Specific pulse excitation, using shaped pulses

The effect of a shaped pulse depends upon both the duration and the rf field intensity of the pulse. Normally the duration of a shaped pulse is adjusted to give the desired excitation range and then the rf field altered to obtain the desired tilt angle. As a rule of thumb the excitation range (selectivity) of shaped pulse in Hz is proportional to the reciprocal of the pulse duration. The RF field profile simulation can be used to study the effect of a shaped pulse and the rf field intensity of the pulse as function of the rf offset. Because of the correlation of pulse duration and tilt angle, the simulation does not accept normalized pulses instead the tilt angles of the individual magnetization vectors are calculated as a function of the rf field intensity of a specific rf offset. 

To be able to understand the effect of a shaped pulse, it would be useful to represent the excitation profile of a shaped pulse as a spectrum. This may be easily achieved in NMR-SIM by using a variable spin system. Since the application of a selective pulse creates a mixture of x- and y-transverse magnetization components over its excitation range, a phase distorted profile would be generated Check it 5.2.2.1(a)). Thus, a specific... 

Photo-excitation of gas-phase ions may result in the photodetachment of an electron rather than photo-fragmentation. Coulombic considerations dictate that this process is more prevalent for anions than for cations. Electron photodetachment action spectroscopy of trapped anions has proved also to be a valuable source of molecular information. In some systems, electron photodetachment and PD compete. The mechanisms for these two processes in large molecules are yet to be understood fully consequently, their branching ratios in specific experimental conditions cannot be predicted as yet. One exciting possibility is the idea of using frequency and phase-shaped pulses to promote selected photochemical pathways. 

MHz frequency as shown. A proton spectrum occurs over a chemical shift range of 10 ppm, which corresponds to 2.5 kHz at 500 MHz. As seen in Figure 3.22, all of the protons in the sample would see 98%-100% of the power of the 500 MHz radiation delivered and all would be excited simultaneously. A pulse programmer is used to control the timing and shape of the RF pulses used to excite the sample. Square wave pulses are commonly used, but multipulse experiments and 2D NMR experiments with other pulse shapes are performed. There are hundreds of pulse sequences and 2D experiments that have been developed, with curious names like attached proton test (APT), DEPT, INEPT, INADEQUATE, COSY, and many more, some of which will be discussed later in the chapter. Each pulse sequence provides specific and unique NMR responses that enable the analyst to sort out the NMR spectrum and deduce the chemical structure of a molecule. 

An analytical theory for the study of CC of radiationless transitions, and in particular, IC leading to dissociation, in molecules possessing overlapping resonances is developed in Ref. [33]. The method is applied to a model diatomic system. In contrast to previous studies, the control of a molecule that is allowed to decay during and after the preparation process is studied. This theory is used to derive the shape of the laser pulse that creates the specific excited wave packet that best enhances or suppresses the radiationless transitions process. The results in Ref. [33] show the importance of resonance overlap in the molecule in order to achieve efficient CC over radiationless transitions via laser excitation. Specifically, resonance overlap is proven to be crucial in order to alter interference contributions to the controlled observable, and hence to achieve efficient CC by varying the phase of the laser field. 

This type of research uses pulsed and tunable la.sers as an excitation source. The rare earth ion is excited selectively with a laser pulse,and its decay is analyzed. The shape of the decay curve is characteristic of the physical processes in the compound under study. For a detailed review the reader is referred to the literature [1-3]. Here we give some results for specific situations. We assume that the object of our study consists of a compound of a rare earth ion S which contains also some ions A which are able to trap the migrating excitation energy of S by SA transfer. 

It is well known from previous experimental [9-12] and theoretical works [13, 21, 24-26] that after excitation to the bright B2u t t ) state, pyrazine undergoes an ultrafast radiationless decay process in a few tens of femtoseconds. This process was directly observed recently by TRPES using sub-20 fs pulses [11, 12]. In this section, we present quantum dynamics simulations of the excited state dynamics of pyrazine triggered by a 14 fs sine-squared shaped laser pulse resonant with the transition from the ground to the B2u Tnr ) state. Specifically, the total Hamiltonian operator reads... 

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