Absorption Spectra

A spectrum with dark lines in it is called an absorption spectrum. To help understand how absorption spectra occur we will use the Spectroscopy Lab Suite computer software.

Drag the figure of the hydrogen gas tube to the gas lamp socket. Figure 3 illustrates what should appear on the screen.

Figure 3: A screen from the Spectroscopy Absorption Computer Program

Instructions

• The dark (absorption) lines appear in the spectrum at the top of the screen.
• To duplicate this spectrum, create and move energy levels and then put in the transitions.
• An absorption transition, unlike an emission transition (which we constructed in the previous activity) is created by starting with the lowest energy level (largest negative energy value) and ending with the highest energy level (smallest negative energy value).
• Notice that the arrows that represent the allowed transitions point upward. The arrows point upward to represent that the hydrogen gas absorbs these energies emitted by the light source. As a result, electrons acquire enough energy (equal to the difference of the allowed total energies) to move up to the higher allowed energies.

Make a sketch of the energy levels and transitions for a hydrogen absorption spectrum. Then repeat the process for two other gases of your choice.

Once you have completed your diagrams compare them.
How are the diagrams similar?
How are they different?

Now, compare these energy levels and transitions to those of the emission spectrum for hydrogen that you created in the previous tutorial.

You may like to return to the Emission program to check.

How are the emission and absorption spectrum different?

Now, use energy considerations to describe how an atom absorbs a photon from a light source containing all visible energies.

Suppose that we create a beam of light that has photons of energies only in region
of 2.0 to 2.4 eV. That beam passes through a tube with hydrogen. Would you see dark lines in the resulting spectrum? Why or why not?

Suppose this light (2.0 - 2.4 eV) passes through nitrogen. Would the resulting spectrum have dark lines? Why or why not? (The nitrogen spectrum is shown in Figure 2 or on page 5-2 of VQM Activities.)