Monolayer transition-metal dichalcogenides (TMDCs) show a wealth of exciton physics. Here, we report the existence of a new excitonic species, the high-lying exciton (HX), in single-layer WSe₂ with an energy of ~3.4 eV, almost twice the band-edge A-exciton energy, with a linewidth as narrow as 5.8 meV. The HX is populated through momentum-selective optical excitation in the K-valleys and is identified in upconverted photoluminescence (UPL) in the UV spectral region. Strong electron-phonon coupling results in a cascaded phonon progression with equidistant peaks in the luminescence spectrum, resolvable to ninth order. Ab initio GW-BSE calculations with full electron-hole correlations explain HX formation and unmask the admixture of upper conduction-band states to this complex many-body excitation. These calculations suggest that the HX is comprised of electrons of negative mass. The coincidence of such high-lying excitonic species at around twice the energy of band-edge excitons rationalizes the excitonic quantum-interference phenomenon recently discovered in optical second-harmonic generation (SHG) and explains the efficient Auger-like annihilation of band-edge excitons.

单层过渡金属二硫属化物 (TMDC) 显示出丰富的激子物理学。在这里，我们报告了在单层 WSe _{2 中}存在一种新的激子物质，即高位激子（HX），其能量约为 3.4 eV，几乎是带边 A 激子能量的两倍，线宽为窄至 5.8 meV。HX 通过K谷中的动量选择性光学激发填充，并在紫外光谱区域的上转换光致发光 (UPL) 中识别。强电子-声子耦合导致级联声子级联在发光光谱中具有等距峰，可解析为九阶。从头算GW-具有完整电子-空穴相关性的 BSE 计算解释了 HX 的形成，并揭示了上导带态与这种复杂的多体激发的混合。这些计算表明 HX 由负质量的电子组成。这种高位激子物种的巧合大约是带边激子能量的两倍，使最近在光学二次谐波产生 (SHG) 中发现的激子量子干涉现象合理化，并解释了带边激子的有效俄歇式湮灭.