Fig. 4: Summary of vibrational dynamics and electronic processes.

a Selection of calculated vibrational modes and the corresponding atomic motions in ODASn₂I₆. Schematic illustrations of vibrational and electronic dynamics: b 2D ODASnI₄ and c 1D ODASn₂I₆ thin films. FE: free exciton; STE: self-trapped exciton; GS: ground state; SUT: surface trap states. The ultrashort pulse excitation initiates coherent wavepackets on the electronic states via ISRS (oscillation on the ground state) or DECP (oscillation on the excited state). The damping time of each dominant mode is denoted by τ_d, with the mode frequencies labeled M for the 1D system and V for the 2D system. The dominant mode M5 of the 1D system is assigned to the 104 cm⁻¹ mode in the calculations. Dashed double arrows depict oscillatory wavepacket propagation. In the 2D structure, the high-energy excitation populates the hot FE state that undergoes carrier cooling (~210 fs). Some carriers are trapped to form the surface trap state and the second-order recombination, respectively. In the 1D structure, the initial rapid dynamics consist of carrier cooling and self-trapped exciton formation combined with wavepacket dynamics (~480 fs). Electronic relaxation happens between the two STE states. After that, ~60% of excitations decay nonradiatively, while the remaining ~40% contribute to PL emission.