Spanning from pico- to milliseconds…
Pulse radiolysis optical spectroscopy
On average ELYSE accelerator provides electron pulses of 5-10 ps width at a 5 Hz repetition rate. This source of ionizing radiation works as a 'pump' pulse for generating radicals in the samples (see sample holder below): liquid or solid, the evolution of which could be analyzed by probe pulses of broadband white light. So here we come to the point of understanding that pulse radiolysis is a variation of classical pump-probe spectroscopy, where the light excitation pulse is replaced by the electron one, whereas the light probe stays.
The ELYSE platform’s accelerator (Paris-Saclay University, ICP UMR8000 CNRS) utilizes short pulses of electrons to produce 150 fs broad supercontinuum. Two experimental areas developed at the ELYSE platform allow conducting pulse radiolysis studies, using the transient absorption pulse-probe set-up and nanosecond to the millisecond time scale, using the streak camera set-up. Laser (@260 nm) driven Cs2Te photocathode allows short electron pulses production with a typical half width of 7 ps and a charge of ~6C of ~7.8 MeV, at a repetition rate of 5-10Hz.
For the picosecond pump-probe experiments, the broadband supercontinuum (380–750 nm), generated by focusing a small part of the fundamental laser (780 nm) into a CaF2 crystal, is split 60/40: probe/reference paths. Both probe and reference are coupled into optical fibers, transmitted to a spectrometer, and dispersed onto a cooled CCD camera. The sample cells used for the pump-probe measurements are 0.5 cm optical path length synthetic fused silica cells through which the sample is continuously flown (ca. 20 cm3/min). The cells’ optical windows are of 200mum thickness to minimize contributions to the signal from the transient species generated in quartz by the electron pulse.
For the nanosecond to millisecond pulse radiolysis experiments, we use a highly dynamic Hamamatsu C7700-01 streak camera, coupled to an Andor Kymera 328i spectrograph for detection. At the same time, the analyzing light is delivered by a home built flash xenon lamp.
Direct line (VD)
- time-window: 0-11.5ns
- spectral window: 340-1350nm
- time-resolution 150fs
- accuracy on average 1mO.D.
The stroboscopic optical detection in pump-probe spectroscopy by far is the most simple one. It consists of following after the changes in optical absorption spectra in time by delaying the light probe's arrival after the pump (electron pulse) using a mechanical delay stage. By forcing light to travel a longer distance, the time-delay can be introduced. Our experiment uses four path delay stages of 1 m length, giving us in total delays up to 11.5 ns.
- time-window: 1ns-1ms
- spectral window: 280-800nm
- time-resolution 50ps in the shortest 1ns time-window
- accuracy on average 5mO.D.
A streak camera (Hamamtsu C7700 streak-camera) operates by transforming the temporal profile of a light pulse into a spatial profile on a detector by causing a time-varying deflection of the light across the detector's width. In particular, a light pulse enters the instrument through a narrow slit along one direction. It then gets deflected in the perpendicular direction so that photons that arrive first hit the detector at a different position than photons that arrive later. The resulting image forms a "streak" of light, from which the duration and other temporal properties of the light pulse can be inferred. Usually, to record periodic phenomena, a streak camera needs to be triggered accordingly, similarly to an oscilloscope.
Middle pressure binary mixture liquid-gas radiolisys
The system consists of a closed water loop. The circulation of solutions, to guarantee refreshment of solution in measurement volume, was achieved using home made pump driven by electromagnets. The principle of action follows the sequence: 1) the current is applied to the electromagnets (1 and 2), working in the opposite direction: electromagnet 1 is pushing, while 2 is pulling the permanent magnets fixed on the piston; 2) once the piston is displaced, the current in electromagnets is reversed, and the piston goes down. The flow of solution during reverse movement of the piston is not affected since the lock-balls are released, letting the solution flow through the six holes in the piston.
The spectroscopic cell fracture test with Ar guaranteed the safe pressure to operate below 75 bar with 1 mm input window and 5 mm output one, respectively. The pressure was controlled by a gas pressure regulator (0-100 bar, Swagelok) directly attached to a gas cylinder.
Once the pressure of the gas was applied using (valve 3), the equilibration for 30 minutes were undertaken. To intensify a gas exchange, a bubble of gas was formed over the water to increase the surface area for an exchange. Each measurement with a different pressure was conducted with a freshwater volume (1.5 mL) pumped into the cell.
The filling of the closed water circuit was accomplished by a peristaltic pump (valve 1, 2); after that, the system was isolated and pressurized to a given pressure.
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