written as , as a consequence the time evolution of this observable is: While the first two terms do not depend on time, the third, instead, does. This creates a dynamic for the observable with a characteristic time, , given by . As a consequence, for energy levels in the range of 10 eV, which is the typical electronic energy range in matter, the characteristic time of the dynamics of any associated physical observable is approximately 400 as. To measure the time evolution of , one needs to use a controlled tool, or a process, with an even shorter time-duration that can interact with that dynamic. This is the reason why attosecond light pulses are used to disclose the physics of ultra-fast phenomena in the few-femtosecond and attosecond time-domain. Generation of attosecond pulses To generate a traveling pulse with an ultrashort time duration, two key elements are needed: bandwidth and central wavelength of the electromagnetic wave. From Fourier analysis, the more the available spectral bandwidth of a light pulse, the shorter, potentially, is its time duration. There is, however, a lower-limit in the minimum duration exploitable for a given pulse central wavelength. This limit is the optical cycle. Indeed, for a pulse centered in the low-frequency region, e.g. infrared (IR) 800 nm, its minimum time duration is around 2.67 fs, where is the speed of light; whereas, for a light field with central wavelength in the extreme ultraviolet (XUV) at 30 nm the minimum duration is around 100 as. Thus, a smaller time duration requires the use of shorter, and more energetic wavelength, even down to the soft-X-ray (SXR) region. For this reason, standard techniques to create attosecond light pulses are based on radiation sources with broad spectral bandwidths and central wavelength located in the XUV-SXR range. The most common sources that fit these requirements are free-electron lasers (FEL) and high-harmonics generation (HHG) setups. Physical observables and attosecond experiments Once an attosecond light source is available, one has to drive the pulse towards the sample of interest and, then, measure its dynamics. The most suitable experimental observables to analyze the electron dynamics in matter are: Angular asymmetry in the velocity distribution of molecular photo-fragment. Quantum yield of molecular photo-fragments. XUV-SXR spectrum transient absorption. XUV-SXR spectrum transient reflectivity. Photo-electron kinetic energy distribution. The general strategy is to use a pump-probe scheme to "image" through one of the aforementioned observables the ultra-fast