Time-resolved pump-probe experiments

Because of the severe congestion of the spectrum due to overlapping resonances, information about the time dependence of the dissociation process can be obtained only through time-resolved pump-probe experiments. They have been carried out by Ionov et al. [34] and by Kirmse et al. [35] as described in 3.1. Exciting a molecule by a pulse creates a wave packet,... 

This review describes some of the recent developments in materials which exhibit enhanced two-photon absorption that can initiate photopolymerization or up-converted emission. Various optical methods including femtosecond time-resolved pump-probe experiments to characterize the two-photon properties are discussed. Finally, the applications of two-photon processes to optical power limiting, up-converted lasing, 3-D data storage, 3-D micro-fabrication, two-photon fluorescence microscopy and bio-imaging, and two-photon photodynamic therapy are presented. 

The Cl of the adiabatic PESs is a common phenomenon in molecules [11-13], The singular nonadiabatic coupling (NAC) associated with Cl is the origin of ultrafast non-Born-Oppenheimer transitions. For a number of years, the effects of Cl on IC (or other nonadiabatic processes) have been much discussed and numerous PESs with CIs have been obtained [11, 12] for qualitative discussion. Actual numerical calculations of IC rates are still missing. In this chapter, we shall calculate IC rate with 2-dependent nonadiabatic coupling for the pyrazine molecule as an example to show how to deal with the IC process with the effect of CL Recently, Suzuki et al. have researched the nn state lifetimes for pyrazine in the fs time-resolved pump-probe experiments [13]. The population and coherence dynamics are often involved in such fs photophysical processes. The density matrix method is ideal to describe these types of ultrafast processes and fs time-resolved pump-probe experiments [14-19]. 

Figure 2 illustrates the basic concept of a typical pump-probe spectroscopy used in most ultrafast spectroscopy techniques. In its simplest form the output pulse train of an ultrafast laser is divided in two by a beam splitter. One pulse in train (called pump) first excites the sample under investigation. The second pulse train (called probe) will probe the sample with a suitable time delay with respect to the pump by introducing an optical delay in its path and some optical property (e.g., reflectivity, absorption, Raman scattering, luminescence, optical nonlinear responses) of the sample is then detected to investigate the changes produced by the pump. In most of the time-resolved pump-probe experiments, the time resolution is limited only by the pulse width of the laser or the jitter between the laser systems. 

We return now to considering the detailed form of the PAD in time-resolved pump-probe PES experiments. It is convenient to describe the excited-state population dynamics in terms of the density matrix, defined by [40, 87]... 

Although steady-state experiments have yielded a wealth of knowledge about the mechanism of charge transfer in DNA, the results obtained with this type of measurementsare insufficient to decide how fast a charge can be transferred over a certain distance. Absolute values of the charge transfer rate can be obtained from time-resolved pump-probe laser experiments, such as those performed by Lewis and coworkers [20]. These experiments were carried out on DNA sequences containing a stilbenedicarboxamide (Sa) electron acceptor. Figure 3 shows a few examples ofthe DNA sequences studied. 

Time-resolved pump-probe laser-induced fluorescence experiments... 

Figure 11.13 Time-resolved pump-probe vibrational relaxation of a C—H stretch vibration in liquid acetonitrile at room temperature detected by (Raman) emission from the excited state. In the isolated molecule such a v = 1 stretch mode will relax on the nanosecond time scale. The experiment maps out the energy transfer pathways in the liquid. The essential point is that the relaxation is by solvent-aided intramolecular transfer, as discussed in the text and indicated by arrows. Energy dumping into the liquid is far slower, requiring about 260ps (adapted from Deak etal. (1998) see also Iwaki and DIott (2001), Wang eta/. (2002)).

While both populations are equivalent in principle, being related by a unitary transformation, one of them may be more clo.sely related to experiment than the other. For example, if there are dipole. selection rules forbidding the optical transition to or from a subset of the interacting electronic states, these selection rules are usually obeyed to a much larger extent in the diabatic basis than in the adiabatic ba.si.s. Then the diabatic electronic populations are monitored via the intensities of spontaneous and induced emission (the adiabatic populations may be more relevant if the optical transition takes place within the interacting manifold). More specifically, in the limit of ideally short pump and probe pulses the time-resolved pump-probe signal as a function of the delay time has been shown to be proportional to the diabatic population, equation (51). For the more realistic case of finite pulse durations the situation is more complex. In the present article we leave these problems aside and focus on the purely intramolecular aspects of the vibronic dynamics. The various aspects associated with their detection in real time have been surveyed in a recent review article. ... 

An interferometric method was first used by Porter and Topp [1, 92] to perfonn a time-resolved absorption experiment with a -switched ruby laser in the 1960s. The nonlinear crystal in the autocorrelation apparatus shown in figure B2.T2 is replaced by an absorbing sample, and then tlie transmission of the variably delayed pulse of light is measured as a fiinction of the delay This approach is known today as a pump-probe experiment the first pulse to arrive at the sample transfers (pumps) molecules to an excited energy level and the delayed pulse probes the population (and, possibly, the coherence) so prepared as a fiinction of time. 

