L. Abouaf-Marguin

The rotation and inversion of normal and deuterated ammonia in inert matrices

Abstract Infrared spectra have been recorded for all of the vibrational fundamental regions of NH 3 in argon, krypton, and xenon matrices, for the ν 2 fundamental region of NH 3 in neon and nitrogen matrices, and for the ν 2 , ν ′ 4 , and ν ″ 4 fundamental regions of the deuterated ammonias in an argon matrix. Detailed studies of the temperature and time dependence of absorptions attributed to rotational structure in the vibrational transitions have led to an assignment consistent with almost free rotation of the ammonia molecule in rare gas matrices. The theory of Devonshire has been used to evaluate a barrier to rotation of NH 3 in these matrices. The inversion splittings observed for the ν 2 fundamentals of NH 3 , NH 2 D, and NHD 2 are somewhat smaller than for the gas-phase molecules, but the calculated inversion potential barrier is increased by only about 10% in the rare gases. No structure in the ν 2 spectral region can be attributed to the rotation or inversion of NH 3 -d n in a nitrogen matrix. However, rotation of the molecule about its C 3 axis may occur in the nitrogen matrix.

Vibrational relaxation of CH3F in a krypton matrix at low temperatures. Influence of the rotation

Abstract The vibrational relaxation time of CH 3 F isolated in a krypton matrix is measured by infrared double resonance, using two CO 2 lasers. A previous

Vibrational energy transfer at low temperatures: CD2F in rare gas and nitrogen matrices☆

Abstract The v3 vibrational mode of matrix isolated CD3F molecules is excited by a Q switched CO2 laser. The energy is transferred to the lowest mode, v6, by a very fast intramolecular process. An experimental study of the relaxation of the v6 mode, and of other V—V energy transfer processes, is performed with an IR—IR double resonance method. The influence of the nature of the matrix on the relaxation is discussed and a correlation is found between the rate constant and the vibrational shift of v6.

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).

Movements of matrix-isolated methyl fluoride and isotopic species studied by infrared absorption spectroscopy

Abstract Infrared absorption spectra were recorded in the 10-μm region for the three isotopic methyl fluoride molecules 12CH3F, 12CD3F, 13CD3F, in the four rare gas matrices and in nitrogen. The two lowest fundamental modes ν3 (CF stretching) and ν6 (bending) have been systematically studied, as well as the ν2 and ν5 modes of 12CD3F. Relative intensity measurements show that the weak ν6 transition is enhanced by the matrix, compared to its gas-phase value. Frequency measurements yield vibrational shifts which can be compared to a theoretical estimate of the ν3 shift based on the Lennard-Jones-Devonshire cell model. These molecules are almost freely rotating in the matrices and a rotational analysis in the ν3 region brings out quantitative values of the barrier to free rotation as defined in Devonshires theory. Frequency and relative intensity measurements are also reported for dimeric species.

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.

Infrared time-resolved study of matrix isolated SF6

Abstract An infrared-infrared double resonance technique is used to study the vibrational energy relaxation of SF 6 in the ν 3 = 1 level, trapped in Ne, Ar, Kr, Xe and N 2 matrices. With different CO 2 laser lines, fundamental and hot bands can be probed. A general scheme for intramolecular V-V transfer is proposed, consistent with the observed signals. The energy cascades from ν 3 to the lowest mode ν 6 in the nanosecond range. Then ν 6 relaxes directly to the phonons, with time constants at 8 K of 12.2 × 10 3 , 6.5 × 10 3 , 200, 7 and 2 respectively. The temperature dependence of the rate constant is discussed in the frame of direct multiphonon processes as well as of binary collisions.

IR detection by matrix isolation technique of radicals produced in a silane discharge

Abstract Infrared absorption spectra of free radicals trapped in a solid argon matrix, after extraction from a low pressure silane plasma hot cathode discharge, have been clearly observed. First partial results concerning the assignments of the detected absorption peaks and the energy dependence of neutral fragmentation pattern of silane by electron impact are presented and discussed. Matrix isolation technique appears to be a promising tool to characterize neutral species in discharges devoted to thin film deposition.

IR high-resolution spectroscopy of the ν3 and ν6 modes of CH3F and CD3F isolated in nitrogen and neon matrices: Intramolecular V-V transfer

Abstract After some general considerations about the different contributions to the linewidths observed in infrared for matrix isolated molecules, the case of the ν 3 and ν 6 modes of CH 3 F and CD 3 F in nitrogen and neon matrices is discussed. When reducing the inhomogeneous contributions by annealing and dilution, the higher of these two modes remains at least three times larger than the lower. The temperature effect on these linewidths is consistent with an intramolecular energy transfer, assisted by the lowest vibrational mode of the solid nitrogen at 32 cm −1 .

Infrared dynamical studies of matrix isolated NH3. I. Vibrational relaxation

Abstract IR-IR double-resonance experiments are performed on NH3 trapped in inert matrices. In rare gas matrices, the vibrational relaxation time of the ν2= 1 level is less than 5 ns in Ne, and 115, 580, 2600 ns in Ar, Kr, Xe respectively. In N2-Ar mixed matrices, NH3 molecules having one N2 as a nearest neighbour relax faster than in a pure argon cage (35 ns instead of 125 ns). In N2 matrices, experiments probing ν4 show that the relaxation is fast (