Single-photon excitation

A more versatile approach that permits monitoring of ground-state products requires two lasers. A pump-probe experiment is one example of such an experiment. The second laser, time delayed from the first, can probe either the parent molecule or the newly formed product molecule. The probe can be fluorescence excitation, single photon (PI) or multiphoton (MPI) ionization, transient absorption, and so on. The time resolution will be limited by the pulse widths of the two lasers. This approach thus spans the time range from seconds to femtoseconds. 

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. 

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]. 

Figure C1.5.12.(A) Fluorescence decay of a single molecule of cresyl violet on an indium tin oxide (ITO) surface measured by time-correlated single photon counting. The solid line is tire fitted decay, a single exponential of 480 5 ps convolved witli tire instmment response function of 160 ps fwiim. The decay, which is considerably faster tlian tire natural fluorescence lifetime of cresyl violet, is due to electron transfer from tire excited cresyl violet (D ) to tire conduction band or energetically accessible surface electronic states of ITO. (B) Distribution of lifetimes for 40 different single molecules showing a broad distribution of electron transfer rates. Reprinted witli pennission from Lu andXie [1381. Copyright 1997 American Chemical Society.

Multiphoton Absorption and Ionization. High laser powers can induce the simultaneous absorption of two or more photons that together provide the energy necessary to excite a transition this transition may be one that is forbidden as a single-photon process (8,297). Such absorption can be made Doppler-free by propagating two laser beams of frequency V in opposite directions, so the Doppler shifts cancel and a two-photon transition occurs at 2v for any absorber velocity. The signal is strong because aU absorbers contribute, and peak ampHtudes are enhanced by, which may... 

Here, po is time independent density matrix and can be defined for initial state I. The excitation of electrons caused by absorption of a single photon is regarded as a polarization of the electron density, which is measured by the linear polarizability = Tr p uj)6). The equation of motion for the... 

Figure 4.6 shows an apparatus for the fluorescence depolarization measurement. The linearly polarized excitation pulse from a mode-locked Ti-Sapphire laser illuminated a polymer brush sample through a microscope objective. The fluorescence from a specimen was collected by the same objective and input to a polarizing beam splitter to detect 7 and I by photomultipliers (PMTs). The photon signal from the PMT was fed to a time-correlated single photon counting electronics to obtain the time profiles of 7 and I simultaneously. The experimental data of the fluorescence anisotropy was fitted to a double exponential function. 

Palladium clusters deposited on amorphous carbon have been studied by XPS and UPS [28] and both techniques show broadening of the d-band peak as cluster size increases. The d-threshold shifts towards Ep as cluster size increases. In UPS studies the d-emission of the single atom has its peak at 3.0 eV below Ep, whereas the d-threshold is 2eV below Ep. Palladium clusters evaporated onto Si02 have been studied by UPS [38]. At large coverages of the Pd metal evaporated (> 10 atoms/cm ), a high emission intensity at Ep excited with photons of 21.2 eV (He(I)) or 40.8 eV (He(II)) as excitation source, is observed. This feature is characteristic in the spectra from bulk Pd samples. At the lowest metal coverage (3 x 10 atoms/cm ),... 

Fig. 3.4. Schematic diagram of a TG setup. The TG electronics need careful synchronization with the excitation pulse. Here, time-gated single photon... 

Volkmer, A., Hatrick, D. A. and Birch, D. J. S. (1997). Time-resolved nonlinear fluorescence spectroscopy using femtosecond multiphoton excitation and single-photon timing detection. Meas. Sci. Technol. 8, 1339 19. 

An important feature that influences the performance of the probe to a large extent is the selection of the FRET pair. Generally, fluorophores with high excitation wavelengths are preferred over blue fluorophores in single photon microscopy for applications in tissue since there is no autofluorescence in this region and the ratio... 

The introduction and diversification of genetically encoded fluorescent proteins (FPs) [1] and the expansion of available biological fluorophores have propelled biomedical fluorescent imaging forward into new era of development [2], Particular excitement surrounds the advances in microscopy, for example, inexpensive time-correlated single photon counting (TCSPC) cards for desktop computers that do away with the need for expensive and complex racks of equipment and compact infrared femtosecond pulse length semiconductor lasers, like the Mai Tai, mode locked titanium sapphire laser from Spectra physics, or the similar Chameleon manufactured by Coherent, Inc., that enable multiphoton excitation. 

Fluorescent chemical sensors occupy nowadays a prominent place among the optical devices due to its superb sensitivity (just a single photon sometimes suffices for quantifying luminescence compared to detecting the intensity difference between two beams of light in absorption techniques), combined with the required selectivity that photo- or chemi-luminescence impart to the electronic excitation. This is due to the fact that the excitation and emission wavelengths can be selected from those of the absorption and luminescence bands of the luminophore molecule in addition, the emission kinetics and anisotropy features of the latter add specificity to luminescent measurements8 10. 

The dynamics of the SHL intensity after subpicosecond UV laser excitation of RuC18B LB films is shown in Figure 32[115,116]. The SHL intensity decreased to 70 % of its initial value upon excitation and returned to almost the initial value within several hundred picoseconds as shown by a bold line. The fluorescence decay of RuC18B LB films measured by the single photon-counting... 

Only one photon at a time may interact with matter. This means that the energy available to each recipient atom or molecule is the same as the energy possessed by the single photon with which it interacts. This truth was refined by Stark and Einstein, who called it the second law of photochemistry. If a species absorbs radiation, then one particle (molecule, ion, atom, etc.) is excited for each quantum of radiation (photon) that is absorbed . 

For EPy-doped PMMA film, a 308 nm excimer laser (Lumonics TE 430T-2, 6ns) was used as as exposure source. We used a tine-correlated single photon counting systen (18) for measuring fluorescence spectra and rise as well as decay curves of a snail ablated area. The excitation was a frequency-doubled laser pulse (295 nm, lOps) generated from a synchronously punped cavity-dumped dye laser (Spectra Physics 375B) and a CW mode-locked YAG laser (Spectra Physics 3000). Decay curves under a fluorescence microscope were measured by the same systen as used before (19). 

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