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Ultrafast lasers are versatile tools used in many scientific areas, from welding to eye surgery. They are also used to coherently manipulate light–matter interactions such as chemical reactions, but so far control experiments have concentrated on cleavage or rearrangement of existing molecular bonds. Here we demonstrate the synthesis of several molecular species starting from small reactant molecules in laser-induced catalytic surface reactions, and even the increase of the relative reaction efficiency by feedback-optimized laser pulses. We show that the control mechanism is nontrivial and sensitive to the relative proportion of the reactants. The control experiments open up a pathway towards photocatalysis and are relevant for research in physics, chemistry, and biology where light-induced bond formation is important.

We experimentally demonstrate a method to generate shaped femtosecond laser pulses in the ultraviolet at a central wavelength of 267 nm, the third harmonic of conventional titanium-sapphire femtosecond laser systems. Employing a 128-pixel liquid-crystal spatial light modulator, we impose variable spectral phase modulations upon the near-infrared laser pulses. By this, complex laser pulses can be shaped whose overall spectrum is still conserved. Our experiments show that it is possible to easily transfer these pulses into the ultraviolet at 267 nm via sum-frequency mixing in nonlinear crystals and to predictably generate multistructured ultraviolet femtosecond laser pulses. We analyze the temporal and spectral composition of these pulses after frequency conversion into the ultraviolet using difference-frequency cross-correlation and XFROG (cross-correlation frequency-resolved optical gating) techniques with an unmodulated fundamental laser pulse. The method can be employed to facilitate adaptive quantum control experiments in the ultraviolet wavelength regime, where the major absorption bands of many organic molecular systems are located.

We experimentally demonstrate molecular quantum control on the photoisomerization of retinal in bacteriorhodopsin with pump and shaped-dump femtosecond laser pulses. The objective is a reversion of regular control schemes for optimal excitation in which the pump pulse is shaped. Instead, we seek optimal de-excitation with a shaped-dump pulse. The pump–shaped-dump–probe scheme allows control of molecular systems away from the initial Franck–Condon window in regions of the potential-energy landscape where the decisive reaction step occurs. In addition, this method has the potential to provide information on wave packet evolution and underlying potential-energy surfaces.

We present the advances in x-ray femtosecond pulse generation and the most recent discoveries in the field of ultrashort (femtosecond) x-ray science. Nowadays x-rays show their potential not only when it comes to resolving atomic spatial scale but also the inherent temporal scale of quantum dynamics in atoms, molecules and solids. We discuss ultrafast x-ray sources that are currently used to generate femtosecond duration pulses of soft and hard x-ray radiation. Several techniques of x-ray pulse characterization are presented along with a method to control the shape of coherent soft x-rays. A large number of experiments using femtosecond x-ray pulses have been conducted recently and we review some of them. The field of ultrafast x-ray science draws its strength from the large variety of different sources of femtosecond duration x-ray pulses that are complementary rather than competing.

Optimization of multiphoton ionization of atomic calcium has been studied by adaptive femtosecond laser pulse shaping together with ion and electron spectroscopy. The photoelectron spectrum obtained following ionization of calcium utilizing 800 nm femtosecond laser pulses consists of three major contributions which can be assigned to ionization channels via intermediate ionic and doubly excited resonances. On the basis of the results found in the optimization experiments performed on the ion and electron signals, we conclude that non-sequential double ionization involves the Ca**(3d2) bound doubly excited state as well as states of the Ca**(3dnl) and the Ca**(4pnl) manifolds as intermediate resonances.

Femtosecond photoelectron spectroscopy was employed to study the excitation of trans-stilbene above the isomerization reaction barrier. Apart from the S1 contribution, evidence of a second electronic state is found based on two different transients measured across the photoelectron spectrum. Time Dependent Density Functional Theory calculations on S0, S1, S2 and D0, together with simulations of the electron energy distribution support the experimental findings for selective photoelectron energies of the S0, S1,… electronic states.

