Rotational rainbow scattering from an off-center rigid shell model

A two-dimensional classical calculation based on a rigid off-centre ellipsoid potential model is used to derive explicit formulas which combine the measurable quantities of rotational rainbow scattering with potential parameters. Application are presented for the scattering of D₂ + CO and K + CO.     

State-to-state dynamics of the hydrogen exchange reaction

Nascent product quantum state distributions have been measured for reactive, H + D₂ → HD + D, and inelastic, H + D₂ → D₂? + H, collisions of H with D₂, at collision energies of 0.98 eV, 1.1 eV, and 1.3 eV. These distributions are extracted from coherent anti-Stokes Raman scattering (CARS) spectra of HD and D₂ (recorded under single-collision conditions) following laser photodissociation of HI to generate translationally hot H atoms in a D₂/HI gas mixture. Variation of the photolysis wavelength allows selection of the H atom translational energy. These rotational and vibrational state distributions are compared with those obtained from quasiclassical trajectory calculations on an ab initio potential energy surface. The agreement between the calculated and measured distributions is extremely good. TheHD and D₂ product quantum state distributions are well represented by simple linear surprisal functions, with large positive vibrational and rotational surprisal.     

Collision and magnetic field effects on time-resolved fluorescence of D2CO excited by a N2 laser

Time-resolved fluorescence spectra of D₂CO were measured as a funtion of sample pressure. The result indicates the occurence of rotational and vibrational relaxation between excited states. The Magnetic field effect on fluorescence from D₂CO excited by a N₂ laser depends on the nature of the vibronic levels.     

IR and Raman study of rotational relaxation of furan and thiophene. I. Bandwidths. Comparison of the rotations about the principal axes of inertia

IR absorption and Raman scattering bandwidths of some fundamental modes of A₁, B₁ and B₂ species of furan and thiophene and their temperature dependence have been studied. This analysis has been carried out on the pure liquid phase. Rotational A, bandwidths are directly obtained. Rotational B₁ and B₂ bandwidths are deduced by using the Rakov model. The same activation energy U accounts for all the results concerning the reorientations around the three principal axes of inertia. The rotational diffusion coefficients D_x,D_y and D_z are then deduced from the rotational IR bandwidths. The results allow us to conclude that quasi isotropic molecular rotations occur in the liquid phase.     

Author index to volume 80

Time-resolved infrared-ultraviolet double-resonance spectroscopy provides a sensitive means of studying collision-induced rotational relaxation rates and mechanisms. Relaxation of the 12_3,10 rotational state in the v₄ = 1 vibrational level of D₂CO is found to follow electric-dipole selection rules with rates determined absolutely to be as much as five times gas kinetic.     

Low temperature vibrational relaxation of carbon monoxide by light mass species

The rates of deactivation of CO(v=1) by ⁴He and by ³He have been measured between 300 K and 80 K using a pulsed laser fluorescence technique. The results show the usual strong deviation from Landau—Teller behaviour and a marked isotope effect. Comparison is made with theoretical predictions including a one-dimensional treatment which takes account of attractive forces. The present results are compared also with earlier work on the deactivation of CO by H₂, HD and D₂. It has been found that at and below 300 K D₂ is less efficient than ⁴He in the (VT) deactivation of CO (v=1) and HD is anomalously efficient. The latter effect is attributed to the involvement of rotational transitions.     

Measurements of inelastic cross sections for the (j,m) = (2,0)→(3,0) rotational transition of CsF in collisions with atoms and molecules

Individual state-to-state rotational transitions have been resolved in small angle scattering of polarized CsF molecules on Ne, Ar, C₂, H₆, N₂, CO, CO₂, CHF₃ at center of mass energies of about 0.1 eV. The absolute inelastic cross sections range from 5Ų up to 600Ų.     

Measurements of rotational relaxation cross sections for H2CO

Cross sections for the 1₁₁, 1₁₀, 2₁₂, and 2₁₁ rotational states and for total beam attenuation of H₂CO were measured using a beam maser spectrometer. Significant inelastic scattering in the forward direction was obtained for He and CF₃H. Significant inelastic contributions were not observed using H₂ as a scattering gas. Observed cross sections for the 1₁₁ state are larger than those for the 1₁₀ state. The relations of the present cross sections to various models for obtaining a low excitation temperature for the 1₁₁-1₁₀ doublet of formaldehyde in interstellar space is discussed.     

