L. Manceron

Performance of the AILES THz‐Infrared beamline at SOLEIL for High resolution spectroscopy

The new THz beamline (AILES) located at the third generation Synchrotron Radiation source SOLEIL is now operating for applications in a wide variety of research themes. In particular, this source with its adapted optics allows high resolution spectroscopic measurements of molecules in the entire infrared and THz range. This presentation focuses on the performances concerning flux, spectral range and stability for molecular spectroscopy. Thanks to these performances, the coupling of synchrotron radiation from a highly stable third generation source with high resolution FTIR spectrometer and with a long path cell (150 m or more) can be particularly advantageous. This fact is related to the optics of the beamline permitting the entire source to be used without aperture stop (entrance iris), even for measurements at highest‐resolution of ∼0.1 μeV (10−3u2009cm−1).

THE VIBRATIONAL SPECTRUM OF ISOLATED ALH4- : AN INFRARED MATRIX ISOLATION AND AB INITIO STUDY

An aluminium tetrahydride species has been formed after electronic excitation at about 5 eV of AlH2(H2) van der Waals aggregates isolated in argon matrices. Isotopic studies characterized the five isotopic species (corresponding to −H4, −H3D, −H2D2, −HD3 and −D4 containing molecules) and demonstrated that this species has four equivalent Al–H bonds with a tetrahedral or very nearly tetrahedral structure. Ab initio studies using the coupled‐cluster singles and doubles [CCSD (T,E4T)] and density functional theory (DFT) methods confirmed the highly unstable character of an hypothetical neutral AlH4 species as previously established by other workers, which would spontaneously dissociate back into AlH2(H2). On the other hand, the calculated vibrational spectrum of the tetrahydroaluminate anion, AlH−4, would perfectly fit that of the observed species. Identification of the isolated AlH−4 is thus proposed.

Observing microscopic structures of a relativistic object using a time-stretch strategy

Emission of light by a single electron moving on a curved trajectory (synchrotron radiation) is one of the most well-known fundamental radiation phenomena. However experimental situations are more complex as they involve many electrons, each being exposed to the radiation of its neighbors. This interaction has dramatic consequences, one of the most spectacular being the spontaneous formation of spatial structures inside electrons bunches. This fundamental effect is actively studied as it represents one of the most fundamental limitations in electron accelerators, and at the same time a source of intense terahertz radiation (Coherent Synchrotron Radiation, or CSR). Here we demonstrate the possibility to directly observe the electron bunch microstructures with subpicosecond resolution, in a storage ring accelerator. The principle is to monitor the terahertz pulses emitted by the structures, using a strategy from photonics, time-stretch, consisting in slowing-down the phenomena before recording. This opens the way to unpreceeded possibilities for analyzing and mastering new generation high power coherent synchrotron sources.

A density functional study of M–C2H4 complexes (M=Li, Na, K): Singularity of the Li atom

Quantum chemical calculations on the Li–C2H4 complex have been performed with coupled‐cluster and density functional methods. For both methods the electronic ground state of the complex is calculated to be 2B2, with a C2v symmetry equilibrium structure, and the calculated binding energy is quite small (around 2 kcal/mol), and therefore very much basis set dependent. The vibrational spectrum has been calculated at the harmonic approximation, including 13C/12C, 7Li/6Li, and H/D isotopic substitutions. The agreement between experimental and calculated infrared frequencies is correct, except for the low frequency symmetric Li–C stretching mode. These calculations also allow to propose an assignment for the observed C–H/C–D stretching modes. The observed blue‐shift of the symmetric CH2 bending mode as well as the red‐shift of the antisymmetric CH2 bending, CD2 bending, and C–C stretching modes with respect to the free ethylene have been confirmed by the density functional calculations. The Na...C2H4 complex ha...

First far-infrared high-resolution analysis of the ν2 band of sulphur dioxide 32S16O18O and 32S18O2

ABSTRACT High-resolution Fourier transform spectra of 18O-enriched isotopic samples of sulphur dioxide (32S16O18O and 32S18O2) have been recorded at 0.00102 cm−1 resolution in the 400–620 cm−1 region at Synchrotron SOLEIL. These spectra have been recorded at low temperature 185 K using a 3.14 m optical path length cryogenic cell. This enables the first detailed infrared analysis of the ν2 bands of the 32S16O18O and 32S18O2 isotopologues of sulphur dioxide located at 507.36541(1) and 496.59988(1) cm−1, respectively. Using a Watson-type Hamiltonian model to compute the upper and lower state energy levels, it was possible to reproduce the observed transitions. For both species, accurate rotational and centrifugal distortion constants were derived for the upper (0,1,0) vibrational state, while those of the (0,0,0) ground state were significantly updated. For this task, we combined the results of the present infrared measurements with the available literature microwave data in the (0,0,0) and (0,1,0) vibrational states. Finally, we took the opportunity of this study to compare the quality of the fit using an A- and S-type reduction for the Watson Hamiltonian.

