I recently developed a demonstration that I’d like to call “The Chemical Laser Show”. I stumbled upon the procedure as I was trying out an experiment reported in the Journal of Chemical Education.¹ The original experiment involves shining a green laser through iodine vapor (I used a green laser pointer with a wavelength, l,  of 532 nm). When this is done, the laser light appears to shift to a yellow color as it passes through the iodine gas (Figure 1, left). This emission of yellow light is a case of fluorescence, where an electronically excited molecule releases light.

To generate the iodine vapor for the experiment, I simply placed a few crystals in the bottom of a beaker, which I then placed on a hot plate.² The “laser show” unexpectedly occurred when the iodine vapor began to crystallize on the sides of the beaker. The crystals caused the laser beam to scatter, which created a really nice effect (Figure 1, right). The “laser show” was best viewed in a very dark room, whereas the yellow fluorescence was easier to see with the lights on in a dimly lit room.

Figure 1: (Left) Light from a green laser appears to shift to a yellow color when it passes through iodine vapor. (Right) The same experiment when viewed in a very dark room, after iodine crystals have formed on the sides of the beaker.

In the video below you can watch how to set up and carry out this experiment. You can also see a discussion of a few chemical topics that connect to this particular experiment.

Video 1: A Chemical Laser Show, Tommy Technetium YouTube Channel, 10/26/19.

In what follows I’ll discuss some of the chemical concepts associated with this experiment. First, there are quite a few phase changes going on. The production of the purple iodine vapor is a striking visual of gas formation from a solid. This phase change is due to the fact that solid iodine has a substantial vapor pressure (~0.1 atm)³ at temperatures near 100^oC:

I_2 (s) à I_{2 (g)}               (Equation 1)

Your students may be surprised to learn that solids have vapor pressures, just like liquids! A full treatment of what is occurring when iodine forms a gas upon heating is beyond the scope of this article, but see reference 4 for an interesting, in-depth discussion – phase diagrams included.

The reverse phase change, gas to solid, is also observed in this experiment:

I_2 (g) à I_2 (s)                (Equation 2)

The crystals on the sides of the beaker are necessary for the creation of the “laser show” because laser light reflecting off these crystals creates this effect. Iodine crystallization on the sides of the beaker can be explained by noting that the hot vapor loses kinetic energy as it strikes the sides of the cooler glass. As a result, the vapor cools and becomes a solid.

The green laser beam inducing yellow fluorescence as it passes through the vapor provides several inroads for discussing quantum chemistry. This process is initiated by green photons that have energy that corresponds to the promotion of ground state I₂(g) to electronically excited I₂*(g) (Figure 2):

I_2 (g) + green photon à I₂*_(g)            (Equation 3)

The color shift from green to yellow occurs because of the following sequence of events: Green photons cause both electronic and vibrational excitations in I_2 (g).^1,5 After absorption of a green photon (green “up” arrow in Figure 2), the excited state iodine loses some vibrational energy through collisions with other iodine molecules (black “down” arrows in Figure 2). After losing vibrational energy, the electronically excited iodine molecule releases a photon to relax back to the ground state (yellow “down” arrow in Figure 2). Because the iodine molecule lost a bit of vibrational energy through collisions, the emitted photon is of lower energy, and different color.  

Figure 2: The energy of a green photon (green arrow) matches the energy difference between ground (I₂) and excited state (I₂*) iodine. After losing vibrational energy (black arrows), excited state iodine releases a photon (yellow arrow) that is lower in energy than the green photon which caused excitation.  

It is generally the case during fluorescence that the emitted photon is of lower energy than the photon which caused excitation. A quantitative description of the color shift can be made using the following equation:

E = hc/l                      (Equation 4)

Where E is the energy of a photon, h is Planck’s constant (6.626 x 10^-34 J s) and c is the speed of light in a vacuum (3.0 x 10⁸ m s^-1). Insertion of the appropriate values into Equation 4 for the wavelength of the green laser light (532 nm), and that of the emitted fluorescent yellow light is (550 nm),⁶ yields 3.7 x 10^-19 J and 3.6 x 10^-19 J for a green and yellow photon, respectively.

I also shined light from violet (l = 405 nm) and red (l = 650 nm) laser pointers through the iodine vapor. Interestingly, the violet laser caused no fluorescence (Figure 3, left), but the red laser did. Because of this, no light was visible from the violet laser as it passed through the vapor (Figure 3). 

Figure 3: Iodine vapor excited with (L to R) a violet laser (405 nm), a green laser (532 nm), and a red laser (650 nm.

The absence of fluorescence upon excitation of the vapor with violet light is probably because violet photons have enough energy (almost 5 x 10^-19 J using Equation 4) to not only electronically excite I_2 (g), but also to cause the excited iodine to dissociate. In essence, the violet photos "break apart" the iodine molecules:

I_2 (g) + violet photon à I* _(g) + I _(g)            (Equation 5)

I was very surprised to see that the red laser caused fluorescence (Figure 3, right). However, after doing a bit of looking through the literature I found that it is well known that red light can cause fluorescence in iodine vapor.^6,7 This can occur because vibrationally excited iodine molecules do not require as much energy to excite electronically (Figure 4).

Figure 4: A red photon (red “up” arrow) has sufficient energy to excite a ground I₂ molecule that has substantial vibrational energy. After losing vibrational energy (black arrow), the excited state iodine releases a photon (red “down” arrow) that is lower in energy than the photon that caused excitation. In this case, both wavelengths appear red.

I don’t know about you, but I found this to be a fascinating experiment. I just loved all the colors involved – violet, green, yellow, and red – and the flashing lights made it all the better. I’d love to hear about any additional observations you make if you try this experiment out for yourself. Furthermore, do let me know if you have ideas for other chemistry topics that connect to this demonstration.

Happy experimenting!

Notes and References:

1. Tellinghuisen, Joel, Laser-Induced Fluorescence in Gaseous I₂ Excited with a Green Laser Pointer, Journal of Chemical Education, 2007 84 (2), 336 

2. If you choose to do this experiment for your students, take note that the iodine vapor should be generated in the hood. Contact with solid iodine can irritate and burn the skin and eyes. Inhalation of the vapor can cause severe irritation of the nose, throat, and respiratory system. High expose may result in pulmonary edema may result with Although brief exposure of the eyes to light from laser pointers is not considered hazardous,¹ such exposure should be limited.

3. Baxter, Gregory Paul, Hickey, Charles Hendee, and Holmes, Walter Chapin, The Vapor Pressure of Iodine, Journal of the American Chemical Society, 1907 29 (2), 127-136 

4. Jansen, Michael P., Sublimation of Iodine: The Rise and Fall of a Misconception, Chem13News Magazine, October 2015 

5. Williamson, Charles J., Molecular Iodine Fluorescence Using a Green Helium-Neon Laser, Journal of Chemical Education, 2011, 88, 816–818 

6. Tellinghuisen, Joel, Laser-Induced Molecular Fluorescence, Journal of Chemical Education, 1981, 58, 438 - 441

7. Singh, Sadhu M. and Tellinghuisen, Joel, The visible emission spectrum of iodine, Journal of Molecular Spectroscopy, Volume 47, Issue 3, Sept 1973, 409 - 419

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