Excitation of atoms

The Goeppert-Mayer two- (or multi-) photon absorption, mechanism (ii), may look similar, but it involves intennediate levels far from resonance with one-photon absorption. A third, quasi-resonant stepwise mechanism (iii), proceeds via smgle- photon excitation steps involvmg near-resonant intennediate levels. Finally, in mechanism (iv), there is the stepwise multiphoton absorption of incoherent radiation from themial light sources or broad-band statistical multimode lasers. In principle, all of these processes and their combinations play a role in the multiphoton excitation of atoms and molecules, but one can broadly... 

The focus of this section is the emission of ultraviolet and visible radiation following thermal or electrical excitation of atoms. Atomic emission spectroscopy has a long history. Qualitative applications based on the color of flames were used in the smelting of ores as early as 1550 and were more fully developed around 1830 with the observation of atomic spectra generated by flame emission and spark emission.Quantitative applications based on the atomic emission from electrical sparks were developed by Norman Lockyer (1836-1920) in the early 1870s, and quantitative applications based on flame emission were pioneered by IT. G. Lunde-gardh in 1930. Atomic emission based on emission from a plasma was introduced in 1964. 

Principles and Characteristics Atomic fluorescence spectrometry (AFS) is based on excitation of atoms by radiation of a suitable wavelength (absorption), and detection and measurement of the resultant de-excitation (fluorescence). The only process of analytical importance is resonance fluorescence, in which the excitation and fluorescence lines have the same wavelength. Nonresonance transitions are not particularly analytically useful, and involve absorption and fluorescence photons of different energies (wavelength). 

Nuclear magnetic resonance spectroscopy is a technique that, based on the magnetic properties of nuclei, reveals information on the position of specific atoms within molecules. Other spectroscopic methods are based on the detection of fluorescence and phosphorescence (forms of light emission due to the selective excitation of atoms by previously absorbed electromagnetic radiation, rather than to the temperature of the emitter) to unveil information about the nature and the relative amount specific atoms in matter. 

FIGURE 3.2 Track formation according to Mott (1930). Simultaneous excitation of atoms at 1 and 2 has negligible probability in second-order perturbation theory unless the interatomic separation vector R is well aligned with the incoming and outgoing momentum vectors of the incident particle. Reproduced from Mozumder (1969), by permission of John Wiley Sons, Inc. ... 

The most commonly observed line emission arises from excitation of atomic electrons to higher level by an energy source. The energy emitted by excited atoms of this kind occurs at wavelengths corresponding to the energy level difference. Since these levels are characteristic for the element involved, the emission wavelength can be characteristic for the element involved. Sodium and potassium produce different line spectra. 

If flame emission is based on excitation of atoms formed by combustion in the flame, why does flame emission work well for sodium, potassium, cesium and some of the transition metals but not vanadium, molybdenum or the lanthanides ... 

The dissociation of molecules and excitation of atoms usually occur at a specific temperature. 

Whereas flame emission photometry relies on the excitation of atoms and the subsequent emission of radiation, atomic absorption spectrophotometry relies on the absorption of radiation by non-excited atoms. Because the proportion of the latter is considerably greater than that of the excited atoms, the potential sensitivity of the technique is also much greater. 

Atomic line emissions are produced by the excitation of atoms as discussed previously. The emission of the light occurs at positions in the spectrum corresponding to definite wavelengths or frequencies. 

Excitation of atoms in the gas phase within an electrical discharge ... 

All the methods used to evaporate metals for atom synthesis were developed originally for the deposition of thin metal films. The more important of these techniques are shown schematically in Fig. la-d. Most of the evaporation devices can be scaled to give amounts of metal ranging from a few milligrams per hour for spectroscopic studies to 1-50 gm/hour for preparative synthetic purposes. Evaporation of metals from heated crucibles, boats, or wires (Fig. la-c) generally gives metal atoms in their ground electronic state. Electronic excitation of atoms is possible when metals are vaporized from arcs, by electron bombardment, or with a laser beam (Fig. Id). The lifetime of the excited states of... 

Electric dipole radiation is the most important component involved in normal excitation of atoms and molecules. Ttu electric dipole operator has the form TejXf where e is the electronic charge in esu and xt is the displacement vector for the jth electron in the oscillating electromagnetic field. 

Figure 14.6—Hollow cathode lamp. Schematic of a typical lamp. The cathode is made from a hollow cylinder whose axis of revolution corresponds to the optical axis of a lamp. On the right is a diagram of the excitation of atoms in the cathode under impact with neon ions.

Mossbauer 107 to 109 Excitation of atomic nuclei Electric-field gradients at the nucleus produced by differences in bond types (Section 27-6)... 

Figure 8. Schematic scattering geometry with laser excitation of atom beam.

Fig. 22.1 (a) Experimental setup for three laser excitation of atomic vapors with ionization detection and reference signal for calibration, (b) Energy level diagram of a typical alkaline earth atom showing the sequence which allows easy excitation of T° states (from ref. 1). 

These are produced by autoionization transitions from highly excited atoms with an inner vacancy. In many cases it is the main process of spontaneous de-excitation of atoms with a vacancy. Let us recall that the wave function of the autoionizing state (33.1) is the superposition of wave functions of discrete and continuous spectra. Mixing of discrete state with continuum is conditioned by the matrix element of the Hamiltonian (actually, of electrostatic interaction between electrons) with respect to these functions. One electron fills in the vacancy, whereas the energy (in the form of a virtual photon) of its transition is transferred by the above mentioned interaction to the other electron, which leaves the atom as a free Auger electron. Its energy a equals the difference in the energies of the ion in initial and final states ... 

The excitation of atoms into discrete states with E>Il usually leads to autoionization. This is so because the autoionization in atoms takes 10 I4-10 n s, and the only competing channel is the radiative one with the time range 10 9-10-6 s. Only in rare cases does the emission occur from autoionization states. For example, for an oxygen atom, the autoionization peaks at A = 878-879 and 791-793 A have the ionization efficiency 77(E) <1. Owing to the constraints imposed by selection rules, the lifetime of such states is 10 8 s, which makes the radiative decay possible. For atoms this is an exception. However, the situtation is different in the case of molecules. 

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