2 research outputs found
Radiative Enhancement of Linear and Third-Order Vibrational Excitations by an Array of Infrared Plasmonic Antennas
Infrared gold antennas
localize enhanced near fields close to the
metal surface, when excited at the frequency of their plasmon resonance,
and amplify vibrational signals from the nearby molecules. We study
the dependence of the signal enhancement on the thickness of a polymer
film containing vibrational chromophores, deposited on the antenna
array, using linear (FTIR) and third-order femtosecond vibrational
spectroscopy (transient absorption and 2DIR). Our results show that
for a film thickness beyond only a few nanometers the near-field interaction
is not sufficient to account for the magnitude of the observed signal,
which nevertheless has a clear Fano line shape, suggesting a radiative
origin of the molecule–plasmon interaction. The mutual radiative
damping of plasmonic and molecular transitions leads to the spectroscopic
signal of a molecular vibrational excitation to be enhanced by up
to a factor of 50 in the case of linear spectroscopy and over 2000
in the case of third-order spectroscopy. A qualitative explanation
for the observed effect is given by the extended coupled oscillators
model, which takes into account both near-field and radiative interactions
between the plasmonic and molecular transitions
Band-Selective Ballistic Energy Transport in Alkane Oligomers: Toward Controlling the Transport Speed
Intramolecular
transport of vibrational energy in two series of
oligomers featuring alkane chains of various length was studied by
relaxation-assisted two-dimensional infrared spectroscopy. The transport
was initiated by exciting various end-group modes (tags) such as different
modes of the azido (νÂ(Nî—¼N) and νÂ(Nî—»N)),
carboxylic acid (νÂ(Cî—»O)), and succinimide ester (ν<sub>as</sub>(Cî—»O)) with short mid-IR laser pulses. It is shown
that the transport via alkane chains is ballistic and the transport
speed is dependent on the type of the tag mode that initiates the
transport. The transport speed of 8.0 Ã…/ps was observed when
initiated by either νÂ(Cî—»O) or ν<sub>as</sub>(Cî—»O).
When initiated by νÂ(Nî—¼N) and νÂ(Nî—»N), the
transport speed of 14.4 ± 2 and 11 ± 4 Å/ps was observed.
Analysis of the vibrational relaxation channels of different tags,
combined with the results for the group velocity evaluation, permits
identification of the chain bands predominantly contributing to the
transport for different cases of the transport initiation. For the
transport initiated by νÂ(Nî—¼N) the CH<sub>2</sub> twisting
and wagging chain bands were identified as the major energy transport
channels. For the transport initiated by νÂ(Cî—»O), the
C–C stretching and CH<sub>2</sub> rocking chain bands served
as major energy transporters. The transport initiated by νÂ(Nî—»N)
results in direct formation of the wave packet within the CH<sub>2</sub> twisting and wagging chain bands. These developments can aid in
designing molecular systems featuring faster and more controllable
energy transport in molecules