Rotational relaxation measurements on OCS using a beam maser

Rotational relaxation cross sections are reported for J = 0, 1 and 2 rotational states of OCS using a variety of polar and non-polar scattering gases. Cross sections are reported for pure states, superposition states and total beam attenuation. Inelastic cross sections for symmetric top scattering molecules are much larger than corresponding cross sections for linear scattering molecules. Upper and lower states for both J = 0 → 1 and J = 1 → 2 transitions were selected using quadrupole or co-axial focusers. These preliminary results show differences in cross sections for different rotational states.     

Rotation-vibration-translation energy transfer in laser excited Li2 (B1Πu)

Using the method of laser fluorescence, inelastic collisions with rare gas atoms of electronically excited ⁷Li₂ molecules in the υ = 2 and 4 levels were studied. Vibrational transitions ranging from Δ = +2 to ?4 were observed. The simultaneous rotational transitions were completely resolved, and detailed rate constants k_{Δυ, ΔJ} for specific collision- induced quantum jumps Δυ, ΔJ were determined. The effect of secondary rotational relaxation was eliminated by an extrapolation to zero pressure. By integration over ΔJ, rate constants k_Δυ, were found. They are, within the error limits, independent of the collision partner and on the initial υ (2 or 4) and depend rather weakly on Δυ. These findings are compared with theoretical results from various methods, generally based on a collinear collision model. The apparent disagreement in all respects suggests strongly the importance of rotational degrees of freedom in the collision. Experimental evidence for this is the large amount of V — R transfer observed, which about equals the V — T transfer. The mean cross sections σ(Δυ) for specific vibrational transitions Δυ range between 6 and 15 A², among the largest ever observed.     

Rotational energy transfer in NO (A2Σ+, v = 0 and 1) studied by two-color double-resonance spectroscopy

Collision-induced rotational relaxation in the A²Σ⁺, v = 0 and 1 states of NO has been measured by using step-wise double-resonant ionization spectroscopy. Multiple quantum rotational energy transfer is occurring to at least |ΔJ| = 6 and the observed cross sections ranging from tens to hundreds of A² are larger than the gas-kinetic collisional cross section. The energy-transfer efficiency is slightly enhanced by the vibrational excitation. Energy-based scaling laws are successfully applied to reproduce the observed rotational distribution.     

Quasi-resonant energy transfer in collisions: Na2(A1Σ u + )+K(4S)

Cross sections for electron energy transfer from the initial rotational stateJ′of the two lowest vibrational levelsv′=0 andv′=1 of excited dimers Na₂(A) to potassium atoms as described by Na₂(A¹Σ _u ⁺ ,v′J′)+K(4S)→Na₂ (X¹Σ _g ⁺ ,v″J″)+K(4P)+ΔE have been examined by laser-induced fluorescence. A strong increase of the cross section by as much as an order of magnitude has been observed for those dimerv′J′-levels for which the dipole transitions are close to resonance of the 4S-4P transitions in the atom (ΔE<4 cm^?1). The absolute cross sections for energy transfer have been calculated by the Rabitz approximation of first-order perturbation theory. In the case of closest energy resonance (ΔE=0.9 cm^?1) the cross section is Q=7.8×10^?13 cm².     

Dynamics of the reaction H2+(He,H)HeH+. Influence of various forms of reactant energy on the total and differential cross section

Results of quasiclassical trajectory calculations of reactive processes between He atoms and H₂⁺ (υ, J) molecular ions in the collision energy interval 0.5–5.0 eV (c.m.) for a large number of selected υ, J combinations are analyzed with respect to the influence of the initial translational, vibrational, and rotational energy on the total and differential reaction cross sections. Vibrational energy is more effective in promoting the reaction than translational energy. Small rotational excitation has a negligible effect, whereas high rotational excitation has a similar influence on the reaction cross sections as the vibrational excitation of the same magnitude.     

Experimental determination of the mj distribution in inelastic scattering of Na2 by He

The orientational distribution of Na₂ molecules scatterd by He has been determined in a molecules beam experiment. At large angle scattering where inelastic collisions are dominant, a high degree of alignment has been observed. This alignment depends strongly on the rotational quantum number J after scattering. The J dependence can be explained by assuming that during collisions m_j is conserved when the quantization axis is chosen parallel with the geometric apse.     

