Electronic spectroscopy and ionization potential of UO2 in the gas phase

Abstract

The electronic spectroscopy of UO(2) has been examined using multiphoton ionization with mass-selected detection of the UO(2) (+) ions. Supersonic jet cooling was used to reduce the spectral congestion. Twenty-two vibronic bands of neutral UO(2) were observed in the range from 17,400 to 32,000 cm(-1). These bands originated from the U(5fphi(u)7ssigma(g))O(2) X (3)Phi(2u) and (3)Phi(3u) states. The stronger band systems are attributed to metal-centered 7p<--7s transitions. Threshold ionization measurements were used to determine the ionization potentials of UO and UO(2). These were found to be higher than the values obtained previously from electron impact measurements but in agreement with the results of recent theoretical calculations.

A Fock space coupled cluster study on the electronic structure of the UO2, UO2+, U4+, and U5+ species

“…Gagliardi et al, 4 who had done accurate complete active space second order perturbation theory ͑CASPT2͒ calculations that gave an IP of 6.27 eV, proposed that the experimental data were in error. A new measurement by Han et al 5 using resonantly enhanced multiphoton ionization ͑REMPI͒, gave a value of 6.13 eV, in very good agreement with the theoretical values.…”

“…Gagliardi et al 18 do not disregard this possibility, but point out that this would make it very difficult to interpret the fluorescence and REMPI data of Heaven and co-workers. 5,21 The relative energy of the 5f 2 3 H g state, as calculated by the SFDC-IHFSCCSD is in qualitative disagreement with these results, as the energy gap with the ground state is so large that matrix-induced ground state switching is improbable.…”

“…These values are significantly below the experimental value of 6.13 eV. 5 A possible reason could be the fact that both states are described using the orbitals from the dication, thus biasing the cation, for which orbital relaxation effects should be smaller than the neutral. To test the influence of this common orbital approximation we compared the multireference approach with the single-reference CCSD approach in which both the cation and the neutral system are treated with optimized orbitals.…”

“…If the 3 H g state would be the lowest state in the argon matrix, the next gerade state is expected to lie several thousands of cm −1 higher. Han et al 5 and later Gagliardi et al 4 discussed the difficulties in explaining both experimental findings but could not present definite theoretical or experimental data to settle the issue of matrix-induced ground state swapping.…”

“…In this case there are, however, also complementary experimental data available. Heaven and co-workers carried out electron spectroscopy in gas phase 5,20 and in Ar matrices. 21 These experiments, using the REMPI technique in the gas phase and electronic emission spectroscopy in the matrix, do not indicate a reordering of the states.…”