Picosecond laser pulses, reliability

When the generation of picosecond laser pulses first became possible in the laboratory, methods of electronic detection which currently are in such widespread use for measurements performed on the picosecond time scale, e.g. streak cameras and two-dimensional photodiode arrays, were not readily available to the experimenter. Techniques had to be developed not only to measure accurately the kinetics of photoinitiated events which occur on this ultrashort time scale, but more importantly to monitor the width and shape of the laser pulse to ensure reliable and reproducible generation of these pulses. 

Because the picosecond continuum is generated through a highly nonlinear process, its detailed spatial, spectral, and intensity characteristics vary from shot to shot more severely than do the laser pulses used to generate it. In order to achieve a high degree of reliability in our spectral measurements, it is therefore necessary to obtain double beam spectra in which the data are corrected for continuum fluctuations for every shot. 

Typically, in measurements of time-resolved luminescence in the time regime of tens of picoseconds, data obtained from 10 to 20 laser shots are averaged to improve the signal-to-noise ratio and to minimize the effects of shot-to-shot variations in the laser pulse energy and shape. Once the reliability of the data has been ensured by application of the corrections described above and made necessary by detector-induced distortions, the time-resolved fluorescence data is analyzed in terms of a kinetic model which assumes that the emitting state is formed with a risetime, xR, and a decay time, Tp. Deconvolution of the excitation pulse from the observed molecular fluorescence is performed numerically. The shape of the excitation pulse to be removed from the streak camera data is assumed to be the same as the prepulse shape, and therefore the prepulse is generally used for the deconvolution procedure. Figure 6 illustrates the quality of the fit of the time-dependent fluorescence data which can be achieved. 

During the last decade, the technology of laser-driven picosecond accelerators has been successfully applied to pulse radiolysis experiments in diverse areas of chemistry The reliability of the accelerators and their beam characteristics, and the development of real-time, non-destructive beam diagnostics to monitor them, have provided a... 

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