Ortho-para mixing hyperfine interaction in the H2O+ ion and nuclear spin equilibration

Area of Science:

• Theoretical chemistry
• Astrochemistry
• Quantum mechanics

Background:

• Ortho and para states of molecules like H2O(+) differ in nuclear spin orientation.
• Interconversion between these states is typically a slow,

Purpose of the Study:

• To theoretically investigate the ortho to para conversion mechanism in H2O(+).
• To calculate the spontaneous emission lifetime between ortho and para levels of H2O(+).
• To assess the implications of this conversion for interstellar H2O(+) populations.

Main Methods:

• Quantum mechanical calculations of electron spin-nuclear spin interaction.
• Analysis of mixing terms between ortho (I=1) and para (I=0) states.
• Calculation of spontaneous emission lifetimes for specific transitions.

Main Results:

• Electron spin-proton spin interaction in H2O(+) enhances ortho to para conversion by 4 orders of magnitude compared to neutral H2O.
• Calculated mixing is significant for near-degenerate levels but occurs at high energies (~300 K).
• Low-lying levels show mixing of ~10(-4), resulting in long spontaneous emission lifetimes (520-5200 years).

Conclusions:

• The enhanced ortho to para conversion in H2O(+) due to unpaired electron interactions is theoretically significant.
• This process may explain anomalous ortho-para ratios observed in interstellar H2O(+).
• The conversion is unlikely to affect thermalization of interstellar H2O(+) under typical conditions.