Narrow Results

High-order harmonic spectra and attosecond pulse generation from Rydberg atom (He${}^{+}$) driven by the spatially inhomogeneous field have been theoretically investigated. (i) Firstly, with an electron initially in a single excited Rydberg state (nth), the harmonic yield can be enhanced due to the decreased ionization potential, and a maximum enhancement can be obtained when the initial state is prepared as the third excited state (n = 3). However, the low cutoff energy from the excited state is unbeneficial to the generation of the higher photon pulse. Thus, with the further introduction of the laser chirp, not only the harmonic cutoff is extended, but also the harmonic modulation is reduced. As a result, five super-bandwidths from 63 eV to 267 eV can be found. (ii) Secondly, by preparing the initial state as a coherent superposition of excited state, the harmonic yield can be further enhanced, especially for the coherent superposition of the first and the third (n = 1 + 3) and the second and the fourth (n = 2 + 4) excited states, the harmonic yield is enhanced by 4–8 orders of magnitude compared with the case of the single ground initial state. Furthermore, by properly adding the laser pulse into the spatially inhomogeneous region (gap center $x_0=0.0$ a.u.) from left $(x_0<0.0\mathrm{\text{a.u.}})$ to right $(x_0>0.0\mathrm{\text{a.u.}})$, much higher cutoff energies can be obtained in the left region. As a consequence, two super-bandwidths of 248 eV and 496 eV can be obtained. Finally, by properly superposing the harmonics, a series of sub-25-as pulses with intensity enhancement of 5–8 orders of magnitude can be produced.

A potential scheme to produce the water window single attosecond pulses (SAPs) from the multi-cycle mid-infrared (MIR) laser field has been investigated with the help of the chirp gating modulation. It is found that with the introduction of the laser chirp (i.e. up-chirp or down-chirp modulation), the extension of the harmonic cutoff can be achieved and the single harmonic radiation peak (HRP) can be selected during the harmonic emission process. Moreover, the chirp gating modulation on the high-order harmonic generation (HHG) is sensitive to the few-cycle pulse duration, but it is not very sensitive to the multi-cycle pulse duration. Moreover, a larger extension of the harmonic cutoff covering the whole water window region can be found with the help of the multi-cycle down-chirp modulation. Further, by properly choosing the coherent superposition of the ground state and the high Rydberg state as the initial state, the efficiency of HHG can be enhanced by 5 orders of magnitude. Finally, a series of high-intensity sub-40as pulses covering the water window region can be obtained.

The effect of the coupling between the valence state B²Π and the Rydberg state C²Π on the absorption spectrum of the NO molecule is studied by using the quantum wave packet dynamics method. The results show that the coupling between the valence state B²Π and the Rydberg state C²Π affects the C²Π ← X²Π absorption spectrum both in the intensity and on the location of spectrum peaks. The dynamics of the wave packet of the excited states is also described. One part of the wave packet evolves on the Rydberg state C²Π and the other is trapped in the valence state B²Π.