Daniel Maillard

Study of hydracids trapped in monatomic matrices. I. Near infrared spectra and aggregate structures

The infrared spectra of H35Cl, D35Cl, HBr, DBr and their mixtures in solid argon and krypton in the range 10–30 K are presented. Studies have been made over a wide concentration range, for matrix/hydracid ratio from 1000 to 20 which, together with annealing experiments and H/D isotopic dilutions, allow a complete discussion of polymeric species spectra. In the monomer region, Q features are clearly identified at low M/R ratio and assigned to lattice distortion effects on isolated molecules. A new assignment for dimer bands is proposed, in which the two molecules are not equivalent and characterized by different values of force constants and moment derivatives. The trimer is found cyclic, a configuration already mentioned, the three molecules being equivalent and oriented along the sides of an equilateral triangle. After annealing, a new polymeric species associated with a strong band at lower frequency appears and is identified, in the light of H/D mixture spectra, as a cyclic planar tetramer. From accura...

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.

Study of hydracids trapped in monatomic matrices. II. Intermolecular potential calculations of the structure and spectral response of polymers

Intermolecular potential calculations are developed for hydracids trapped in rare gas matrices and the results concerning both geometries and spectral responses of low polymers are compared to the analysis of experimental data previously described. Geometrical structures, including matrix distortion, are calculated from a minimization of the total energy. Cyclic and planar conformations found for trimers and tetramers satisfy both criteria: the (molecular) packing tendency imposed by the isotropic Lennard‐Jones potential and the highest order symmetry for this packing required by the electrostatic potential. In a second step, the corresponding librational and vibrational frequencies are calculated according to the usual techniques of separation of angular and vibrational variables. For librations, it is moreover possible to separate in‐plane and out‐of‐plane motions, the first ones lying at higher frequency than the second; intensities of the corresponding bands are also calculated, taking into account th...

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.

Convergency of some properties of electrostatically interacting molecules from accurate CI multipole function calculations

The electric multipole moments up to the hexadecapole, as well as their first and second derivatives with respect to internuclear distance, have been calculated from near Hartree-Fock wavefunctions supplemented by medium size CI treatments. For HF the octupolar and hexadecapolar moments are respectively Ω zzz = 1·699 DA2, Φ zzzz = 1·804 DA3, while for HCl Ω zzz = 2·446 DA2 and Φ zzzz = 4·704 DA3. Lower moments are in agreement with experimental values. The convergency of two expansions of the electrostatic potential, that differ in the way they are quenched, as well as the corresponding geometries is then discussed. It is concluded that the expansion should be exponent truncated (ET) rather than moment truncated (MT).

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...

Why are perturbations of HCl by N2 and CO so different?: Part II. Matrix isolation experiments

Abstract The infrared and raman spectra of hydracids (HCl, DCl, H 35 Cl, D 35 Cl, HBr) codeposited with dopants P (CO, 13 CO, 13 C 18 O, N 2 ) in argon matrices are presented. Studies have been carried out over a wide concentration range, for matrix/dopant ratios from 1000 to 10 which, together with H D isotopic dilutions, allows identification of several (HX) m (P) n aggregates (P = CO, N 2 ) such as m = 1, n =1, 2, 3 (CO case), and m = 2, n = 1, 2 (CO and N 2 cases). The straightforward identification of the bands assignable to the HClCO codimer for six isotopic species allows an accurate force field calculation to be done. For the (HCl) 2 (P) n aggregates, intensity and frequency measurements are used for a comparison between the HCl molecules which are non-equivalent. From the whole data a comparison is drawn between the perturbing power of CO and N 2 ; this second molecule is a poorer electron donor than CO but is able, however, to destroy the HCl dimer structure. Finally, the relative stability of the various copolymers (HCl) m (P) n is discussed using the results of careful diffusion experiments. It is shown that stability generally increases with m and decreases with n .

Vacuum UV photolysis of CO2. Rare-gas oxide formation in matrices

Abstract In this paper, we report experimental results obtained upon photolysis of CO 2 trapped at low concentrations (0.1%) in neon, argon and krypton matrices. The mixture are photolysed using a xenon resonance lamp emitting photons at 8.4 eV. Only in a neon matrix does the photolysis of CO 2 lead to O( 1 D) → O( 3 P) emission. Furthermore, oxygen atoms are shown to diffuse at all temperatures in a neon matrix (as detected by molecular oxygen emission), whereas in the other matrices this occurs only when the sample is warmed even though irradiation is performed at high temperature. In argon and krypton matrices, O( 1 S) → O( 1 D) emission is observed even though there is not enough energy to form an O( 1 S) atom in a primary photodecomposition process. This suggests that O( 1 D) becomes bound to argon or krypton atoms as a stable rare-gas oxide RGO excimer, which is subsequently excited and photolysed by 8.4 eV photons. These observed features are explained using a model for the interaction between oxygen ( 3 P, 1 D, 1 S) and rare-gas atoms ( 1 S) which had previously been proposed to account for the emission spectra of oxygen atoms observed during photolysis of oxygenated compounds.

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).