Short magnetic

Figure 1 shows the gas phase fractional abundance of certain species included in the model as a function of time. It can be seen from the diagram that there is a very steep fall in the abundances as accretion dominates. The effect is exaggerated over that determined for quiescent clouds because, in a free-fall collapse, the density rises rapidly in the latter stages, and the accretion timescale, proportional to 1/n, becomes very short. Magnetic fields and turbulence caused by rotation may slow the collapse. We have modelled cases in which the collapse is slowed by factors of 10 and 100 and found no qualitative difference from the results presented here. As already mentioned, the inclusion of accretion means that... 

The sample is again subjected to a constant magnetic field but all the nuclei are excited by a very short radio frequency pulse. The frequency e (e.g., 400 MHz for a proton at 9.4 tesla) is applied over a period of several... 

The magnetic regulators allow to synthesize in one module of the X-ray apparatus main cir-euit commutator, form converter, noncontact smooth ac voltage amplitude regulator, para-metrie stabilizer, ac supply filter, fimetional protection against the short-circuit in the X-ray tube and protection against emergencies in the control circuits. 

The original method employed was to scan eitiier the frequency of the exciting oscillator or to scan the applied magnetic field until resonant absorption occiined. Flowever, compared to simultaneous excitation of a wide range of frequencies by a short RF pulse, the scanned approach is a very time-inefficient way of recording the spectrum. Flence, with the advent of computers that could be dedicated to spectrometers and efficient Fourier transfomi (FT) algoritluns, pulsed FT NMR became the nomial mode of operation. 

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