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) Thumbwheel is a switch located in the front panelof the gradient controller which helps to perform image data acquisition with proper resolution and speed.The thumb wheel knob is adjusted to position the probe at the proper phase in the RF field and to select a particular set of measurement parameters to improve the null.As a result, the reflected signal at the detector gets reduced to almost one by tenth of a millivolt.Thumbwheel control also help to choose a particular 2D or 3D image from a pull down menu and corresponding image mode can be set.

The micrometer adjustment is done to ensure that the probe tip doesnot extend into the waveguide (It is adjusted to counter clock wise half inch).The DC detector output can be observed on the oscilloscope.Care should be taken that the probe is not unscrewed completely.Next step is the null adjustment .By tuning the cavity termination adjustment for a null (dip)in the RF output ,the resonant frequency of the cavity is matched with that of the oscillator.Now, we have to adjust the cavity for a minimumTo improve the null, the micrometer probe insertion of the slide screw tuner is adjusted.

b) Optimization of analytical results is by selecting proper slit width. Adjust the width of the collimator slit by adjusting the screw at the slit end of the collimator.If the slit is too narrow, the images are too faint but the resolution is better.If the slit is too wide, the angular position of its center cannot be accurately read but the signal to noise ration will be better.Set the excitation and emission slit dials of the fluorescence spectrophotometer at around 4 to 5 nm since the optimum slit widths are not known.For testing whether the slit width is narrow enough for the required resolution,first run a spectrum , then decrease the slit width and again run the spectrum.The slit widths are adequate if there is no significant change.

c) Graphite is a well know periodic crystal which is composed of only one atom- carbon.Due to this periodical structure, the image can be observed in graphite very easily.So , graphite is a good choice for STM (Scanning Tunneling Microscope),which is an instrument for imaging atomic level surfaces.

d) ESR experiment is an experiment which detects null (dip),when the resonant frequency of the cavity is equal to the frequency of the oscillator.A sharp dip can be observed which helps us to read the frequency of resonance for the cavityfrom the pre caliberated dial.

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