Andrea Merli

Coherent control with shaped femtosecond laser pulses applied to ultracold molecules

We report on coherent control of excitation processes of translationally ultracold rubidium dimers in a magneto-optical trap by using shaped femtosecond laser pulses. Evolution strategies are applied in a feedback loop in order to optimize the photoexcitation of the Rb{sub 2} molecules, which subsequently undergo ionization or fragmentation. A superior performance of the resulting pulses compared to unshaped pulses of the same pulse energy is obtained by distributing the energy among specific spectral components. The demonstration of coherent control to ultracold ensembles opens a path to actively influence fundamental photoinduced processes in molecular quantum gases.

Learning from the acquired optimized pulse shapes about the isotope selective ionization of potassium dimers

Selective optimization of the 39,39K2 and 39,41K2 isotopomers in a three-photon ionization process is presented by applying evolution strategies on shaped fs pulses in a feedback loop. The optimizations at different center wavelengths show considerably large enhancements of one isotope compared to the other and reversed. We compare the acquired optimized pulse shapes for combined phase and amplitude with pure amplitude modulation. Particularly from their spectra we are able to extract information about the optimally chosen differing ionization paths via the involved vibrational states. Furthermore, a comparison of the temporal shape of the optimized pulse forms for combined phase and amplitude with pure phase optimization is given. The presented pulse form analysis demonstrates the potential of restricted optimization to gain insight into the underlying dynamical processes. Our approach reveals how the optimization algorithm precisely addresses the vibrational wave functions both spectrally and temporally.

Characteristic oscillations in the coherent transients of ultracold rubidium molecules using red and blue detuned pulses for photoassociation

We investigate the interaction of femtosecond laser pulses with an ensemble of ultracold rubidium atoms by applying shaped excitation pulses with two different types of spectral filtering. Although the pulses, which are frequency filtered with a high pass, have no spectral overlap with molecular states, we observe coherent molecular transients. Similar transients obtained with nearly transform-limited pulses, where only the atomic resonance is removed, reveal two differing oscillatory components. The resulting transients are compared among themselves and supported with quantum dynamical simulations which indicate a photoassociation process. The effect is due to the strong field interaction of the pulse with the colliding atom pair.

Optimized isotope-selective ionization of 23Na39K and 23Na41K by applying evolutionary strategies

We study specific properties of different isotopes by applying optimal control. Selective optimization for 23Na39K and 23Na41K isotopes is reported at two different central wavelengths by employing evolutionary strategies on shaped femtosecond laser pulses. The optimized ionization processes exhibit high enhancements of one isotope compared to the other and reversed. We analyze the pulse spectra for extracting information about the optimally chosen ionization paths and observe vibrational transitions to differing electronic states for the different isotope selections. To get a deeper insight we compare simultaneous phase and amplitude modulation with pure amplitude modulation and as well pure phase modulation. Our approach reveals how the optimization algorithm precisely addresses the vibrational wave functions by coherent interaction with the corresponding pulse components.

Femtosecond investigations on the ultrafast photo-dissociation dynamics of CpMn(CO)3 and its fragment ions

This work is devoted to investigation on the ultrafast dissociation phenomena of the cyclopentadienyl manganese tricarbonyl–CpMn(CO)3− molecule and its emerging fragments by means of mass spectrometry and pump–probe spectroscopy. Studies on the laser intensity and wavelength dependence observed in the recorded mass spectra are reported. Time-resolved mass spectra were measured in order to investigate the fragmentation time scale. From the recorded two-color pump and probe spectra the lifetimes of the electronically excited states for all CpMn(CO)n+ (1 ≤ n ≤ 3) ions were extracted. By employing laser pulses of ca. 40 fs, wave packet dynamics of the parent ion CpMn(CO)3+ as well as for the first fragment have been resolved. These findings have already been successfully exploited in optimal control experiments. The present studies are extended to different pump pulse intensities.

Electronic and Optical Properties of Methylated Adamantanes

Recent theoretical work has identified functionalized diamondoids as promising candidates for the tailoring of fluorescent nanomaterials. However, experiments confirming that optical gap tuning can be achieved through functionalization have, up until now, found only systems where fluorescence is quenched. We address this shortcoming by investigating a series of methylated adamantanes. For the first time, a class of functionalized diamondoids is shown to fluoresce in the gas phase. In order to understand the evolution of the optical and electronic structure properties with degree of functionalization, photoelectron spectroscopy was used to map the occupied valence electronic structure, while absorption and fluorescence spectroscopies yielded information about the unoccupied electronic structure and postexcitation relaxation behavior. The resulting spectra were modeled by (time-dependent) density functional theory. These results show that it is possible to overcome fluorescence quenching when functionalizing diamondoids and represent a significant step toward tailoring the electronic structure of these and other semiconductor particles in a manner suitable to applications.

From isolated diamondoids to a van-der-Waals crystal: A theoretical and experimental analysis of a trishomocubane and a diamantane dimer in the gas and solid phase

The electronic properties of sp2/sp3 diamondoids in the crystalline state and in the gas phase are presented. Apparent differences in electronic properties experimentally observed by resonance Raman spectroscopy in the crystalline/gas phase and absorption measurements in the gas phase were investigated by density functional theory computations. Due to a reorganization of the molecular orbitals in the crystalline phase, the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy gaps are lowered significantly by 0.5 eV-1 eV. The π → π* transition is responsible for large absorption in both gas and crystalline phases. It further causes a large increase in the Raman intensity of the C=C stretch vibration when excited resonantly. By resonance Raman spectroscopy we were able to determine the C=C bond length of the trishomocubane dimer to exhibit 1.33 Å in the ground and 1.41 Å in the excited state.

Multi-objective optimization on alkali dimers

We present experimental results of a multi-objective evolutionary algorithm that controls the ionization of the NaK molecule and the fragmentation/ionization of potassium dimers. The two physically contradictive goals are dimer ionization and fragmentation into the atomic isotopes. In the NaK case, we use an approach to simplify the usually complex results of feedback-loop coherent control experiments and implement the ‘pulse cleaning’ method. It defines the objective function in a way that unnecessary spectral features are reduced and the most important vibronic transitions are enhanced. This method will be developed in terms of a classical multi-objective optimization and we present some corresponding Pareto-optimal solutions.

Controlling the dynamical pathways in the ionization processes of NaK dimers

This chapter analyzes the control of the dynamical pathways in the ionization processes of NaK dimers. Active control by means of feedback optimization using tailored femtosecond laser pulses is carried out in a resonant 3-photonic ionization process for optimizing the 23 Na 39 K isotopomer. The acquired optimized pulses are analyzed regarding their nuclear dynamics, and a simple model that is based on full quantum-mechanical calculations at similar conditions is proposed. Novel coherent control methods such as parametric optimization and control pulse cleaning are applied to receive information about the involved oscillations and the vibronic transitions within the optimized ionization path. When optimizing the isotopomer ratio, considerable optimization factors are obtained and intrinsic properties of the photo-induced process emerges in the resulting pulse shapes. This latter experiment reveals how the algorithm precisely addresses the vibrational wavefunctions both spectrally and temporally by coherent interactions with the shaped light field.

Coherent Control of Isotope Selective Ionization and Fragmentation of Potassium

By applying evolution strategies we performed isotope selctive ionization and fragmentation of potassium dimers. The optimized pulse structures revealed the ionization process. For optimizing the fragmentation of potassium dimers to atoms we performed a new two-criterium optimization.