Increased Excited State Metallicity in Neutral Cr2On Clusters (n < 5) Upon Sequential Oxidation

Abstract

Excited state lifetimes of neutral Cr2On (n < 5) clusters were measured using femtosecond pump-probe spectroscopy. Density functional theory calculations reveal that the excited state dynamics are correlated with changes in the cluster’s electronic structure with increasing oxidation. Upon absorption of a UV (400 nm) photon, the clusters exhibit features attributed to three separate relaxation processes. All clusters exhibit similar sub-picosecond lifetimes, attributed to vibrational relaxation. However, the ~30 fs transient signal fraction grows linearly with oxidation, matching the amount of O to Cr charge transfer character of the photoexcitation and highlighting a gradual transition between semiconducting and metallic behavior at the molecular level. A long-lived (>2.5 ps) response is recorded only in clusters with significant d-electron character, suggesting that adiabatic relaxation back to the ground state is efficient in heavily oxidized clusters, due to the presence of terminal O atoms. The simple picture of sequential oxidation of Cr2On reveals a linear variation in the contributions of each relaxation component to the total transient signals, therefore opening possibilities for the design of new molecular spintronic materials.

Tuning the photodynamics of sub-nanometer neutral chromium oxide clusters through sequential oxidation

“…Transient ion signals were fit using a series of exponential decay functions convoluted with a Gaussian function as described in detail previously. 38,[40][41][42] Two decay functions were used to account for a fast signal (τ 1 = 30 fs) and a sub-picosecond signal lifetime (τ 2 ). The instantaneous decay (on the time scale of the laser pulse) is attributed to e-e scattering and is shown to be related to the LMCT, or excitation from the O-2p orbitals to the Cr-3d orbitals.…”

“…We previously reported the transient signals of Cr 2 O n (n < 5) clusters collected using identical experimental conditions. 38 Photoexcitation with a 3. Although the oxidation state in Cr 2 O n clusters changes by integer values with sequential addition of each O atom, the change in oxidation state of each Cr with each O in larger clusters is smaller (non-integer values).…”

“…[30][31][32] Only a few measurements have determined the properties of chromium oxide clusters containing several metal atoms. 8,12,[33][34][35][36][37] Our recent work on the transient dynamics of Cr 2 O n (n < 5) clusters revealed an increased metallic-like relaxation mechanism with higher O content, 38 related to the percent ligand-tometal charge-transfer (LMCT) character of the photoexcitation. 38 However, larger chromium oxides need to be examined to elucidate the connection between excited state lifetimes and electronic transitions as chromium oxides grow in size to explore their potential for material applications.…”

“…8,12,[33][34][35][36][37] Our recent work on the transient dynamics of Cr 2 O n (n < 5) clusters revealed an increased metallic-like relaxation mechanism with higher O content, 38 related to the percent ligand-tometal charge-transfer (LMCT) character of the photoexcitation. 38 However, larger chromium oxides need to be examined to elucidate the connection between excited state lifetimes and electronic transitions as chromium oxides grow in size to explore their potential for material applications. Here, we report the transient dynamics of CrO n , Cr 3 O n and Cr 4 O n clusters and show how their behavior is nearly consistent with increases in cluster size.…”