Femtosecond time scale chemical studies

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. 

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. 

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. 

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. 

Table 4.11. Examples of studying the kinetics of chemical reactions in the femtosecond time scale...

The flash lamp teclmology first used to photolyse samples has since been superseded by successive generations of increasingly faster pulsed laser teclmologies, leading to a time resolution for optical perturbation metliods tliat now extends to femtoseconds. This time scale approaches tlie ultimate limit on time resolution (At) available to flash photolysis studies, tlie limit imposed by chemical bond energies (AA) tlirough tlie uncertainty principle, AAAt > 2/j. 

Some ingenious experimental innovations have now made it possible to conduct flash photolysis on time scales < 10-11 s. They are anything but routine, especially as they approach a resolution of some femtoseconds, which is the approximate current state of the art. The implementation of these methods allows the study of chemical and physical events on time scales approaching and even exceeding those of molecular vibrations. Indeed, it is studies of vibration, including ligand motion, and (especially) electron transfer that have benefited most. 

The events that happen to an atom in a chemical reaction are on a time scale of approximately 1 femtosecond (1 fs = 10 ",5 s), the time that it takes for a bond to stretch or bend and, perhaps, break. If we could follow atoms on that time scale, we could make a movie of the changes in molecules as they take part in a chemical reaction. The new field of femto-cbemistry, the study of very fast chemical processes, is bringing us closer to realizing that dream. Lasers can emit very intense but short pulses of electromagnetic radiation, and so they can be used to study processes on very short time scales. 

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. 

The technique we employ to measure a rate of reaction depends on how rapidly the reaction takes place. Some biologically important reactions may take weeks to show significant changes in composition, but some chemical reactions are very rapid. Special techniques have to be used when the reaction is so fast that it is over in seconds. With lasers, chemists can study reactions that are complete in a picosecond (1 ps = 10 12 s). The newest techniques can even monitor reactions that are complete after a few femtoseconds (1 fs = 10 15 s), as described in Box 13.1. On that time scale, atoms are hardly moving at all, and they are caught red-handed in the act of reaction. 

Some reactions occur very slowly, such as when a nail rusts. Other occur very rapidly, such as when methane is combusted in a Bunsen burner. Studying very fast reactions requires very special techniques, usually involving lasers—devices that produce high-energy bursts of light with very precise frequencies. The study of very fast reactions is one of the most important areas of chemical research, as demonstrated by the fact that the 1999 Nobel Prize in chemistry was awarded to Ahmed H. Zewail of the California Institute of Technology in Pasadena, California. Zewail s studies involve reactions that occur on the femtosecond (10—15 s) time scale—the time scale for molecular vibrations. 

Laser ionization mass spectrometry of explosives and chemical warfare simulants has been studied using nanosecond laser pulses. Primary ions observed in many of these studies were NO and PO, which are not unique signatures of the parent molecules. It is now widely accepted that after absorption of the first photon, the parent molecule dissociates on a time scale of about 100 femtoseconds (fs). We can attempt to compensate for this rapid dissociation by using ultrafast laser pulses of a corresponding time duration." Here we compare the nanosecond, ultrafast, and SPI approaches. 

---