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Title: Influence of dark excitons on the electro-absorption spectrum of polyacetylene
This study revisits the electroabsorption (EA) spectrum of polyacetylene, (CH)x thin-films, for both the cis- and trans-isomers, as functions of the electric field strength, isomerization degree, and light polarization states. The EA spectrum of cis-(CH)x reveals an oscillatory feature that follows the Stark shift-related first derivative of the material’s absorption spectrum that contains v(0-1) and v(0-2) sidebands of the excited C=C stretching vibration that agrees well with the Raman scattering (RS) spectrum. In contrast, the EA spectrum of trans-(CH)x does not match the first derivative of the material’s absorption spectrum, and the phonon sideband frequency does not agree with the RS spectrum. In addition, the EA spectrum of trans-(CH)x reveals a band below the first allowed 1Bu exciton. We interpret this feature as due to the electric field activated even-parity dark (forbidden) exciton, namely mAg (m>2), showing that the ‘non-luminescent’ trans-(CH)x is due to the reverse order of the excited states, where a series of dark mAg exciton lies below the allowed 1Bu exciton. This agrees with the unusual phonon sideband in trans-(CH)x absorption, since the excited state attenuation caused by the fast internal conversion from 1Bu to mAg influences the apparent frequency that determines the phonon sideband. Consequently, from the EA and RS spectra we estimate the 1Bu lifetime in trans-(CH)x to be ~30 fs. Moreover, the integrated EA spectrum of trans-(CH)x shows a traditional Huang-Rhys type series with a relaxation parameter, S ~ 0.5. This indicates that the EA spectrum of the trans isomer is also determined by a Stark shift related to the first derivative of the absorption spectrum, but preferentially for the longest chains in the film’s chain lengths distribution. This is due to the N3 response of the non-linear susceptibility, (3) (~EA), dependence on the chain length having N monomers.  more » « less
Award ID(s):
2206653
PAR ID:
10590320
Author(s) / Creator(s):
; ; ; ; ;
Corporate Creator(s):
Editor(s):
NA
Publisher / Repository:
American Physical Society
Date Published:
Journal Name:
Physical Review B
Edition / Version:
1
Volume:
110
Issue:
7
ISSN:
2469-9950
Page Range / eLocation ID:
075150
Subject(s) / Keyword(s):
Polyacetylene electro-absorption excitons
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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