Two-color experiments in the gas phase at FLASH

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Abstract

Intense ultra-short XUV-pulses from the Free Electron Laser in Hamburg (FLASH) were used in combination with the synchronized near-infrared (NIR) radiation from a femtosecond laser to perform two-color experiments on rare gas atoms and small molecules. Results of atomic photoionization in the presence of a NIR dressing field are presented and discussed for the low field regime, i.e. for intensities of less than 1011 W/cm2. In particular, the analysis of two-color Above Threshold Ionization (ATI) as a function of the relative orientation of the polarization vectors of both photon beams provided detailed information about the partial photoionization crosssections by comparing the experimental results with theoretical values obtained by employing second-order perturbation theory and the “soft-photon” approximation. Furthermore, a first time-resolved study was performed on the photodissociation of molecular hydrogen. In this proof-of-principle experiment, the excited atomic fragments, produced in the primary interaction with the intense XUV pulse, are probed by a time delayed NIR laser pulse that ionizes these fragments.

Introduction

The recent developments in the technology of linear accelerators have enabled the production of intense ultra-short light pulses in the XUV and X-ray regime [1], [2]. The application of this Free Electron Laser (FEL) radiation to research on free atoms and molecules has thereby opened up unique possibilities to study new aspects of photoionization and photodissociation processes related to the dynamics and high field phenomena. Numerous experiments have already been performed at the Free Electron Laser in Hamburg (FLASH) [1], which became a user facility in 2005 (e.g. [3], [4] and references therein). The extremely high photon flux (about 1012–1013 photons/pulse) arriving within a time interval of 20–30 fs on the target gives rise to multiple ionization and non-linear processes involving electrons from inner subshells. Many new experimental results, such as the observation of very high charge states in atomic Xe (up to 21+), when irradiated by FEL photons of about 90 eV photon energy [5], or the sequential two and three photon double-ionization in Ne and Ar [6], [7], have prompted theoretical developments aiming to explain the underlying complex mechanism (e.g. [8], [9]). In addition, the short pulse durations can give access to the dynamics of these processes via time-resolved experiments. Experiments using the same FEL pulse, which was split into two independently controllable parts, have provided first results, while demonstrating at the same time the short pulse duration of the FEL [10]. Particularly interesting for time-resolved pump-probe studies is the possibility to select independently the wavelength of both the pump and the probe process in so-called two-color experiments combining the XUV FEL pulses with those of a strong synchronized optical femtosecond laser. Temporal stability and general performances of this two-color pump-probe facility at FLASH have been determined by a series of measurements on free atoms and molecules [11], [12], [13], [14].

Having characterized this facility, we have used it for two specific applications reported here. In the case of atoms, we investigated the Above Threshold Ionization (ATI) of rare gases. The strong optical field gives rise to so-called “sidebands” in the photoelectron spectrum when it is applied during the photoionization process. In addition to the absorption of a XUV photon, leading to ionization, the electron also interacts with the optical field and can absorb or emit an additional NIR photon (see Fig. 1a). The intensity of these sidebands is extremely sensitive to the spatial and temporal overlap between both pulses and has been widely used to characterize high order harmonic generation (HHG) sources [15], [16], [17]. However, the presence of different harmonics leads also to interference effects in the photoemission, which often hamper the interpretation of the observed ATI process [18], [19]. By making use of the FEL radiation and, in particular, of its monochromaticity, the investigation of two-photon two-color photoionization unperturbed by interferences becomes possible. In order to illustrate this application, aspects of the two-color ATI in atomic He and its photon energy dependence are discussed below. For molecular samples, the set-up enables us to perform time-resolved measurements, i.e. to follow the temporal evolution of a dissociation process (see Fig. 1b). After fragmentation of the molecule, which is induced by the XUV pulse, the NIR pulse can be used to track the dissociation products, e.g. excited atomic fragments, via photoionization. Access to the dynamics of the process is then obtained by varying the temporal delay between the XUV and the NIR pulse. In a first proof-of-principle experiment, we have studied the dissociation of molecular hydrogen by analyzing the laser-induced photoelectron spectrum of excited hydrogen atoms.

After a short presentation of the experimental set-up in Section 2, the two-color ionization of atomic He and the fragmentation of molecular hydrogen is discussed in Section 3 as showcases for typical experiments in the gas phase. A short conclusion and an outlook for future studies is given in Section 4.

Section snippets

Experiment

The experiments were performed at FLASH using a two-color pump-probe set-up. Briefly, the FEL and the optical laser beams were introduced into the vacuum chamber in a collinear geometry and intersected an effusive gas jet within the acceptance volume of a magnetic bottle electron spectrometer (MBES) (see Fig. 2).

For our experiments FLASH was operated at 25.5 nm (48.5 eV) and 13.7 nm (90.5 eV) at a repetition rate of 5 Hz in single bunch mode. Typical pulse energies were in the order of 30–50 μJ

Results and discussion

In a first application, two-color ATI in atomic He was investigated, in particular the intensity variation of the electron emission as a function of the relative orientation between the linear polarization vectors of the FEL and the NIR ‘dressing’ laser [25]. A selection of typical spectra recorded with optimal spatial and temporal overlap between both pulses is displayed in Fig. 3. Beside the main He+ 1 s−1 photoline at a kinetic energy of 66 eV, the sideband is seen at 67.5 eV, i.e. separated

Conclusion and outlook

Two-color Above Threshold Ionization in rare gas atoms has been accomplished by combining XUV pulses from the Free Electron Laser FLASH and strong NIR pulses from an external synchronized femtosecond laser. For low dressing fields of the optical laser it was possible to determine the ratio of the relative partial photoionization crosssections by making use of the well-defined linear polarization of both beams. The experimental results are in good agreement with the theoretical results obtained

Acknowledgements

We greatly appreciate the work of the scientific and technical team at FLASH, in particular the machine operators and run coordinators. Support from the EU RTD-project X-Ray FEL Pump-Probe HRPI-CT-1999-50009, from Science Foundation Ireland, IRCSET and from the France-Ireland ULYSSES program are acknowledged.

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