Excited atoms, number

X-ray fluorescence A method of analysis used to identify and measure heavy elements in the presence of each other in any matrix. The sample is irradiated with a beam of primary X-rays of greater energy than the characteristic X-radiation of the elements in the sample. This results in the excitation of the heavy elements present and the emission of characteristic X-ray energies, which can be separated into individual wavelengths and measured. The technique is not suitable for use with elements of lower atomic number than calcium. 

The conmron flash-lamp photolysis and often also laser-flash photolysis are based on photochemical processes that are initiated by the absorption of a photon, hv. The intensity of laser pulses can reach GW cm or even TW cm, where multiphoton processes become important. Figure B2.5.13 simnnarizes the different mechanisms of multiphoton excitation [75, 76, 112], The direct multiphoton absorption of mechanism (i) requires an odd number of photons to reach an excited atomic or molecular level in the case of strict electric dipole and parity selection rules [117],... 

Figure Cl.4.13. Trap modulation experiment showing much greater deptli of ion intensity modulation (by more tlian one order of magnitude) tlian fluorescence or atom number modulation, demonstrating tliat excited atoms are not tire origin of tire associative ionizing collisions.

In addition to qualitative analysis of nearly all the elements of the periodic table, EEL spectra also enable determination of the concentration of a single element which is part of the transmitted volume and hence gives rise to a corresponding ionization edge. As in all comparable spectroscopic techniques, for quantification the net edge signal, which is related to the number N of excited atoms, must be extracted from the raw data measured. The net intensity 4 of the feth ionization shell of an individual element is directly connected to this number, N, multiplied by the partial cross-section of ionization ) and the intensity Iq of the incident electron beam, i.e. ... 

Fig. 4.18. Detection limit ofTXRF for the residues of aqueous solutions, depending on the atomic number of the analyte element. Three excitations modes were used (a) W-tube, 50 kV Ni-fil-ter, cutoff 35 keV (b) Mo-tube 50 kV Mo filter, cutoff 20 keV (c) W-tube,...

The atomic numbers (Z), electronic configurations, and numbers of unpaired electrons for five ions are listed in the following table. Assume that all unpaired electrons have parallel spins. Indicate the element symbol, charge, and energy state (that is, ground state or excited state) for each of the five cases. 

Nuclide—A species of atom characterized by the constitution of its nucleus. The nuclear constitution is specified by the number of protons (Z), number of neutrons (N), and energy content or, alternatively, by the atomic number (Z), mass number A (N+Z), and atomic mass. To be regarded as a distinct nuclide, the atom must be capable of existing for a measurable time. Thus, nuclear isomers are separate nuclides, whereas promptly decaying excited nuclear states and unstable intermediates in nuclear reactions are not so considered. 

Flame emission photometry is used mainly for the determination of alkali metals and some easily excited elements (Na, K, Li, Ca, etc.). This is related to the fact that the number of excited atoms in the flame decreases exponentially with increasing excitation energy. Moreover, at variance to AAS, where the sensitivity is directly proportional to the number of atoms in the ground state, the sensitivity of AES increases with an increasing number of atoms in the excited state. 

In a competitive process, the excess energy can be dissipated by emission of a second or Auger electron from an outer shell of the atom, leaving it in a doubly ionised excited state. The relative importance of AES and XRF depends upon the atomic number (Z) of the element involved. High Z values favour fluorescence, whereas low Z favours AES. This fact, taken together with X-ray absorbance in air, makes XRF into a method which is not very sensitive for elements with atomic numbers below Z 10. Measurements of solid samples are normally made under vacuum, as the absorption of air renders analysis of elements lighter than Ti impossible. 

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