3) Experimental aspects of spectroscopy (20 points, 5 each) a)In the ESR experim

Question

3)         Experimental aspects of spectroscopy (20 points, 5 each)

a)In the ESR experiment, you adjusted a thumbwheel and micrometer drive attached to the microwave cavity. Explain what each of these did and why it was necessary to adjust them to achieve good sensitivity.

b)In the pyrene experiment, the slit widths on the spectrometer were preset. If this were not the case and you were writing the lab manual, however, what experimental procedure would you suggest to students for choosing them optimally?

c)What properties of graphite made it a good choice for the STM experiment?

d)Characterize the ESR and pyrene experiments in terms of whether they are essentially null experiments or not. Explain your answer in a sentence or two.

Explanation / Answer

(a) In the ESR expt. the sample is placed inside a glass tube within a micrometer cavity between a pair of electromagnet. The sample is placed in such a way that it should be within the field in order to maximize the effect. The RF oscillator genertaes the microwave to an outside wall of the cavity through a hole. The detector detects the fraction of the incident RF power which gets relected inside the cavity. Now the the frequency of the cavity should match with the oscillator frequency by adjusting the cavity terminator arrangement. Here a micrometer drive (probe) is used to reduce the power reflected in the cavity. Thumbwheel knob is used to make proper phase in RF field of the position of the probe and minimize the reflection. This arrangement reduce once again the reflected signal at the detector. Therefore, maximum power transfers into the cavity with minimum reflection.

c) Graphite is a good choice for STM expt. because it consists of single atom, 'Carbon' and it has very good crystalline nature which is known. Therefore, the image can be easily observed due to higly periodic nature of graphite.

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