Population trapping in bound states during IR-assisted ultrafast photoionization of ${\mathrm{Ne}}^{+}$

We have investigated the photoionization of ${\mathrm{Ne}}^{+}$ in the combined field of a short infrared laser pulse and a delayed ultrashort pulse of the infrared laser's 23rd harmonic. We observe an ionization yield compatible with a picture in which one electron gets excited into Rydberg states by the harmonic laser field and is subsequently removed by the infrared laser field. Modulations are seen in the ionization yield as a function of time delay. These modulations originate from the trapping of population in low members of the Rydberg series with different states being populated at different ranges of delay times. The calculations further demonstrate that single-threshold calculations cannot reproduce the ${\mathrm{Ne}}^{+}$ photoionization yields obtained in multithreshold calculations.

Figure 1

Time profile of the combined electric field of the IR and the XUV pulse. The shown delay of the XUV pulse is 7.8 fs, corresponding to three cycles of the IR field. Since the delay is determined from the onset of the pulse, the delay means that the maximum in the XUV pulse occurs when the IR field is decreasing.

Figure 2

${\mathrm{Ne}}^{+}$ ionization yield obtained as a function of time delay between the IR pulse and the XUV pulse. The XUV pulse is the 23rd harmonic of the IR pulse of 780 nm. The ionization yield (solid black line) is given by the sum of the direct ionization yield (direct ionization, red dashed line) and the population left in Rydberg states associated with excited thresholds (indirect ionization, blue dot-dashed line). The difference between the ionization yield and the total population in the outer region (purple dotted line with circles) can be assigned to population in Rydberg states associated with the ground-state threshold.

Figure 3

Final-state population in $2s^{2}2p^4{(}^3{P}^e)3d$ ${}^2{D}^e$ (black full line) and $2s^{2}2p^4{(}^3{P}^e)4p{}^2{P}^o$ (red dashed line) as a function of XUV time delay.

Figure 4

Final-state population in $2s^{2}2p^4{(}^1{D}^e)3s$ ${}^2{D}^e$ (red dashed line) and $2s^{2}2p^4{(}^1{S}^e)3s$ ${}^2{S}^e$ (black solid line) as a function of XUV time delay.

Figure 5

Final-state population in $2s^{2}2p^4{(}^3{P}^e)3p$ ${}^2{P}^o$ (solid black line), $2s^{2}2p^4{(}^1{D}^e)3p{}^2{P}^o$ (red dashed line), and $2s^{2}2p^4{(}^1{D}^e)3p$ ${}^2{F}^o$ (blue dot-dashed line) as a function of XUV time delay.