Excitation, multi-frequency

Burdick, D.S., Tu, X.M., McGown, L.B., and Millican, D.W., Resolution of multi-component fluorescent mixtures by analysis of the excitation-emission-frequency array, J. Chemom., 4, 15-28, 1990. 

The probe has been long and successfully commercialised (see http //www.aber-instruments.co.uk) and since we have reviewed this approach on a number of occasions (e.g. Kell et al. 1990, Davey 1993a,b, Davey et al. 1993 a, b) we will not do so here, save to point out (in the spirit of this review) the trend to the exploitation of multi-frequency excitation for acquiring more (and more robust) information on the underlying spectra. [124, 125]. Most recently, we have also devised a number of novel routines for correcting for the electrode polarisation that can occur under certain circumstances [126, 127], and have turned our attention to the nonlinear dielectric spectra of biological systems. 

Fig. 5.3.13 Hadamard encoding and decoding for simultaneous four-slice imaging. The encoding is based on four experiments, A-D. In each experiment, all four slices are excited by a multi-frequency selective pulse. Its phase composition is determined by the rows of the Hadamard matrix H2. The image response is the sum of responses for each individual, frequency selective part of the pulse. Thus, addition and subtraction of the responses to the four experiments separates the information for each slice. This operation is equivalent to Hadamard transformation of the set of image responses. Adapted from [Miil21 with permission from Wiley-Liss. Inc., a division of John-Wiley Sons, Inc.

The measured responses to the combinations of multi-frequency selective pulse excitation can be unscrambled for each volume element by transformation with a super-Hadamard matrix. The dimension of this matrix equals the product of the dimensions of the Hadamard matrices used for encoding each space axis. 

Wc notice that for the 6 applied frequencies, tp < tm Thus, the dcca) is heterogeneous and at least two fluorescence lifetimes exist. In fact, the dsta yield two fluorescence lifetimes 2.320 and 0.333 ns Nith fiactional uitensities of 0.9 and O.I. respectively. The mean fluorescence lifetime is equal to 2.213 ns Actually, the possibility of measuring fluorescence lifetimes in the multi frequency method allows in principle to obtain much more accurate results Oian with small numbers of frequencies. However, below one nanosecond, the higher modulation frequencies necessary to make accurate measurements require an expensive laser-based excitation source. 

When using a multi-frequency excitation signal the in-phase and the quadrature currents were smoothed simultaneously. 

Fig.9 shows the original ac polarographic data obtained by a multi-frequency excitation signal and Fig. 10 represents the same data after two-dimensional filtering. The cut-ofF was chosen equal (=8) in every row of the Fourier spectrum as is described in Fig.8. 

The stability analysis for parametrically excited systems with delay can be performed by numerical techniques. Such a technique is the semi-discretization method (Insperger and Stepan 2011), which is a time-domain method, or the multi-frequency solution... 

In contrast, MFI involves the application of a resonant excitation waveform consisting of several frequency components while the value of is held constant. The frequency components of the waveform bracket the entire anticipated range of the secular frequency so as to compensate for frequency shifts. Other variants of multi-frequency irradiation (March 1998) include random noise, swept frequency and broadband excitation. A rather different ion activation method (Paradisi 1992, 1992a Curcuruto 1992) is accessible with ion trap instruments equipped with a DC power supply so that non-zero values of U and thus a are available. The method involves moving the working point of a given ion species to either the P,. or the p, boundary of the stability diagram... 

Mass spectrometry of these compounds in a quadrupole ion trap, comparison with single frequency modulation and multi-frequency resonant excitation modes... 

Fokker Bond Tester. An ultrasonic inspection technique commonly used for aircraft structures is based on ultrasonic spectroscopy [2]. Commercially available instruments (bond testers) used for this test operate on the principle of mechanical resonance in a multi-layer structure. A piezoelectric probe shown in Figure 3b, excited by a variable frequency sine signal is placed on the surface of the inspected structure. A frequency spectrum in the range of some tens of kHz to several MHz is acquired by the instrument, see Figure 3a. 

Valuable findings on the electronic ground and excited states of clusters have been derived from laser-induced multi-photon ionization (MPl) investigations, such as laser-induced fluorescence (LIF) and REMPI. This latter technique is particularly promising since it enables mass selection of cluster species and their spectral and thermochemical characterization. The complex is excited from its electronic ground state from a photon and then ionized by a second photon of equal or different frequency, near threshold to avoid cluster fragmentation. ... 

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