  We perform a theoretical investigation of the clamped laser intensity inside
the filament plasma as a function of gas pressure with external focusing.
Unlike the clamped intensity under the selffocusing condition, which is
independent on the gas pressure, the clamped intensity with external focusing
decreases with the gas pressure. Our findings can explain the changes of the
signals of femtosecond-laser-induced 391-nm forward emission and fluorescence
with the nitrogen gas pressure.
