Pulse trains, picosecond

The type of laser source that can be used is exactly the same as for single-photon counting pulse fluorometry (see above). Such a laser system, which delivers pulses in the picosecond range with a repetition rate of a few MHz can be considered as an intrinsically modulated source. The harmonic content of the pulse train - which depends on the width of the pulses (as illustrated in Figure 6.11) - extends to several gigahertz. 

The stroboscopic pulse radiolysis with the single bunch electron pulse instead of pulse trains started in Argonne National Laboratory in 1975 [54]. The research fields have been extended by the stroboscopic pulse radiolysis with the picosecond single electron bunch, although most of researches had been limited to hydrated and solvated electrons in the aqueous and alcoholic solutions. This system was unable to study the kinetics of the geminate ion recombination in liquid hydrocarbons until the modification of the Argonne linac in 1983, which made possible the quality measurements of the weak absorption. 

In the first reported measurements made with picosecond pulses, an optical beam splitter was used to pick off a portion of the pulse train and a variable optical delay path was introduced between the two beams [7]. The main beam was used to excite (pump) a dye sample, and the weak (probe) beam was used to monitor the recovery of dye transmission as a function of delay. Over the past two decades, this pump-probe method has been extended to a variety of measurement geometries and used to measure electronic polarization dephasing times as well as population lifetimes. 

We demonstrated that the field-induced large amplitude vibration of the hg(l) mode persists for a rather long period (a few to several picoseconds), owing to slow intramolecular vibrational energy redistribution (IVR) [28]. Mode selective excitation can therefore be achieved by adjusting the pulse intervals in a pulse train [24], as in the experiment reported by Laarmann et al. [9]. In this chapter, by using the time-dependent adiabatic state approach, we first demonstrate that... 

H. Lehmitz, W. Kattav, H. Harde, Modulated pumping in Cs with picosecond pulse trains, in Methods of Laser Spectroscopy, ed. by Y. Prior, A. Ben-Reuven, M. Rosenbluth (Plenum, New York, 1986), p. 97... 

MODULATED PUMPING IN CS WITH PICOSECOND PULSE TRAINS... 

Synchronization of Two Mode-Locked Titanium Sapphire Lasers. Besides utilizing OPOs - described above - the use of two independently tunable ultrafast laser sources is possible. This is of special interest while using picosecond laser sources, since the efficiency of nonlinear optical processes is, due to the peak power, much lower than for equivalent femtosecond processes. To achieve the conditions for the pumpfeprobe technique the pulse trains of the two independent lasers have to be synchronized. A successful approach to this problem is described below. For further details on the design of the appropriate stabilization see [50]. 

Fig. 4. Temporal pulse characteristics of lasers (a) millisecond laser pulse (b) relaxation oscillations (c) Q-switched pulse (d) mode-locked train of pulses, where Fis the distance between mirrors and i is the velocity of light for L = 37.5 cm, 2L j c = 2.5 ns (e) ultrafast (femtosecond or picosecond) pulse.

It is also possible to switch a single picosecond pulse out of the train of mode-locked pulses using an electrooptic switch. It is possible to obtain a single pulse having duration in the picosecond regime or even less. Pulses with durations in the regime of a few hundred femtoseconds (10 s) are also available (Fig. 4e). 

Pulse radiolysis systems capable of picosecond time resolution use the fine structure of the output from the electron linear accelerator. Electrons in the accelerating tube respond to positive or negative electric field of the radiofrequency, and they are eventually bunched at the correct phase of the radiofrequency. Thus the electron pulse contains a train of bunches or fine structures with their repetition rate being dependent on the frequency of the radiofrequency (350 ps for the S-band and 770 ps for the L-band). 

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