Incident power

The intensity of fluorescence therefore, increases with an increase in quantum efficiency, incident power of the excitation source, and the molar absorptivity and concentration of the fluorescing species. 

The rms noise is measured in a noise bandwidth, The D is called D star lambda when the spectral band is limited to a given interval, and D blackbody when the total blackbody incident power density is used in the calculation. 

French researchers [38c] have investigated the /zetero-Diels-Alder reaction of methylglyoxylate and glyoxal monoacetal with 2-methyl-1,3-pentadiene in a microwave oven under various reaction conditions (Table 4.9). The microwave (MW) irradiation does not affect the diastereoisomeric ratio of adducts trans/cis = 70 30) but dramatically reduces the reaction time. The glyoxal monoacetal, for instance, gives 82 % adducts after 5 minutes when submitted to irradiation with an incident power (IP) of 600 W in PhH and in the presence of ZnCL (Table 4.9, entry 1), while no reaction occurs if carried out for 4h at 140 °C in sole PhH. Similarly, methylgloxylate in water at 140 °C gives 82% adducts after 3h, whereas microwave irradiation reduces the reaction time to 8 minutes (Table 4.9, entry 5). 

In Raman measurements [57], the 514-nm line of an Ar+ laser, the 325-nm line of a He-Cd laser, and the 244-nm line of an intracavity frequency-doubled Ar+ laser were employed. The incident laser beam was directed onto the sample surface under the back-scattering geometry, and the samples were kept at room temperature. In the 514-nm excitation, the scattered light was collected and dispersed in a SPEX 1403 double monochromator and detected with a photomultiplier. The laser output power was 300 mW. In the 325- and 244-nm excitations, the scattered light was collected with fused silica optics and was analyzed with a UV-enhanced CCD camera, using a Renishaw micro-Raman system 1000 spectrometer modified for use at 325 and 244 nm, respectively. A laser output of 10 mW was used, which resulted in an incident power at the sample of approximately 1.5 mW. The spectral resolution was approximately 2 cm k That no photoalteration of the samples occurred during the UV laser irradiation was ensured by confirming that the visible Raman spectra were unaltered after the UV Raman measurements. 

On their way from the source to the resonator the intensity of the microwaves must be attenuable for two reasons (1) full power may be too much for the sample leading to saturation (treated in Chapter 4) or (2) it may be impossible to critically couple the cavity at full incident power (e.g., because the sample contains too much water). Therefore, the main waveguide contains an attenuator, usually of the dissipative, rotary-vane type. Dissipative means that the eliminated power is converted to heat and is not reflected as radiation to the source. Rotary vane means that it contains a section of circular waveguide, in which a flat piece of material is located that can be rotated over an angle 0, where 0 = 0 means no attenuation and 0 = 90° causes full attenuation. The amount of attenuation is expressed in decibels, a non-SI,... 

Three reactions of 1 successively with diethyl fumarate, maleic anhydride, and dimethyl acetylenedicarboxylate (DMAD) are highly representative of the variety of experimental conditions used in the GS/MW process [26, 27]. Continuous MW irradiation (CMWI) with an incident power of 120 W for 1 min led to a high increase in temperature (Tmax> 300 °C). Adduct 4 was obtained almost quantitatively (Tab. 7.1, entry 1), whereas only traces of adducts 5 and 6 were detected. When the incident power was reduced (30 W) and sequential MW irradiation (SMWI) was used, adducts 5 and 6 were obtained in good yield (Tab. 7.1, entries 3 and 4). This controlled irradiation enabled the temperature to be limited (Tmax < 200 °C) and avoided the retro-DA reaction. In the reaction between 1 and diethyl fumarate similar SMWI conditions also gave the adduct 4 in high yield (Tab. 7.1, entry 2). 

Sequential MW irradiation (SMWI) applied incident power time and number of irradiations interval between two irradiations 2 min (entries 2-5), 1 min (entries 6-9) e Not given f This work... 

Under classical conditions, the reaction between 3 and styrene required 50 h of heating at 110 °C, and gave the dihydropyridazine adduct 10a [24], After SMWI with 30 W incident power for 5 min (Tmax = 154 °C), the adduct 10a was not detected whereas the totally dehydrogenated product, pyridazine 10b, was isolated in almost quantitative yield (Tab. 7.1, entry 8). Ethyl vinyl ether and 3 gave the same product, pyridazine 11, under both classical heating [25] and MW irradiation conditions (Tab. 7.1, entry 9). In this instance the DA adduct lost nitrogen and ethanol. 

