Halogen Spectroscopy

The halogens, particularly bromine and iodine are unique elements. They are brightly colored diatomic molecular elements. Since they are colored, they are assured to have some sort of spectral features. Also these two members of the halogen family are the only halogens that can be safely handled. You will, however, want to take precautions with these materials, especially in the vapor state, where they can be choking and quite toxic when inhaled.

Bromine is a brown liquid that evaporates rapidly at room temperature to make a dense red vapor:

All halogens exist natively as diatomic molecules. As molecules, they have molecular orbitals rather than atomic orbitals. The spectra of molecules have wider bands rather than sharp lines as atomic orbital exhibit. Diatomic molecules have a wide series of narrow bands in increasing then decreasing intensity. As bromine vapor is brownish red, one would expect the blue and green regions of the spectrum to be attenuated.

In order to observe the molecular orbital electronic transitions, a vapor cell was made to hold a sample of halogen vapor. The cell was to hold a small but macro sized quantity of each substance, allow the elements to be vaporized have big enough optical windows on the ends, and keep the vapor in for our comfort and safety.

A fifty centimeter length of glass tubing with an outer diameter of nine centimeters was prepared with ground, level ends. A glass plate was glued to one end with 'hot-melt' glue, sealing the end gas tight. The other open end was fitted with a 90 centimeter crystallizing dish that could be held tightly to the glass tube and sealed with clear packing tape. The completed cell was held with a tubing clamp to a stand post. This way it could be positioned in all three dimensions for optimum alignment.

A microscope illuminator was focused through the assembled sample cell onto the slit of a spectroscope. A two megapixel digital camera was used at the eyepiece to record the spectrum. In the picture below, Dr. Lee observes the spectrum of bromine on the LCD of the camera:

Here is the detector end of the spectroscope, clearly showing the exit window of the halogen cell:

This spectrum, clearly illustrating a series of bands from about 550 nm to about 650 nm. In addition, no blue light is observed in the spectrum at all. This makes sense considering the color of the vapor, Here is the spectrum of bromine from (left to right) 535 nm to 725 nm:

Iodine is the next halogen down the periodic table. This element is a purplish grey chunky solid which, like bromine, is volatile - evaporating to a choking vapor. Both bromine and iodine have an odor that is reminiscent of bleach or swimming pool chlorine. Chlorine, of course, is the halogen above bromine on the periodic table.

Since iodine is a solid, and its vapor pressure is much less than bromine's the cell was heated to keep all the iodine from condensing on the glass. Two hotplates were placed under, and in contact with the cell, near the ends. This kept crystals of iodine from growing on the inside of the windows. Heated in this manner, the tube quickly filled with dense iodine vapor. Below, Dr Ong watches an iodine spectrum while the cell and light source are aligned to focus onto the slit of an antique three arm spectroscope:

The iodine spectrum appeared similar to bromine's spectrum, but it appeared to be shifted toward the red end of the spectrum. The bands were observed between 600 and 675 nm, and the spectrum cut off at about 600 nm. The bands appear sharper than were observed in the spectrum of bromine. The picture below is the spectrum iodine vapor from 540 nm on the left to 775 nm on the right:

These elemental molecules provide an excellent way to observe a quantum phenomena as elegant and tiny as molecular orbitals using decidedly low-tech equipment. Presumably chlorine and fluorine will have similar spectra, imaginably shifted further toward the blue for each lighter element. Although colored, these gasses are less optically dense, and would require a longer (and more difficult to align) sample cell. These gases are also much more toxic, reactive and harder to handle than are the heavier halogens. Overheard: "There are good fluorine chemists, but there are no old fluorine chemists."