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The so-called aromatic infrared bands are attributed to emission of polycyclic aromatic hydrocarbons. The observed variations toward different regions in space are believed to be caused by contributions of different classes of PAH molecules, i.e. with respect to their size, structure, and charge state. Laboratory spectra of members of these classes are needed to compare them to observations and to benchmark quantum-chemically computed spectra of these species. In this paper we present the experimental infrared spectra of three different PAH dications, naphthalene$^{2+}$, anthracene$^{2+}$, and phenanthrene$^{2+}$, in the vibrational fingerprint region 500-1700~cm$^{-1}$. The dications were produced by electron impact ionization of the vapors with 70 eV electrons, and they remained stable against dissociation and Coulomb explosion. The vibrational spectra were obtained by IR predissociation of the PAH$^{2+}$ complexed with neon in a 22-pole cryogenic ion trap setup coupled to a free-electron infrared laser at the Free-Electron Lasers for Infrared eXperiments (FELIX) Laboratory. We performed anharmonic density-functional theory calculations for both singly and doubly charged states of the three molecules. The experimental band positions showed excellent agreement with the calculated band positions of the singlet electronic ground state for all three doubly charged species, indicating its higher stability over the triplet state. The presence of several strong combination bands and additional weaker features in the recorded spectra, especially in the 10-15~$\mu$m region of the mid-IR spectrum, required anharmonic calculations to understand their effects on the total integrated intensity for the different charge states. These measurements, in tandem with theoretical calculations, will help in the identification of this specific class of doubly-charged PAHs as carriers of AIBs., Comment: Accepted for publication in A&A

The picosecond dynamics of excited charge carriers in the silicon substrate of THz metamaterial antennas was studied at different wavelengths. Time-resolved THz pump-THz probe spectroscopy was performed with light from a tunable free electron laser in the 9.3–16.7 THz frequency range using fluences of 2–12 J/m2. Depending on the excitation wavelength with respect to the resonance center, transient transmission increase, decrease, or a combination of both was observed. The transient transmission changes can be explained by local electric field enhancement, which induces impact ionization in the silicon substrate, increasing the local number of charge carriers by several orders of magnitude, and their subsequent diffusion and recombination. The studied metamaterials can be integrated with common semiconductor devices and can potentially be used in sensing applications and THz energy harvesting. ispartof: Optics Express vol:29 issue:1 pages:170-181 ispartof: location:United States status: published

Third-order non-linearities are important because they allow control over light pulses in ubiquitous high-quality centro-symmetric materials like silicon and silica. Degenerate four-wave mixing provides a direct measure of the third-order non-linear sheet susceptibility χ(3)L (where L represents the material thickness) as well as technological possibilities such as optically gated detection and emission of photons. Using picosecond pulses from a free electron laser, we show that silicon doped with P or Bi has a value of χ(3)L in the THz domain that is higher than that reported for any other material in any wavelength band. The immediate implication of our results is the efficient generation of intense coherent THz light via upconversion (also a χ(3) process), and they open the door to exploitation of non-degenerate mixing and optical nonlinearities beyond the perturbative regime., Light: Science & Applications, 10 (1), ISSN:2047-7538

The absorption of multiple photons when there is no resonant intermediate state is a well-known nonlinear process in atomic vapours, dyes and semiconductors. The N-photon absorption (NPA) rate for donors in semiconductors scales proportionally from hydrogenic atoms in vacuum with the dielectric constant and inversely with the effective mass, factors that carry exponents 6N and 4N, respectively, suggesting that extremely large enhancements are possible. We observed 1PA, 2PA and 3PA in Si: P with a terahertz free-electron laser. The 2PA coefficient for 1s-2s at 4.25 THz was 400,000,000 GM (= 4 x 10(-42) cm(4) s), many orders of magnitude larger than is available in other systems. Such high cross-sections allow us to enter a regime where the NPA cross-section exceeds that of 1 PA-that is, when the intensity approaches the binding energy per Bohr radius squared divided by the uncertainty time (only 3.84 MW cm(-2) in silicon)-and will enable new kinds of terahertz quantum control.

We present time-resolved measurements of the relaxation between the orbital states of the shallow acceptor boron in silicon. The silicon host was enriched Si-28, which exhibits life-time broadened absorption lines. We observed a wide range of T1 lifetimes from 6ps to 130ps depending on the excited state and the pump intensity. The fastest transients have not been observed previously in the time domain, and they are caused by the phonon relaxation responsible for the small-signal frequency domain line-width. We identify the slower components with an ionisation/recombination/cascade pathway.

The results of the experimental observation of stimulated terahertz emission under optical intracenter excitation of uniaxially strained bismuth-doped silicon are presented. Pumping in the presented experiment is performed using a FELIX free-electron laser. It is shown that uniaxial strain of the silicon crystal leads to a significant change in the stimulated emission spectrum of the impurity.

