Absorption Spectra of Electrified Hydrogen Molecules

Molecular hydrogen normally has only weak, quadrupole transitions between its rovibrational states, but in a static electric field it acquires a dipole moment and a set of allowed transitions. Here we use published ab initio calculations of the static electrical response tensors of the H 2 molecule...

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Bibliographic Details
Published in:The Astrophysical journal Vol. 932; no. 1; pp. 4 - 29
Main Author: Walker, Mark A.
Format: Journal Article
Language:English
Published: Philadelphia The American Astronomical Society 01.06.2022
IOP Publishing
Subjects:
Absorption
Absorption spectra
Angular momentum
Astronomical data
Astrophysics
Diffuse interstellar bands
Dipole moments
Eigenvectors
Electric fields
Electrification
Hydrogen
Infrared spectra
Internal conversion
Interstellar chemistry
Interstellar dust
Interstellar dust extinction
Interstellar line absorption
Interstellar matter
Interstellar medium
Molecular physics
Molecular spectroscopy
Near infrared radiation
Quadrupoles
Tensors
ISSN:0004-637X, 1538-4357
Online Access:Get full text
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Summary:Molecular hydrogen normally has only weak, quadrupole transitions between its rovibrational states, but in a static electric field it acquires a dipole moment and a set of allowed transitions. Here we use published ab initio calculations of the static electrical response tensors of the H 2 molecule to construct the perturbed rovibrational eigensystem and its ground state absorptions. We restrict attention to two simple field configurations that are relevant to condensed hydrogen molecules in the interstellar medium (ISM): a uniform electric field and the field of a pointlike charge. The energy eigenstates are mixtures of vibrational and angular momentum eigenstates so there are many transitions that satisfy the dipole selection rules. We find that mixing is strongest among the states with high vibrational excitation, leading to hundreds of absorption lines across the optical and near-infrared. These spectra are very different from that of the field-free molecule, so if they appeared in astronomical data they would be difficult to assign. Furthermore, in a condensed environment the excited states likely have short lifetimes to internal conversion, giving the absorption lines a diffuse appearance. We therefore suggest electrified H 2 as a possible carrier of the diffuse interstellar bands (DIBs). We further argue that in principle it may be possible to account for all of the DIBs with this one carrier. However, despite electrification, the transitions are not very strong and a large column of condensed H 2 would be required, making it difficult to reconcile this possibility with our current understanding of the ISM.
Bibliography:AAS33764
Interstellar Matter and the Local Universe
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ISSN:0004-637X
1538-4357
DOI:10.3847/1538-4357/ac68ee