Nonselective excitation

Muns ENDOR mvolves observation of the stimulated echo intensity as a fimction of the frequency of an RE Ti-pulse applied between tlie second and third MW pulse. In contrast to the Davies ENDOR experiment, the Mims-ENDOR sequence does not require selective MW pulses. For a detailed description of the polarization transfer in a Mims-type experiment the reader is referred to the literature [43]. Just as with three-pulse ESEEM, blind spots can occur in ENDOR spectra measured using Muns method. To avoid the possibility of missing lines it is therefore essential to repeat the experiment with different values of the pulse spacing Detection of the echo intensity as a fimction of the RE frequency and x yields a real two-dimensional experiment. An FT of the x-domain will yield cross-peaks in the 2D-FT-ENDOR spectrum which correlate different ENDOR transitions belonging to the same nucleus. One advantage of Mims ENDOR over Davies ENDOR is its larger echo intensity because more spins due to the nonselective excitation are involved in the fomiation of the echo. 

A homonuclear spin-system may be excited with radiofrequency (r.f.) pulses that are so Intense (in the order of p.s), compared to the frequency width of the spectrum, that all resonances are excited essentially uniformly. This is a nonselective excitation. A homonuclear spin-system may also be excited with a relatively weak, r.f. pulse (in the order of ms), in the sense that all components of a given multiplet are inverted at time zero, whereas the other resonances in the spectrum remain essentially unperturbed this is a selective excitation. The r.f. pulse may be single-selective, that is, there is an inversion of one multiplet in the spectrum, or double-selective, triple-selective, and so on, where two, three, or more separate multiplets in the spectrum are inverted simultaneously while the remaining resonances remain unperturbed. 

Compared with the sensors for atoms and radicals, the calibration of EEP sensors is also somewhat specific. To calibrate detectors of atomic particles, it will be generally enough to determine (on the basis of sensor measurements) one of the literature-known constants, say, tiie energy of parent gas dissociation on a hot Hlament. For the detection of EEPs when nonselective excitation of gas is taking place, in order to calibrate a sensor use should be made of some other selective methods detecting EEPs. The calibration method may be optical spectroscopy, chemical and optic titration, emission measurements, etc. 

With nonselective excitation of oxygen, a problem arises of distinguishing between the 02( a ) and other active particles, say, O-atoms... 

Compared to photoluminescence processes, no external light source is required, which offers some advantages such as the absence of scattering or background photoluminescence signals, the absence of problems related to instability of the external source, reduction of interferences due to a nonselective excitation process, and simple instrumentation. 

Similarly to the situation found for Pt(4,6-dFppy)(acac) in /7-octane, several discrete sites are observed in the nonselectively excited emission spectrum of Ir(4,6-dFppy)2(acac) in CH2C12 at 4.2 K (not shown, but compare [50]). However, for the Ir(III) compound the inhomogeneously broadened background is much more intense. In Fig. 7, site-selectively excited emission spectra at different temperatures and a site-selectively detected excitation spectrum are displayed for the region of the electronic 0-0 transitions of the site of lowest energy, denoted as site A. 

Fig. 5.3.3 [Houl Relationships between excitation spectrum X (w) (a) of the excitation transfer function K (a>) (b), and spectrum K (a>) (c) of the linear system response M, ) =, Vi (/) (d) for nonselective excitation (left) and selective excitation (right).

Fig. 7.2.31 Filters for homonuclear coherent magnetization transfer. All filters start from and end with longitudinal magnetization, (a) Selective excitation and reconversion of coherences with a nonselective mixing period, (b) Realization of a nonselective mixing period in a z filter via longitudinal magnetization, (c) Nonselective excitation and reconversion of coherences with a selective mixing period, (d) Realization of a selective mixing period by a multi-quantum filter, (e) Selective exchange of transverse magnetization within the multiplets of coupled homonuclear spin pairs by a homonuclear version of the INEPT method.

In one-dimensional usage, a ir/2 nonselective excitation pulse can be applied before the WATERGATE sequence. In cases where it is desired to observe exchangeable protons, a selective n/2, pulse (selective of the water resonance, for example an EBURP2 pulse ) can precede the nonselective 7t/2 pulse. WATERGATE can be incorporated into two- and higher-dimensional experiments (see Section 5). 

Figure 1 ISIS. In eight separate acquisitions combinations of up to three frequency-selective RF pulses invert the longitudinal magnetization in three orthogonal slices prior to a nonselective excitation pulse and collection of the free induction decay. The inversion pulses modify the resultant phase of the transverse magnetization existing after excitation. For three-dimensional localization, the inversion pulses are applied and the data added or subtracted according to the protocol in Table 1.

Beam-Foil Techniques. The beam-foil method has been discussed in Sect. 6.1. It is a very general method for measuring lifetimes of atoms and ions. However, the non-selective excitation, leading to cascading decays, places heavy demands on the data analysis and sometimes a detailed study of the different cascade channels is necessary for reliable lifetime evaluations. While the nonselective excitation frequently constitutes a problem, it is also an advantage of the method since a multitude of excited states are populated. For measurements of multiply charged ions in particular, the technique provides unique measurement possibilities where other techniques are not applicable. 

Fig. 11.6 Model of photodissociation of a polyatomic molecule by an intense IR field left, scheme of vibrational-energy acquisition by the molecule in the regions of mode-selective and mode-nonselective excitation right evolution of the fundamental IR absorption band spectrum with increasing vibrational energy of the molecule. Even at the dissociation limit the molecule is capable of absorbing, in a quasi-resonant fashion, IR radiation at a laser frequency i l tuned to the long-wavelength wing of the fimdamental absorption band.

The very rich structure, which occurs as satellite pattern to the electronic origin in the emission spectrum, reveals a considerable amount of information about properties of the electronic states involved. Figure 3 a shows the nonselectively excited spectrum, while Fig. 3 b reproduces the resonantly excited one of the low-lying [Pt(bpy-hg)2] " trap in neat [Pt(bpy-hg)2] ( 104)2. Due to the selective excitation, the spectrum in Fig. 3 b is line-narrowed and therefore better resolved than the one shown in Fig. 3 a. 

It is highly instructive to compare spectroscopic properties of [Os(bpy-h8)2(bpy-d8)], [Os(bpy-h8)(bpy-d8)2]. and [Os(bpy-d8))3] + to those of [Os(bpy-h8)3]. All of these compounds can be doped into [Zn(bpy-118)3] (0104)2, iid one always obtains highly resolved emission spectra[98, 104]. Those of the two partially deuterated compounds represent superpositions of spectra of different sites if nonselectively excited. In [104] the occurrence of three sites A, B, and C for both complexes is reported, and it is shown that it is suitable to investigate sites B in more detail. These specific sites are easily singled out with the methods of site-selective spectroscopy. The spectra obtained are compared in Fig. 31 to those of the two per-complexes. 

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