Excitation Secondary

The evaluation of the various XRF measurements will be discussed for different effects in EDXRS the spectra evaluation is perfonned by different programs with varying assumptions, partially different mass attenuation coefficients are used, the calibration procedures are principally different (e.g., thin foils with given thickness, or, infinitely thick samples), measurement under atmospheric pressure or in vacuum, secondary excitation (enhancement) mainly of Al by Si radiation. 

Similar accuracies have been found for thick, homogeneous, complex specimens when corrections for secondary excitation are also included. With appropriate standards, total accuracies of 2% have been demonstrated. Because the determination of the lighter elements (i.e., 5 < Z< 15) are more sensitive to the uncertainties in the data base items listed above, less accuracy should be expected for these elements. 

In the consideration of excited but not ionized states we would like to make a distinction between primary excited states, those formed directly by the initial interaction of the molecule with the primary radiation or secondary electrons, and secondary excited states, those formed by primary energy transfer or by charge neutralization. 

Secondary excited molecules are probably much more common, being... 

The highly excited states of molecules produced by high-energy radiation that arc chemically important are mainly the ionic states because of the rapidity of internal conversion processes. Primary excitation is relatively unimportant while secondary excitation is quite common. In the condensed phases energy dissipation is very rapid because of colli-sional deactivation, the cage effect, and excitation energy transfer processes all of which act to negate the chemical effects of secondary excitation,... 

Absorption by the matrix can cause either an underestimation of the results due to optical quenching, or an overestimation of the results when some of the X-ray fluorescence causes secondary excitation of other elements. For example, the presence of iron in aluminium causes an increase in the fluorescence of the latter because fluorescence emanating from iron excites aluminium. 

The de-excitation of the excited primary products may produce secondary excited products. The latter, in turn, may undergo deexcitation or dissociation within < 10-1° sec. The species that survive this time interval may still be highly reactive intermediates that react with each other, with the solvent or with other solutes. These intermediates seldom survive longer than milliseconds thereafter they form the secondary or tertiary more stable products. 

Two-color pump-probe absorption spectroscopy is carried out with moderate pump energies producing small depletions of the vibrational ground state of only a few percent in order to avoid secondary excitation steps and minimize the temperature increase of the sample due to the deposited pump energy. 

A second matrix effect, called the enhancemeni effect, can also yield results that arc greater than expected. This behavior occurs when the sample contains an element whose characteristic emission spectrum is excited by the incident beam, and this spectrum in turn cause.s a secondary excitation of the nnalylical hue. 

Figure 14.5. Secondary excitation of iron using various x-ray tube targets.

Azizova et al. 7 > showed that the production of free radicals involved a mechcmism in which the triplet level of the aromatic chromophore appears as an intermediate state. The mechanism proposed electron photoejection after absorption of a second photon by an aromatic molecule already excited to its phosphorescent triplet state 77,78). In acidic media the photoejected electron can be trapped by a proton and the EPR signal of hydrogen atoms is observed. In basic media the EPR signal from 0(—) ions is observed 79). In some instances secondary excitation with visible light induces radical recombination by freeing the trapped electrons 79). 

In Fig. 11.20, the spectra that result from direct excitation, secondary excitation and polarized direct excitation of a standard oil sample are compared, containing 21 elements at the 30 gg level. The relative detection limits obtained from the three spectra are summarized in Tab. 11.6 they indicate that the DL values by means of polarized excitation are on average 5 times better than those determined with direct excitation. The secondary target results are better by a factor of 2.5 than the polarized excitation values for elements efiSdently excited by the Mo-Ka line (e. g. Pb) however, elements such as Sn and Cd cannot be determined with the Mo secondary target while they are well excited by the polarized brems-strahlung radiation. 

In secondary excitation nnits, the source is used to illuminate a selectable secondary target, which is made of specific material to either scatter the beam or to act as a new source. The geometric arrangement is such that the signal from the secondary target illuminates the sample and also ensures that the X-ray beam is polarized (Figure 8.16). 

In Figure 7.3, the fluorescence process for a multi-level molecule is illustrated. In the scheme shown in Figure 7.3, the laser is tuned to one absorption transition. Fluorescence decay is then observed to all lower lying levels that can be reached via allowed transitions. In addition, collisions may populate levels adjacent to the excited state thus, fluorescence from those secondary excited states will be observed as well. If the fluorescence is spectrally resolved, then each individual transition can be observed, and the result is called the fluorescence spectrum. If several different transitions are excited in sequence and the total fluorescence signal observed in each case, then the result is called the excitation spectrum. In any case, the total fluorescence energy collected from each transition is always of the form given by Equation (7.B2). 

With increasing vapor pressure these peaks become stronger, and additional equidistant peaks become observable. This is ascribed to secondary excitations, according to... 

Beside X-ray satellites mentioned in the section Source Satellites, characteristic of a nonmonochromatic source, some ghost lines generated by a deteriorated or contaminated anode, by misalignment or by cross linking between Mg and Al anodes may appear. X-ray fluorescence from a subsfrate may induce a secondary excitation in a thin film, creating ghost peaks. 

Principles and Characteristics Laser microprobe mass spectrometry (LMMS, LAMMS), sometimes called laser probe microanalysis (LPA or LPMA) and often also referred to as laser microprobe mass analysis (LAMMA , Leybold Heraeus) [317] or laser ionisation mass analysis (LIMA , Cambridge Mass Spectrome-try/Kratos) [318], both being registered trademarks, is part of the wider laser ionisation mass spectrometry (LIMS) family. In the original laser microprobe analyser, emitted light was dispersed in a polychro-mator. Improved sensitivity may be obtained by secondary excitation of ablated species with an electric spark. In the mass spectrometric version of the laser microprobe, ions formed in the microplasma... 

Figure ID. If a tunable laser is used for secondary excitation of molecular ions, resonance dissociation spectroscopy of molecular cations may be performed. Here excited cationic levels are subject to laser spectroscopy. They serve as intermediate states for the process of resonance enhanced multiphoton dissociation. This is quite similar to resonance ionization spectroscopy of neutrals. The difference is that a dissociation instead of an ionization continuum is finally reached by multiphoton excitation. The advantage of this technique is that it is independent of high ion numbers (as necessary for absorption spectroscopy), fluorescence [necessary for laser-induced fluorescence (LIF)] or predissociation and therefore is fairly general. In addition, mass selectivity is intrinsic and one may benefit from state selective ion formation if resonance multiphoton ionization is used as an ion source.

Since multiphoton excitation in mass spectrometry takes place in the more or less tight laser focus, which can easily be shifted in space and time or be subject to other variations, it can be combined with different ion optical or mechanical arrangements (e.g. sources of neutral molecular systems) without the need for much additional hardware. Thus, by combination with chromatography (particularly gas chromatography), species selection has successfully been realized. Another very promising combination, which has frequently been applied in the recent past for the study of involatile molecules (e.g. polycyclic aromatics, biomolecules), is that of laser desorption of neutral molecules and resonance enhanced multiphoton ionization. All the benefits of multiphoton mass spectrometry, such as soft ionization, selective ionization, controllable fragmentation or secondary excitation for tandem mass spectrometry, may be used in this field. 

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