Pump-probe techniques time domains

Pump-probe spectroscopy Time-domain spectroscopic technique in which an intensive pump pulse is used to perturb some physical property of the sample system and subsequently a weak time-delayed probe pulse is applied to monitor the pump induced effect as a function of the delay time. 

ESIPT processes are usually faster than 1 ps and the subsequent relaxation processes occur typically in less than 1 ns. The observation of the dynamics calls for techniques with a time resolution higher than what can be achieved by electronic means. This is the domain of pump-probe techniques [1, 2]. An ultrashort laser pulse excites the sample and initiates the process of interest. A second ultrashort pulse probes the properties of the sample after a delay, which is the temporal separation between the two pulses. The measurement is repeated with systematically varied delay times to sample the complete evolution of an observable during the process under... 

This discourse tries to give an overview of the current state-of-the-art instrumentation in real-time pulse radiolysis experiments utilizing optical, conductometric and other methods. Pump-and-probe techniques for the sub-nanosecond time domain are believed to be beyond the scope of this discussion. 

A simplified view of the early processes in electron solvation is given in Figure 7. Initially, electron pulse radiolysis was the main tool for the experimental study of the formation and dynamics of electrons in liquids (Chapter 2), first in the nanosecond time range in viscous alcohols [23], later in the picosecond time range [24,25]. Subsequently, laser techniques have achieved better time resolution than pulse radiolysis and femtosecond pump-probe laser experiments have led to observations of the electron solvation on the sub-picosecond to picosecond time scales. The pioneering studies of Migus et al. [26] in water showed that the solvation process is complete in a few hundreds of femtoseconds and hinted at the existence of short-lived precursors of the solvated electron, absorbing in the infrared spectral domain (Fig. 8). The electron solvation process could thus be depicted by sequential stepwise relaxation cascades, each of the successive considered species or... 

Second, what is the nature of IVR at higher vibrational energies in excited electronic states While time-domain fluorescence techniques are not well suited to answer this question, pump-probe ionization experiments have the potential of doing so. 

The extent of new and insightful knowledge regarding metal complex photophysics that can now be derived from a diverse variety of time-resolved pump-probe spectroscopic techniques is illustrated by recent examples in the field of spin-state crossover complexes. This is especially so in the solution state/ but also in the solid, crystalline state straddling several time domains, from the steady-state to femtoseconds. Examples are discussed in Section 4 below on Molecular bi-stability in solution and the solid state . First however we look at recent examples where Raman spectroscopy in both steady-state and time-resolved modes has been applied to the investigation of metal-centred species of bioinorganic and catalytic interest. 

For measuring the transient photoexcitation response in the Is to ns time domain we have used the fs two-color pump-probe correlation technique with linearly polarized light beams. We have used two laser systems a high repetition rate, low power laser for the mid-lR spectral range [92] and a relatively lower repetition rate, high power laser system for the near-lR and visible spectral range [93]. 

In this section, the real-time spectra for two different pump probe cycles, i.e. two-color and one-color real-time experiments, are presented. First, the two essentially different real-time spectra I t) are analyzed in the time domain, followed by a detailed analysis in the frequency domain (/(u )). Introducing the spectrogram technique I At,uj)) enables detailed insight into the investigated wave packet dynamics. In particular, it visualizes directly several total and fractional revivals of induced vibrational wave packets. By comparing the spectrograms of the different pump probe cycles, one can easily assign the different ionization pathways of the laser-induced processes. This nicely demonstrates the different excitation mechanisms in the two experiments. 

In order to monitor the real-time dynamics of gas molecules interacting with surface, time-resolved study is required. It is generally known that the time domains for the gas adsorption/desorption on surface are within pico-second regime while the molecular vibration on surface is within femto-second regime. To accommodate this time-requirement as well as chemical analysis on surface, a type of pump and probe experiment is required, which makes use of synchronization between a laser pulse and a synchrotron radiation pulse of AP-XPS endstation. For example, the carrier dynamics and reaction mechanism of photocatalysts under AP conditions can be an ideal system to look at with this time-resolved experimental set-up. At present, the synchronization technique has been well developed as shown in a block diagram (Fig. 9.24). This time-resolved set-up can be further refined and adapted into advanced system when the free electron X-ray source is available. 

A method less common for lifetime measurements is the so-called pump-probe or double-pulse approach. Like time- and frequency-domain detection, the technique originates in non-spatially-resolved fluorescence spectroscopy [ 19]. In this technique, two very short excitation pulses follow each other. The first pulse excites fluorochromes inside the detection volume to full or partial saturation. The second pulse, or probe pulse, arrives at a variable (ns) time delay. If the time delay between the pulses is short compared to the fluorescence lifetime, most of the fluorochromes will still be in the excited state when the second pulse arrives so that the second pulse cannot excite additional fluorochromes and thus does not lead to additional fluorescence. If the time delay is long, most fluorochromes will have relaxed back to their ground state, so that the second pulse leads to... 

Two main techniques are available to perform 2D-IRS, namely a pulsed-Jrequency-domain technique [88] and a time-domain-pulsed-Fourier transform technique [89]. A hybrid method using acousto-optic modulation has also been proposed recently [90]. In the pulsed-frequency-domain experiments, an intense broadband femtosecond pulse is split into a pump pulse, which passes a filter to reduce the band width to sections of typically 10 cm, and an unfiltered probe pulse. Both pulses are focused into the sample within an adjustable time delay. The probe pulse (full bandwidth) measures the spectral changes of the sample after the arrival of the pump pulse. For this purpose, the intensity of the probe light beam is recorded using a spectrometer equipped with a broadband array detector. 

Time resolved fluorescence measurements have been used for decades because they are such a powerful tool to investigate fluorophore-metal composites. Due to insufficient time resolution, mostly long lived luminescence like that from triplet states has been investigated. When fluorophOTes are attached to metal nanostructures, fluorescence decay times are in the sub nanosecond time range. To measure those dect times accurately, techniques such as time correlated single photon counting, frequency domain fluorescence measurements, streak camera measuremets, and femtosecond pump SHG-probe have been used. 

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