Femtosecond pump-probe spectroscop

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. 

The spectroscopic tool to be considered here is femtosecond pump/probe spectroscopy. This experimental technique uses two ultrashort laser pulses which are time-delayed with respect to each other. They are sent into a molecular sample and a signal is recorded as a function of the delay-time between the pulses. To be more specific, we assume the molecule to be in an inital state 0o) O). Here o) denotes the wave function for the nuclear motion and 0) the wave function of the electrons (the adiabatic separation of nuclear and electronic motion is assumed throughout). The pump pulse induces a transition and the resulting wave function which describes the molecule after the interaction with the electric field may be assigned as 0i l). We treat electronic excitation so that the molecule is prepared in another electronic state 1). After the pump pulse passed the sample, the molecule evolves unperturbed until the probe pulse starts interacting. This interaction results in a second excitation to (in our case) a final electronic state 2) with the respective nuclear wave function 1 2) The scheme just described is depicted in Figure 1 and illustrates the idea of many pump/probe experiments. 

This approach has the potential to resolve the time evolution of reactions at the surface and to capture short-lived reaction intermediates. As illustrated in Figure 3.23, a typical pump-probe approach uses surface- and molecule-specific spectroscopies. An intense femtosecond laser pulse, the pump pulse, starts a reaction of adsorbed molecules at a surface. The resulting changes in the electronic or vibrational properties of the adsorbate-substrate complex are monitored at later times by a second ultrashort probe pulse. This probe beam can exploit a wide range of spectroscopic techniques, including IR spectroscopy, SHG and infrared reflection-adsorption spectroscopy (IRAS). 

Figure 5. Set of time-resolved UV-near-IR spectroscopic data (3.44-0.99 eV) following the femtosecond UV excitation of an aqueous sodium chloride solution ([H20]/[NaCl] = 55). An instrumental response of the pump-probe configuration at 1.77 eV (n-heptane) is also shown in the middle part of the figure. The ultra-short-lived components discriminated by UV and IR spectroscopy correspond to low or high excited CTTS states (CTTS, CTTS ), electron-atom pairs (Che pairs), and excited hydrated electrons (ehyd )- The spectral signature of relaxed electronic states (ground state of a hydrated electron, (ehyd) electron-cation pairs, a e hyd) observed in the red spectral region.

The methods discussed so far, fluorescence upconversion, the various pump-probe spectroscopies, and the polarized variations for the measurement of anisotropy, are essentially conventional spectroscopies adapted to the femtosecond regime. At the simplest level of interpretation, the information content of these conventional time-resolved methods pertains to populations in resonantly prepared or probed states. As applied to chemical kinetics, for most slow reactions (on the ten picosecond and longer time scales), populations adequately specify the position of the reaction coordinate intermediates and products show up as time-delayed spectral entities, and assignment of the transient spectra to chemical structures follows, in most cases, the same principles used in spectroscopic experiments performed with continuous wave or nanosecond pulsed lasers. 

Undoubtedly, a key development that will clarify basic aspects in connection to ET dynamic is the effective control on the distance separating the redox species. It can be envisaged that redox couples linked by functionalized spacers, which allow highly ordered self-assembling structures, could provide very valuable information on the rate of interfacial ET and the effect of the interfacial potential. In this case, electronic coupling between reactants could be quite substantial and sophisticated modeling will be required to fiiUy rationalize these findings. Ultrafast photoinduced processes can be effectively approached by spectroscopic Pump-Probe methods down to the femtosecond timescale. 

Two examples which emerged from a fruitful cooperation of theory and experiment are discussed in this book. First, once again the K2 molecule excited to its A state acts as a model system (Sect. 3.1.5). Its special spectroscopic properties combined with the dynamics induced by femtosecond state preparation facilitate the transition from pump probe to control spectroscopy. The intensity of the pump pulse serves as the control parameter, allowing the forcing of the molecule to perform either the A state vibrations or its ground state dynamics. 

In the last chapter, we used a steady-state treatment to relate the shape of an absorption band to the dynamics of relaxations in the excited state. Because a period on the order of the electronic dephasing time will be required to establish a steady state, Eqs. (10.43) and (10.44) apply only on time scales longer than this. We need to escape this limitation if we hope to explore the relaxation dynamics themselves. Our first goal in this chapter is to develop a more general approach for analyzing spectroscopic experiments on femtosecond and picosecond time scales. This provides a platform for discussing how pump-probe and photon-echo experiments can be used to probe the dynamics of structural flucmations and the transfer of energy or electrons on these short time scales. 

In this chapter we describe advances in the femtosecond time-resolved multiphoton photoemission spectroscopy (TR-MPP) as a method for probing electronic structure and ultrafast interfacial charge transfer dynamics of adsorbate-covered solid surfaces. The focus is on surface science-based approaches that combine ultrafast optical pump probe excitation to induce nonlinear multi-photon photoemission (MPP) from clean or adsorbate covered single crystal surfaces. The photoemitted electrons transmit spectroscopic and dynamical information, which is captured by their energy analysis in real or reciprocal space. We examine how photoelectron spectroscopy and microscopy yield information on the unoccupied molecular structure, electron transfer and relaxation processes, light induced chemical and physical transformations and the evolution of coherent single particle and collective excitations at solid surfaces. 

We are beginning to develop a detailed understanding of these methods (18,21,30,33,34,37-40,42,44,47-49), many of which are described in this book. We have recently demonstrated a series of novel nonlinear all-IR spectroscopic techniques (IR-pump-IR-probe, IR-three-pulse photon echoes, IR-dynamic hole burning, IR-2D spectroscopy), all of them utilizing intense femtosecond IR pulses, with the intention to develop new multidimensional spectroscopic tools to study the structure and the dynamics of proteins (30,31,41,42,50-53). We shall summarize in this contribution our work, its underlying principles, and its applications. 

Following the above-mentioned spectroscopic study by Johnson and co-workers [55], Neumark and co-workers [56] explored the ultrafast real-time dynamics that occur after excitation into the CTTS precursor states of I (water) [n — 4-6) by applying a recently developed novel method with ultimate time resolution, i.e., femtosecond photoelectron spectroscopy (FPES). In anion FPES, a size-selected anion is electronically excited with a femtosecond laser pulse (the pump), and a second femtosecond laser pulse (the probe) induces photodetachment of the excess electron, the kinetic energy of which is determined. The time-ordered series of the resultant PE spectra represents the time evolution of the anion excited state projected on to the neutral ground state. In the study of 1 -(water), 263 nm (4.71 eV) and 790 nm (1.57 eV) pulses of 100 fs duration were used as pump and probe pulses, respectively. The pump pulse is resonant with the CTTS bands for all the clusters examined. 

Electron injection dynamics in the conduction band of metal oxide materials from dye molecules or metal nanoparticles, which is important when applied to sensitized solar cells, can be monitored in the infrared by 100 fs time resolution. In this chapter, technical details of femtosecond visible-pump/IR-probe transient absorption spectroscopy and some typical spectroscopic data revealing the mechanism of electron injection process were described. A great advantage of this technique is that one can observe transient absorption of injected electrons easily because of the intense intraband transition of an electron at the bottom of or at the trap level just below the conduction band of the metal oxide that forms an electrode. In the case of dye-sensitized solar cells, the effects of metal oxide, dye, solvent and additive ions on the rate and efficiency of electron injection were discussed in detail. One recent discovery, plasmon-induced electron injection from a gold nanoparticle to a Ti02 nanoparticle, was presented to show how femtosecond visible-pump/IR-probe transient absorption spectroscopy is useful in studying this kind of new charge transfer dynamics in a nano-structured system. 

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