Transient Absorption Spectroscopy (Pump-probe)
Transient absorption spectroscopy uses two laser pulses, a strong pump and a weak probe which are overlapped in the sample. The linear absorption of the sample is recorded as the probe is delayed in time relative to pump using an optical delay line. This technique is used to measure ground state recovery times and the induced absorption of any new species resulting from a photochemical reaction, e.g., the charge separated state for a Donor-Bridge-Acceptor system.
Figure 1. Basic Pump-probe experimental arrangement and levels electronic levels diagram. Note the thickness of the lines indicates the relative intensity of the laser pulses the probe is weak to avoid unwanted perturbation of the system.
This technique will shortly be extended to incorporate a super-continuum or “white-light” probe which will give information across the visible range with a time resolution of approximately 50 fs.
Photon Echo Peak Shift and Transient Grating Spectroscopy
The photon echo peak shift (PEPS) and transient grating (TG) techniques use three laser pulses of equal intensity overlapped in the sample with a triangular geometry (Fig. 2). The signal is measured in a unique direction that is dependent on the pulse sequence for the incoming pulses (Fig. 2). In the scenario where pulse one proceeds in time pulse two and pulse two proceeds pulse three the signal is measured in the phase matching directions ks = k3 + k2 – k1 and ks’ = k3 – k2 + k1. Here the two time variables are referred to as the Coherence and Population times, respectively. The purpose of these experiments is to measure the coherence lifetime of a sample and in the case of PEPS distinguish between slowly varying decoherence processes such as inhomogeneous broadening and the random and usually rapid decoherence processes such as homogeneous broadening.
Figure 2. Basic experimental arrangement for a three pulse photon echo experiment from the top and side view and the corresponding energy level diagrams for pulse sequence E1–E2–E3. Note photon echo signal is measured in the phase matching directions ks = k3 + k2 – k1 and ks’ = k3 – k2 + k1 whereas a transient grating signal (t12 = 0) is the sum of ks and ks’.
Coherent Raman Scattering (CSRS and CARS)
Using the sample optical arrangement above it is also possible to perform femtosecond Raman scattering experiments by simply detuning the frequency ω2 of pulse two E2 such that the energy difference dω = 2ω1 – ω2 = ωn, where ω1 = ω3, ωn is the frequency of a vibrational mode and the signal is measured in the phase matching direction ks’. When ω2 > ω1 pulse two is an anti-Stokes pulse and the experiment is referred to as CARS whereas when ω1 > ω2 the second pulse is Stokes shifted and referred to as CSRS. The major reason for performing CARS and/or CSRS experiments is their ability to separate ground from excited state contributions that are convoluted in a degenerate four wave mixing (DFWM) signal when all the laser pulses are of the same frequency and resonant with the two electronic levels.
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