Switched Wave Packets with Spectrally Truncated Chirped Pulses

A new technique for obtaining switched wave packets using spectrally truncated chirped laser pulses is demonstrated experimentally and numerically by one-dimensional alignment of both linear and asymmetric top molecules. Using a simple long-pass transmission filter, a pulse with a slow turn on and a rapid turn off is produced. The degree of alignment, characterized by $\langle\cos^2\theta_\text{2D}\rangle$ rises along with the pulse intensity and reaches a maximum at the peak of the pulse. After truncation $\langle\cos^2\theta_\text{2D}\rangle$ drops sharply but exhibits pronounced half and full revivals. The experimental alignment dynamics trace agrees very well with a numerically calculated trace based on solution of the time-dependent Schr\"odinger equation. However, the extended periods of field-free alignment of asymmetric tops following pulse truncation reported previously is not reproduced in our work.Comment: 7 pages, 4 figure

Molecular movie of ultrafast coherent rotational dynamics of OCS

Recording molecular movies on ultrafast timescales has been a longstanding goal for unravelling detailed information about molecular dynamics. Here we present the direct experimental recording of very-high-resolution and -fidelity molecular movies over more than one-and-a-half periods of the laser-induced rotational dynamics of carbonylsulfide (OCS) molecules. Utilising the combination of single quantum-state selection and an optimised two-pulse sequence to create a tailored rotational wavepacket, an unprecedented degree of field-free alignment, 〈cos2θ2D〉 = 0.96 (〈cos2θ〉 = 0.94) is achieved, exceeding the theoretical limit for single-pulse alignment. The very rich experimentally observed quantum dynamics is fully recovered by the angular probability distribution obtained from solutions of the time-dependent Schrödinger equation with parameters refined against the experiment. The populations and phases of rotational states in the retrieved time-dependent three-dimensional wavepacket rationalises the observed very high degree of alignment

Picosecond pulse-shaping for strong three-dimensional field-free alignment of generic asymmetric-top molecules

We demonstrate three-dimensional (3D) field-free alignment of the prototypical non-rotation-symmetric molecule indole using elliptically polarized, shaped, off-resonant laser pulses. A truncated laser pulse is produced using a combination of extreme linear chirping and controlled phase and amplitude shaping using a spatial-light-modulator (SLM) based pulse shaper of a broadband laser pulse. The angular confinement is detected through velocity-map imaging of H$^+$ and C$^{2+}$ fragments resulting from strong-field ionization and Coulomb explosion of the aligned molecules by intense femtosecond laser pulses. The achieved three-dimensional alignment is characterized by comparing the result of ion-velocity-map measurements for different alignment directions and for different times during and after the alignment laser pulse to accurate computational results. The achieved strong three-dimensional field-free alignment of $\langle\cos^{2}\delta\rangle=0.89$ demonstrates the feasibility of both, strong three-dimensional alignment of generic complex molecules and its quantitative characterization

Molecular-Frame Angularly-Resolved Photoelectron Spectroscopy

One of the big technical and scientific challenges today is to accomplish the ultimate dreamof filming chemical reactions with atomic spatial and temporal resolution and recording themolecular movie. Important prerequisites toward this goal are, on the one hand, methodsto create cold, controlled molecular samples and, on the other hand, imaging techniquesthat combine the required spatial and temporal resolution. In recent years, especially due tothe fast progress in the development of laser and electron sources, more and more refinedimaging techniques have become accessible. The combination of quantum state selectionwith laser-induced field-free alignment and orientation, allow to precisely control and preparethe molecules under study, before being imaged. Using ultrafast, high-intensity laser sourcesin the mid-infrared spectral range, self-imaging methods, such as laser-induced electrondiffraction (LIED), have emerged and their full potential can be explored today to image thestructure and dynamics of molecules with atomic spatio-temporal resolution.This work can be divided into two major parts, the control and the imaging part.In the control part, the focus lies on the optimization of field-free alignment using tailoredlight fields. Strong field-free alignment will be presented for three different molecules, rangingfrom the relatively simple linear molecule carbonyl sulfide (OCS) up to the complex asymmetrictop rotor indole, which lacks rotational symmetries and marker atoms. Different experimentaland numerical schemes of increasing complexity will be presented, depending on the complexityof the molecule under study, that allow to achieve strong field-free alignment and to accessthe molecule-fixed frame (MFF).In the imaging part, the LIED method will be employed to image and to retrieve thestatic structure of molecules with atomic resolution, applied on the example of OCS. Theunprecedented degree of field-free alignment of OCS, achieved in the control part, is employedto record angularly-resolved photoelectron momentum distributions (PEMDs) for differentrotational wavepackets and for different orientations of the molecular axis with respectto the ionizing laser polarization. These molecular-frame angularly-resolved photoelectronspectra (MF-ARPES) exhibit large differences, indicating a dependence of the emitted electroncontinuum wavepacket and its dynamics on the shape of the highest occupied molecular orbital(HOMO). In the low-energy region of the PEMDs, strong-field photoelectron holography(SFPH) is observed, revealing diverse interference patterns for different molecular orientations.Moreover, measurements of angle-dependent ionization yields of direct, low-energy electronsand of rescattered, high-energy electrons will be presented, showing clear alignment-dependentfeatures. From these aforementioned observations, conclusions will be drawn about the impactof the underlying molecular orbital on strong-field ionization and field-driven recollisions

Molekülrotation gefilmt

Die Kombination eines ultrakalten Molekülensembles mit präzise ab­gestimmten Laserpulsen ermöglicht die Erzeugung und zeitlich hoch­aufgelöste Beobachtung eines molekularen Rotationswellenpakets. Der Molekülfilm zeigt den Quantenteppich der Interferenzen über die gesamte Rotationsperiode und demonstriert die hohe Kontrolle über die Ausrichtung der Moleküle im Raum

Picosecond pulse-shaping for strong three-dimensional field-free alignment of generic asymmetric-top molecules

Fixing molecules in space is a crucial step for the imaging of molecular structure and dynamics. Here, we demonstrate three-dimensional (3D) field-free alignment of the prototypical asymmetric top molecule indole using elliptically polarized, shaped, off-resonant laser pulses. A truncated laser pulse is produced using a combination of extreme linear chirping and controlled phase and amplitude shaping using a spatial-light-modulator (SLM) based pulse shaper of a broadband laser pulse. The angular confinement is detected through velocity-map imaging of H$^+$ and C$^{2+}$ fragments resulting from strong-field ionization and Coulomb explosion of the aligned molecules by intense femtosecond laser pulses. The achieved three-dimensional alignment is characterized by comparing the result of ion-velocity-map measurements for different alignment directions and for different times during and after the alignment laser pulse to accurate computational results. The achieved strong three-dimensional field-free alignment of $\langle \cos^{2}\delta \rangle=0.89$ demonstrates the feasibility of both, strong three-dimensional alignment of generic complex molecules and its quantitative characterization