Nonlinear optical spectroscopy of excited states in disubstituted polyacetylene

Abstract

We used a variety of nonlinear optical (NLO) spectroscopies for studying the excited states order and primary photoexcitations in disubstituted polyacetylene (DPA). The NLO spectroscopies include ultrafast pump-probe photomodulation in a broad spectral range of 0.2\char21{}2.5 eV, two-photon absorption in the range of 3.2\char21{}4.2 eV, and electroabsorption (EA) in the range of 2.3\char21{}4.1 eV. For completeness we also measured the linear-absorption and photoluminescence spectra. We found that the primary photoexcitations in DPA are singlet excitons with three photoinduced absorption (PA) bands, ${\text{PA}}_{0}$ at 0.6 eV, ${\text{PA}}_{1}$ at 1 eV, and ${\text{PA}}_{2}$ at 1.6 eV, respectively, and a stimulated emission band at $\ensuremath{\sim}2.5\text{ }\text{eV}$. This is in contrast to other $\ensuremath{\pi}$-conjugated polymers where the singlet excitons are characterized by two PA bands and a stimulated emission (SE) band. We conclude that in addition to the four essential excited states known to determine the NLO spectra of $\ensuremath{\pi}$-conjugated polymers, namely, $1{A}_{g}$, $1{B}_{u}$, $\text{m}{A}_{g}$, and $\text{n}{B}_{u}$; in DPA there is another important complex of states at $\ensuremath{\sim}3.3\text{ }\text{eV}$ that contains almost degenerate states with odd and even parity symmetries (dubbed here transverse excited state or TES) that is probably reminiscent of the $2{A}_{g}$ state in unsubstituted trans-polyacetylene, which lies in between the $1{B}_{u}$ and $\text{m}{A}_{g}$ of which dipole moment has a strong component perpendicular to the chain direction. In support for this model we found that the depolarization ratio of the EA spectrum drastically changes around the TES from a value of $\ensuremath{\sim}2$ for $\ensuremath{\hbar}\ensuremath{\omega}l3\text{ }\text{eV}$ to $\ensuremath{\sim}0.5$ for $\ensuremath{\hbar}\ensuremath{\omega}g3.3\text{ }\text{eV}$.