Two-photon excitation spectroscopy

They have carried out similar kind of study with BSA also. They have noticed the formation of AU25 along with the Aug intermediate and concluded that the core might be the same but the growth depends completely on the nature of the protein. The above mentioned proteins are bigger (583 amino acid residues) and so it is expected that they can accommodate smaller 1 nm cluster core within. But when the protein size is smaller like insulin, there is a chance that clusters cannot form inside the protein rather multiple proteins can stabilize one core. Such type of studies were done with insulin where clusters were grown uniformly inside micro-crystals of protein and that was proved by depth dependent two-photon excitation spectroscopy and RAMAN spectroscopy but mass spectrum is not available for this specific system. ... 

Another important aspect about the optical properties of QDs is the multiphoton process which has been widely applied in recent years in biological and medical imaging after the pioneer work of Goeppert-Mayer (1931), Lami et al. (1996), Helmchen et al. (1996), Yokoyama et al. (2006). The multiphoton process has largely been treated theoretically by steady-state perturbation approaches, for example, the scaling rules of multiphoton absorption by Wherrett (1984) and the analysis of two-photon excitation spectroscopy of CdSe QDs by Schmidt et al. (1996). Non-perturbation time-dependent Schrodinger equation was solved to analyze the ultrafast (fs) and ultra-intense (in many experiments the optical power of laser pulse peak can reach... 

Schmidt, M. E., Blanton, S. A., Hines, M. A., 8c Guyot-Sionnest, P. (1996). Size-dependent two-photon excitation spectroscopy of CdSe nanocrystals. Physical Review B, 53,12629-12632. 

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... 

Piston D. W. (1996) Two-photon Excitation Microscopy in Wang X. F. and Herman B. (Eds), Fluorescence Imaging Spectroscopy and Microscopy, Chemical Analysis Series, Vol. 137, John Wiley Sons, New York, pp. 253-72. 

Finally, in Chapter 11 some advanced techniques are briefly described fluorescence up-conversion, fluorescence microscopy (confocal excitation, two-photon excitation, near-field optics, fluorescence lifetime imaging), fluorescence correlation spectroscopy, and single-molecule fluorescence spectroscopy. 

Following a two photon excitation of hydrated hydroxyl ions (FhO/NaOH = 55) with femtosecond UV pulses (/.purnp = 310 nm, Eexdtation = 2x4 eV), short-time electron transfer trajectories have been investigated by near-IR and UV absorption spectroscopies at room temperature. The energy of the pump beam is 1011 W cm 2. 

G. Gerber By applying two-photon ionization spectroscopy with tunable femtosecond laser pulses we recorded the absorption through intermediate resonances in cluster sizes Na with n = 3,. 21. The fragmentation channels and decay pattern vary not only for different cluster sizes but also for different resonances corresponding to a particular size n. This variation of r and the fragmentation channels cannot be explained by collective type processes (jellium model with surface plasmon excitation) but rather require molecular structure type calculations and considerations. 

Figure 1. Experimental set-up for performing transient two-photon ionization spectroscopy on metal clusters. The particles were produced in a seeded beam expansion, their flux detected with a Langmuir-Taylor detector (LTD). The pump and probe laser pulses excited and ionized the beam particles. The photoions were size selectively recorded in a quadrupole mass spectrometer (QMS) and detected with a secondary electron multiplier (SEM). The signals were then recorded as a function of delay between pump and probe pulse.

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]. 

The laser-excited fluorescence from dibenzofuran in a biphenyl host at 4.2 K and three different sites in the lattice have been identified. The two-photon excitation spectrum of single crystals of carbazole at 4.2 K has been analysed in detail. Very extensive vibrationally unrelaxed fluorescence is observed following dye-laser excitation of single vibronic levels of naphthazanine and the rates and pathways of relaxation are examined by picosecond emission spectroscopy. The photophysics of rubrene peroxide have been measured for the first time by Bayrakceken. Picosecond fluorescence has been used to study the spectra and kinetics for Sj— So and Si transitions for... 

This experiment teaches us that the energy-level structure of an ion in a host lattice, the absorption of which covers the higher part of the energy levels of the dopant ion, can be unraveled by two-photon spectroscopy. It shows also that two-photon excitation may occur via an intermediate level or directly. Our discussion continues now with the latter case. 

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