Gas phase vibrational spectra of [C3H3]+ isomers and their fully deuterated isotopologues measured in a cryogenic 22-pole ion trap are presented. The widely tunable free electron laser for infrared experiments, FELIX, was employed to cover the frequency range 500–2400 cm−1, complemented with an OPO/OPA system covering 2800–3400 cm−1. Spectral assignments for both the linear and cyclic isomeric form (H2C3H+ and c-C3H3+, respectively) are made based on various high-level computational studies. The effect of ion source conditions and different precursors (allene and propargyl chloride) for the preferential production of a specific isomer is discussed. The perturbation of the vibrational band position due to complexation with neon in the recorded infrared-predissociation (IRPD) spectra are also reported in this study.

We investigate the spin relaxation under conditions of optical excitation between the Rydberg orbital states of phosphorus donor impurities in silicon. Here we show that the spin relaxation is less than a few percent, even after multiple excitation/relaxation cycles. The observed high level of spin preservation may be useful for readout cycling or in quantum information schemes where coupling of neighbor qubits is via orbital excitation.

The gas phase vibrational spectrum of CH3CNH+ is investigated using a messenger infrared predissociation (IRPD) action spectroscopic method. Vibrational bands were recorded in the 300-1700 cm−1 and 2000-3300 cm−1 regions making use of the widely tunable free electron laser for infrared experiments, FELIX, coupled to a cryogenic ion trap instrument. Band assignments were aided by high-level quantum-chemical calculations, which showed excellent agreement with the experimental data. Effects of the neon atom used as messenger in the IRPD method are investigated in detail. The data presented here will support astronomical searches for the CH3CNH+ ion in space, and provide a basis for high-resolution ro-vibrational and pure rotational studies in vibrationally excited states.

© The Royal Society of Chemistry 2016. Understanding of the nature and extent of chemical bonding in uranyl coordination complexes is crucial for the design of new ligands for nuclear waste separation, uranium extraction from seawater, and other applications. We report here the synthesis, infrared spectroscopic characterization, and quantum chemical studies of a molecular uranyl-di-15-crown-5 complex. The structure and bonding of this unique complex featuring a distinctive 6-fold coplanar coordination staggered sandwich structure and an unusual non-perpendicular orientation of the uranyl moiety are evaluated using density functional theory and chemical bonding analyses. The results provide fundamental understanding of the coordination interaction of uranyl with oxygen-donor ligands.

International audience; Transient-type stimulated emission in the terahertz (THz) frequency range has been achieved from phosphorus doped silicon crystals under optical excitation by a few-picosecond-long pulses generated by the infrared free electron lasers FELIX and CLIO. The analysis of the lasing threshold and emission spectra indicates that the stimulated emission occurs due to combined population inversion based lasing and stimulated Raman scattering. Giant gain has been obtained in the optically pumped silicon due to large THz cross sections of intracenter impurity transitions and resonant intracenter electronic scattering. The transient-type emission is formed under conditions when the pump pulse intervals exceed significantly the photon lifetime in the laser resonator.

Experimental investigations of THz emission from silicon (Si) doped by shallow donors under uniaxial stress along the [001] crystal axis upon selective intracenter excitation by an infrared free electron laser are presented. spectral dependences of Si output integral emission on the pump wavelength and Si laser output terahertz emission spectra have been analyzed. Two mechanisms of stimulated emission have been recognized as inversion-population-based lasing and stimulated Raman scattering. It is shown that uniaxial stress allows to control the Stokes shift and by this the output Raman emission frequency in the active medium. The obtained results demonstrate the feasibility of broad range THz tuning in doped silicon by means of uniaxial stress.

Our microscopic understanding of intermolecular interactions has increased considerably from the studies on isolated molecular clusters. The intermolecular modes for these clusters are located in far-infrared and terahertz regions and provide a sensitive test of the potential energy surface. Here we report a helium droplet setup (BoHeNDI@FELIX), which is coupled with free electron lasers, FEL-I and FEL-II, at the FELIX laboratory in Nijmegen. These lasers provide tunable radiation in the range of 66–3600 cm−1, covering the intermolecular modes, with average output power of 0.1–0.8 W and spectral bandwidth of 0.2–5% of the central frequency. Mass-selective infrared spectra of propargyl alcohol were recorded in the frequency range of 560–1600 cm−1, which demonstrate the performance of the new setup. The observed vibrational bands could be assigned to the gauche conformer of propargyl alcohol. The high power and micropulse structure of the FELIX radiation allow multiple excitations of the embedded molecules allowing spectral detection with a signal to noise of 10, even in the low-frequency region.

