Photoabsorption measurements

Ma, Y., Chen, C. T., Meigs, G., Randall, K., and Sette, F. (1991). High-resolution K-shell photoabsorption measurements of simple molecules. Phys. Rev. A 44,1848-1858. 

Photoabsorption measurements were also performed using a deuterium lamp (Hamamatsu Photonics L544) as a light source. The transmitted light was resolved by a monochromator (Nikon P-250) and detected by a photomultiplier (Hamamatsu Photonics R928). 

In femto-second pump-prove transition photoabsorption measurements, titania photocatalysts gave visible-light photoabsorption of trapped e" within ca. 100 fs pump pulse without showing photoabsorption of e in the CB. See Section IV.C. 

Thus far, the strengths 5(cu) or detailed energies with transition moment (N D 0) describe the gross features of the spectra as they are accessible, e.g. by photoabsorption measurements. There is much more information carried in the normal modes of a many-body system. One can thus look one step further into it by considering the whole transition density... 

The experimental access to all these other modes has to use methods beyond simple photoabsorption measurements. They have been studied extensively in the case of nuclei by various sorts of scattering experiments (with electrons, protons, ions), which could even provide the full transition formfactors Detailed scattering experiments in the... 

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. ... 

In PPV films deposited onto silver substrate, parallel chainlike features were reported, with larger separation (typically 1.8 nm) than expected from the interchain unit cell [132]. This was observed independently of polymer film thickness and material used as substrate and was tentative attributed to some surface reconstruction, possibly due to adsorption of oxygen. Polarization-dependent photoabsorption measurements made in PPV mono-layers prepared by the Langmuir-Blodgett precursor technique onto Mo 2 substrate showed that the phenylene and vinylene moieties are nearly parallel to the surface [125]. 

Steps in the photoconductivity spectra after optical excitation confirm the existence of the trapping levels known from absorption and photoabsorption measurements [Fig. 15a (105)]. 

Figure 15 (a) Second derivatives of the real and the imaginary parts of the dielectric function of P-rhombohedral boron in the spectral range of fundamental absorption (104). (b) Photoconductivity of P-rhombohedral boron in the low-energy tail of the fundamental absorption. Conditions of measurement 155 K, sample annealed at 450 K, cooled down to 155 K, 1-h excitation by a xenon arc lamp, quickly cooled down to 90 K, then measured 320 K, sample annealed at 450 K, cooled down to 320 K, then measured without (unexcited) or after 1-h excitation with a xenon arc lamp. The ionization energies of the electron traps known from absorption and photoabsorption measurements are indicated (105). 

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. 

The quantity L(0) = In I, where I is the mean excitation potential introduced by Bethe, which controls the stopping of fast particles (see Sect. 2.3.4) L(2) = In K, where K is the average excitation energy, which also enters into the expression for Lamb shift (Bethe, 1947). Various oscillator sum rules have been verified for He and other rare gases to a high degree of accuracy. Their validity is now believed to such an extent that doubtful measurements of photoabsorption and electron-impact cross sections are sometimes altered or corrected so as to satisfy these. 

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. 

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

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

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... 

The light intensities need not be measured to obtain the photoabsorption cross sections (o ), as will be shown later using Eq. (20). Therefore we do not need to consider the absorption of light by gas that effuses through the entrance and exit apertures of the chamber. 

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... 

According to Eq. (20), we measure only q, q, and the pressure of a gas in the chamber to obtain the absolute values of the photoabsorption cross sections (cr), and then we obtain the values of from a following Eq. (18) or Eq. (19). If we use a rare gas as reference, of which is unity in the whole range above its first ionization potential, then /q is obtained. Thus the relation between Iq and the signal from the incident photon detector,... 

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. 

In the measurements of the PHOFEX spectrum, we scanned the VUV wavelength while the photofragment of S(]5) was monitored by exciting the S(3D )-S(lS) transition by UV laser light. Since only the fluorescence emitted from the S(3D]> fragments in the central region of the free-jet expansion was collected, the photoabsorption of ultracold (-5 K) OCS was selectively... 

The energy of a single photon is obviously insufficient to ionize an organic compound. As early as the nineteen forties (3, 4), however, it -was observed that Wurster blue cation radical is produced by photoirradiation of 3-methylpentane glass containing N,N-tetramethyl p-phenylenediamine (TMPD) at 77° K. The recent detailed study of this system by electric conductivity measurement (5, 6) and electronic spectroscopy (7) provided conclusive evidence that the ionization is brought about via excitation to the triplet state followed by successive photoabsorption at the triplet state. This mechanism is supported by the facts that the life-time of the photochemical intermediate is identical with that of phosphorescence and the formation of Wurster blue, and that phosphorescence is inhibited in the presence of triplet scavengers. 

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