Another example of the retention of volatile DA reagents is that of cyclopentadiene in a tandem retro-DA/DA prime reaction [15, 16, 38], This reaction type is the thermal decomposition of a DA adduct (A) and the generation of a diene (generally the initial diene) which is trapped in situ by a dienophile leading to a new adduct (B) [39]. Cyclopentadiene (22) (b.p. 42 °C) is generated by thermolysis of its dimer at approximately 160 °C [40]. An equimolar mixture of commercial crude dicyclopenta-diene (21) and dimethyl maleate was irradiated in accordance with the GS/MW process, in an open reactor, under 60 W incident power, for 4 min (8 x 30 s). The expected adduct 23 was isolated in 40% yield (Scheme 7.1). The isomeric composition of 23 (endo-endoIexo-exo = 65/35) was identical with that obtained under classical conditions from 22 and methyl maleate [41]. The overall yield of this tandem reaction can be increased from pure dimer 21 (61%) and the same tandem reaction has also been reported using ethyl maleate as dienophile [31]. 

Pinene (26), a more reactive ene than 24, reacted with ethyl mesoxalate under CC14 reflux in 90% yield after 5 h conventional or MW heating [42]. The reaction supported on graphite occurred in only 2x1 min of MW irradiation with an incident power of 60 W. The adduct 27 was obtained in 67% isolated yield (Scheme 7.3) [30]. This yield was obtained after 2 h of reaction under CC14 reflux. 

These redistribution reactions are possible at atmospheric pressure under the action of MW irradiation is performed for a few minutes in the presence of the same catalysts [57]. These reactions with the less volatile germanium tetrabromide (44b) (b.p. 184 °C) have also been performed by use of the GS/MW process, without added catalyst (Tab. 7.4, entries 1 and 3) [15, 16]. In this instance, despite the use of weaker incident power, the temperature reached 420 °C, very much higher than that obtained under the action of MW irradiation of a reaction mixture containing AlBr3 (200 °C to 250 °C) (Tab. 7.4, entries 2 and 4). The presence of this catalyst considerably favors redistribution towards the dibrominated products (46b) (84% for R = nBu, 85% for R = Ph) relative to the monobrominated compounds (46a), which are the major products of the GS/MW process (78% and 43% respectively). The tri-brominated products (46c), the most difficult to prepare, have been obtained with a rather high selectivity (73 to 80%) by use of the catalytic process under the action of MW [57]. In this reaction, therefore, the GS/MW process seems less effective than the MW-assisted and AlX3-catalyzed process. 

An equimolar (10 mmol) mixture of benzoyl chloride and n-butyl oxide adsorbed on 5 g graphite A was sequentially irradiated with 90 W incident power. The conversion reached 80% (Yield of isolated n-butyl benzoate (69) 62%). With ethyl oxide, the yield of ethyl benzoate (68) was lower, but noteworthy considering the volatility of this oxide, and the significant retentive power of graphite towards organic compounds. These preliminary results have not yet been expanded, but it is certain that more reactive ethers, like those substituted with sec- or teri-alkyl, benzylic or allylic groups, are deavable in the same way. 

Mass of 70 2.19 g (15 mmol) mass of graphite 5 g b Sequential MW irradiation controlled to a maximum temperature of 450 °C c Applied incident power and irradiation time interval between two irradiations 2 min d Yield of cyclopentanone (74) from GC analysis... 

The temperature rise T2 = P2/G expected from an increase of the incident power P2 is modified by the fact that R(T) changes, and, as a consequence, the bias heating varies. This effect can be expressed as an effective thermal conductance ... 

Figure 6.18. (a) Transmittance versus wavelength for a CuGaS2 thin film (I = transmitted power, I0 = incident power) (b) plot of the derivative of the transmission data versus energy. 

Figure 5.2 shows that the onset of collapse in barium fluoride as the incident power is varied from 110 PCT to 1,200 Pcr. The onset of filamentation, z0f, the filament start position (the arrow in each panel marks the onset position), varies from 4.6 mm inside the crystal at 110 PCI to a position that is located on the incident face of the crystal at 1,200 PCI. Note that the images in Fig. 5.2 indicate that multiple filamentation occurs within the crystal at such high power levels. 

Typical images of filamentation parallel to the laser propagation direction are shown in Fig. 5.5 for various values of incident power. 

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