Malte Oppermann

Comparison of high-order harmonic generation in uracil and thymine ablation plumes

We present studies of high-order harmonic generation (HHG) in laser ablation plumes of the ribonucleic acid nucleobase uracil and its deoxyribonucleic acid counterpart thymine. Harmonics were generated using 780 nm, 30 fs and 1300 nm, 40 fs radiation upon ablation with 1064 nm, 10 ns or 780 nm, 160 ps pulses. Strong HHG signals were observed from uracil plumes with harmonics emitted with photon energies >55 eV. Results obtained in uracil plumes were compared with those from thymine, which did not yield signs of harmonic generation. The ablation plumes of the two compounds were examined by collection of the ablation debris on a silicon substrate placed in close proximity to the target and by time-of-flight mass spectrometry. From this evidence we conclude that the differences in HHG signal are due to the different fragmentation dynamics of the molecules in the plasma plume. These studies constitute the first attempt to analyse differences in structural properties of complex molecules through plasma ablation-induced HHG spectroscopy.

Molecules in Strong Laser Fields

The purpose of this chapter is to provide the background and context for interpreting the experiments presented in this thesis.

Experimental Methods and Setup

The experiments presented in this thesis deal with the molecular dynamics following strong field ionisation. This implies that the resulting reaction products—molecular fragments and parent ions—always include charged particles.

Characterisation and Optimisation of Impulsive Molecular Alignment in Mixed Gas Samples

Molecules in gas phase are randomly oriented within the laboratory frame of reference. When interacting with a linearly polarised laser field, the response of the molecular ensemble is thus an average over all orientations of the molecule with respect to the laser polarisation. This makes it impossible to resolve the effect of the molecular structure on the studied laser induced processes. In this chapter laser induced impulsive molecular alignment is presented as a technique to study laser-molecule interactions in the molecular frame.

Strong Field Control of Dissociative Excitation in CO_2

In this chapter the molecular structure dependence of strong field induced dissociation in CO\(_2\) is investigated. In particular, the dissociative excitation of CO\(_2^+\) was studied in the molecular frame as a function of probe laser intensity, ellipticity and polarisation direction with respect to the molecular bond.

Multichannel Contributions in Nonsequential Double Ionisation of \mathrm{{CO}}_2

In this chapter the molecular alignment dependence of recollision induced double ionisation is studied.

Dynamical coupling of molecular rotation and Coulomb explosion

Summary form only given. The first observation of 1/8<;sup>th<;/sup> rotational revival in impulsively aligned CO<;sub>2<;/sub> molecules probed by Coulomb explosion is reported. Such a dynamic gives insight in the coupling arising between rotation wavepacket and strong field dynamic. Field free molecular rotational alignment is nowadays a routine technique to achieve structural and dynamical measurements in the molecular frame [1]. It is a unique tool to study electron recollision in strong field physics as in High Harmonic Generation (HHG) spectroscopy and recollision-induced double ionisation and fragmentation. A rotational wavepacket is launch by an ultrashort pulse leading to periodic alignment of the molecules, so called “revivals”. Various experimental techniques are able to give insight in the rotational dynamic of the wavepacket as for example the direct visualization of the molecular bond orientation using velocity map imaging [2]. While the temporal evolution of such alignment is well represented by the expectation value <;cos<;sup>2<;/sup> θ> (0 is the angle between molecular bond and electric field polarisation), the experimental measurement of rotational periodic alignment can lead to significantly distorted revivals, especially with the presence of higher order terms as <;cos<;sup>n<;/sup> 0>with n=4,6. This has been shown especially with HHG, where the coupling of rotational wavepacket and angular yield of HHG is modulated differently, depending the molecule structure [3,4].

Attosecond Generation and High Field Physics

Attosecond physics is concerned with the dynamics of electrons in atoms and molecules. Resolving and controlling their motion during excitation and relaxation processes in matter is an essential contribution our understanding of atomic scale processes. This chapter presents a general introduction to the key aspects of attosecond measurement technology and the associated strong laser field physics that underlie most of it. In this context High Harmonic Generation (HHG) is one of the most intensely studied phenomena and its use for attosecond pulse generation and HHG spectroscopy of molecules are discussed. Here, the attosecond streaking technique and its use for attosecond pulse and electron dynamics measurement is outlined briefly. In addition, other routes to attosecond sources such as Raman processes, relativistic HHG and free electron lasers are also introduced and compared.

Development of High Power Infrared Optical Parametric Amplification Laser System Seeded by Self-difference Frequency Generation Pulses

We report on our recent development of a femtosecond infrared laser system based on self-phase-stabilized seed by difference-frequency generation of supercontinuum obtained with so-called hollow fiber compression technique and two-stage optical parametric amplification. After the final amplifier, we obtained pulses with duration of 500 μJ and as a first demonstration of this new laser system, we generated high-harmonics up to 47th order in a tube target filled with Xe gas.

Controlling ionisation and fragmentation processes in CO2 via laser driven inelastic electron recollisions

For the first time, the angular dependence of nonsequential double ionisation and dissociation induced by laser driven inelastic electron rescattering was investigated experimentally in aligned CO2. A strong dependence on the recollision angle was found.