Intense Free-electron Laser Research Articles

Neutral and charged photofragment coincidence imaging with soft x rays on molecular ion beams: Breakup ofH3O+at 13.5 nm

The fragmentation of the hydronium cation ${\text{H}}_{3}{\text{O}}^{+}$ after photoabsorption at 13.5 nm has been investigated with a crossed-photon-and-ion-beams experiment making pulsing and trapping techniques available for fragment momentum imaging at the intense Free-electron LASer in Hamburg. The observed photofragmentation patterns demonstrate that the photolysis of ${\text{H}}_{3}{\text{O}}^{+}$ proceeds by valence ionization into ${\text{H}}_{3}{\text{O}}^{2+}$ which subsequently fragments to mainly $\text{OH}+2{\text{H}}^{+}$ and ${\text{H}}_{2}{\text{O}}^{+}+{\text{H}}^{+}$ with a branching ratio of up to 0.6:1 and with different degrees of excitation of the molecular fragment. The cross section for fragmentation into $\text{OH}+2{\text{H}}^{+}$ is found to be $(0.37\ifmmode\pm\else\textpm\fi{}0.18)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}\text{ }{\text{cm}}^{2}$, while the total photoabsorption cross section is estimated to be greater than $0.95\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}\text{ }{\text{cm}}^{2}$. The data suggest that ionization mainly occurs from the $3{a}_{1}$ and $1e$ valence orbitals and that initial ionization from $3{a}_{1}$ mainly leads to fragmentation into ${\text{H}}_{2}{\text{O}}^{+}(A\text{ }{^{2}A}_{1})+{\text{H}}^{+}$ while initial ionization from the $1e$ orbital predominantly populates the ${\text{H}}_{2}{\text{O}}^{+}(B\text{ }{^{2}B}_{2})+{\text{H}}^{+}$ and $\text{OH}(X^{2}\ensuremath{\Pi})+2{\text{H}}^{+}$ channels. The results are of significance for astrophysical models of gas clouds in the vicinity of hot radiating objects and for models of the chemistry of planetary and lunar ionospheres under solar irradiation.

Frustration of direct photoionization of Ar clusters in intense extreme ultraviolet pulses from a free electron laser

We have measured the kinetic energies of fragment ions from Ar clusters (average cluster size ⟨N⟩∼ 10–600) exposed to intense extreme ultraviolet free electron laser pulses (λ ∼ 61 nm, I∼ 1.3× 1011 W cm−2). For small clusters (⟨N⟩≲ 200), the average kinetic energy of ions strongly increases with increasing the cluster size, indicating a promotion of the multiple ionization, whereas the average kinetic energy is observed to be saturated for ⟨N⟩≳ 200. Considering how many photoelectrons can escape from the cluster, it was found that the size dependence of the ion kinetic energy exhibited the frustration of direct photoionization, which resulted from the strong Coulomb potential of the highly ionized cluster.

Multiple K-shell excitation of lithium clusters: Implications for hollow-atom solids

Systematic molecular-orbital calculations are performed for Li 9 z + clusters in which N exc (=0–9) core electrons are excited to valence orbitals. For neutral ( z = 0) clusters, the magnitudes of work functions, cohesive energies, and average core-electron excitation energies increase with N exc, owing to relaxation of valence electrons around localized core holes. Total energy of a multiply charge ion, produced by a series of Auger decay, remains lower than the corresponding dissociation limit as far as z ⩽ 3. We thereby discuss a possibility of realizing a hollow-atom solid, multiply core-excited state with temporal crystalline order, by utilizing intense free-electron lasers.

Dissociative two-photon ionization of N2 in extreme ultraviolet by intense self-amplified spontaneous emission free electron laser light

Dissociative multiple ionization processes of N2 were investigated by irradiating N2 with an intense extreme ultraviolet (XUV) light at 50.3nm generated by a compact self amplified spontaneous emission free electron laser light source. From the analysis of the momentum distribution of N+ ejected through the Coulomb explosion of N2 and by the single-shot correlation between the yields of N2+ and N+, it was confirmed that double ionization of N2 occurred by the two-photon absorption of the XUV light.

