Multi photon spectroscopy

All the techniques already developed for photodissociation dynamics can be used here (laser-induced fluorescence, time resolved spectroscopy, multi photon spectroscopy, etc). 

Extensive research has been conducted in the field of multi-photon spectroscopy for the past several decades. However, until recently, multi-photon processes did not find widespread applications due to the small multi-photon absorptivity of materials. The contributions from several research groups to develop a new generation of multifunctional organic materials with sufficiently large multi-photon absorption cross-sections have opened up a number of novel applications in photonics and biophotonics. 

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. 

Following a brief discussion of the basic physics of two-photon transitions, we illustrate the relevance of multi photon spectroscopy by several examples. The Doppler-free multi photon spectroscopy will be presented in Sect.10.6. 

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. 

Many other applications of multi-photon absorption spectroscopy have meanwhile been reported in photochemistry and also in solid state physics, for instance, a new assignment of the band gap in alkali bromides by Froehlich et al Some further examples will be discussed in Section 111.10). 

Tunable laser spectroscopic techniques such as laser-induced fluorescence (LIF) or resonantly enhanced multi-photon ionization (REMPI) are well-established mature fields in gas-phase spectroscopy and dynamics, and their application to gas-surface dynamics parallels their use elsewhere. The advantage of these techniques is that they can provide exceedingly sensitive detection, perhaps more so than mass spectrometers. In addition, they are detectors of individual quantum states and hence can measure nascent internal state population distributions produced via the gas-surface dynamics. The disadvantage of these techniques is that they are not completely general. Only some interesting molecules have spectroscopy amenable to be detected sensitively in this fashion, e.g., H2, N2, NO, CO, etc. Other interesting molecules, e.g. 02, CH4, etc., do not have suitable spectroscopy. However, when applicable, the laser spectroscopic techniques are very powerful. 

S. Kielich. Multi-photon scattering molecular spectroscopy. Prog. Optics, 20 155, 1983. 

The theoretical problems associated with calculating nonlinear polarizabilities is closely linked to the field of charge transfer spectroscopy and reactivity as well as the field of multi-photon and excited state spectroscopy. It is likely that theoretical methods from these fields will contribute to a deeper understanding of nonlinear optical phenomena in organic, inorganic, and organometallic compounds. 

Lakowicz JR, Gryczynski I, Tolosa L, Dattelbaum JD, Castellano FN, Li L, Rao G. Advances in fluorescence spectroscopy multi-photon excitation, engineered proteins, modulation sensing and microsecond rhenium metal-ligand complexes. Acta Physica Polonica A 1999, 95, 179-196. 

J. Laane, Application of Raman spectroscopy to structural and conformational problems, in Advances in Multi-photon Processes and Spectroscopy (S. H. Lin, ed.), Vol. 1. World Scientific Press, Singapore, 1984. 

Y. Fujimura, E. Gonzalez, K. Hoki, J. Manz, Y. Ohtsuki, and H. Umeda, Advances in Multi-Photon Processes and Spectroscopy, 14, 30 (2001). 

Several articles and reviews on different aspects of multi-photon excitation of biomolecule system are available. For example, Birch [11] consideraticms concentrate mainly on the impact of multi-photon techniques to the time-resolved fluorescence spectroscopy. Lakowicz and Gryczynski [12] have discussed examples of three-photon excited fluorescence. Rehms and Callis studied the two-photon excited fluorescence emission of aromatic amino acids [13]. Kierdasz et al analyzed emission spectra of Tyrosine- and Tryptophan-containing proteins using one-photon (270-3 10 nm) and two-photon (565-6 10 nm) excitation [14]. 

High intensity and monochromaticity, resulting in a high spectral intensity, are ideal tools for spectroscopic investigations, especially for fluorescence measurements with low quantum yields, for the study of multi-photon processes and excited states, and for Raman spectroscopy. For example, important biomolecules like nucleic acids have an extremely low fluorescence quantum yield at room temperature. 

Two-color, Resonance Enhanced Multi-Photon Ionization (REMPI) spectroscopy is similar to OODR, differing only in that the... 

Conventional photoelectron spectroscopy uses a rare-gas discharge lamp to produce radiation at the wavelength of the He 2p <— Is atomic transition (hu = 21.218 eV). Synchrotron radiation is now widely used for PES because its photon energy is widely tunable yet monochromatic. The initial state, in the first PES experiments, has been the molecular ground state but now, by exploiting Resonance Enhanced Multi-Photon Ionization (REMPI) excitar tion/detection schemes (see Section 1.2.2.3), any excited state of the molecule can be used as the initial state for PES (for a review, see Pratt, 1995). 

There are several classes of optical effects induced by an internal perturbation, such as saturation of absorption, coherent Raman spectroscopy, multi-photon absorption processes, coherent transient spectroscopy (see Table 0.3). Section 5.1 of this chapter deals with saturation of absorption and multi-photon absorption processes. Section 5.2 outlines the principles of coherent anti-Stokes Raman spectroscopy (CARS), Raman-induced Kerr effect spectroscopy (RIKES), four-wave mixing (FWM), and photon echo. 

Ohtsuld Y, Nakagami K, Fujimura Y (2001) In Lin SH, ViUaeys AA, Fujimura Y (eds.) Advances in multi-photon processes and spectroscopy, vol 13. Woild Scientific, Singapore, pp 1-127... 

In contrast to the other listed single molecule techniques, measurements based on fluorescence correlation spectroscopy (FCS) can already be performed both routinely and rapidly. Moreover, FCS is applied in many scientific disciplines and the number of applications of this technique is growing very rapidly. Thus, its principles will be briefly outlined Usually, a sharply focused laser beam illuminates a volume element of about 10 1 by using confocal or multi-photon microscopy. 

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