# Delayed Resonant Two-Photon Ionization

The advancements in the understanding of the attosecond streaking camera technique
(see, e.g., our work below) opens the perspective to retrieve time-resolved
information about the dynamics of electrons during transitions inside an atom or molecule.
As a prototype example, we have shown how the time delay in resonant two-photon ionization
as compared to the instantaneous transition from the ground state to the continuum in a
nonresonant process can be retrieved from numerical results for attosecond streaking traces.
Using perturbative analysis we systematically investigated the absorption time delay through an intermediate resonance.

C. Goldsmith et al., *J. Phys. B* **51**, 155602 (2018);
J. Su et al., *Phys. Rev. Lett.* **113**, 263002 (2014)

JILA research highlight:
An Ultrafast Photoelectric Adventure

# Finite-Range Attosecond Time Delays

Measurements of the photoelectron momentum as a function of the delay between an ionizing XUV
and a superimposed infrared streaking pulse have recently revealed temporal offsets for
the electron emission from different shells of an atom. We have shown that results of related
numerical simulations and classical analysis (see Figure) can be interpreted as due to
the dynamics of the photoelectron in the Coulomb field of the parent ion and the streaking field.
The time delay is accumulated over a finite range in space, which the photoelectron probes after its transition into the
continuum until the streaking pulse ceases and can be approximately obtained via analytical expressions.
Through the effect of attochirp on the streaking time delay photoemission at frequencies near the Cooper minimum can be probed.

C. Goldsmith et al., *Phys. Rev. A* **96**, 053410 (2017); J. Phys. B **51**, 025601 (2018)

J. Su et al., *Phys. Rev. A* **87**, 033420; **88**, 023413 (2013); **89**, 013404 (2014)

# Attosecond High Harmonic Spectroscopy

High harmonic generation is a nonlinear process that involves the release of an electron wave packet,
its laser-driven propagation in the continuum followed by its recombination. Each of these steps
occurs on a femto- or sub-femtosecond time span and, hence, high harmonic generation provides a
spectroscopic tool to resolve electron dynamics on the attosecond time scale. Our analysis shows
that the nonadiabatic electron motion inside a molecule over one cycle of the driving laser
field (lower panel, see also below) imprints a minimum in the generated high harmonic spectrum (upper panel).
Even changes to the nonadiabatic dynamics from one field cycle to the next can be traced in the
emitted signal.

M.R. Miller et al., *Phys. Rev. A* **93**, 013406 (2016); *Mol. Phys.* **115**, 1758 (2017).

# Nonadiabatic Attosecond Electron Dynamics

Attosecond laser technology is expected to provide movies
of the ultrafast quantum world of electrons in atoms, molecules and other materials.
We have shown that an intense laser field can drive the electron in between the
two protons of the hydrogen molecular ion nonadiabatically.
This leads to the unexpected
result that the electron may not leave the molecule through the tunnel exit
when the field of the laser is
strongest, in contrast to the predictions of popular quasistatic ionization pictures.
In contrast, our numerical results have shown that there are multiple bursts of electron ejection
at low field strengths in the oscillating electric field of a laser (see Figure).
This nonadiabatic attosecond intramolecular dynamics can be mapped onto
the momenta of the electron in the continuum or the high harmonic spectrum where it becomes observable
- in collaboration with R. Dörner (Universität Frankfurt, Germany).

M.R. Miller et al., *Phys. Rev. A* **93**, 013406 (2016)

M. Odenweller et al., *Phys. Rev. Lett.* **107**, 143004 (2011)

N. Takemoto and A. Becker, *Phys. Rev. Lett.* **105**, 203004 (2010)

F. He et al., *Phys. Rev. Lett.* **101**, 213002 (2008)

**Reviews:**

M.R. Miller et al., Optica **3**, 259-269 (2016)

A. Becker et al., in *Attosecond Physics: Attosecond Measurements and Control
of Physical Systems*, Springer Series in Optical Sciences, Vol. **177**
(Springer, Berlin - Heidelberg, 2013) p. 207-229

Discussion of our work with the group of R. Dörner in NewScientist:
Attoclock turns electrons into movie stars

JILA research highlight:
Quantum Body Swapping

# Past Projects

**Instantaneous Stark Shifts**

We have proposed a method to use attosecond pulse technology for observing
instantaneous Stark shifts of atomic states in an oscillating intense laser field.

F. He et al., *J. Phys. B* **44**, 211001 (2011)

**Coherent Control Schemes in the Time Domain**

We have provided a complementary view on few-photon coherent control schemes
by analyzing the electron dynamics in the time domain.

S. Chen et al., *Phys. Rev. A* **82**, 013414 (2010)

J. Su et al., *Phys. Rev. A* **84**, 065402 (2011)

**Attosecond Coherent Control**

We have studied how the electron in the dissociating hydrogen molecular
ion can be localized with high probability at one of the two nuclei using
two time-delayed ultrashort intense laser pulses.

F. He et al. *Phys. Rev. Lett.* **99**, 083002 (2007)

F. He and A. Becker, *J. Phys. B* **41**, 074017 (2008)

F. He et al. *J. Phys. B* **41**, 081003 (2008)