Experimental investigation of the optical spectrum of the
sodium atom
Introduction to atomic physics
Joachim Mortensen, Edin Ikanović, Nadja Frydenlund
June 1, 2009
Introduction to atomic physics
1
J. Mortensen, E. Ikanović, N. Frydenlund
Introduction
The purpose of this excercise is to get a feel for the behaviour of the spectral lines of atoms, when they
are excited under different pressure conditions. We used two sodium lamps with comparatively low or
high pressure, respectively.
2
Measurements
We used the same spectrometer (OP8223) with both lamps, a sketch of which is attached to these pages.
2.1
Calibration with the low pressure lamp
Using the low pressure lamp, we measured the wavelength of the centre of the yellow douplet (λ = 588.63
nm) moving in on the doublet centre from both the blue and red part of the spectrum 10 times each. This
was done in order to find out, which side allowed us to perform the most accurate reading possible.
We also made sure, we did not touch the spectrometer tube in order to avoid random errors from its
instability. We found the following best estimates of the wavelength and uncertainty of the douplet
centre:
Read off from
hλi/nm
σλ /nm
Red side
Blue side
588,63
588,49
0,21
0,17
Table 1: The best estimate (average) of the centre of the yellow duplet used for calibration of the spectrometer.
The readings from the blue side of the spectrum have the lowest uncertainty, so in the following we only
use measurements read from this side of the spectrum.
2.1.1
Fitting
In order to calibrate the spectrometer, we plotted the line-values of the respective lines from NIST against
the values we read in the spectrometer (fig. 1) and fitted with a parabolic function f (λ) = aλ2 + bλ + c,
resulting in coefficient values:
a = 0.000122073 ± 0.00003846 nm−2
b = 0.865315 ± 0.004264 nm
−1
c = 37.3835 ± 1.175 nm
2.2
(1)
(2)
(3)
Comparing lines of the high pressure lamp with NIST
In the table below is shown the lines that we observed together with a description of the particular
appearence of the different lines.
We notice that the yellow doublet that we observed in the low pressure lamp spectrum now appear as
an absorption line rather than an emmission line. The reason for this is due to the fact that, although most
of the atoms in the lamp are hot enough to emit at higher level energies, atoms closer to the surface of the
lamp move very little about and are mostly neutral and thus more apt to absorption. Also, considering the
residual values of the calibrated wavelengths and those from the NIST website, the calibration function
still seem reasonable in adjusting our measurements for the high pressure lamp.
Experimental investigation of the optical spectrum of the sodium atom
*1
Introduction to atomic physics
J. Mortensen, E. Ikanović, N. Frydenlund
Figure 1: Calibration fit for the low pressure lamp.
Appearence/da
meget svag rød
mellemsvag rød
stærk rød
mellem rød
mellem orange duplet
svag orange
klar gul duplet m. abs.linier
mellem gul-grøn
mellem gul-grøn
mellem-stærk gul-grøn duplet
meget svag grøn
svag grøn
mellem-stærk grøn
meget svag udtværet
mellem-svag udtværet
stærk grøn m. abs.linie
svag grøn-blå duplet
mellem blå udtværet
stærk blå m. abs.linie
stærk blå
λ/nm
λNIST /nm
residual/nm
687.64
682.60
668.91
653.60
615.13
609.70
589.10
579.16
576.88
568.70
558.71
553.11
546.29
540.27
538.56
535.04
515.18
498.26
449.27
434.57
653.070
615.749
589.294
568.820
540.046
515.112
498.281
449.418
434.411
0.53
-0.62
-0.19
-0.12
0.22
0.07
-0.02
-0.15
0.16
LineID
NaII
NaI
NaI
NaI
NaII
-a
NaI
NaI
NaIb
NaII
a Højst
sandsynligt er NaIII grunden til absoptionslinien, mens den grønne linie kan skyldes Hg.
stærke blå lys skyldes overgangen ved 449.087 nm i NaII, mens absorptionslinien skyldes
449.418 nm overgangen i NaI.
b Det
Table 2: Table showing the calibrated values of the observed lines in the high pressure lamp as well as the corresponding lines in the NIST table.
As it appears in the table, we have found a number of linbes that have no matching line in the NIST table
for the sodium atom. These lines must correspond to other elements also present in the sodium lamp. We
are not able to account for all the other lines, but our knowledge of high pressure sodium lamps leads us
to believe that some of the lines might be due to the presence of mercury inside the lamp.
Experimental investigation of the optical spectrum of the sodium atom
*2