A reaction microscope ia a powerfull tool which allows the complete kinematical analysis of all charged particles (ions and electrons) from a molecular reaction induced for example by laser radiation. Coupling a reaction microscope to a high order harmonic source driven by intense fsec laser pulses opens the doorway to the analysis of extremely fast electronic processes in molecules on a sub-fsec time scale in great detail. In order to exploit the enormous possibilities of such a setup it is essential to be able to stabilize delay lines used in typical pump-probe experiments for long time intervals (many hours) to better than about 20 attosec. In terms of the wavelength l of the laser pulses driving the high order harmonic source this means long term interferometric stability to better than l/100. We implemented a setup capable of reaching the required stability and demonstrated the delay stability by analysing an attosecond pulse train in the extreme ultraviolet (XUV) spectral range using the RABITT (resolution of attosecond beating by interference of two-photon transitions) method [M. Böttcher et al.].

Our experimental setup which combines a high order harmonic source with a reaction microscope. A delay line which allows to delay the XUV harmonic pulses with respect to infrared laser pulses is made up by the split mirror. The center part reflects the XUV harmonic radiation and the annular part the infrared radiation which generates the harmonics. The split mirror pair is part of an external interferometer which actively fixes the relative position of the two mirror parts (for the details see [M. Böttcher et al.]).

High Harmonics - How are they generated?

The experimental setup shows the generation schematically. A millimeter size cell filled with the nonlinear medium, typically a noble gas, inside a vacuum chamber is irradiated by an ultrashort near infrared focused laser pulse. Harmonics are generated in the focal spot at light intensities of ~10¹⁴ W/cm². Harmonics at odd multiples of the driving laser frequency are generated. They exit the cell together with the transmitted infrared (IR) beam. The IR light is then either removed from the harmonics by a thin aluminium foil or single harmonics are selected by a grating monochromater as shown in the experimental setup and the figure below (from A. L'Huillier and Ph. Balcou).

High harmonics - What are their characteristics?

A characteristic feature of the high harmonics is that they are generated over a broad range of orders with nearly constant intensity. This part of the spectrum forms the so-called plateau. It is followed by a short cut-off region where the intensity of the harmonics drops to a negligible value over only a few harmonic orders. This behaviour can well be identified in the spectrum above. The harmonic orders are coupled by a specific phase relation. Under particular generation conditions this relation allows to superimpose harmonic orders coherently. In this way it is possible to generate a beam of XUV radiation which consists of a train of pulses separated by half the period of the carrier wave of the driving laser radiation. The width of the individual XUV pulses has been shown by several research groups to reach down to ~0.1 fsec, i.e. significantly less than the oscillation period of the driving radiation. A typical train pulse we generated and reconstructed from a RABITT scan with our setup is shown below (full line, for details see [M. Böttcher et al.]). The harmonic orders 11 - 17 contribute to the pulse train shown. The individual pulses have a width of ~330 attosec.

Starting with an driving pulse which consists of only few optical cycles it is even possible to generate single XUV pulses through harmonic generation.

High harmonics - What is the generation mechanism?

Emitted spectrum:
The emitted photon has an energy of the atomic ionization potential plus the energy the recombining electron has gathered in the course of its acceleration by the driving laser field. The periodicity of the driving light wave demands that harmonic photons can be emitted only at multiples of the fundamental driving frequency. Parity conservation further restricts the harmonics to odd multiples of this frequency. The harmonic radiation emitted by the individual atoms in the gaseous nonlinear medium adds up coherently. The cutoff of the harmonic spectrum is determined either by the intensity of the driving NIR laser pulses or by the medium specific light intensity, where ionization becomes saturated, whichever is higher.

Application of high harmonics in photo double ionization studies

We have started to use single high order harmonics of Ti:Sapphire laser radiation to study 1-photon and few-photon two-color photo double ionization (PDI) of Xenon atoms near the PDI threshold. This is done in an ideal way with our reaction microscope. The harmonics we used have been generated in Argon gas as nonlinear medium. A single harmonic with order in the range between 17 (l = 47.65 nm) and 25 (l = 32.4 nm) was selected from the beam of harmonic radiation by a monochromator and directed into the reaction microscope. In the reaction microscope a split off part of the NIR Ti:Sapphire laser radiation was superimposed on the XUV harmonic beam to study the two-color excitation processes. Synchronization in time of the XUV and NIR pulses is achieved by a delay line.

