Multiphoton emission

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]. 

For > 0, one has absorption for < 0, emission. Multiphoton absorption and emission fall into this class. The Class I Raman spectroscopies clearly exliibit a net absorption of energy in Stokes scattering and a... 

Figure B2.3.8. Energy-level sehemes deseribing various optieal methods for state-seleetively deteeting ehemieal reaetion produets left-hand side, laser-indueed fluoreseenee (LIF) eentre, resonanee-enlianeed multiphoton ionization (REMPI) and right-hand side, eoherent anti-Stokes Raman speetroseopy (CARS). The ionization oontinuiim is denoted by a shaded area. The dashed lines indieate virtual eleetronie states. Straight arrows indieate eoherent radiation, while a wavy arrow denotes spontaneous emission.

Figure 8.2e shows the dependence of the fluorescence intensity on the excitation power of the NIR light for the microcrystals measured with a 20x objective. In this plot, both axes are given in logarithmic scales. The slope of the dependence for the perylene crystal is 2.8, indicating that three-photon absorption is responsible for the florescence. On the other hand, slopes for the perylene and anthracene crystals are 3.9 for anthracene and 4.3 for pyrene, respectively. In these cases, four-photon absorption resulted in the formation of emissive excited states in the crystals. These orders of the multiphoton absorption are consistent with the absorption-band edges for each crystal. The four-photon absorption cross section for the anthracene crystal was estimated to be 4.0 x 10 cm s photons by comparing the four-photon induced fluorescence intensity of the crystal with the two-photon induced fluorescence intensity of the reference system (see ref. [3] for more detailed information). 

The general principle of detection of free radicals is based on the spectroscopy (absorption and emission) and mass spectrometry (ionization) or combination of both. An early review has summarized various techniques to detect small free radicals, particularly diatomic and triatomic species.68 Essentially, the spectroscopy of free radicals provides basic knowledge for the detection of radicals, and the spectroscopy of numerous free radicals has been well characterized (see recent reviews2-4). Two experimental techniques are most popular for spectroscopy studies and thus for detection of radicals laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI). In the photochemistry studies of free radicals, the intense, tunable and narrow-bandwidth lasers are essential for both the detection (via spectroscopy and photoionization) and the photodissociation of free radicals. 

Zipfel, W. R., Williams, R. M., Christie, R., Nikitin, A. Y, Hyman, B. T., and Webb, W. W. 2003. Live tissue intrinsic emission microscopy using multiphoton-excited native fluorescence and SHG. Proc. Natl. Acad. Sci. USA 100 7075-80. 

In praetiee, one nses femtosecond lasers, which necessarily feature a broader spectrum the shorter the pulses are. This provides another benefit for multiphoton imaging ultrashort pulses (sub-20 femtoseconds [fs]) possess a laser bandwidth increased in such a way that several typical flurophores with different emission wavelengths can be excited at once. Usually, fluorescence signals in microscopy are recorded in a single... 

Heger, H.J., Zimmermann, R., Dorfner, R., Beckmann, M., Griebel, H., Kettrup, A., Boesl, U. (1999) Online emission analysis of polycyclic aromatic hydrocarbons down to pptv concentration levels in the flue gas of an incineration pilot plant with a mobile resonance-enhanced multiphoton ionization time-of-flight mass spectrometer. Anal. Chem. 71 46-57. 

Figure 3. Electron emission time profiles from resonant multiphoton detachment of Cg at various photon energies. The lime width at full width at half maximum (FWHM) of the laser pulse is 30 ns.

Figure 4. Schematic drawing of Cg resonant multiphoton detachment mechanism for excitation at the band origin of the C2ng - Y2I1U transition (IC, internal conversion TE, thermionic emission).

The use of lanthanides are common for optical purposes because of their narrow and sharp bands, and distinguishable long lifetimes, accomparied by low transition probabilities due to the forbidden nature of the transitions [10-13]. Thus chromophoric sensitization of ligand to metal has been subjected to numerous theoretical and experimental investigations [14—16]. However, only limited classes of organic-lanthanide complexes have been developed and shown to display nonlinear processes [17-19]. Common nonlinear processes from lanthanide complexes include harmonic generation, photon up-conversion and multiphoton absorption induced emission. 

This chapter reviews the important aspects of multiphoton absorption sensitization of lanthanide complexes and their nonlinear behavior some typical nonlinear processes from the conversion of long-wavelength excitations to give short-wave-length emissions will be presented in Section 7.2. Their basic features and their differences will be described. 

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