Wave‐packet dynamics in the Li2 E(1Σ+g) shelf state: Simultaneous observation of vibrational and rotational recurrences with single rovibronic control of an intermediate state

Characterizing coherences in chemical dynamics with attosecond time-resolved x-ray absorption spectroscopy

Coherence can drive wave-like motion of electrons and nuclei in photoexcited systems, which can yield fast and efficient ways to exert materials’ functionalities beyond the thermodynamic limit. The search for coherent phenomena has been a central topic in chemical physics although their direct characterization is often elusive. Here, we highlight recent advances in time-resolved x-ray absorption spectroscopy (tr-XAS) to investigate coherent phenomena, especially those that utilize the eminent light source of isolated attosecond pulses. The unparalleled time and state sensitivities of tr-XAS in tandem with the unique element specificity render the method suitable to study valence electronic dynamics in a wide variety of materials. The latest studies have demonstrated the capabilities of tr-XAS to characterize coupled electronic–structural coherence in small molecules and coherent light–matter interactions of core-excited excitons in solids. We address current opportunities and challenges in the exploration of coherent phenomena, with potential applications for energy- and bio-related systems, potential crossings, strongly driven solids, and quantum materials. With the ongoing developments in both theory and light sources, tr-XAS holds great promise for revealing the role of coherences in chemical dynamics.

Reaction microscope for investigating ionization dynamics of weakly bound alkali dimers

We report on the implementation of a far-off-resonant, optical dipole force trap in a reaction microscope combined with a magneto-optical trap. Kinematically complete multi-photon ionization experiments were performed on optically trapped ⁶Li atoms and photo-associated ⁶Li₂ molecules in their highest vibrational state. The apparatus allows us to distinguish different ionization mechanisms related to the presence of the IR field of the optical dipole trap that can occur during ionization of ⁶Li and ⁶Li₂ in strong fields. In a series of proof-of-principle experiments, we detect weakly bound dimers via three-photon ionization with femtosecond pulses (τ = 30 fs) at a central wavelength of 780 nm and measure directly the momenta of the photoelectrons in coincidence with recoil ions.

Control of wave packets in Li(2) by shaping the pump and probe pulses for a state-selected pump-probe analysis of the ionization continuum

Wave packet signals in Li(2) prepared by shaped pump pulses are also detected with state-selected shaped probe pulses in the ionization continuum. The results show that the final states are discrete Rydberg states instead of continuum states. Final autoionizing states in the continuum are observed and characterized. By selecting specific resonant rovibrational electronic transitions from the superposition states prepared in the wave packets to the final autoionizing states with the pulse shaping system, the modulation depths of the wave packet signals are increased by as much as 5.20+/-0.03 times. Control of the wave packets is also realized by shaping the probe pulses to select specific resonant transitions between the states in the wave packets and the highly excited Rydberg states. The detected amplitude ratio of one specific vibrational quantum beat to one specific rotational quantum beat can be decreased by ten times.

Control of Li2 wave packet dynamics by modification of the quantum mechanical amplitude of a single state

Sequences of pulses with different spectra are used to control rotational wave packet dynamics in Li(2) by exploiting quantum interference phenomena. Wave packet superpositions are excited in a two-step resonant Raman process by two different pulses. Interferences between individual states shared by both wave packets can be used to enhance or destroy specific components of a superposition by varying the time delay between the pulses and/or the relative phase within the pulses. Elimination of selected quantum beats is achieved by greater than 94% for each case. A simple, yet effective, method for generating different color phase-locked pairs of laser pulses in a liquid-crystal pulse shaper setup without the need for interferometric stabilization schemes is described. The ability to manipulate single states of a superposition is an important advancement for intuitive control schemes and provides a potential new approach for initialization schemes in the field of quantum information.

Active control of the dynamics of atoms and molecules

There has been much progress in the control of chemical reactions since methods of active control were first proposed by Brumer & Shapiro and by Tannor & Rice ten years ago. This chapter reviews both theoretical and experimental advances in the field. Control schemes based on quantum mechanical interference between competing paths and the manipulation of wave packets with tailored laser pulses are discussed. The theory of optimal control, the limitations of control theory applied to many-body dynamics, and the effects of constraints on the trajectory of the controlled observable are presented. Experimental progress in controlling the population of specific quantum states, in manipulating the dynamics of bound wave packets, and in the control of chemical reactions are reviewed, and current problems in the field are summarized.