Photoionization, principles

The general principle of detection of free radicals is based on the spectroscopy (absorption and emission) and mass spectrometry (ionization) or combination of both. An early review has summarized various techniques to detect small free radicals, particularly diatomic and triatomic species.68 Essentially, the spectroscopy of free radicals provides basic knowledge for the detection of radicals, and the spectroscopy of numerous free radicals has been well characterized (see recent reviews2-4). Two experimental techniques are most popular for spectroscopy studies and thus for detection of radicals laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI). In the photochemistry studies of free radicals, the intense, tunable and narrow-bandwidth lasers are essential for both the detection (via spectroscopy and photoionization) and the photodissociation of free radicals. 

The two most common LG/MS interfaces used for routine quantitative analyses are APCI and APT electrospray. The principles of these techniques in direct infusion analyses have been described earlier (see Sections 3.3 and 3.4). As APTelectrospray has a broader application profile, its use is more widespread than APCI. Other configurations including El, atmospheric pressure photoionization (APPl), and thermospray interfaces with liquid chromatographs are available but are less commonly used for high throughput or routine analysis. 

The H2 molecule is a system for which quite recently it has been possible to measure in unprecedented detail state-selected vibrationally and rotation-ally resolved photoionization cross sections in the presence of autoionization [27-29]. The technique employed has been resonantly enhanced multiphoton ionization. The theoretical approach sketched above has been used to calculate these experiments from first principles [30], and it has thus been possible to give a purely theoretical account of a process involving a chemical transformation in a situation where a considerable number of bound levels is embedded in an ensemble of continua that are also coupled to one another. The agreement between experiment and theory is quite good, with regard to both the relative magnitudes of the partial cross sections and the spectral profiles, which are quite different depending on the final vibrational rotational state of the ion. 

The resolution of photoion laser microscopy is limited by two fundamental factors [7] the Heisenberg principle of uncertainty and the presence of the nonzero tangential component of the velocity of the ejected photoion (photoelectron). The same factors restrict the spatial resolution of the field-ion microscopy. It must be emphasized again that the key difference lies in the fact that for photoion microscopy there is no need for a strong (ionizing) electric field that distorts and desorbs the molecules. And also, the femtosecond laser radiation allows the photoion to be photoselectively extracted from certain parts of a molecule. 

The uncertainty principle causes a spread of the transversal velocity of the photoion, Av - h/2M Ax its coordinate is accurately determined to be Ax, where M is the mass of the photoion. This spread, Aux, results in a circle of ion scattering on the screen with a diameter d = 2t Ax, where r is the flight time of the particle from the tip to the screen and R is the distance between the tip and screen. On the other hand, the circle of scattering on the screen, d, is also related to the indeterminacy of the coordinate at the point of detachment Ax(Ax = d/K), where K = R/r is the projector magnification coefficient. 

The simple estimations show that the uncertainty principle limits the spatial resolution of photoelectron microscopy on the level 20-30 A for photoion microscopy this is below 1 A. 

If the isotopic shift of a spectral line in an atom or in a molecule is more Hi... the Doppler width, it is in principle possible to selectively excite a parti, id... isotopic species from isotopic mixtures by monochromatic light of w.u. length in coincidence with the absorption of the particular isotopic spe> < In a typical example, 2°2Hg atoms in natural Hg vapor containing 204. o 201, 200, 199, and 198 isotopes are preferentially excited by the 2b i V resonance line of 202Hg atoms. It has recently been demonstrated tlm 235U atoms are enriched in the photoionization processes of Mi. t... 

T.N. Rescigno, W. Vanrose, D.A. Homer, F. Martin, C.W. McCurdy, First principles study of double photoionization of H2 using exterior complex scaling, J. Electron Spectros. Relat. Phenom. 161 (2007) 85. 

As demonstrated in the previous section, satellites are due to electron correlation effects, and, in principle, all types which are classified in a configurational picture as initial state configuration interaction (ISCI), final ionic state configuration interactions (FISCI) and final state configuration interactions (FSO which includes interchannel interactions in the continuum) have to be taken into account. In certain cases, however, one type of correlation is more important than the others, and in the present case of 3s and 3p photoionization in argon this is FISO. This property allows a rather transparent analysis of the implications which these correlations have on the corresponding satellites. 

Tellinghuisen, J. (1985). The Flranck-Condon principle in bound-free transitions, in Photodissociation and Photoionization, ed. K.P. Lawley (Wiley, New York). 

Lucchese, R.R., Takatsuka, K. and McKoy, V. (1986). Applications of the Schwinger variational principle to electron-molecule collisions and molecular photoionization, Phys. Rep. 131, 147-221. 

In conclusion, much interest is presently focussed on the dynamics and excitation spectra of transition-metal carbonyls and adsorbate systems, and I have little doubt that the problems discussed in this section will be understood before long. However, the problem of first principles descriptions of photoionization spectra of these systems will remain a very important field of application of many-body theory for a long time to... 

J. W. Rabalais, Principles of Ultraviolet Photoelectron spectroscopy, Wiley, New York, 1977, is mentioned among the numerous monographs because of its literature review and the lucid presentation of difficult topics, such as photoionization cross-sections. 

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