Absolute zero and vibrating atoms

[kjamha]

I am trying to get a better understanding of what it means for an atom to vibrate. Let's say there is a chunk of iron in deep space that starts out with a temperature of 50 degrees celsius and is rapidly losing thermal energy. I will make the following assumptions:

1. The iron has thermal energy because atoms in the solid are wiggling and bumping into each other which results with electrons in a higher energy state. (is wiggling and bumping correct? is there a lot more to the "vibrating")

2. As electrons return to a lower energy state photons are emitted and the iron loses energy.

3. Almost all of the electrons in the iron atoms have returned to their lowest possible energy level and the temperature of the iron chunk is near absolute zero.

Thank you for any contributions you may have.

 

[gadong]

The important point to note is that the vibrations of atoms in a solid or molecule do not cease at absolute zero. Put another way, atoms vibrate in their ground state, no matter how cool the sample.

Looking at individual atoms (in molecular dynamics simulations), one sees that their vibrations consist of oscillatory excursions to/from/around their ideal lattice site with a period on the order of 0.1 ps (the actual behaviour depends on the material). In this sense, the word "wiggle" seems to be a reasonable description (but not "bump" which suggests a collision). However, you need to recognize that the wiggles of neighbouring atoms are connected ("correlated') to a high degree since the wiggles are caused by interactions with neighbouring atoms. So while all of the atoms are wiggling, they are are not doing so randomly (as in Brownian motion) but in a kind of orchestrated fashion.

Edit: It is worth mentioning that a typical vibration in a metal at room temperature involves displacements of ca. 0.01 nm, whereas the interatomic spacings are much larger at ca. 0.3 nm.

 

[marcusl]

Eventually all materials are in equilibrium with their surrounding. If your 50C iron is placed in a 0C freezer, it will lose energy until its temperature is also 0C. At this point equilibrium is established and equal amounts of energy are exchanged from the iron to its surroundings and vice versa. It can never approach absolute zero so long as it is in communication with surroundings that have finite temperature.

 

[MikeGomez]

2. As electrons return to a lower energy state photons are emitted and the iron loses energy.

I’m a bit confused here. Doesn’t thermal energy have to do with the motion of the atom (or molecule) as a whole, rather than the energy state of the electron? The motion of the constituent parts of an atom such as the electron are of course part of this overall motion, but it seems that the discussion of electrons getting bumped into a higher energy states has more to do with the subject of photon emission/absorption than the subject of thermal radiation (although there may be some overlap between the two concepts).

It has been my understanding that thermal radiation is due to the acceleration of the charged particles of the atoms, and that this radiation can and does occur even when the electrons of an atom remain in the same energy state. Is this correct?

 

[Khashishi]

Either picture is ok. The thermal energy is divided among all the degrees of freedom of the stuff, including vibration and rotation of the molecules. But molecules are built up of nuclei and electrons, and the electrons generally carry most of the kinetic energy since they are lighter. It's not unreasonable to refer to an atomic orbital as an electron orbital (even though the electron and nuclei are both moving in sync, the electron moves much more), and energy levels as electron energy levels.

Thermal radiation is probably not a useful term or concept, unless you are talking about blackbody radiation. Thermal radiation isn't any particular kind of radiation. It has more to do with the spectrum of the emission than the source. The source of any particular photon could be any process. When there are enough of them, and the source is thick enough, then the thermal statistical properties appear.

 

[Khashishi]

Molecules continue to vibrate at absolute 0. This is known as zero point energy. Vibration is some deviation from some equilibrium distance, and the spring motion is approximately a harmonic oscillator. The energy in this spring motion goes something like $(1/2 + n) \hbar \omega$, where n is some vibrational quantum number, which is a positive integer. Even at the lowest value of n, the energy is not zero.