Download Lab 11: Atomic Spectra

LAB 11
Atomic Spectra and Fluorescence
OBJECTIVES
1. Observe and study the bright line spectra produced by hydrogen in the Balmer
series.
2. Predict the wavelengths of the photons in the Balmer series.
3. Measure the wavelengths and correlate them with photon energies and electron
energy level diagrams.
EQUIPMENT
Diffraction gratings, laser, gas lamps, and spectrometers.
THEORY
The hydrogen atom is of particular theoretical interest because it is the simplest of all
atoms: one electron and one proton. The hydrogen atom is observed to have spectral lines
as a result of photons being emitted by electron transitions between different atomic
energy levels. From solving the three-dimensional Schrodinger’s Equation for the
hydrogen atom, it is found that the energy of an electron with principle quantum number
n is given by:
4
me 1
1
En =
− 2 2 2 =
( −13.6 eV) 2
8e 0 h n
n
When an electron jumps from an excited state (n >1) to a lower energy state, it emits a
photon whose energy is equal to the difference in energy between the two different states.
The wavelengths of these photons (or spectral lines) are given theoretically by
=
λ hc / ∆E .
The Balmer series produces a line spectrum in the visible region when the electron
transitions have a final state n f = 2 as seen below:
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: Light Spectra of Other Gases
As stated above, each element can only emit specific frequencies of light. Astronomers
use this fact to determine what a star is composed of (i.e. H, He, C, …) by observing the
spectrum of light emitted by the star. In this part of the lab, you will play astronomer and
determine what type of gas is contained in each of three gas lamps.
(a) Use a diffraction grating to observe the spectra of each of the three different gas
lamps.
(b) Sketch the spectra for each of the three different gas lamps.
(c) Using the chart of different line spectra, identify what kind of gas is in each of the
three lamps.
Part 3: Hydrogen Gas Spectra
a. Using the hydrogen energy levels, create an Excel table that predicts all of the visible
photon wavelengths for all jumps for the first 5 electron energy jumps. Setup the
columns of n, m, λ(nm), Spectrum, and Color.
b. Which visible wavelengths are observable in the hydrogen atomic spectrum?
c. Use the calibrated blue spectrometers to identify the color and measure the
wavelengths of the observed atomic spectrum of hydrogen.
d. Compare the measured and predicted wavelengths using a percent difference. How
do they compare?
Part 4: Fluorescence
Fluorescence occurs when an atom absorbs light at one frequency but then emits light at
lower frequencies. For example, an atom can absorb a UV photon and jump to a higher
energy state. Rather then jumping directly back to the initial energy state (and emitting
the same energy UV photon), the atom can make several smaller jumps and emit lower
energy photons (such as visible light). This is how black lights work. In this station, you
will observe the light emitted by different fluorescent paints that are illuminated by
different frequency lasers.
(1) Shine the red laser onto each of the three different color fluorescent paints (green, red,
and yellow). Use a diffraction grating to observe the spectra emitted by each paint.
(2) Shine the green laser onto each of the three different color fluorescent paints (green,
red, and yellow). Use a diffraction grating to observe the spectra emitted by each paint.
(3) Shine the blue laser onto each of the three different color fluorescent paints (green,
red, and yellow). Use a diffraction grating to observe the spectra emitted by each paint.
(4) Which color paints does the red laser cause to fluoresce? Which color paints does the
green laser cause to fluoresce? Which color paints does the blue laser cause to
fluoresce?
(5) Can you come up with a general rule for which color laser will cause which colors of
paint to fluoresce? Explain your reasoning in terms of fluorescence and photon energies.