Femtosecond time scale spectroscopy

Because of the generality of the symmetry principle that underlies the nonlinear optical spectroscopy of surfaces and interfaces, the approach has found application to a remarkably wide range of material systems. These include not only the conventional case of solid surfaces in ultrahigh vacuum, but also gas/solid, liquid/solid, gas/liquid and liquid/liquid interfaces. The infonnation attainable from the measurements ranges from adsorbate coverage and orientation to interface vibrational and electronic spectroscopy to surface dynamics on the femtosecond time scale. 

Pump-probe absorption experiments on the femtosecond time scale generally fall into two effective types, depending on the duration and spectral width of the pump pulse. If tlie pump spectrum is significantly narrower in width than the electronic absorption line shape, transient hole-burning spectroscopy [101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112 and 113] can be perfomied. The second type of experiment, dynamic absorption spectroscopy [57, 114. 115. 116. 117. 118. 119. 120. 121 and 122], can be perfomied if the pump and probe pulses are short compared to tlie period of the vibrational modes that are coupled to the electronic transition. 

Luminescence lifetime spectroscopy. In addition to the nanosecond lifetime measurements that are now rather routine, lifetime measurements on a femtosecond time scale are being attained with the intensity correlation method (124), which is an indirect technique for investigating the dynamics of excited states in the time frame of the laser pulse itself. The sample is excited with two laser pulse trains of equal amplitude and frequencies nl and n2 and the time-integrated luminescence at the difference frequency (nl - n2 ) is measured as a function of the relative pulse delay. Hochstrasser (125) has measured inertial motions of rotating molecules in condensed phases on time scales shorter than the collision time, allowing insight into relaxation processes following molecular collisions. 

Recently, Zewail and co-workers have combined the approaches of photodetachment and ultrafast spectroscopy to investigate the reaction dynamics of planar COT.iii They used a femtosecond photon pulse to carry out ionization of the COT ring-inversion transition state, generated by photodetachment as shown in Figure 5.4. From the photoionization efficiency, they were able to investigate the time-resolved dynamics of the transition state reaction, and observe the ring-inversion reaction of the planar COT to the tub-like D2d geometry on the femtosecond time scale. Thus, with the advent of new mass spectrometric techniques, it is now possible to examine detailed reaction dynamics in addition to traditional state properties." ... 

In addition to the natural improvements expected in the accuracy of the measurements, and the increased scope in the types of systems examined, new techniques go beyond the issue of thermochemistry to allow for very detailed studies of reaction dynamics. The investigation by Zewail and co-workers of the reactivity of planar COT" on the femtosecond time scale is likely only the beginning. Time-resolved photoelectron spectroscopy, for example, has recently been used to map the potential energy surfaces for the dissociation of simple ions IBr and l2. " Although applications in the field of organic reactive molecules are likely far off, they are now possible. 

The question is then, first, how often has such a complete match between experiment and theoretical simulation been achieved Second, are there good examples where complete simulations have been carried out but lead to two or more equally acceptable models to interpret the experimental results I refer to this question of ambiguity also in relation to a very similar problem arising in the interpretation of nontime-resolved high-resolution spectroscopy data [1,2], which provided in fact, the first experimental results on nontrivial three-dimensional wavepacket motion on the femtosecond time scale [3]. 

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 most cases, the two latter processes have been studied individually by fast techniques (flash photolysis, transient spectra measurements, Raman spectroscopy) in nano-, pico-, and femtosecond time scales as processes accompanying photophysical deactivation steps [64-66]. In Table 3 the data for such individual steps are reported. The data can be summarized as follows ... 

Photoinduced electron injection is by no means a new development. This process has already been applied in areas such as silver halide photography. In this discussion, only sensitized TiC>2 surfaces will be considered. Many experiments have shown that the charge injection into the semiconductor surface is very fast. In order to study these processes, fast spectroscopic techniques are preferred. Whether or not charge injection takes place can be studied conveniently on the nanosecond time-scale by using transient absorption spectroscopy. However, to address the injection process directly, experiments are carried out on the femtosecond time-scale, while recombination and charge separation require the nanosecond to microsecond range. 

Abstract A challenging task in surface science is to unravel the dynamics of molecules on surfaces associated with, for example, surface molecular motion and (bimolecular) reactions. As these processes typically take place on femtosecond time scales, ultrafast lasers must be used in these studies. We demonstrate two complementary approaches to study these ultrafast molecular dynamics at metal surfaces. In the first, the molecules are studied after desorbing from the surface initiated by a laser pulse using the so called time-of-flight technique. In the second approach, molecules are studied in real time during their diffusion over the surface by using surface-specific pump-probe spectroscopy. 

