A theoretical study of the high-order harmonics of a 200 nm laser from H₂ and HeH⁺

Abstract

Using a time-dependent quantum mechanical method, based on the evolution of the electron density in real time, a theoretical study is made of the high harmonic generation in H₂ and HeH⁺, corresponding to a 200 nm laser. For H₂, four laser intensities in the range 10¹³-10¹⁶ W cm^-2 are employed while only one intensity ( ≈ 10¹⁶ W cm^-2) is employed for HeH⁺. Up to the 45th order (4.44 nm) harmonic for H₂ and the 53rd order (3.77 nm) harmonic in HeH⁺ are predicted. For H₂, increase in laser intensity I does not change the structure of the high harmonic spectrum but increases the yield of each harmonic according to a fractional law of I^1.5. Polar molecules/molecular ions might serve as excellent sources for higher harmonics, the yield and order of the harmonic increasing with increasing mass, ionization potential and polarizability.