Figure 1

(a) Spectrum of the UV pulses used in the experiment shown in relation to the ionization potentials of helium and argon. The inset shows the temporal profile of the attosecond pulses in the train. Experimental photoelectron momentum distributions from single-photon ionization by the APT in helium (b) and argon (c) with the polarization of the light parallel to the $p_y$ axis.Reuse & Permissions

Figure 2

Experimentally measured ion yields, $P_{\mathrm{ion}}$, for ${\mathrm{He}}^{+}$ (blue circles) and ${\mathrm{Ar}}^{+}$ (red squares) as a function of the delay $\tau$ between the attosecond pulses and the IR field. Calculated ion yields in the same conditions (${\mathrm{He}}^{+}$, blue solid line; ${\mathrm{Ar}}^{+}$ red solid line).Reuse & Permissions

Figure 3

(a) Calculated probabilities for removal of an electron from the ground state ($P_{\mathrm{abs}}$, blue line), ionization ($P_{\mathrm{ion}}$, red line) and remaining in an excited bound state ($P_{\mathrm{trp}}$, green line) as a function of the phase of the IR field at the time of the attosecond pulses. (b) Absorption probability, $P_{\mathrm{abs}}$, versus delay for different APTs, normalized to the excitation probability for zero delay in each case. The FWHM of the APT intensity envelope is 1 fs (single attosecond pulse, blue line), 2 fs (red line), 4 fs (green line) or 8 fs (black line). (c) Contrast (defined as the maximum excitation probability divided by the minimum) for various peak intensities of the IR field for a single pulse (red line) and for a train with 10 fs FWHM (blue line). The dashed vertical line indicates the experimental intensity.Reuse & Permissions

Figure 4

Momentum distributions from argon [(a) and (b)] and helium [(c) and (d)] for the IR-UV delays corresponding to lowest [(a) and (c)] and highest [(b) and (d)] maximum electron momentum along the $p_y$ axis. The polarization directions of the UV and IR fields are parallel to the $p_y$ axis.Reuse & Permissions