We study ionization of atoms in strong orthogonal two-color (OTC) laser fields numerically and analytically. The calculated photoelectron momentum distribution shows two typical structures: a rectangular-like one and a shoulder-like one, the positions of which depend on the laser parameters. Using a strong-field model which allows us to quantitatively evaluate the Coulomb effect, we show that these two structures arise from attosecond response of electron inside an atom to light in OTC-induced photoemission. Some simple mappings between the locations of these structures and response time are derived. Through these mappings, we are able to establish a two-color attosecond chronoscope for timing electron emission, which is essential for OTC-based precise manipulation.

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We study single-photon ionization of aligned H$_2^+$ in a high-frequency
low-intensity laser field. We focus on the cases where the laser frequency is near
to or somewhat larger than the ionization potential of the target. The calculated
photoelectron momentum distribution through numerical solution of time-dependent
Schr¨odinger equation shows clear interference patterns. However, different from the
cases of photon energy far larger than the ionization potential usually explored in
experiments, the positions of interference maxima and minima for the present cases
can not be explained by the interference of the electronic wave with the observed
momentum between these two atomic centers of the molecule. By developing a theory
model applicable for ionization of molecules in high-frequency laser field, we show
that the Coulomb potential plays an important role in the momentum component
of the emitting electronic wave near the nuclei, resulting in a remarkable shift of
the interference pattern. A simple expression with Coulomb-modified momentum is
obtained to predict the interference extrema, which gives suggestions for probing the
structure and electron dynamics of the aligned molecule in single-photon ionization
with lower photon energy.

No abstract