| dc.description.abstracteng | 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 13C18O 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 orientation (Na+ – CO) in the highest vibrational states. The well-resolved emission
spectra of the two orientational isomers show characteristic vibrational blue- and
red-shifts relative to the frequency of gas phase CO (+7.6 cm−1 and −9.3 cm−1 for
the fundamental frequencies of the C-down and O-down isomer, respectively). The
distinct frequency shifts are explained and modeled based on the orientation-dependent
electrostatic interactions of the CO molecule with its environment. The O-down isomer
has a comparatively long lifetime for back-conversion to the C-down isomer, which is
estimated between 0.1 and 100 s at 7 K. By adsorption of additional CO overlayers on
top of the monolayer, this lifetime can be made indefinitely long.
For the 13C18O monolayer covered by 100 12C16O overlayers, it is found that the
O-down isomer is most efficiently formed when the overlayer, which can absorb 100
times more photons, is excited. The resulting vibrational excitation transferred from
multilayer to monolayer is found to be 30 times higher than the excitation achieved with
direct excitation of the monolayer. Furthermore, the preferred direction of energy transfer
across the CO monolayer/multilayer interface is investigated for several different isotopic
combinations of 13C18O and 12C16O. Vibrational energy can be efficiently transferred
from the 12C16O to the 13C18O layer, rationalized by the energetic preference that results
from the 100 cm−1 difference in the fundamental vibrational frequencies. Reverse energy
flow from 13C18O to 12C16O is not observed.
In addition, coverage-dependent infrared absorption measurements of CO on NaCl(100)
are used to determine the infrared absorption cross section of the 𝑣 = 0 → 1 transition of
an isolated CO molecule on the NaCl surface without assumptions about its polarizability.
The determined integrated cross section of (2.51 ± 0.08) × 10−17 cm/molecule is 18 %
lower than that of the gas phase molecule but significantly larger than the effective cross
section in the monolayer. The 18 % reduction for the isolated adsorbate is consistent
with previous theoretical work that considered the interaction of CO and the surface
electric field.
The well-resolved vibrational spectra of both orientational isomers make CO on
NaCl(100) an interesting system for future studies on quantum state-resolved isomerization dynamics. In addition, the mechanism for controlled vibrational energy transport
suggests that large amounts of vibrational energy could be transferred from CO to
infrared-active vibrations of more reactive acceptor molecules. It is worth noting that the
phenomena observed in this thesis can mostly be explained by electrostatic interactions
of CO with its environment and they should therefore apply to other vibrationally excited
physisorbed molecules as well. In conclusion, the presented results not only provide a
deeper understanding of the vibrational dynamics in the CO/NaCl(100) system but of
physisorbed molecules in general. | de |