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

Area of Science:

• Physical Chemistry
• Materials Science
• Quantum Chemistry

Background:

• Chromium oxide clusters are crucial in catalysis.
• Understanding their excited-state dynamics is key to optimizing properties.
• Gas-phase cluster studies offer insights into fundamental electronic behavior.

Purpose of the Study:

• To investigate the excited-state lifetimes of sub-nanometer chromium oxide clusters.
• To correlate electronic structure with observed lifetimes using theoretical calculations.
• To explore the influence of size and oxidation state on cluster properties.

Main Methods:

• Gas-phase production of chromium oxide clusters via laser ablation.
• Femtosecond pump-probe spectroscopy for measuring excited-state lifetimes.
• Time-dependent density functional theory (TD-DFT) for electronic structure analysis.

Main Results:

• Excited-state lifetimes of ~30 fs and sub-picosecond observed for clusters with low oxygen content, attributed to electron scattering and carrier relaxation.
• Long-lived (>2 ps) excited states in small clusters and those with low oxidation, linked to terminal Cr-O bonds facilitating relaxation.
• A near-linear increase in the ~30 fs transient signal with oxidation, correlating with O-2p to Cr-3d charge transfer, indicating a semiconducting-to-metallic transition.

Conclusions:

• Excited-state relaxation in chromium oxide clusters is strongly influenced by size and oxidation state.
• Terminal Cr-O bonds are effective in promoting excited-state relaxation.
• Photocatalytic properties of chromium oxides can be tuned by controlling cluster size and oxidation.