Pump-probe spectroscopy,

In order to obtain quantitative results, pump-probe spectra have to be corrected for the optical density of the sample following the Lambert-Beer law of absorption. The correction can be performed in the following way  

The optical density OD at the excitation wavelength Aexc is the product of the absorption coefficient a and the film thickness z. Combining the above three 

If the absorption depth L within the sample decreases exponentially according to the Lambert-Beer law, the density of populated excited states N(z) obeys 

The fraction of absorbed probe photons in an infinitesimal thickness dz is estimated by 

The change in transmission of the probe due to the pump source, considering only the normalised fraction, is given by 

These expressions apply to a variety of experiments that depend on the third-order polarization, including pump-probe and photon-echo experiments. Three-pulse photon echo experiments, for example, depend on R2. 

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 

11 Pump-Probe Spectroscopy, Photon Echoes and Vibrational Wavepackets 

The radiation fields that enter into Eq. (11.35) are combinations of the fields from the pump and probe pulses, and are given by 

Probably the most interesting of the pathways shown in Fig. 11.6 is 3. Although this pathway yields state c on the fourth interaction with the radiation field, state b is never actually populated the route proceeds entirely through the coherences pab, Pac and pi,c The pathway should, therefore, be particularly sensitive to dephasing 

An elegant way to study the dynamics from excited vibrational levels of the electronic ground state is to make use of the femtosecond pump dump probe scheme in transient absorption experiments. The molecular system is initially excited to the Si state and, using a second pulse of longer wavelength (the dump pulse), excited vibrational levels of the So state are populated via stimulated emission.197 

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Diffey W M and Beck W F 1997 Rapid-scanning interferometer for ultrafast pump-probe spectroscopy with phase-sensitive detection Rev. Sci. Instrum. 3296-300... 

Jeiezko F, Lounis B and Orrit M 1997 Pump-probe spectroscopy and photophysicai properties of singie di-benzanthanthrene moiecuies in a naphthaiene crystai J. Phys. Chem 107 1692-702... 

Figure 8-1. Layout of the experimental scl-up for field-assisted pump-probe spectroscopy. PD photodiode OMA optical multichannel analyzer.

A qualitatively different approach to probing multiple pathways is to interrogate the reaction intermediates directly, while they are following different pathways on the PES, using femtosecond time-resolved pump-probe spectroscopy [19]. In this case, the pump laser initiates the reaction, while the probe laser measures absorption, excites fluorescence, induces ionization, or creates some other observable that selectively probes each reaction pathway. For example, the ion states produced upon photoionization of a neutral species depend on the Franck-Condon overlap between the nuclear configuration of the neutral and the various ion states available. Photoelectron spectroscopy is a sensitive probe of the structural differences between neutrals and cations. If the structure and energetics of the ion states are well determined and sufficiently diverse in... 

H. Ueba, Vibrational relaxation and pump-probe spectroscopies of adsorbates on solid surfaces, Progress in Surf. Sci. 55, 115 (1997). 

Schwartz, B. J. and Rossky, P. J. Pump-probe spectroscopy of the hydrated electron a quantum molecular dynamics simulation, J. Chem.Phys., 101 (1994), 6917-6926... 

This interpretation is supported by a thorough study of the photophysics of PCT-1 and its complexes with Li+ and Ca2+ (Martin et al., 1994). In particular, subpicosecond pump-probe spectroscopy provided compelling evidence for the disruption of the link between the crown nitrogen atom and the cation. A photo-... 

Transient bleaching and recovery rates of CdS excitonic absorption, determined by picosecond pump-probe spectroscopy, depended on [H20]/[A0T] ratio and micellar surface. Fluorescence spectra and lifetimes depended on [Cd2+]/[S2 ] ratios... 

The double proton transfer of [2,2 -Bipyridyl]-3,3 -diol is investigated by UV-visible pump-probe spectroscopy with 30 fs time resolution. We find characteristic wavepacket motions for both the concerted double proton transfer and the sequential proton transfer that occur in parallel. The coherent excitation of an optically inactive, antisymmetric bending vibration is observed demonstrating that the reactive process itself and not only the optical excitation drives the vibrational motions. We show by the absence of a deuterium isotope effect that the ESIPT dynamics is entirely determined by the skeletal modes and that it should not be described by tunneling of the proton. 