Pump-probe spectroscopy is the most common time-resolved technique for investigation of electronic dynamics, and the results provide the incoherent population decay time T1. Here we use a modified pump-probe experiment to investigate coherent dynamics, and we demonstrate this with a measurement of the inhomogeneous dephasing time T2* for phosphorus impurities in silicon. The pulse sequence produces the same information as previous coherent all-optical (photon-echo-based) techniques but is simpler. The probe signal strength is first order in pulse area but its effect on the target state is only second order, meaning that it does not demolish the quantum information. We propose simple extensions to the technique to measure the homogeneous dephasing time T2, or to perform tomography of the target qubit.

Stimulated emission in the terahertz frequency range has been realized from boron acceptor centers in silicon. Population inversion is achieved at resonant optical excitation on the 1Γ_{8}^{+} → 1Γ_{7}^{−}, 1Γ_{6}^{−}, 4Γ_{8}^{−} intracenter transitions with a midinfrared free-electron laser. Lasing occurs on two intracenter transitions around 1.75 THz. The upper laser levels are the 1Γ_{7}^{−}, 1Γ_{6}^{−}, and 4Γ_{8}^{−} states, and the lower laser level for both emission lines is the 2Γ_{8}^{+} state. In contrast to n-type intracenter silicon lasers, boron-doped silicon lasers do not involve the excited states with the longest lifetimes. Instead, the absorption cross section for the pump radiation is the dominating factor. The four-level lasing scheme implies that the deepest even-parity boron state is the 2Γ_{8}^{+} state and not the 1Γ_{7}^{+} split-off ground state, as indicated by other experiments. This is confirmed by infrared absorption spectroscopy of Si:B.

Shallow group-V donors in silicon may be thought of as hydrogenlike, and shallow acceptors are similarly described by effective-mass theory with similar energy scales, which implies that donor and acceptor excitations should be just as long-lived. Yet, spectral widths of acceptors are considerably wider. We have measured the orbital dynamics of acceptors in silicon using time-domain spectroscopy with a free-electron laser. Both the population and coherence lifetimes for acceptors in natural silicon are substantially longer-e. g. approximately 60 ps for boron-than implied by the spectral linewidths; our experiments also establish the recombination time for ionized acceptors to be, at approximately 500 ps, nearly an order of magnitude longer. We show that there are no extra sources of decoherence introduced by the host crystal, other than the population relaxation. In this sense, the crystal acts as an atom trap, and, by introducing quantum coherent control of acceptors to that previously established for donors, we open the way to optically controllable nanoscale p-n devices.

The low-temperature (∼5 K) phonon-assisted relaxation of the 2p0 state of phosphorus donors in isotopically pure, monocrystalline 28Si has been studied in the time domain using a pump-probe technique. The lifetime of the 2p0 state in 28Si:P is found to be 235 ps, which is 16% larger than the lifetime of a reference Si:P sample with a natural isotope composition. The interaction of the 2p0 state with intervalley g-type longitudinal acoustic and f-type transverse acoustic phonons determines its lifetime. This interaction, which depends on the homogeneity of the crystal, becomes weaker in 28Si because of its more perfect crystal lattice compared to natural Si, and this leads to a longer lifetime. The difference between the linewidths of the 1s(A1) → 2p0 transition in 28Si:P and natural Si:P is more than a factor of two. It follows that linewidth broadening due to isotopic composition is an inhomogeneous process.

International audience; Midinfrared strong-field laser ionization offers the promise of measuring holograms of atoms and molecules, which contain both spatial and temporal information of the ion and the photoelectron with subfemtosecond temporal and angstrom spatial resolution. We report on the scaling of photoelectron holographic interference patterns with the laser pulse duration, wavelength, and intensity. High-resolution holograms for the ionization of metastable xenon atoms by 7-16 m light from the FELICE free electron laser are presented and compared to semiclassical calculations that provide analytical insight.

International audience; Ionization is the dominant response of atoms and molecules to intense laser fields and is at the basis of several important techniques, such as the generation of attosecond pulses that allow the measurement of electron motion in real time. We present experiments in which metastable xenon atoms were ionized with intense 7-micrometer laser pulses from a free-electron laser. Holographic structures were observed that record underlying electron dynamics on a sublaser-cycle time scale, enabling photoelectron spectroscopy with a time resolution of almost two orders of magnitude higher than the duration of the ionizing pulse.

We demonstrate THz photon echoes from silicon doped with phosphorus donors. We provide experimental evidence showing that the echo emerges at the predicted angle and that it arrives at our detector at the predicted time. We use this to demonstrate coherent control of the silicon donor states.

