Precise and Accurate Measurements of Strong-Field Photoionization and a Transferable Laser Intensity Calibration Standard

Ionization of atoms and molecules in strong laser fields is a fundamental process in many fields of research, especially in the emerging field of attosecond science. So far, demonstrably accurate data have only been acquired for atomic hydrogen (H), a species that is accessible to few investigators. Here, we present measurements of the ionization yield for argon, krypton, and xenon with percent-level accuracy, calibrated using H, in a laser regime widely used in attosecond science. We derive a transferable calibration standard for laser peak intensity, accurate to 1.3%, that is based on a simple reference curve. In addition, our measurements provide a much needed benchmark for testing models of ionization in noble-gas atoms, such as the widely employed single-active electron approximation.

Attosecond electron spectroscopy using a novel interferometric pump-probe technique

We present an interferometric pump-probe technique for the characterization of attosecond electron wave packets (WPs) that uses a free WP as a reference to measure a bound WP. We demonstrate our method by exciting helium atoms using an attosecond pulse (AP) with a bandwidth centered near the ionization threshold, thus creating both a bound and a free WP simultaneously. After a variable delay, the bound WP is ionized by a few-cycle infrared laser precisely synchronized to the original AP. By measuring the delay-dependent photoelectron spectrum we obtain an interferogram that contains both quantum beats as well as multipath interference. Analysis of the interferogram allows us to determine the bound WP components with a spectral resolution much better than the inverse of the AP duration.

Molecular dissociative ionization and wave-packet dynamics studied using two-color XUV and IR pump-probe spectroscopy

We present a combined theoretical and experimental study of ultrafast wave-packet dynamics in the dissociative ionization of H_{2} molecules as a result of irradiation with an extreme-ultraviolet (XUV) pulse followed by an infrared (IR) pulse. In experiments where the duration of both the XUV and IR pulses are shorter than the vibrational period of H_{2};{+}, dephasing and rephasing of the vibrational wave packet that is formed in H_{2};{+} upon ionization of the neutral molecule by the XUV pulse is observed. In experiments where the duration of the IR pulse exceeds the vibrational period of H_{2};{+} (15 fs), a pronounced dependence of the H;{+} kinetic energy distribution on XUV-IR delay is observed that can be explained in terms of the adiabatic propagation of the H_{2};{+} wave packet on field-dressed potential energy curves.

A velocity map imaging detector with an integrated gas injection system

We present the design of a velocity map imaging spectrometer where the target gas is injected from a capillary that is integrated in the repeller plate of the ion optics assembly that drives electrons/ions formed by ionization or dissociation to a two-dimensional detector. The geometry of this design allows the use of gas densities in the interaction region that are two to three orders of magnitude higher than the densities that are used in standard velocity map imaging spectrometers, making the detector suitable for working with weak light sources such as newly developed attosecond pulse sources, or (quasi-)cw sources such as synchrotrons. In a test where monoenergetic photoelectrons were generated by six-photon ionization of Xe (utilizing the second harmonic of a neodymium doped Nd:YAG), the kinetic energy resolution of the spectrometer was found to be DeltaE/E=1.8%. This number was found to be in good agreement with Monte Carlo simulations.