Maximizing the Response of a Helium Atom at the Third Harmonic of an Intense Femtosecond Ultraviolet Pulse

The optimal regime of three-photon resonant excitation of a helium atom via a femtosecond ultraviolet (UV) pulse was discovered and numerically studied, at which the maximum power of the third harmonic of the UV field is achieved in the spectrum of dipole acceleration (the second time derivative of the induced dipole moment) of the atom. It is shown that the optimal frequency of the UV field nearly coincides with the frequency of the three-photon transition |1s2 & rang;-|1s2p & rang;, taking into account its shift as a result of the dynamic Stark effect, and the intensity of the UV field is dictated by the condition of maximizing the product of the populations of the |1s2 & rang; and |1s2p & rang; states, averaged over the time interval during which the UV field is non-zero. For the considered UV field durations, from 10 to 100 cycles of the carrier frequency (from units to tens of femtoseconds), the optimal intensity lies in the range from 1014 W/cm2 to several units of 1014 W/cm2. It is shown that with an optimal choice of the frequency and intensity of the UV field, the dynamics of excitation of bound and continuum states, as well as the shape of the time envelope of the dipole acceleration of the atom, weakly depend on the duration of the UV field envelope; only their time scale changes significantly. In addition, under optimal conditions, the average power of the third harmonic signal in the dipole acceleration spectrum is practically independent of the duration of the UV field envelope.