Femtosecond time scales

Gaspard P and Burghardt I (ed) 1997 XXth Solvay Conf on Chemistry Chemical Reactions and their Control on the Femtosecond Time Scale (Adv. Chem. Phys. 101) (New York Wiley)... 

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. 

One of the early examples for kinetic studies on the femtosecond time scale is the photochemical predissociation of Nal [74] ... 

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. 

Following a description of femtosecond lasers, the remainder of this chapter concentrates on the nuclear dynamics of molecules exposed to ultrafast laser radiation rather than electronic effects, in order to try to understand how molecules fragment and collide on a femtosecond time scale. Of special interest in molecular physics are the critical, intermediate stages of the overall time evolution, where the rapidly changing forces within ephemeral molecular configurations govern the flow of energy and matter. 

From a theoretical perspective, the object that is initially created in the excited state is a coherent superposition of all the wavefunctions encompassed by the broad frequency spread of the laser. Because the laser pulse is so short in comparison with the characteristic nuclear dynamical time scales of the motion, each excited wavefunction is prepared with a definite phase relation with respect to all the others in the superposition. It is this initial coherence and its rate of dissipation which determine all spectroscopic and collisional properties of the molecule as it evolves over a femtosecond time scale. For IBr, the nascent superposition state, or wavepacket, spreads and executes either periodic vibrational motion as it oscillates between the inner and outer turning points of the bound potential, or dissociates to form separated atoms, as indicated by the trajectories shown in Figure 1.3. 

Mass spectrometric studies are not limited to the investigation of stable intermediates they have also been carried out on reaction transition states. The ultrafast studies by Zewail, for example, are nominally mass spectrometric based, where photoionization is used to detect reactive species on exceedingly short (femtosecond) time scales.Time resolved studies provide insight into the rates of unimo-lecular reactions, but do not provide direct thermochemical insight. 

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. 

In order to directly probe the dynamics of CT between Et and ZG, and to understand how the intervening DNA base stack regulates CT rate constants and efficiencies, we examined this reaction on the femtosecond time scale [96]. These investigations revealed not only the unique ability of the DNA n-stack to mediate CT, but also the remarkable capacity of dynamical motions to modulate CT efficiency. Ultrafast CT between tethered, intercalated Et and ZG was observed with two time constants, 5 and 75 ps, both of which were essentially independent of distance over the 10-17 A examined. Significantly, both time constants correspond to CT reactions, as these fast decay components were not detected in analogous duplexes where the ZG was re-... 

Diels JC, Rudolph W (1996) Ultrashort laser pulse phenomena fundamentals, techniques, and applications on a femtosecond time scale. Academic, San Diego CA, pp 365-399... 

Various ultrafast phenomena occuring in the femtosecond time-scale in the condensed phase have been studied by fluorescence up-conversion (for a review, see Mialocq and Gustavsson, 2001). As already mentioned in Chapter 7 (Box 7.1),... 

However, systems with localized atoms represent only a first challenge. The next challenge is monitoring atomic motions in systems that vary in time. Following atomic motions during a chemical process has always been a dream of chemists. Unfortunately, these motions evolve from nanosecond to femtosecond time scales, and this problem could not have been overcome until ultrafast detection techniques were invented. Spectacular developments in laser technology, and recent progress in constmction of ultrafast X-ray sources, have proved to be decisive. Two main techniques are actually available to visualize atomic motions in condensed media. 

The nanosecond and femtosecond realms are bridged by the picosecond regime not only in time but also in the level of complexity associated with the experiments. One must be prepared to evaluate critically the spectroscopic information that is obtained in the picosecond domain—more so than the nanosecond regime, but less so than the femtosecond time scale. Cautions will be noted and explained at appropriate times. 

Molecular hydrogen has a vibrational period of only 7.6 fs, while vibrational modes involving heavier atoms are slower. The motions of atoms in activated molecules, the various vibrations and rotations leading to chemical reactions, will generally take place over time periods of 10 to 100 or 1000 fs, the femtosecond time scale. One femtosecond is 10 s femton is Swedish for 15, making the femtosecond unit, the next beyond the nicely classical miUi-, micro-, nano-, picosecond sequence, easy to remember. 

Norrish type-I reaction, has been studied over the years in extreme detail, with every imaginable physical and theoretical method at hand. Data gathered through studying such reactions on the femtosecond time scale, together with new theoretical work prompted by the dynamics observed, have provided a detailed picture of the processes involved and a fresh perspective on nonconcerted ot-cleavage events. 

Experimentally, it has become possible to follow chemical reactions on a femtosecond time scale with excellent time resolution— with precise definitions of f = 0, time separations between pulses, and with very narrow pulse widths. 

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