Effects of initial electronic state on elliptically polarized attosecond pulses

We investigate the ellipticity dependence of the high-order harmonic generation from molecular hydrogen ions with different initial states (1s(g) and 1s(u)). The ellipticity of the harmonics can be tuned and is more sensitive to the alignment angle with the ground state. The position of the maximum ellipticity shifts with the increase of the alignment angles, which can be illustrated by the two-center interference model and molecular orbital symmetry. The underlying physical mechanisms of the ellipticity can be demonstrated by the phase difference and amplitude difference of the x and y components of the harmonics. In addition, with specific carrier envelope phase of the laser fields, the elliptically polarized attosecond pulse with high intensity and large ellipticity e = 0.77 is synthesized from the first excited state, which can be used as the bright elliptically polarized EUV light source.