Ultrafast laser techniques

A typical example of this is in the preparation of extraordinary divalent carbon intermediates (carbenes) by application of ultrafast laser techniques. Thus, photoexcitement of diphenyldiazomethane (DPDM) to an excited singlet state breaks the C=N bond, releasing diphenvlcarbene (DPC) in a singlet state and ultimately allows its stabilization in the triplet ground state,... 

We will in the presentation below review some of our recent work in this area. Those results which have already been published elsewhere will only be briefly summarized, while previously unpublished material will be presented in greater detail. In particular, the material in Section 2, and parts of 3.2, and 4.1, 4.2, and 4.3 is essentially new. Some outlooks for the future, connected especially with the application of ultrafast laser techniques, are given in section 5. 

We have utilized this wealth of background information in the design of our experiments. We use ultrafast laser techniques to drive sustained shocks into thin films of energetic materials, which are then interrogated using several different kinds of ultrafast spectroscopic and interferometric probes. The remainder of this chapter will describe these experiments in detail, especially the ultrafast laser shock production and characterization methods and the spectroscopic and interferometric anomalies caused by working with thin films, and present... 

Applications of ultrafast laser techniques for studies in solids, optoelectronics, condensed phase, and in biological systems. 

The study of chemical dynamics concerned with the ultrashort time interval when a chemical bond is formed or broken may be called real-time femtochemistry [1412], It relies on ultrafast laser techniques with femtosecond time resolution [1413]. 

The high costs associated with specialist ultrafast laser techniques can make their purchase prohibitive to many university research laboratories. However, centralised national and international research infrastructures hosting a variety of large scale sophisticated laser facilities are available to researchers. In Europe access to these facilities is currently obtained either via successful application to Laser Lab Europe (a European Union Research Initiative) [35] or directly to the research facility. Calls for proposals are launched at least annually and instrument time is allocated to the research on the basis of peer-reviewed evaluation of the proposal. Each facility hosts a variety of exotic techniques, enabling photoactive systems to be probed across a variety of timescales in different dimensions. For example, the STFC Central Laser Facility at the Rutherford Appleton Laboratory (UK) is home to optical tweezers, femtosecond pump-probe spectroscopy, time-resolved stimulated and resonance Raman spectroscopy, time-resolved linear and non-linear infrared transient spectroscopy, to name just a few techniques [36]. 

One of the most exciting possibilities of ultrafast laser techniques is to follow the course of fundamental chemical reactions on the relevant timescale at which they occur. Previously, it was only possible to know the individual states of molecules A and B before reacting and the final state of the compound molecule AB. In contrast, the details of the chemical reaction can now be followed on a femtosecond scale with information on how chemical bonds are formed and broken. In particular, the existence of transition states has been demonstrated. This new field of science is frequently referred to as femtochemistry [9.191-9.204], for which A. Zewail was awarded a Nobel prize in chemistry (1999). 

Time-resolved X-ray absorption is a very different class of experiments [5-7]. Chemical reactions are triggered by an ultrafast laser pulse, but the laser-induced change in geometry is observed by absorption rather than diffraction. This technique permits one to monitor local rather than global changes in the system. What one measures in practice is the extended X-ray absorption fine structure (EXAFS), and the X-ray extended nearedge strucmre (XANES). 

This chapter discusses the apphcation of femtosecond lasers to the study of the dynamics of molecular motion, and attempts to portray how a synergic combination of theory and experiment enables the interaction of matter with extremely short bursts of light, and the ultrafast processes that subsequently occur, to be understood in terms of fundamental quantum theory. This is illustrated through consideration of a hierarchy of laser-induced events in molecules in the gas phase and in clusters. A speculative conclusion forecasts developments in new laser techniques, highlighting how the exploitation of ever shorter laser pulses would permit the study and possible manipulation of the nuclear and electronic dynamics in molecules. 

Absorption and Ensuing Ion-Molecule Reactions via Ultrafast Laser Pump-Probe Techniques.196... 

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. 

Today, ultrafast pulsed-laser techniques, high-speed computers, and other sophisticated instrumentation make it possible to measure the time evolutions of reactants, intermediates, transition structures, and products following an abrupt photoactivation of a starting material. Detailed theoretical calculations, experienced judgments based on the literature, and newly accessible femtosecond-domain experimental data providing observed intensities of chemical species versus time can provide insights on the atomic-scale events responsible for overall reaction outcomes. 

In addition to using imaging as a technique to obtain spatially and temporally patterned chemical data from a sample, one can also pattern chemical reactions in space and time using similar methods. Figure 2.18 demonstrates an example in which multiphoton scanning with ultrafast laser pulses was used to polymerize a photoresist resin with about 120 nm spatial resolution in three dimensions. 

In recent years there has been significant interest in the extension of nonlinear optical spectroscopy to higher orders involving multiple time and/or frequency variables. The development of these multidimensional techniques is motivated by the desire to probe the microscopic details of a system that are obscured by the ensemble averaging inherent in linear spectroscopy. Much of the recent work to extend time domain vibrational spectroscopy to higher dimensionality has involved the use of nonresonant Raman-based techniques. The use of Raman techniques has followed directly from the rapid advancements in ultrafast laser technology for the visible and near-IR portions of the spectrum. Time domain nonresonant Raman spectroscopy provides access to an extremely... 

Ultrafast TRIR. The most fundamental processes of bond making, bond breaking and electron transfer have ultrafast dynamics. Access to these ultrafast time scales by TRIR requires a different approach from real-time measurements. Instead, pulsed-laser techniques based upon optical delay for measurement of time must be used. There are several approaches to measuring TRIR on the 10 — 10 s... 

Kinetic experiments are performed in two different ways. In one an initial disequilibrinm exists between two or more reactants, which after being rapidly mixed, combine to react toward equilibrium see Rapid Scan, Stopped-Flow Kinetics). Ideally, the mixing time is short with respect to the timescale of the reaction or actually with respect to the formation of intermediates. In contrast, in the relaxation experiment, the reactants are together and in equilibrium, and the whole system is instantaneously displaced from equilibrium. Subsequently, the system relaxes to the same or a new equilibrium state. Table 1 suimnarizes the approximate time resolution of various commonly applied mixing and relaxation techniques. The table indicates the superiority of the relaxation methods with respect to time resolution, mainly due to the development of ultrafast lasers. Mixing liquids on the (sub)microsecond time scale appears to present an important experimental barrier. 

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