Block diagram of the pulsed-ESR system. The instruments used for the timing control and the data acquisition are a data timing generator TEKTORONIX DTG5334, a delay generator Stanford DG535, and an AD converter Acqiris AP240. These are programed and controlled by a personal computer (PC). PC also controls a JEOL spectrometer that concerns cw microwave unit and magnetic field control. A dielectric resonator was composed of alumina ceramic purchased from Nippon Tungsten Co. that installed in a continuous flow-type helium cryostat JANIS Research Co. STVP-200 and matched to the bridge via a Gordon coupler.
Acquisition time required for sampling and accumulating 4096 transient signals examined with repetition rates ranging between and .
Data flow for attributing data to the memory addresses in the phase alternation experiment. The two outputs and from the quadrature IF mixer were digitized and stored in the segment space on the AD converter. During acquisition, the pointer for assigning the segment position of the data to be stored incrementally moves from the first to the segment by receiving every trigger pulse. After receiving trigger, the pointer returns to the first segment and coadds the next series of data, thus completing the accumulation. The result data were transferred to the PC through a PCI bus and then manipulated based upon the phase alternation scheme.
The pulse taken from the TWTA output via a directional coupler and observed by a fast microwave diode detector. Time zero was indicated at the center of the pulse.
(a) Six-pulse sequence for the DQC experiment and the definitions of parameters. (b) The third (P3), the forth (P4), and the fifth (P5) pulses in the six-pulse sequence and the definitions of parameters. (c) Three-pulse sequence for the stimulated echo experiment and the definition of parameters.
Structure of a biradical 1 used as a model compound. Details of synthesis will be published elsewhere.
(a) The dipolar modulation of DQC echoes obtained from biradical 1, and (inset) the DQC echo signal. (b) The fast Fourier transform (FFT) spectrum of the dipolar modulation. The echo amplitude in (a) is obtained by integration of DQC echo signal. The dipolar spectrum was directly obtained by FFT after apodization by the Hanning window. Radical concentration: in toluene-THF (4:3; ) solution, microwave frequency: ; : ; temperature: , the pulse width: ; the microwave power: ; , 64 step phase cycling, 2000 averages; AD converter resolution: , the increment of : ; and repetition time: .
DQC amplitude of TEMPO dissolved in toluene plotted with respect to the pulse-pulse interval . Closed circles are the amplitudes measured by the regular setting of pulse width and pulse-pulse intervals for the third, fourth, and fifth pulses as shown in Fig. 9(a). The solid line is calculated by using the fast linear-prediction method. The open circles are the amplitudes measured by adjusting pulse width and pulse-pulse interval to obtain proper microwave pulses, as described in Fig. 9(b). Incident microwave power: ; microwave frequency: ; : ; sample temperature: , repetition time: ; 100 averages, , , and 64 step phase cycling.
The third (P3), the forth (P4), and the fifth (P5) pulses in the six-pulse sequence taken from the TWTA output via a directional coupler and observed by a fast microwave diode detector. Time zero was indicated at the beginning of the third pulse. The pulse signal was taken from (a) , , and and (b) , , and .
Result of three-pulse stimulated echo experiment on charcoal (a) without and (b) with phase alternation, as listed in Table II. The pulse sequence was shown in Fig. 5(c), where , and was used for the experiment.
The double quantum coherence signal obtained from biradical 1 at . The definition of the pulse sequence is shown in Fig. 5(a) and DQC echo refocuses at . Repetition time: , 64 step phase cycling, 2000 averages, and 50 points along axis.
Assignment of the output channels of the pulse generator.
Phase alternation scheme used to cancel out two-pulse echoes on the stimulated echo experiment.
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