# I Can you tell which solid a substance is by analyzing the phonons only?

#### lucas_

Can you tell what kind of solids by simply having access to the phonons only?

Supposed you join a challenge where you will use any tools just to analyze the phonons (without telling you what kinds of solids). Can you tell what solids is it?

Is there any geometric relationship between the phonons and the solids producing them so you can tell the identity of the latter by knowing the former only?

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#### DrDu

Well, there are huge data bases of IR and Raman spectra of solids which basically contain the information about the frequencies of the phonons. These data are often sufficient to identify a compound.

#### lucas_

Well, there are huge data bases of IR and Raman spectra of solids which basically contain the information about the frequencies of the phonons. These data are often sufficient to identify a compound.
Ok. But what I'd like to know specifically if whether the shape of the phonon vibrations is related to the shapes or structure of the solid?

#### lucas_

My questions are related to this

"That's not true. See Eqs. (32) and (34) in my paper. Eq. (32) describes a collective excitations of atoms, Eq. (34) describes a wave function of a single phonon, and yet those two wave functions represent the same physical state.

So the wave function of collective excitations of atoms is equivalent to the wave function of a single phonon?

That means you can vary the collective excitations of atoms to come up with the same phonon? Therefore different combinations of atoms can have the same phonon? Example?

#### DrDu

Hm, phonons are the solutions of the nuclear part of the Schrödinger equation in Born-Oppenheimer approximation and using a quadratic expansion of the potential energy surface around the equilibrium position.
So the potential part reduces to the force matrix which depends on the electronic properties of the atoms making up the material.
The kinetic energy part contains the mass matrix of all the atoms in the material which is unique.
In general, the spectrum of the resultign hamiltonian will also be unique, so meaning that there are no two different substances having the same phonon spectrum.

#### lucas_

Hm, phonons are the solutions of the nuclear part of the Schrödinger equation in Born-Oppenheimer approximation and using a quadratic expansion of the potential energy surface around the equilibrium position.
So the potential part reduces to the force matrix which depends on the electronic properties of the atoms making up the material.
The kinetic energy part contains the mass matrix of all the atoms in the material which is unique.
In general, the spectrum of the resultign hamiltonian will also be unique, so meaning that there are no two different substances having the same phonon spectrum.
Ok. Say. What are the different ways the phonon spectrum can be influenced by doing something to the substance. Is it by heating it only, or breaking it that can affect the phonons? Increasing the temperature can change the infrared and visible characteristics which change the phonons, what else can change the phonons?

#### DrDu

I think some of these questions you may answer yourself, e.g. the effect of heating on the phonon spectrum.

#### lucas_

I think some of these questions you may answer yourself, e.g. the effect of heating on the phonon spectrum.
So only Raman and IR spectrometer can detect the phonons? how about thermal imagers. I already have thermal imager. I want to purchase either Raman or IR spectrometer. What is the cheapest one you know available.. at least just for initial learning?

#### Lord Jestocost

Gold Member
2018 Award
There are a number of different experimental techniques which allow to determine phonon dispersion relations experimentally, each focusing on a different part of the vibration spectrum.
The most powerful technique, however, is inelastic neutron scattering which can map the entire dispersion relation even of complex solids. Experimentally, phonon dispersion curves are thus primarily measured using inelastic neutron scattering,
Have a look at:
http://www.chembio.uoguelph.ca/educmat/chm729/Phonons/method.htm

#### lucas_

There are a number of different experimental techniques which allow to determine phonon dispersion relations experimentally, each focusing on a different part of the vibration spectrum.
The most powerful technique, however, is inelastic neutron scattering which can map the entire dispersion relation even of complex solids. Experimentally, phonon dispersion curves are thus primarily measured using inelastic neutron scattering,
Have a look at:
http://www.chembio.uoguelph.ca/educmat/chm729/Phonons/method.htm
What is the longest laser range for a Raman spectroscope? In Mars, they seemed to use long distant on rocks so can you aim one meters away to get a good reading (for an unknown sample)?

"Can you tell which solid a substance is by analyzing the phonons only?"

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