Session T04: Control of Ultrafast and Strong-Field Processes

Live

Attosecond and femtosecond pump-probe schemes provide a sensitive tool for understanding the coupled electronic and nuclear dynamics in molecules. In this context, we report a theoretical investigation of single ionization of the LiH molecule using a two-color UV-UV sequence. Our approach is based on the accurate solution of the time-dependent Schr\"odinger equation in its full dimensionality, using the eigenstates of the Hamiltonian as the basis set with ionization amplitudes from the Schwinger method. This scheme allows for a detailed analysis of the correlated electron-nuclear dynamics of the system. While total ionization yields (dissociative and non-dissociative) provide information about the amplitudes and phases that build up the molecular wave-packet in the neutral states, molecular frame photoelectron angular distributions (differential in both electronic and nuclear energies) exhibit the changing character of those states, i.e., from ionic to covalent. In addition, the time dependent mean momentum of the wave-packet on neutral states is mapped onto the kinetic energy release of the atomic fragments produced by the probe ionization pulse.   

Live

Attosecond probing of core-level electronic transitions provides a sensitive tool for real-time observation of chemical dynamics. Here, we employ ultrafast XUV transient absorption spectroscopy to investigate the electronic and nuclear dynamics in a prototype molecule, molecular iodine (I$_{\mathrm{2}})$. A few-femtosecond pulse ranging between 500 and 900 nm is employed to excite the I$_{\mathrm{2}}$ molecule while an attosecond extreme-ultraviolet pulse is used to probe the dynamics through the 4$d$ iodine core-to-valence transition. Different processes are unraveled involving one-photon excitation and dissociation producing I atoms as well as multiphoton dissociative ionization leading to the formation of I$^{\mathrm{+}}$ ions. Vibrational coherences are observed in both the I$_{\mathrm{2}}$ (X$^{\mathrm{1}}\Sigma_{\mathrm{g}}^{\mathrm{+}})$ ground electronic state and the I$_{\mathrm{2}}$ (B$^{\mathrm{3}}\Pi _{\mathrm{0u+}})$ excited state, which exhibit different behaviors. While an exponential decay is observed in the excited state mainly attributed to rotational dephasing, long-lived (\textgreater 1~ps) coherences in the ground state are retrieved. The presented results demonstrate the ability of XUV transient absorption to directly map the potential energy curves of the molecule and resolve molecular dynamics with a few-femtosecond resolution.   

Live

Nonadiabatic effects appear due to avoided crossings (AC) or conical intersections (CIs) that are either intrinsic properties in field-free space or induced by a classical laser field in a molecule. It was demonstrated that avoided crossings in diatomics can also be created in an optical cavity. Here, the quantized radiation field mixes the nuclear and electronic degrees of freedom creating hybrid field- matter states called polaritons. In the present theoretical study we go further and create CIs in diatomics by means of a radiation field in the framework of cavity quantum electrodynamics. By treating all degrees of freedom, that is the rotational, vibrational, electronic and photonic degrees of freedom on an equal footing we can control the nonadiabatic quantum light-induced dynamics by means of CIs. First, the pronounced difference between the the quantum light-induced avoided crossing and the CI with respect to the nonadiabatic dynamics of the molecule is demonstrated. Second, we discuss the similarities and differences between the classical and the quantum field description of the light for the studied scenario.   

Live

Few-cycle pulse generation has largely been achieved through the nonlinear propagation of short (\textless 10-cycle) pulses in noble gas-filled capillaries, where the ``instantaneous'' optical nonlinearity can be efficiently exploited to generate octave spanning bandwidths. At much longer input pulse durations (\textgreater 100-cycle), generating comparable bandwidths using noble gases requires employing larger levels of nonlinearity, prolonged interaction lengths, or multiple compression stages. By instead using linear molecular gases, where the field-driven alignment of the molecular ensemble boosts the optical nonlinearity, multi-octave supercontinua supporting sub-cycle durations can be generated. We demonstrate the enhanced supercontinuum generation by propagating 280 fs pulses in N$_{\mathrm{2}}$, N$_{\mathrm{2}}$O, and CO$_{\mathrm{2}}$ filled hollow-core fiber. We compress pulses below two-optical cycles and use them to generate high-order harmonics.   

