Time-resolved pump-probe

In many respects the time-resolved pump-probe technique is similar to the CW counterpart. The use of pulsed laser light permits direct probing of both the magnitude of the PA and its dynamics. The experimental arrangement is practically the same as for the CW version, i.e., both pump and probe beams are focused and overlapped onto same spot on a sample. In addition, the pump and probe pulses are synchronized so that the lime interval t between them is constant and confined to a certain time range (in our case up to 3 ns). 

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... 

More recently the application of sub-picosecond, time-resolved pump-probe methods revealed the timescale for vibrational relaxation of a diatomic molecule at a metal surface directly. See for example Refs. 19-21. In comparison to vibrational relaxation on NaCl salts,22 which occurs on the millisecond timescale, another relaxation mechanism is clearly at play. Theory of vibrational relaxation based on excitation of electron-hole pairs gave agreement with observed ps timescales for CO on copper.23... 

Two redox states of one complex, (168) and (169), exhibit very similar respective values of ca. 0.6 J cm-2 and 0.7 J cm-2 with 32 ps pulses at 532 nm (in benzene).452 A 532 nm OL study of the two neutral complexes (170) and (171) using ns and ps pulses has also been reported.453-455 Low values of ca. 0.3 Jem-2 are observed with ps pulses in benzene, and both ps time-resolved pump-probe and Z-scan measurements reveal that RSA and nonlinear refraction are responsible for the OL behavior.453-455 Because (170) and (171) are transparent in the region 400-900 nm, their OL responses should cover a wider range than those of fullerenes and MPcs.453-455 Dai et al. have applied ps 532 nm DFWM to the tetrahedral Zn11 or Cd11 complexes (172) and (173), the modest 7 responses of which are resonance enhanced by the n- 7r transition at 512nm.456 The dimeric square pyramidal Zn11 complex (174) exhibits a broad n —> tt absorption with Amax = 497 nm in DMF and is shown by 532 nm Z-scan to exhibit SF behavior.457... 

As a last example of a molecular system exhibiting nonadiabatic dynamics caused by a conical intersection, we consider a model that recently has been proposed by Seidner and Domcke to describe ultrafast cis-trans isomerization processes in unsaturated hydrocarbons [172]. Photochemical reactions of this type are known to involve large-amplitode motion on coupled potential-energy surfaces [169], thus representing another stringent test for a mixed quantum-classical description that is complementary to Models 1 and II. A number of theoretical investigations, including quantum wave-packet studies [163, 164, 172], time-resolved pump-probe spectra [164, 181], and various mixed... 

Py-dU group, as in P2 this is indicative of ET from the Py-dU group to the adjacent T or C. This ET interpretation is supported by time-resolved pump-probe laser spectroscopy measurements. 

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]... 

The pump pulse in time-resolved pump-probe absorption spectroscopy is often linearly polarized, so photoexcitation generally creates an anisotropic distribution of excited molecules. In essence, the polarized light photoselects those molecules whose transition moments are nominally aligned with respect to the pump polarization vector (12,13). If the anisotropy generated by the pump pulse is probed on a time scale that is fast compared to the rotational motion of the probed transition, the measured anisotropy can be used to determine the angle between the pumped and probed transitions. Therefore, time-resolved polarized absorption spectroscopy can be used to acquire information related to molecular structure and structural dynamics. 

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,... 

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. 

Fig. 1.25. Time-resolved pump-probe photoelectron spectra of Au2CO. The topmost spectrum of AU2CO is obtained with solely the probe laser beam (400nm wavelength). The corresponding 400nm probe-only photoelectron spectrum of Au2 is shown in the bottom trace [149]...

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 spectroscopic methods are based on time-resolved pump-probe schemes where the collision-free regime is usually attained by using low pressure conditions. Application of various linear and non-linear laser techniques, such as LIF (laser-induced fluorescence), REMPI (resonant-enhanced multiphoton ionization) and CARS (coherent antistokes Raman spectroscopy) have provided detailed information on the internal states of nascent reaction products [58]. Obviously, an essential prerequisite for the application of these techniques is the knowledge of the spectroscopic properties of the products. 

Based on ab initio classical trajectories and assuming Gaussian femtosecond envelopes for the laser fields, analytic expressions for the time-resolved pump-probe and pump-dump signals in the framework of the Wigner distribution approach are given by Eq. (6) (cf. Ref. 20). This ab initio Wigner distribution... 

Fig. 20. Time-resolved pump-probe signal for NaK+ compared with the tcquired shape for producing a maximum yield of NaK+.

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]. 

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. 

Figure 10.3 Result of time-resolved pump-probe spectroscopy in combination with TOF-MS. The graphs show femtosecond ionization signals for four DNA and RNA bases, namely cytosine, guanine, thymine, and uracil. The transient of dimethylether (DME) was displayed along with the transient of guanine for comparison [8]. Reprinted with permission from Canuel, C., Mans, M., Piuzzi, F, Tardivel, B., Dimicoli, ., Elhanine, M. (2005) Excited States Dynamics of DNA and RNA Bases Characterization of a Stepwise Deactivation Pathway in the Cas Phase. y. Chem. Phys. 122 074316. Copyright (2005) AlP Publishing EEC...

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

While the computational strategies outlined above represent numerically exact methods, several workers have suggested an approximate calculation of time-resolved pump-probe spectra. T00-i04 jjjg gjjfj jjj... 

Coppens P (2011) Molecular excited-state structure by time-resolved pump probe X-ray diffraction. What is new and what are the prospects for further progress J Phys Chem Lett 2 616-621... 

Relaxations of solvent-chromophore interactions can be studied experimentally by hole-burning spectroscopy, time-resolved pump-probe measurements, and photon-echo techniques that we discuss in the next chapter. If the temperature is low enough to freeze out pure dephasing, and a spectrally narrow laser is used to bum a hole in the absorption spectmm (Sect. 4.11), the zero-phonon hole should have the Lorentzian lineshape determined by the homogeneous lifetime of the excited state. The hole width increases with increasing temperature as the pure dephasing associated with tP comes into play [36, 37]. 

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