This work studied the emission of XUV radiation from water microdroplets under excitation with either a single or a pair of intense femtosecond laser pulses (Ti:Sa, 80 fs, ∼1014 W/cm2 , 800 nm, 1 kHz). When varying the delay between the two pulses a transition from pure incoherent plasma emission to coherent high-harmonic generation was observed. Under optimized conditions high-harmonic radiation up to the 27th order was obtained.

It is shown that the efficiency of attosecond pulse and high-harmonic generation in the ionization of excited molecular structures by a powerful femtosecond optical pulse can appreciably exceed the efficiency of analogous processes in atomic systems. This is due to the presence of a delocalized electron wave-packet component in the nonequilibrium molecular states, resulting in an increase of the number of particles that are effectively involved in the bremsstrahlung generation in the course of recollisions of laser-accelerated electrons with molecular core. Calculations suggest that, by optimizing the nonlinear response of molecular systems in the ionization process, one can develop compact sources of coherent vacuum ultraviolet and X-ray radiation with luminance at a level that is presently achieved only at large-scale accelerator facilities with free-electron lasers.

Obtaining active control over quantum-mechanical systems is a fascinating prospect in modern physics. In recent years, quantum control has developed as a means of actively controlling the outcome of light-matter interactions. The basic principles of adaptive femtosecond quantum control are discussed. Specifically shaped femtosecond laser pulses are used to direct the dynamical evolution of quantum wavefunctions into desired target channels. This is achieved in a ‘closed-loop’ implementation wherein experimental feedback signals are evaluated by a learning algorithm. In this iterative optimization scheme, knowledge of the Hamiltonian is not required. Specific experimental examples are automated compression of amplified femtosecond laser pulses and control of gas-phase molecular photodissociation in Fe(CO)5, CpFe(CO)2Cl and CH3CHOHCOOH. Additionally, selective control of molecular excitation is demonstrated under liquid-phase conditions. As a further technological development, the technique of femt...

Femtosecond adaptive pulse shaping of 800-nm laser pulses is applied to control the multiphoton molecular excitation of the charge-transfer coordination complex [Ru(dpb)3](PF6)2 (where dpb=4,4′-diphenyl-2,2′-bipyridine) dissolved in methanol. A phase-only femtosecond pulse shaper provides a mechanism for multiparameter (128) variation of the incident field, and a closed-loop evolutionary algorithm optimizes pulse shapes within the vast search space. Molecular emission at 620 nm is used as experimental feedback which is proportional to the excited-state population in the long-lived 3MLCT (metal-to-ligand charge-transfer) state. The dominant intensity dependence of the multiphoton excitation process is removed by using second-harmonic generation (SHG) in a thin optical crystal as a general “reference” signal. Successful control of the emission/SHG ratio demands that the field adapt to the electronic structure or dynamic needs of the molecule in solution. This suggests that adaptive femtosecond pulse shaping can provide a general means of finding field shapes capable of selectively exciting molecules based on their unique optical properties.

Adaptive femtosecond laser pulse shaping is employed to achieve bond selective photodissociation/photoionization of CH2ClBr in the gas phase. The photoproduct signal measured in a reflectron time-of-flight mass spectrometer is used as feedback to improve iteratively the spectral phases of the laser pulse via an evolutionary algorithm. We observe an increase of the fission of the stronger versus the weaker carbon halogen bond by 100%. Single parameter control schemes proved unable to achieve bond-selectivity. The complexity of the control problem is addressed by mapping it onto the well-known problem of maximizing second-harmonic generation (SHG). Further spectroscopic results indicate that the control involves manipulation of wave-packet dynamics on the neutral surfaces.

We report femtosecond pump–probe observations of thermal Na + Kr collisions using a crossed atomic beam arrangement. Though the signal level is very small, a detectable signal is observed which shows the characteristic expected features with respect to intensity, product particle velocity, and pump–probe delay.

