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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.

High-harmonic generation provides an attractive light source of coherent radiation in the extreme-ultraviolet (XUV) and soft-x-ray regions of the spectrum and allows for the production of single attosecond pulses or pulse trains. This Colloquium covers the control of high-harmonic spectra by temporal and spatial pulse shaping of the driving laser pulses and its implications on time-resolved XUV spectroscopy and attosecond pulse shaping. It summarizes important steps for extending existing pulse shaping techniques and control schemes from the near-infrared or visible part to shorter wavelengths. Using adaptive pulse shaping of the driving laser pulses, several groups have demonstrated control of the high-harmonic spectrum, including the author's work on the complete control over the XUV spectrum of high-order harmonics, generated in a gas-filled hollow fiber. It is possible to achieve both the enhancement and the suppression of single or several selected harmonic orders. These arbitrarily shaped soft-x-ray spectra will allow for important modifications of the resulting harmonic pulses in the temporal domain. This constitutes first steps towards direct attosecond pulse shaping in the soft-x-ray domain. Moreover, high-harmonic generation in a hollow-core fiber can be enhanced by coupling into a single fiber mode using a feedback-controlled adaptive two-dimensional spatial light modulator.

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.

We demonstrate the generation of arbitrarily shaped spectra of coherent soft X-ray radiation by adaptive control of the driving laser pulse. The selection of both narrow and wide spectral ranges as well as suppression of high-harmonic emission in a selected wavelength region is achieved. Feedback-controlled spectral engineering of coherent soft X-rays paves the way to shaping of soft X-ray pulses and coherent control of electron dynamics in the sub-femtosecond regime.

We show that the efficiency of single attosecond pulse generation during atomic ionization by an intense few-optical-cycle laser pulse can be enhanced dramatically when pre-excited electronic states are used. This is due to the slowed-down spreading of the laser-driven free-electron wave packet. We develop an analytical approach to the treatment of the attosecond burst production due to the electron recollision with a parent ion, assuming that the atom is fully ionized during one optical cycle. Our analytical and numerical simulations show that, when a hydrogen atom is ionized from the 2s-state by an ultrashort pulse of a Ti:sapphire (800 nm) laser, the energy of the generated attosecond pulse is several tens of times higher, its duration is much shorter, and the spectral intensity of radiation in the VUV and soft–X-ray range is several orders of magnitude superior to that in the case of ionization from the ground state. We determine the excited states preferable for experiments using the fundamental or the second harmonic of a Ti:sapphire laser.

We investigate at TD-DFT/PBE0/6-31+G(d,p) level of theory, the two lowest Bu states of the trans isomer of stilbene and two stiff-stilbenes, whose phenyl rings are blocked in the molecular plane by alkylic bridges, and whose two first Bu bands have been observed. At the Franck–Condon point, computations have been performed both in gas and in condensed phase by PCM method, and the results for the two stiff-stilbenes are in very good agreement with the experiments, predicting a strongly absorbing Bu state with a main HOMO → LUMO character and, about 0.6–0.7 eV higher in energy, a second weak absorbing Bu state, combination of the two HOMO → LUMO + 1 and HOMO-1 → LUMO excitations. A very similar picture is predicted for trans-stilbene in agreement with the experimental data and estimate. The dependence of the vertical excitation energies on the C–C ethylenic bond-distances and on the rotation of the phenyl rings has been explored for trans-stilbene in gas phase.

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.

The time behaviour of ultrashort laser pulses can be changed and optimized for a specific application by the combination of pulse-compression and pulse-shaping techniques. A pulsed laser beam at a central wavelength of 800 nm and 80 fs pulse duration is guided through a hollow fibre of 70cm length, filled with argon gas. Due to the nonlinear optical effect of self-phase modulation the pulse spectrum broadens during propagation. After separating the spectral components of the modified pulse with a prism and lens setup, a deformable-membrane mirror is used to control the spectral phase. By means of an evolutionary algorithm, it was possible to remove higher-order phase contributions in the final pulse shape as well as additional dispersion.

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.

We compare high-harmonic generation (HHG) for an atomic and a diatomic molecular system. This is done by studying the cllipticity dependence for the generation process in the two representative gases Ar and N2 as they have almost identical ionization potential and average static polarizabilities. The experiment was carried out by changing the initial polarization state of an 800 nm, 80 fs laser pulse, which was focused to an intensity of 3 × 1014 W cm−2. We report an overall weaker dependence on ellipticity for the molecular system. In addition, a simulation based on the three-step model is presented, which is then used to discuss the experimental results. We point out that to explain molecular HHG fully, a theory which goes beyond the three-step model has to be established.

