Gaussian pulse calibration

The Gaussian pulse is truncated at 2.5% in order to keep its duration finite without introducing distortions in the profile its half-height full width is 43.5% of its total duration. The flip angle is calibrated to 270° in order to use the self-refocussing properties of this pulse [15]. 

We use common sense to find the correct power level We know we want lower power, and for Bruker that means a larger number. So we add this to 3 dB to get a power setting of 58.4 dB. As this power level corresponds to the maximum power of the Gaussian shaped pulse, we can set this power level for our shaped pulse and get a 180° rotation. This would be the starting point for the pulse calibration. 

The excitation profile of soft pulses is defined by the duration of the pulse, these two factors sharing an inverse proportionality. More precisely, pulse shapes have associated with them a dimensionless bandwidth factor, which is the product of the pulse duration. At, and its effective excitation bandwidth, Af for a correctly calibrated pulse. This is fixed for any given pulse envelope and represents its time efficiency. It is used to estimate the required pulse duration for a desired effective bandwidth Table 10.3 summarises these factors for some common pulse envelopes. Thus, an excitation bandwidth of 100 Hz requires a 21-ms 90° Gaussian pulse but a 49-ms EBURP2 pulse clearly the Gaussian pulse is more time-efficient but has a poorer excitation profile. 

Universal pulses act equally on any initial magnetisation state whereas excitation and inversion pulses are designed to act on longitudinal magnetisation only. The bandwidth factor is the product of the pulse duration. At, and the excitation bandwidth, Af, which is here defined as the excitation window over which the pulse is at least 70% effective (net pulse amplitude within 3 dB of the maximum other publications may define this value for higher levels and so quote smaller bandwidth factors). Use this factor to estimate the appropriate pulse duration for the desired bandwidth. The attenuation factor is used for approximate power calibration and represents the amount by which the transmitter output should be increased over that of a soft rectangular pulse of equal duration. The Gaussian based profiles are truncated at the 1% level. 

The volumes of product S atoms sampled by the ionization detector in the pulsed beam mode and in the gas mode are different. The determination of the intersection volume of the ionization laser beam and the CH3SCH3 beam, as well as that of the ionization laser beam and the CS molecules in the gas cell, is necessary for the calibration. The ionization dye laser beam profile is again assumed to conform with the Gaussian profile. The size of the CH3SCH3 beam is determined by the skimmer opening and the nozzle-skimmer distance. 

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