On the possibility of orienting rotationally cooled polar molecules in an electric field

We have investigated the effects of initial rotational stateJ and the gas temperature (T) on the rotationally elastic, inelastic and summed processes in CH₄ and SiH₄ molecules by low-energy electron impact. While rotationally summed differential, integral, momentum transfer and energy-loss cross sections are independent of initial stateJ (and hence independent onT), it is found that the same rotationally inelastic differential and integral cross sections, when averaged over rotational distribution function, show a qualitative improvement over the unaveraged results when compared with experimental results. Various theorems regarding the dependence of scattering parameters on the rotational distribution, as described in a series of papers by Shimamura, are discussed by presenting actual calculations on the two spherical tops, namely the CH₄ and SiH₄ molecules.     

Rotationally inelastic scattering from surfaces CO(g) + LiF(OO1)

The rotationally inelastic scattering of a supersonic, rotanonally cold (T_rot^I = 4–30 k) CO molecular beam from a clean LiF(001) surface has been investigated by VUV laser-induced fluorescence. Rotational accommodation was incomplete for surface temperatures 350 <T_s < 900 K. The degree of accommodation decreased with T_s and was independent of collision energy.     

Two-photon spectra of gases by three-wave mixing

We show that the two-photon resonances in the third-order susceptibility can be exploited to yield two-photon spectra of molecular gases at moderately high spectral resolution. This form of spectroscopy does not depend on the occurrence of processes (such as fluorescence of photoionization) leading to indirect methods of two-photon absorption. The method is direct and leads in principle to values for a two-photon cross section. Comparisons of two-photon and coherent anti-Stokes Raman resonances leads to ratios of Raman and two-proton cross sections independent of the laser powers and spatial characteristics. The technique is documented with rotationally resolved spectra of SO₂ and NO. A value for |α_xxα_yy| of 1.5 × 10^?52 cm⁶ was measured for the O₁₂ (6$\text{built:1}\text{2}$) component of the (A)²Σ ← (X)² Π_{$\text{1}\text{2}$} transition of nitric oxide.     

Polarization in elastic scattering: close-coupling studies on ArN2

We present the results of close-coupling calculations of m_j-dependent differential and integral cross sections forj₁ = 2 → j₂ = 2 rotationally elastic ArN₂ collisions. Two potential surfaces were used with differing long-and short-range anisotropies. If the anisotropy is long-ranged the scattering of an isotropic beam results in a significant angle dependent polarization of the elastically scattered products. To a certain extent this reflects a selective loss of m_j-state population due to rotationally inelastic transitions. For quantization along the initial relative velocity vector or perpendicular to the scattering plane, the depolarization of an initially m_j-state selected beam vanishes in the forward direction and is significantly less than the statistical limit at all angles, which indicates a dynamical conservation of the direction of the molecular rotational angular momentum. By contrast, in the helicity frame depolarization is much more pronounced. The oscillatory structure present in the rotationally inelastic differential cross section does not appear to be quenched by the interference between various m → m′ transitions.     

Emission spectra of supersonically cooled chloroacetylene cations

The ā²Π_Ω → X?²Π_Ω, Ω = $\text{3}\text{2}$, $\text{1}\text{2}$ emission spectra of rotationally cooled chloro- and deuterochloro-acetylene cations have been obtained by electron-impact ionisation of a seeded helium supersonic free jet. The resultant spectral improvements lead to the identification of the spin-orbit components and isotope splittings and to a vibrational assignment of the prominent bands. The vibrational frequencies could be determined to within ±1 cm^?1 for many of the fundamentals for the cations in the X?²Π_Ω and ā²Π_Ω electronic states.     

Reorientation,polarization and scaling laws in rotational transfer experiments

Reorientation of angular momentum in molecules excited by polarized light, following rotationally inelastic collisions results in depolarization of the light emitted by these molecules. We show here how this effect can be quantitatively predicted from degeneracy, i.e. orientation-averaged rotational transfer rates, as usually measured in cell experiments. For this we first develop a classical phenomenology of rotational relaxation of diatomic molecules in the ¹Σ state, based on a vector model for angular momenta. Different simple scaling relationships are then given and applied to analyze the Li₂

+He system for which both orientation-averaged rate constants and polarization measurements for rotationally inelastic collisions already exist. It is found indeed that the latter data set agrees quite well with what can be predicted from the former data set. Finally we show that a propensity for conserving 0, the orientation of the angular momentum with respect to a laboratory-fixed axis results generally from geometrical reasons and is fairly independent of the collision dynamics.     

