Molecular bond stabilization in the strong-field dissociation of O2+

Abstract

We theoretically examine the rotational and vibrational dynamics of ${\mathrm{O}}_{2}{}^{+}$ molecular ions exposed to intense, short laser pulses for conditions realized in contemporary pump-probe experiments. We solve the time-dependent Schr\odinger equation within the Born-Oppenheimer approximation for an initial distribution of randomly aligned molecular ions. For fixed peak intensities, our numerical results show that total, angle-integrated ${\mathrm{O}}_{2}{}^{+}\ensuremath{\rightarrow}\mathrm{O}(^{3}P)+\mathrm{O}{}^{+}(^{4}S^{0})$ dissociation yields do not monotonically increase with increasing infrared-probe pulse duration. We find this pulse-duration-dependent stabilization to be consistent with the transient trapping of nuclear probability density in a light-induced (bond-hardening) potential-energy surface and robust against rotational excitation. We analyze this stabilization effect and its underlying bond-hardening mechanism (i) in the time domain, by following the evolution of partial nuclear probability densities associated with the dipole-coupled ${\mathrm{O}}_{2}{}^{+}(a\phantom{\rule{0.16em}{0ex}}^{4}\mathrm{\ensuremath{\Pi}}_{u})$ and ${\mathrm{O}}_{2}{}^{+}(f\phantom{\rule{0.16em}{0ex}}^{4}\mathrm{\ensuremath{\Pi}}_{g})$ cationic states, and (ii) in the frequency domain, by examining rovibrational quantum-beat spectra for the evolution of the partial nuclear probability densities associated with these states. Our analysis reveals the characteristic timescale for the bond-hardening mechanism in ${\mathrm{O}}_{2}{}^{+}$ and explains the onset of bond stabilization for sufficiently long pulse durations.