Picosecond pump-delay probe

Since there are a large number of different experimental laser and detection systems that can be used for time-resolved resonance Raman experiments, we shall only focus our attention here on two common types of methods that are typically used to investigate chemical reactions. We shall first describe typical nanosecond TR spectroscopy instrumentation that can obtain spectra of intermediates from several nanoseconds to millisecond time scales by employing electronic control of the pnmp and probe laser systems to vary the time-delay between the pnmp and probe pnlses. We then describe typical ultrafast TR spectroscopy instrumentation that can be used to examine intermediates from the picosecond to several nanosecond time scales by controlling the optical path length difference between the pump and probe laser pulses. In some reaction systems, it is useful to utilize both types of laser systems to study the chemical reaction and intermediates of interest from the picosecond to the microsecond or millisecond time-scales. 

The characterization of the laser pulse widths can be done with commercial autocorrelators or by a variety of other methods that can be found in the ultrafast laser literature. " For example, we have found it convenient to find time zero delay between the pump and probe laser beams in picosecond TR experiments by using fluorescence depletion of trans-stilbene. In this method, the time zero was ascertained by varying the optical delay between the pump and probe beams to a position where the depletion of the stilbene fluorescence was halfway to the maximum fluorescence depletion by the probe laser. The accuracy of the time zero measurement was estimated to be +0.5ps for 1.5ps laser pulses. A typical cross correlation time between the pump and probe pulses can also be measured by the fluorescence depletion method. 

A computer-controlled motorized translation stage mounted with a retro-reflector is used to vary the pump laser beam path relative to the probe laser beam path and this controls the relative timing between the pump and probe laser beams. Note that a one-foot difference in path length is about 1 ns time delay difference. The picosecond TR experiments are done essentially the same way as the nanosecond TR experiments except that the time-delay between the pump and probe beams are controlled by varying their relative path lengths by the computer-controlled motorized translation stage. Thus, one can refer to the last part of the description of the nanosecond TR experiments in the preceding section and use the pump and probe picosecond laser beams in place of the nanosecond laser beams to describe the picosecond TR experiments. 

Figure 17. Snapshots of wavepacket propagation during the picosecond pump-probe excitation for Naa for selected delay times between pump and probe pulse [15].

Figure 11. Time-resolved PADs from ionization of DABCO for linearly polarized pump and probe pulses. Here, the optically bright S E state internally converts to the dark 5i state on picosecond time scales, (a) PADs at 200 fs time delay for pump and probe polarization vector both parallel to the spectrometer axis. The difference in electronic symmetry between S2 and Si leads to significant changes in the form of the PAD. (b) The PADs at 200 fs time delay for pump polarization parallel and probe polarization perpendicular to the spectrometer axis, showing the effects of lab frame molecular alignment, (c) and (d) The PADs evolve as a function of time due to molecular axis rotational wavepacket dynamics. Taken with permission from C. C. Hayden, unpublished.

In CARS two ultrashort pulses of laser light (from femtoseconds to picoseconds in duration) arrive simultaneously at the sample of interest (Mukamel, 2000 Fourkas, 2001 and references herein). The difference between the frequencies (W) - w2) matches the frequency of a Raman active vibrational mode in the sample. A probe pulse (w3) emits a signal pulse of frequency Wj - w2 + w3 in a unique special direction. By scanning the delay time between the pump and probe pulses, the delay of the vibrational coherence can be measured. The distinct advantage of CARS is that it is a background free technique, since the signal propagates in a unique direction. 

Figure SO. Polarization-dependent transients measured by picosecond pump-probe ionization techniques [r(c) is defined in the figure] as a function of the delay time and the excess vibrational energy of jet-cooled c-stilbene.

The pump and probe technique using picosecond lasers also allows us to investigate the I2 (C02) cluster ion photofragmentation dynamics in real time. An example of such an investigation is shown in Figure 25.5 for cluster size n= 12-17, where the absorption recovery of I2 is displayed as a function of the pump-probe delay. 

Fig. 3.48. Snapshots during the picosecond pump probe excitation of Nas B for selected delay times between pump and probe pulses (taken from [382]). Bottom X state and top B state, shown in the pseudorotational coordinates Qx, Qy for a Qs value of 7.2ao...

Besides excitation and probing with infrared laser pulses the CARS technique (Sect.8.4) is a promising technique to study these relaxation processes. An example is the measurement of the dephasing process of the OD stretching vibration in heavy water D2O by CARS [11.111]. The pump at w = Wl is provided by an amplified 80-fs dye-laser pulse form a CPM ring dye laser. The Stokes pulse at Wg is generated by a synchronized tunable picosecond dye laser. The CARS signal at = 2wL-Wg is detected as a function of the time delay between the pump and probe pulses. 

In the picosecond and femtosecond range, the pump-and-pulse technique is more suitable where the time delay between pump and probe can be accurately realized by an optical delay line (Figure 13) where one of the laser pulses travels a variable path length before it is again superimposed on the pump pulse direction. 

In the systems for femto-picosecond transient spectroscopy it is used a special detection technique, known as pump and probe . The idea behind this technique is to use the same laser source to generate the excitation pulse (PUMP), and the analysis beam (PROBE). The path of the PROBE beam is varied in length by a delay line, i.e. a mobile platform on which are mounted mirrors that reflect the laser beam with high efficiency. The change in the optical path allows the control of the temporal distance between excitation and analysis (Fig. 8.14). 

The application of picosecond TR spectroscopy by Doig el al. to study the J, K, and KL intermediates of the BR photocycle will be briefly described here. The 550 nm pump and 589 nm probe pnlses in the TR experiments were chosen to be near the absorption maxima of gronnd state BR (568 nm) and K (590nm) respec-tively. " Stokes TR spectra were obtained with time delays varying from Ops to 13 ns between the pnmp and probe pnlses in order to examine the structure and kinetics of the J —> K —> KL seqnence of the BR photocycle. ... 

---