The dynamics of a THz Rydberg wavepacket

An optically excited Rydberg wavepacket can be generated by exciting the electron from a low-lying state to a coherent superposition of high-lying states with a short broadband optical pulse. A special kind of Rydberg wavepacket is generated in the case of a interaction of a weak THz half cycle pulse with a stationary Rydberg state, called the THz wavepacket. This THz wavepacket is a coherent superposition of the initial Rydberg state and its neighbouring states. We have investigated the time evolution of THz wavepackets by measuring the impact of two in time delayed half cycle pulses ( ≈ 200 V cm^-1) on the population of a stationary (n = 40) Rydberg state in rubidium. The first half cycle pulse creates the THz wavepacket and the second half cycle pulse probes the dynamics of the THz wavepacket. We support our experimental data by numerically solving the Schr?dinger equation and with a semi-classical picture. Whereas an optically excited wavepacket is initially localized, a THz wavepacket is initially delocalized and becomes localized after half a revival time. Received 23 August 2000 and Received in final form 27 March 2001     

Analysis and control of ultrafast photodissociation processes in organometallic molecules

In this paper we characterize the ultrafast fragmentation in electronically excited Fe(CO)₂(NO)₂ and CpMn(CO)₃ by means of femtosecond time-resolved spectroscopy combined with mass spectrometry. From the transient two-color multi-photon ionization data, it was possible to record the transients of the parent molecule ions and their photofragment ions. The experimentally observed decay times indicated an ultrafast loss of the first ligands (sub-100 fs decay times). Further we performed a feedback control experiment on the photofragmenting CpMn(CO)₃ molecular system in order to maximize the yield of desired ionic products through pulse modulation. The shape of the pulses obtained from optimization reflect well the intrinsic molecular dynamics during photofragmentation and the change of the CpMn(CO)⁺/CpMn(CO)₃ ⁺ ratio shows a clear evidence for the capability of the optimization method to find tailor-made system-specific pulses. Received 9 January 2001     

General theory of population transfer by adiabatic rapid passage with intense, chirped laser pulses

We present a general theory of adiabatic rapid passage (ARP) with intense, linearly chirped laser pulses. For pulses with a Gaussian profile and a fixed bandwidth, we derive a rigorous formula for the maximum temporal chirp rate that can be sustained by the pulse. A modified Landau-Zener formula displays clearly the relationships among the pulse parameters. This formula is used to derive the optimal conditions for efficient, robust population transfer. As illustrations of the theory, we present results for two- and four-level systems, and selective vibronic excitation in the I₂ molecule. We demonstrate that population transfer with chirped pulses is more robust and more selective than population transfer with transform-limited pulses. Received 6 September 2000 and Received in final form 25 September 2000     

Theoretical study of the time-resolved photoelectron spectrum of Na 2F: effects of thermal initial conditions

The excited state dynamics of the Na₂F cluster initiated by a femtosecond laser pulse is studied considering a thermally excited initial sample. Within a pump-probe set-up, the time-dependent photoelectron spectrum is calculated, which is shown to be a sensitive tool to study intramolecular motion of the cluster. Temperature effects are taken into account through thermal averaging over the time-dependent spectra obtained from different initial vibrational states of the cluster. The nuclear motion upon laser excitation is described by full-dimensional quantum wavepacket propagation using explicit, realistic pump and probe pulses. The characteristic features of the time-resolved photoelectron spectra of the Na₂F cluster, identified as due to periodic bending motion of the cluster as well as to the excitation of the stretching mode, are found to be robust against increasing vibrational temperature of the cluster beam. This finding is important for possible future experiments.     

Orienting molecules using half-cycle pulses

Using a rigid-rotor model, we study the orientation dynamics of polar diatomic molecules excited by experimentally available half-cycle pulses. The results of the numerical solution of the time-dependent Schr?dinger equation are compared to those of an approximate “sudden-impact” impulsive model neglecting the molecular rotation during the pulse. We show that efficient orientation is achieved during time periods of several picoseconds for LiCl. For short pulses, where the kicked molecule model is valid, orientation turns out to be mainly sensitive to the time-integrated field amplitude and not the shape or rise time of the pulse. Received 16 August 2000 and Received in final form 4 December 2000     

Coupled longitudinal and transverse stimulated Brillouin scattering in single-mode optical fibers

Due to their finite numerical aperture, both longitudinal and transverse stimulated Brillouin scattering can occur in single-mode fibers. We discuss the role of the fiber structure and propose a coherent model accounting for both effects. We show experimentally and numerically that, in some cases, the perturbative cladding Brillouin scattering (CBS) can severely affect the dynamics of SBS Brillouin fiber lasers. New dynamical regimes of long-fiber Brillouin ring lasers are presented, including stable trains of modulated pulses. Received 17 September 2001 / Received in final form 5 March 2002 Published online 28 June 2002     

Control of bond-selective photochemistry in CH BrCl using adaptive femtosecond pulse shaping

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. Received 21 December 2001 / Received in final form 2 April 2002 Published online 28 June 2002     

Isotopomer selective optimization of <Superscript>39</Superscript>K<Superscript>85</Superscript>Rb<Superscript>+</Superscript> and <Superscript>41</Superscript>K<Superscript>87</Superscript>Rb<Superscript>+</Superscript> using optimal control

We report on isotope selective three-photon ionization of two isotopomers of KRb by applying evolution strategies. The particularity of this experiment is based on the high resolution phase and amplitude modulation of the fs-laser pulses provided by a 2 × 640 pixel pulse shaper. The optimization in a closed feedback loop performed with spectrally broad pulses centered at 840 nm shows high enhancements of one isotopomer at the expense of the other isotopomer and vice versa. From the optimal laser field we aim to gain details about the selective ionization sequence and the wavepacket evolution on the involved vibrational states.     

