Conventional Pump-Probe

Figure 8-3. Layout of the experimental set-x / up for conventional pump-probe experi-...

In this section experimental results are described, which are obtained by applying the conventional pump-probe technique to m-LPPP films kept in vacuum at the temperature of liquid nitrogen [25], These results allow the identification of the primary excitations of m-LPPP and the main relaxation channels. In particular, the low and high excitation density regimes are investigated in order to get an insight into the physical processes associated with the emission line-narrowing phenomenon. 

Although very detailed, fundamental information is available from ultrafast TRIR methods, significant expertise in femtosecond/picosecond spectroscopy is required to conduct such experiments. TRIR spectroscopy on the nanosecond or slower timescale is a more straightforward experiment. Here, mainly two alternatives exist step-scan FTIR spectroscopy and conventional pump-probe dispersive TRIR spectroscopy, each with their own strengths and weaknesses. Commercial instruments for each of these approaches are currently available. 

In conventional nanosecond pump-probe dispersive TRIR experiments, also described previously, kinetic data are collected at one frequency at a time. These data can then be used to construct a series of time-resolved IR spectra. Thus, in the dispersive experiment kinetic data are used to construct spectra, and in the step-scan experiment spectral data are used to derive kinetics. 

The experimental configuration of the pump-probe experiment is similar to Ref. [5]. A home built non-collinear optical parametric amplifier (nc-OPA) was used as a pump, providing Fourier-transform-limited 30 fs pulses, which could be spectrally tuned between 480-560 nm. In all experiments white-light generated in a sapphire crystal using part of the fundamental laser (800 nm), was used as probe light. In the pump-probe experiments the pump was tuned to the S2 0-0 band for carotenoids with n>l 1. In the case of M9, it was not possible to tune the nc-OPA to its 0-0 transition, and hence another nc-OPA tuned to 900 nm was frequency doubled and used for excitation. In addition to conventional transient absorption pump-probe measurements, we introduce pump-deplete-probe spectroscopy, which is sensitive to the function of an absorbing state within the deactivation network. In this technique, we... 

H. Rabitz The information in the recurrence time alone is minimal. However, the temporal structure of the recurrence signal contains detailed information on the surface explored by the wandering scout wavepacket during its excursion. Further experiments may be necessary to follow (i.e., track) the wavepacket through its excursion over the potential surface. Such pump-probe experiments go beyond conventional spectroscopy. 

We have adopted the excitonic band model not only because it describes conventional absorption spectroscopy (linear spectroscopy), but because it enables an extremely convenient description of nonlinear experiments, such as pump-probe, dynamical hole burning, or photon echoes. In these third-order experiments one has to consider not only transitions from the ground state to the one-excitonic states but also transitions from the one-excitonic to the two-excitonic states (see Fig. 13). These additional transitions reveal the required information to deduce, at least in principle, the complete coupling scheme. 

In the case of non-degenerate frequencies, the nonlocal third-order effects may give rise to chiral pump-probe spectroscopies. The only observation of a coherent Raman optical activity process to date is also due to a third-order pseudoscalar. Spiegel and Schneider have observed Raman optical activity in coherent anti-Stokes Raman scattering in a liquid of (-l-)-trans-pinane and report chiral signals that are 10 of the conventional electric-dipolar CARS intensity [23],... 

The methods discussed so far, fluorescence upconversion, the various pump-probe spectroscopies, and the polarized variations for the measurement of anisotropy, are essentially conventional spectroscopies adapted to the femtosecond regime. At the simplest level of interpretation, the information content of these conventional time-resolved methods pertains to populations in resonantly prepared or probed states. As applied to chemical kinetics, for most slow reactions (on the ten picosecond and longer time scales), populations adequately specify the position of the reaction coordinate intermediates and products show up as time-delayed spectral entities, and assignment of the transient spectra to chemical structures follows, in most cases, the same principles used in spectroscopic experiments performed with continuous wave or nanosecond pulsed lasers. 

In conventional nanosecond pump-probe dispersive TRIR experiments, spectra are measured one frequency at a time. The pump source is typically a nanosecond laser the probe source can be broadband IR light from a globar or tunable IR light from a CO laser or a semiconductor diode laser. 

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