Pressure dependence of high order harmonic generation in nitrogen molecule and atmospheric air

Dependence of the variation of the gas pressure on the high harmonic generation (HHG) from nitrogen molecule (N2) and atmospheric air using ultrashort intense laser pulses is measured. The optimum pressure point for generation of maximum harmonic signal is found for both sample. Enhancement and extension of the HH orders are observed at around optimum pressure value. A theoretical calculation based on one-dimensional model is used to explain this effect.