Interferometry with entangled atoms

A quantum gravity-gradiometer consists of two spatially separated ensembles of atoms interrogated by pulses of a common laser beam. The laser pulses cause the probability amplitudes of atomic ground-state hyperfine levels to interfere, producing two, motion-sensitive, phase shifts, which allow the measurement of the average acceleration of each ensemble, and, via simple differencing, of the acceleration gradient. Here we propose entangling the quantum states of atoms from the two ensembles prior to the pulse sequence, and show that entanglement encodes their relative acceleration in a single interference phase which can be measured directly, with no need for differencing. Received 6 June 2002 / Received in final form 25 October 2002 Published online 28 January 2003     

Ultrafast dynamics in the excited hydrogen atom transfer states of ammonia clusters

Neutral ammonia clusters (NH₃)_m are photo-excited to the electronic state by a deep UV femtosecond laser pump pulse. Within a few hundred femtoseconds a significant fraction of the clusters rearrange to form an H-transfer state (NH₃)_m-2NH₄(3s)NH₂ with the subunit NH₄ in its 3s electronic ground state. This state is then electronically excited by a time-delayed infrared control pulse of variable wavelength. Finally, a third (probe) pulse in the UV ionizes the clusters for detection. The lifetime of the excited (NH₃)_m-2NH₄(3p)NH₂ states is found to vary between 2.7 and 0.13 ps depending on cluster size and excitation energy. It increases drastically upon deuteration. The corresponding cluster size-dependent photoelectron spectra allow us to disentangle the underlying energetics of the excitation and ionization process and reveal additional processes, such as nonresonant ionization or dissociative ionization. The experimental findings suggest that the excited H-transfer ammonia complexes with m > 2 are deactivated by an internal conversion process back to the electronically lowest H-transfer state followed by fast dissociation. Received 22 September 2001 and Received in final form 31 January 2002     

Electron dynamics in Na and Pt clusters from time-dependent Hartree-Fock theory

We present a method for the numerical investigation of the electron dynamics in small metallic clusters in intense laser fields. We obtain information about collective excitations and relaxation processes in the Na ₉ ⁺ and Pt₃ clusters analyzing the power spectrum of the dipole moment within a mean-field approach. The power spectrum is computed for various laser pulse parameters as well as for the limit of an infinitely short laser pulse. Due to the basis set expansion of the wave function our method is capable to follow the dynamics not only of the whole electron cloud, but of any particular molecular orbital. Received 28 March 2002 / Received in final form 31 May 2002 Published online 24 September 2002 RID="a" ID="a"e-mail: pavlyukh@mpi-halle.de     

A new powerful source for coherent VUV radiation: Demonstration of exponential growth and saturation at the TTF free-electron laser

We present experimental evidence that the free-electron laser at the TESLA Test Facility has reached the maximum power gain of 10⁷ in the vacuum ultraviolet (VUV) region at wavelengths between 80 and 120 nm. At saturation the FEL emits short pulses with GW peak power and a high degree of transverse coherence. The radiation pulse length can be adjusted between 30 fs and 100 fs. Radiation spectra and fluctuation properties agree with the theory of high gain, single-pass free-electron lasers starting from shot noise. Received 26 April 2002 Published online 28 June 2002     

Highly localized vibronic wavepackets in large reactive molecules

The ultrafast dynamics of the internal conversion of S₁ azulene and the excited-state intramolecular proton transfer in 2-(2^′-hydroxyphenyl)benzothiazole (HBT) were investigated by pump–probe spectroscopy with tunable pulses as short as 20 fs. In both cases we find very pronounced oscillatory contributions to the transients, which are due to vibrational wavepacket motion in the excited state. The damping times are on the order of 1 ps even for these large reactive molecules in solution at room temperature. For azulene only 2 out of 48 vibrational modes participate and in HBT only 4 out of 69. This high degree of localization of the wavepacket seems to be a general feature, and supports hopes that even for systems of chemical interest coherent control might be possible. Received: 24 January 2000 / Published online: 24 July 2000     

波包动力学-飞秒激光操纵波包过程

本文介绍了波包动力学的基本理论和数值模拟方法,尤其是在飞秒激光与分子相互作用领域中的应用,探讨了飞秒激光引起的波包过程（包括波包的制备、波包的干涉）,以及利用调频脉冲、脉冲串和优化脉冲形状控制波包动力学的方案。     