CO Blocking of D2 Dissociative Adsorption on Ru(0001)

The influence of pre‐adsorbed CO on the dissociative adsorption of D₂ on Ru(0001) is studied by molecular‐beam techniques. We determine the initial dissociation probability of D₂ as a function of its kinetic energy for various CO pre‐coverages between 0.00 and 0.67 monolayers (ML) at a surface temperature of 180 K. The results indicate that CO blocks D₂ dissociation and perturbs the local surface reactivity up to the nearest‐neighbour Ru atoms. Non‐activated sticking and dissociation become less important with increasing CO coverage, and vanish at θ_CO≈0.33 ML. In addition, at high D₂ kinetic energy (>35 kJ mol^?1) the site‐blocking capability of CO decreases rapidly. These observations are attributed to a CO‐induced activation barrier for D₂ dissociation in the vicinity of CO molecules.     

Classical differential cross sections and rotational energy transfer distributions for realistic ion—molecule potentials in the CPST limit

Classical differential cross sections, rotational energy transfer distributions at specified scattering angles and the first moments of the rotational energy transfer distributions are calculated for two ion—molecule systems: K⁺ ?CSCl and Li⁺ ?CO. The deflection angles and change in angular momentum are calculated using classical perturbation scattering theory (CPST). Monte Carlo techniques are then used to calculate the orientation averaged total differential cross sections and the rotational energy transfer distributions. Results are compared with experiment and agreement is found to be satisfactory. These two systems represent two extremes in anisotropy. For Li⁺ ?CO a strong classical rainbow peak is still seen in the differential cross section, while in the K⁺ ?CSCl system the rainbow is complete quenched. In the rotational energy transfer distributions of both systems, rotational rainbow peaks are clearly observed. The calculations also predict a leveling off of the first moment of the rotational energy transfer distribution at high angles, corresponding to the transition to repulsive scattering. On the basis of these results some comments are made on the nature of classical rainbow scattering for anisotropic systems.     

Low frequency motions of water isolated in low temperature matrices

Far infrared spectra of monomer H₂O, D₂O, HOD in argon and nitrogen and of H₂O in krypton and methane are recorded at different temperatures. Assignments of the bands to rotational and translational motions are given for argon and krypton on the basis of rotational constants, derived from mid infrared vibration rotation matrix spectra. Motions of water in nitrogen are interpreted as librations around the OH bond axes, because the HOD spectra do not fit the picture of librations around the proper rotational axes.     

Depolarised Rayleigh scattering from liquid carbon monoxide,nitrogen and oxygen

The spectral distributions and scattering cross sections of depolarised Rayleigh scattering have been determined for liquid carbon monoxide, nitrogen and oxygen at 77 K and atmospheric pressure. It is shown that the scattering arises predominantly from molecular orientational fluctuations. The experimental scattering cross sections at 488 nm are 7 ± 1 for CO, 16 ± 1 for N₂ and 46 ± 4 for O₂ in units of 10^?30 cm² sr^?1 molecule^?1, based on a recently determined value for the absolute depolarised scattering intensity for liquid argon. The estimated proportions of induced anisotropy scattering are 10% for CO, 2.5% for N₂ and 0.8% for O₂. It is shown that there is appreciable free rotation of N₂ and CO in the liquids at this temperature but for O₂ this motion is dissipated much more efficiently by molecular interactions.     

Magnetic resonance studies of hydrogen atom transfer

The experimental results obtained on four different types of Raman spectra: pure rotational lines, the I_VV and _VH components of the vibrational Q-branch and the vibrational rotational lines are presented for H₂, D₂, HF and N₂ dissolved at low concentration in inert solvents. The line broadening and motional narrowing due to the solvent interaction is discussed.     

Photodissociation Dynamics of OCS at 217 nm

The S(¹D₂)+CO(X¹Σ⁺) product channel from photodissociation of OCS at 217 nm has been measured using the DC slice velocity map imaging (VMI) technique in combination with resonance enhanced multiphoton ionization (REMPI). Two diflerent REMPI intermediate states (¹F₃ and ¹P₁) and several pump-probe laser polarization geometries are used to detect the angular momentum polarization of the photofragmented S(¹D₂). The molecular- frame polarization parameters, as well as the laboratory-frame anisotropy parameters, for individual rotational states of co-fragment CO, are determined using two diflerent full quantum theories. The measured total kinetic energy release spectrum from photodissociation of OCS indicates two dissociation channels, corresponding to the fast and slow recoiling velocities of S(¹D₂), respectively. The slow channel is concluded to originate from an initial photoexcitation to the A(¹A') state, followed by a non-adiabatic transition to the ground state. The fast channel is found to follow a coherent excitation to A(¹A') and B(¹A') states, where contributions of the two states are almost equal at 217 nm. The determined alignment and anisotropy parameters further indicate that the slow channel follows an incoherent excitation, while the fast channel follows a coherent excitation to A(¹A') and B(¹A') states with a phase di erence of π/2.     

