Excited atoms and molecules

Flames are also plasmas, characterized by electron densities of about 10 /cm and electron energies of about 0.5 eV. Many excited species are present in the flame, namely free radicals, ions, excited atoms and molecules, and electrons [43]. Excited species that have been observed include O, OH, NH, NO, and CH [44]. 

A distinguishing feature of electronically excited atoms and molecules is that they have one or a few excited orbitals of an electron. The principal properties of these particles are represented by a high internal energy potential localized on the excited orbitals and the structure of electron shell essentially different from the electron ground state. 

In the near-UV, visible and near-IR regions of the spectrum, emission is due to electronic transitions in excited atoms and molecules, while in the near- and mid-IR it is due to vibrational transitions within molecules. 

Reactions of Low-Energy Electrons, Ions, Excited Atoms and Molecules, and Free Radicals in the Gas Phase as Studied by Pulse Radiolysis Methods... 

Advances in pulse radiolysis studies in the gas phase have been summarized in several review papers. In a comprehensive review by Sauer [4], a review presented by Firestone and Dorfman [5] in 1971 was referred to as the first review on gas-phase pulse radiolysis. Experimental techniques and results obtained were summarized by one of the present authors [6], with emphasis on an important contribution of pulse radiolysis to gas-phase reaction dynamics studies. Examples were chosen by Sauer [7] from the literature prior to 1981 to show the types of species that were investigated in the gas phase using pulse radiolysis technique. Armstrong [8] reviewed experimental data obtained from gas-phase pulse radiolysis together with those from ordinary steady-state radiolysis. Advances in gas-phase pulse radiolysis studies since 1981 were also briefly reviewed by Jonah et al. [9], with emphasis on an important contribution of this technique to free radical reaction studies. One of the present authors reviewed comprehensively the gas-phase collision dynamics studies of low-energy electrons, ions, excited atoms and molecules, and free radicals by means of pulse radiolysis method [1-3]. An important contribution of pulse radiolysis to electron attachment, recombination, and Penning collision studies was also reviewed in Refs. 10-15. 

Collisions of Electronically Excited Atoms and Molecules (Muschlitz) 10 121... 

Figure 21-26 Emission from a plasma produced by laser irradiation of the high-temperature superconductor YBCI2CU3O7. Solid is vaporized by the laser and excited atoms and molecules such as YO emit light at characteristic wavelengths. [From w.a. warmer. "Plasma Emission from Laser Ablation of YBa2Cuj07." Appf. Phys. Lett. 1988, 52.2171]...

As nonthermal plasma is a mixture of electrons, highly excited atoms and molecules, ions, radicals, photons, and so on, its chemistry is extremely complex, and highly selective products should not be expected via plasma chemistry. The basic reactions for controlling both the direction and reaction rate of plasma C02 utilization can be summarized as follows (here, A and B represent atoms, A2 and B2 molecules, e represents an electron, M is a temporary collision partner, and S represents a solid surface site. The excited species is indicated by an asterisk). 

J. P. Schermann, J. P. Astruc, C. Desfrangois and R. Barbe, Electron Transfer Between Excited Atoms and Molecules, in Photoinduced Electron Transfer, Part A, edited by M. A. Fox and M. Chanon, Chapter 1.2, pp. 60-122, Elsevier, Amsterdam, 1988. 

The study of electronically excited atoms and molecules by mass spectrometry is a relatively recent development. In mu mass spectrometer the fiight time of a molecule from entrance slit to ionization chamber is a few hundred microseconds, so that only those excited states whose transitions to lower electronic states are highly forbidden can be investigated. Since these metastable species are often readily destroyed by wall collisions, it is generally necessary to employ a collision-free sampling system. 

If the electric current and thus the density of electrons and excited atoms and molecules grows in the plasma, electron collisions with excited atoms and molecules and the Coulomb interaction between the electrons become increasingly important and have to be included in the kinetic study of the electron behavior. 

A brief survey is given of physicochemical aspects of atomic and molecular processes that are of great importance in reactive plasmas. The processes are composed of the interaction of molecules, in most cases polyatomic molecules, with reactive species such as electrons, ions (both positive and negative), free radicals, and excited atoms and molecules. Topics are chosen from recent studies of some elementary processes in reactive plasmas. Some comments are also given on future problems that call for more work in reactive-plasma research from the viewpoints of physicochemical studies of gas-phase reaction dynamics and kinetics, such as radiation chemistry and photochemistry. 

The development of techniques to prepare well-characterized targets of species in excited states (long-lived metastable atoms and molecules, electronically excited atoms and molecules, and vibrationally excited, hot molecules) for electron and ion collisions, including attachment and dissociative attachment studies and spectroscopic measurements. 

Ionizing radiation produces ionized and excited atoms and molecules in all materials. Excited molecules formed directly or by recombination reactions between electrons and cations decompose in the vast majority of systems to highly reactive free radicals. The reactive species formed on radiolysis are precursors of further reactions, such as reduction, oxidation, polymerization, cross linking and so on. It should therefore be possible to apply radiation chemical methods to industrial processes and, consequently, extensive applied research and development of radiation chemistry has been carried out during the past three decades. 

Relaxation of electronically excited atoms and molecules is due to different mechanisms. Superelastic collisions (energy transfer back to plasma electrons) and radiation are essential mostly in thermal plasma. Relaxation in collision with other heavy particles dominates in non-thermal discharges. Relaxation of electronic excitation into translational degrees... 

Electronically excited atoms and molecules transfer energy not only into translational, but also into vibrational and rotational degrees of freedom, which is less adiabatic and faster. Fast relaxation takes place by formation of intermediate ionic complexes. For example, electronic energy transfer from excited metal atoms Me to vibrational excitation of nitrogen takes place for almost every collision ... 

The high rates of plasma-chemical reactions are often due to a high concentration of excited atoms and molecules in electric discharges. Vibrational and electronic excitations play the most important role in the stimulation of endothermic processes in plasma. We next focus on reactions of vibrationally excited molecules, which are easier to analyze. The kinetic relations, however, can also be applied to some extent to reactions of electronically excited particles. 

Vibrationally and electronically excited atoms and molecules, which make a significant contribution to plasma-chemical kinetics, can be described in terms of distribution functions. We already introduced the vibrational distribution functions (population of vibrationally... 

Kinetics of Different Mechanisms of NO Synthesis in Non-Equilibrium Plasma Conditions. Based on (6-10) estimate the electron temperatures and plasma ionization degrees required for the Zeldovich mechanism of NO synthesis, (6-2) and (6-3), to be kinetically faster than alternative mechanisms related to the contribution of positive ions and electronically excited atoms and molecules. 

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