波包动力学-飞秒激光操纵波包过程

本文介绍了波包动力学的基本理论和数值模拟方法,尤其是在飞秒激光与分子相互作用领域中的应用,探讨了飞秒激光引起的波包过程（包括波包的制备、波包的干涉）,以及利用调频脉冲、脉冲串和优化脉冲形状控制波包动力学的方案。     

Control of the pulse duration in one- and two-axis passively Q-switched solid-state lasers

We study theoretically and experimentally different methods to control the pulses emitted by solid-state lasers passively Q-switched by a saturable absorber. We explore one- and two-axis laser schemes allowing to control the pulse duration, which is ruled by the saturation powers of the transitions in the absorber and in the gain medium. In one-axis lasers, it is shown that the adjustment of the pump and laser beam sizes in the active medium and in the absorber provides an efficient means to control the pulse temporal shape and duration. Furthermore, a two-axis laser cavity supporting so-called forked-eigenstate operation permits to freely adjust the parts of the mode power which circulate in the gain medium and in the absorber. In this case, a lengthening of the pulse duration up to 500 ns is obtained with an increase of the average output power. The theoretical results obtained by using rate equations adapted to each cavity geometry are in close agreement with experiments performed on a diode-pumped Nd³⁺:YAG laser Q-switched by a Cr⁴⁺:YAG saturable absorber. The relevance of the different techniques to control the pulse durations in the framework of potential applications is discussed. Received 3 December 2001     

Ad-hoc design of temporally shaped fs laser pulses based on plasma dynamics for deep ablation in fused silica

We have analyzed the ablation depth yield of fused silica irradiated with shaped pulse trains with a separation of 500 fs and increasing or decreasing intensity envelopes. This temporal separation value is extracted from previous studies on ablation dynamics upon irradiation with transform-limited 100 fs laser pulses. The use of decreasing intensity pulse trains leads to a strong increase of the induced ablation depth when compared to the behavior, at the same pulse fluence, of intensity increasing pulse trains. In addition, we have studied the material response under stretched (500 fs, FWHM) and transform-limited (100 fs, FWHM) pulses, for which avalanche or multiphoton ionization respectively dominates the carrier generation process. The comparison of the corresponding evolution of the ablated depth vs. fluence suggests that the use of pulse trains with decreasing intensity at high fluences should lead to enhanced single exposure ablation depths, beyond the limits corresponding to MPI- or AI-alone dominated processes.     

Dichromatic squeezing generation

We propose a novel scheme for the joint generation of two squeezed beams at arbitrary frequencies ω ₁ and ω ₂. The scheme consists of two successive steps, both involving nonlinear interactions in χ⁽²⁾ crystals. The dynamics of the setup is analyzed both quantum mechanically and classically within the parametric approximation. An experimental implementation involving the fundamental and the harmonics of a Nd:YAG laser pulse, and β-BaB ₂ O ₄ nonlinear crystals is suggested. Received 17 May 2000 and Received in final form 9 October 2000     

Isotopic effects on the control of molecular handedness of H 2 POSH by ultrashort laser pulses

The selective preparation of an enantiomer starting from a pure state of H₂POSH representing a 50%:50% mixture of two enantiomers with opposite chiralities is extended to its deuterated counterpart, H₂POSD. A one-dimensional model involving the torsional angle of S-H/D around a pre-oriented P-S axis is used. The design of an appropriate sequence of ultrashort infrared laser pulses to achieve molecular handedness for H₂POSH/D is based on the characteristic level splitting and tunneling times of both molecules. A simple scheme of two linearly polarized laser pulses involving a three level system serves to convert the different isotopomers to opposite enantiomers, for any given mixture of H₂POSH and H₂POSD molecules. Received 31 August 2000 and Received in final form 23 November 2000     

Resonant coupling of small size-controlled lead clusters with an intense laser field

We exposed small size-controlled lead clusters with a few hundreds of atoms to laser pulses with peak intensities up to 10¹⁵ W cm^-2 and durations between 60 fs to 2.5 ps. We measured kinetic energies and ionic charge of fragments as a function of the laser intensity and pulse duration. Highly charged Pbⁿ⁺ ions up to n = 26 have been detected presenting kinetic energies up to 15 keV. For comparison with our experimental results, we have performed simulations of the laser coupling with a cluster-sized lead nanoplasma using a qualitative model that was initially proposed by Ditmire and co-workers at LLNL for the case of rare gas clusters. From these simulations we conclude that two mechanisms are responsible for the explosion dynamics of small lead clusters. As already observed for large rare gas clusters (n = 10⁶), fragments with charge states below +10 are driven by Coulomb forces, whereas the higher charged fragments are accelerated by hydrodynamic forces. The latter mechanism is a direct consequence of the strong laser heating of the electron cloud in the nanoplasma arising from a plasmon-like resonance occurring at n _e = 3n _c. In order to obtain an optimized laser-nanoplasma coupling, our results suggest that the plasma resonance should occur at the peak intensity of the laser pulse. Due to inertial effects, even for such small-sized clusters, the observed optimum pulse duration is in the order of 1 ps which is in good agreement with our theoretical results. Received 18 March 2002 Published online 19 July 2002