Directly modulated detection schemes

A second detection scheme for observation of periodic excitation resonances is shown in Fig. 1b. The fluorescence radiation of the atoms which are optically excited by the pulse train, is monitored as a function of the pulse rate. At resonant coherent excitation of the atomic sample the incoming light is less absorbed and reduced fluorescence radiation is observed. In this case a cheap Ga(Al)As injection laser has been applied which is directly modulated and supplied with electrical pulses from a comb generator. The laser generates optical pulses of about Uo ps duration at a pulse rate of 1 GHz. 

As pointed out above, the most accurate and reliable information obtained by ENDOR is the resonance frequency that directly gives the hyperfine coupling con.stant. In this respect, the firequency-derivative ENDOR spectra obtained by cw-ENDOR detection by employing the frequency modulation scheme, are particularly important because of high-frequency resolution. Combined with ENDOR-induced ESR, the maximum ENDOR frequency, important in identifying the peak value of the spin density distribution, can be determined very accurately [7]. Specific examples are discussed in the cases of solitons in polyacetylene and polarons in poly(paraphenylene vinylene) in Sections 3.2 and 3.3. 

A further small tweaking of the reaction conditions involves great changes a further new cascade channel is breaking. This is accomplished by modulation of the nucleophilicity of the activated methylene compounds applied. When used with isocyanoacetate 87 instead of cyanoacetates 86 as a methylene-activated component, an Ugi-like multicomponent cascade reaction is observed. This reaction was first detected by deployment of ethyl isocyanoacetate 87 in reactions of ribose and proline. In these experiments, proline—once the catalyst in the Knoevenagel condensation/ketalization/oxa-Michael cascade reactions—is directly incorporated into the product. As a result of that, seven-membered lactone 89 is formed. This sharp difference is demonstrated in Scheme 2.18 [40]. 

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