Visualizing ultrafast weak-field-induced rotational revivals of air molecules at room temperature.

The ability to observe quantum coherence and interference is crucial for understanding quantum effects in nonlinear optical spectroscopy and is of fundamental interest in quantum mechanics. Here, we present an experimental study combined with theoretical analysis and numerical simulations to identify the underlying process behind the rotational revivals induced by a pair of time-delayed ultrafast femtosecond laser pulses for air molecules under ambient conditions. Our time-resolved two-dimensional alignment measurements confirm that one-step non-resonant Raman transitions from initial states of mixed molecules play a dominant role, showing a signature of weak-field-induced rotational revivals. Furthermore, we demonstrate that such rotational revival spectra can simultaneously measure the entire pure rotational Raman spectra and observe the quantum interference between two transition pathways from a given initial state. This work provides a powerful tool to observe, control, and identify the rotational dynamics of mixed molecular samples under weak-field excitations.

Triple ionization and fragmentation of benzene trimers following ultrafast intermolecular Coulombic decay.

Intermolecular interactions involving aromatic rings are ubiquitous in biochemistry and they govern the properties of many organic materials. Nevertheless, our understanding of the structures and dynamics of aromatic clusters remains incomplete, in particular for systems beyond the dimers, despite their high presence in many macromolecular systems such as DNA and proteins. Here, we study the fragmentation dynamics of benzene trimer that represents a prototype of higher-order aromatic clusters. The trimers are initially ionized by electron-collision with the creation of a deep-lying carbon 2s-1 state or one outer-valence and one inner-valence vacancies at two separate molecules. The system can thus relax via ultrafast intermolecular decay mechanisms, leading to the formation of C[Formula: see text]C[Formula: see text]C[Formula: see text] trications and followed by a concerted three-body Coulomb explosion. Triple-coincidence ion momentum spectroscopy, accompanied by ab-initio calculations and further supported by strong-field laser experiments, allows us to elucidate the details on the fragmentation dynamics of benzene trimers.

Femtosecond Time-Resolved Neighbor Roles in the Fragmentation Dynamics of Molecules in a Dimer.

How the neighbor effect plays its role in the fragmentation of molecular clusters attracts great attention for physicists and chemists. Here, we study this effect in the fragmentation of N_{2}O dimer by performing three-body coincidence measurements on the femtosecond timescale. Rotations of bound N_{2}O^{+} triggered by neutral or ionic neighbors are tracked. The forbidden dissociation path between B^{2}Π and ^{4}Π is opened by the spin-exchange effect due to the existence of neighbor ions, leading to a new channel of N_{2}O^{+}→NO+N^{+} originating from B^{2}Π. The formation and dissociation of the metastable product N_{3}O_{2}^{+} from two ion-molecule reaction channels are tracked in real time, and the corresponding trajectories are captured. Our results demonstrate a significant and promising step towards the understanding of neighbor roles in the reactions within clusters.

Revealing Molecular Strong Field Autoionization Dynamics.

The novel strong field autoionization (SFAI) dynamics is identified and investigated by channel-resolved angular streaking measurements of two electrons and two ions for the double-ionized CO. Comparing with the laser-assisted autoionization calculations, we demonstrate the electrons from SFAI are generated from the field-induced decay of the autoionizing state with a following acceleration in the laser fields. The energy-dependent photoelectron angular distributions further reveal that the subcycle ac-Stark effect modulates the lifetime of the autoionizing state and controls the emission of SFAI electrons in molecular frame. Our results pave the way to control the emission of resonant high-harmonic generation and trace the electron-electron correlation and electron-nuclear coupling by strong laser fields. The lifetime modulation of quantum systems in the strong laser field has great potential for quantum manipulation of chemical reactions and beyond.

Tracing the Coherent Manipulation of Rotational Dynamics by Shaped Femtosecond Pulse-Induced Two-Dimensional Rotational Coherent Spectrum.

The temporal delayed orthogonal pulse pairs generated by the phase shaping technique are used to study the coherent control of the rotational wave packet dynamics in air. By continuously changing the intrapulse delay of the pump pulse, we measured the corresponding revival signals and obtained a two-dimensional rotational coherent spectrum (2D RCS). An additive property of the rotational dynamics is observed from the revival signals. Moreover, combining with the coherent control model, we find that the 2D RCS can be used to demonstrate the control over the underlying Raman rotational excitation. A beat frequency-dependent oscillation of each rotational transition is obtained. The transition process is revealed from the Fourier transformation about the pump delay. The scheme of this work can be used for further control and detection of the rotational wave packet and can be extended to other molecular dynamic researches.

Accurate in situ Measurement of Ellipticity Based on Subcycle Ionization Dynamics.