The solvated electron in methanol. Novel time- and frequency-resolved pump-probe spectroscopy of short-lived precursors. 

The present study shows a combined investigation of both the generation process of solvated electrons and of pump-probe-spectroscopy of intermediate states including the final equilibrated ground state. As shown previously [5], the formation of the solvated electron in methanol takes place within -10 ps. About an order of magnitude slower than in water, the timescale of this process allows to investigate the precursor states in much more detail using a state-of-the-art laser system. 

The photodynamics of electronically excited indole in water is investigated by UV-visible pump-probe spectroscopy with 80 fs time resolution and compared to the behavior in other solvents. In cyclohexane population transfer from the optically excited La to the Lb state happens within 7 ps. In ethanol ultrafast state reversal is observed, followed by population transfer from the Lb to the La state within 6 ps. In water ultrafast branching occurs between the fluorescing state and the charge-transfer-to-solvent state. Presolvated electrons, formed together with indole radicals within our time resolution, solvate on a timescale of 350 fs. 

Coherent nuclear motion of reacting excited-state molecules in solution observed by ultrafast two color pump-probe spectroscopy... 

Unlike the case of simple diatomic molecules, the reaction coordinate in polyatomic molecules does not simply correspond to the change of a particular chemical bond. Therefore, it is not yet clear for polyatomic molecules how the observed wavepacket motion is related to the reaction coordinate. Study of such a coherent vibration in ultrafast reacting system is expected to give us a clue to reveal its significance in chemical reactions. In this study, we employed two-color pump-probe spectroscopy with ultrashort pulses in the 10-fs regime, and investigated the coherent nuclear motion of solution-phase molecules that undergo photodissociation and intramolecular proton transfer in the excited state. 

An additional piece of information can be obtained by studying a synthetic compound derived from the GFP chromophore (1-28) fluorescing at room temperature. In Fig. 3a we show the chemical structure of the compound that we studied in dioxan solution by pump-probe spectroscopy. If we look at the differential transmission spectra displayed in Fig. 3b, we observed two important features a stimulated emission centered at 508 nm and a huge and broad induced absorption band (580-700 nm). Both contributions appear within our temporal resolution and display a linear behavior as a function of the pump intensity in the low fluences limit (<1 mJ/cm2). We note that the stimulated emission red shifts with two characteristic time-scales (500 fs and 10 ps) as expected in the case of solvation dynamics. We conclude that in the absence of ESPT this chromophore has the same qualitative dynamical behavior that we attribute to the relaxed anionic form. 

It is very likely that the metal-insulator transition, the unusual catalytic properties, the unusual degree of chemical reactivity, and perhaps even some of the ultramagnetic properties of metal clusters are all linked intimately with the dynamic, vibronic processes inherent in these systems. Consequently, the combination of pump-probe spectroscopy on the femtosecond time scale with theoretical calculations of wavepacket propagation on just this scale offers a tantalizing way to address this class of problems [5]. Here we describe the application of these methods to several kinds of metal clusters with applications to some specific, typical systems first, to the simplest examples of unperturbed dimers then, to trimers, in which internal vibrational redistribution (IVR) starts to play a central role and finally, to larger clusters, where dissociative processes become dominant. 

In this section we apply the same formalism to pump-probe spectroscopy, where one measures the absorption of a probe pulse 2(0 by a molecule excited by a pump pulse E (t). The pump-probe signal can be written as... 

Ellington et al.40) used femtosecond pump-probe spectroscopy to probe directly the arrival of electrons injected into the TiOz film with near- and mid-IR that probe the absorption at 1.52 jum and in the range of 4.1-7.0 jUm. Their measurements indicate an instrument limited 50 fsec upper limit on the electron injection time. These observations suggest that electron injection from Dye 2 to... 

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