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.

We report a systematic experimental study of the influence of metalligand bonding properties on the ultrafast photofragmentation dynamics and on quantum control of organometallic molecules in the gas phase. In the first part, the femtosecond pump–probe technique is combined with reflectron time-of-flight mass spectrometry to record transient mass spectra of the photodissociation products of CpFe(CO) 2 X (Cp=C 5 H 5 ; X=Cl, Br, I). Sequential dissociation steps are observed for the X=Cl species while concerted and more complex mechanisms are seen in the molecules involving the larger halogens. The overall dissociation rate increases from the lighter toward the heavier halogen ligands in this series which can be explained qualitatively by the different metalhalogen bonding properties and their stabilizing effects on unsaturated intermediate species. In the second part, adaptive quantum control experiments are performed on the X=Cl and X=Br molecules. A femtosecond laser pulse shaper provides spectrally phase-modulated electric fields which are then iteratively optimized by a learning evolutionary algorithm. Direct experimental feedback from the photoproduct mass spectra is used in this search for specific optimization goals. A comparison between the two molecular species shows that laser fields optimized for one molecule also lead to an improvement in the species with the substituted halogen ligand, but not to the same degree as when the substituted molecule is directly optimized. Despite their chemical similarities, adaptive pulse shaping is sensitive enough to detect differences in the investigated metal complexes which are due to the electronic metalhalogen bonding properties. These results suggest an application in chemical analysis where specific signatures are desired for the individual compounds in mixtures of chemically similar molecules.

High harmonic generation is compared in the dependence on the ellipticity of the fundamental laser radiation for an atomic and a molecular system. In particular argon and nitrogen are compared employing molecular beams and intense ( 3×1014 W/cm2) and ultrashort (80 fs) 800 nm laser pulses. It turns out that for all the harmonics under investigation (H5, H13 and H21) the harmonic yield decreases slower with the ellipticity for the molecule than for the atom. This indicates differences in atomic and molecular high harmonic generation.

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.

Photofragmentation of Na{sup +}{sub 2} molecules in well prepared vibrational levels has been studied employing intense (10{sup 11}{endash}10{sup 14} W/cm{sup 2} ) and ultrashort (80fs) 790nm laser fields. Four fragmentation channels with different released kinetic energies are observed. Depending on the applied laser intensity, the fragmentation of Na{sup +}{sub 2} is governed by photodissociation on light-induced potentials and field ionization followed by Coulomb explosion. Below 1{times}10{sup 12} W /cm{sup 2} , only photodissociation on light-induced potentials is seen. For intermediate laser intensities, field ionization at large internuclear distances competes with photodissociation, thus preventing the observation of above threshold dissociation. Field ionization at small internuclear distances dominates for the highest laser intensities used.

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.

Mithilfe der Femtosekunden-Spektroskopie lassen sich atomare Bewegungen in Molekulen zeitaufgelost beobachten. Seit kurzem ist es mit speziell geformten Femtosekunden-Laserpulsen sogar moglich, uber diese blose Beobachtung hinauszugehen, „von ausen” steuernd einzugreifen und den Ablauf molekularer Dynamik „mikroskopisch” gezielt zu beeinflussen. Durch den Einsatz lernender Algorithmen mit experimenteller Ruckkopplung lassen sich komplexe Licht-Materie-Wechselwirkungen in vielen Anwendungsbereichen optimieren. In diesem Artikel wird schwerpunktmasig die Quantenkontrolle der Femtochemie, also die Steuerung chemischer Prozesse auf einer Femtosekunden-Zeitskala, diskutiert.

