Anisotropic atomic displacement drives morphological transformation of adjacent ion tracks in polyimide

Deciphering the formation mechanism and fine structures of ion tracks in polymers is crucial for understanding ion-matter interactions and exploring new applications, particularly when they are in proximity. Here, two sequentially generated and closely spaced ion tracks in polyimide are investigated at the atomic scale. We reveal that the occurrence of the first track breaks the cylindrical symmetry of transverse atomic displacement during the second track formation, leading to non-uniformity in track size and deformed shape that significantly deviates from the well-recognized core-halo concentric structure of an isolated track. More interestingly, this anisotropic atomic displacement induces a low-density region between two tracks. This region is fundamentally different from the isolated track cores, where mass loss occurs primarily through radiolytic outgassing. Mechanistic analysis further discloses that these direction-dependent atomic displacement dynamics are driven by the distinct chemical and mechanical responses of the pre-existing track and bulk material to ion energy deposition.