1.) 1-Photon Photo Double Ionization of Xenon

In order to demonstrate the capabilities of the setup we only show a few exemplary results. The graph above shows the kinetic energy distribution for photoelectrons from 1-photon PDI of Xenon. Only electrons detected together with doubly charged xenon ions are included (experimental data: black curve). Here the 25^th harmonic of the 810 nm radiation of the Ti:Sapphire laser has been used to photo double ionize Xe. The colored lines represent a decomposition of the measured spectrum according to the different PDI mechanisms of Xe. Two mechanisms are present, a direct and an indirect one. The direct PDI pathway yields two photoelectrons which are emitted simultaneously in a highly correlated way. They share the excess energy arbitrarily, leaving the the doubly charged ion in one of several accessible final states (green curves). On the indirect pathway Xe first looses one electron leaving Xe⁺ in a highly excited autoionizing Rydberg state beyond the PDI threshold. Autoionization of this state results in PDI of Xe. The yellow curve represents the kinetic energy distribution of the photoelectron and the orange one that of the electron from the autoionization step (the detailes of this experiment can be found in our publication).

The figure above shows the momentum correlation of the two photoelectrons for the momentum components parallel to the direction of polarization of the high harmonic radiation. As is typical for a reaction microscope the momenta are determined from the measured momentum of one of the photoelectrons and the recoil momentum of the doubly charged Xe ion. The contours are given for constant event counts per bin. The structure of the momentum distribution (its deviation from rotational symmetry) points to a correlated emission of the two photoelectrons. A back-to-back emission is preferred.

2.) Few-Photon 2-Colour Photo Double Ionization of Xenon

Pulse width of a selected harmonic (21^st harmonic)

The pulse width of the 21^st harmonic has been determined by 2-color PDI of Xenon using the XUV and the NIR laser beams (see the experimental setup). The energy of the XUV photons of this harmonic is not sufficient for 1-photon PDI of Xe, further photons from the superimposed NIR laser beam (at least one) have to be absorbed for PDI. This allows a cross-correlation measurement of the pulse width of the high harmonic pulses by determining the Xe⁺⁺ ion yield as a function of the delay of the NIR pulses with respect to the harmonic pulses (see the figure above). When the harmonic pulse arrives after the NIR pulse no Xe⁺⁺ ions are formed. The Xe⁺⁺ yield increases when the pulses start to overlap. When the NIR pulse follows the harmonic pulse the Xe⁺⁺ ion yield becomes constant. The rising part of the curve can be used to determine the XUV pulse width to be approximately 2.8 psec. This pulse width is completely determined by the monochromator used to select one harmonic. The Xe⁺⁺ ion yield stays at a constant level different from zero when the NIR pulse follows the harmonic pulse because the main PDI pathway is stepwise via excited states of the Xe⁺ ion which are populated by the XUV pulse. They live for a time long compared to the delay times used for the NIR laser pulse.

Electron kinetic energy distributions

As an example the figure above shows the photoelectron kinetic energy distribution for 2-color PDI of Xenon using the 17^th harmonic and the NIR Ti:Sapphire laser pulses focused to a light intensity of ~1 x 10¹³ W/cm² within the XUV beam. The spectrum for single ionization of Xe by absorption of one XUX photon is shown (SI) and the PDI spectrum (DI). PDI at this XUV wavelength needs the absorption of at least 5 photons of the NIR radiation in adition to the XUV photon.

PDI is mainly sequential at this XUV wavelength. First, one XUV photon is absorbed leaving Xe⁺ in an excited state. As can be seen in the (SI) spectrum a state with a 5s-shell hole [5s(5p)⁶] or the accompanying satellites [(5s)²(5p)⁵] may become populated in this first step. Subsequently, the NIR pulse photoionizes these excited Xe⁺ states by multiphoton absorption leaving Xe⁺⁺ in one of the [(5s)²(5p)⁴] open shell fine structure states. A qualitative analysis of the PDI spectrum shows that mainly the minimum number of NIR photons is absorbed for PDI. If at all only a small above threshold ionization contribution is found. One of the two low energy peaks in the PDI spectrum stems from XUV single ionization of Xe the other one can be attributed to NIR multiphoton ionization of Xe⁺ excited states. They show completely different angular distributions. An important fact is, that the [5s(5p)⁶] 5s-shell hole state does not contribute to PDI. This means, a filling of the 5s-hole during multiphoton ionization is not possible. The full details of this experiment can be found in our publication.