Chemical dynamics femtosecond time scale

In summary, spectrally resolved 3-pulse 2-colour photon echoes provide a potential tool to study the molecular structure dynamics on a femtosecond time scale and will be used to study chemical and physical processes involving nonequilibrium relaxation in both ground and excited states of molecules. 

Reaction dynamics on the femtosecond time scale are now studied in all phases of matter, including physical, chemical, and biological systems (see Fig. 1). Perhaps the most important concepts to have emerged from studies over the past 20 years are the five we summarize in Fig. 2. These concepts are fundamental to the elementary processes of chemistry—bond breaking and bond making—and are central to the nature of the dynamics of the chemical bond, specifically intramolecular vibrational-energy redistribution, reaction rates, and transition states. 

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. 

Third, it makes it possible to observe on a real-time basis the evolution of the most short-lived transient of reacting particles, that is, to study the molecular dynamics of chemical reactions in a femtosecond time scale [2]. Many papers presented at the XXth Solvay Conference were devoted to this possibility of implementing one-dimensional resolution along the reaction coordinate. The time resolution At corresponds to the spatial resolution Az At(v), where (v) is the average velocity along the reaction coordinate, for example, the velocity of the reaction products. At v) 3 x 104 cm/s, the quantity At - 100 fs corresponds to a one-dimensional (longitudinal) resolution of Az - 0.3 A. 

Techniques employing the ultraviolet (UV), visible, and near-infrared parts of the spectrum have the advantage of high sensitivity (single photon), high time resolution (femtoseconds), and moderate spatial resolution (on the order of 100 nm). Structural information is obtainable by infrared to radio-frequency techniques (e.g., magnetic resonance). Together, these techniques have enabled the visualization of individual molecules and the measurement of excited state dynamics from such molecules on the picosecond time scale. It is also possible to follow the time course of chemical reactions on the femtosecond time scale when... 

Recently, two basic questions of chemical dynamics have attracted much attention first, is it possible to detect ( film ) the nuclear dynamics directly on the femtosecond time scale and second, is it possible to direct (control) the nuclear dynamics directly as it unfolds These efforts of real-time detection and control of molecular dynamics are also known as femtosecond chemistry. Most of the work on the detection and control of chemical dynamics has focused on unimolecular reactions where the internuclear distances of the initial state are well defined within, of course, the quantum mechanical uncertainty of the initial vibrational state. The discussion in the following builds on Section 7.2.2, and we will in particular focus on the real-time control of chemical dynamics. It should be emphasized that the general concepts discussed in the present section are not limited to reactions in the gas phase. 

The exploration of ultrafast molecular and cluster dynamics addressed herein unveiled novel facets of the analysis and control of ultrafast processes in clusters, which prevail on the femtosecond time scale of nuclear motion. Have we reached the temporal boarders of fundamental processes in chemical physics Ultrafast molecular and cluster dynamics is not limited on the time scale of the motion of nuclei, but is currently extended to the realm of electron dynamics [321]. Characteristic time scales for electron dynamics roughly involve the period of electron motion in atomic or molecular systems, which is characterized by x 1 a.u. (of time) = 24 attoseconds. Accordingly, the time scales for molecular and cluster dynamics are reduced (again ) by about three orders of magnitude from femtosecond nuclear dynamics to attosecond electron dynamics. Novel developments in the realm of electron dynamics of molecules in molecular clusters pertain to the coupling of clusters to ultraintense laser fields (peak intensity I = lO -lO W cm [322], where intracluster fragmentation and response of a nanoplasma occurs on the time scale of 100 attoseconds to femtoseconds [323]. 

Lasers are the precision tools of photochemistry and they have been used to both pump (initiate) and probe (analyse) chemical processes on time-scales that are short enough to allow the direct observation of intramolecular motion and fragmentation (i.e. on the femtosecond time-scale). Thus, laser-based techniques provide us with one of the most direct and effective methods for investigating the mechanisms and dynamics of fundamental processes, such as photodissociation, photoionization and unimolecu-lar reactions. Avery wide variety of molecular systems have now been studied using laser techniques, and only a few selected examples can be described here. 

Another class of chemical reactions also covered here is that of proton-transfer reactions. These processes play a key role in solution chemistry, and more specifically in acid—base reactions. In this class of reactions the cmcial step involves the motion of the hydrogen atom, which typically occurs on the picosecond or femtosecond time-scale. By investigating the time dynamics of these processes in size-selected clusters, for a given system, information is gained at which specific cluster size the onset of the proton transfer reaction occurs. 

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. 

Su T, Chesnavich WJ. (1982) Parametrization of the ion-polar molecule collision rate-constant by trajectory calculations. J. Chem,. Phys. 76 5183-5185. Troe J, Lorquet JC, Manz J, Marcus RA, Herman M. (1997) Recent advances in statistical adiabatic channel calculations of state-specific dissociation dynamics. Chemical Reactions and Their Control on the Femtosecond Time Scale XXth Solvay Conference on Chemistry, Vol. 101, pp. 819-851. 

Femtosecond lasers represent the state-of-the-art in laser teclmology. These lasers can have pulse widths of the order of 100 fm s. This is the same time scale as many processes that occur on surfaces, such as desorption or diffusion. Thus, femtosecond lasers can be used to directly measure surface dynamics tlirough teclmiques such as two-photon photoemission [85]. Femtochemistry occurs when the laser imparts energy over an extremely short time period so as to directly induce a surface chemical reaction [86]. 

Under the simulation conditions, the HMX was found to exist in a highly reactive dense fluid. Important differences exist between the dense fluid (supercritical) phase and the solid phase, which is stable at standard conditions. One difference is that the dense fluid phase cannot accommodate long-lived voids, bubbles, or other static defects, whereas voids, bubbles, and defects are known to be important in initiating the chemistry of solid explosives.107 On the contrary, numerous fluctuations in the local environment occur within a time scale of tens of femtoseconds (fs) in the dense fluid phase. The fast reactivity of the dense fluid phase and the short spatial coherence length make it well suited for molecular dynamics study with a finite system for a limited period of time chemical reactions occurred within 50 fs under the simulation conditions. Stable molecular species such as H20, N2, C02, and CO were formed in less than 1 ps. 

Beyond imaging, the combination of CRS microscopy with spectroscopic techniques has been used to obtain the full wealth of the chemical and the physical structure information of submicron-sized samples. In the frequency domain, multiplex CRS microspectroscopy allows the chemical identification of molecules on the basis of their characteristic Raman spectra and the extraction of their physical properties, e.g., their thermodynamic state. In the time domain, time-resolved CRS microscopy allows the recording of the localized Raman free induction decay occurring on the femtosecond and picosecond time scales. CRS correlation spectroscopy can probe three-dimensional diffusion dynamics with chemical selectivity. 

More recently, the use of picosecond and femtosecond lasers in reaction dynamics opened up the field of femtochemistry, which was pioneered by Zewail [51-54]. The idea of these reactions is to photoinitiate the reactive process in a van der Waals complex. Sometimes, the process that is initiated is a simple dissociation or the isomerization of a free molecule. In each case, the reaction is initiated by a first ultrashort laser pulse (the pump pulse). It is analyzed after a certain delay by a second pulse (the probe pulse). This gives access to the reaction dynamics on the pertinent time-scale where chemical bonds are broken and others are formed. Depending on the system, this typically lasts between a few tenths of femtoseconds to hundredths of picoseconds. Recently the techniques of stereodynamies have been combined by Zewail and co-workers with femtosecond analysis [55, 56] to label specific reaction channels in electron-transfer reactions. 

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