Fig. 1. (a) Potential as a function of the distance from the nucleus and (b) the corresponding potential depths as a function of I_p.

Fig. 2. Calculated HHG spectra through numerical simulation of 1D-TDSE, considering potentials given in Fig. 1.

Fig. 3. (a) Recombination amplitudes for all potentials given in Fig. 1. (b) Calculated ionization probabilities corresponding to each system given in Fig. 1, when these are irradiated by an external electric field of fixed laser parameters.

Fig. 4. Electron wavefunction density for systems having (a) I_p = 2.0 a.u. and (b) I_p = 2.5 a.u.

Fig. 5. Time-frequency distribution of the HHG spectra when model potentials given in Fig. 1 are exposed to a single 5 fs/800 nm laser field with a peak intensity of 1.0 × 10¹⁵ W/cm². Long and short quantum trajectories are calculated only for 1.5 cycles for a cosine-like pulse. These laser parameters are kept to be constant for each system, where (a) I_p = 2.0 a.u., (b) I_p = 2.1 a.u., (c) I_p = 2.2 a.u., (d) I_p = 2.3 a.u., (e) I_p = 2.4 a.u., (f) I_p = 2.5 a.u.

Fig. 6. (a) The temporal profile of the attosecond pulses by superposing harmonics from the 130^th to 165^th order for the system with I_p = 2.0 a.u. and (b) the temporal profile of the attosecond pulses by superposing harmonics from the 140^th to 175^th order for the system with I_p = 2.5 a.u.