Claude Girardet

HX, N2 double doping experiments in monatomic matrices: Near infrared spectra and symmetry properties of the intermolecular potential

The infrared spectra of monomeric and dimeric hydrogen halides trapped in mixed nitrogen‐rare gas matrices are recorded for various nitrogen–rare gas ratios. The analysis of the displacement of the monomer and dimer band frequencies strongly suggests the formation of HX–nN2 aggregates inside the monatomic crystal when varying this ratio from pure rare gas to pure nitrogen. A convenient analytical form is proposed in order to describe the interaction potential energy between the foreign molecules and the matrix. Numerical procedures based on realistic but tractable models for the representation of the doped crystal structure are then used in order to calculate the conformation of some aggregates inside the crystal and to prove the strong tendency for foreign molecules to agglomerate each other. We can thus explain the behavior of the HX monomer and dimer bands frequencies that decrease when doping with N2 molecules down to a minimum for a N2–rare gas ratio 60%–70% and then increase up to the HX–pure nitrog...

Properties of the hydracids trapped in a nitrogen matrix: Experimental and static field approximation study

The properties of the hydracid monomers and dimers trapped in N2 matrices are studied through an experimental reinvestigation of the near and far infrared spectra. In the nir spectra, two kinds of dimers are identified: a nearest‐neighbor (nn) dimer in which the two molecules are nonequivalent and a next‐nearest‐neighbor (nnn) dimer giving rise to a specific absorption very close to the monomer signal. New data in the fir confirm the previous assignment of the librational modes of the monomer but do not allow the identification of the corresponding dimer bands. These results are interpreted according to intermolecular potential calculations, taking into account the angular distortion of the matrix molecules around the HX impurity. An analytical description of the orientation quantum states for the impurities is developed and conveniently explains the fir monomer data by the lifting of the symmetry properties of the perfect crystal. The theoretical frequencies are in much better agreement with experimental when the N2 crystal dynamics is included. In the same way, calculations performed on the dimers show they lose their entity properties in the N2 crystal. Moreover, the librational response of the (HX)2 species, found in the same frequency range as the monomer, may account for a surprising discrepancy between experimental and calculated fir monomer band intensity ratio. The results obtained in the nir lead to a crucial modification of the electric multipole derivatives with respect to the internuclear coordinate in the hydracid dimers, which is explained in terms of a charge transfer process between the two molecules.

Crystal‐split electronic states of an atom in a rare gas crystal. Calculation of the absorption and fluorescence spectra of trapped oxygen (3P, 1D, 1S) atoms

A method is proposed to determine the (crystal perturbed) energy levels of a guest species at any point inside the trapping site of an inclusion distorted host crystal. It relies on available pair potential energy curves, usually deduced from ab initio data supplemented by long‐range dispersion energy when necessary. It is applied here to oxygen in its atomlike 3P, 1D or 1S state imbedded in Ne, Ar, Kr, and Xe matrix and allows reproduction of transition energies within 0.03 eV, and other experimental features. Its broad field of application could find many other exploitations, some of which are proposed.

Self‐consistent interaction potential for a molecule adsorbed on a dielectric surface: A symmetric top molecule on an ionic crystal

An iterative self‐consistent determination of the long range interaction energy between an admolecule and a ionic crystal is performed within the scheme of local and response potentials and the definition of the generalized electric susceptibilities of the two partners. The multipolar (electrostatic+induction) contributions and the quantum (dispersion+empirical short range) terms are determined as a sum of interactions between the molecule and the atomic planes parallel to the surface, constituting the crystal. The discrete structure of each plane is described with an increasing accuracy by increasing the order of the Fourier expansion in the reciprocal planar lattice. A semianalytical expression of each contribution is given for a symmetric top molecule adsorbed on a NaCl surface as a function of the location of the center of mass and of the orientation of the molecule.

Interaction potential and chiral discrimination between an alanine molecule and a quartz surface

The calculation of the interaction energy between a quartz crystal surface and a chiral molecule is considered. The various contributions (electrostatic, dispersion, hydrogen bond, and repulsion) to this energy are separately treated and computed using atom–atom summation or Fourier expansion techniques. The discussion of the resulting potential surface allows us to exhibit an influence of the chiral character of the adsorbed molecule; this may amount to a few percent of the total interaction energy for closest approaches. A possible connection with the experimental results is suggested.

