Excitation multiple pulse

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... 

The idea of using phase increment to achieve frequency-shifted excitation can be extended virtually to any sort of RF pulses, including the most complicated adiabatic inversion pulses where a non-linear phase increment has already been applied. Using the phase increment, double or multiple pulses can be constructed with only a single waveform generator in order to excite different regions of a NMR spectrum or to compensate the BSFS, BSPS, as well as BSOS. 

Artifacts may be roughly categorized into those due to inherent limitations (e.g. pulses cannot excite unlimited bandwidths even if all hardware components work perfectly) and those that result from improper set-up of the experiment or nonideal functioning of the NMR spectrometer system. In this chapter we will mainly focus on the latter two. These artifacts are more likely to appear in multiple-pulse experiments. Quite often, they are avoided by clever programming of the experiments (e.g. interleaved acquisition of data for NOE spectra, use of pulsed-field gradients instead of phase-cycling). 

Locking of Dephasing and Energy Redistribution in Molecular Systems by Multiple-Pulse Laser Excitation, E. T. Sleva, M. Glasbeek, and A. H. Zewail, J. Phys. Chem. 90, 1232 (1986). 

Optical Multiple Pulse Sequences for Multiphoton Selective Excitation and Enhancement of Forbidden Transitions, W. S. Warren and A. H. Zewail, J. Chem. Phys. 78 (11), 3583 (1983). 

A pumping laser satisfying these requirements is the Cu vapor laser, first advocated for the same reasons by J. G. Anderson in about 1980. Thus Stimpfle et al. (93, 94) used a Cu vapor laser pumped dye laser that was frequency-doubled to 282 nm to determine stratospheric HO during balloon-borne descent in the stratosphere. At the 17-kHz repetition rate of this laser, multiple-pulse photolytic HO accumulation appears to have been avoided by the use of a fan downstream of the excitation zone to increase the air velocity beyond that provided by balloon descent. 

Analogous to the principal concept of multiplex CARS microspectroscopy (cf. Sect. 6.3.5), in multiplex SRS detection a pair of a broad-bandwidth pulse, eg., white-light femtosecond pulse, and a narrow-bandwidth picosecond pulse that determine the spectral width of the SRS spectrum and its inherent spectral resolution, respectively, is used to simultaneously excite multiple Raman resonances in the sample. Due to SRS, modulations appear in the spectrum of the transmitted broad-bandwidth pulse, which are read out using a photodiode array detector. Unlike SRS imaging, it is difficult to integrate phase-sensitive lock-in detection with a multiplex detector in order to directly retrieve the Raman spectrum from these modulations. Instead, two consecutive spectra, i.e., one with the narrow-bandwidth picosecond beam present and one with that beam blocked, are recorded. Their ratio allows the computation of the linear Raman spectrum that can readily be interpreted in a quantitative manner [49]. Unlike the spectral analysis of a multiplex CARS spectrum, no retrieval of hidden phase information is required to obtain the spontaneous Raman response in multiplex SRS microspectroscopy. 

Thus, using very sophisticated spectral resolving techniques, one can obtain important structural information on coal. Hence, multiple pulse/multidimensional spectroscopy offers an exciting new analytical tool for the study of complex materials such as coal and coal macerals. 

Figure 1. Multiple Quantum NMR pulse. Growth curves are obtained by incrementing either the interpulse delays or the number of multiple pulse excitation trains in the conversion and reconversion sequences. (Reproduced from reference 13. Copyright 2005 American Chemical Society.)...

If coq coi, the degree of excitation depends on coi, coq and on the molecular orientation in the applied field. In a powdered sample, nuclei in equivalent sites but different crystallites may be excited differently, resulting in distorted powder patterns and in intensities that are not proportional to the number of nuclear spins corresponding to the different sites. The evolution of the system is not periodic and in this case it is possible to excite multiple-quantum coherences with a single pulse. The rf excitation becomes less de-... 

---