Stimulated terahertz emission at different frequencies between 4.5 THz and 6.4 THz has been realized from a single silicon crystal doped simultaneously by two hydrogenlike donor centers, which were optically excited on intracenter transitions at low lattice temperature.

Silicon-based semiconductors offer optically low-loss and high-thermal-conducting lattice for the broad-band terahertz active media that can be used in the range of 5-7 THz. We report on realization of the terahertz-range stimulated emission from monocrystalline natural and isotopically enriched silicon crystals doped by group-V donor centers due to nonlinear frequency conversion. Lasing in the frequency bands of 1.2 - 1.8 THz; 2.5 - 3.4 THz has been achieved from silicon crystals doped by phosphorus and in the frequency band of 4.6 - 6.4 THz from different donors under optical pumping by radiation of mid-infrared free electron laser at cryogenic temperatures. Analysis of the data shows that the emission in high-frequency band corresponds to electronic Stokes-shifted Raman-type lasing. The low-frequency bands indicate on high-order nonlinear frequency conversion processes similar to four-wave mixing accompanied by highenergy intervalley g-phonons and f-phonons of host lattice. These lasers supplement terahertz silicon lasers operating on transitions between donor states.

For decades, protonated methane, CH5+, has provided new surprises and challenges for both experimentalists and theoreticians. This is because of the correlated large-amplitude motion of its five protons around the carbon nucleus, which leads to so-called hydrogen scrambling and causes a fluxional molecular structure. Here, the infrared spectra of all its H/D isotopologues have been measured using the ‘Laser Induced Reactions’ technique. Their shapes are found to be extremely dissimilar and depend strongly on the level of deuteration (only CD5+ is similar to CH5+). All the spectra can be reproduced and assigned based on ab initio quantum simulations. The occupation of the topologically different sites by protons and deuterons is found to be strongly non-combinatorial and thus non-classical. This purely quantum-statistical effect implies a breaking of the classical symmetry of the site occupations induced by zero-point fluctuations, and this phenomenon is key to understanding the spectral changes studied here. Stepwise deuteration of protonated methane CH5+ — a fluxional structure that undergoes ‘hydrogen scrambling’ — leads to dramatic changes in the infrared spectra of the isotopologues. The spectra can be assigned using ab initio quantum simulations that account for the non-classical occupation — by H and D atoms — of topologically different sites within the molecule.

Raman-type stimulated emission at frequencies between 5.0 and 5.2 THz as well as between 6.1 and 6.4 THz has been realized in silicon crystals doped by phosphorus donors. The Raman laser operates at around 5 K under optical excitation by a pulsed, frequency-tunable infrared free electron laser. The Stokes shift of 3.16 THz is equal to the difference between the energies of the phosphorus ground state, 1s(A1), and the 1s(E) split off state.

Under optical pumped by mid-infrared lasers, silicon crystals doped by group-V donor centers reveal stimulated emission in terahertz range. Different features of the population inversion mechanisms in the silicon lasers allow covering the frequency range from 1 to 7 THz with a few tenths of mW pulse peak powers at low lattice temperatures.

Stimulated Stokes emission has been observed from silicon crystals doped by antimony donors when optically excited by radiation from a tunable infrared free electron laser. The photon energy of the emission is equal to the pump photon energy reduced by the energy of the intervalley transverse acoustic (TA) g phonon in silicon (approximately 2.92 THz). The emission frequency covers the range of 4.6-5.8 THz. The laser process occurs due to a resonant coupling of the 1s(E) and 1s(A1) donor states (separation approximately 2.97 THz) via the g-TA phonon, which conserves momentum and energy within a single impurity center.

Protonated methane, CH 5 + , continues to elude definitive structural assignment, as large-amplitude vibrations and hydrogen scrambling challenge both theory and experiment. Here, the infrared spectrum of bare CH 5 + is presented, as detected by reaction with carbon dioxide gas after resonant excitation by the free electron laser at the FELIX facility in the Netherlands. Comparison of the experimental spectrum at ∼110 kelvin to finite-temperature infrared spectra, calculated by ab initio molecular dynamics, supports fluxionality of bare CH 5 + under experimental conditions and provides a dynamical mechanism for exchange of hydrogens between CH 3 tripod positions and the three-center bonded H 2 moiety, which eventually leads to full hydrogen scrambling. The possibility of artificially freezing out scrambling and internal rotation in the simulations allowed assignment of the infrared spectrum despite this pronounced fluxionality.

Optical pumping of excited donor states in silicon crystals doped by antimony, phosphor, arsenic, and bismuth impurity centers by emission from a free electron laser yields stimulated terahertz emission (4-7 THz). The detailed study of silicon emission spectra provides information on intracenter phonon-assisted relaxation mechanisms in silicon at low lattice temperatures as well as on binding energies of excited states of donor centers.