Clocking Enhanced Ionization of Hydrogen Molecules with Rotational Wave Packets

Abstract

Laser-induced rotational wave packets of H 2 and D 2 molecules were experimentally measured in real time by using two sequential 25-fs laser pulses and a reaction microscope. By measuring the timedependent yields of the above-threshold dissociation and the enhanced ionization of the molecule, we observed a few-femtosecond time delay between the two dissociation channels for both H 2 and D 2. The delay was interpreted and reproduced by a classical model that considers enhanced ionization and thus additional interaction within the laser pulse. We demonstrate that by accurately measuring the phase of the rotational wave packet in hydrogen molecules we can resolve dissociation dynamics which is occurring within a fraction of a molecular rotation. Such a rotational clock is a general concept applicable to sequential fragmentation processes in other molecules.

Low-Energy Protons in Strong-Field Dissociation of H 2 + via Dipole-Transitions at Large Bond Lengths

“…Then, the quantitatively reproduced MF-PADs can be obtained if n is set to be 1 and 4 for KER regions of C and D, as shown by green and yellow curves in Figure 4(b). This conclusion is consistent with the wellaccepted picture of the one-photon and net-two-photon dissociation pathways [11][12][13][14][15]. Here the KER region D, i.e., the net-two-photon pathway, is utilized to confirm the analysis process, and we will focus on the comparison between KER regions A, B, and C. To match the orange and violet Internuclear distance R (a.u.)…”

“…when exposed to an intense laser field [11,12]. The produced H 2 + stretches and further absorbs photons from the remaining laser field and dissociates via the well-known one-photon and net-two-photon pathways [11][12][13][14][15], shown as the red and blue arrows between black curves in Figure 2. For example, in the one-photon pathway, the stretching H 2 + absorbs one photon at R ω , transits to the 2pσ u state, and dissociates along the 2pσ u curve.…”

“…In the classical model, the single ionization of H 2 in the PS laser pulse is described by P i ðtÞ using the Ammosov-Delone-Krainov theory [26]. Then, the bondstretching time τ of H 2 + is calculated by solving the Newtonian equation μd 2 R/dt 2 = -dVðRÞ/dR [27] of a classical particle with reduced mass μ = 0:5m H + , representing the nuclear wave packet (NWP) propagating on a certain potential energy curve VðRÞ of H 2 + [12][13][14][15], where R denotes the internuclear distance. Different initial kinetic energies are assigned for the classical particles to take into account the population of different vibrational states of the photoionization-launched NWPs.…”

“…For molecules in intense laser fields, different dissociation pathways contribute to different nuclear fragment energies. For instance, the two-hump energy structure peaked around 0.7 and 1.2 eV of the ejected protons in the dissociation of H 2 + driven by 790 nm lasers discloses the one-photon and net-two-photon pathways [11][12][13][14][15]. The vibrational states from which the dissociation starts are further identified by carefully analyzing the fine structure of the discrete energy peaks in the kinetic energy spectrum of the nuclear fragments [16,17].…”

Signatures of magnetic-field effects in nonsequential double ionization manifesting as backscattering for molecules versus forward scattering for atoms

“…This occurs mostly when the polarization of the electric field is along the molecular axis. Our prediction is experimentally accessible, since diatomic molecules such as H 2 , which is the focus of the current work, can be aligned [26]. Moreover, it takes place for intensities significantly smaller than expected from β 0 ≈ 1 a.u.…”