Ultrafast Electronic Relaxation and Vibrational Cooling Dynamics of Au144(SC2H4Ph)60 Nanocluster Probed by Transient Mid-IR Spectroscopy

Energy relaxation dynamics of a gold nanocluster with atomically precise composition, Au-144(SC2H4Ph)(60), is studied by transient mid-IR spectroscopy. The experiment is designed to simultaneously probe electronic and vibrational dynamics by using excitation at 652 nm to prepare an electronic state localized on the gold core (as shown by high level DFT calculations) and by probing a stretching vibration localized on phenyl ring of the ligand molecules. We found that electronic relaxation proceeds with a time constant of 1.5 ps simultaneously heating the phonon bath of the cluster. The heat is further dissipated to solvent with a time constant of 29 Ps. The electronic relaxation time increases with increasing pump power. Absence of long-lived electronic states and power dependence of relaxation time indicate metallic behavior. The metal-ligand interface modes are strongly anharmonically coupled to the probed mode which provides connection between the cluster core temperature and the vibrational shift of the ligand molecules. The obtained results are relevant for understanding energy relaxation dynamics of nanoclusters and together with the measured absolute molar absorption coefficient of the duster allow designing experiments for controlled heating of the cluster by continuous wave irradiation.