We demonstrate the generation and complete characterization of femtosecond laser pulses which change their intensity, frequency, and light polarization almost arbitrarily within a single pulse employing the new technique of femtosecond polarization pulse shaping. Specifically, the degree of polarization ellipticity as well as the orientation of the elliptical principal axes can be varied as a function of time in a completely controllable manner, using a 128-pixel, two-layer, liquid-crystal display (LCD) inside a zero-dispersion compressor. A mathematical formalism is presented with which polarization-shaped pulse parameters can be calculated and used to generate intuitive quasi-three-dimensional electric field representations. However, laboratory realization requires accurate and complete experimental pulse characterization methods. For this purpose, the technique of dual-channel spectral interferometry is employed. Furthermore, Jones calculus for polarization-shaped pulses with experimentally determined Jones matrices is developed. It can be used to predict and account for all pulse-shape modifications occurring in various optical elements of the pulse shaper and the experimental setup.

High harmonic generation at 1 kHz repetition rate is reported. A piezoelectric pulsed valve together with a femtosecond Ti:Sapphire laser both operating at 1 kHz are employed to generate high harmonic radiation in xenon and argon. The characteristics of the valve have been analyzed by using a fast ion gauge and a time-of-flight mass-spectrometer.

Adaptive femtosecond laser pulse shaping is employed to achieve active quantum control in selected gas phase photodissociation and photoionization processes of CpFe(CO)2Cl. An evolutionary optimization algorithm uses experimental feedback from the photofragment mass spectra in a learning loop setup to improve the applied spectral laser pulse phases iteratively. The significance of the resulting complex temporal electric-field profiles is investigated by artificial reduction of the optimization parameter space. The degree of problem complexity is further studied by correlation measurements of the molecular control objective versus second-harmonic generation (SHG). In the case of direct ionization, the previously unknown molecular control problem can be mapped one-to-one onto the search space topology of SHG, whereas in the case of photoproduct ratio optimization, no clear correlation is observed. The control mechanism in the latter example is therefore much more complex and cannot be explained by a simple laser pulse intensity variation. Correlation techniques in a general sense are considered to be very helpful in solving the so-called problem of inversion, i.e., obtaining information about the underlying photophysical and photochemical processes.

Femtosecond-laser spectroscopy is used to study the photoionization and photofragmentation of large neutral silicon clusters in a beam. Silicon clusters Sin with sizes up to n≈6000, corresponding to nanoparticles with diameters up to 6 nm, are generated in a laser vaporization source. Nanosecond- and femtosecond-laser ionization are employed to characterize the free silicon nanoparticles. Excitation with intense femtosecond-laser pulses leads to prompt formation of doubly and triply charged Sin clusters. Additionally, strong fragmentation of charged clusters occurs by Coulomb explosion, resulting in high released kinetic energies. Multiply charged atoms up to Si4+ with initial kinetic energies in the range of 500 eV are observed for laser intensities of about 1013 W/cm2. Pump–probe spectroscopy yields decay times of the intermediate resonances of the nanoparticles.

We present the application of a time-dependent model to Fe(CO)5 dissociation after femtosecond excitation to understand transient signals obtained in recent pump/probe experiments. The calculation of time-dependent energy distributions allows us to interpret the decay behaviour of the various fragments. The results indicate that at least two dissociation pathways have to be considered to explain the transient signals obtained for the different fragments. Furthermore, it is shown that, within each of the pathways discussed, concerted and sequential fragmentation compete.

We report on time-resolved multiphoton ionization and fragmentation dynamics of free Hg n -clusters (n≤110) with femtosecond laser pulses. Already at laser intensities of 1011 W/cm2 we observe prompt formation of singly, doubly, and triply charged Hg n clusters. In pump-probe experiments short-time (0–5 ps) oscillatory modulations of the transient Hg n + and Hg n 2+ signals are observed which indicate an intermediate state dynamics common to all observed cluster sizes and charge states. Long-time (0–200 ps) transient ionization signals show caged dissociation and recombination chromophore dynamics.

