Photonic excitation

Some recent advances in stimulated desorption were made with the use of femtosecond lasers. For example, it was shown by using a femtosecond laser to initiate the desorption of CO from Cu while probing the surface with SHG, that the entire process is completed in less than 325 fs [90]. The mechanism for this kind of laser-induced desorption has been temied desorption induced by multiple electronic transitions (DIMET) [91]. Note that the mechanism must involve a multiphoton process, as a single photon at the laser frequency has insufScient energy to directly induce desorption. DIMET is a modification of the MGR mechanism in which each photon excites the adsorbate to a higher vibrational level, until a suflBcient amount of vibrational energy has been amassed so that the particle can escape the surface. 

Mikami N and Ito M 1975 Two-photon excitation spectra of naphthalene and naphthalene-dg Chem. Rhys. Lett. 31 472-8... 

B1.18.5.5 CONTRAST ENHANCEMENT AND PRACTICAL LIMITS TO CONFOCAL ONE-PHOTON-EXCITATION FLUORESCENCE MICROSCOPY... 

One-photon excitation has lunitations due to the unwanted out-of-focus fliiorophore absorption and bleaching, and light scattering. These drawbacks can be circumvented if multiphoton excitation of the fliiorophore is used. Since it increases with the nth power of the photon density, significant absorption of the exciting light will only occur at the focal point of the objective where the required high photon density for absorption is reached. Consequently, only... 

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

Wokosin D L and White J G 1997 Optimization of the design of a multiple-photon excitation laser scanning fluorescence imaging system Proc. SPIE 2984 25-9... 

This teclnhque can be used both to pennit the spectroscopic detection of molecules, such as H2 and HCl, whose first electronic transition lies in the vacuum ultraviolet spectral region, for which laser excitation is possible but inconvenient [ ], or molecules such as CH that do not fluoresce. With 2-photon excitation, the required wavelengdis are in the ultraviolet, conveniently generated by frequency-doubled dye lasers, rather than 1-photon excitation in the vacuum ultraviolet. Figure B2.3.17 displays 2 + 1 REMPI spectra of the HCl and DCl products, both in their v = 0 vibrational levels, from the Cl + (CHg) CD reaction [ ]. For some electronic states of HCl/DCl, both parent and fragment ions are produced, and the spectrum in figure B2.3.17 for the DCl product was recorded by monitoring mass 2 (D ions. In this case, both isotopomers (D Cl and D Cl) are detected. 

In the ideal case for REMPI, the efficiency of ion production is proportional to the line strength factors for 2-photon excitation [M], since the ionization step can be taken to have a wavelength- and state-mdependent efficiency. In actual practice, fragment ions can be produced upon absorption of a fouitli photon, or the ionization efficiency can be reduced tinough predissociation of the electronically excited state. It is advisable to employ experimentally measured ionization efficiency line strengdi factors to calibrate the detection sensitivity. With sufficient knowledge of the excited molecular electronic states, it is possible to understand the state dependence of these intensity factors [65]. 

The Goeppert-Mayer two- (or multi-) photon absorption, mechanism (ii), may look similar, but it involves intennediate levels far from resonance with one-photon absorption. A third, quasi-resonant stepwise mechanism (iii), proceeds via smgle- photon excitation steps involvmg near-resonant intennediate levels. Finally, in mechanism (iv), there is the stepwise multiphoton absorption of incoherent radiation from themial light sources or broad-band statistical multimode lasers. In principle, all of these processes and their combinations play a role in the multiphoton excitation of atoms and molecules, but one can broadly... 

Quack M 1998 Multi photon excitation Encyclopedia of Computational Chemistry o 3, ed P v R Schleyer et al (New York Wiley) pp 1775-91... 

The vast majority of single-molecule optical experiments employ one-photon excited spontaneous fluorescence as the spectroscopic observable because of its relative simplicity and inlierently high sensitivity. Many molecules fluoresce with quantum yields near unity, and spontaneous fluorescence lifetimes for chromophores with large oscillator strengths are a few nanoseconds, implying that with a sufficiently intense excitation source a single... 

Two-photon excited fluorescence detection at the single-molecule level has been demonstrated for cliromophores in cryogenic solids [60], room-temperature surfaces [61], membranes [62] and liquids [63, 64 and 65]. Altliough multiphoton excited fluorescence has been embraced witli great entluisiasm as a teclmique for botli ordinary confocal microscopy and single-molecule detection, it is not a panacea in particular, photochemical degradation in multiphoton excitation may be more severe tlian witli ordinary linear excitation, probably due to absorjDtion of more tlian tire desired number of photons from tire intense laser pulse (e.g. triplet excited state absorjDtion) [61],... 

