Photon excitations, experimental technique

For many years, investigations on the electronic structure of organic radical cations in general, and of polyenes in particular, were dominated by PE spectroscopy which represented by far the most copious source of data on this subject. Consequently, attention was focussed mainly on those excited states of radical ions which can be formed by direct photoionization. However, promotion of electrons into virtual MOs of radical cations is also possible, but as the corresponding excited states cannot be attained by a one-photon process from the neutral molecule they do not manifest themselves in PE spectra. On the other hand, they can be reached by electronic excitation of the radical cations, provided that the corresponding transitions are allowed by electric-dipole selection rules. As will be shown in Section III.C, the description of such states requires an extension of the simple models used in Section n, but before going into this, we would like to discuss them in a qualitative way and give a brief account of experimental techniques used to study them. 

The nonlinear susceptibility is evaluated using third-order perturbation theory, and resonant enhancement is readily demonstrated to occur. Four-wave mixing is a useful experimental technique to extend the energy range available to tunable dye lasers [468]. It is also of interest that processes involving excitation by three photons allow transitions between even and odd parity states to be excited, as do single-photon transitions. 

The second volume of Laser Spectroscopy covers the different experimental techniques, necessary for the sensitive detection of small concentrations of atoms or molecules, for Doppler-free spectroscopy, laser-Raman-spectroscopy, doubleresonance techniques, multi-photon spectroscopy, coherent spectroscopy and time-resolved spectroscopy. In these fields the progress of the development of new techniques and improved experimental equipment is remarkable. Many new ideas have enabled spectroscopists to tackle problems which could not be solved before. Examples are the direct measurements of absolute frequencies and phases of optical waves with frequency combs, or time resolution within the attosecond range based on higher harmonics of visible femtosecond lasers. The development of femtosecond non-collinear optical parametric amplifiers (NOPA) has considerably improved time-resolved measurements of fast dynamical processes in excited molecules and has been essential for detailed investigations of important processes, such as the visual process in the retina of the eye or the photosynthesis in chlorophyl molecules. 

The main luminescence parameters traditionally measured are the frequency of maximal intensity Vmax, intensity I, the quantum yield < >, the hfetime of the exited state T, polarization, parameters of Raman spectroscopy, and excited-state energy migration. The usefulness of the fluorescence methods has been greatly enhanced with the development of new experimental techniques such as nano-, pico-, and femtosecond time-resolved spectroscopy, single-molecule detection, confocal microscopy, and two-photon correlation spectroscopy. 

Under incident radiation or bombardment by an electron beam surfaces emit photons, electrons, or both. The emission properties of solid surfaces differ widely, just as their mechanisms or relaxation after excitation by high-energy radiation differ. Many surface-sensitive experimental techniques providing information related to the electronic properties of surfaces are based on these processes, for example. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). These are discussed below. 

The experimental techniques of fluorescence line narrowing and hole burning were invented, in part, to access this dynamic information. They each involve selective excitation by a narrow-band laser of a nearly resonant subset of chromophores. The resulting fluorescence line shape or hole shape reflects the spectral dynamics of the members of this subset, unobscured by the other chromophores. In a similar vein, in the time-domain photon echo experiment, after the application of a short pulse the inhomogeneous dephasing of all of the chromophores is then rephased by a second pulse, and so the echo decay again reflects only transition frequency fluctuations. 

As in most other experimental techniques, there is also a constant development and improvement involved in FCS which broadens its range, applicability, and accuracy. These developments resulted in advances such as dual-focus FCS, total internal reflection FCS, and STED-FCS which will be discussed in this section. Apart from this progress, a multitude of other variations have been reported which cannot be covered within this book chapter. These include FCS with two-photon excitation [57-61], spatial fluorescence cross-correlation spectroscopy (FCCS) which can be used to investigate microflows [62], dual-color FCCS to correlate... 

Nonlinear Absorption. In theory, any of the above measurement techniques could be applied to materials in the presence of nonlinear absorption and interpreted to provide the two-photon absorption coefficient or the two-photon cross-section The study of multiphoton absorption is, of course, inherently spectroscopic, probing the multiphoton resonances that impact various nonlinear phenomena. Third-harmonic generation in thin films has been used to characterize the phase of the complex x and thus but the experimental technique requires additional steps and interpretation may be complicated by excited state absorption and three-photon resonances (282). The presence of an imaginary component of x significantly complicates DFWM measurements by adding... 

All the previous discussion in this chapter has been concerned with absorption or emission of a single photon. However, it is possible for an atom or molecule to absorb two or more photons simultaneously from a light beam to produce an excited state whose energy is the sum of the energies of the photons absorbed. This can happen even when there is no intemrediate stationary state of the system at the energy of one of the photons. The possibility was first demonstrated theoretically by Maria Goppert-Mayer in 1931 [29], but experimental observations had to await the development of the laser. Multiphoton spectroscopy is now a iisefiil technique [30, 31]. 

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