C. Lupulescu

Feedback optimization of shaped femtosecond laser pulses for controlling the wavepacket dynamics and reactivity of mixed alkaline clusters

Abstract In this paper we describe the branching control of different ionization and fragmentation channels of coherently excited Na 2 K by means of feedback optimization of shaped femtosecond laser pulses. For this purpose the system was first excited with one photon into a pair of intersecting electronic states, from where it was ionized in a two-photonic process, so the resulting ions could size selectively be detected. By employing an evolutionary algorithm for optimizing phase and amplitude of the applied laser field, the relative signal intensities of the resulting mother and fragment ions could significantly be influenced. The peak intensities in the obtained pulse shapes correspond very well to the cross sections of the irradiated transitions; their temporal structure reflects perfectly the wavepacket dynamics of the resulting particles. Hence it was possible to extract with the self-learning algorithm – as postulated by Judson and Rabitz [Phys. Rev. Lett. 68 (1992) 1500] – intrinsic properties from the reactive system in real time.

Analysis and control of laser induced fragmentation processes in CpMn(CO)3

Abstract In this paper we present experimental and theoretical findings about the dynamics of ultrafast fragmentation processes which occur during resonant multiphoton ionization of CpMn(CO) 3 with femtosecond laser pulses. Employing a two-color pump and probe scheme, it was possible to retrieve lifetimes of the electronically excited parent molecule and its first fragment CpMn(CO) 2 . The observed time of 66 fs for the loss of the first CO-ligand is in good agreement with the results of one-dimensional quantum dynamical model simulations based on three relevant adiabatic ab initio potentials and the related components of the transition dipole matrix elements, in C s symmetry. Subsequently, smaller fragments appear somewhat later during approximately 100 fs. Based on these findings we performed feedback control experiments on the system in order to optimize individual fragmentation/ionization paths. With the routine a considerable increase of – for example – the CpMn(CO) + /CpMn(CO) + 3 intensity ratio was achieved. The obtained optimized laser pulses correlate well with the fast dynamics of the photoinduced preparation of CpMn(CO) + 3 versus CpMn(CO) + product ions, respectively.

One parameter fs-pulse form control on NaK and Na2K

One parameter control experiments are carried out on two simple model systems with different complexities, the dimer NaK and the trimer Na2K. The influence of the most important pulse features in formally found optimal control results are investigated independently for both molecules: the chirp and the separation of subpulses in pulse trains. For both molecules, the photoinduced processes under control are the transient three photon ionizations. Studies are presented that reveal the dependence of the measured ion yield on the applied chirps and the pulse separations. The pulse trains consisting of several subpulses are created by applying a sinusoidal spectral phase modulation. These studies may as well aid the interpretation of generated complex pulse shapes obtained in optimal control experiments.

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.

Optimal control of multi-photon dissociation and ionization processes in small NamKn clusters

Closed loop optimization of fs pulse shapes is investigated on the model systems consisting of small mixed alkali clusters. By controlling the photo-excited wave packet dynamics the multi-photon fragmentation and ionization of the NamKn clusters are optimized. The resultant optimized pulse shapes carry fingerprints of the chosen dynamical pathways of the involved molecules. The dependence of fragmentation with cluster size is additionally investigated by gradually shifting the cluster size distribution towards larger masses. The resulting modification of the optimized pulse forms offers new insights into competing fragmentation channels during the control process. Characteristic motions of the involved wave packets are proposed to explain the optimized dynamic pathways. In particular, the steering of the wave packet in the trimer Na2K by the optimized pulse can be described in terms of avoiding the predissociative curve crossing and thereby achieving an effective and selective fragmentation.

Matrix-assisted laser desorption/ionization by using femtosecond laser pulses in the near-infrared wavelength regime

We observe a substantial matrix-assisted laser desorption/ionization (MALDI) signal when irradiating femtosecond laser pulses in the near-infrared spectral range centered around 800 nm and using standard MALDI matrices with absorption bands in the ultraviolet (UV) regime. The laser pulse energy dependence of this novel phenomenon is investigated in comparison with MALDI with near-UV laser pulses. Our observations show that multiphoton absorption/ionization could be a major issue among the MALDI processes when the sample is irradiated with ultra-short laser pulses.

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.

Coherent control of alkali cluster fragmentation dynamics

Abstract Metal clusters exhibit extraordinary chemical and catalytic properties, which sensitively depend upon their size. This behavior makes them interesting candidates for the real-time analysis of ultrafast photo-induced processes—ultimately leading to coherent control scenarii. We have performed transient multi-photon ionization experiments on small alkali clusters of different size in order to probe their wave packet dynamics, structural reorientations, charge transfers and dissociative events in different vibrationally excited electronic states including their ground state. The observed processes were highly dependent on the irradiated pulse parameters, like its phase, amplitude and duration; an emphasis to employ a feedback control system for generating the optimum pulse shapes. Their spectral and temporal behavior reflects interesting properties about the investigated system and the irradiated photochemical process. We present first the vibrational dynamics of bound, dissociated, and pre-dissociated electronically excited states of alkali dimers and trimers. The scheme for observing the wave packet dynamics in the electronic ground state using stimulated Raman-pumping is shown. Since the employed pulse parameters significantly influence the efficiency of the irradiated dynamic pathways photo-induced fragmentation experiments on bifurcating reaction channels were carried out. In these experiments different branching ionization and fragmentation pathways of electronically excited Na 2 K were investigated. By employing an evolutionary algorithm for optimizing the phase and amplitude of the applied laser field, the yield of the resulting parent or fragment ions could significantly be influenced and interesting features could be concluded from the obtained optimum pulse shapes revealing the characteristic molecular oscillation period. Moreover, the influence on the optimal pulse shape due to fragmentation from larger clusters into NaK is obtained. The substructure of the optimal pulse shape thereby offers new insight into the fragmentation channel during the control process. Characteristic motions of the involved wave packets are proposed, in order to explain the optimized dynamic dissociation pathways.

Chapter 24 - Free phase optimization of CpMn(CO) + 2 as a fragment of CpMn(CO) 3 by means of shaped femtosecond laser pulses

The dissociative tendencies of organometallic compounds with transition metals and CO ligands make these species apt model systems for influencing the dynamics of photodissociation processes. Active control experiments on the photo-excited CpMn(CO) 3 by using the computer controlled feedback optimization loop have been carried out. By employing an evolutionary algorithm for optimizing the phase and/or the amplitude of the applied laser field, the yield of the resulting parent and/or fragment ions can be influenced. Moreover, interesting features about the investigated system and the irradiated photochemical process can be extracted from the obtained pulse shapes. In many cases free optimizations lead to optimal pulse shapes that are very complicated and difficult to interpret. The chapter deals with optimal control experiments of fragmentation/ionization ratio in the photo-excited CpMn(CO) 2 , as a fragment arising from cyclopentadienyl manganese tricarbonyl (CpMn(CO) 3 ) by using shaped femtosecond laser pulses. The resulting optimized pulse form reflects the capability of the optimization method to find tailored pulses, rather than an unshaped pulse. The optimal laser field should correlate with the ultrafast molecular dynamics during the photo-fragmentation process.

Closed loop optimization of the ionization process in NaK. Learning from the optimal pulse shape.

ABSTRACT The ionisation process of the NaK dimer is investigated by applying evolution strategies to optimize the spectral phase of fs pulses interacting with the molecules. The obtained optimal pulse structure with three intensity maxima is presented. As an explanation of the ionization process a simple model of wave packet propagation on given potential energy surfaces is proposed.