Atomic versus molecular plasmas for frequency conversion of laser radiation: stronger harmonics, larger cutoffs, better stability, resonance, and quasi-phase-matching enhancement

High-order harmonic generation in laser-induced plasmas is analyzed from the point of view of presence of either molecular or atomic components. Comparison of boron carbide, carbon, and boron plasmas demonstrates the worsened characteristics of coherent extreme ultraviolet radiation in the case of B₄C (smaller conversion efficiency, lower cutoff of generated harmonics, worsened stability). Similar features, alongside the disappearance of resonance-induced enhancement of harmonics and weakened quasi-phase-matching conditions, were observed in other molecular plasmas (Ag₂S, GaAs, Cr₃C₂) compared with plasmas comprising atomic components (Ag, As, Cr). We discuss the reasons of worsening conditions in the case of high-order harmonic generation in few-atomic molecular species compared to the single-atomic ones.