Temperature Dependence of the Vibrational Wave-Packet Dynamics of Cu ₅

Vibrational wave-packet dynamics on the electronic ground state of the neutral copper pentamer (Cu₅) are studied by femtosecond (fs) pump-probe spectroscopy using the 'negative ion to neutral to positive ion' excitation scheme (NeNePo). A vibrational wave packet is prepared on the electronic ground state (²A₁) of Cu₅ via photodetachment of a mass-selected, cryogenically cooled Cu₅⁻ anion using the first fs pump pulse. The temporal evolution of the vibrational wave packet is then probed by a second ultrafast probe pulse via resonant multiphoton ionization to Cu₅⁺. A frequency analysis of the femtosecond NeNePo transients for pump-probe delay times from 0.2 to 20.0 ps reveals two primary beating frequencies at 148 and 108 cm⁻¹ as well as weak and transient frequency features at 222, 216, 76, and 40 cm⁻¹. A comparison of experimentally obtained beating frequencies to the harmonic frequencies of normal modes obtained from quantum chemistry calculations confirms that Cu₅ in the gas phase adopts a planar trapezoidal geometry. NeNePo transients measured at ion-trap temperatures from 20 to 270 K probe the influence of the ion temperature on the wave-packet dynamics obtained. The inverse correlation between the oscillation lifetime τ1/2 and the square root of the temperature indicates a vibrational decoherence channel originating from the anharmonicity of high-energy vibrational levels.