Laser-induced phenomena

TDDFT is a tool particularly suited for the study of systems under the influence of strong lasers. We recall that the time-dependent Kohn-Sham equations yield the exact density of the system, including all non-linear effects. To simulate laser induced phenomena it is customary to start from the ground-state of the system, which is then propagated imder the influence of the potential... 

Although site effects are not as prevalent in UV-vis absorption as they are in IR spectra, they do exist and manifest themselves sometimes very clearly in band systems that comprise sharp peaks. An example is the radical cation of all-trans-octatetraene whose first absorption band consists of multiple peaks that can be selectively bleached by highly monochromatic light. The site stmcture can become more evident in laser-induced fluorescence, where excitation of individual sites is possible down to the level of single molecules in favorable cases, but a discussion of this fascinating phenomenon is beyond the scope of this chapter. 

Houcine et al. (64) used a non-intrusive laser-induced fluorescence method to study the mechanisms of mixing in a 20 dm CSTR with removable baffles, a conical bottom, a mechanical stirrer, and two incoming liquid jet streams. Under certain conditions, they observed an interaction between the flow induced by the stirrer and the incoming jets, which led to oscillations of the jet stream with a period of several seconds and corresponding switching of the recirculation flow between several metastable macroscopic patterns. These jet feedstream oscillations or intermittencies could strongly influence the kinetics of fast reactions, such as precipitation. The authors used dimensional analysis to demonstrate that the intermittence phenomenon would be less problematic in larger CSTRs. 

The extremely small cross sections for conventional Raman scattering, typically 10 111 to 10-25 cm2/molecule has in the past precluded the use of this technique for single-molecule detection and identification. Until recently, optical trace detection with single molecule sensitivity has been achieved mainly using laser-induced fluorescence [14], The fluorescence method provides ultrahigh sensitivity, but the amount of molecular information, particularly at room temperature, is very limited. Therefore, about 50 years after the discovery of the Raman effect, the novel phenomenon of dramatic Raman signal enhancement from molecules assembled on metallic nanostructures, known as surface-enhanced Raman spectroscopy or SERS, has led to ultrasensitive single-molecule detection. 

Laser-induced desorption via the DIET process is a structure-sensitive phenomenon. Firstly, we describe the recent results for adsorbed NO on Pt(l 1 1), since the adsorption structure of this system has been misunderstood for a long time. Adsorbed species giving rise to the 1490 cm-1 NO stretching vibrational mode had been believed to be adsorbed at bridge sites [34, 35]. Recently it has been shown that this species is adsorbed at the threefold fee hollow site. This problem was pointed at first using LEED analysis by Materer et al. [36, 37]. A similar problem is the occupation of the fee and hep threefold hollow sites in a ratio of 50/50 described by Lindsay et al. [38] on the basis of a photoelectron diffraction investigation of NO on Ni(l 1 1) at a coverage of 0.25 monolayer. 

Another related phenomenon that results in a lower quantum yield than expected is called concentration quenching. This can occur when a macromolecule, such as an antibody, is heavily labeled with a fluorophore, such as fluorescein isothiocyanate. When this compound is excited, the fluorescence labels are in such close proximity that radiationless energy transfer occurs. Thus, the resulting fluorescence is much lower than expected for the concentration of the label. This is a common problem in flow cytometry and laser-induced fluorescence when attempting to enhance detection sensitivity by increasing the density of the fluorescing label. 

It should be noted that the diameter of a laser-produced cavity may differ significantly from the Gaussian beam diameter 2w0. Figure 8 illustrates this phenomenon. The distance 2w0 is indicated on the x-axis (projection at F0le2 to the x-axis). For ablation, a material- and laser-dependent fluence threshold Fth has to be surpassed. On the sample, one can realize a laser-induced hole with a diameter smaller than 2w0 (projection at Fth to the x-axis) for example. By adjusting F0 very close to Fth (with F0>Ft l), the production of tiny structures should be possible. 

Laser-induced fluorescence (LIF) is (spontaneous) emission from atoms or molecules that have been excited by (laser) radiation. The phenomenon of induced fluorescence was first seen and discussed back in 1905 by R. W. Wood, many decades before the invention of the laser. The process is illustrated schematically in Figure 7.1. 

Becaii.se of the phenomenon of self-absorption the ideal sample for conventional emission studies is a thin layer (e.g.. a polymer film), on both metal and semiconductor. surfaces [81]. A sample is usually heated from below the emitting surface, the lower surface thus having a higher temperature than the upper one. Therefore, radiation emitted from below the upper surface is absorbed before it reaches the surface, and this self-absorption of previously emitted light severely truncates and alters features in the emission spectra of optically thick samples. This problem is overcome by using a laser for controlled heat generation within a thin surface layer of the sample, self-absorption of radiation thus being minimized. These methods, known as laser-induced thermal emission (LITE) spectroscopy [85], [86] and transient infrared emission spectroscopy (TIRES) [87], [88] can produce analytically useful emission spectra from optically thick samples. Quantitative applications of infrared emission spectroscopy are described in [89]-[91]. 

The electric breakdown of a dielectric liquid under high electric stress is a complex phenomenon where many elementary processes contribute to the change of the electrical current through the test gap from values of pico- to nano-amperes to values of kilo-amperes on a time scale of nanoseconds. Electronic processes are always involved in the initiation of the electric breakdown of nonpolar dielectric liquids. Unambiguous experimental evidence is scarce in the literature since a multitude of other effects obscures the electronic contribution of the breakdown process. In addition, many breakdown tests were performed on industrial-grade liquids, such as transformer oil etc., which are not pure liquids but rather mixtures of several components. More unambiguous information on electronic processes can be obtained from breakdown measurements with impulse voltages of nanosecond to microsecond duration. Complementary are studies of the laser-induced breakdown. 

Laser-induced breakdown is a phenomenon which occurs when the light is focused into a small volume and the electric field strength exceeds the breakdown threshold. From a quantum point of view, one may define the breakdown process as the formation of a plasma region due to multiphoton ionization processes. 

---