Photoabsorption and photoionization

The adiabatic ionization potential (1A) of a molecule, as shown in Figure 4.1, equals the energy difference between the lowest vibrational level of the ground electronic state of the positive ion and that of the molecule. In practice, few cases would correspond to adiabatic ionization except those determined spectroscopically or obtained in a threshold process. Near threshold, there is a real difference between the photoabsorption and photoionization cross sections, meaning that much of the photoabsorption does not lead to ionization, but instead results in dissociation into neutral fragments. 

The distinction between photoabsorption and photoionization is important, particularly near threshold, where the probability that ionization will not occur upon photoabsorption is significant. Thus, the ionization efficiency is defined by TJ. = a. /photoionization cross section and photoabsorption cross section, is related to the absorption coefficient a by a = n(7, n being the absorber density. 

PHOTOABSORPTION AND PHOTOIONIZATION OF MOLECULES OF CHEMICAL INTEREST, NORMAL ALKANES—AN OVERVIEW... 

An examination of the photoelectron (PE) spectra of 42 five-membered heteroaromatics and simple derivatives thereof, including furan and furan derivatives, showed only minor perturbations of the aromatic structures by the substituents <1993MI811>. Threshold PE spectra as well as photoabsorption and photoion spectra of furan and furan-, 4 have been measured and interpreted <1998CPH(236)365, 2001CPH(263)149>. 

The absolute values of the photoabsorption, photoionization, and photodissociation cross sections are key quantities in investigating not only the interaction of photons with molecules but also the interaction of any high-energy charged particle with matter. The methods to measure these, the real-photon and virtual-photon methods, are described and compared with each other. An overview is presented of photoabsorption cross sections and photoionization quantum yields for normal alkanes, C H2 + 2 n = 1 ), as a function of the incident photon energy in the vacuum ultraviolet range and of the number of carbon atoms in the alkane molecule. Some future problems are also given. 

Figure 2 The photoabsorption (c), photoionization (o-,-), and photodissociation (cr

In this chapter, we aim at giving an overview of the photoabsorption (c), photoionization (ff ), and photodissociation (cross sections of molecules in the vacuum ultraviolet range, where as stated the significant part of the photoabsorption cross sections lies. 

The real-photon method is essentially more direct and easier compared to the dipole-simulation method in obtaining absolute values of photoabsorption cross sections (o ), photoionization cross sections and photoionization quantum yields (t],). In the real-photon method, however, there is a practical need to use the big and dedicated facilities of synchrotron radiation where, in many cases, one should change the beam lines equipped with different types of monochromators depending on used photon-wavelengths—and to develop some specific new experimental techniques in the range from the vacuum ultraviolet radiation to soft X-ray. 

J. W. Gallagher, C. E. Brion, J. A. R. Samson, and P. W. Langhoff, Absolute cross sections for molecular photoabsorption, partial photoionization, and ionic photofragmentation processes, Journal of Physical and Chemical Reference Data, vol. 17, no. 1, pp. 9-153, 1988. 

The temi action spectroscopy refers to those teclmiques that do not directly measure die absorption, but rather the consequence of photoabsorption. That is, there is some measurable change associated with the absorption process. There are several well known examples, such as photoionization spectroscopy [47], multi-photon ionization spectroscopy [48], photoacoustic spectroscopy [49], photoelectron spectroscopy [, 51], vibrational predissociation spectroscopy [ ] and optothemial spectroscopy [53, M]. These teclmiques have all been applied to vibrational spectroscopy, but only the last one will be discussed here. 

In Eq. (4.9), V is the frequency of radiation and a>. and (Ox are the statistical weights of the initial and final states. It should be remembered that Eq. (4.9) refers to the photoionization cross section, not the total photoabsorption cross section (see Sect. 4.2). 

J. Berkowitz, Photoabsorption, Photoionization, and Photoelectron Spectroscopy, Academic Press, New York, 1979. 

The surface sensitivity is ensured by detecting the decay products of the photoabsorption process instead of the direct optical response of the medium (transmission, reflection). In particular one can measure the photoelectrons, Au r electrons, secondary electrons, fluorescence photons, photodesorbed ions and neutrals which are ejected as a consequence of the relaxation of the system after the photoionization event. No matter which detection mode is chosen, the observable of the experiment is the interference processes of the primary photoelectron with the backscattered amplitude. 

Interaction of Photons with Molecules Photoabsorption Photoionization and Photodissociation Cross Sections... 

EXPERIMENTAL METHODS TO MEASURE THE PHOTOABSORPTION, PHOTOIONIZATION, AND PHOTODISSOCIATION CROSS SECTIONS... 

Photoionization quantum yields ( / ) and, therefore, photoionization cross sections (o ) are obtained together with the photoabsorption cross sections (cr). 

The features in C1-C4 normal alkanes discussed in Section 3 seem to be generalized to a wide range of molecules, and thus we conclude that the major part of the photoabsorption cross sections of molecules (cr) is associated with the ionization and excitation of the outer-valence electrons. Hence, there is a strong need to measure the absolute values of a in the vacuum ultraviolet range, particularly in the range of the incident photon energy 10-30 eV, which is covered by the normal incidence monochromator used to monochromatize synchrotron radiation. The photoionization (cr ) and photodissociation (cd) cross sections. 

The apparatus has been already described in Ref. [7]. The clusters are produced by laser vaporization of a sodium rod, with helium at about 5 bars as a carrier gas and a small amount of SF6. The repetition rate is 10 Hz. In this configuration, the vibrationnal temperature of the formed clusters is roughly 400 K,[10] that gives 85% of C2V geometry and 15% of C3V for a Boltzman distribution. The laser beams are focused onto the cluster beam between the first two plates of an axial Wiley Mac-Laren Time-Of-Flight mass spectrometer with a reflectron. The photoionization efficiency curve as well as the photoabsorption spectrum determined by a photodepletion experiement are displayed on Fig. 1(b) and 1(c) respectively. The ionization threshold is at 4.3 eV, close to the 4.4 eV calculated for the C3V isomer and 4.9 eV for the C2V isomer (see the Fig. 1 (b)). The conclusion arising out of the photodepletion spectrum shown on Fig 1(c) and from ab initio calculations of the excited states, [5] is that the observed... 

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