The photodissociation dynamics of [CpFe(CO) 2 ] 2 in the gas phase is investigated with femtosecond time resolution. Using femtosecond laser pulses at 400 and 800 nm, the parent molecular ion and corresponding fragment ions are observed with a reflectron time-of-flight (TOF) spectrometer. Employing the pump–probe technique their ionization transients are measured. We find that the photodissociation dynamics of this molecule having two different metal–ligand bonds and one metal–metal bond is dominated by the dynamics of three different intermediate states.

Optical second-harmonic generation (SHG) at the surface of sodium and potassium clusters was studied using fs laser pulses. The size dependence of the SHG intensity can be explained by resonant enhancement by surface plasmon–polariton excitation. At different cluster sizes the second-order interferometric autocorrelation of a 25 fs pulse was recorded using the clusters as nonlinear medium. Compared to a reference autocorrelation, the trace exhibits no broadening within the experimental error of 1 fs. Taking into account inhomogeneous line broadening due to the particle size distribution, the single cluster plasmon dephasing time is found to be τ ≤15 fs. A lower limit of τ ≥1 fs is obtained from the width of the linear extinction spectrum.

We have investigated the radial Rydberg wave-packet dynamics in the two-valence-electron atom calcium by femtosecond pump-probe spectroscopy. The classical periodic motion of the Kepler electron as well as the quantum mechanical revival structure were observed when employing the phase sensitive detection technique. Preliminary results are reported on Rydberg wave-packet autoionization under the influence of an isolated excitation of the remaining outer core electron.

Femtosecond laser spectroscopy was employed to study photoionization and photofragmentation processes of neutral CdTe clusters produced in a laser vaporization source. With time-of-flight mass spectrometry we studied cluster size distributions and identified Te n clusters ( n ⩽30) as well as Cd x Te y hetero-clusters. A strong signal at 4275 amu observed only with fs-laser ionization is tentatively assigned to a Cd 5 Te 29 cluster. First results are reported on the ultrafast dynamics of CdTe-cluster photoionization using pump–probe spectroscopy.

Femtosecond pump-probe techniques are applied to the measurement of above-threshold ionization (ATI) electrons to observe the influence of vibrational wave-packet dynamics on molecular above threshold ionization in real time. When ionized by a single laser pulse structural changes in the ATI electron spectra map the changes of the vibrational wave-packet evolution in the resonant intermediate electronic states as a function of laser intensity. Molecular vibrational motion in the coherently coupled electronic states dominates the electron energy distribution rather than the energy splitting induced by Rabi oscillations.

Direct illumination of the tunneling gap in an ultrahigh vacuum scanning tunneling microscope with ultrashort pump-probe laser pulses may offer ultimate spatial and temporal resolution in surface experiments. The electronic bandwidth of the tunneling gap (\(

Coherent control of molecular multiphoton ionization by a single phase shaped femtosecond laser pulse is reported. Electron and ion spectra of the sodium dimer are recorded in a molecular beam experiment and both show a strongly chirp- and pulse-length-dependent behavior. The measured spectra reveal that although for one chirp direction the population in all intermediate molecular states may be higher, the opposite chirp leads to a higher ionization yield. In addition, a strongly chirp-dependent photoionization of atomic sodium is observed.

Femtosecond time resolved pump-probe experiments studying wave packet dynamics in the (2)1Σu+ double minimum state of Na2 are reported. The experiments were performed in a molecular beam with ion Time of Flight (TOF) detection. By Fast Fourier Transformation (FFT) of the observed time domain data the energy spacings of the coherently coupled vibrational levels in the (2)1Σu+ potential are obtained with an accuracy of 0.02 cm−1, although an ultrafast laser source with its inherent spectral width was used in the experiment.

