Free induction decay detection

Although the idea of generating 2D correlation spectra was introduced several decades ago in the field of NMR [1008], extension to other areas of spectroscopy has been slow. This is essentially on account of the time-scale. Characteristic times associated with typical molecular vibrations probed by IR are of the order of picoseconds, which is many orders of magnitude shorter than the relaxation times in NMR. Consequently, the standard approach used successfully in 2D NMR, i.e. multiple-pulse excitations of a system, followed by detection and subsequent double Fourier transformation of a series of free-induction decay signals [1009], is not readily applicable to conventional IR experiments. A very different experimental approach is therefore required. The approach for generation of 2D IR spectra defined by two independent wavenumbers is based on the detection of various relaxation processes, which are much slower than vibrational relaxations but are closely associated with molecular-scale phenomena. These slower relaxation processes can be studied with a conventional... 

This effect induces a free induction decay (FID) signal in the detection circuit. The FID can be measured, and the normal absorption spectrum can be obtained by means of an inverse Fourier transform. A variety of experimental extensions have been developed for this approach. By means of particular pulse sequences it is possible to detect spin resonances selectively on the basis of a broad ensemble of properties such as spatial proximity and dipolar coupling strengths. The central fundamental quantity of interest is, however, still the energy spectrum of the nuclear spin,... 

A pre-requisite for the successful extraction of key NMR parameters from an experimental spectrum is the way it is processed after acquisition. The success criteria are low noise levels, good resolution and flat baseline. Clearly, there are also experimental expedients that can further these aims, but these are not the subject of this review per se. In choosing window functions prior to FT, the criteria of low noise levels and good resolution run counter to one another and the optimum is just that. Zero filling the free induction decay (FID) to the sum of the number acquired in both the u and v spectra (in quadrature detection) allow the most information to be extracted. 

Figure 4. Coherent transients observed in gases and molecular beams. Shown are the photon echo (detected by spontaneous emission), the free induction decay, and Ti for different pressures (iodine gas and beam).

Fig. 2.4 outlines the concept of pulsed NMR, including the formation of transverse magnetization My, by the rf pulse (b), followed by the free induction decay (c) and the corresponding time-dependent signal detectable in the resonance and off-resonance situation (d, e). 

Preparation, evolution and detection are the time periods of pulse sequences described for. /-modulated spin-echo and polarization transfer experiments. The basic FT NMR technique operates without any evolution period tt immediately after generation of transverse magnetization (preparation) its free induction decay S(t2) is detected. Subsequent Fourier transformation provides the FT NMR spectrum S (f5). 

FID free induction decay flame ionization detection... 

While in the frequency domain all the spectroscopic information regarding vibrational frequencies and relaxation processes is obtained from the positions and widths of the Raman resonances, in the time domain this information is obtained from coherent oscillations and the decay of the time-dependent CARS signal, respectively. In principle, time- and frequency-domain experiments are related to each other by Fourier transform and carry the same information. However, in contrast to the driven motion of molecular vibrations in frequency-multiplexed CARS detection, time-resolved CARS allows recording the Raman free induction decay (RFID) with the decay time T2, i.e., the free evolution of the molecular system is observed. While the non-resonant contribution dephases instantaneously, the resonant contribution of RFID decays within hundreds of femtoseconds in the condensed phase. Time-resolved CARS with femtosecond excitation, therefore, allows the separation of nonresonant and vibrationally resonant signals [151]. 

Natural abundance 13c spectra were obtained using quadrature detection modified Bruker HX-270 and HFX-90 spectrometers operating at 67.9 and 22.6 MHz, respectively. Free induction decays were accumulated using 4K/4K data points and a 3kHz spectral window. A fast inversion recovery (FIRFT) pulse sequence (21) was employed to measure T] s T] s were calculated using a nonlinear three parameter fitting procedure(22). 

Figure 1. Schematic representation of the 27A1 quadrupole nutation experiment. The rf pulse of length ti is followed by the detection of the free induction decay in die absence of rf fields. Two-dimensional Fourier transformation of the series of FIDs gives the nutation spectrum.

Free induction decay after a 90° pulse. (A) A 90° pulse rotates M from (close) to the Z axis to the Y axis, where a pick-up coil detects the signal. (B) Signal detected decays with time this is the induced signal in the /-axis coils. 

In two-dimensional techniques, prior to the observation pulse with the detection period t2, an rf pulse is applied with the evolution period y between the two pulses. A second time dimension (COSY) is created by repeating the same experiment with the incrementation of H. For each value of ti a free induction decay (FID) is recorded and, after 2D Fourier transformation, the desired 2D frequency spectrum S(wi,w2) is obtained. In the NOESY spectroscopy, the mixing period consisting of two 90° pulses separated by the mixing time Tm is used. The general experimental scheme for... 

Rhee H, JuneYG KZH et al (2009) Phase sensitive detection of vibrational optical activity free-induction-decay vibrational CD and ORD. J Opt Soc Am B 26 1008-1017... 

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