Vanadium dioxide (VO₂) exhibits an insulator-to-metal transition accompanied by a structural transition near room temperature. This transition can be triggered by an ultrafast laser pulse. Exotic transient states, such as a metallic state without structural transition, were also proposed. These unique characteristics let VO₂ have great potential in thermal switchable devices and photonic applications. Although great efforts have been made, the atomic pathway during the photoinduced phase transition is still not clear. Here, we synthesize freestanding quasi-single-crystal VO₂ films and examine their photoinduced structural phase transition with mega-electron-volt ultrafast electron diffraction. Leveraging the high signal-to-noise ratio and high temporal resolution, we observe that the disappearance of vanadium dimers and zigzag chains does not coincide with the transformation of crystal symmetry. After photoexcitation, the initial structure is strongly modified within 200 femtoseconds, resulting in a transient monoclinic structure without vanadium dimers and zigzag chains. Then, it continues to evolve to the final tetragonal structure in approximately 5 picoseconds. In addition, only one laser fluence threshold instead of two thresholds suggested in polycrystalline samples is observed in our quasi-single-crystal samples. Our findings provide essential information for a comprehensive understanding of the photoinduced ultrafast phase transition in VO₂.

二氧化钒（VO ₂ ）在室温附近表现出绝缘体到金属的转变并伴随着结构转变。这种转变可以由超快激光脉冲触发。还提出了奇异的瞬态，例如没有结构转变的金属态。这些独特的特性让VO ₂在热开关器件和光子应用中具有巨大的潜力。尽管人们付出了巨大的努力，但光诱导相变过程中的原子路径仍然不清楚。在这里，我们合成了独立式准单晶VO ₂薄膜，并通过兆电子伏特超快电子衍射检查了其光致结构相变。利用高信噪比和高时间分辨率，我们观察到钒二聚体和锯齿链的消失与晶体对称性的转变并不相符。光激发后，初始结构在 200 飞秒内发生强烈改变，形成没有钒二聚体和锯齿链的瞬态单斜结构。然后，它在大约 5 皮秒内继续演化为最终的四方结构。此外，在我们的准单晶样品中只观察到一个激光注量阈值，而不是多晶样品中建议的两个阈值。我们的研究结果为全面了解 VO ₂中的光致超快相变提供了重要信息。