The real-time dynamics of multiphoton ionization and fragmentation of molecules Na2 and Na3 has been studied in molecular beam experiments employing ion and electron spectroscopy together with femtosecond pump-probe techniques. Experiments with Na2 and Na3 reveal unexpected features of the dynamics of the absorption of several photons as seen in the one- and three-dimensional vibrational wave packet motion in different potential surfaces and in high laser fields: In Na2 a second major resonance-enhanced multiphoton ionization (REMPI) process is observed, involving the excitation of two electrons and subsequent electronic autoionization. The possibility of controlling a reaction by controlling the duration of propagation of a wave packet on an electronically-excited surface is demonstrated. In high laser fields, the contributions from direct photoionization and from the second REMPI process to the total ion yield change, due to different populations in the electronic states participating in the multiphoton ionization (MPI) processes. In addition, a vibrational wave packet motion in the electronic ground state is induced through stimulated emission pumping by the pump laser. The 41Σ+g shelf state of Na2 is given as an example for performing frequency spectroscopy of high-lying electronic states in the time domain. Pure wave packet effects, such as the spreading and the revival of a vibrational wave packet, are investigated. The three-dimensional wave packet motion in the Na3 reflects the normal modes in the X and B states, and shows in addition the pseudorotational motion in the B state in real time.

We report on femtosecond laser-induced catalytic reactions of carbon monoxide and hydrogen on single crystal surfaces under high vacuum conditions. Several product molecules are synthesized, among them also species for whose formation at least three reactants are required. By applying closed-loop optimal control, we manipulate these reactions and selectively optimize the ratio of different bond-forming reaction channels, in contrast to previous quantum control experiments aiming at bond-cleavage. Further experiments explore the nontrivial control mechanism and its sensitivity to the relative proportion of the two reactant gases.

We report time-resolved experiments studying the dynamics of the Na 3 B—X system. Femtosecond pump-probe techniques combined with ion time-of-flight (TOF) and zero kinetic energy (ZEKE) photoelectron spectroscopy allow us to observe the three-dimensional wavepacket motion in the excited Na 3 B state and in the Na 3 X state. The ground state wavepacket is induced by stimulated emission pumping during the pump pulse. The X-state dynamics is dominated by the three vibrational modes of the Na 3 . Furthermore we observed pseudorotational wavepacket motion in the B state. We do not observe a fragmentation of the B state within a time interval of 10 ps.

We present a comparison between experimental and theoretical results for pump/probe multiphoton ionizing transitions of the sodium dimer, initiated by femtosecond laser pulses. It is shown that the motion of vibrational wave packets in two electronic states is probed simultaneously and their dynamics is reflected in the total Na2+ ion signal which is recorded as a function of the time delay between pump and probe pulse. The time dependent quantum calculations demonstrate that two ionization pathways leading to the same final states of the molecular ion exist: one gives an oscillating contribution to the ion signal, the other yields a constant background. From additional measurements of the Na+-transient photofragmentation spectrum it is deduced that another ionization process leading to different final ionic states exists. The process includes the excitation of a doubly excited bound Rydberg state. This conclusion is supported by the theoretical simulation.

The real-time dynamics of Nan (n=3–21) cluster multiphoton ionization and fragmentation has been studied in beam experiments applying femtosecond pump-probe techniques in combination with ion and electron spectroscopy. Three dimensional wave packet motions in the trimer Na3 ground state X and excited state B have been observed. We report the first study of cluster properties (energy, bandwidth and lifetime of intermediate resonancesNan*) with femtosecond laser pulses. The observation of four absorption resonances for the cluster Na8 with different energy widths and different decay patterns is more difficult to interpret by surface plasmon like resonances than by molecular structure and dynamics. Time-resolved fragmentation of cluster ions Nan+ indicates that direct photo-induced fragmentation processes are more important at short times than the statistical unimolecular decay.

We report on the first femtosecond time-resolved experiments in cluster physics. The photofragmentation dynamics of small sodium cluster ions ${\mathrm{Na}}_{\mathit{n}}^{+}$ have been studied with pump-probe techniques. Ultrashort laser pulses of 60-fs duration are employed to photoionize the sodium clusters and to probe the photofragments. We find that the ejection of neutral dimer ${\mathrm{Na}}_{2}$ and, observed for the first time, neutral trimer ${\mathrm{Na}}_{3}$ photofragments occur on ultrashort time scales of 2.5 and 0.4 ps, respectively. This and the absence of cluster heating reveals that direct photoinduced fragmentation processes are important at short times rather than the statistical unimolecular decay.

