Photon methods

UHV techniques are usually classified in terms of the electron/photon method, as is shown in Table 2.3 which lists the common electron bombardment and emission techniques that have been employed in electrochemical studies. A detailed description of UHV surface analysis techniques is beyond the scope of this book there are many excellent reference texts that can be consulted for this purpose (see further reading list). It is sufficient to note that methods involving electron bombardment or emission are inherently surface-sensitive as a result of the low pathlength, or escape depth, of electrons in condensed media. In addition, Table 2.3 briefly describes the type of information each method provides. 

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. 

Double Ionization-Chamber Method (Real-Photon Method)... 

In most cases, except those in earlier comparative studies between the real-photon method and the dipole-simulation method, the absolute cross-section values obtained by both methods agree with each other [27]. Comparison of obtained cross-section values between the two methods were discussed in detail [27, 2, and references therein] and summarized in conclusion [5]. It should be noted, at least briefly, that it is essentially difficult to accurately obtain the absolute values of photoabsorption cross sections (u) in the dipole-simulation experiments, and it is necessary to use indirect ways in obtaining those values as the application of the TKR sum rule, Eq. (3), to the relative values of the cross sections obtained partly with theoretical assumptions. Moreover, in some cases, in relatively earlier dipole-simulation experiments, particularly of corrosive molecules upon their electron optics with poorer energy resolutions, serious discrepancy from the real-photon experiments was clearly pointed out in the obtained absolute values of photoabsorption cross sections [5,20,25-28]. 

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. 

The very large signal to noise ratio of 1 in 106 and better makes this method highly sensitive to small fractions of three photons, where the standard 3-to-2 photon method fails. It has been proposed to use this technique to observe rare 3 photon decays of positrons in metals [72],... 

Later, with refinements, the two-photon method yielded a more accurate value ... 

By October 1997, Hansch and his co-workers had a new most accurate value. Still using the provocative S-2S transition and building on the two-photon method used to get the 1992 result, the 1997 result was... 

FAD allows to study rather precisely and unambiguously the OACF of polymer chains. Thanks to the unique statistical nature of the single photon method, and to the performances of the synchrotron source, it has proved very useful in the discussion of the nature of motions. For instance, it led to the first observation of the diffusion of orientational motions along the chains in polymer melts. Of course, the labeling nature of this kind of experiments implies two limitations. The first one, technical, is the necessity of labeling chains. Indeed, many different polymers can be labelled and several species have been or are studied in our laboratory (Polystyrene polybutadienes -20.37) polyisoprenes poly-... 

Recent reviews on alkali metal beam studies, theoretical and experimental determinations of electron affinities using photon methods, and atomic electron affinities and an Internet source for electron affinities all give compilations [113-117]. The evaluation of molecular electron affinities is a major objective of this book. 

The equilibrium, beam, and photon methods have been used to measure Ea. The electron capture detector (ECD), magnetron (MGN), and swarm equilibrium... 

The reaction AB —) I hv AB e is the basis of photoelectron spectroscopy and photodetachment methods. Many precise and accurate ionization potentials of molecules have been obtained by studying the photoionization of neutral molecules. The same principles apply to the photon methods for determining electron affinities, except that negative ions are studied. The electron affinities of over 1,000 atoms, radicals, clusters, and small molecules have been determined using... 

As new values were obtained, atomic electron affinities were reviewed periodically beginning in 1953 [1-13]. All the available experimental, extrapolated, and theoretical values were tabulated in 1984 [7]. Presently, experimental values are available at the NIST website [12]. Prior to 1970 the majority of the values for the main group elements were determined by the Born Haber cycle, electron impact, or relative and absolute equilibrium surface ionization techniques. However, values for C, O, and S had been measured by photodetachment [1-3]. By the mid-1970s virtually all the Ea of the main group elements in the first three rows had been measured by photon methods [4-7]. By the early 1980s values were obtained for the transition elements by photon techniques [7, 8]. In the 1990s the values of Ca, Sr, and Ba were measured [9-13]. Recently, experimental values have been reported for Ce, Pr, Tm, and Lu [14-17],... 

Electron Affinities Determined Using Photon Methods... 

Thus far, we have examined vibrational spectroscopy using IR absorption spectroscopy, what we called in Ch. 3 one photon method , a general type that encompasses most experiments in spectroscopy. There exist, however, other types of spectroscopy to observe vibrations. These are for instance Raman spectroscopy, which is also of a current use in chemical physics and may be considered a routine method. Other less known methods are modem time-resolved IR spectroscopies. All these methods are two-photon or multiphoton spectroscopies. They do not involve a single photon, as in absorption, but the simultaneous absorption and emission of two photons, as in Raman and in other scattering experiments, or the successive absorption(s) and emission(s) of photons that are coherently delayed in time, as in time-resolved nonlinear spectroscopies. By coherently , we assume the optical waves that carry these two photons keep a well-defined phase difference. In this latter type of spectroscopy, we include all modem set-ups that involve time-controlled laser spectroscopic techniques. We briefly sketch the interest of these various methods for the study of H-bonds in the following subsections. 

Still there is a lot to be done. While the security of single-photon methods is already quite well defined, the security of cryptography with correlated pairs is much less understood. It would also be useful to perform a rigorous analysis of what is the optimum protocol. QKD itself is now well elaborated, but its further propagation to other areas of cryptography (or discovery of another quantum primitive) still stands ahead. 

Relative to mass spectrometry and photon methods, NMR is a weak technique for other than proton observation, but high magnetic fields (900 MHz) allow detection of tens of microgram quantities in a matter of seconds. 

The two photon method uses the same remark as the saturated absorption method, namely that the sign of the Doppler shift changes with the sign of the wavevector k when the Laser beam is reflected back on itself by a mirror. 

In the limited pages of these lectures, we cannot explain in details all applications of the Doppler-free two-photon method, which can be found in the review papers 1391 for the earliest experiments or 40l for more recent experiments. 

Set-up with an atomic beam. In the current experiments, the two-photon method permits to obtain the cancellation of the Doppler broadening in vapours and does not need the use of an atomic beam. But in that particular experiment Biraben and Julien work on an atomic beam in order to avoid broadening and shift due to collisions in the Rydberg levels. That explains the experimental set-up 56 illustrated in Figure 24 ... 

In conclusion, we will try to compare succintly the two-photon method with other Doppler-free methods. We first carry out the comparison with the saturated absorption technique ... 

The Doppler-free two-photon method must also be compared with othei classical Doppler-free methods, which can be used in vapours as the leve] crossing technique or the double resonance technique. 

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