Laser excitation source

Detailed information on the conformation and orientation of polysilane thin films has been obtained by anisotropic PL.80 The alignment of poly (methylphenylsilane-co-methacryloxypropyltriethoxysilane) [P(MPS-co-MPTES)] adsorbed on silica substrates has been studied at 14 K using an attenuated He-Cd polarized laser beam (X = 325 nm) as the excitation source. Laser irradiation decreases the PL intensity, blue shifts the PL peak position... 

In the future, we can expect the ratho expensive picosecond dye lasers and Ti sapphire lasexs to be t laced by sin ler and less eiq>ensive device. A diode-punq>ed Nd YAG laser has already been used fw Ume-resolved detection in capillary zone eleftto(4ioresis, and one can purchase a streak camera with apulsed lasCT diode excitation source. Laser diodes have also been used as the excitation source for FD fluwiMnctry. The wavelengths are usually limited to 600-700 nm, but some laso diodes can be frequency-doubled to 410 nm. It is also likely that... 

Figure 8. Schematic diagram of standoffRaman spectrometer. Essential parts are (a) excitation source laser (b) collector telescope (c) fiber optics coupling (d) fast spectrograph equipped with edge or notch filters. Details of the optical coupling of the telescope to the fiber optics bundle are shown in the...

A Raman microscope consists of five basic components excitation source (laser), focusing component (microscope), signal analyser (spectrometer or interferometer), photon detector (either monochannel or 2D array) and mapping unit such as a computer-controlled micromanipulator. Raman microscopes are usually equipped with low-power UVA IS or NIR lasers, with laser spot sizes (focused laser beam) below 10 pm. Raman microscopy... 

The eombination in a compact system of an infrared sensor and a laser as excitation source is called a photothermal camera. The surface heating is aehieved by the absorption of the focused beam of a laser. This localisation of the heating permits a three-dimensional heat diffusion in the sample to be examined. The infrared (IR) emission of the surface in the neighbourhood of the heating spot is measured by an infrared detector. A full surface inspection is possible through a video scanning of the excitation and detection spots on the piece to test (figure 1). 

Better detection limits are obtained using fluorescence, particularly when using a laser as an excitation source. When using fluorescence detection, a small portion of the capillary s protective coating is removed and the laser beam is focused on the inner portion of the capillary tubing. Emission is measured at an angle of 90° to the laser. Because the laser provides an intense source of radiation that can be focused to a narrow spot, detection limits are as low as 10 M. 

The high performance of modem spectrographs means that low power lasers can be used as excitation sources. These are typically 10—100-mW devices which are air-cooled and can be operated from 117-V a-c lines. In the green, the Ar" (514.5-nm) laser remains the most popular but is being challenged by the smaller and more efficient frequency doubled Nd YAG (532-nm). In the nir, diode lasers (784-nm) and diode-pumped alexandrite... 

Lasers act as sources and sometimes as amplifiers of coherent k—uv radiation. Excitation in lasers is provided by external particle or photon pump sources. The high energy densities requked to create inverted populations often involve plasma formation. Certain plasmas, eg, cadmium, are produced by small electric discharges, which act as laser sources and amplifiers (77). Efforts that were dkected to the improvement of the energy conversion efficiencies at longer wavelengths and the demonstration of an x-ray laser in plasma media were successful (78). 

Unlike these light sources, laser technology relies on a concept known as stimulated emission. Wlien an excited atom is stimulated by a photon, light is emitted at precisely the same wavelength and precisely in ph ase with the light wave that stimulated it. 

The first successful application of the continuous wave (CW) He-Ne gas laser as a Raman excitation source by Kogelnik and Porto (14) was reported in 1963. Since that time, significant improvements in instrumentation have been continually achieved which have circumvented a great number of problems encountered with mercury lamp sources. The renaissance of Raman spectroscopy has also been due to improvements in the design of monochromators and photoelectric recording systems. 

In general, the choice of a laser for use as a Raman excitation source is based on a number of considerations. The laser excitation wavelength, for experimental and theoretical reasons, must lie in the visible region, i.e. 400-700 nm. The laser should have many emission lines over a wide range of the visible region and the excitation frequency should not correspond... 

A list of the principal laser lines useful as Raman excitation source is... 

The preferable excitation source is an Nd-YAG laser because of its bandwidth (which is only about 0.3 ps), its easier mode locking, and its wavelength range. The use of an ultrafast laser creates the need for time resolution in a similarly short regime. The fastest photodiodes and oscilloscopes cannot resolve times <50 ps, and so other methods have been developed. One of them is the streak camera but it is not all that fast (0.5-5 ps resolution), and it is none too sensitive to small signals. 

Figure lb shows the transient absorption spectra of RF (i.e. the difference between the ground singlet and excited triplet states) obtained by laser-flash photolysis using a Nd Yag pulsed laser operating at 355 nm (10 ns pulse width) as excitation source. At short times after the laser pulse, the transient spectrum shows the characteristic absorption of the lowest vibrational triplet state transitions (0 <— 0) and (1 <— 0) at approximately 715 and 660 nm, respectively. In the absence of GA, the initial triplet state decays with a lifetime around 27 ps in deoxygenated solutions by dismutation reaction to form semi oxidized and semi reduced forms with characteristic absorption bands at 360 nm and 500-600 nm and (Melo et al., 1999). However, in the presence of GA, the SRF is efficiently quenched by the gum with a bimolecular rate constant = 1.6x10 M-is-i calculated... 

Figure 5 shows the dependence of the total emission intensity on the excitation intensity and its spectral width obtained from DCM-encapsulated dendrimers. A nitrogen laser (wavelength of 337 run, pulse duration of 4 ns, and repetition rate of 10 Hz) was used as the excitation source. A cylindrical lens focused the excitation beam onto a stripe 200 pm wide on a quartz cuvette... 

Figures 6a-c report the effect of H2O adsorption on Ti(IV) sites on the Raman features of TS-1 using three different excitation sources. As far as the asymmetric modes of the [Ti(OSi)4] units are concerned, we observed a blue shift of the 960 cm band whichever laser was used in the experiment [48,52,64]. More interesting are the consequences that water adsorption has on the to-...

Figure 6. Tempcraiure dependence of the fluorescence lifetime of BMPC in 1 1 ethanol-mcihanol. Measurements were carried out at the LENS laboratory of Florence by a picosecond apparatus using as an excitation source (at 380 nm) a dye laser pumped by a frequency-doubled cw Nd-YAG laser and recording the fluorescence time jirofiles by a streak camera. Since the overall insuumental response time was 75-80 ps, decays with t>200 ps, observed at T<130 K, were analyzed without deconvolution. At 177, 178 and 193 K, the lifetimes were roughly estimated as i=(FWHM -77 ), where FWHM was the width at half maximum of the decay. Because of the rather high sample absorbances (An,x=2), self absorption may have reduced the lifetimes to some extent.

In addition, combining the microscope with the use of a pulsed laser light source provides temporal information on these systems in a small domain. The dispersion of refractive index, however, strongly affects the temporal resolution in the measurements of dynamics under the microscope and typical resolution stays around 100 fs when a Ti Sapphire laser is used as an excitation source. 

Confocal Microscope with a Chromium Forsterite Ultrafast Laser as an Excitation Source... 

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