The motion of a vibrational wave packet in the bound A (1Σu+) electronic state of the sodium dimer is detected in a femtosecond pump/probe molecular beam experiment. For short times harmonic motion is seen in the total ion yield of Na2+ as a function of delay time between the two laser pulses. The spreading of the wave packet results in the loss of the periodic variation of the ion signal. For longer delay times (47 ps) the wave packet regains its initial form which is reflected in the revival structure of the Na2+ signal. Time-dependent quantum calculations reproduce the measured effects.

Optimal control techniques combined with femtosecond laser pulse shaping are applied to steer and enhance the strong-field induced emission of highly charged atomic ions from silver clusters embedded in helium nanodroplets. With light fields shaped in amplitude and phase we observe a substantial increase of the Ag$^{q+}$ yield for $q>10$ when compared to bandwidth-limited and optimally stretched pulses. A remarkably simple double-pulse structure, containing a low-intensity prepulse and a stronger main pulse, turns out to produce the highest atomic charge states up to Ag$^{20+}$. A negative chirp during the main pulse hints at dynamic frequency locking to the cluster plasmon. A numerical optimal control study on pure silver clusters with a nanoplasma model converges to a similar pulse structure and corroborates, that the optimal light field adapts to the resonant excitation of cluster surface plasmons for efficient ionization., 7 pages, 5 figures, text revised

In recent years, the use of joint time-frequency representations to characterize and interpret shaped femtosecond laser pulses has proven to be very useful. However, the number of points in a joint time-frequency representation is daunting as compared with those in either the frequency or time representation. In this article we introduce the use of the von Neumann representation, in which a femtosecond pulse is represented on a discrete lattice of evenly spaced time-frequency points using a non-orthogonal Gaussian basis. We show that the information content in the von Neumann representation using a lattice of radicalN points in time and radicalN points in frequency is exactly the same as in a frequency (or time) array of N points. Explicit formulas are given for the forward and reverse transformation between an N-point frequency signal and the von Neumann representation. We provide numerical examples of the forward and reverse transformation between the two representations for a variety of different pulse shapes; in all cases the original pulse is reconstructed with excellent precision. The von Neumann representation has the interpretational advantages of the Husimi representation but requires a bare minimum number of points and is stably and conveniently inverted; moreover, it avoids the periodic boundary conditions of the Fourier representation.

We investigate femtosecond laser-induced surface reactions by varying the properties of the surface, the reactant gases, and the laser. In optimal control experiments, we selectively manipulate the bond-forming catalytic reactions.

We report here the first experimental study of femtosecond time-resolved molecular multiphoton ionization. Femtosecond pump-probe techniques are combined with time-of-flight spectroscopy to measure transient ionization spectra of ${\mathrm{Na}}_{2}$ in a molecular-beam experiment. The wave-packet motions in different molecular potentials show that incoherent contributions from direct photoionization of a singly excited state and from excitation and autoionization of a bound doubly excited molecular state determine the observed transient ionization signal.

the methyl iodide molecule as quasi-diatomic, the cytindrical symmetry of the problem allows us to place the perpendicular transition dipole vector anywhere in the x-y plane. Lao et al. define their perpendicular transition dipole as being in the (l,l,O) direction, but the algebra is somewhat less cumbersome if the dipole is chosen along either the x or y molecule-fixed axes. The electric vectors for the incident and scattered radiation are given in our coordinate system by 8, = 2 cos 7 + 2 sin 7 8, = 2 cos 8 + P sin 8 (A.4)

Publisher Summary Doubly excited states play a major role in the reaction dynamics of diatomic molecules as these states directly couple the different continua of dissociation and ionization. Electronic autoionization and neutral dissociation are therefore competing processes, but information from both channels may be used to characterize the doubly excited molecular states. Application of femtosecond (fs) laser pulses considerably facilitates the interpretation of fragmentation processes because the time duration of a laser pulse is much shorter than the fragmentation separation time. Femtosecond pulses are generated in a home-built colliding-pulse-mode-locked ring dye laser (CPM) with four intracavity prisms to adjust for the group. This chapter illustrates the use of fs-laser in combination with ion- and electron spectroscopy to clarify the ionization and fragmentation pathways of molecules excited by multiphoton processes.

