Photonic excitation theory

Schrader M, Bahimann K and Hell S W 1997 Three-photon-excitation microscopy Theory, experiment, and applications Optik 104 116-24... 

The NIR femtosecond laser microscope realized higher order multi photon excitation for aromatic compounds interferometric autocorrelation detection of the fluorescence from the microcrystals of the aromatic molecules confirmed that their excited states were produced not via stepwise multiphoton absorption but by simultaneous absorption of several photons. The microscope enabled us to obtain three-dimensional multiphoton fluorescence images with higher spatial resolution than that limited by the diffraction theory for one-photon excitation. 

Braun and Scott (1987) used two-photon ionization of benzene and azulene in n-hexane and followed the e-ion recombination process by monitoring the transient absorption of the electron. The results are not very different from those obtained by the IR stimulation technique. A mean thermalization length of 5.0 nm was inferred at 223 K using a two-photon excitation at 266 nm. Hong and Noolandi s theory was used for the analysis. The absorption technique was... 

The triplet state of the unpaired electrons of oxygen play a key role in both the photon excitation and the potential relaxation mode of the excited chromophores of vision. The paramagnetic properties of oxygen provide a definitive method of determining whether oxygen is present in the chromophores of vision, a condition that would eliminate the Shiff-base theory of retinol reaction with opsin to form rhodopsin. The evaluation of the electron paramagnetic resonance of the chromophores of vision is discussed in Chapter 7. 

One-photon spectroscopy is due to the linear term, whereas the nonlinear terms lead to the simultaneous absorption of two or more photons. Although the theory was worked out almost 50 years ago, observation of multiphoton absorption was made feasible only after the development of lasers. This chapter deals with the application of two-photon excitation (TPE) to kinetic studies in low-pressure gas-phase samples. For a systematic, extensive discussion of spectroscopic applications, one of the excellent reviews available should be consulted. ... 

A. Esposito, F. Federici, C. Usai, F. Cannone, G. Chirico, M. Collini, A. Diaspro, Notes on theory and experimental conditions behind two-photon excitation microscopy, Microsc. Res. and Techn. 62, 12-17 (2004)... 

As a consequence of the IVR-mediated nature of the multiple-photon excitation process, the vibrational excitation is randomized as the dissociatirai threshold is approached. Hence, the molecule has no memory of the vibrational coordinate that was originally excited. Dissociation therefore occims statistically and can be modelled using the Arrhenius equation or phase-space theories. Mode-selective dissociation is normally not observed. 

Two-photon excitation was predicted in her doctoral thesis by Maria Goppert-Mayer [88], who recognized that it was a corollary of the Kramers-Heisenberg-Dirac theory of light scattering. It was not observed experimentally until 30 years later, when pulsed ruby lasers finally provided the high photon flux that was required [89]. Goppert-Mayer received the physics Nobel prize in 1963 for imre-lated work on nuclear structure. 

The two-photon absorption (2PA) process was predicted by M. Goppert-Mayer in 1931 during her PhD thesis [19]. Based on the quantum theory of radiation, M. Goppert-Mayer introduced the concept of virtual or intermediate states and provided the theoretical expression the 2PA probability. However, due to the very low value of this feature, the first experimental observation of this phenomenon could be only obtained for the first time at the advent of power lasers in 1960s, by the two-photon excited (2PE) up-eonverted fluoreseenee of a CaF2 Eu(II) crystal using 694.3 nm ruby laser exeitation, as described in detail below [20]. 

A good example is the spectnun of naphthalene. The two lowest excited states have 62 and synnnetries and are allowed for one-photon transitions. A weak transition to one of these is observable in die two-photon spectnun [33], presumably made allowed by vibronic effects. Much stronger two-photon transitions are observable at somewhat higher energies to a and an A state lying quite close to the energies predicted by theory many years earlier [34]. 

For two Bom-Oppenlieimer surfaces (the ground state and a single electronic excited state), the total photodissociation cross section for the system to absorb a photon of energy ai, given that it is initially at a state x) with energy can be shown, by simple application of second-order perturbation theory, to be [89]... 

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