,9,10,11]). It is well known that the two-photon photoelectron angular distributions (PAD) are directly related to the relative amplitudes and the relative phase between different partial waves [8,12,13,14,15,16]. Free-electron lasers (FELs) as well as intense ultrashort high-harmonic sources has led to renewed interest in two-photon processes in the EUV to X-ray regimes (see, e.g., [11,21,22,23,24,25,26,27,28]). A similar experiment was performed by O’Keeffe et al [43] using synchrotron radiation for the excitation and a laboratory laser for the ionization Both these experiments confirmed that the relative phase extracted from measured PADs resulting from sequential two-color excitation and ionization agrees well with the theoretically predicted scattering phase shift difference.
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