Pulsed electron sources, very high intensity

Because of the unique features of the x-ray radiation available at synchrotrons, many novel experiments ate being conducted at these sources. Some of these unique features are the very high intensity and the brightness (number of photons per unit area per second), the neatly parallel incident beam, the abihty to choose a narrow band of wavelengths from a broad spectmm, the pulsed nature of the radiation (the electrons or positrons travel in bunches), and the coherence of the beam (the x-ray photons in a pulse are in phase with one another). The appHcations are much more diverse than the appHcations described in this article. The reader may wish to read the articles in the Proceedings of the Materials Research Society Hsted in the bibhography. 

The Dosimetry of Very High Intensity Pulsed Electron Sources Used for Radiation Chemistry II. Dosimetry for Gaseous... 

An alternative track which has been pursued recently is time-resolved X-ray diffraction. An important goal is to improve the time resolution such that direct observation of the dynamics of the chemical bond is possible, corresponding to the observation of the time-dependent distribution of atomic positions. Pulsed X-rays are, e.g., obtained from synchrotron radiation or plasma sources. The temporal duration of these pulses is currently in the range of 100 ps-100 fs [4], Very recently, the first free-electron laser has produced short 100-10 fs coherent and highly intense bursts of X-rays [4—6]. 

There is first the fundamental of a strong infrared laser. For the sake of illustration, we have chosen the case of a Ti sapphire laser operated at coi = 1.55 eV, i.e. a>i = 0.057 a.u., which is representative of the recently operated "femtosecond" sources. We have considered intensities li around lO l W/cm, which are typical. Note that, although such intensities are very high by laboratory standards, they remain quite moderate when compared to the "atomic" intensity, Iq = 3.5 lOl W/cm, which is associated to the field strength experienced by an electron on the first Bohr orbit in hydrogen, namely Fq = 5.1 10 V/cm. At intensities li =10 W/cm the atom can experience multiphoton ionization and even ATI, as shown in Fig, 1, which displays the simulation of a photoelectron spectrum for a peak intensity 1l = 2. 10 W/crri. Here, the pulse shape is assumed to be trapezoidal with linear tum-on and turn-off durations of one laser period Ti = 110 a.u., i.e. Ti = 2.6 fs), the total duration of the pulse being 8Tl. 

Lasers with short pulses are not used in Raman spectrometers, mainly because the detectors in Raman spectrometers are tuned to high sensitivity. Such detectors are very easy to saturate and this is a case where short and intense laser pulses are employed for excitation of Raman scattering. It must be noted, that gas lasers are not perfect sources of monochromatic radiation. Together with intense coherent radiation such lasers produce weak incoherent radiation, caused by a different transition between electronic energy levels of the gas. The intensity of this incoherent and noncollimated radiation can be suppressed by increasing the distance between the laser and the sample, by placing a spatial filter (consisting of two lenses and a pinhole) or a narrow-band filter (usually an interference filter) into the laser beam. 

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