User:Brian P. Josey/Notebook/Junior Lab/2010/09/27

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Steve Koch 02:46, 21 December 2010 (EST): As always, excellent primary lab notebook!

Neon

This week we are doing the excitation and ionization levels of Neon. The purpose of this experiment is to demonstrate the energy quantization of atoms. To do this, we bombarded neon atoms with electrons generated by a power supply. By varying the potential of the electrons bombarding the neon, we were able to measure the current across the neon and plot the two sets of data against each other. From these plots we were able to determine the ionization points for neon.

Equipment and Set Up

General Set Up
General Set Up
Close up on picoamplifier
Close up on picoamplifier
Close up on power supplies
Close up on power supplies

This is all of the equipment that we used in the experiment:

  • The tube containing the Neon, and a wire ring
  • Alarm Meter
  • Picoamplifier
  • SOAR Corporation DC Power Supply 7403
  • Digital Multimeter
  • Battery and Battery Unit
  • Kepco Power Supply

We connected them with wires in the following way:

  • The ground from the Soar power supply was input into ground, middle of three connectors, on the picoamplifier
  • Third of three on picoamplifier connected to negative of the battery.
  • Positive of battery connected to positive on kepco power supply.
  • Also from positive on kepco power supply:
    • Red input on multimeter
    • A1 on neon tube stand
  • Kepco Negative:
    • f3 on stand
    • black input on multimeter
  • From stand
    • F3 to possitive on soar
    • F4 to C5 on stand
    • negative on soar

These wires were connected based on this illustrations, which is a sharper reproduction of the image in Gold's manual:

This was taken from Tomas Mondragon's notebook. However, please note that F5 is mislabeled, and should read F3.

Procedure

Initially, we set the voltage on the Soar Power Supply to 2.1 V, and the voltage on the Kepco Power Supply to zero. We measured the voltage from the multimeter, and the current from the picoamplifier. When we had set the voltage to zero, we adjusted the picoamplifier, by using the knob on the upper right of the base, to zero. We then steadily increased the voltage in 1V steps up to 25V, and measured the current. At one point, we had to readjust the scale on the picoamplifier by switching the power from -12 to -11, and changing the given value so that the two scales matched. Initially, we had the polarity reversed from where we were supposed to have it. This should have been the final step, used to illustrate the importance of setting up the equipment properly, but we did manage to learn our lesson well. The current as a function of the voltage for the reversed polarity:


We then reversed the polarity on the battery, by switching the wires on the battery stand, and repeated the measurements. First we did a course sweep, with steps of 1V and then a second sweep. For the second sweep, we ran between 15 and 22 V in steps of 0.1 V. Plotting these data points, we generated these two graphs:

The graph on the left is for the course sweep of through the various voltages, and as you can see, there are two sharp peaks that developed between 15 and 22 V. When we did the fine sweep for data, we focused in on this area producing the more detailed plot on the right. To get a second set of data, we then dropped the potential on the Soar Power Supply to 1.8 V, and repeated the data collection, this produced the following plots:

Again, the course graph is given on the left, and the more detailed graph is on the right. Once again, we were able to produce peaks in the range between 15 and 22V, which gave us our range for the second data collection. The primary difference, however, is that the peaks in these two plots are not as sharp as in the initial sweep at a higher voltage. The reason for this is the higher potential creates a greater energy on each of the electrons, so a higher potential will result in more current from the ones that are picked up, creating the sharper peaks.

Here is all of our raw data summarized on a single spreadsheet

View/Edit Spreadsheet

Calculations and Results

We know, from Ritter from fall of '07, that the accepted values of for the two first peaks are given at 16.7eV and 18.65eV, while the ionization energy is 21.56 eV. To find our ionization energies, we found the places in our data, where the current jumped as a result of some given voltage. This voltage is also the energy, in eV that an incident electron would have when striking the neon atom, and the jump in current is the result of the release of energy. Careful combing of the data reveals that we detected two peaks. On the 2.1 V run, these occurred at 16.51 V, and at 19.2V. In the 1.8 V run, the peaks where at 16.49 V, and at 19.71V. This then gives a value of 16.5 ± 0.01 eV, for the first and, 19.46±.3 eV for the second.

The ionization energy is the point at which the current levels out and then begins to drop again. Examining the data reveals that our ionization energies were 21.41 eV for Vf=2.1V and 21.82 eV for Vf=1.8V. This gives a value of 21.6 ± 0.2 eV.

Conclusions

Overall, the experiment was a success as we were able to measure two of the peaks for the excitation level, and the ionization energy level. However, our exact measured values where not perfect. For the first peak, we measured 16.5 ± 0.01 eV, while the accepted value is 16.7 eV. Because of our small standard error of mean, we know that these two values greatly disagree. The most likely cause of this discrepancy is the difficulty of finding the peak in this range as a result of the small differences in the current, especially in the 1.8 V case. The second value of the peak, where we measured 19.46±.3 eV also differs greatly from the accepted value of 18.65 eV. However, in contrast our ionization energy measurement, 21.6 ± 0.2 eV, is so close to the accepted value, 21.56 eV, that it is clear they are in agreement. It is clear there is some difficulty in measuring the peaks in this experiment, but not in the ionization energies.

Acknowledgments and Resources

Again, I want to thank my lab partner for this experiment, Kirstin. For this experiment, we used these following resources:

  • Dr. Gold's manual for the course,
  • Energy levels in helium and neon atoms by electron-impact method By N. Taylor, K.D. Bartle, and D. Mills, a paper in the appendix of Gold's manual. American Journal of Physics link here.
  • Alex Benedict's notebook
  • Joseph Frye's notebook
  • Brian Ritter's notebook

Alex and Joseph were both very helpful as they had just done this experiment before we did, and answered all of our questions, and save us a ton of time by pointing our Tomas' diagram from above. Also, Dr. Koch and Katie the T.A. both helped us out a lot.


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