Strong Laser Field Fragmentation of ${\mathrm{H}}_2$: Coulomb Explosion without Double Ionization

We observe fragmentation of ${\mathrm{H}}_2$ molecules exposed to strong laser fields into excited neutral atoms. The measured excited neutral fragment spectrum resembles the ionic fragmentation spectrum including peaks due to bond softening and Coulomb explosion. To explain the occurrence of excited neutral fragments and their high kinetic energy, we argue that the recently investigated phenomenon of frustrated tunnel ionization is also at work in the neutralization of ${\mathrm{H}}^{+}$ ions into excited ${\mathrm{H}}^{*}$ atoms. In this process the tunneled electron does not gain enough drift energy from the laser field to escape the Coulomb potential and is recaptured. Calculation of classical trajectories as well as a correlated detection measurement of neutral excited ${\mathrm{H}}^{*}$ and ${\mathrm{H}}^{+}$ ions support the mechanism.

Figure 1

Kinetic energy distribution of ionic and excited neutral fragments after strong field interaction at an intensity of $I=3\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. Shown are the kinetic energy distribution of (a) ${\mathrm{H}}^{+}$ (black curve) and (b) ${\mathrm{H}}^{*}$ (red curve). For comparison it has been assured that the solid angle of detection is the same in both cases. The spectra have been normalized to the maximum of the second low energy peak. The inset shows the ${\mathrm{H}}^{*}$ time-of-flight spectrum at an extraction voltage of 100 V. Curves taken at other extraction voltages fall on top of this curve as expected for neutral fragments.

Figure 2

From a correlated TOF measurement of neutral excited and ionic fragments a color coded correlated diagram of the kinetic energy of neutral excited ${\mathrm{H}}^{*}$ and ${\mathrm{H}}^{+}$ is extracted. The black line indicates where correlated events should be located. Clearly visible are correlated events originating from the Coulomb explosion (CE). For further explanation, see text.

Figure 3

Classical trajectory calculation for the Coulomb explosion of ${\mathrm{H}}_2^{+}$ after tunnel ionization. Shown are the components of the trajectories of the two nuclei and the electron along the direction of the laser polarization as a function of time.

Figure 4

Comparison of simulated kinetic energy distributions of excited neutral fragments ${\mathrm{H}}^{*}$ after Coulomb explosion with experimental data (solid line). Shown are the results from classical Monte Carlo simulations of the Coulomb explosion, where one of the two escaping ${\mathrm{H}}^{+}$ recaptures the tunneled electron to form an excited neutral ${\mathrm{H}}^{*}$ atom. We assumed a Gaussian distribution of nuclear separations centered at $R_0=3.5\mathrm{a}.\mathrm{u}.$ (○), 4 a.u. (■), and 5 a.u. (▴) with a width of 1 a.u.