Session J04: Atoms and Molecules in Strong Fields

Live

We have measured the strong field ionization of diatomic iodine molecules as a function of wavelength and internuclear separation. The latter was done by launching a wavepacket on the B-state of iodine and probing the ionization as a function of time delay. In both wavelength and time delay, we find critical points where the ionization increases significantly. All of the results are consistent with ionization from deep sigma orbitals and resonant interactions. We present a unified view of the strong field ionization of diatomic molecules.   

Live

We study the strong-field induced ionization and multi-body fragmentation of formic acid (HCOOH) and its isotopologue (HCOOD) using coincident three-dimensional momentum imaging. In particular, we implement the native frames method [1] to elucidate the multi-body sequential fragmentation dynamics of formic acid as well as study several bond rearrangement processes. \newline [1] J. Rajput \emph{et al.}, Phys. Rev. Lett. \textbf{120}, 103001 (2018).   

Live

We use few cycle intense ultrafast laser pulses and coincidence velocity map imaging to investigate strong field Channel Resolved Above Threshold Ionization (CRATI) of D$_2$O. We use a time stamping camera to make coincidence vector momentum measurements of electrons and ions, allowing us to distinguish multiple ionization channels. Individual Above Threshold Ionization (ATI) peaks can be assigned to leaving the molecule in different states of the molecular cation, indicating that we have direct ionization to multiple states of the cation, involving the removal of HOMO and HOMO-2 electrons. Additional information from electrons and fragment ions measured in coincidence also shows that there is a competition between sequential and non-sequential double ionization.   

Live

We present the results of experiments conducted on several carbon ring molecules when subject to ultrafast ionizing radiation fields. By employing either pulses of a strong-field near-infrared laser or an electron-synchrotron generated X-ray beam, we excite these molecules to a highly charged cationic state which will dissociate into several neutral or charged ionic fragments. By employing coincident ion momentum imaging techniques, we can measure the relative yields, angular distributions, and kinetic energies of these photoproducts. We show evidence that these charged carbon rings predominantly fragment sequentially, where the later fragmentation step(s) occur on a timescale longer than the rotational period of the molecule.   

Live

Creating transient, field-free orientation is an important capability for exploring and controlling strong-field processes within molecules. It has been demonstrated using phase-controlled, 2-color (1 $\omega$ + 2 $\omega$) laser pulses and single-cycle THz fields. While some measurements are fairly insensitive to some sample ionization, caused by the orienting fields prior to the initiation of the strong-field process of interest, others are less forgiving and require that the molecular sample remain neutral with negligible electronic or vibrational excitation. We have used Coulomb explosion induced by an intense, time-delayed 780nm 35 fs laser pulse, to characterize the effectiveness of 2-color, 780nm+390nm, 50 fs laser pulses for orienting rotationally cooled OCS molecules, as a function of the 2-color intensity and the degree of ionization induced by the 2-color field alone. Our comparison of the results obtained in two cases - one in which a 780nm pre-alignment pulse precedes the 2-color field, and the other when the 2-color pulse acts alone - enables a determination of the suitability of these approaches in experiments seeking to measure angle-dependent strong-field single ionization. Future measurements will extend the comparison to non-ionizing THz induced orientation.   

Live

We apply time-dependent density functional theory, strong field approximation, optical Bloch equations, and Floquet theory to study the interaction of high intensity ultrashort laser pulses with molecules in the context of high harmonic generation, strong field ionization and nonadiabatic electron localization. For several molecules we discuss the properties of these nonlinear processes, which can be related to dynamic electron localization of coherent electron wavepacket, during the laser pulse. We show how the properties change for two color laser case and how changing the polarization of the two components influences the harmonics spectra and ionization yield. Finally we consider the modification of harmonic spectra and ionization by the interaction of two linearly polarized pulses with different orientation of polarizations of the components.   

Live

We study the excitation of Rydberg states for the interaction of atoms with intense bi-circular laser pulses using numerical solutions of the time-dependent Schr\"{o}dinger equation. By identifying the pathways in co-rotating and counter-rotating bi-circular laser pulses, we interpret an enhancement in excitation for the counter-rotating case consistent with the experimental results in Ref. [1]. We further identify several other features, e.g. angular momentum selection rules for the populations in the excited states and a cut-off in angular momentum that is a dependent on the frequency and intensity of the two circular laser pulses. [1] C. A. Mancuso, et al., PRA 96, 023402 (2017)   

Live

We study the process of laser-assisted radiative recombination of electron with proton by using a semiclassical approach involving calculation of classical trajectories in combined laser and Coulomb fields. Due to chaotic scattering in the combined fields$^1$, the radiation probability as a function of the impact parameter and the constant phase of the laser field exhibits fractal structures similar to those observed in bremsstrahlung$^2$. We obtain a strong enhancement of the recombination cross section as compared to the laser-free case due to the Coulomb focusing effect$^3$. For sufficiently low incident electron velocities the cross section becomes infinite, and we limit it by assuming a finite laser pulse duration of about 1 ps. With this assumption we obtain the gain factor for capture into the ground state of the hydrogen atom of about 74 for a terawatt field, and about 49 for a gigawatt field. $^1$L. Wiesenfeld, Phys. Lett. A {\bf 144}, 467 (1990). $^2$H. B. Ambalampitiya and I. I. Fabrikant, Phys. Rev. A {\bf 99}, 063404 (2019). $^3$Th. Brabec, M. Yu. Ivanov, and P. B. Corkum, Phys. Rev. A {\bf 54} R2551 (1996).   

On Demand

Separation of the chiral enantiomers is of crucial importance in many biological, pharmaceutical, chemical and physical processes, and a variety of methods have been developed for this purpose over the years. Here we theoretically demonstrate enantio-selective control of molecular orientation with the help of strong THz pulses. We show that for properly polarization-shaped pulses, the induced orientation is enduring, and survives long after the field is over. This orientation is in all three laboratory axes, and the two enantiomers are oriented in opposite directions along and against the light propagation direction and perpendicular to the THz field polarization. We discuss the underlying mechanism of this phenomenon, explore effects of temperature and field parameters, and propose the observation of this effect by Second Harmonic Generation induced by interaction of the oriented molecules with probe optical pulse.   

On Demand

In standard lasers, light amplification requires population inversion between an upper and a lower state to break the reciprocity between absorption and stimulated emission. But, in a medium prepared in a specific superposition state, quantum interference may fully suppress absorption while leaving stimulated emission intact, opening the possibility of lasing without inversion (LWI). Conventional LWI schemes generally strive to maintain a specific phase relationship between the lower-lying states that carry most of the population. We will present a scheme that does not follow this tradition. It uses only the natural dynamics of a multi-level quantum system and requires no coherence between the excited and the lower electronic states; effectively, LWI comes "for free". We will show that this mechanism is active in the highly efficient generation of 391 nm radiation during propagation of intense femtosecond laser pulses in air, under standard conditions where the process known as "laser filamentation" leads to self-guiding of light. Identifying the mechanism responsible for this "air lasing" effect has been a long-standing puzzle. We show how it arises naturally; triggered by the combination of molecular ionization and molecular alignment, both unavoidable in intense light fields.