Excited states and their relaxation dynamics in trans-polyacetylene.

Abstract

We investigate excited states in polyacetylene, taking account of electron correlation by transforming the electronic degrees of freedom into a quantum phase variable via boson representation. When the lattice is fixed at the ground-state configuration, there are the following species of excitations: soliton ${S}^{e}$, antisoliton S${\ifmmode\bar\else\textasciimacron\fi{}}^{e}$, the first breather ${B}_{1}^{e}$, and the second breather ${B}_{2}^{e}$. ${S}^{e}$ should not be confused with the usual neutral soliton ${S}^{e\mathrm{\ensuremath{-}}l}$; in the former, the amount of phase change is 2\ensuremath{\pi} and the lattice is in the uniformly dimerized state, while in the latter the amount of phase change is \ensuremath{\pi} and the lattice has a usual soliton structure. ${S}^{e}$, ${B}_{1}^{e}$, and S${\ifmmode\bar\else\textasciimacron\fi{}}^{e}$ are three components of the triplet exciton, and ${B}_{2}^{e}$ is the singlet dipole-forbidden exciton which corresponds to the 2 $^{1}A_{g}$ state in finite polyenes. The electronic structure of ${B}_{2}^{e}$ can be regarded as a bound state of ${S}^{e\mathrm{\ensuremath{-}}l}$'s. We study the dynamics of the lattice relaxation from ${B}_{2}^{e}$ by applying a semiclassical approximation to the boson system. Since the lattice is driven by the electronic structure of ${B}_{2}^{e}$, a neutral soliton pair or their bound state is formed as a relaxation product.