Theory predicts that double-core-hole (DCH) spectroscopy can provide a new powerful means of differentiating between similar chemical systems with a sensitivity not hitherto possible. Although DCH ionization on a single site in molecules was recently measured with double-and single-photon absorption, double-core holes with single vacancies on two different sites, allowing unambiguous chemical analysis, have remained elusive. Here we report that direct observation of double-core holes with single vacancies on two different sites produced via sequential two-photon absorption, using short, intense X-ray pulses from the Linac Coherent Light Source free-electron laser and compare it with theoretical modeling. The observation of DCH states, which exhibit a unique signature, and agreement with theory proves the feasibility of the method. Our findings exploit the ultrashort pulse duration of the free-electron laser to eject two core electrons on a time scale comparable to that of Auger decay and demonstrate possible future X-ray control of physical inner-shell processes.multi-photon ionization | ultrafast | two-photon spectroscopy

We report on the first storage of ion beams in the Double ElectroStatic Ion Ring ExpEriment, DESIREE, at Stockholm University. We have produced beams of atomic carbon anions and small carbon anion molecules (C − n , n = 1, 2, 3, 4) in a sputter ion source. The ion beams were accelerated to 10 keV kinetic energy and stored in an electrostatic ion storage ring enclosed in a vacuum chamber at 13 K. For 10 keV C − 2 molecular anions we measure the residual-gas limited beam storage lifetime to be 448 s ± 18 s with two independent detector systems. Using the measured storage lifetimes we estimate that the residual gas pressure is in the 10 −14 mbar range. When high current ion beams are injected, the number of stored particles does not follow a single exponential decay law as would be expected for stored particles lost solely due to electron detachment in collision with the residual-gas. Instead, we observe a faster initial decay rate, which we ascribe to the effect of the space charge of the ion beam on the storage capacity. © 2013 AIP Publishing LLC. [http://dx

International audienceWe present experimental evidence for the dominance of prompt single-atom knockout in fragmenting collisions between large polycyclic aromatic hydrocarbon cations and He atoms at center-of-mass energies close to 100 eV. Such nonstatistical processes are shown to give highly reactive fragments. We argue that nonstatistical fragmentation is dominant for any sufficiently large molecular system under similar conditions

We have investigated the effectiveness of molecular hydrogen (H2) formation from Polycyclic Aromatic Hydrocarbons (PAHs) which are internally heated by collisions with keV ions. The present and earlier experimental results are analyzed in view of molecular structure calculations and a simple collision model. We estimate that H2 formation becomes important for internal PAH temperatures exceeding about 2200 K, regardless of the PAH size and the excitation agent. This suggests that keV ions may effectively induce such reactions, while they are unlikely due to, e.g., absorption of single photons with energies below the Lyman limit. The present analysis also suggests that H2 emission is correlated with multi-fragmentation processes, which means that the [PAH-2H](+) peak intensities in the mass spectra may not be used for estimating H2-formation rates.

A recent study of soft X-ray absorption in native and hydrogenated coronene cations, C 24 H + 12+m m = 0-7, led to the conclusion that additional hydrogen atoms protect (interstellar) Polycyclic Aromatic Hydrocarbon (PAH) molecules from fragmentation [Reitsma et al., Phys. Rev. Lett. 113, 053002 (2014)]. The present experiment with collisions between fast (30-200 eV) He atoms and pyrene (C 16 H + 10+m , m = 0, 6, and 16) and simulations without reference to the excitation method suggests the opposite. We find that the absolute carbon-backbone fragmentation cross section does not decrease but increases with the degree of hydrogenation for pyrene molecules.

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We have performed X-ray two-photon photoelectron spectroscopy (XTPPS) using the Linac Coherent Light Source (LCLS) X-ray free-electron laser (FEL) in order to study double core-hole (DCH) states of CO 2 , N 2 O and N 2 . The experiment verifies the theory behind the chemical sensitivity of two-site (ts) DCH states by comparing a set of small molecules with respect to the energy shift of the tsDCH state and by extracting the relevant parameters from this shift.The LCLS X-ray free-electron laser, at the SLAC National Accelerator Laboratory, produces ultra-short laser pulses with extremely high peak intensities in both the soft and hard X-ray domain [1,2]. These characteristics enable the exploration of hitherto virtually unmapped scientific territories, such as the non-linear interaction between matter and X-ray photons, and allows for a natural continuation of the already well established field of optical non-linear laser spectroscopy [3]. An intriguing example of such an X-ray-induced multiphoton process is the production of DCH states via the sequential absorption of two soft-X-ray photons on a time-scale on the order of the molecular Auger lifetime (~4-8 fs) [4]. The formation of molecular tsDCH states in particular shows great promise as a powerful tool for chemical analysis [5,6], and recently has attracted considerable attention [7][8][9][10]. The unique properties of the LCLS permit the search for these double core vacancies located at different atomic sites using XTPPS [11][12][13][14].A compelling motivation for the study of tsDCH states is their ability to probe the local chemical environment more sensitively than either single core-hole (SCH) [15] In the difference spectra a number of features can be discerned that are unambiguously related to the sequential absorption of two soft X-ray photons. First, at kinetic energies that are about 50-100 eV lower than the ordinary 1s -1 photoline (depending on the atom involved) a peak is observed that can be confidently assigned to the ssDCH state [6]. The tsDCH states are located much closer to the main photoline, typically shifted to lower energy by about 10-20 eV [6]. If the pulse duration exceeds the Auger lifetime, Auger decay can take place before the absorption of a second photon. This gives rise to the so-called Photoemission-AugerPhotoemission (PAP) peaks in the photoelectron spectra, whose location can be calculated from the energies of the relevant doubly and triply ionized states of the molecule [10,18,[22][23][24]. Generally, PAP peaks appear at kinetic energies of about 20-40 eV lower than the main photoline.The relative intensities of the ssDCH, tsDCH and PAP peaks, as well as that of the main photoline, can be simulated on the basis of a straightforward kinetic model which has been shown to produce reliable results [25]. For our experimental conditions and for the molecules studied here, the various ssDCH and tsDCH peaks for a particular molecule are expected to have very similar integrated intensities, typically within a factor of 2 [19]. This ...