Energy partitioning in the reaction O(1D) + H2O → OH + OH. V. Rotational relaxation of OH(X2Π, ν″, J″)

The rotational relaxation of OH(X²Π, ν″, J″) in ν″, = 0, 1, and 2 produced from the reaction of O (¹D) with H₂O has been studied as a function of H₂O vapor pressure and added argon. Water molecules are extremely efficient in bringing about relaxation and the experiments performed indicate that, on the average, the high temperature distribution is relaxed to nearly room temperature at a gas kinetic rate. This observation is rationalized by assumming a collision complex between OH and H₂O having a quasichemical interaction similar to weak hydrogen bonding. The nascent OH internal energy distribution does not depend upon the translational energy of the O(¹D) reactant. Translational relaxation of the nascent OH product by H₂O is fast, as fast as rotational relaxation.     

An experimental study of collisional transfer of electronic excitation,collisional dissociation of excited molecules and photodissociation in alkali vapour

A single-frequency laser is used to excite Na₂ molecules to the electronic B state. Besides the molecular fluorescence also atomic Na resonance radiation is observed. This is caused by: collisional transfer of electronic excitation from a Na₂(B) molecule to a Na atom, collisional dissociation of Na₂(B) molecules and photodissociation of Na₂ from very high vibration—rotation (v. J) levels of the ground state. We show how the contributions of these processes can be separated experimentally and characterized quantitativily over a wide range of temperatures, using a free-jet expansion. Illustrative results are given for one laser frequency (i.e. one molecular transition). Effective collision cross sections for excitation transfer and for collisional dissociation are given. The probability of photodissociation is compared to the probability of the discrete (B ← X) transition. The relaxation of the number of molecules in high (v. J) levels in a free jet is obtained.     

Quantum-mechanical phase interference and optical polarization in low-energy Na+Hg inelastic collisions

Cross sections for production of Hg(6³P₁) and Na(3²P) have been measured for low energy (?3 keV) Na⁺Hg collisions. An antiphase oscillatory structure in the energy dependence has been observed, and attributed to phase interference between charge-exchange and direct excitation channels. From the measurement of polarization of the Hg-resonance line, another pronounced antiphase oscillatory behaviour is found for the two cross sections for production of the magnetic sublevels, m_J=0 and m_J = ± 1, of the Hg(6³P₁) state.     

Noble-gas broadening of an optical transition of I2 measured by coherent transients. II. Three-pulse stimulated photon echo experiments

Solutions are derived for the decay of a three-pulse stimulated photon echo, including the separate treatment of the spectral diffusion in the Doppler profile within and out of the spectral bandwidth of the excited molecules. Thermal cross sections are obtained for the X Σ_g⁺ (ν″ = 2, J″= 59) → B³Π_0⁺u(ν′ = 15, J′= 60) transition of iodine molecules perturbed by noble gases. It is argued that under certain experimental conditions using the results of two-pulse delayed emission measurements, direct information can be obtained for spectral diffusion restricted within the bandwidth of excited molecules.     

Collision cross sections for excitation energy transfer in Na* (3P 1/2)+K(4S 1/2)⇔Na*(3P 3/2)+K(4S 1/2) processes

The cross sections for the excitation energy transfer between the 3² P _J states of sodium atoms by collisions with ground-state potassium atoms have been measured by resonant Doppler-free two-photon spectroscopy, where the population densities of directly pumped and collisionally excited Na(3P _J )(J=1/2, 3/2) levels were probed by counter-propagating Na(3P _J ) → Na(4D _{3/2, 5/2}) excitation and detected with the thermionic diode. Cross sections of σ(3P _1/2 → 3P _3/2)=190 Ų±20% and σ(3P _3/2 → 3P _1/2)=100 Ų±20% were found. The theoretical calculations taking into account the long-range interaction termsR ^?6,R ^?8 andR ^?10 yield a value σ(3P _1/2 → 3P _3/2)=165 Ų. On the basis of these long-range interaction potentials the differential cross section has been calculated and compared with recently published experimental data. Very good agreement between the theoretical and experimental data was found.     

