# Rotationally Resolved Electronic Spectroscopy of UO2

## TL;DR

The paper presents detailed electronic spectroscopy of UO2, revealing its ground state configuration and vibrational properties.

## Contribution

The study provides new insights into the electronic structure and vibrational modes of gaseous UO2 using rotationally resolved spectroscopy.

## Key findings

- The ground state of UO2 is confirmed as X3Φ2u with vibrational fundamentals at 853 cm⁻¹ and 133 cm⁻¹.
- Fluorescence decay lifetimes of excited states range from 1.5 to 4.7 μs, unusually long for dipole-allowed transitions.
- Long lifetimes are attributed to mixing with vibronic dark states.

## Abstract

Rotationally resolved
electronic spectra have been recorded for
gas-phase UO2. Analysis of the rotational fine structure
confirms that the ground state is X3Φ2u, derived from the O2–U4+(5f7s)­O2– electronic configuration. All transitions observed
below 14500 cm–1 were assigned to the [11.51]­3g ↔ X3Φ2u transition, which
correlates with the metal-centered 5f7p ↔ 5f7s electron promotion.
The symmetric stretch and bending vibrational modes were active in
the spectrum. Ground-state vibrational fundamentals of 853(5) and
133(5) cm–1 were determined along with a vibrationally
averaged bond length of 1.781 Å. Spectra for the [17.68]­4g ↔ X3Φ3u transition exhibited
the same lower state vibrational frequencies, confirming that the
X3Φ3u state also arises from the 5f7s
configuration. Fluorescence decay lifetimes for the excited states
were in the range of 1.5–4.7 μs, values that were anomalously
long for electric dipole-allowed transitions. The long lifetimes are
attributed to extensive mixing with a dense background of vibronic
dark states.