Two-photon photoemission spectroscopy using femtosecond laser pulses is used to investigate the excitation and decay mechanisms of the surface plasmon resonance in Ag nanoparticles grown on graphite. The resonant excitation of this collective excitation leads to a two-orders-of-magnitude-enhanced two-photon photoemission yield from a graphite surface with Ag nanoparticles compared to the yield from pure graphite. From the shape of the photoemission spectra, the polarization dependence of the photoemission yield and the excitation probabilities for different excitation pathways we conclude that excitation with 400-nm femtosecond laser pulses leads to the coherent multiple excitation of the surface plasmon in the Ag nanoparticles. This multiply excited plasmon mode can decay via the coupling to a single-particle excitation leading to the emission of an electron if its final state is located in the continuum. The surface plasmon in metallic nanoparticles is a model system to investigate collective excitations in multiphoton 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.

are the constituents of a molecular beam. Of importance are parameters such as laser intensity, pulse duration, and laser wavelength for a quantitative characterization and thereby improvement of MBE sources. This would help to optimize the epitaxial growth process. We demonstrate that nonresonant multiphoton ionization (MPI) by fs laser pulses leads to detection of the parent molecule, whereas nonresonant MPI with ns laser pulses leads to strong fragmentation.

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 have recorded the 351 nm photoelectron spectra of Bi−2, Bi−3, and Bi−4. The spectrum of Bi−2 shows transitions to at least seven electronic states of Bi2 neutral, four of which are observed with vibrational resolution. Term energies, bond lengths, and vibrational frequencies are obtained for the anion ground state and for the first three excited states of Bi2. These results are compared to previous spectroscopic measurements and to the ab initio calculations presented in the accompanying paper. The photoelectron spectrum of Bi−3 reveals some of the electronic structure of Bi3 and the results are discussed in comparison to recent theoretical work. Adiabatic electron affinities are obtained for Bi2 [1.271(8) eV] and for Bi3 [1.60(3) eV]. The electron affinity of Bi4 is estimated from the onset of photodetachment to be 1.05(10) eV.

We report the 351 nm photoelectron spectra of Sb−, Sb2−, Sb3−, and Sb4−. The electron affinity of atomic Sb is measured to be 1.046(5) eV. The Sb2− photoelectron spectrum displays rich vibrational and electronic structure. Low‐lying electronically excited states are observed for both the anion and the neutral. Several features in both the 351 and 364 nm photoelectron spectra of Sb2− cannot be explained as Franck–Condon processes, indicating that we are accessing autodetaching resonances of the negative ion at these wavelengths. The adiabatic electron affinity of Sb2 is determined to be 1.282(8) eV. For the photoelectron spectra of Sb3− and Sb4−, the observed electronic structure is explained in terms of recently reported ab initio calculations. The adiabatic electron affinity of Sb3 is estimated to be 1.85(3) eV, and an upper bound on the electron affinity of Sb4 is reported, EA(Sb4)≤1.00(10) eV. The vertical detachment energies of Sb3− and Sb4− to the neutral ground states are determined to be 1.90(2) an...

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.

The photofragment spectrum of N2+2→N+(3Pg)+N+(3Pg) has been studied using a coaxial laser–ion beam spectrometer. Transitions from the excited 3Σ+u(v=0,1) state of N2+2 to the predissociative 3Πg(v=0) state result in two bands with dense rotational structure at 15 300 and 13 100 cm−1. A complete analysis of the 27 rotational branches associated with the 3Πg(v=0)←3Σ+u(v=0) transition provides bond lengths and spectroscopic constants for both states. A perturbation is observed in the (0,0) band, caused by the interaction of 3Σ+u(v=0) with 3Πu(v≊10). From a deperturbation analysis, the coupling constants and the energy difference between the 3Πu(v≊10) and 3Σ+u(v=0) states are determined—ξ=1.5±0.1 cm−1, η=0.220±0.003 cm−1, and Tpert=365±25 cm−1. A broad unstructured band at 16 400 cm−1 arises from a transition out of v=0 in the 3Σ+u state into a very short‐lived v=1 level of the 3Πg state. Vibrational spacings between v=0 and v=1 in both electronic states are determined—3Σ+uω0=2210±20 cm−1 and 3Πg ω0≊1100 cm−1...

Two-photon ionization and fragmentation of the metal cluster Na 3 via the D (3 2 A 2 ) state have been studied with nanosecond pulsed dye lasers in the wavelength range 410–440 nm. We report on a molecular-beam experiment where, for this neutral metal cluster, the photoelectron spectrum and the mass and released kinetic energies of the ionic fragments Na + and Na + 2 as well as their angular distributions have been measured. On the basis of the ion and electron spectra, we conclude that ionization of Na 3 and subsequent fragmentation of Na + 3 into Na + 2 (X) + Na (3s) and Na + + Na 2 (X) occur rather than fragmentation of the excited neutral Na 3 cluster with subsequent excited-fragment photoionization.