Pump-probe signal integral

Lee S-Y 1995 Wave-packet model of dynamic dispersed and integrated pump-probe signals in femtosecond transition state spectroscopy Femtosecond Chemistry ed J Manz and L Wdste (Heidelberg VCH)... 

Finally, it is instructive to discuss the relation of the integral pump-probe signals derived above from the electronic polarization to the corresponding expressions obtained from the calculation of electronic population probabilities. To this end, we consider the stimulated-emission polarization (33) and rewrite the the integral signal (8) in the suggestive form... 

The integral pump-probe signal is then given as integral over all emission frequencies uj... 

Being mainly interested in the dynamics associated with the conical intersection of the and S2 excited electronic states, we focus in the following on the excited-state contribution to the pump-probe spectrum. Figures 2 and 3 compare three different excited-state pump-probe signals, namely the integral stimulated-emission spectrum (2b), the time-resolved fluorescence spectrum (3a), and the dispersed stimulated-emission spectrum (3b). As has been discussed above, the integral stimulated-emission spectrum and the time-resolved fluorescence spectrum are rather similar. Because of the... 

All TOF spectra are recorded with a 500-MHz digital oscilloscope (LeCroy) and averaged over several thousand laser shots. Boxcar averagers (Stanford Research) are used to integrate the signal of individual ion mass or electron eneigy peaks in the pump-probe experiments. The data are later corrected for laser fluctuations that are monitored by a photodiode. 

To obtain TRIR spectra with sufficient sensitivity, we typically signal average several thousand laser shots at each IR frequency of interest. A flow cell, therefore, is necessary to prevent excessive sample decomposition, especially when photo-irreversible processes are monitored. A reservoir of solution (typically 10-20 mL) is continually circulated between two calcium or barium fluoride salt plates. To maintain sample integrity for non cyclic systems, one is usually forced in the dispersive TRIR experiment to acquire data in a series of short (e.g., 100-200 cm ) scans rather than in one complete scan. Thus, a substantial amount of sample may be required. Sample integrity is also of significant concern in the step-scan FTIR experiment because data must be collected at each mirror position. To address this concern, very large reservoirs of solution are required alternatively, a sample changing wheel [33] or very focused pump-probe beams in combination with sample translation [34] have been used with thin film samples. 

Fig. 3. (a-c) Time resolved changes of the O-H stretching absorption of OH/OH dimers as measured with spectrally integrated probe pulses centered at Epr and corrected for rotational diffusion (symbols, pump pulses centered at Ep=2950 cm"1). The solid lines represent numerical fits based on exponential kinetic components with time constants of 200 fs, 1 ps and 15 ps. Inset of Fig. (c) Time evolution up to a 70 ps delay time, (d-f) Oscillatory component of the signals in Figs, (a-c) and Fourier transforms (insets). 

The simplest signal to be measured in a time-resolved photoionization experiment is the total ion yield following the action of pump and probe pulses. It is given by the integral population of the ionization continua in the limit t —> oo. ... 

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