Physics307L:People/Le/Notebook/071119

Excitation of Neon

--Linh N Le 15:24, 19 November 2007 (CST)

Purpose

The purpose of this lab is to investigate energy quantization of atoms. We achieve this by inelastically scattering electrons off of Ne atoms. Electrons have to have a certain minimum energy to excite the Ne atom. We can assume this energy is the energy needed to excite the Ne atom. Once an electron with this energy collides with the Ne atom, it losses much of its energy to the excitation of the atom. These low energy electrons are now easily collected with an anode and measured. By adjusting the energy of the electrons, we can find areas of higher collection, that signifies we have reached one of the excitation energies of Ne.

Equipment

• Fluke 111 true rms multimeter
• Hertz Critical Potentials Tube filled with neon
• Picoamplifier and alarmed meter
• 2 HP power supplies

Procedure

For more details see Lab manual pg 45

Circuit of the apparatus

Following the circuit to the right, we connect the probe to 2 voltages (one for the electron heater and another for the accelerating voltage for the electrons). We also connect the probe to a battery (to control polarity) and to the picoamplifier and alarmed meter to measure the small variations in the current.

Set the heater voltage at some value (making sure the heater voltage does not exceed 2.5V) keeping the accelerator voltage to zero. After a suitable voltage has been found, zero the meter, using a zeroing knob on the base of the amplifier. Now we start to increase the voltage across the accelerating plates, watching the current in the probe on the meter.

We want to do a quick "sweep" of the accelerating voltage and look for drastic changes in the current. Once these "peaks" are found, we can start using smaller iterations of the voltage to try to measure them (Note: the alarmed meter can be set to beep when the current gets too low or too high, but that was more annoying then helpful, so we didn't use that feature.) Continue looking for peaks until the current starts to grow unbounded. After this point we have ionized the Ne gas.

Data

(battery - to - and + to +)

Measuring peaks

Vf=2.5V

VA (volts) Ic pA
14.00 -6
14.25 -6
14.50 -6
14.75 -6
15.00 -6
15.25 -6
15.50 -7
15.75 -7
16.00 -6
16.25 -5
VA (Volts) Ic pA
20.00 -3
20.25 -2
20.50 -2
20.75 -2
21.00 -2
21.25 0
21.50 0
21.75 1
22.00 2
22.25 3
22.50 4
22.75 5
23.00 6

[Thanks to Dr. Koch shoving in the tube, we are getting some better numbers]

VA (Volts) Ic pA
15.00 -1.516
15.25 -1.547
15.50 -1.579
15.75 -1.610
16.00 -1.640
16.25 -1.659 X
16.50 -1.620
16.75 -1.651 X
17.00 -1.569
17.25 -1.235
17.50 -1.138
17.75 -1.186
18.00 -1.057
• day 2

(battery + to + and - to -) Vf=2.000V

Va (V) Current (mA)
17.00 -.063
17.25 -.086
17.50 -.140
17.75 -.103
18.00 -.093
18.25 -.119 X
18.50 -.107
18.75 -.102
19.00 -.092
19.25 -.086
20.00 -.093
20.25 -.109
20.50 -.125 X
20.75 -.118
21.00 -.106
21.25 -.118
21.50 -.118
21.75 -.120
22.00 -.121
• continues to grow unbounded after this point

Battery flip (+to- and -to+) Vf 2.00A

Va (V) Current (mA)
17.50 -.015
17.75 ~-.0003
18.00 .028
18.25 .040
18.50 .044
18.60 .051
18.70 .062
18.80 .069
18.90 .075
19.00 .078
19.10 .80
19.20 .082
19.30 .084
19.40 .086
19.50 .088
19.60 .090
19.70 .093
19.80 .096
19.90 .098
20.00 .100
20.10 .102
21.50 .135
21.60 .139
21.70 .142
21.80 .144
21.90 .148
22.00 .151
22.10 .154
22.20 .158
22.30 .163
22.40 .167
22.50 .173
22.60 .181
22.70 .188
22.80 .196
22.90 .205
23.00 .213
• Battery (+ to - and - to +)

Vf=2.476V

Va (V) Current (mA)
17.00 -.071
17.25 -.001
17.30 .026
17.40 .081
17.50 .142
17.60 .180
17.70 .201
17.75 .206
17.80 .210
17.90 .213
18.00 .221
18.10 .255
18.20 .304
18.30 .344
18.40 .374
18.50 .389
18.60 .397
18.70 .406
18.80 .416
18.90 .425
19.00 .437
19.10 .449
19.20 .462
19.30 .473
19.40 .489
19.50 .497
19.60 .506
19.70 .514
19.80 .524
19.90 .545
20.00 .555

Analysis

Sample ionization graph A
Sample ionization graph B

Since the apparatus was not set up properly, we will ignore the first 2 sets of data. As for the second 2, we believe them to be like that of graph A, and the last 2 to be like that of graph B. The reasoning is that the trend for the first 2 was to keep growing, whereas the second 2 started off in the negative region before "flipping" and going positive.

Looking at the 3rd and 4th sets of data, I have marked areas of a "peak" with an 'X'. We found peaks at voltages:16.25, 16.75, 18.25, 20.50. The last 2 sets are much harder to read. We don't see "peaks" but more like stair steps.

After 21.25V, the data grows unbounded, so we conclude that the ionization energy of Ne is approximately 21.25V.

Conclusion

According to a paper included in last year's lab manual (see page 91), the resonance voltages for Ne are: 16.7, 18.65, 19.75, 20.10. Our values are very very close to these reported values.

For a quantitative look at the success of this experiment, let us compare the 2 values. (Note: we have values that the paper did not report, and vice versa. I will only compare those that match)

$%error= \frac{|Actual-Experimental|}{|Actual|}x100$

$%error= \frac{|16.7-16.75|}{|16.7|}x100$

%error = .3

$%error= \frac{|19.75-18.25|}{|19.75|}x100$

%error = 7.59

$%error= \frac{|20.1-20.5|}{|20.1|}x100$

%error = 1.9

now taking the average of our error, Average Error= 3.26%

Sources of Error

• The biggest source of error is the sheer sensitivity of the ammeter. Moving around the apparatus or talking near it changed the readings from the ammeter.
• The need to zero the meter could cause errors. If the zero was not set properly, or for some reason the zero shifted during measurements, it would skew our numbers

Possible Fixes

• If are able to isolate the experiment from any outside noise or movement we can avoid slight deviations
• If we were to take finer iterations in the measurements, the numbers may be more exact.
• Have a system that auto zeroed