The time-dependent response of transient current to incident laser pulses in molecular junctions is an important method to obtain the information about molecular structures and excited-state dynamics. In this work, the transient charge transport is studied theoretically through a model polyacetylene molecular junction driven by Gaussian-type femtosecond laser pulses. The molecule is described by the extended Su-Schrieffer-Heeger model, which explicitly includes electron-phonon interactions and involves both electronic and lattice degrees of freedom. The transient transport dynamics are calculated by combining the non-equilibrium Green’s function formalism with the hierarchical equations of motion, enabling a fully non-adiabatic description of the coupled electron-lattice evolution.

The results show that the central frequency of the incident laser pulse is one of the key factors affecting the transient current response. When electrons resonate with the optical field, the current amplitude is significantly enhanced, and the temporal profile becomes asynchronous with the laser field, indicating strong non-linear response. The corresponding current spectra exhibit broadened main peaks accompanied by multiple sidebands, suggesting the coexistence of various frequency components due to dynamic coupling between electrons and lattice vibrations.

Further analysis of the evolution of instantaneous energy levels demonstrates that, under resonant excitation, electrons are efficiently excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The excited electrons induce lattice relaxation through electron–phonon coupling, resulting in local structural distortion and the formation of self-trapped excitonic states. These excitonic effects lead to additional energy transfer channels, thus amplifying the current response and broadening the frequency spectrum.

In contrast, when the lattice motion is artificially frozen, both the current amplitude and frequency broadening are greatly suppressed, and only a single sharp spectral peak corresponding to the laser frequency is observed. This comparison clearly demonstrates that electron-phonon coupling is a key factor governing the transient transport behavior in molecular junctions under optical excitation.

This study reveals the microscopic mechanism of light-induced transient transport in organic molecular junctions and highlights the essential role of lattice dynamics in modulating non-equilibrium charge transfer. These findings provide theoretical guidance for designing novel optoelectronic molecular devices and contribute to the fundamental understanding of non-adiabatic transport processes in low-dimensional quantum systems.