Strickler J FI and Webb W W 1990 Two-photon excitation in laser scanning fluorescence microscopy Proc. SPIE 13948107-18... 

Sanchez E J, Novotny L, Floltom G R and Xie X S 1997 Room-temperature fluorescence imaging and spectroscopy of single molecules by two-photon excitation J. Chem. Phys. A 101 7019-23... 

Sonnieitner M, Schutz G J and Schmidt T 1999 Imaging individual molecules by two-photon excitation Chem. Phys. Lett 300 221-6... 

Brand L, Eggeling C, Zander C, Drexhage K FI and Seidel CAM 1997 Single-molecule identification of coumarin-120 by time-resolved fluorescence detection comparison of one- and two-photon excitation in solution J. Chem. Phys. A 101 4313-21... 

Mertz J, Xu C and Webb W W 1995 Single-molecule detection by two-photon-excited fluorescence Opt Lett. 20 2532-4... 

Figure 9.51 A zero kinetic energy photoelectron (ZEKE-PE) resonant two-photon spectrum of 1,4-difluorobenzene in which the first photon excites the molecule of the zero-point level of the S-[ excited electronic state of the molecule. (Reproduced, with permission, from Reiser, G., Rieger, D., Wright, T.G., Muller-Dethlefs, K. and Schlag, E.W., J. Phys. Chem., 97, 4335, 1993)...

Other techniques in which incident photons excite the surface to produce detected electrons are also Hsted in Table 1. X-ray photoelectron Spectroscopy (xps), which is also known as electron spectroscopy for chemical analysis (esca), is based on the use of x-rays which stimulate atomic core level electron ejection for elemental composition information. Ultraviolet photoelectron spectroscopy (ups) is similar but uses ultraviolet photons instead of x-rays to probe atomic valence level electrons. Photons are used to stimulate desorption of ions in photon stimulated ion angular distribution (psd). Inverse photoemission (ip) occurs when electrons incident on a surface result in photon emission which is then detected. 

Tab. 1.1. Surface-specific analytical techniques using particle or photon excitation. The acronyms printed in bold are those used for methods discussed in more details in this publication.

The primary reason for studying aqueous plutonium photochemistry has been the scientific value. No other aqueous metal system has such a wide range of chemistry four oxidation states can co-exist (III, IV, V, and VI), and the Pu(IV) state can form polymer material. Cation charges on these species range from 1 to 4, and there are molecular as well as metallic ions. A wide variety of anion and chelating complex chemistry applies to the respective oxidation states. Finally, all of this aqueous plutonium chemistry could be affected by the absorption of light, and perhaps new plutonium species could be discovered by photon excitation. 

Figure 20. Energy levels of neutral sodium atom mostly involved in the resonant incoherent 2-photon excitation for the polychromatic LGS. Wavelengths (nm), lifetimes (ns) and homogeneous widths (MHz).

Fig. 3a, b. Schematic representation of (a) conventional fluorescent sensor and (b) fluorescent sensor with signal amplification. Open rhombi indicate coordination sites and black rhombi indicate metal ions. The curved arrows represent quenching processes. In the case of a den-drimer, the absorbed photon excites a single fluorophore component, which is quenched by the metal ion regardless of its position... 

Figure 14. Mode selectivity in photodissociation of V (OCO). The ratio of the reactive (VO + CO) to nonreactive (V + CO2) product is measured at the peaks of the vibronic bands labeled in Fig. 13. The data below 16,600 cm is from bands accessed by one-photon excitation data at higher energy was obtained by vibrationally mediated photodissociation exciting the OCO antisymmetric stretch.

Nonlinear optical phenomena, as well as near-field optics, provide us with super resolving capability [20]. The probability of nonlinear optical phenomena is proportional to the number of photons which participate in the phenomenon. For example, the intensity distribution of two-photon excited fluorescence corresponds to the square of the excitation light. Thus, we proposed a combination of the field... 

Sanchez, E. J., Novotny, L. and Xie, X. S. (1999) Near-field fiuorescence microscopy based on two-photon excitation with metal tips. Phys. Rev. Lett., 82, 4014-4017. 

Near-Field Two-Photon Excitation Images of Gold Nanorods 47... 

FigureS.7 Near-field two-photon excitation images of single gold nanorods detected by two-photon induced photoluminescence. Nanorod dimensions (length, diameter) are 540 nm, 20 nm for (a) and 565 nm, 21 nm in (b). Scale bars lOOnm. (Reproduced with permission from The Chemical Society of Japan [11]).

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