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

Trans-polyacetylene [t-(CH)(x)] possesses twofold ground state degeneracy. Using the Su-Schrieffer-Heeger Hamiltonian, scientists predicted charged solitons to be the primary photoexcitations in t-(CH)(x); this prediction, however, has led to sharp debate. To resolve this saga, we use subpicosecond transient photomodulation spectroscopy in the mid-IR spectral range (0.1-1.5 eV) in neat t-(CH)(x) thin films. We show that odd-parity singlet excitons are the primary photoexcitations in t-(CH)(x), similar to many other nondegenerate pi-conjugated polymers. The exciton transitions are characterized by two photoinduced absorption (PA) bands at 0.38 and 0.6 eV, and an associated photoluminescence band at similar to 1.5 eV having similar polarization memory. The primary excitons undergo internal conversion within similar to 100 fs to an even parity (dark) singlet exciton with a PA band at similar to 1.4 eV. We also find ultrafast photogeneration of charge polarons when pumping deep into the polymer continuum band, which are characterized by two other PA bands in the mid-IR and associated photoinduced IR vibrational modes.