Allowed infrared absorption in simple London crystals

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  • Thread starter snorkack
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  • #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 possess 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 possess simple symmetry. Can they legally absorb in multiphonon manners alone, like diamond but at much lower energies?

Liquid noble gases do not possess "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?
 

Answers and Replies

  • #2
DrDu
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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
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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?
 

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