Attosecond pulses are exploited in time-resolved spectroscopy since they can excite and probe matter on temporal scales typical of electron dynamics. This ability discloses unresolved aspects of atomic and molecular physics.
One of the intriguing questions about molecules is how they respond to quick removal of one of their electrons: does this information propagate along the molecular chain or it stays confined to the ionization site? Attosecond science recently answered this query: an oscillating charge migration was observed along the molecule after one-electron removal; this phenomenon evolves over temporal scales too fast to be detected by usual femtosecond laser pulses.
How these measurements are performed in practice? A widespread approach in attosecond spectroscopy is based on electron streaking: an attosecond light pulse promptly excites an electron from the ground state of an atom or a molecule, lifting it to an unbound (or weakly bound) state. Then, a second pulse in the infrared (IR) spectral range arrives and accelerates the electron to a final momentum.
A suitable electron spectrometer, like time-of-flight or a velocity-map-imaging one, detects the final electron energy; several measurements are performed at different delays between the two light pulses, leading to a streaking trace, from which the electron dynamics after excitation can be retrieved by suitable analysis.
Despite the much longer duration of the second pulse, the attosecond temporal resolution is still preserved provided that the IR pulse has a shot-to-shot reproducible electric field; this requirement calls for a stable Carrier-Envelope Phase offset of the laser pulses, a prerequisite that is also essential for attosecond pulse generation.
