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5.1: Experimental Techniques

In the previous sections we have discussed time-resolved fluorescence measurements, or the intrinsic emission of excited molecules. In this chapter we will discuss another widely used technique of time-resolved spectroscopy, based on the absorption measurement. This is so-called pump-probe (PP) spectroscopy.

 

Fig. 1) The idea of pump-probe experiment: pump pulse excites the sample, and probe is used to measure the transmittance. The variable delay between pump and probe pulses provides the time dependence of the difference absorption signal.

 

 

            An idea of pump-probe measurement is simple: a sample (e.g. biologically relevant light absorbing molecule) is excited using a short laser pulse. Another pulse that arrives some time after the first one is used to measure how the absorption of the sample was altered by the first pulse. By varying the arrival time of the second pulse with respect to the first one, we can measure the entire time-dependence of the absorption change. Retroreflector on a motorized translation stage is used to delay one of the pulses, similarly to an autocorrelator (Fig. 11B), or fluorescence upconversion experiment. Thus, for PP experiment, two laser pulses are required: the pump and the probe. Obviously, the pump should be more intense than the probe, because it produces the changes in molecules, whereas the probe merely interrogates them. It would be best, if the probe had completely no influence on the sample being investigated. PP spectroscopy has an advantage compared to time-resolved fluorescence in that the changes of the absorption are sensitive to both the changes in the ground state and the excited state. Fluorescence always looks at the transitions between the excited and the ground state, and, as soon as the excited state is lost, no signal can be observed any more (even though, the resulting ground state may still be different from the original one, i.e. the effects of the excitation pulse may persist). Free cheese can only be found in mouse traps, and this advantage of PP technique comes with the penalty that the signals contributing to the PP spectra ar more complicated and harder to interpret than those in time-resolved fluorescence. Therefore, both techniques are sometimes combined in order to isolate excited-state information (fluorescence) and obtain a complete ultrafast description of the systems being investigated (pump-probe).

 

Fig. 2) A: Typical layout of time-resolved difference absorption experiment: amplified pulses from Ti:Sapphire laser are split into two parts using a beamsplitter BS. One part is used to pump an optical parametric amplifer that produces tunable pump pulses. The other part is delayed in a delay line (a retroreflector RR on a motorized translation stage) and then focused into a nonlinear medium NM, where white light supercontinuum is generated (see section 2.1.5). The generated probe light is overlapped with pump light in the sample. The chopper is periodically closing and opening the pump beam in order to take the probe intensity measurements of the excited and non-excited sample).

 

 

            The sample receives two pulses one after the other: the pump and the probe. After passing the sample, the pump is usually blocked, and the probe intensity is measured (Fig. 25). According to the absorption law, probe light intensity behind the excited sample is

 

                                                                                                                      .

Here I0 is the intensity of the incident pulse and Aexc is the absorption of the excited sample. Analogously, when the pulse is not excited

                                                                                                                  .

By dividing by and taking a logarithm of both sides of the equation, we obtain

                                                                                                   .

 

This means that in order to record the absorption change in the sample induced by the excitation pulse, we do not need to measure the intensity of the incident pulse I0. Absorption change expressed by Eq. is the signal measured in pump-probe spectroscopy. In practice, the layout of the experiment will look something like the one shown in Fig. 25. Ti:Sapphire laser and amplifier produces femtosecond pulses. They are further used in an optical parametric amplifier (or harmonic generator, see section 2.1.5) to obtain the pulses at the wavelengths absorbed by the sample. These become pump pulses. They are periodically blocked and unblocked by an optical chopper (a rotating disc with slots in it) in order to measure both  and in Eq . In the visible and near-IR and near-UV spectral range,[1] white light supercontinuum generated in nonlinear medium (sapphire crystal, thin water cell or CaF2 window) is normally used as a probe. Diffraction grating or prism-based spectral device is used to select desired probe wavelength. The intensity of probe pulses is measured using appropriate detectors (photodiodes or array detectors like CCD, whereby the entire spectrum can be recorded at once). It is important for the detector to be synchronized with the chopper in order to separate the measurements of pumped and unpumped sample. In summary, the spectral dimension of the transient absorption comes from the spectrograph or a monochromator placed behind the sample, whereas the time-resolution is obtained by moving a mechanical delay line. The measured signal is the function of delay time (period between the pump and the probe pulses) and probe wavelength:

                                                                                                                        

In the further section we will discuss the factors determining the transient absorption spectrum. .

 


[1] When probing in UV, VIS and nIR spectral ranges, we usually interact with electronic molecular excited states, because these wavelengths match the energy gaps between electronic states. If the laser pulses were in the mid-IR spectral range (3-15 mm), we would be doing vibrational time-resolved spectroscopy, because the energies of mid-IR photons correspond to the energies of vibrational transitions. In the following, we will stick to electronic time-resolved spectroscopy unless stated otherwise.