Band-Selective Ballistic Energy Transport in Alkane
Oligomers: Toward Controlling the Transport Speed
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Abstract
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 (ν(NN) and ν(NN)),
carboxylic acid (ν(CO)), and succinimide ester (ν<sub>as</sub>(CO)) 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 Å/ps was observed when
initiated by either ν(CO) or ν<sub>as</sub>(CO).
When initiated by ν(NN) and ν(NN), 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 ν(NN) the CH<sub>2</sub> twisting
and wagging chain bands were identified as the major energy transport
channels. For the transport initiated by ν(CO), the
C–C stretching and CH<sub>2</sub> rocking chain bands served
as major energy transporters. The transport initiated by ν(NN)
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