Driving high-flux soft X-rays with SRS shifted pulses

In 1999 the Nobel Prize in Chemistry was awarded to Ahmed Zewail for his studies on the transition states of chemical reactions using femtosecond spectroscopy. Chemical reactions were initiated at a very precise time by an extremely short pulse (on the time scale of femtoseconds, 1fs=10-15s) of laser light, and the subsequent molecular dynamics were probed at different delays by a second ultrashort laser pulse. This technique in which an excitation pulse is used to initiate molecular dynamics, followed by a second pulse to probe the subsequent evolution, is called pump-probe. Its implementation with ultrafast, femtosecond laser pulses constitutes the basis of Femto-chemistry. The novelty, as stated in the motivation for the award of the Nobel prize, lies in the possibility to monitor the extremely short-lived transition states of chemical reactions. The following years saw impressive technological improvements in the field of ultrafast lasers: the possibility to generate laser pulses with shorter and shorter duration, selectable wavelengths within an increasingly wider spectral range, an unprecedented control over the pulse properties. As an example, it became possible to precisely measure and control the shape of the electric field of the light within the pulse, so that the temporal resolution of pump-probe measurements could be improved to even less than 1 femtosecond. These progresses in laser technologies allowed researchers to perform more and more accurate experiments to disclose faster and faster molecular and even electronic dynamics, as well as the investigation on increasingly more complex samples. A key tool for these experiments were laser pulses in the extreme ultra-violet (XUV) spectral range, as they offer both unprecedented short duration (below 1fs) as well as the direct access to core electrons in atoms and molecules. As we know from our everyday experience, tissues and bones under flesh and skin can be visualized by using X-rays, rather than more intense visible light: the same applies to the quantum world of atoms, where visible lights interacts mostly with the outer electrons (the skin), while XUV and soft X-ray can interact directly with core electrons (the bones). The possibility to probe the transition states of photo-induced chemical reactions with soft X-rays laser pulses gains particular importance when dealing with complex samples. This is due to the fact that each element has its own characteristic soft X-ray signature, which is also sensitive to the chemical environment. For example, by monitoring the soft X-ray spectra it is possible to know which molecular bonding are formed by the compounding elements. The technological problem is to generate short laser pulses with both the appropriate extension in the soft X-ray spectral region and enough brightness to allow the required experimental sensitivity. Such soft X-ray pulses are routinely generated by converting conventional near infra-red (NIR) pulses from laser amplifiers. In detail, pulses from laser amplifiers are, in a first step, converted to slightly longer wavelengths. The output pulses from this first conversion step are then used to generate soft X-ray pulses which are needed for investigations in Femtochemistry. The main technological limitations are given by the limited conversion efficiencies for each of these processes, as well as by the strict requirements on the specifications of the laser amplifiers, which are incompatible with the desirable increase in the laser power. Our group has developed a new scheme for this first step, with two advantageous: not only it operates at higher efficiency, but it is also compatible with a class of more powerful laser amplifiers. In other terms, it is possible to use simpler and more powerful laser amplifiers and in combination with a more efficient conversion process, it allows us to generate brighter soft X-ray laser pulses. This project aims to demonstrate the validity of the proposed approach and to apply its higher potential in further studies in Femtochemistry.