Monochromatic ionization sources

The use of laser (qv) radiation as an excitation source has had a major impact on analytical chemistry (36—39). A powerful, coherent, monochromatic laser source can serve a variety of appHcations, including sample-induced fluorescence and chemiluminescence (qv). Because ionization efficiency is high within the laser probe zone and because all ions can be collected and efficiendy measured, the sensitivity of the method is high. Also, because ionization depends on resonant transitions of the analytes, a relatively high degree of selectivity is obtained. 

Using a monochromatic photon source, the ionization energies of M corresponding to the energy differences between the ground state and the different cation states of M (Fig. 1) can be obtained as... 

Lasers are used to deliver a focused, high density of monochromatic radiation to a sample target, which is vaporized and ionized. The ions are detected in the usual way by any suitable mass spectrometer to produce a mass spectrum. The yield of ions is often increased by using a secondary ion source or a matrix. 

Radiations outside the ultraviolet, visible and infrared regions cannot be detected by conventional photoelectric devices. X-rays and y-rays are detected by gas ionization, solid-state ionization, or scintillation effects in crystals. Non-dispersive scintillation or solid-state detectors combine the functions of monochromator and detector by generating signals which are proportional in size to the energy of the incident radiation. These signals are converted into electrical pulses of directly proportional sizes and thence processed to produce a spectrum. For radiowaves and microwaves, the radiation is essentially monochromatic, and detection is by a radio receiver tuned to the source frequency or by a crystal detector. 

The XPS mechanism, which can be used for quantitative and qualitative chemical analysis of surfaces, is based on the photoelectric effect. A monochromatic soft Mg or Al anode X-ray source is used to irradiate the surface. The absorbed X-rays ionize die core shell, and in response, the atom creates a photoelectron that is transported to the surface and escapes. The ionization potential of a photoelectron that must be overcome to escape into vacuum is the binding energy (BE) plus the work function of the material. The emitted photoelectrons have a remaining kinetic energy (KE), which is measured by using an electron analyzer. Individual elements can be identified on the basis of their BE. The resulting XP spectrum is a characteristic set of peaks for a specific element, with BE as the abscissa and counts per unit time as... 

By use of a monochromatized synchrotron radiation source it is possible to obtain better instrumental resolution, and the variable photon energy enables one to find the optimum cross section for valence and core ionization studies. These instrumental improvements are demonstrated by the XPS studies of a number of solid Sn, In, Sb and Pb organometallic and inorganic compounds . Sb 4d(3/2,5/2) binding energies were determined for Ph SbCl, the only Sb-organic derivative in the series, with improved instrumental resolution (0.43 eV) and line width (1.01 eV). The corresponding values are 36.12 and... 

Although the effects of spin coherence have been mainly studied using radiation-chemical processes as an example, published are the first works on the MARY spectra of radical ion pairs produced in solutions by photoionization. Probably, there are no principle obstacles to the application of the method of quantum beats to these systems. Interpretation of results is expected to be more simple, in this case, because of the use of monochromatic sources of ionization and the absence of cross recombination effects typical of the ionization track. Another manifestation of spin coherence, observed experimentally but omitted in this review, is the beats induced by resonance microwave pumping [36-38]. The range of applications of this phenomenon for studying spin-correlated radical ion pairs has yet to be outlined. 

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