  Quantum-mechanical investigation is presented to study single attosecond
pulse generation by gating high-order harmonic emission from H$_2^+$ molecule
in intense laser pulses with time-dependent ellipticity. The high-order
harmonic generation from H$_2^+$ molecule in superposition of a left and a
right-hand circularly polarized Gaussian pulse is studied and the effect of
time duration and carrier-envelope phase of laser field on single attosecond
pulse generation is investigated. Using laser field formed by combination of a
left and a right-hand elliptically polarized Gaussian pulse, the effect of
ellipticity of field on duration and intensity of generated attosecond pulses
is studied. The numerical calculations show that, with the conventional
polarization gating, an intense single attosecond pulse can be isolated from
the pulse train emitted by H$_2^+$ molecule in the laser field.
