Atomic photoabsorption measurements

Since the photoabsorption measurements of Madden and Codling autoionization processes have been investigated by various methods. Excita-tion has been initiated by electrons, " by heavy particles, or by beam-foil interaction. Whereas the number of states that can be excited by photon impact is limited by selection rules, this limitation is less stringent for electron collisions, especially at low impact energies. For ion-atom or atom-atom collisions it is possible to provoke or suppress the excitation of certain types of autoionization states by careful selection of the collision partners. ... 

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. 

The double ionization-chamber method [19] provides an excellent means of measuring the photoabsorption cross sections of atoms and molecules in the range of the incident photon... 

Fig. 4 shows the illustration of a double ionization chamber. We describe the process of measuring the photoabsorption cross sections as follows, /q denotes the incident photon flux coming into the chamber filled with atoms or molecules of the number density n, I and I denote the photon fluxes entering and leaving plate 1, respectively, and I2 and I2 denote the photon fluxes entering and leaving plate 2, respectively. The ion currents q and q collected by plates 1 and 2, respectively, are expressed as... 

Recently in our laboratory we have initiated a program to study the photoabsorption processes of metal vapors throughout the UV and EUV region. Our research interests are (1) to obtain the absolute cross section measurement of atomic and molecular metal vapors, (2) to study the photoionization processes of molecular metal species, and (3) to study the photodissociation processes of molecular metal ions. Several experimental methods such as the heat-pipe absorption spectroscopy, photoionization mass spectroscopy, and electron-ion coincidence technique, will be used in the study. This report summarizes our first experiment using heat-pipe absorption spectroscopy. 

The photoabsorption of atomic and molecular potassium were measured in the extreme ultraviolet (EUV, 50-70 nm) and ultraviolet (UV, 210-400 nm) region. In the first experiment the... 

In this survey of linear unsubstituted CPs some mention of polysilanes must be made. This polymer, sketched in Figure 17.1d, has an all silicon backbone and, to first order, many structural characteristics common to the aforementioned carbon-based polymers. However, there is only a single cr-bond between every Si atom and the onset of the cr—cr transition, as measured by photoabsorption, is typically located in the UV portion of the spectrum. Thin films are perfectly transparent to the eye. When photoexcited, polysilanes exhibit high hole mobilities [113-115]. Thus, these a-conjugated polysilanes are just as much a conducting polymer as their ir-conjugated counterparts. 

If for each photon at least one atom is ejected one can equate the measured photofragmentation to the desired photoabsorption cross-section. It is still possible to extract a cross-section if two photons are necessary for ejection of an atom [16, 17]. 

Figure 5.10 shows photoabsorption cross-sections measured at a temperature of 105 K in the size range of 3 to 64 atoms per cluster. The data agree within experimental accuracy with earlier measurements for Na2i made by the Orsay group [13] (only the small dip in the spectrum was not seen here) and with the data of the Copenhagen group [27] for n = 14 to 48. 

The x-ray absorption spectra of an element recorded for free atoms, atoms in molecules, or atoms in various chemical compounds exhibit small but significant differences. Apparently the photoabsorption by an atom depends on its chemical environment. Much progress has been made in the interpretation of the detailed features of absorption spectra and nowadays absorption measurements provide a powerful tool to extract information on a variety of materials including three-atom molecules and small clusters. Principles and results of experimental investigations have been compiled by several authors. " ... 

Fig. 4.2 (a) Photoabsorption and electron production in a sample consisting of substrate atoms B and an adsorbate layer A. Only electrons created within a depth L from the surface contribute to the measured electron yield signal, (b) Electron mean free path in solids as function of the electron kinetic energy above the Fermi level. The shaded area represents the distribution tipically found for different materials (Reprinted from Stdhr [2], with kind permission of Springer Science (2009))... 

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