Measurement of the temperature dependence of midinfrared optical absorption spectra of germanium in intense laser fields

We have investigated the temperature dependence of midinfrared optical absorption in germanium under intense free-electron laser fields. We observed that the absorption decreases as temperature decreases. We find that the temperature dependence of the absorption is consistent with calculations based on the Keldysh theory, in which the temperature dependence of the direct-gap energy of germanium is considered.

Infrared spectroscopy of organometallic ions in the gas phase: From model to real world complexes

Gas phase mid-infrared spectroscopy of molecular ions can nowadays be performed with high performance mass spectrometers coupled to free electron lasers (FEL). The wide and continuous tunability of highly intense FELs in the mid-infrared region can be exploited for performing infrared multiple photon dissociation (IRMPD) spectroscopy of molecular ions. This review will focus on gas phase IRMPD spectroscopic investigations aiming at probing the structure and the reactivity of transition metal complexes. The performance of infrared spectroscopy for characterizing the coordination mode of polydentate ligands and the spin state of the metal will be illustrated. Infrared spectroscopy has also been exploited to probe the reactivity of metal complexes, and a special attention will be given to the infrared spectroscopy of reactive intermediates.

Autocorrelation measurements of free-electron laser radiation using a two-photon QWIP

The two-photon QWIP comprises three equidistant subbands, namely two bound states localized in the quantum well and an extended state in the continuum. This device is very promising for quadratic autocorrelation measurements of pulsed mid-infrared lasers due to its resonantly enhanced optical nonlinearity and sub-ps time resolution. We report on interferometric autocorrelation measurements of ps optical pulses from a free-electron laser (FEL). The intense FEL radiation further allows us to study the saturation properties of two-photon QWIPs at liquid nitrogen temperature and their detection properties at 300K. The device is well suited for standard diagnostics of the FEL pulse shape via interferometric autocorrelation.

The MIT X-ray laser project

Recent advances in accelerator, laser, and undulator technology have created the possibility of constructing a national user facility based on an intense free-electron laser at extreme ultraviolet and X-ray wavelengths. MIT is exploring the construction of such a facility at its Bates Laboratory site, 20 miles north of Cambridge. The science that is foreseen spans many disciplines including atomic and fundamental physics, condensed matter physics and materials sciences, femtochemistry, structural biology, and various fields of engineering. The source we propose, and the experimental methods it will spawn, will generally be qualitatively new and have high impact in many fields of science and technology. The strength of the science and technology base in the northeast region, and in particular at MIT, make this a superb location for such a national user facility.

A compact laser FEL in a X-ray regime

A new Free Electron Laser (FEL) using high power laser system as a wiggler/undulater and an electron beam of transverse laser cooling in a storage ring and a combination of chicane for longitudinal compression is proposed. This scheme will open an intense FEL hard X-ray laser with very compact system.

Neutron diagnostic for MTX

A neutron diagnostic system has been installed on the MTX tokamak. The diagnostic will be used to measure both the peak ion temperature and the ion response to intense free-electron laser microwave pulses (140–250 GHz, 1–8 GW, 20–50 ns). High-speed measurements are important for studying the intense microwave absorption process. A plastic scintillator with a polymer-clad silica bundle is used for fast time response detection. Five-channel proportional counters and fission counters are used to monitor the neutron yield for a source strength from 108 to 1013 n/s. This sytem has been calibrated with a 252Cf neutron source in the vacuum vessel and is being tested using the MTX ohmic plasma.

Dynamical self-focusing of high-power free electron laser radiation in a magnetized plasma

Under the action of very intense free electron laser (FEL) radiation, a homogeneous plasma can manifest nonlinear behavior and dramatically change the spatial distribution of the incident electromagnetic beam. The dominant mechanism is the constriction depletion of the plasma due to an averaged ponderomotive force. The unstable character of the FEL-beam plasma system and the consequent self-focusing are studied in the dynamical ‘‘supersonic limit.’’ The self-similar structure of the high-frequency field and density distributions is computed and the form of a ‘‘trace,’’ left by a moving ‘‘focus’’ in the plasma is determined.