LAB 12
Atomic Spectra & Fluorescence
OBJECTIVES
1. Observe and explain fluorescence.
2. Predict and measure the atomic spectrum of hydrogen using spectrometers.
EQUIPMENT
Fluorescent tubes, fluorescent samples, hydrogen gas lamps, blue spectrometers.
THEORY
The hydrogen energy levels are defined as En = −E0/n2 (n, m = integers) such that when
an excited electron jumps to a lower energy level, the energy of the emitted photon is
given by
E mn  hfmn 
hc
1 
 1
 E0  2  2  where n  m and
mn
n m 
E0  13.6eV
http://phet.colorado.edu/en/simulation/hydrogen-atom
PROCEDURE
Part 1: The Atomic Spectrum of Fluorescent Lights
a. Look at the fluorescent lights with the blue spectrometers. Write down the
wavelengths of the most prominent lines you can see, along with their colors. (Some
will be smeared so just do the best you can).
b. Using the jpg image of the spectra of different gases, (i) identify the wavelengths you
observed and determine what gas (es) are in the fluorescent bulbs. You might not
find all the lines in the list of gases - if not, discuss where these lines might come
from. (ii) Which ones (color and wavelength) were not found among the list of gases?
Part 2: Hydrogen Atomic Transitions
a. Using the hydrogen energy levels, create an Excel table that predicts all of the visible
photon wavelengths for all electron jumps for the first five energy-levels. Setup the
columns of n, m, (nm), Spectrum, and Color.
b. Which visible wavelength are observable in the hydrogen atomic spectrum?
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c. Use the calibrated blue spectrometers to identify the color and measure the
wavelengths of the observed atomic spectrum of hydrogen. Be sure to adjust the
Variac to the lowest possible voltage setting that gives a nice pink color.
d. Compare the measured and predicted wavelengths using a percent difference. How
do they compare?
Part 3: Fluorescence
Look at the fluorescent samples that are set up. For each sample, describe what range
of incoming wavelengths will cause it to fluoresce. Notice the spectrum of the fluorescent
light from each sample? Are they discrete wavelengths or a range of wavelengths?
Describe and comment.
a. What is Fluorescence? What is Two-photon absorption? Hint: Wikipedia
b. Look at the fluorescent samples that are set up. For each sample, describe what
range of incoming wavelengths will cause it to fluoresce. Notice the spectrum of the
fluorescent light from each sample? Are they discrete wavelengths or a range of
wavelengths? Describe and comment.
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