Studio Session 12

Atomic spectra and lasers

In this studio session you will measure the wavelengths of the visible spectral lines in the Balmer series of hydrogen, inspect the spectral lines of helium, neon, and the output of a He-Ne laser, and use a simulation to build a laser.

Equipment needed:

Open a Microsoft Word document to keep a log of your procedures, results and discussions.  This log will become your lab report.  Address the points highlighted in blue.  Answer all questions.

Experiment 1

In quantum mechanics confinement leads to energy quantization.  The energy levels of the electron in a hydrogen atom are quantized.  The allowed energies are

En = -13.6 eV/n2.

When an electron changes from one energy level to another, the energy of the atom must change as well.  This energy can be supplied by a photon whose energy E = hf = hc/λ.

Since the energy levels are quantized, only certain photon wavelengths can be absorbed.  If a photon is absorbed, the electrons will be promoted to higher energy levels and will then fall back down into the lowest energy state (ground state) in a cascade of transitions.  Each time the energy level of the electron changes, a photon will be emitted and the energy (wavelength) of the photon will be characteristic of the energy difference between the initial and final energy levels of the atom in the transition.  The energy of the emitted photon is just the difference between the energy levels of the initial (ni) and final (nf) states.
The set of spectral lines for a given final state nf are generally close together.  The lines for which nf = 2 are called the Balmer series and many of these spectral lines are visible.  You will be measuring the wavelengths of the Balmer series lines.  The photon energies E = hf for the Balmer series lines are given by the formula

hf = -13.6 eV(1/ni2 - ½2) = 13.6 eV(1/4 - 1/ni2).

We may write hc/λ = 13.6 eV(1/4 - 1/ni2), or

1/λ = (13.6 eV/(hc))(1/4 - 1/ni2) = R(1/4 - 1/ni2).

The constant R is called the Rydberg constant.  You will determine the Rydberg constant in this experiment.

imagePlug in and turn on the hydrogen discharge lamp.  Hydrogen gas is excited by a current flowing through the gas.  Look at the light emitted by the excited gas through your spectral glasses.  You will see the line spectrum of hydrogen.


To measure the wavelengths of the spectral lines, connect the "Red Tide" spectrometer to a USB port of your computer. 

Make sure the Pasco 850 interface is turned on.  Open the Capstone program.

You should see 4 peaks in the visible region with very different intensities.  The peaks correspond to the 4 longest wavelength lines of the Balmer series.  From ni = 3, 4, 5, and 6 to nf = 2.  Determine the wavelength of each peak as accurately as possible.  Enter each wavelength in units of nm into a spreadsheet.

ni color λ (nm) 1/λ (nm-1) (1/4 - 1/ni2)
6 v      
5 vb      
4 bg      
3 r      

Exploration 1

Lasers produce highly coherent light via stimulated emission.  A pumping mechanism has to produce a population inversion and a cavity is needed to recycle the photons and increase the probability of stimulated emission.  The class notes introduce you to the He-Ne laser.  Use the spectral glasses to observe the spectrum of He, Ne, the He-Ne mixture in a He-Ne laser, and the output of the He-Ne laser.  Your instructor will also use the "Red Tide" spectrometer and show the spectra on the screens.

Download the spreadsheet He-Ne.xlsx and open it.  Examine plots of the spectra.

The class notes introduce you to a 4-level optical pumping scheme.  In a 4-level scheme, the upper laser level may or may not be metastable.

image3-level optical pumping schemes are also possible, but in a 3-level scheme the upper laser level has to be long-lived or metastable.
In the diagram on the right, a 3-level optical pumping scheme is shown.  Level 3 is the metastable upper laser level.

Use an on-line simulation from the University of Colorado PhET group to build a laser.
Link to the simulation

Explore the interface.

Open the One Atom Panel in the Laser Simulation and start exploring the two-level atom.

Explore the three-level atom.

Switch to the Multi-Atom Panel and build a laser.

You want to produce many identical photons.  Describe what you have to do to achieve this goal.
First try to establish a population inversion.

Now build a laser.

Convert your log into a session report, certify with you signature that you have actively participated, and hand it to your instructor.