Qinying Ji

Photon Energy Deposition in Strong-Field Single Ionization of Multielectron Molecules

Molecules exposed to strong laser fields may coherently absorb multiple photons and deposit the energy into electrons and nuclei, triggering the succeeding dynamics as the primary stage of the light-molecule interaction. We experimentally explore the electron-nuclear sharing of the absorbed photon energy in above-threshold multiphoton single ionization of multielectron molecules. Using CO as a prototype, vibrational and orbital resolved electron-nuclear sharing of the photon energy is observed. Different from the simplest one- or two-electron systems, the participation of the multiple orbitals and the coupling of various electronic states in the strong-field ionization and dissociation processes alter the photon energy deposition dynamics of the multielectron molecule. The population of numerous vibrational states of the molecular cation as the energy reservoir in the ionization process plays an important role in photon energy sharing between the emitted electron and the nuclear fragments.

Directional bond breaking by polarization-gated two-color ultrashort laser pulses

We experimentally investigate directional bond breaking in dissociative single ionization of H2 driven by circularly polarized two-color ultrashort pulses of counter- and co-rotating laser fields. Trefoil or semilunar patterns of directional proton emission in a two-dimensional space spanned by the laser fields are observed, which can be finely controlled by varying the relative phase of the counter- or co-rotating circularly polarized two-color fields, respectively. Our results open new possibilities to manipulate two-dimensional directional bond breaking of molecules by strong laser fields.

Visualizing molecular unidirectional rotation

A molecule can be optically accelerated to rotate unidirectionally at a frequency of a few terahertzes which is many orders higher than the classical mechanical rotor. Such a photon-induced ultrafast molecular unidirectional rotation has been well explored as a controllable spin of the molecular nuclear wave packet. Although it has been observed for more than 10 years, a complete imaging of the unidirectional rotating nuclear wave packet is still missing, which is essentially the cornerstone of all the exploring applications. Here, for the first time, we experimentally visualize the time-dependent evolution of the double-pulse excited molecular unidirectional rotation by Coulomb explosion imaging the rotational nuclear wave packet. Our results reveal comprehensive details undiscovered in pioneering measurements, which exhibits as a joint of the quantum revival of the impulsively aligned rotational wave packet and its unidirectional rotation following the angular momentum conservation. The numerical simulations well reproduce the experimental observations and intuitively revivify the thoroughgoing evolution of the rotational wave packet.

Directional deprotonation ionization of acetylene in asymmetric two-color laser fields

We experimentally investigate the deprotonation dissociative double ionization of an acetylene molecule by an asymmetric two-color laser pulse. We find that the ejection direction of the proton, and hence the directional C–H bond breaking of a polyatomic hydrocarbon molecule, can be controlled by finely tuning the phase of a two-color laser pulse.

High-order above-threshold dissociation of molecules

Significance Above-threshold ionization of atoms in strong laser fields is extensively studied for its overwhelming importance and universality. However, its counterpart, above-threshold dissociation of molecules in strong laser fields, is hard to be observed, although it has been predicted for decades. In this paper, by measuring the momenta of photoelectron and dissociative fragments coincidently, we successfully obtained distinct nuclear energy peaks of the high-order above-threshold dissociation, which must appear simultaneously with the above-threshold ionization. The coexistence of high-order above-threshold dissociation and high-order above-threshold ionization in molecular dissociative ionization offers a perspective to disentangle the complex electron–nuclear correlation in molecules and to image the molecular orbitals, and so on. Electrons bound to atoms or molecules can simultaneously absorb multiple photons via the above-threshold ionization featured with discrete peaks in the photoelectron spectrum on account of the quantized nature of the light energy. Analogously, the above-threshold dissociation of molecules has been proposed to address the multiple-photon energy deposition in the nuclei of molecules. In this case, nuclear energy spectra consisting of photon-energy spaced peaks exceeding the binding energy of the molecular bond are predicted. Although the observation of such phenomena is difficult, this scenario is nevertheless logical and is based on the fundamental laws. Here, we report conclusive experimental observation of high-order above-threshold dissociation of H2 in strong laser fields where the tunneling-ionized electron transfers the absorbed multiphoton energy, which is above the ionization threshold to the nuclei via the field-driven inelastic rescattering. Our results provide an unambiguous evidence that the electron and nuclei of a molecule as a whole absorb multiple photons, and thus above-threshold ionization and above-threshold dissociation must appear simultaneously, which is the cornerstone of the nowadays strong-field molecular physics.

Energy-Resolved Ultrashort Delays of Photoelectron Emission Clocked by Orthogonal Two-Color Laser Fields

A phase-controlled orthogonal two-color (OTC) femtosecond laser pulse is employed to probe the time delay of photoelectron emission in the strong-field ionization of atoms. The OTC field spatiotemporally steers the emission dynamics of the photoelectrons and meanwhile allows us to unambiguously distinguish the main and sideband peaks of the above-threshold ionization spectrum. The relative phase shift between the main and sideband peaks, retrieved from the phase-of-phase of the photoelectron spectrum as a function of the laser phase, gradually decreases with increasing electron energy, and becomes zero for the fast electron which is mainly produced by the rescattering process. Furthermore, a Freeman resonance delay of 140±40 attoseconds between photoelectrons emitted via the 4f and 5p Rydberg states of argon is observed.

Orientation-dependent strong-field dissociative single ionization of CO

The dissociative ionization of CO in orthogonally polarized femtosecond laser pulses are studied in a pump-probe scheme. The ionization of CO by the pump pulse and the dissociation of the created CO+ by the probe pulse can be fully disentangled by identifying the photoelectron momentum distributions. Different from the dissociative ionization by a single pulse in which the CO molecule mostly breaks along the field polarization, in this pump-probe strategy, the CO+ ion created from ionization by the pump pulse is favored to dissociate when it orients orthogonal to the polarization direction of the probe pulse. It is attributed to the laser-coupling of various electronic states of the molecular ion in the dissociation process, supported by the numerical simulation of a modeled time-dependent Schrödinger equation.

Disentangling the role of laser coupling in directional breaking of molecules

The directional control of molecular dissociation with a laser electric field waveform is a paradigm and was demonstrated for a variety of molecules. In most cases, the directional control occurs via a dissociative ionization pathway. The role of laser-induced coupling of electronic states in the dissociating ion versus selective ionization of oriented neutral molecules, however, could not be distinguished for even small heteronuclear molecules such as CO. Here, we introduce a technique, using elliptically polarized pump and linearly polarized two-color probe pulses, that unambiguously distinguishes the roles of laser-induced state coupling and selective ionization. The measured photoelectron momentum distributions governed by the light polarizations allow us to coincidentally identify the ionization and dissociation from the pump and probe pulses. Directional dissociation of

Comparison Study of Strong-Field Ionization of Molecules and Atoms by Bicircular Two-Color Femtosecond Laser Pulses

{\mathrm{CO}}^{+}

Pathway-resolved photoelectron emission in dissociative ionization of molecules

as a function of the relative phase of the linearly polarized two-color pulse is observed for both parallel and orthogonally oriented molecules. We find that the laser-induced coupling of various electronic states of