A. Assion

Coherent control by a single phase shaped femtosecond laser pulse

Abstract 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.

Compact, robust, and flexible setup for femtosecond pulse shaping

We present an improved design and adjustment concept for femtosecond pulse shaping. The concept results in a compact and robust pulse shaping setup. A systematic adjustment procedure, high reproducibility and stability, as well as easy adaptability to different femtosecond laser sources are the key features of the presented design. The constructed prototype pulse shaper was tested in an open loop and feedback-controlled adaptive pulse shaping on two different femtosecond laser sources.

Femtosecond spectroscopy of the (2)1Σ u + double minimum state of Na2: time domain and frequency spectroscopy

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 wavelengths of the pump and probe laser pulses are chosen such that in this two color experiment we can control ionisation versus ionisation induced fragmentation.In order to study the influence of the potential barrier on a vibrational wave packet motion we performed simulations based on time dependent quantum calculations.

Filling a spectral hole via self-phase modulation

The effect of spectral amplitude modulation on self-phase modulation is studied. To that end we remove a small interval of frequency components from the broad spectrum of a femtosecond laser pulse. We investigate the regeneration of these missing frequency components via self-phase modulation. A water jet serves as a transparent sample. A physical model is given which explains the observation that the removed frequency components are not only replenished by self-phase modulation but can even overshoot their adjacent frequencies in power spectral density. In addition, we suggest possible applications in the field of nonlinear microscopy.

Femtosecond Pump-Probe Photoelectron Spectroscopy: Mapping of Vibrational Wave Packet Motion

Energy-resolved photoelectron spectroscopy in combination with femtosecond pump-probe and molecular beam techniques are used to map molecular dynamics along the internuclear coordinate. The capabilities of this method are demonstrated on the one-dimensional wave-packet motion on electronically excited states of the sodium dimer: by recording transient photoelectron spectra in a pump-probe experiment we are able to follow the molecular wave-packet motion on neutral electronic states along all energetically allowed internuclear distances simultaneously. @S1050-2947~96!51312-0#

Automated Coherent Control of Chemical Reactions and Pulse Compression by an Evolutionary Algorithm with Feedback

We experimentally demonstrate automated coherent control of a photodissociation reaction as well as femtosecond pulse compression using a pulse shaper and an evolutionary algorithm with feedback.

Short and Ultrashort Laser Pulses

In this contribution some basic properties of femtosecond laser pulses are summarized. In Sect. 12.1 we start with the linear properties of ultrashort light pulses. Nonlinear optical effects that would alter the frequency spectrum of an ultrashort pulse are not considered. However, due to the large bandwidth, the linear dispersion is responsible for dramatic effects. For example, a 10 fs laser pulse at a center wavelength of 800 nm propagating through 4 mm of BK7 glass will be temporally broadened to 50 fs. In order to describe and manage such dispersion effects a mathematical description of an ultrashort laser pulse is given first before we continue with methods how to change the temporal shape via the frequency domain. The chapter ends with a paragraph on the powerful technique of pulse shaping, which can be used to create complex-shaped ultrashort laser pulses with respect to phase, amplitude and polarization state.

Chapter 28 - Quantum control beyond spectral interference and population control: Can resonant intense laser pulses freeze the population?

The ability to measure and to control the quantum mechanical phase is the key step towards a deeper understanding of quantum control. Since the energy resolved photoelectron spectra from simultaneous excitation and ionization are directly related to the temporal evolution of the excited state (population and phase), this technique is most suited to elucidate details of the quantum control dynamics. In particular, the use of pulse sequences has proven a strong tool to study interference effects in atomic and molecular systems in detail. This scheme was extended to the continuum in order to demonstrate the coherence transfer from femtosecond laser pulses to ultrashort free electron wave packets. A variety of important control mechanisms is only accessible when strong laser fields are employed. Examples of coherent control by intense sequential laser pulses are coherent transients such as the photon echo and Ramsey fringes as well as the STIRAP. In this contribution, recent results on the control of the quantum mechanical phase of an atomic state in strong laser fields studied using the Autler-Townes (AT) effect in the photoionization of the K (4p) state are discussed. This chapter demonstrates quantum control beyond population control and spectral interference.

FEMTOSECOND PUMP-PROBE PHOTOELECTRON-SPECTROSCOPY ON ELECTRONIC STATES OF NA2: A TOOL TO STUDY ULTRAFAST CONTROL OF CHEMICAL REACTIONS

Microscopic control of the outcome of a chemical reaction is a long-standing dream in physical chemistry. Recently femtosecond lasers have emerged as a particularly suitable tool for quantum control of reaction dynamics. Assion et al. [1] were the first to demonstrate the use of tailored femtosecond laser pulses from a computer-controlled pulse shaper to control the branching ratios of various photodissociation channels in CpFe(CO)2Cl. Moreover, quite recently Levis et al. [2] have shown that high intensity laser pulses permit to control a dissociative rearrangement reaction in which chemical bonds are not only selectively broken, but also newly formed. Quantum control over chemical reactions may be obtained employing various so-called one parameter schemes [3-6]. Currently much attention has been focused on the utilization of adaptive feedback controlled femtosecond pulse shaping making available a most versatile instrument for multiparameter control schemes [1, 7-10]. These techniques have proven to be universal in the sense that they enable to optimize virtually any conceivable quantity and are capable to deliver the optimal electric field without the knowledge of the underlying potential energy surfaces (PES). However, the individual control mechanisms may be inferred, if at all, only for very simple systems. In order to get a better physical insight into the multi-parameter control driving such an experiment it is essential to investigate one parameter control schemes in detail on pertinent model systems.

Femtosecond pump-probe spectroscopy of surface bound alkali nanoparticles

Second harmonic generation at the surface of Na and K clusters was studied using femtosecond laser pulses. The observed size dependence of the second harmonic intensity can be explained by resonant enhancement of 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. There is no broadening of the cluster autocorrelation with respect to a non resonant reference autocorrelation within the experimental error of 1 fs. This experimental finding is independent of particle size, resonance of the fundamental (potassium) or the second harmonic (sodium) or surface modification by chemisorption of oxygen.