Live

An optical centrifuge is a type of laser pulse capable of controlling unidirectional molecular rotation up to rotational frequencies of several THz. The efficiency of an optical centrifuge to spin a molecule depends on whether the molecule can follow the accelerated rotation of the centrifuge adiabatically. We discuss the development and characterization techniques for a new optical centrifuge built in our laboratory with a lower rotational acceleration that offers substantial improvements on the adiabaticity of spinning molecules. The improvements are quantified by an increase of dimensionless 'spinnability' by a factor of $\sim$20, which allows for the spinning of molecules with a higher moment of inertia and/or weaker anisotropic polarizability. Intended applications include the orientation of ensembles of chiral molecules and the study of rotational dynamics in superfluid helium nanodroplets.   

Live

In pump-probe experiments the pumping laser excites a quantum system to a linear combination of excited states before it is tested by a second probe pulse. Observables such as ionization measured at various relative delays can be used in order to gain information about excited states of these systems. We apply numerical solutions of the time-dependent Schrodinger equation using a basis state method to analyze such scenarios.   

Live

We investigated AC Stark-shifted resonances in Ar with ultrashort near-infrared light of two different durations (30 fs and 6 fs). With the 30 fs pulses, we clearly observed a periodic enhancement of the excitation yield in the intensity regions where the absorption of 13 and 14 photons occurs. When few-cycle 6 fs pulses are used, these enhancements become ambiguous due to the bandwidth of the pulse. We have a good agreement compared to simulations of the process using the TDSE and demonstrate evidence that the enhancements are due to AC Stark-shift resonances efficiently populating the 5g and 6h states.   

Live

Time-resolved x-ray photoelectron spectroscopy (tr-XPS) is a well-established tool for studying the temporal evolution of excited-state dynamics of atoms, molecules, and solids. The technique has been enabled by the latest generation of bright, short-pulse x-ray light sources. Here, we present recent progress in developing a tr-XPS technique that does not rely on traditional pump--probe settings, but instead exploits the temporal correlations between photoelectrons generated by different pulses of the x-ray pulse train. The point-in-time properties of a molecular ensemble in chemical equilibrium statistically fluctuate around a mean (expectation) value. These fluctuations are encoded in the time-dependent photoelectron signal, and its temporal autocorrelation can reveal the underlying, time-invariant quantities, like diffusion constants and reaction rates. We demonstrate that, in a traditional pump--probe experiment, auto- and cross-correlations of the time-stamped photoelectron signal stream provide access to much of the same dynamics information as the pump--probe spectra. The results represent an important milestone toward the long-term goal of developing XPS techniques capable of determining the microscopic, short-term processes that underlie dynamic equilibria.   

Live

We investigate many-body effects in strong-field impulsive alignment of linear molecules by a linearly polarized laser pulse in the essentially non-perturbative regime of transient nonadiabatic charge redistribution (TNCR). The electron-electron interaction in model diatomic and polyatomic molecules is considered within the framework of the Hubbard Hamiltonian and expressed by the effective on-site repulsion parameter. During the ultrashort laser pulse, the nonadiabatic electron localization competes with the inter-electron repulsion, and this interplay determines the patterns of angular dependence of the resulting cumulative torque on the molecule. As a result, the semiclassical dynamics of the subsequent field-free alignment in the molecular ensemble in the wake of the laser pulse depend specifically on the intensity and shape of the pulse, as well as on the tunnel matrix element and the repulsion parameter in the model molecules. We discuss applications of these model many-body effects in situations where considerable nonresonant excitation or ionization of molecules occurs during the laser pulse.   

On Demand

Coherent control experiments in molecules are often done with shaped laser fields. The electric field is described classically and control over the time evolution of the system is achieved by shaping the laser pulses in the time or frequency domain. Moving on from a classical to a quantum description of the light field allows one to engineer the quantum state of light to steer chemical processes. The quantum field description of the photon mode allows one to manipulate the light-matter interaction directly in phase space. In this work we demonstrate the basic principle of coherent control with quantum light on the avoided crossing in lithium fluoride (Phys. Rev. A 100 053421, 2019). Using a quantum description of light together with the nonadiabatic couplings and vibronic degrees of freedoms opens up alternative perspective on quantum control. We show the deviations from control with purely classical light field and how back-action of the light field becomes important in a few-photon regime.