Ultrafast processes in photochromic material YH x O y studied by excited-state density functional theory simulation

Oxygen-containing rare-earth metal hydride, YH x O 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 x O 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 x O 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 x O 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.