Oxygen-containing rare-earth metal hydride, YH_xO_y, is a newly found photochromic material showing fast photoresponse. While its preparation method, optical properties and structural features have been studied extensively, the photochromic mechanism in YH_xO_y remains unknown. Here, using excited-state molecular dynamics simulation based on the recently developed real-time time-dependent density functional theory (RT-TDDFT) method, we study the photochemical reactions in YH_xO_y. We find that under photoexcitation, dihydrogen defects are formed within 100 fs. The dihydrogen defect behaves as a shallow donor and renders the material strongly n-type doped, which could be responsible for the photochromic effect observed in YH_xO_y. We also find that oxygen concentration affects the metastability of the dihydrogen species, meaning that the energy barrier for the dihydrogen to dissociate is related to the oxygen concentration. The highest barrier of 0.28 eV is found in our model with O/Y=1:8. If the oxygen concentration is too low, the dihydrogen will quickly dissociate when the excitation is turned off. If the oxygen concentration is too high, the dihydrogen dissociates even when the excitation is still on.

含氧稀土金属氢化物YH _x O _y是一种新发现的光致变色材料，显示出快速的光响应。尽管已经对其制备方法，光学性质和结构特征进行了广泛研究，但是在YH _x O _y中的光致变色机理仍然未知。在这里，使用基于最新开发的实时时变密度泛函理论（RT-TDDFT）方法的激发态分子动力学模拟，我们研究了YH _x O _y中的光化学反应。我们发现在光激发下，在100 fs内会形成二氢缺陷。二氢缺陷表现为浅的供体，使材料强烈地被n型掺杂，这可能是在YH _x O _y中观察到的光致变色效应的原因。我们还发现氧浓度会影响二氢物种的亚稳性，这意味着二氢解离的能垒与氧浓度有关。在我们的模型中，O / Y = 1：8的最高势垒为0.28 eV。如果氧浓度太低，则在关闭激励时，二氢会迅速离解。如果氧浓度太高，即使激发仍在进行，二氢也会离解。