Excited states of atom

Luminescence can be defined as the emission of light (intended in the broader sense of ultraviolet, visible, or near infrared radiation) by electronic excited states of atoms or molecules. Luminescence is an important phenomenon from a basic viewpoint (e.g., for monitoring excited state behavior) [1] as well as for applications (lasers, displays, sensors, etc.) [2,3]. 

The basic research in our fields is now done largely in universities. It can have incredibly important practical results, but those results cannot normally be predicted in advance. Who would have thought that the basic study of induced energy emission from excited states of atoms and molecules that led to the laser would wind up giving us a better way to record music, or read supermarket prices Would a music company have funded that research Who would have thought that our increased understanding of the chemistry of life would have led to the creation of biotechnology as an entirely new industry The industry that benefited from the basic research could not have funded it, since it did not yet exist. 

Photophysics and photochemistry are relatively young sciences, a real understanding of light-induced processes going back some 50 or 60 years. The development of quantum mechanics was an essential step, as classical physics cannot account for the properties of excited states of atoms and molecules. In the past 30 years the advent of new experimental techniques has given a major impetus to research in new areas of photochemistry, and these are the subject of this final chapter. It must of course be realized that these developments advance all the time, and that we talk here of a moving frontier, as it is in 1992. 

K. Harth, H. Hotop, and M.-W. Ruf, in International Seminar on Highly Excited States of Atoms and Molecules, Invited Papers, eds. S. S. Kano and M. Matsuzawa (Chofu, Tokyo, 1986). 

Fig. 14.1 Two atoms, A and B have energy levels as shown. Initially atom A is in its excited state, and atom B is in its ground state. If the energy of the excited state of atom B could be tuned by some means, we would expect the cross section for resonant energy transfer from atom A to atom B to increase at resonance as shown on the right (from ref. 5).

By using either a continuous or pulsed source of radiation and by measuring the amount of radiation absorbed by the reaction products, it is possible to determine product state distributions. The source of radiation can either be monochromatic (resonance lamp or laser) or broad-band (flash lamp or arc lamp) used in conjunction with a form of monochromator at the detector. The amount of absorption is monitored by an appropriate photosensitive or energy-sensitive detector. Particular care must be taken in the case of resonance lamps to avoid self-reversal of the output of the source, as this will complicate the quantitative analysis of product densities [17]. Similarly, laser sources must not be operated at such high output powers that the transitions involved become saturated, as this also complicates the analysis. Absorption measurements can be used for a wide range of reaction products, both ground and excited states of atoms, radicals and molecules [9,17, 22]. 

At 1236 A., there is a possibility that all processes, (49)-(52), are involved. There is a noticeable change in the distribution of products (Table XI). However, this neither proves nor disproves the participation of these processes. Information on reactions involving excited states of atoms and molecules is almost entirely lacking. More information on the quantum yields at various wavelengths and pressures is badly needed, as well as that on emission bands occurring during photolysis. 

The spectrum of radiation from electronically excited states of atoms appears as lines, when the emission from a hot gas is diffracted and photographed, whereas radiation from these excited states of molecules appears as bands because of emission from different vibrational and rotational energy levels in the electronically excited state. Equation (26) shows that the intensity of radiation from a line or band depends upon the temperature and concentration of the excited state and the transition probability (the rate at which the excited state will go to the lower state). Since the temperature term appears in the exponential, as the temperature rises the exponential term approaches unity, as does the ratio of the concentration of the excited (emitting) state to the ground state (as T approaches oo, Ng = Nj). The concentrations of both the ground and excited states, however, reach a maximum, and then decrease due to the formation of other species. The line or band intensity must also reach a maximum and then decrease as a function of temperature. This relationship can be used to determine the temperature of a system. 

Examine now the determination of exponents for polarization functions. Obviously, the atomic ground state calculations that are so useful in the optimization of valence shell exponents cannot help us. There is a possibility of performing calculations for excited states of atoms. This approach is, however, not appropriate. The role of polarization functions is to polarize valence orbitals in bonds so that the excited atomic orbitals are not very suitable for this purpose. Chemically, more well-founded polarization functions are obtained by direct exponent optimization in molecules. Actually, this was done for a series of small molecules in both Slater and Gaussian basis sets. Among the published papers, we cite. Since expo-... 

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. 

Excited states of atoms result from the promotion of one or more electrons into orbitals of higher energy. Excited electronic states in molecules arise similarly and can be described in the framework of molecular orbital theory. 

Mossbauer spectroscopy measures the resonant absorption of nuclear gamma rays involved with transitions between the ground and excited states of atomic nuclei with nonzero angular momenta. The precise energy of such transitions is... 

The arguments are supported by a number of results on prototypical ground and excited states of atoms and molecules. Most of these are compared with results from conventional methods of quantum chemistry, where single basis sets, orbital- or Flylleraas-type, are used. 

B. Skutnik, I. Oksiiz, O. Sinanoglu, Correlation effects in the excited states of atoms. The ls 2s"2p configuration of Carbon, Nitrogen and Oxygen, Int. J. Quantum Chem. 2 (1968) 1. 

In 1993, the National Institute of Standards and Technology (NIST) brought into use a caesium-based atomic clock called NIST-7 which kept international standard time to within one second in 10 years the system depends upon repeated transitions from the ground to a specific excited state of atomic Cs, and the monitoring of the frequency of the electromagnetic radiation emitted. 

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