Since H₃S was discovered to have high T_c superconductivity (203 K at 155 GPa), high pressure phases of other chalcogen hydrides (e.g. H_nSe, H_nTe) exist only in theoretical researches, let alone their high-temperature superconductivity^[1-3]. Regarding experimental researches, both hydrogen selenide and (H₂Se)₂H₂ dissociate to constituent elements, selenium and hydrogen, above 24 GPa^[4]. In this research, through a series of experiments in diamond anvil cells, we have synthesis an unpredicted molecular compound H₂Se(H₂)₂. X-ray diffraction reveals the structure to adopt a beta-Sn type structure with space group I4₁/amd. Combining Raman spectroscopy and density functional theory calculations, we find the structure to contain both H₂Se molecular units and quasi-molecular H₂. Unlike molecules gradually dissociating in hydrogen-sulfur system^[5,6], dense selenium-hydrogen system exhibit a unique molecular dissociation-recombination process. Transport measurements indicate that H₂Se(H₂)₂ is an electrical insulator at megabar pressure.

[1] A. P. Drozdov, M. I. Eremets, I. A. Troyan, V. Kseno-fontov, and S. I. Shylin, Conventional superconductivity at 203 kelvin at high pressures in the sulfur hydride system, Nature 525, 73 (2015)
[2] S. Zhang, Y. Wang, J. Zhang, H. Liu, X. Zhong, H.-F Song, G. Yang, L. Zhang, and Y. Ma, enPhase Diagram
and High-Temperature Superconductivity of Compressed Selenium Hydrides, Scientific Reports 5, 15433 (2015)
[3] Zhong, X.; Wang, H.; Zhang, J.; Liu, H.; Zhang, S.; Song, H.-F.; Yang, G.; Zhang, L.; Ma, Y. Tellurium hydrides at high pressures: High-temperature superconductors. Phys. Rev. Lett. 2016, 116, 057002.
[4] Pace, E. J.; Binns, J.; Peña Alvarez, M.; Dalladay-Simpson, P.; Gregoryanz, E.; Howie, R. T. Synthesis and stability of hydrogen selenide compounds at high pressure. J. Chem. Phys. 2017, 147, 184303.
[5] Pace, E. J.; Liu, X.-D.; Dalladay-Simpson, P.; Binns, J.; Peña-Alvarez, M.; Attfield, J. P.; Howie, R. T.; Gregoryanz, E. Properties and phase diagram of (H2S)2H 2. Phys. Rev. B: Condens. Matter Mater. Phys. 2020, 101, 174511.
[6] Duan, D.; Liu, Y.; Tian, F.; Li, D.; Huang, X.; Zhao, Z.; Yu, H.; Liu, B.; Tian, W.; Cui, T. Pressure-induced

high pressure,Raman spectroscopy,XRD,hydrogen selenide