We review the progress in controlling quantum dynamical processes in the condensed phase with femtosecond laser pulses. Due to its high particle density the condensed phase has both high relevance and appeal for chemical synthesis. Thus, in recent years different methods have been developed to manipulate the dynamics of condensed-phase systems by changing one or multiple laser pulse parameters. Single-parameter control is often achieved by variation of the excitation pulse’s wavelength, its linear chirp or its temporal subpulse separation in case of pulse sequences. Multiparameter control schemes are more flexible and provide a much larger parameter space for an optimal solution. This is realized in adaptive femtosecond quantum control, in which the optimal solution is iteratively obtained through the combination of an experimental feedback signal and an automated learning algorithm. Several experiments are presented that illustrate the different control concepts and highlight their broad applicability. These fascinating achievements show the continuous progress on the way towards the control of complex quantum reactions in the condensed phase.

We experimentally demonstrate a novel quantum-control scheme involving pump and optimally-shaped dump laser pulses. This allows selective control of molecular dynamics all the way to the product state and provides information on potential energy surfaces.

We report on femtosecond laser-assisted catalytic reactions of syngas (CO+H2) on a Pd(100) single crystal surface under high vacuum conditions. Several product molecules are synthesized (e.g. CH3 + and H2CO+). Using closed-loop optimal control, we manipulate these reactions and selectively optimize the ratio of different bond-forming reaction channels, in contrast to previous quantum control experiments aiming at bond-cleavage.

Fitness landscapes based on a limited number of laser pulse shape parameters can elucidate reaction pathways and can help to find the underlying control mechanism of optimal pulses determined by adaptive femtosecond quantum control. In a first experiment, we employ colored double pulses and systematically scan both the temporal subpulse separation and the relative amplitude of the two subpulses to acquire fitness landscapes. Comparison with results obtained from a closed-loop experiment demonstrates the capability of fitness landscapes for the revelation of possible control mechanisms. In a second experiment, using transient absorption spectroscopy, we investigate and compare the dependence of the excitation efficiency of the solvated dye molecule 5,5{sup '}-dichloro-11-diphenylamino-3,3{sup '}-diethyl-10,12-ethylene thiatricarbocyanine perchlorate (IR140) on selected pulse shapes in two parametrizations. The results show that very different pulse profiles can be equivalently adequate to maximize a given control objective. Fitness landscapes thus provide valuable information about different pathways along which a molecular system can be controlled with shaped laser pulses.

The molecule 3,3'-diethyl-2,2'-thiacyanine (NK88) isomerizes after irradiation with light of the proper wavelength. After excitation, it undergoes a transition, in which one or more conical intersections are involved, back to the ground state to form different product photoisomers. The dynamics before and directly after the transition back to the ground state is investigated by transient absorption spectroscopy in a wavelength region from 360~nm to 950~nm, as well as by fluorescence upconversion. It is shown that the excited-state dynamics are governed by two timescales: a short one with a decay time of less than 2~ps and a long one with about 9~ps. A thorough comparison of the experimental results with those of CIS (configuration interaction singles) and TD-DFT (time-dependent density functional theory) calculations suggests that these dynamics are related to two competing pathways differing in the molecular twisting on the excited surface after photoexcitation. From the experimental point of view this picture arises taking into account the timescales for ground-state bleach, excited-state absorption, stimulated emission, fluorescence and assumed hot ground-state absorption both in the solvent methanol and ethylene glycol.

