Two-color experiment

Consider, by contrast, a two-color experiment where the continuum is accessed by two laser fields with a well defined relative phase, a and , . A schematic illustration of the experiment envisioned is provided in Fig. 1 a, where we consider the specific case of excitation with one- and three-photon fields of... 

Fluorescent labeling of cDNA can be a potential source of technical variability. In a typical two-color experiment, fluorescently labeled cDNA probes are transcribed from separate mRNA populations (e.g., cerebral ischemia versus sham). One set of cDNA probes is labeled with one fluorescent dye (typically Cy5) and the second set with a different fluorescent dye (Cy3). A number of methods for making labeled cDNA from the RNA samples have been tested and reviewed (Stears et al., 2000 Vernon et al., 2000 Li et al., 2002) and a number of potential sources for variation must be appreciated. First, the molecular structure of the fluorescent dyes used in making labeled cDNA can affect efficiency of dye incorporation. Second the mode of dye incorporation (direct verses indirect labeling) can affect subsequent hybridization kinetics (Stears... 

To this aim we study the DHA/VHF photoconversion in an approach that combines broadband transient absorption with 100 fs pulses and two color experiments with sub-30 fs pulses. The former provides a clear identification of the transient states involved in the process, while the later allows us to determine precisely the kinetics of the process. Through the analysis of the coherent signal observed in addition we are able to identify the structural evolution of DHA directly after the application of the ultrashort pump pulse. 

Figure 23. Pump and probe spectra of a two-color experiment probing bound-free transitions in K (3 < n < 9). For At < 0, Epump = 1.47 eV and Eprobe = 2.94 eV for At > 0, Epump — 2.94 eV and Eprobe = 1.47 eV [23],...

Both proposed two-color experiments have not yet been performed for problems of internal twisting therefore, from the experimental viewpoint, this contribution is confined mainly to fluorescence spectroscopic results, that is, quantum yields, spectral quanta distribution, polarization, and rise and decay functions. 

Given that triplet-triplet energy transfer proceeds via a Dexter (electron exchange) mechanism, it is not surprising that electron transfer can also occur via upper triplet states. Two-color experiments with anthracene in acetonitrile in the presence of ethylbromoacetate, a dissociative electron acceptor, showed that excitation to an upper triplet state led to depletion of the T-T absorption and concurrent production of the anthracene cation radical as a result of electron transfer (Scheme 1) [52]. 

In initial two-color experiments performed with 90, and other compounds such as benzophenone (70), in benzene solvent, extensive T-T depletion was observed. This led to the assumption that upper state chemistry such as type I cleavage was efficient. However, as was pointed out above, benzene and other aromatic solvents can quench upper triplet states by energy transfer and thereby cause depletion. A reexamination of several reluctant type I systems using other solvents (particularly acetonitrile) revealed that energy transfer quenching by aromatic solvent is usually a more important upper triplet decay pathway than cleavage. 

Figure 3.2 Difference gel electrophoresis (DIGE). Ettan DIGE workflow three-color and two-color experiments including the internal standard. For fluorescence proteins tagging, two different CyDyes techniques are available. Minimal fluors allow consideration of three different CyDyes (Cy2, Cy3 and Cy5) in a multiplexing experiment.

No emission at 1269 nm was detected, which was taken to mean that no 02 was formed or that its formation quantum yield, fl>A, was lower than the sensitivity limit of the detector (5 x 10-3). In two-laser, two-color experiments, after excitation at 355 nm the merocyanine formed was excited (2-ns delay) with a green layer (532 nm), and again no 02 was detected, a clear indication that the (photo)merocyanine did not participate in the sensitized formation of singlet oxygen, the species that could induce oxidative degradation of the photochromes. The only noticeable exception was found for 11 for which fl>Awas measured to be 0.15. 

Experimental Setup. An obvious extension of the one-color pump-probe experiments is the application of two-color experiments in which two independently tunable dye lasers share the same pump laser. One can use the same high repetition rate and obtain spectral evolutions on excitation at selected wavelengths. The measurements are performed in essentially the same way as one-color experiments.A disadvantage is the broadened instrument function (cross-correlation function) caused by time jitter between the two pulses, since they are not obtained from the same dye laser. This leads to a full-width half-maximum (fwhm) value of the instrument function of approximately 5-10 psec. 

Time Domain. The results of the one-color and two-color experiments are compared in Fig. 3.7. The Figures 3.7 a, b show the raw pump probe data. The output voltage U on of the secondary electron multiplier is plotted as a function of the delay time At between the pump and probe pulses. In both cases (two-color experiment. Fig. 3.7 a and one-color experiment. Fig. 3.7 b), an oscillatory time-dependent variation superimposed on a time-independent... 

Frequency Domain. Now, the frequency spectra of the one-color and two-color experiments are compared. To get comparable Fourier spectra, the normalized data presented in Figs. 3.7c, d for delay times 0ps< < 30 ps are used for the calculation of the Fourier transform. 

The Fourier spectra are presented in Figs. 3.8 a (two-color experiment) and 3.8b (one-color experiment). Frequency components appear in both... 

The logarithmic contour plots of the spectrograms are presented in Fig. 3.9a (two-color experiment) and Fig. 3.9b (one-color experiment). The Fourier amplitude is plotted with increasing gray from white to black. The... 

Fig. 3.10. Excitation scheme of 39,39K2 for the two-color experiment (a) and one-color experiment (b) (taken from [52]). In both cases a wave packet is prepared on the A state s PES by the pump pulse. In the two-color experiment the subsequent ionization by one photon is slightly favored at both the inner and at the outer turning points of the PES. In the one-color experiment the ionization is strongly enhanced by the (2) /7g state acting as a Franck-Condon window. Potential-energy curves are based on data given in [324, 325]...

Fig. 4.15. Real-time spectra of the two-color experiment for different cluster sizes n (taken from [307]). For At Epump = 1.47eV, Eprobe = 2.94 eV for At>0 Epump = 2.94 eV, F probe = 1.47eV. Dots, experimental data lines, fitted curves based on the extended fragmentation model...

In the two-color experiments the pulses of the Tiisapphire laser (1.47 eV) were frequency-doubled by a 1 mm BBO crystal with a conversion efficiency of 15%. A dichroic mirror separated the fundamental from the second harmonic (2.94 eV). For convenience, a positive delay here means the pump beam is the second harmonic at 2.94 eV and the probe beam is the fundamental at 1.47 eV. For negative delay times the energies of the pump and probe pulses are interchanged. 

Fig. 4.20. Possible contributions to the ion signal of Ks caused by ionic fragmentation (taken from [307]). Energy representation of the two-color-experiment, a, pump pulse excited state b, ionization potential c, energetic threshold for ionic fragmentation d, maximum energy of the ionic fragments...

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