Pump-probe signal dispersed

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)... 

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... 

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. 2 Experimental arrangement for time-resolved FSRS (femtosecond stimulated raman spectroscopy). The femtosecond actinic pump pulse excites the sample electronically. After a delay the femtosecond probe pulse and picosecond Raman pump pulse arrive together to interrogate the instantaneous molecular structure. The self-heterodyned signal is emitted in the probe direction, dispersed, and detected by a kHz readout CCD. Data collection is best performed by division of subsequent Raman pump-on by Raman pump-off laser shots (lower trace), however this has been performed by other groups as a subtraction of subsequent pulses (upper trace). Reproduced from ref 2 with permission from the PCCP Owner Societies (2012).

In a typical pump-probe experiment, a sample is excited with a pulse with frequency < i and wavevector ki, and is probed by a second pulse with frequency C02 and wavevector 2- The optical path of (Mie of the pulses is varied to change the delay between the two pulses. The measured signal is the difference between the intensities of the transmitted probe pulses in the presence and absence of the excitation pulses, and usually is averaged over many pulses (Fig. 1.9). In a system with only two electronic states, the difference can reflect either stimulated emission from the excited state or bleaching of the absorption band of the ground state. The probe frequency often is selected by dispersing a spectrally broad probe beam after... 

The transient transmittance PP signal is defined through the difference polarization Ppp (0 = Ppp(t) —Ppf(f) [1]. Here Ppp ( ) is the pump-offpolarization induced solely by the probe pulse, which is obtained from Eqs. 9.29 and 9.30 with = 0. Within the RWA and the slowly varying envelope approximation, the integral (int) and dispersed... 

The method has been applied so far to the spectroscopy of Na2 [10.63] and I2 [10.64]. Figure 10.47a shows saturated absorption signals in I2 obtained with a dye laser at X = 600 nm with 10 mW pump power and 1 mW probe power. Figure 10.47b displays the first derivative of the spectrum in a) and Fig.10.47c, the first derivative of the saturated dispersion signal. 

Recently picosecond photoinduced absorption studies of emeraldine base polymer in film and NMP solutions have been reported [48]. They show that for pumping at 2.0 eV and probing ant 2.9 eV a dispersive decay is obtained with more than 50% of the photoinduced signal for the thin film remaining after 5 ns. In contrast, only fast decay components (<100 ps) were found in solution. These differences between solution and film dynamics are attributed to difTerences in the barriers to ring rotation and differences in interchain processes. In contrast to the luminescence observed from the exciton in LEB, no luminescence is observed for EB [24]. 

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