Excited atoms or molecules

If we consider a set of atoms or molecitles of the same natrrre, it is known that they store energy that is distributed into various forms  

At rest and at a certain temperatirre, these forms present an energy that has a Gaussian shape with a mean value and a standard deviation that increases with temperature. It is possible by various methods to provide energy well above the average to a relatively significant number of these atorrrs or molecules. The esserrtial activation methods are  

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Table C2.13.1 Collision processes of electrons and heavy particles in non-thennal plasmas. The asterisk denotes short-lived excited particles, the superscript m denotes long-lived metastable excited atoms or molecules.

In Section 2.2 we saw that emission of radiation by an excited atom or molecule M may be by a spontaneous (Equation 2.4) or by an induced, or stimulated, process... 

Spectroscopy The science of analyzing the spectra of atoms and molecules. Emission spectroscopy deals with exciting atoms or molecules and measuring the wavelength of the emitted electromagnetic radiation. Absorption spectroscopy measures the wavelengths of absorbed radiation. 

Spontaneous emission occurs when an excited atom or molecule emits a photon of energy equal to the energy difference between the two states without the influence of other atoms or molecules (Figure 1.3(a)) ... 

In contrast to spontaneous emission, induced emission (also called stimulated emission) is coherent, i.e. all emitted photons have the same physical characteristics - they have the same direction, the same phase and the same polarization. These properties are characteristic of laser emission (L.A.S.E.R. = Light Amplification by Stimulated Emission of Radiation). The term induced emission comes from the fact that de-excitation is triggered by the interaction of an incident photon with an excited atom or molecule, which induces emission of photons having the same characteristics as those of the incident photon. 

F+Ha HF- +. H AH =-139.9 kj is also exothermic and can produce energy rich HF molecules. The heat of chemical reaction is distributed in various vibrational-rotational modes to give vibrationally excited HF or HC1 in large numbers. Emission from these hot molecules can be observed in the infrared region at h 3.7 (j-m. The reaction system in which partial liberation of the heat of reaction can generate excited atoms or molecules is capable of laser action (Section 3.2.1). They are known as chemical lasers. The laser is chemically pumped, without any external source of radiation. The active molecule is born in the excited state. Laser action in these systems was first observed by Pimental and Kasper in 1965. They had termed such a system as photoexplosion laser. 

The simplest case of reaction kinetics occurs when the excitation and emission occur in the same atom, molecule, or luminescence center. The recombination can then be treated as a first-order unimolecular reaction. The decay time is independent of the number of other similarly excited atoms or molecules. 

Second, a tunable laser radiation can excite atoms or molecules of a certain species (or even of a certain isotopic composition) in a mixture, that is, can ensure the intermolecular selectivity of the subsequent photochemical processes [3]. 

The word laser stands for fight amplification by stimulated emission of radiation. The process of stimulated emission is the release of a photon by an excited atom or molecule under the impact of an incoming photon. This can be written as... 

This technique is known as the passive Q-switch. The dye acts as an absorber for weak light, so that the population of excited atoms or molecules in the active material can increase until its maximum level is reached. The dye cell is in fact a high-speed shutter. 

A study of such spectra shows that the lines or bands therein accurately coincide in frequency with certain lines or bands of the emission spectra of the same substances, This was formerly attributed to resonance of electronic vibrations, but is now more satisfactorily explained by quantum theory on Ihe assumption that those quanta of the incident radiation which are absorbed are able to excite atoms or molecules of the medium to some (but not all) of the energy levels involved in the production of the complete emission spectrum,... 

Emission Emission is the loss of energy from an excited atom or molecule in the form of light. Although a long-lived form of light emission (known as phosphorescence) does occur, in flow cytometry the light emission with which we are primarily concerned occurs rapidly after excitation and is called fluorescence. 

When an electron neutralizes a positive ion, the energy released can be dissipated either in photon emission (radiative recombination), or by a third body encounter with the transient excited atom or molecule (three-body recombination) or by the fragmentation of the transient excited molecule (dissociative recombination). Radiative recombination only occurs with a very small probability and three-body recombination only occurs at high pressures or high charge densities, neither of these being appropriate to the atmospheric plasma. It is the dissociative process, exemplified by reactions (5a) and (5b), which is dominant in the ionosphere. In fact, reactions (5a) and (5b) are almost entirely responsible for the loss of ionization in the ionosphere above 85 km altitude (with N2 recombination contributing somewhat) as is readily shown by simple calculations based on laboratory determinations of dissociative recombination coefficients, are, for the dominant molecular ions 02 and NO+. 

The mechanism consists of using a conventional energy source (flash-lamp or other) to excite atoms or molecules from the ground state to some excited state, so that an inversion of population occurs in the Arrhenius55 sense this is usually best understood in a three-level laser, although two-level lasers are also discussed (see Problem 10.10.1), and many are four-level lasers (see Fig. 10.14). 

The random matrix was first introduced by E. P. Wigner as a model to mimic unknown interactions in nuclei, and it has been studied to describe statistical natures of spectral fluctuations in quantum chaos systems [17]. Here, we introduce a random matrix system driven by a time-dependent external field E(t), which is considered as a model of highly excited atoms or molecules under an electromagnetic field. We write the Hamiltonian... 

The ions M+ and the excited atoms or molecules M produced in the primary reactions (6.1) and (6.2), respectively, give rise to further (secondary) reactions ... 

These effects overlap and lead to ionization, emission of electrons from the electron shell and fluorescence. They may cause secondary reactions in molecules and subsequent reactions of the ions or excited atoms or molecules produced by these effects. 

If the ojjtical field that excites atoms or molecules is strong enough, it can create Zeeman coherences not only in the excited state of atoms or molecules, but also in the ground state. In a slightly different context this effect for the first time was studied as an optical pumping of atomic states. 

The radiant power P of a line or a band depends directly on the number of excited atoms or molecules, which in turn is proportional to the total concentration c of the species present in the source. Thus, we can write... 

Fluorescence is a photoluminescence process in which atoms or molecules are excited by absorption of electromagnetic radiation, as shown in Figure 24-22a. The excited species then relax back to the ground state, giving up their excess energy as photons. As we noted in Section 24D, the lifetime of an excited species is brief because there are several mechanisms by which an excited atom or molecule can... 

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