Figure 1

The measured kinetic energy of protons created by CE of ${\mathrm{H}}_2$ as a function of the delay between two identical 25 fs laser pulses. Plotted are only events where two protons have been detected in coincidence. Dashed lines: classically simulated wave packet propagation for dissociation of $\mathrm{H}_2{}^{+}$ via $1\omega$ and $2\omega$ channels. The arrow indicates an enhancement corresponding to the maximum of the $\mathrm{H}_2{}^{+}$ ionization rate around $R=10\mathrm{a}.\mathrm{u}.$ Inset: a scheme of the pump-probe experiment. Thin lines indicate potential curves for the two lowest electronic states of $\mathrm{H}_2{}^{+}$ (solid for $1s{\sigma }_g$, dashed for $2p{\sigma }_u$) dressed with $n$ ($n=1,2,3$) photons. The $\mathrm{H}_2{}^{+}$ ion can dissociate either by absorbing one photon ($1\omega$ channel) or absorbing three photons in the first avoided crossing followed by a reemission of one photon at the second crossing ($2\omega$). Since for each photon the parity of the molecular state must change, there is no direct two-photon process. For a detailed description of the dissociation mechanisms see 8, 14, 15, 16, 17, 20.Reuse & Permissions

Figure 2

Noncoincident proton kinetic energy spectrum for the delays from 17.5 to 45 fs. The lines 1, 2, and 3 show the delays corresponding to 7, 14, and 14.5 optical cycles, respectively.Reuse & Permissions

Figure 3

(a) Time-dependent laser intensity profile for a single pulse (thin gray), and for two pulses separated by the delay of 14 (solid black, line 2 in Fig. 2) and 14.5 (dashed, line 3 in Fig. 2) optical cycles. (b) Corresponding noncoincident proton kinetic energy spectra. The two-pulse data were integrated over the delay region $\pm 0.3\mathrm{fs}$ around lines 2 and 3 of Fig. 2 for constructive and destructive case, respectively. Thus, the relative height of these two curves reflects the ionization probabilities for corresponding field configurations, whereas the single-pulse data were arbitrarily normalized for a visual convenience. (c) Same as (b) but measured in coincidence.Reuse & Permissions