Rovibrational Characterization of High-Lying Electronic States of Cu₂ by Double-Resonant Nonlinear Spectroscopy

Abstract

The available knowledge of the electronically excited states of the copper dimer is limited. This is common for transition metals, as the high density of states hinders both experimental assignment and computation. In this work, two-color resonant four-wave mixing spectroscopy was applied to neutral Cu in the gas phase. The method yielded accurate positions of individual rovibrational lines in the I-X and J-X electronic systems. This revealed the term symbols for the I and J states as Π (1) and Σ (0), respectively. For the Cu isotopologue, accurate molecular constants were obtained. The characterization of the J state finally allowed decisive determination of its electron configuration. The J state is obtained from the ground state by promotion of a 3dπ electron into the weakly bonding 4pπ molecular orbital. From the data analysis, lifetimes of the I state (between 10 ps and 5 ns) and J state (66 ns) were inferred.

Observation of a gerade symmetry state of Cu2 using two‐color resonant four‐wave mixing

“…Interestingly, this value is roughly equal to the vibrational energy of the aforementioned J0 + u state in the "u"-symmetry manifold with its origin located at 37437.107(95) cm −1 . [15,33] The strong transition moment observed for the [37.5(v)]0 + g ↔ B0 + u transition further suggests significant ion-pair state character (Cu + Cu − ) of the observed "g"-symmetry state similarly to the J0 + u Ass. state.…”

“…The experimental setup has been described in our previous publications. [15,26] In brief, the copper dimer molecules are produced in a cluster source by laser ablation of a copper target disk. Pure helium gas with a static background pressure of 50 bar is introduced into the source by using a pulsed valve (General valve, series 9), which generates a typical gas pulse of ≈ 250 s width.…”

“…By opening the d-shell upon excitation of an electron, a complex electronic configuration is obtained that is typical for transition metals thus providing an unique opportunity to understand the role of the d-orbitals in chemical bonding. [1,2] As such, extensive experimental [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] and theoretical work [19][20][21][22][23][24] have focused on the ground and excited electronic levels of Cu 2 . The electronic ground state has been identified as X 1 Σ + g state in Hund case (a) notation.…”

“…Among these, the A ′ 1 u , A0 + u , B0 + u , G0 + u , F0 + u , I1 u , and J0 + u states were high-resolution investigations studied by resolving the rotational structure of the vibronic bands and determined accurate molecular constants. [4,6,11,12,14,15,18] For excited levels, total angular momentum Ω labeling in Hund case (c) notation is applied, because spin-orbit interaction is not negligible for excited states in Cu 2 . [14] All previous studies have provided much insight into the "u" symmetry electronic structures as well as the d-orbital bonding properties of Cu 2 .…”

“…[14,15,26] Actually, the combination of TC-RFWM and a homemade laser vaporization cluster source yielding internally cold Cu 2 dimers in a molecular beam environment [15] allowed a systematic investigation of the electronic states of dicopper and resulted in the identification of the A ′ 1 u state and the characterization of the I1 u and J0 + u states. [14,15] Furthermore, a recent detailed study of the complex electronic structure of excited "u"-symmetry levels has been performed in this laboratory by applying the same technique. [33] In this work, we report on our method to unravel the "g" symmetry electronic states of Cu 2 .…”