Muonic molecules in superintense laser fields

We study theoretically the ionization and dissociation of muonic molecular ions (e.g., dd\u3bc) in superintense laser fields. We predict that the bond breaks by tunneling of the lightest ion through a bond-softened barrier at intensity I 6510\ub2\ub9 W/cm\ub2. Ionization of the muonic atomic fragment occurs at much higher intensity I 656 710\ub2\ub2 W/cm\ub2. Since the field controls the ion trajectory after dissociation, it forces recollision of a 3c10\u2075\u201310\u2076 eV ion with the muonic atom. Recollision can trigger a nuclear reaction with sub-laser-cycle precision. In general, molecules can serve as precursors for laser control of nuclear processes.Peer reviewed: YesNRC publication: Ye

Photon-momentum transfer in multiphoton ionization and in time-resolved holography with photoelectrons

In most models and theoretical calculations describing multiphoton ionization by infrared light, the dipole approximation is used. This is equivalent to setting the very small photon momentum to zero. Using numerical solutions of the two-dimensional (2-D) time-dependent Schr\uf6dinger equation for one electron (H-like) systems, we show that, for linear polarization, the radiation pressure on photoelectrons is very sensitive to the details of the ionization mechanism. The directly ionized photoelectrons, those that never recollide with the parent ion, are driven in the direction of the laser photon momentum, whereas a fraction of slower photoelectrons are pushed in the opposite direction, leading to the counterintuitive shifts observed in recent experiments [Phys. Rev. Lett. 113, 243001 (2014)]. This complex response is due to the interplay between the Lorentz force and the Coulomb attraction from the ion. On average, however, the photoelectron momentum is in the direction of the photon momentum as in the case of circular polarization. The influence of the photon momentum is shown to be discernible in the holographic patterns of time-resolved atomic and molecular holography with photoelectrons, thus suggesting a new research subject in multiphoton ionization.Peer reviewed: YesNRC publication: Ye

Photon momentum sharing between an electron and an ion in photoionization: from one-photon (photoelectric effect) to multiphoton absorption

We investigate photon-momentum sharing between an electron and an ion following different photoionization regimes. We find very different partitioning of the photon momentum in one-photon ionization (the photoelectric effect) as compared to multiphoton processes. In the photoelectric effect, the electron acquires a momentum that is much greater than the single photon momentum \u210f\u3c9/c [up to (8/5) \u210f\u3c9/c] whereas in the strong-field ionization regime, the photoelectron only acquires the momentum corresponding to the photons absorbed above the field-free ionization threshold plus a momentum corresponding to a fraction (3/10) of the ionization potential Ip. In both cases, due to the smallness of the electron-ion mass ratio, the ion takes nearly the entire momentum of all absorbed N photons (via the electron-ion center of mass). Additionally, the ion takes, as a recoil, the photoelectron momentum resulting from mutual electron-ion interaction in the electromagnetic field. Consequently, the momentum partitioning of the photofragments is very different in both regimes. This suggests that there is a rich, unexplored physics to be studied between these two limits which can be generated with current ultrafast laser technology.Peer reviewed: YesNRC publication: Ye

Attosecond photoionization of a coherent superposition of bound and dissociative molecular states: effect of nuclear motion

We study numerically the possibility for monitoring electron motion in a dissociating molecule using an attosecond XUV probe pulse which photoionizes a coherent superposition of two nuclear wave packets. We present the photoelectron spectra and forward\u2013backward asymmetries in these spectra obtained from a numerical solution of the time-dependent Schr\uf6dinger equation for one electron and two protons both moving in 1D, along the laser polarization vector. In our (non-Born\u2013Oppenheimer) approach the 1D dissociative ionization of the model H\u207a\u2082 with softcore potential is described exactly. We find that in general the nuclear motion in a fast moving light molecule does not wash out the oscillations as function of the time delay between XUV probe and the pump pulses as expected from the model with fixed nuclei.Peer reviewed: YesNRC publication: Ye

Tabletop imaging using 266nm femtosecond laser pulses, for characterization of structural evolution in, single molecule, chemical reactions

We have demonstrated a generally applicable tabletop approach utilizing a 266nm femtosecond laser pulse pump, 800nm pulse probe, coupled with Coulomb explosion imaging (CEI). We have investigated two simple chemical reactions in C2H2 + simultaneously: proton transfer and C=C bond-breaking, triggered by multiphoton ionization to excited states. Too and fro proton migration results are in excellent agreement with new ab initio trajectory simulations which predict isomerization timescales and pathways.Peer reviewed: YesNRC publication: Ye

Tabletop imaging of structural evolutions in chemical reactions demonstrated for the acetylene cation

The introduction of femto-chemistry has made it a primary goal to follow the nuclear and electronic evolution of a molecule in time and space as it undergoes a chemical reaction. Using Coulomb Explosion Imaging, we have shot the first high-resolution molecular movie of a to and fro isomerization process in the acetylene cation. So far, this kind of phenomenon could only be observed using vacuum ultraviolet light from a free-electron laser. Here we show that 266 \u2030nm ultrashort laser pulses are capable of initiating rich dynamics through multiphoton ionization. With our generally applicable tabletop approach that can be used for other small organic molecules, we have investigated two basic chemical reactions simultaneously: proton migration and C=C bond breaking, triggered by multiphoton ionization. The experimental results are in excellent agreement with the timescales and relaxation pathways predicted by new and quantitative ab initio trajectory simulations.Peer reviewed: YesNRC publication: Ye

Tabletop Imaging of Structural Evolutions in Chemical Reactions

The first high-resolution molecular movie of proton migration in the acetylene cation is obtained using a tabletop multiphoton pump-probe approach\u2014an alternative to demanding free-electron-lasers and other VUV light sources when ionizing from the HOMO-1.Peer reviewed: YesNRC publication: Ye