Resonance ionization requirements

As illustrated in Fig. 3.41, several laser schemes can be used to ionize elements and molecules. Scheme (a) in this figure stands for non-resonant ionization. Because the ionization cross-section is very low, a very high laser intensity is required to saturate the ionization process. Scheme (b) shows a simple single-resonance scheme. This is the simplest but not necessarily the most desirable scheme for resonant post-ionization. Cross-... 

Resonance ionization methods (RIMS) have also been explored for improving Th ionization efficiency for mass spectrometric measurement (Johnson and Fearey 1993). As shown in Figure 3, two lasers are required, a continuous resonant dye laser for resonance of thorium atoms, and a continuous UV argon laser for transition from resonance to ionization. Consequently, sophisticated laser instrumentation is required for these methods. 

Whereas in LIMS only one laser with defined wavelength (e.g., Nd YAG - 1064 nm) is used for direct vaporization and ionization of solid samples in laser plasma, in resonance ionization mass spectrometry (RIMS) " one or more lasers are tuned precisely to the wavelength required for the excited states and ionization of evaporated atoms in order to get a highly selective ionization of the analyte. The basic principles of resonant ionization were first described by Hurst and coworkers at Oak Ridge National Laboratory as well as by Letokhov et in Russia. The technology... 

In a resonance ionization mass spectrometer, one or more lasers are applied which are tuned precisely to the wavelength required for optical resonance excitation from the atomic ground state... 

Apart from the galvanic detection of the ion currents, direct mass spectrometric detection of the ions can also be applied, as is the case with resonance ionization mass spectrometry (RIMS) [676]. In addition, ionization can be performed by multiphoton absorption, which requires very intense primary sources. 

Inductively coupled plasma mass spectrometry (ICP-MS), laser ablation ICP-MS (LA ICP-MS), thermal ionization mass spectrometry (TIMS), secondary ion mass spectrometry (SIMS), glow discharge mass spectrometry (GDMS), resonance ionization mass spectrometry (RIMS), and accelerator mass spectrometry (AMS) have been used successfully to measure uranimn concentrations and isotope ratios in a wide range of sample matrices. The specific details of the methods are described fully in the relevant sections of this encyclopedia. There are specific advantages associated with each method, which depend on the sample of interest and the information required. 

Technetium isotopes in the PWR primary coolant show a behavior which is quite different from that of the other fission products treated in this section. Whereas the short-lived Tc is only of minor radiological relevance, its long-lived isomer c (halflife 2.1 HP a) deserves some attention with respect to safety analyses for the final storage of radioactive wastes. Normally, c is not analyzed in the reactor coolant because of its very low activity concentration and of its unfavorable radiation properties (P 0.3 MeV, negligible y transition). For this reason, the Tc mass is frequently determined rather than its radioactivity suitable techniques for this task are neutron activation analysis, inductively-coupled mass spectrometry and laser resonance ionization mass spectrometry (see, for example, the comparative evaluation given by Trautmann, 1993). In each case, very expensive analytical procedures are required therefore, the greatest part of the available information on Tc behavior in reactor primary circuits has been derived from measurements of y-emitting Tc. 

The control scheme that we have discussed does not require high-power lasers. The electric field of the light is weak enough that the levels of the system are not shifted by it. The laser induces transitions but these can be described as transitions between states of the free molecule. This is no longer the case when the laser is strong. The very potential on which the nuclei move can be altered by the external field because, if it is strong enough, its effect on the electrons can become comparable to the electrical field due to the nuclei or that of the other electrons. Once that happens a variety of other processes, such as non-resonant ionization,... 

B) The multiphoton excitation of electronic levels of atoms and molecules with visible or UV radiation generally leads to ionization. The mechanism is generally a combination of direct, Goeppert-Mayer, and quasi-resonant stepwise processes. Since ionization often requires only two or tln-ee photons, this type of multiphoton excitation is used for spectroscopic purposes in combination with mass-spectrometric detection of ions. 

The result of this resonance stabilization of the anion by delocalization of the negative charge makes the overall energy change required for ionization to be smaller for phenol than it is for ethanol. This can be shown in terms of energies as follows ... 

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