Ultrafast Transient Spectroscopy of Trans-Polyacetylene in the Midinfrared Spectral Range.

Area of Science:

• Condensed Matter Physics
• Polymer Science
• Spectroscopy

Background:

• Trans-polyacetylene [t-(CH)x] exhibits ground state degeneracy, leading to theoretical predictions of charged solitons as primary photoexcitations.
• This prediction has been a subject of significant scientific debate for decades.

Purpose of the Study:

• To experimentally resolve the debate on the primary photoexcitations in trans-polyacetylene [t-(CH)x].
• To investigate the photophysical processes and identify the nature of excited states in t-(CH)x.

Main Methods:

• Utilized subpicosecond transient photomodulation spectroscopy.
• Employed mid-infrared (mid-IR) spectral range (0.1-1.5 eV) on neat t-(CH)x thin films.

Main Results:

• Identified odd-parity singlet excitons as the primary photoexcitations in t-(CH)x.
• Observed two photoinduced absorption (PA) bands at 0.38 and 0.6 eV, and a photoluminescence band at ~1.5 eV, characteristic of exciton transitions.
• Documented internal conversion of primary excitons to a dark singlet exciton (~1.4 eV PA band) within ~100 fs.
• Observed ultrafast photogeneration of charge polarons with distinct PA bands and photoinduced IR vibrational modes when pumping into the polymer continuum band.

Conclusions:

• The study concludes that odd-parity singlet excitons are the dominant photoexcitations in trans-polyacetylene, challenging previous theoretical models.
• Provides a comprehensive understanding of the excited-state dynamics, including exciton relaxation and polaron formation in t-(CH)x.