High-resolution analysis of the ν1 and ν5 bands of phosgene 35Cl2CO and 35Cl37ClCO

Fourier transform spectra of phosgene (Cl2CO) have been recorded in the 11.75 and 5.47 μm spectral regions using a Bruker IFS125HR spectrometer at resolutions of 0.00102 and 0.0015 cm−1, respectively, leading to the observation of the ν5 and ν1 vibrational bands of the two isotopologues 35Cl2CO and 35Cl37ClCO. The corresponding upper state ro-vibrational levels were fit using Watson-type Hamiltonians and/or a Hamiltonian matrix accounting for resonance effects when necessary. In this way, it was possible to reproduce the upper state ro-vibrational levels to within the experimental accuracy, i.e. ∼0.17 × 10−3 cm−1. Very accurate rotational and centrifugal distortion constants were derived from the fit together with the following band centres: ν0(ν5, 35Cl2CO) = 851.012737(20) cm−1, ν0(ν5, 35Cl37ClCO) = 849.995451(90) cm−1, ν0(ν2 + ν3, 35Cl37ClCO) = 864.42370(50) cm−1, ν0(ν1, 35Cl2CO) = 1828.202514(40) cm−1 and ν0(ν1, 35Cl37ClCO) = 1827.246444(20) cm−1.

On the Electronic Structure and Photochemistry of Coordinatively Unsaturated Complexes: The Case of Nickel Bis-dinitrogen, Ni(N2)2

The electronic ground and excited states of the coordinatively unsaturated complex Ni(η(1) -N2 )2 , isolated in an Ar matrix, are analyzed in detail by vibrational and electronic absorption and emission spectroscopies allied with quantum chemical calculations. The bond force constants are determined from a normal coordinate analysis and compared with those of the isoelectronic carbonyl complex. The consequences for the bond properties are discussed, and the trend in the force constants is compared with the standard formation enthalpies. The linear complex Ni(η(1) -N2 )2 with two terminal dinitrogen ligands can be photoisomerized to two isomeric, metastable forms Ni(η(1) -N2 )(η(2) -N2 ) and Ni(η(2) -N2 )2 , with one and two side-on coordinated dinitrogen ligands, respectively.

Terahertz coherent synchrotron radiation at the synchrotron SOLEIL

We present experimental and numerical results on Terahertz coherent synchrotron radiation obtained in the beamline AILES at the synchrotron SOLEIL. Depending on the storage ring configuration and on the electron bunch current we observed either stable or instable signals.

Direct Observation of Spatiotemporal Dynamics of Short Electron Bunches in Storage Rings

In recent synchrotron radiation facilities, the use of short (picosecond) electron bunches is a powerful method for producing giant pulses of terahertz coherent synchrotron radiation. Here we report on the first direct observation of these pulse shapes with a few picoseconds resolution, and of their dynamics over a long time. We thus confirm in a very direct way the theories predicting an interplay between two physical processes. Below a critical bunch charge, we observe a train of identical THz pulses (a broadband Terahertz comb) stemming from the shortness of the electron bunches. Above this threshold, a large part of the emission is dominated by drifting structures, which appear through spontaneous self-organization. These challenging single-shot THz recordings are made possible by using a recently developed photonic time stretch detector with a high sensitivity. The experiment has been realized at the SOLEIL storage ring.

High sensitivity photonic time-stretch electro-optic sampling of terahertz pulses

Single-shot recording of terahertz electric signals has recently become possible at high repetition rates, by using the photonic time-stretch electro-optic sampling (EOS) technique. However the moderate sensitivity of time-stretch EOS is still a strong limit for a range of applications. Here we present a variant enabling to increase the sensitivity of photonic time-stretch for free-propagating THz signals. The ellipticity of the laser probe is enhanced by adding a set of Brewster plates, as proposed by Ahmed et al. [Rev. Sci. Instrum. 85, 013114 (2014)] in a different context. The method is tested using the high repetition rate terahertz coherent synchrotron radiation source of the SOLEIL synchrotron radiation facility. The signal-to-noise ratio of our terahertz digitizer could thus be straightforwardly improved by a factor ≈6.5, leading to a noise-equivalent input electric field below 1.25 V/cm inside the electro-optic crystal, over the 0-300 GHz band (i.e., 2.3 μV/cm/Hz). The sensitivity is scalable with respect to the available laser power, potentially enabling further sensitivity improvements when needed.