Vibrational Energy Transfer Between CO Molecules on a NaCl(100) Surface Studied by Infrared Fluorescence Spectroscopy

Abstract

The breaking of chemical bonds in surface reactions is inherently connected to highly excited molecular vibrations. Therefore, understanding the vibrational energy transfer dynamics of adsorbed molecules, which effectively determine the lifetime of vibrational excitation, is of great importance. CO adsorbed on NaCl(100) is possibly the best studied physisorbed molecule. Despite that, most previous experiments have focused on the vibrational ground state and the 𝑣 = 0 → 1 transition of CO-mainly because dispersed fluorescence from high vibrational states could not be observed with conventional infrared detectors. In this thesis, I thus investigate the vibrational energy transfer dynamics of CO on NaCl(100) in highly vibrationally excited states up to 𝑣 = 30.Dispersed and time-resolved laser-induced fluorescence (LIF) is used to observe the vibrational dynamics. For this, an improved version of a recently developed mid-infrared emission spectrometer based on superconducting nanowire single-photon detectors (SNSPDs) is used. The current setup is capable of detecting infrared fluorescence from a single adsorbate layer with spectral and temporal resolution of 7 nm and ∼1 µs, respectively. High vibrational states, CO(𝑣), are prepared by pulsed infrared laser excitation of CO to 𝑣 = 1 at cryogenic temperatures around 7 K. Subsequent vibrational energy pooling (VEP), driven by the anharmonicity of the CO oscillators, concentrates many vibrational quanta in single molecules via vibration-to-vibration (V-V) energy transfer from the surrounding molecules: CO(n) + CO(m) → CO(n+1) + CO(m -1).Kinetic Monte Carlo simulations of the vibrational dynamics in a 13 C 18 O monolayer show that VEP proceeds via a sequential mechanism, in which adsorbates in high vibrational states are further excited by collecting vibrational quanta from molecules in lower vibrational states over increasingly large distances and timescales, up to 100 µs. The shape of the phonon spectrum, to which the excess energy in the V-V transfer processes is dissipated, causes a distinct peak structure in the vibrational state distribution. Furthermore, dissipation to transverse phonons that involve Na atom motion in the surface plane is found to be most effective. Vibrational relaxation to the NaCl substrate is slower than VEP and occurs on the millisecond time scale for 𝑣 ≤ 23. The 𝑣-dependent relaxation rates can be explained by a classical electrodynamic mechanism, whereby energy is transferred non-radiatively to the absorbing NaCl medium via the near-field of the oscillating CO dipole. This finding is in strong contrast to the dominating mechanism for more strongly bound adsorbates, where energy is dissipated via anharmonic couplings between the CO vibration and the surface phonons.The improved resolution of the emission spectrometer revealed a previously unknown metastable O-down orientation (Na + -OC), which is formed from the stable C-down v First and foremost, I would like to express my deepest gratitude to my supervisor Alec Wodtke for getting the chance...