High harmonics generation from excited states of atomic lithium

We present a calculation of the harmonics yield from the lithium atom exposed to an intense 3.5 νm mid-infrared laser pulse. Our calculation shows that a considerable increase of the yield of high harmonics generation (HHG) can be achieved if initially the atom is prepared in an excited 2p state. We show that this increase can be regarded as a resonant process due to appearance of a multiphoton resonance between the initial state and a quasienergy state. We also consider HHG from lithium prepared initially in a superposition of the ground and excited states.