Pump-probe absorption experiments on the femtosecond time scale generally fall into two effective types, depending on the duration and spectral width of the pump pulse. If the pump spectrum is significantly narrower in width than the electronic absorption line shape, transient hole-burning spectroscopy [101. 102. 

In siammary, the preliminary results presented in this contribution already demonstrate that time resolving polarization spectroscopy offers a number of favourable and new features for direct observation of fast evolving events on a femtosecond time scale and detection of oscillations up to the THz-range. The described technique can be applied to free atoms, liquids and solids to measure coherent transients in groimd and excited states. Since the observed beats result from an atomic interference effect, narrow structures which may be hidden by inhomogeneous broadening mechanisms can still be resolved. 

Both linear and nonlinear Raman spectroscopy can be combined with time-resolved detection techniques when pumping with short laser pulses [8.781. Since Raman spectroscopy allows the determination of molecular parameters from measurements of frequencies and populations of vibrational and rotational energy levels, time-resolved techniques give information on energy transfer between vibrational levels or on structural changes of short-lived intermediate species in chemical reactions. One example is the vibrational excitation of molecules in liquids and the collisional energy transfer from the excited vibrational modes into other levels or into translational energy of the collision partners. These processes proceed on picosecond to femtosecond time scales [8.77,8.79]. 

The chapters of this book are all theoretical in character. This reflects the fact that the conical intersection is a theoretical concept, and as such is not directly accessible to experimental observation. Nevertheless, the concepts, techniques and results discussed in this book are crucial for the interpretation of the observations in time-resolved spectroscopy and chemical kinetics on femtosecond time scales. It is hoped, therefore, that this book is of value not only for the theoretician, but also for the practitioneer in molecular spectroscopy, photochemistry, and collision-induced reaction dynamics. 

Chapter 3 treats nuclear motions on the adiabatic potential energy surfaces (PES). One of the most powerful and simplest means to study chemical dynamics is the so-called ab initio molecular dynamics (or the first principle dynamics), in which nuclear motion is described in terms of the Newtonian d3mamics on an ab initio PES. Next, we review some of the representative time-dependent quantum theory for nuclear wavepackets such as the multiconfigurational time-dependent Hartree approach. Then, we show how such nuclear wavepacket d3mamics of femtosecond time scale can be directly observed with pump>-probe photoelectron spectroscopy. 

Development of this kind of knowledge about excited states is likely to be slow, however moreover, the equipment is highly specialized and its use is more likely to be in the hands of spectroscopically oriented chemical physicists than in those of coordination chemists. For this reason, however, we can expect increasing collaboration between laboratories having different and complementary capabilities. There is another point to be made about laser spectroscopy. It is now possible to do state-to-state photochemistry on a long femtosecond time scale, that is, to pinpoint the vibrational level of the excited state and that of the immediately produced product state. At this point I believe that we have left the realm of chemistry and entered that of physics and spectroscopy for their own sake. What seems important to me, as a physical chemist, is to know the structure and electronic properties of thexi states rather than those of spectroscopic states. Notice that there are two distinct usages of the word "state" that of a thermodynamic state or ensemble, i.e. of a thexi state, and that of a particular molecular wave-mechanical or spectroscopic state. 

The first example concerns the superfast dissociation of the electronically excited molecule CH300H (the class of such reactions was discussed in Section 4.1). It should be emphasized here that the methods of femtosecond spectroscopy allowed the study of the kinetics of formation of the photodissociation products, viz., the CH3O and OH radicals, in the femtosecond time scale. 

Recent advances in laser spectroscopy (femtosecond time scale) have made it possible to determine the rate constants for decarboxylation (fcjJ and back-electron transfer (fc et) in the photolysis of electron donor/acceptor salts such as methylviologen/benzilates (MV +/Ar2C(0H)C02) and methylviologen/ary-lacetates (MV VArGHjCOj) (Scheme 7). The values are much higher for the benzilates (2-8 x 10" s ) than for the arylacetates (1-2 x 10 s" ). Decarboxylation of these donors is thus almost a barrier-free... 

For very fast reactions, as they are accessible to investigation by pico- and femtosecond laser spectroscopy, the separation of time scales into slow motion along the reaction path and fast relaxation of other degrees of freedom in most cases is no longer possible and it is necessary to consider dynamical models, which are not the topic of this section. But often the temperature, solvent or pressure dependence of reaction rate... 

These two examples show how femtosecond spectroscopy allows the observation, in real time, of processes whose real-time behaviour could only be inferred, previously, from experimental observations made on a much longer time scale. 

The availability of lasers having pulse durations in the picosecond or femtosecond range offers many possibiUties for investigation of chemical kinetics. Spectroscopy can be performed on an extremely short time scale, and transient events can be monitored. For example, the growth and decay of intermediate products in a fast chemical reaction can be followed (see Kinetic measurements). 

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