Electronic spectra of C<Subscript>6</Subscript>H<Superscript>+</Superscript> and C<Subscript>6</Subscript>H<Stack><Subscript>3</Subscript><Superscript>+</Superscript></Stack> in the gas phase

Measurement of the ³Π-³Π transition of C₆H⁺ in the gas phase near 19486 cm⁻¹ is reported. The experiment was carried out with a supersonic slit-jet expansion discharge using cavity ringdown absorption spectroscopy. Partly resolved P lines and observation of band heads permitted a rotational contour fit. Spectroscopic constants in the ground and excited-state were determined. The density of ions being sampled is merely 2×10⁸ cm⁻³. Broadening of the spectral lines indicates the excited-state lifetime to be ≈100 ps. The electronic transition of HC₆H₂⁺ at 26402 cm⁻¹ assumed to be ¹A₁-X¹A₁ in C_2v symmetry could not be rotationally resolved.     

Selection rules for collisional energy transfer in homonuclear diatomics. rotationally inelastic collisions

We have made a precise study of the circular polarisation of rotationally resolved features of laser-excited iodine. The J′ = 19, ν′ = 16 level of ³II⁺_ou was excited using circularly polarised dye-laser fluorescence and a quantitative data on polarisation features representing inelastic transfer of ΔJ′ = 30 was recorded. The experimental circular polarisation ratios were compared to those predicted by two totally conserving models, ΔM = 0 and Δθ = 0. The agreement between experimental points and the predictions based on the former lead to the formation of a new selection due on rotationally inelastic transfer namely, ΔM = 0.     

Ground and Electronically Excited Singlet‐State Structures of 5‐Fluoroindole Deduced from Rotationally Resolved Electronic Spectroscopy and ab Initio Theory

The structure and electronic properties of the electronic ground state and the lowest excited singlet state (S₁) of 5‐fluoroindole (5FI) were determined by using rotationally resolved spectroscopy of the vibration‐less electronic origin of 5FI. From the parameters of the axis reorientation Hamiltonian, the absolute orientation of the transition dipole moment in the molecular frame was determined and the character of the excited state was identified as L_b.     

Rotational energy transfer (theory). I. Comparison of quasiclassical and quantum mechanical results for elastic and rotationally inelastic HClAr collisions

Integral elastic and rotationally inelastic cross sections for HCl (J_i = 0, 1) + Ar collisions at a collision energy of 1.785 kcal/mole are rep     

High resolution spectra and conformational analysis of 2-butoxy radical

We have recorded five high resolution (200 MHz), rotationally resolved, vibrational bands of the B-X electronic transition of 2-butoxy. Two bands of the 2-butoxy spectrum have been rotationally analyzed and assigned to two different geometrical conformers of the molecule. The analyses allow the determination of the six experimental rotational constants defined by the geometry of the species in the ground (X) and excited (B) electronic states and also four spin-rotation constants for the X electronic state of the conformers. Comparison of the experimental rotational constants with the results of ab initio computations provides unambiguous conformational assignment of these bands. This approach can be extended to assign two other spectral bands to the third 2-butoxy conformer.     

Mode-to-mode vibrational energy transfer in D2CO: Evidence of coriolis-enhanced rotational selectivity

Infrared—ultraviolet double resonance spectroscopy is used to demonstrate rapid collision-induced V-V transfer between the v₆ and v₄ vibrational manifolds of D₂CO. The rate of transfer is at least gas-kinetic and is explained in terms of Coriolis coupling and rotationally specific, quasi-resonant relaxation channels     

Application of the Lie algebraic approach to diffractionally and rotationally inelastic molecule–surface scattering

