Simultaneous excitation

The original method employed was to scan eitiier the frequency of the exciting oscillator or to scan the applied magnetic field until resonant absorption occiined. Flowever, compared to simultaneous excitation of a wide range of frequencies by a short RF pulse, the scanned approach is a very time-inefficient way of recording the spectrum. Flence, with the advent of computers that could be dedicated to spectrometers and efficient Fourier transfomi (FT) algoritluns, pulsed FT NMR became the nomial mode of operation. 

Most infrared spectroscopy of complexes is carried out in tire mid-infrared, which is tire region in which tire monomers usually absorb infrared radiation. Van der Waals complexes can absorb mid-infrared radiation eitlier witli or without simultaneous excitation of intennolecular bending and stretching vibrations. The mid-infrared bands tliat contain tire most infonnation about intennolecular forces are combination bands, in which tire intennolecular vibrations are excited. Such spectra map out tire vibrational and rotational energy levels associated witli monomers in excited vibrational states and, tluis, provide infonnation on interaction potentials involving excited monomers, which may be slightly different from Arose for ground-state molecules. 

Precisely controllable rf pulse generation is another essential component of the spectrometer. A short, high power radio frequency pulse, referred to as the B field, is used to simultaneously excite all nuclei at the T,arm or frequencies. The B field should ideally be uniform throughout the sample region and be on the order of 10 ]ls or less for the 90° pulse. The width, in Hertz, of the irradiated spectral window is equal to the reciprocal of the 360° pulse duration. This can be used to determine the limitations of the sweep width (SW) irradiated. For example, with a 90° hard pulse of 5 ]ls, one can observe a 50-kHz window a soft pulse of 50 ms irradiates a 5-Hz window. The primary requirements for rf transmitters are high power, fast switching, sharp pulses, variable power output, and accurate control of the phase. 

Fluorescence. The fluorescence detection technique is often used in clinical chemistry analyzers for analyte concentrations that are too low for the simpler absorbance method to be appHed. Fluorescence measurements can be categorized into steady-state and dynamic techniques. Included in the former are the conventional simultaneous excitation-emission method and fluorescence polarization. 

Graphite exhibits strong second-order Raman-active features. These features are expected and observed in carbon tubules, as well. Momentum and energy conservation, and the phonon density of states determine, to a large extent, the second-order spectra. By conservation of energy hut = huty + hbi2, where bi and ill) (/ = 1,2) are, respectively, the frequencies of the incoming photon and those of the simultaneously excited normal modes. There is also a crystal momentum selection rule hV. = -I- q, where k and q/... 

Greatly enhanced sensitivity with very short measuring time is the major advantage of PFT (pulse Fourier transform) experiments. In the CW (continuous wave) experiment, the radiofrequency sweep excites nuclei of different Larmor frequencies, one by one. For example, 500 s may be required for excitation over a 1-KHz range, while in a PFT experiment a single pulse can simultaneously excite the nuclei over 1-KHz range in only 250 jits. The PFT experiment therefore requires much less time than the CW NMR experiment, due to the short time required for acquisition of FID signals. Short-lived unstable molecules can only be studied by PFT NMR. 

FIGURE 3.2 Track formation according to Mott (1930). Simultaneous excitation of atoms at 1 and 2 has negligible probability in second-order perturbation theory unless the interatomic separation vector R is well aligned with the incoming and outgoing momentum vectors of the incident particle. Reproduced from Mozumder (1969), by permission of John Wiley Sons, Inc. ... 

Almost all Raman spectra of adsorbed species have therefore relied on enhancement effects. Thus, if the incident radiation can simultaneously excite an electronic absorption (see Figure 2.52), then the polarisabilily change associated with this change in the electronic configuration is very large. The... 

In FT-ICR mass spectrometers, ion isolation and detection occur in the same region. In fact all ions coming from the source are simultaneously excited applying a RF pulse of a... 

This fluorophore has excitation maxima at 375 and 400 nm and an emission maximum at 410 nm. The small Stoke s shift may create some difficulty in discrete excitation without contaminating the emission measurement with scattered or overlapping light. The extinction coefficient of the molecule in water is about 27,000M 1cm 1. Cascade Blue and Lucifer Yellow derivatives can be simultaneously excited by light of less than 400 nm, resulting in two-color detection at 410 and 530 nm. 

The events taking place in the RCs within the timescale of ps and sub-ps ranges usually involve vibrational relaxation, internal conversion, and photo-induced electron and energy transfers. It is important to note that in order to observe such ultrafast processes, ultrashort pulse laser spectroscopic techniques are often employed. In such cases, from the uncertainty principle AEAt Ti/2, one can see that a number of states can be coherently (or simultaneously) excited. In this case, the observed time-resolved spectra contain the information of the dynamics of both populations and coherences (or phases) of the system. Due to the dynamical contribution of coherences, the quantum beat is often observed in the fs time-resolved experiments. 

The methanol trimer is arguably one of the most interesting clusters. The unexpected structure in its O—H stretching spectrum [65, 75, 77, 173] has only recently found a consistent explanation [16]. It is not related to structural isomers [64, 75, 195, 219, 220] but rather to simultaneous excitation and de-excitation of low-frequency methyl umbrella modes [16, 65], that is, a... 

In addition, for thermochemical purposes we are primarily interested in the core-valence correlation, since we can reasonably expect the core-core contributions to largely cancel between the molecule and its constituent atoms. (The partitioning between core-core correlation -involving excitations only from inner-shell orbitals - and core-valence correlation - involving simultaneous excitations from valence and inner-shell orbitals - was first proposed by Bauschlicher, Langhoff, and Taylor [42]). 

In the ICR cell, there is a stringent correlation of cyclotron frequency/c and m/z value. For simplicity, the very first FT-ICR experiment was therefore performed with an excitation pulse of a fixed/c tailored to fit the model analyte, methane molecular ion. [185] However, useful measurements require the simultaneous excitation of all ions in the cell, and this in turn demands for a large RF bandwidth. 

The stray light arising from the simultaneous excitation of multiple regions of the specimen may be substantially reduced by restricting the illumination to a point. This is accomplished by introducing a pinhole (P) into the excitation... 

A solution to improve the overall efficiency of selective ID and 2D experiments is described and demonstrated. It is based on the principle of multiple selective excitation with subsequent data processing to disentangle the superimposed responses of the selectively and simultaneously excited spin systems. Several inherent insensitive and correspondingly modified experiments are presented together with typical applications. 

The basic physical processes involved in Auger neutralization are illustrated in Fig. 3. One electron tunnels into the ground state of the ion from the level in the metal labeled 1 while a second electron is simultaneously excited from... 

The band at 22000 cm-1 (anomalous because other Cr complexes of Cl- and Br- do not absorb strongly here) has been assigned to simultaneous excitation of a pair of antiferro-magnetically coupled Cr11 ions so that the absorption is at ca. 2x11500 cm-1. 

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