Allowed infrared absorption in simple London crystals

In summary, the conversation discusses the legal absorption of infrared photons by bound liquids and crystals of simple particles, such as atoms and homoatomic diatomic molecules. It is stated that diamond, which is bound by strong covalent bonds, does not have legal one phonon absorption due to its high cubic symmetry. However, two-phonon absorption is possible in diamond, but it is weaker compared to crystals with polar bonds. The conversation then moves on to discuss diprotium, which has a fundamental vibration band but no dipole moment for absorption. In dense gas, however, it is possible for diprotium to absorb infrared photons through collision/pressure induced absorption. The legality of absorption in solid diprotium crystal is questioned, as it
  • #1
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Which infrared photons can London force bound liquids and crystals of simple particles (atoms, and homonuclear diatomic molecules) legally absorb?

For comparison: diamond is bound by strong covalent bonds. The restoring forces permit vibrations about the C-C bonds at about 1000/cm.
But since a diamond has high cubic symmetry, the crystal has no transition dipole moment around a C-C bond, and single phonon absorption is illegal.
Interaction between two phonons breaks the symmetry around the C-C bond, and diamond does have a band for two-phonon absorption about 2000/cm (and another for three phonons). But those bands are much weaker than the bands for crystals that do possesses polar bonds with legal one phonon absorption.
Now, diprotium has a fundamental vibration band around H-H bond at about 4200/cm. But it has no dipole moment, so absorption is illegal.
In dense gas, interaction with other molecules or atoms breaks the symmetry around H-H bond, and does allow the collision/pressure induced absorption at 4200/cm. Logically, so would liquids.
But solid diprotium crystal again possesses symmetry around the H-H bond of individual molecules. Does this likewise make 4200/cm absorption illegal in solid diprotium?
Would the reasoning for H2 apply to other simple molecules - D2, N2, O2, F2?

About atoms: solid noble gases are bound by London forces, yet possesses simple symmetry. Can they legally absorb in multiphonon manners alone, like diamond but at much lower energies?

Liquid noble gases do not possesses "restoring force" - atoms displaced will not oscillate around crystal position. Yet they do support phonons of some sort. And "liquid"-s lack of long range order would avoid issues with symmetry around London "bond".
At which energies does liquid He absorb infrared photons to create phonons? What are the legitimate mechanisms - single or multiple phonon? And how absorptive, really, are liquid noble gases in infrared bands matching the bands of their London forces?
 
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  • #2
Homoatomic diatomic gasses are in deed not IR active. As far as crystals of one atom type are concerned, you should look up terms like optical and acoustical phonon branches.
 
  • #3
DrDu said:
Homoatomic diatomic gasses are in deed not IR active.

As isolated molecules/tenuous gases. Dense gases and liquids should have collision/pressure induced violation of symmetry.

Do simple symmetry crystals (cubic of dihydrogen and dinitrogen) forbid the collision induced absorptions that the liquids have?

Do low symmetry crystals of simple homoatomic diatomic molecules (ortorhombic of iodine, bromine, chlorine and dioxygen, monoclinic of fluorine and also oxygen) permit absorption to the molecule's stretching line?
 

1. What is the concept of allowed infrared absorption in simple London crystals?

The concept of allowed infrared absorption in simple London crystals refers to the phenomenon where certain crystals are able to absorb infrared radiation within a specific range of wavelengths. This absorption occurs due to the vibrations of the crystal lattice, which can be excited by the incoming infrared radiation.

2. How is the allowed infrared absorption in simple London crystals related to their atomic structure?

The allowed infrared absorption in simple London crystals is directly related to the atomic structure of the crystal. The atoms in the crystal are arranged in a specific pattern, which determines the crystal's lattice structure. This lattice structure determines the allowed energy levels and vibrations within the crystal, which in turn affects its ability to absorb infrared radiation.

3. What factors determine the strength of allowed infrared absorption in simple London crystals?

The strength of allowed infrared absorption in simple London crystals depends on several factors, including the crystal's lattice structure, the type and arrangement of atoms, and the amount of impurities present. The strength of absorption can also be influenced by temperature, pressure, and external electric or magnetic fields.

4. How is allowed infrared absorption in simple London crystals used in scientific research?

Allowed infrared absorption in simple London crystals is a crucial aspect of many scientific studies, especially in areas such as materials science, chemical analysis, and spectroscopy. By analyzing the absorption spectra of crystals, researchers can gain insights into the atomic and molecular structure of materials and identify their composition and properties.

5. Are there any practical applications of allowed infrared absorption in simple London crystals?

Yes, there are several practical applications of allowed infrared absorption in simple London crystals. These include the development of advanced materials for use in electronics, telecommunications, and energy harvesting. Infrared absorption in crystals is also utilized in medical imaging and in the detection of chemical compounds, such as pollutants or explosives.

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