Collision-induced rotational transitions in nali(1Π)

Collision-induced rotational transitions in the electronically excited NaLi molecule have been studied using laser excited fluorescence. Due to the greater number of allowed transitions as compared with Na₂ and Li₂, more cases were found of transitions +ΔJ and ?ΔJ having greatly different cross sections (“+/ - asymmetry”). This observation is in agreement with recent predictions on the basis of Born's approximation. All observed transitions are classified according to the symmetry of the contributing potential terms. Data are presented which indicate the necessity for refinements of the theory.     

A coupled-states approximation study of Li+-H2 collisions

The recently developed coupled-states approximation for describing atom-molecule collisions is applied in a slightly modified form to the Li⁺-H₂ system. Due to the large anisotropy in the potential, a preferred orientation for rotational excitation exists which suggests the use of l = J-j rather than l = J as the angular momentum quantum number in approximating the eigenvalue of 1² by l(l + 1). Here, J and j are respectively the total and rotator angular momentum quantum numbers. The coupled-states integral and differential cross sections are compared with results of close-coupling calculations at 0.6, 0.9, and 1.2 eV.     

Crystal structure of (glycine) sodium nitrate Gly·NaNO3

The crystal structure of Gly·NaNO₃ was determined by single crystal diffraction methods (monoclinic crystal system, B11n, a = 14.339(3) Å, b = 9.136(3) Å, c = 5.263(3) Å, γ = 119.14(5)°). The structure is built from alternating layers of glycine organic molecules and inorganic layers consisting of Na⁺ ions and planar NO ₃ ^? ions stretching along the b axis. The surroundings of the Na atom include the oxygen atoms of the NO ₃ ^? groups and the oxygen atoms of glycine molecules. The structure has a system of hydrogen bonds.     

Rotational relaxation in S1 glyoxal: Cross sections and a search for propensity rules

Details of rotational energy transfer from a few selected K′J′ levels in the zero point vibrational level of ¹Au(S₁) glyoxal vapor have been studied. The cross section for destruction of an initial K′J′ level by rotational relaxation in collision with ground electronic state glyoxal is about 240 A² or 4.5 times gas kinetic. Much of the rotational transfer within the S₁ state occurs with large ΔK′ and ΔJ′. No strong propensities for △K′ = 0, ± 1, ± 2, or ± 3 with small ΔJ′ changes occur in collisions with ground electronic state glyoxal. The study was made by examination of the rotational structure in the 5₁⁰ emission band at various pressures after excitation in the 0,0 band of the S₁—S₀ system with the 454.5 nm argon ion line.     

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.     

Collision rotational transfers in NaH A1Σ+ by He,Ar or H2 (and in KH A1Σ+ by H2)

The total and relative rotational transfer cross sections σ_total and σ_Ji-Jf, by collisions of NaH A¹Σ with He, Ar or H₂, are measured from υ′ = 4 and υ′ = 11, J₁′ = 6. The σ_total increase as υ′ increases. They are similar for He and H₂ but much greater for Ar especially at large υ′. In NaH A¹Σ⁺ the bond goes from covalent to ionic as υ′ increases: σ_total is very sensitive to an attractive potential due to the interaction of the permanent electric dipole moment of the molecule with the polarizability of the atom (α_Ar = 11 au, α_He = 1.37 au). The σ_Ji-Jf decrease monotonously as |J_f-J_i| increases and may be fitted by a scaling law. The variation with ΔJ depends on the colliding gas but does not change appreciably with υ′: most of the transfers could take place on the repulsive part of the interaction potential, the shape of which would not depend on υ′.     

Experimental verification of the theory of J-diffusion in nitrogen gas

The measured linewidths of NMR spectra for ¹⁵N₂ and ¹⁴N₂ have been used to calculate rotational (τJ) and orientational (τθ₂) relaxation times. Within the density range studied (0–100 amagat) τθ₂ is proportional to τ_J in line with the impact theory of orientational and rotational relaxation.     

The vibrational relaxation of I2 by H2 in a supersonic jet

The vibrational relaxation of I₂ by H₂ has been studied in a supersonic free jet. It was observed that the addition of 5% H₂ to the helium carrier gas greatly reduces the concentration of X ¹Σ⁺_g(ν″ = 1) I₂ in the jet as compared to the concentration in a pure helium carrier. From this observation we have determined that the average vibrational relaxation cross sections of H₂ is 7.1 times as large as that of helium. Since the average vibrational relaxation cross section of deuterium is at least as large as that of hydrogen, the mechanism responsible for this phenomenon appears not to be dominated by mass effects.     

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.