The Lie algebraic approach of Alhassid and Levine [Phys. Rev. A 18 , 89 (1978)] is applied to the molecule–surface scattering. Specially, the diffractionally and rotationally inelastic scattering of a diatomic molecule from a solid surface is dealt with. Within the framework of the close-coupling method, we construct a Hamiltonian for the scattering system and use it to generate a dynamical algebra h₆. By solving equations of motion for the group parameters, the scattering wave functions near the surface are obtained. Computed transition probabilities of diffractively and rotationally inelastic scattering of H₂ from LiF(001) surface with the use of Lie algebraic method are seen to agree well with the coupled-channel calculations. The Lie algebraic method thus appears to have a wide range of validity for describing the dynamics of gas–surface scattering. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 981–989, 1997     

Studies of lifetimes of rotationally cooled NO2 using time-resolved fluorescence excitation spectra

A taunble, pulsed dye laser with output in the region of 5750 to 6000 Å was used to excite rotationally cooled NO₂ which was produced by expansion in conjunction with argon carrier gas through a supersonic nozzle. The resulting time-resolved fluorescence excitation spectra were used the lifetimes of various vibronic bands of NO₂ (²B₂). The lifetimes measured were in the region of 15 to 40 μs which were shorter than those obtained from cell experiments. For each individual excitation wavelength, only a single exponential decay was observed from very early times through 250 μs.     

Production of triplets and cold radicals with a pulsed electric discharge in a pulsed supersonic bea]tl

It is found that a pulsed electric discharge in a pulsed supersonic expansion is an efficient source of rotationally cold triplet metastables and radicals. The spectra of both triplet argon (³P_2,0) and carbon monoxide (a ³ II,) as well as CCl radical (X²II) were recorded with mass selective resonant multiphoton ionization detection.     

The effect of vibrational structure on multiphoton dissociation spectra of CF3Br

Results on IR multiphoton dissociation of rotationally cold CF₃Br molecules are presented and interpreted. Model calculations have been performed extending current theories and using known linear absorption data. Strong dependence of the dissociation spectra on excitation to the first discrete levels has been recognized and discussed.     

The structure of the VO2F3−4 ion: Crystal structure of (NH4)3VO2F4

The VO₂F³⁻₄ ion has a cis octahedral structure, as is shown by single crystal structure analysis of the title compound. The unit cell of (NH₄)₃VO₂F₄ (space group Immm or I222, a = 912.6(2), b = 1881.8(4), c = 626.4(1) pm, Z = 6) contains two symmetrically independent anions. One is rotationally disordered. Oxo and fluoro ligands cannot be distinguished. But the second one has a distorted cis octahedral structure with the lengths 170.0(4), 186.1(4), and 202.3(4) pm for the VO, VF (axial), and VF (equatorial) bonds. Infrared and Raman spectra as well as theoretical considerations support the crystallographic results. The phase transitions at 418 and 215 K were confirmed by variable temperature X-ray powder diffraction. Above 418 K the cubic cryolite type structure is adopted with a = 902.6(2) pm. Plausible mechanisms for the phase transitions are suggested.     

Model discrimination for rotational energy transfer in the Ar-N2 system

The jackknife test of Rothstein et al. is applied to discrimination between several different models used to compute the rotationally inelastic cross sections for the Ar-N₂ system. The modified exponential models are the best models, except for the case where the energy gap is small, when power laws are best.     

Rotational alignment and non-statistical a doublet population in no following (CH3)3CONO photodissociation

The rotational alignment and internal state distribution of the NO fragment ejected from the photodissociation of (CH₃)₃CONO in its first absorption band are determined. The rotational distribution is non-Boltzmann and unaffected by the dissociating wavelength. The NO fragment is rotationally aligned and the Π⁻ antisymmetric. Λ doublet component is preferentially populated, indicating planarity of the fragmentation process.     

Classical rotational cross sections for H2—HD and HD—HD collisions

Classical trajectory calculations of integral cross sections for rotationally inelastic collisions of HD-para H₂ and HD—HD were carried out for a wide variety of transitions over a wide range of initial relative translational energies. The results of the HD—H₂ calculations were compared with the quantum effective potential calculations of Chu. It was found that the classical method is in reasonably good agreement with the quantum method for the calculation of rotational transitions of HD at the higher initial translational energies, but the classical method is in poor agreement with quantum results for HD excitation at low energies and for H₂ excitations at all energies.