Absorption:
The diode laser can be used to study the fine and hyperfine structure of atoms because it has a tunable wavelength. By adjusting the current of the laser slightly, one can achieve a very small change in the wavelength emitted by the laser. By using the diode laser a small range of wavelengths can be generated prompting excitation to the many possible energy levels present in the hyperfine structure of the rubidium atom. If the wavelength of the laser is such as to provide an electron with the energy required to "jump" to a higher energy state, then the electron will absorb a photon emitted by the laser and will be excited to a higher energy level. As the beam travels through a cell containing many Rb atoms, many atoms absorb photons, becoming excited and the laser beam loses some intensity . This effect can be seen in what is known as an absorption spectrum. (See figure below.)
In the spectrum, the beam is of a certain intensity until it reaches a wavelength that allows electrons to be excited to higher energy states. At this point, the intensity of the beam is lessened somewhat because photons are being absorbed by the rubidium atoms. The peak is broad due to the Doppler effect. The laser can excite atoms for a range of wavelengths. This is possible because atoms traveling with a velocity toward the laser beam see it as having a shorter wavelength or higher frequency than it actually does. In a similar fashion, atoms traveling away from the beam see it as having a longer wavelength than it actually has. For this reason, the beam can be slightly above or below the frequency necessary to excite atoms which have no velocity in the x-direction. What results is a Doppler broadened peak where absorption occurs for a range of frequencies. The beam loses the most intensity at a frequency that is the resonance frequency of the rubidium atoms. This is the point at which the atoms having no velocity in the x-direction inside the cell absorb photons and are excited to higher energy levels. The reason the loss of intensity is so great at this frequency is that a majority of the atoms in the tube have no velocity in the x-direction.
Emission:
Upon returning to lower energy states, electrons release bundles, or quanta, of energy in the form of photons. These photons will be of varying wavelength depending on the transition being made by each electron. The direction of the emitted photons will be random. They will exit the sides of the Rb cell as well as the ends. If we hold a photo diode up to the side of the Rb cell, we can see an emission spectrum on an oscilloscope. An example of an emission spectrum is shown below.
The peaks in the spectrum occur at specific frequencies. The frequency of the emitted photon depends upon the energy released during the transition from the higher to lower energy state. Comparing the example absorption and emission spectra given here, one can see that the peaks occur at the same frequencies for both. The reason is because the energy that is absorbed from the laser beam at a certain frequency can be emitted at a later time.
The absorption spectrum shown above does not tell the whole story when it comes to the hyperfine structure of the rubidium atom. In fact, all that we are seeing are the peaks of the ground state hyperfine structure. These peaks are represented by a transitions from the two levels of the 5s1/2 to one of the levels of the 5p1/2 state. The general transitions are shown below.
In actuality, we can see more than just the general transitions shown above. All of the transitions that we studied in this lab from the 5s1/2 to the 5p1/2 states are shown below. These transitions include the excited state hyperfine structure.
In order to see the absorption spectrum with the excited state hyperfine structure, we can not simply look at the laser beam passing in one direction through the Rb cell. Instead what must happen is we must have a "pump" beam passing through the cell in the opposite direction as the original beam or "probe" beam. The pump beam will excite atoms which have either no velocity in the x-direction or those which have a velocity in the x-direction that opposes the beam. The probe beam on the other hand will excite atoms which have either no velocity in the x-direction or those which have a velocity in the x-direction that is in the same direction as the pump beam. If we look at a simplified energy level diagram, the roles of each beam become more clear.
When the frequency of the pump beam is at a level A it facilitates the transition A for electrons having no velocity in the x-direction. At frequency A, the pump beam can also excite electrons that are moving with a velocity in the x direction towards the beam through transition B. It is able to do this because of the Doppler effect. The moving atoms in the tube see the pump beam as having a higher frequency that it actually does because the atoms are moving toward the beam. Therefore, at frequency A the pump beam excites atoms moving at v = 0 through transition and atoms moving at v = -v1 (negative because the atoms are moving to the left), through transition B.
When the frequency of the pump beam increases to a level B it facilitates the transition B for electrons having no velocity in the x-direction. At frequency B, the pump beam can also excite electrons that are moving with a velocity in the x direction away from the beam through transition A. Again, it is able to do this because of the Doppler effect. The moving atoms in the tube see the pump beam as having a lower frequency that it actually does because the atoms are moving away from the beam. Therefore, at frequency B the pump beam excites atoms moving at v = 0 through transition B and atoms moving at v = v1 (positive because the atoms are moving to the right), through transition A.
The same is true for the probe beam. By the same argument, at frequency A, it can excite atoms with velocities v = 0 and v = v1 to both levels in the 5p1/2 state and at frequency B, it can excite atoms with velocities v = 0 and v = -v1 to both levels in the 5p1/2 state.
The absorption spectrum is generated by the probe beam. If the pump beam is exciting atoms at certain frequencies, then there are not as many atoms available for the probe beam to excite. Therefore, the intensity of the probe beam is greater at the frequencies at which the pump beam has already excited atoms. The result is an absorption spectrum where the hyperfine transitions are visible. The small peaks in the broad envelope shown below are the result of a greater intensity beam caused by the fact that there are less atoms available to the probe beam for excitation. This set-up is called a Lamb Dip apparatus and it allows us to see the hyperfine structure as well as the fine structure of the rubidium atom.
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