We experimentally demonstrate the generation and characterization of polarization-shaped femtosecond laser pulses in the ultraviolet at a central wavelength of 400 nm. Near-infrared laser pulses are first polarization shaped and then frequency doubled in an interferometrically stable setup that employs two perpendicularly oriented nonlinear crystals. A new pulse shaper design involving volume phase holographic gratings reduces losses and hence leads to an increase in pulse energy.

We employ a 128-pixel liquid-crystal spatial light modulator to generate variable pulse sequences from a titanium-sapphire femtosecond laser amplifier system centered at 800 nm. By applying phase modulations based on triangular-shaped spectral phase patterns, pulse sequences can be generated whose overall spectrum is still conserved but whose subpulses differ in their spectral composition. We further use nonlinear crystals to analyze the shape of these pulses after frequency conversion. Our experiments show that it is possible to transfer these pulse sequences into the ultraviolet at central wavelengths of 400 nm and 267 nm without losing the ability to spectrally distinguish between the femtosecond subpulses. This is affirmed by measurements using cross-correlation and XFROG (cross-correlation frequency resolved optical gating) techniques with an unmodulated laser pulse. Simulations of the experiments are performed for comparison. We also discuss promising applications in spectroscopy or information encoding.

With the invention of the laser, the dream was realized to actively exert control over quantum systems. Active control over the dynamics of quantum mechanical systems is a fascinating perspective in modern physics. Cleavage and creation of predetermined chemical bonds, selective population transfer in atoms and molecules, and steering the dynamics of bound and free electrons have been important milestones along this way. A promising tool for this purpose is available with femtosecond laser technologies. In this chapter we review some of our work on adaptive femtosecond quantum control where a learning algorithm and direct experimental feedback signals are employed to optimize user-defined objectives. Femtosecond laser pulses are modified in frequency-domain pulse shapers, which apart from phase- and intensity-modulation can also modify the polarization state as a function of time. We will highlight the major advances in the field of optimal control by presenting our own illustrative experimental examples such as gas-phase and liquid-phase femtochemistry, control in weak and strong laser fields, and control of electron dynamics.

We report on optimal control of the photoisomerization of 3,3-diethyl-2,2-thiacyanine iodide dissolved in methanol. Enhancement and reduction of the relative yield of cis to trans isomers are achieved; i.e., the quantum efficiency of the photoisomerization is controlled with optimally phase and amplitude shaped 400 nm femtosecond laser pulses. Single-parameter control schemes, like chirp or intensity variation, fail to change the ratio of the photoproducts. The successful modification of the molecular structure can be regarded as a first step towards controlled stereoselectivity in photochemistry.

This chapter discusses the molecule of stilbene that is a well-known model system for photo-induced isomerization reactions. Due to space interaction of the phenyl rings, the vertical ionization potentials of cis- and trans-stilbene isomers are well separated by about 300meV. Therefore, the photoelectron spectrum following the excitation of the molecule with an ultrashort probe laser pulse can be used as experimental feedback for a potential feedback controlled optimization of photoisomerization reaction. This chapter serves as first step towards this goal and involves measured photoelectron spectra of trans-Stilbene following the excitation with tuneable femtosecond UV laser pulses. The ionization process occurs in a (1+1) REMPI scheme, using the first excited singlet state (Si) of trans-Stilbene as an intermediate resonance. When the excitation is resonant with the origin of the intermediate S 1 state, the measured photoelectron distribution reveals a maximum probability for the 0-0 transition. For higher photon energies (266nm) the photoelectron spectrum exhibits a rather complex distribution, due to the excitation of an alternate (C-C) stretching mode.

Since the advent of quantum mechanics there has been a desire to understand in detail the behavior of quantum systems. This quest has been accompanied by the implicit dream to not only be able to observe in a passive way, but in fact also to actively control quantum-mechanical processes. The key question in quantum control is: Can one find external control parameters which guide the temporal evolution of quantum-mechanical systems in a desired way, even if this evolution is very complex?