Elliptically polarized laser pulses (EPLPs) are widely applied in many fields of ultrafast sciences, but the ellipticity (ϵ) has never been in situ measured in the interaction zone of the laser focus. In this Letter, we propose and realize a robust scheme to retrieve the ϵ by temporally overlapping two identical counterrotating EPLPs. The combined linearly electric field is coherently controlled to ionize Xe atoms by varying the phase delay between the two EPLPs. The electron spectra of the above-threshold ionization and the ion yield are sensitively modulated by the phase delay. We demonstrate that these modulations can be used to accurately determine ϵ of the EPLP. We show that the present method is highly reliable and is applicable in a wide range of laser parameters. The accurate retrieval of ϵ offers a better characterization of a laser pulse, promising a more delicate and quantitative control of the subcycle dynamics in many strong field processes.

Identifying the Multielectron Effect on Chemical Bond Rearrangement of CH3Cl Molecules in Strong Laser Fields.

Strong field double ionization that triggers the chemical bond rearrangement of CH3Cl is investigated by impulsive control of the alignment of molecules. The alignment and laser intensity dependent H2+ and H3+ yields in linearly polarized femtosecond laser have been measured, and the obtained data show that the maximum signal of H2+ appears at the laser polarization parallel to the C-Cl axis of molecules and H3+ species are more likely to eject at the laser polarization parallel to the C-Cl axis at low laser intensity while the H3+ signal peaks at laser polarization perpendicular to the C-Cl axis at high laser intensity. The measurements indicate that electrons from HOMO - 1 and HOMO - 2 orbitals have been ionized for the generation of bond rearrangement at different laser intensity. Our results demonstrate the importance of multielectron effects and also provide an effective control method in the process of chemical bond rearrangement of the molecules in strong laser fields.

Multielectron Effects in the Strong Field Sequential Ionization of Aligned CH3I Molecules.

Strong field sequential ionization of symmetric-top CH3I molecules is studied experimentally by using a combined method of femtosecond laser-induced impulsive alignment and time-of-flight mass spectrometry. Both alignment- and angular-dependent ion yields have been measured, and the sequential ionization of a multielectron has been discussed. It is found that the maximum ionization occurs when the polarization of probe laser is perpendicular to the internuclear axis of molecules, and the signal of fragment ion peaks at the polarization of the probe laser is parallel to the internuclear axis of molecules. The angular distribution of ions indicated that ionization of π-type orbitals corresponds to generation of charged parent ions and ionization of σ-type orbitals corresponds to generation of fragment ions. The sequential release of multielectrons for Coulomb explosion channels is studied by analysis of the time evolutions of multicharged In+ (n = 1-4) signals.

Ultrafast proton migration and Coulomb explosion of methyl chloride in intense laser fields.

We investigated the ultrafast proton migration and the Coulomb explosion (CE) dynamics of methyl chloride (CH3Cl) in intense femtosecond laser fields at the wavelengths of 800 nm (5.5 × 1014 W/cm2) and 400 nm (4 × 1014 W/cm2), respectively. Various fragment channels from molecular dication and trication were observed by coincidence momentum imaging through the measurement of their kinetic energy releases (KERs). The proton migration from different charged parent ions was analyzed from the obtained KER distributions. For the direct CE channel of CH3+ + Cl+ and CH3+ + Cl2+, the contribution of multiply excited electronic states and multicharged states is identified. In addition, the measurements of relative yields of the fragmentation channel at different laser wavelengths provide a selective control of proton migration for CH3Cl molecules in intense laser fields.

Rotational Dynamics of Quantum State-Selected Symmetric-Top Molecules in Nonresonant Femtosecond Laser Fields.

Rotational dynamics of quantum state selected and unselected CH3I molecules in intense femtosecond laser fields has been studied. The orientation and alignment evolutions are derived from a pump-probe measurement and in good agreement with the numerical results from the time-dependent Schrödinger equation (TDSE) calculation. The different rotational transitions through nonresonant Raman process have been assigned from the Fourier analysis of the orientation and alignment revivals. These revivals are derived from a pump-probe measurement and in good agreement with the numerical results from the TDSE calculation. For the molecules in rotational state |1, ±1, ∓1⟩, the transitions can be assigned to ΔJ = ±1, ±2, while for thermally populated molecules, the transitions are ΔJ = ±2. Our results illustrate that the orientation and alignment revivals of the rotational quantum-state-selected molecules give a deep insight into the rotational excitation pathways for the transition of different rotational states of molecules in ultrafast laser fields.

Laser induced alignment of state-selected CH3I.

Hexapole state selection is used to prepare CH3I molecules in the |JKM〉 = |1±1∓1〉 state. The molecules are aligned in a strong 800 nm laser field, which is linearly polarised perpendicular to the weak static extraction field E of the time of flight setup. The molecules are subsequently ionised by a second time delayed probe laser pulse. It will be shown that in this geometry at high enough laser intensities the Newton sphere has sufficient symmetry to apply the inverse Abel transformation to reconstruct the three dimensional distribution from the projected ion image. The laser induced controllable alignment was found to have the upper and lower extreme values of 〈P2(cos θ)〉 = 0.7 for the aligned molecule and -0.1 for the anti-aligned molecule, coupled to 〈P4(cos θ)〉 between 0.3 and 0.0. The method to extract the alignment parameters 〈P2(cos θ)〉 and 〈P4(cos θ)〉 directly from the velocity map ion images will be discussed.