摘要： High-harmonic generation (HHG) is a promising tool for generating XUV and soft X-ray radiations using table-top experimental setup. Whereas the conversion efficiency of HHG is usually low, one alternative for increasing HH yield lies in the use of transition metal plasma as a generating medium, from which several resonantly enhanced harmonic peaks have been observed [1]. In this work, we numerically investigate HHG from transition metal elements. We use state-of-the-art all-electron first-principles simulation methods, TD-CASSCF and TD-ORMAS [2-4], which we have recently developed. The calculated harmonic spectra from Mn and its cation (“active 3p” in Fig. 1) exhibit an enhanced peak at 51 eV, successfully reproducing the experimental observations [5, 6]. The arrows in Fig. 1 mark the cutoff positions (cid:7)(cid:15) of the indicated ionic species predicted by the cutoff law, (cid:7)(cid:15) (cid:20) (cid:8)(cid:16) (cid:17) (cid:5)(cid:2)(cid:4)(cid:6)(cid:9)(cid:16), where (cid:8)(cid:16) and (cid:9)(cid:16) denote the ionization potential and ponderomotive energy, respectively. The extension of the cutoff beyond that corresponding to the starting ionic species indicates a significant production of higher ionic states, i.e., plasma. One of the advantages of our methods is that we can freeze the motion of given orbitals to get physical insights into the underlying mechanism. If we freeze the 3p orbitals, the enhanced peak disappears (“frozen 3p” in Fig. 1). This unambiguously demonstrates that the origin of the peak is the 3p-3d giant resonance, as implied by its position [5], and, thus, the manifestation of multielectron effects. Further, we have analyzed how each of the three 3p-3d transition lines ( (cid:10) (cid:20) (cid:18)(cid:4)(cid:1)(cid:3)(cid:21) contributes to the peak and revealed that they constructively interfere to form the enhanced peak at 51 eV.