D-nuclear quadrupole coupling in the rotational spectrum of 15N2…DF and an interpretation of the hyperfine coupling constants χ aaD and DaaHF within a series of complexes B…HF

Hyperfine structure in the J = 1 ← 0 and J = 2 ← 1 transitions of ¹⁵N₂…DF, arising from D-nuclear quadrupole coupling and D, ¹⁹F nuclear-spin—nuclear-spin coupling, has been observed and measured by using pulsed-nozzle Fourier-transform microwave spectroscopy. The following rotational, centrifugal distortion and hyperfine coupling constants were determined: β_o = 3091.9004 MHz, D_J = 14.75 kHz, χ_aa^D = 278.6(38) kHz and D_aa^HF = ?38.5(38) kHz. An interpretation is presented for the variation of the coupling, constants χ_aa^D and D_aa^HF along the series B…DF and B…HF, where B = Ar, Kr, Xe, ¹⁵N₂, CO, PH₃, H₂S, HC¹⁵N and H₂O.     

Differential Reduction of CO2 by Molybdenum and Vanadium Nitrogenases

The molybdenum and vanadium nitrogenases are two homologous enzymes with distinct structural and catalytic features. Previously, it was demonstrated that the V nitrogenase was nearly 700 times more active than its Mo counterpart in reducing CO to hydrocarbons. Herein, a similar discrepancy between the two nitrogenases in the reduction of CO₂ is reported, with the V nitrogenase being capable of reducing CO₂ to CO, CD₄, C₂D₄, and C₂D₆, and its Mo counterpart only capable of reducing CO₂ to CO. Furthermore, it is shown that the V nitrogenase may direct the formation of CD₄ in part via CO₂‐derived CO, but that it does not catalyze the formation of C₂D₄ and C₂D₆ along this route. The exciting observation of a V nitrogenase‐catalyzed C? C coupling with CO₂ as the origin of the building blocks adds another interesting reaction to the catalytic repertoire of this unique enzyme system. The differential activities of the V and Mo nitrogenases in CO₂ reduction provide an important framework for systematic investigations of this reaction in the future.     

The enthalpies of formation of CH3Mn(CO)5 and of CH3Re(CO)5, and the strengths of the CH3Mn and CH3Re bonds

From measurements of the heats of iodination of CH₃Mn(CO)₅ and CH₃Re(CO)₅ at elevated temperatures using the ‘drop’ microcalorimeter method, values were determined for the standard enthalpies of formation at 25° of the crystalline compounds: ΔH^o_f[CH₃Mn(CO)₅, c] = ?189.0 ± 2 kcal mol^?1 (?790.8 ± 8 kJ mol^?1), ΔH^o_f[Ch₃Re(CO)₅,c] = ?198.0 ± kcal mol^?1 (?828.4 ± 8 kJ mo^?1). In conjunction with available enthalpies of sublimation, and with literature values for the dissociation energies of MnMn and ReRe bonds in Mn₂(CO)₁₀ and Re₂(CO)₁₀, values are derived for the dissociation energies: D(CH₃Mn(CO)₅) = 27.9 ± 2.3 or 30.9 ± 2.3 kcal mol^?1 and D(CH₃Re(CO)₅) = 53.2 ± 2.5 kcal mol^?1. In general, irrespective of the value accepted for D(MM) in M₂(CO)₁₀, the present results require that, D(CH₃Mn) = $\text{1}\text{2}$D(MnMn) + 18.5 kcal mol^?1 and D(CH₃Re) = $\text{1}\text{2}$D(ReRe) + 30.8 kcal mol^?1.     

Molecular reorientation of liquid benzene. Anisotropic Raman scattering of the CH and CD stretching modes in the deuterated molecules

The anisotropic Raman spectra of the CH and CD stretching modes in seven deuterated benzenes of D_6h, D_3h, D_2h and C_2h symmetry are reported. The reorientational linewidths are interpreted within the model of anisotropic rotational diffusion. The data are consistent with NMR relaxation studies. The study covers the temperature range between T/T_c = 0.49 and T/T_c = 0.97.