Monday, July 19, 2010

Why do solids absorb light?

I used to participate in an online discussion group at until I was banned for life earlier this year. I still sometimes notice some topics that I am motivated to comment on, and today was one such occasion. Since I can't post to the newsgroup, I will use my blogsite to put in my two cents worth.

A "newbie" using the name of "Infrasound" posted a question about absorption of light by solids. It's a very good question. Basically he notes that the traditional explanation of exitation of electron orbitals fails to explain where the light actually goes. Every electron that is excited to a higher energy level must sooner or later decay back to the ground state. It's a non-dissipative process which cannot absorb energy. So where does the light go?

Infrasound was quickly put in his place by a several veteran posters including the cryptically named cthugha and alxm, and of course the omniscient ZapperZ. Basically they tell Infrasound to get a life before wasting their time with such simple questions whose answer is to be found in any first-year text. It is not, they say, the atomic energy levels which are excited: those are too high to be susceptible to optical or infrared; it is rather the collective energy levels, which can be much closer spaced.

Infrasound rightly points out that this answer merely evades the question without answering the fundamental point about absorption. As long as the modes are driven by a specific frequency of light, they will give off the same frequency when they decay back to the ground state. So the colors are scattered but never absorbed. This objection is met by scoffing on the part of the above-mentioned smug guardians of truth who dominate the forum.

I'm going to propose an answer to this question, which I don't believe I've seen anywhere else. The mechanism I'm going to propose is most closely related to Compton scattering; and recall that in Compton scattering, the scattered light has a lower frequency than the incident light. So there is actual transfer of energy from light to matter.

When people hear Compton scattering they thing of free electrons being impacted by photons. That's not what I'm talking about. Most people thing that the Compton effect is conclusive proof of the particle nature of light, but in fact back in 1927 Schroedinger showed that the Compton effect can be explained purerly as a wave-on-wave interaction between classical e-m radiation and standing waves of electrons. The feature that characterises this interaction is that the light waves and matter waves interact when they have exactly the same wavelength. This is very different from the semi-classical explanation for the photoelectric effect, which is based on light and matter waves interacting at a shared frequency. Schroedinger's explanation was scoffed at and marginalized by the dominant Copenhagen group at the time, and today hardly anyone remembers it. But its mechanism is essential to a full understanding of how light interacts with matter.

In almost any solid, when the atomic lattice goes into vibration, there is a net displacement of electric charge. It is almost impossible for the positive charge lattice (the nuclei) to vibrate exactly in unison with the sea of negative charge (the electrons), so it is almost inevitable that sheets of charge density will appear along with the vibrations. It is these sheets of charge, at the exact separation of the wavelength of light, which must interact strongly with the light whose wavelength they share. The exact mechanism is difficult to explain in a few words, but the overall effect is that the light is absorbed and converted into mechanical energy of vibration.

My personal discovery of Schroedinger's wave explanation for the Compton effect is a long story which I'm going to have to save for another blogpost.