Since there are a large number of different experimental laser and detection systems that can be used for time-resolved resonance Raman experiments, we shall only focus our attention here on two common types of methods that are typically used to investigate chemical reactions. We shall first describe typical nanosecond TR spectroscopy instrumentation that can obtain spectra of intermediates from several nanoseconds to millisecond time scales by employing electronic control of the pnmp and probe laser systems to vary the time-delay between the pnmp and probe pnlses. We then describe typical ultrafast TR spectroscopy instrumentation that can be used to examine intermediates from the picosecond to several nanosecond time scales by controlling the optical path length difference between the pump and probe laser pulses. In some reaction systems, it is useful to utilize both types of laser systems to study the chemical reaction and intermediates of interest from the picosecond to the microsecond or millisecond time-scales. 

Describing complex wave-packet motion on the two coupled potential energy surfaces, this quantity is also of interest since it can be monitored in femtosecond pump-probe experiments [163]. In fact, it has been shown in Ref. 126 employing again the quasi-classical approximation (104) that the time-and frequency-resolved stimulated emission spectrum is nicely reproduced by the PO calculation. Hence vibronic POs may provide a clear and physically appealing interpretation of femtosecond experiments reflecting coherent electron transfer. We note that POs have also been used in semiclassical trace formulas to calculate spectral response functions [3]. 

A further possibility is that the signals arise from hydrated electrons or base radical ions produced by monophotonic ionization of the polymers. However, the quantum yield for photoionization of adenosine is reported to be approximately the same as that of poly(A) and poly(dA) [25], It is unlikely that photoionization of the polymers can account for the signals seen here since there is no detectable signal contribution from the photoionization of single bases [4], The most compelling argument that our pump-probe experiments monitor excited-state absorption by singlet states is the fact that ps and ns decay components have been observed in previous time-resolved emission experiments on adenine multimers [23,26-28]. 

AT DNA double strand oligomers with sodium counterions and a length of 20 base pairs were obtained from Biotherm, and were dissolved in water and dried on a CaF2 window at 293 K in an atmosphere of 52% relative humidity (saturated solution of NaHSC. IDO at 20° Celsius [60]). This results in DNA samples with approximately 4 to 6 water molecules per base pair [37] (sample thickness 6.5 pm). It has been reported that under these conditions AT DNA oligomers adopt the B -form [35], Femtosecond time-resolved IR pump-probe experiments were performed with two independently tunable femtosecond pulses generated by parametric conversion processes pumped by a regenerative Ti sapphire laser system (800 nm, repetition rate 1 kHz, pulse duration 100 fs) [61]. The central frequency of the pump pulse was varied from 1630 to 1760 cm-1 and the probe was centred around 1650 cm-1 or 3200... 

To help completely understand the ultrafast interface ET between dye molecules and semiconductor nanoparticles, it is desirable to experimentally measure the femtosecond time-resolved spectra (i.e., probing signal in the pump-probe experiment) at various pumping wavelengths. Changing laser pulse-durations will also be useful. 

A pumping laser excites the system from the ground vibronic manifold g to the excited vibronic manifold n. After excitation, a probing laser is applied to induce transitions from the manifold to the manifold g via stimulated emission and/or to higher excited manifolds via induced absorption. This work shall focus on the pump-probe time-resolved stimulated emission experiment. In this case, an expression for the time-resolved profiles is derived in terms of the imaginary part of the transient susceptibility X (copu,copr, x). In the adiabatic approximation and the Condon approximation, it has been shown that [18,21]... 

In order to apply these equations to a femtosecond pump-probe experiment, an additional assumption has to be made regarding the shape of the time resolved signal. We wish to account for the finite relaxation time of the transient polarisation and so the signal must be described by a double convolution of an exponential decay function with the pump and probe intensity envelope functions. We will assume a Gaussian peak shape so that the convolution may be calculated analytically. As we will see, the experimental results require two such contributions, and hence, the following function will be used to fit the experimental data... 

Figure 1 Schematic representation of a time-resolved coherent Raman experiment, (a) The excitation of the vibrational level is accomplished by a two-photon process the laser (L) and Stokes (S) photons are represented by vertical arrows. The wave vectors of the two pump fields determine the wave vector of the coherent excitation, kv. (b) At a later time the coherent probing process involving again two photons takes place the probe pulse and the anti-Stokes scattering are denoted by subscripts P and A, respectively. The scattering signal emitted under phase-matching conditions is a measure of the coherent excitation at the probing time, (c) Four-photon interaction scheme for the generation of coherent anti-Stokes Raman scattering of the vibrational transition.

Ru(bipy)3 + is the prototype of a very large family of MLCT species. In the standard model of the photoprocesses of this compound, a photon excites the molecule to an initial Frank-Condon singlet state, MLCT, that rapidly transforms to a triplet, MLCT, with a quantum yield of near unity. Femtosecond pump probe experiments have established a half-life of about 100 fs for the formation of the triplet state. Recent studies utilizing femtosecond time-resolved fluorescence emission spectroscopy has observed fluorescence emission from the Frank-Condon state itself and the hfetime of this state has been estimated to be 40 15 fs. ... 

---