  Extreme ultraviolet (XUV) attosecond pulses, generated by a process known as
laser-induced electron recollision, are a key ingredient for attosecond
metrology, providing a tool to precisely initiate and probe sub-femtosecond
dynamics in the microcosms of atoms, molecules and solids[1]. However, with the
current technology, extending attosecond metrology to scrutinize the dynamics
of the inner-shell electrons is a challenge, that is because of the lower
efficiency in generating the required soft x-ray \hbar\omega>300 eV attosecond
bursts and the lower absorption cross-sections in this spectral range. A way
around this problem is to use the recolliding electron to directly initiate the
desired inner-shell process, instead of using the currently low flux x-ray
attosecond sources.Such an excitation process occurs in a sub-femtosecond
timescale, and may provide the necessary "pump" step in a pump-probe
experiment[2]. Here we used a few cycle infrared \lambda_{0}~1800nm source[3]
and observed direct evidences for inner-shell excitations through the
laser-induced electron recollision process. It is the first step toward
time-resolved core-hole studies in the keV energy range with sub-femtosecond
time resolution.
