Free-space fluorescence spectrum

Another complementary approach to that presented so far is to study different fluorophores (with different free-space fluorescence spectral profiles) on the same NP arrays. This is illustrated in Fig. 2.7. Four fluorophores are used there Methylene Blue (MB) as before, Rhodamine 6G (Rh6G), Rhodamine B (RhB), and Crystal Violet (CV). All experiments are carried out at 633 nm excitation again, and the corresponding free-space fluorescence spectra are shown in Fig. 2.7(a). Rh6G, RhB, and CV all exhibit a tail-like fluorescence spectrum, very similar to each other. This simply reflects the fact that the excitation (at 633 nm) is close or below (in energy) the main absorption band. In this case, internal relaxation in S is expected to play a negligible role, even... 

Transient terahertz spectroscopy Time-resolved terahertz (THz) spectroscopy (TRTS) has been used to measure the transient photoconductivity of injected electrons in dye-sensitised titanium oxide with subpicosecond time resolution (Beard et al, 2002 Turner et al, 2002). Terahertz probes cover the far-infrared (10-600 cm or 0.3-20 THz) region of the spectrum and measure frequency-dependent photoconductivity. The sample is excited by an ultrafast optical pulse to initiate electron injection and subsequently probed with a THz pulse. In many THz detection schemes, the time-dependent electric field 6 f) of the THz probe pulse is measured by free-space electro-optic sampling (Beard et al, 2002). Both the amplitude and the phase of the electric field can be determined, from which the complex conductivity of the injected electrons can be obtained. Fitting the complex conductivity allows the determination of carrier concentration and mobility. The time evolution of these quantities can be determined by varying the delay time between the optical pump and THz probe pulses. The advantage of this technique is that it provides detailed information on the dynamics of the injected electrons in the semiconductor and complements the time-resolved fluorescence and transient absorption techniques, which often focus on the dynamics of the adsorbates. A similar technique, time-resolved microwave conductivity, has been used to study injection kinetics in dye-sensitised nanocrystalline thin films (Fessenden and Kamat, 1995). However, its time resolution is limited to longer than 1 ns. 

From the rotational and vibrational spacings of the fluorescence lines terminating on bound levels below the dissociation limit an accurate determination of the potential V(R) is possible. From the structure in the continuous fluorescence spectrum of bound-free transitions the difference between upper and lower potential can be determined37. Figure 12 illustrates both kinds of fluorescence spectra. It shows the long wavelength section of the fluorescence spectrum of Csg excited on a transition x1e+(v"=0, J =49) DlEjj(v =50,... 

Lighting should be provide by a few banks of wide spectrum fluorescent tubes fairly evenly distributed across the ceiling and turned on for 10-12 hours regularly each day. These are great dust catchers, however, and must be wiped clean periodically. The work table should also be painted with a hard smooth, white finish. If the table is metal, a small, clean cutting board must be provided on which to pin down mushroom caps when disecting them. Shelf boards on the wall next to the table may be extended above the table to provide space for storage of work equipment and ready containers. A hood should be constructed around the table to protect it from dust, etc. A fume hood with a flu vent and spark-free exhaust fan should be constructed over the extraction area to remove toxic and combustible methanol vapors. 

Lanthanide impurity-ion spectra consist of a series of sharp lines that appear in groups of closely spaced sublevels that correspond to transitions between crystal-field split free-ion levels. The simplest absorption spectra occur at very low temperatures ( 4K), at which only the lowest Stark level is populated, in general. As the temperature is raised, transitions originating from thermally accessible excited levels are possible, thus complicating the spectrum. In fluorescence spectra, transitions arise at low temperature only from the lowest lying sublevel of the excited free-ion level. At higher temperatures, other transitions become possible. 

The Fourier analysis of the time-dependent signal (7.27a-7.27b) yields a Doppler-free spectrum /( ), from which the energy spacing A as well as the width y of the two levels ) can be determined, even if A is smaller than the Doppler width of the detected fluorescence (Fig. 7.9c). Quantum-beat spectroscopy therefore allows Doppler-free resolution [868]. 

Laser induced fluorescence (LIF) techniques in supersonic free jets can also yield useful information on the potential energy curves of open shell atomic systems. This type of studies has provided high quality data on tbe ground and excited states of NaAr type of molecules. The LIF technique was also successfully applied for probing the potential surfaces of XeF. The B<-X fluorescence excitation spectrum of XeF in a supersonic free jet is sufficiently simplified that rotational analysis and accurate vibrational spacing are readily obtained, overcoming the complexity of gas phase emission spectroscopy, mainly due in this case to isotopic richness of natural Xe. 

---