Structure of the HX-HX′ dimers

With the advent of accurate measurements on simple molecular systems, the usual electrostatic + polarization + exchange potential has been more and more subject to severe criticism. Owing to the universality of the sole analytical form actually at our disposal and to its reliability in interpreting the spectral response of hydracids trapped in low temperature rare gas matrices, we apply it to the determination of the geometry of molecular dimers HX-HX′, to the subsequent study of the low frequency coupled internal motions (the three coupled anharmonic librational modes describing the orientation of the internuclear axes and the intermolecular vibration mode) and to the calculation of the principal moments of inertia of the rotating complex. Although the calculated mean intermolecular distance appears 5 per cent shorter than the measured one, the close agreement between the internal zero point motion, averaged angular geometries and the beam experiment data clearly shows that such an analytical potential c...

Potential energy calculations for argon and methane adsorbed on MgO(001) substrate

Abstract A semi-empirical potential model is used to calculate the interaction energy of a rare gas atom or a methane molecule adsorbed on a MgO substrate with square symmetry. The potential surfaces are drawn and compared with the results obtained on the hexagonal (0001) face of graphite. MgO appears as a corrugated surface for both argon and methane whereas graphite is a nearly perfect planar surface. The calculated holding and corrugation energies are in agreement with experimental data.

NH3 trapped in nitrogen and rare gas matrices. I. Potential surface analysis

Abstract New results obtained from the high-resolution study of the ν2 mode of nitrogen-trapped ammonia have prompted us to investigate both the internal (vibration and inversion) and external (translation and orientation) degrees of freedom of the symmetric top. In a first step, the overall potential is built from the usual internal NH3 term supplemented by an a priori NH3 interaction contribution. This latter contribution appears as a sum over the lattice of a pairwise potential containing molecular and atomic terms. Large eccentricity values (0.36 A) are calculated for the NH3 equilibrium configurations. A high barrier to flip-flap reorientation (= 850 cm−1) and a comparatively moderate one (= 90 cm−1) to proper rotation (spinning) around the C3 axis are predicted. Twelve different sets of equilibrium coordinates involving inversion, orientation, proper rotation and translation (eccentricity) yield equivalent potential wells. A number of sections through the potential hypersurface - those connecting two by two the equivalent wells - is critically described. The adequacy of the potential is guaranteed by comparing the calculated librational frequency (νL = 167 cm−1) to the experimental one (νL = 174 cm−1).

NH3 trapped in nitrogen and rare gas matrices. II. High-resolution spectroscopy of ν2 and calculation of the inversion-translation doubling and proper rotation quadrupling

Abstract The high-resolution IR spectrum of the ν 2 absorption band of NH 3 embedded in solid N 2 at 5.5 K exhibits a quadruplet structure. This structure includes the previously mentioned inversion doublet while the additional splitting shows striking nuclear spin species conversion over a very large timescale. The inversion doubling, ⋍ 1.65 cm −1 , is considerably smaller than in rare gas matrices (⋍ 24 cm −1 ) and in the gas phase (37 cm −1 ). The temperature dependence of the quadruplet frequencies shows in N 2 a larger blue shift than is usually expected and a typical motional narrowing for the doublet structure in the range 8–17 K. The a priori determination of the motions of NH 3 around the equilibrium configurations of the potential surface described in the first paper of this series, shows that the strong coupling between intrinsic inversion and translational space inversion is responsible for the doubling decrease. Such a feature is due to the large equilibrium eccentricity in a N 2 matrix. As this eccentricity is much smaller in rare gas matrices, the coupling is much weaker and the spacing closer to the gas-phase value. The quadruplet structure is due to the vibrational dependence of the hindered proper rotational (spinning) motion in the three-fold wells, characteristically coupled to the nuclear spin species. All numerical predictions are in agreement with experimental measurement.

Substrate-mediated interactions between adsorbed atoms and molecules - a discrete solid theory

Abstract The formalism of the generalized susceptibility theory is applied to the calculation of the substrate-mediated long-range interaction energy between atoms or molecules adsorbed on metal or dielectric surfaces. It is shown that a discrete description of the substrate can increase the magnitude of the metal substrate-mediated dispersion energy between rare gas atoms by a factor 2 with respect to the results of the continuum approach. The influence of the energy is maximum when the atomic density of the surface is the greatest, which corresponds to a minimum of corrugation. For ionic crystals, the substrate-mediated multipolar energy between adsorbed molecules can also play a significant role in the determination of the orientational and translational configurations of an adsorbed molecular overlayer.