Laser-induced fluorescence measurement of the nascent rotational distribution of N+2 (X2Σ+g) formed by electron impact on N2

The internal state distribution of ground state N⁺₂ ions formed from N₂ by electron impact ionization is measured under collision-free conditions using laser-induced fluorescence. Analysis of the B–X (0, 0) band shows the rotational distribution to be characterized by a temperature which increases slightly with decreasing electron energy (60–100 eV). Cascade contributions are unimportant.     

Low-pressure collisional narrowing in CO2

We report the observation of collisional narrowing of the Q branch of the Raman spectrum for the (000) → (100) transition in CO₂ at very low pressures. The minimum linewidth is reached at ≈250 Torr. An estimate for the difference in rotational constants of the (100) state and the ground state is obtained. The narrowing of the linewidth and the changes in line-shape are interpreted in terms of velocity changing collisions and rotationally inelastic collisions between the CO₂ molecules.     

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 transfer in reactive and nonreactive collisions of Na with Na2

Stimulated by the experimental finding of vibrationally and rotationally cold dimers in supersonic nozzle molecular beams of sodium, we have studied energy transfer in collisions of Na with Na₂ over a wide range of initial relative translation energies E and impact parameters b by a classical mechanical trajectory method. The vibrational and rotational energies were initialized using Boltzmann distributions characterized by temperatures T_vib = 150 K, T_rot = 50 K. We find that for large values of E the energy transfer in reactive collisions increases with b while it decreases with b for the nonreactive collisions. For low values of E, energy transfer is a decreasing function of b for both reactive and nonreactive encounters. Both the reactive and nonreactive mechanisms are very efficient in effecting transfer, between 40–70% of the initial relative translational energy is converted into internal energy of the diatom, leading to the conclusion that the reverse collisions would result in the rapid relaxation observed in experiment.     

Propensity rules in rotationally inelastic collisions of CO2

Propensity rules for the rotational quantum number dependence of cross sections for CO₂ collisions with atoms are predicted to have subtle characteristics for transitions involving levels in excited vibrational angular momentum states.     

Resolution of angular rotational rainbows in Na2 + Ne,Ar collisions

State-to-state dfferential cross sections for rotational excitation in Na₂-Ne collisions have been measured and the characteristics of such processes involving repulsive surfaces demonstrated. The rotational rainbow maxima shift to larger scattering angles and broaden as the angular momentum transfer increases. Good agreement with IOS calculations is found. Results for Na₂-Ar are included.     

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.     

Calculation of differential cross sections for rotational excitation in D2 + CO collisions at 87.2 meV

Differential cross sections for rotational excitation in D₂ + CO collisions are calculated at six scattering angles using an electron gas surface and a semiclassical scattering theory in which the translational and the rotational motion of CO are treated classically whereas the D₂ rotation is quantized.     

Relaxation in a Na/Na2 nozzle expansion

Atom—molecule collisions in a Na/Na₂ nozzle expansion are investigated. The atoms are marked near the nozzle exit by preparing them alternatively into the 3²S and 3²P state. The effect of collisions between these atoms and the molecules is studied far downstream the beam. We find that the internal energy distribution of the molecules depends on the internal energy of the atoms. The specific change of the internal energy distribution gives information on the energy relaxation in the nozzle expansion.     

Classical superposition phenomena in H+2 (v = O) - He reactive collisions

A simple classical model is used to analyze reactive processes in H⁺₂-He collisions at relative energies above a few eV. Detailed structure of the cross section occurs and is a consequence of the superposition of probabilities due to distinct binary collision mechanisms which depend on impact parameter and initial orientation. The predicted structure correlates favorably with available experimental data. Comparison is also made with the (quantum) statistical phase space model.     

Trajectory studies of energy transfer: CH3NC with He,Xe, H2 and N2

A trajectory program was used to simulate the collisions of CH₃NC with He, Xe, H₂ and N₂. Calculated energy transfer is in accord with experiment. The pattern of CH₃NC vibrational mode energization is found to be noticeably non-random. The approximate sampling methods used in thermal unimolecular trajectory studies produce a more uniform state distribution.     

Vibrational energy transfer in H2 + He

A semiclassical approach is developed to study vibrational energy transfer in H₂ + He by use of the a priori interaction potential including all nonzero impact parameter collisions. The calculated values of the rate coefficient are found to be in excellent agreement with experimental data which are available in the temperature ranges 60–450 K and 1350-3000 K. The temperature dependence is shown to seriously deviate from the Landau-Teller prediction below 1000 K. The calculation was carried out over the temperature range of 30 to 10000 K.     

Rotational analysis of the 201 and 211 bands in the Ã2Σ+-X̃ 2Π spectrum of N2O+

Analysis of new emission spectra of òΣ⁺-X? ²Π in N₂O⁺, including the forbidden components with Δν₂ = 1, have made it possible to locate all four vibronic components of the ν″₂ = 1 manifold. Principal rotational constants and spinvibronic terms are given. A previous estimate of the Renner parameter is revised to a value of ?0.179 ± 0.002.     

Comparisons of energy disposal by the reactions of H atoms with Cl2, SCl2, S2Cl2 and SO2Cl2 from observation of HC1 infrared chemiluminescence

Arrested relaxation infrared chemiluminescence studies of the H + Cl₂, SCl₂, S₂Cl₂, SOCl₂ and SO₂-Cl₂ reactions have been made. The mean fraction of vibrational (stational) energy released to HCl is 0.40 (0.10); 0.40 (0.13); 0.38 (? 0.02); 0.33 (?0.02) and 0.36 (?0.02) for the series. Only the H + SCl₂ reaction shows a two component initial rotational distribution. The larger (f_V) and (f_R) from H + SCl₂, relative to the other polyatomic reagents, is consistent with the observation that this is the only reaction that shows forward scattering. The room temperature rate constants also were measured, relative to H + Cl₂, and were found to decline in the series from 0.68 for SCl₂ to 0.02 for SO₂Cl₂. All of these data support the suggestion (first made by Heydtmann and Polanyi) that the unusual rotational energy disposal pattern from H + SCl₂ is a consequence of migration of H from the initially encountered C1 to the second C1, which then forms the HCI product; this pathway augments the direct reactive pathway, which gives HCI in lower J states.     

Collision-assisted multiple step excitation of CH3F by irradiation with a TEA CO2 laser

By irradiating a mixture of CH₃F highly diluted in Ar with a TEA CO₂ laser, the v₃ overtone emission rises with a rate much larger than the “up-the-ladder” V-V pumping rate indicating that the CH₃F is excited up to the 3v₃ level almost instantly. This occurs via multiple step excitation assisted by rotational transitions in collisions.     

Excitation and decay of the C2Σ+u state of N+2 following collisions of He+ ions with N2 isotopes

A quantitative analysis is made of the N⁺₂ “2nd negative” emission (“2N”: C²Σ⁺_u → X²Σ⁺_g) produced by the impact of 500 eV to 25 keV He⁺ beams on ¹⁴N₂, ¹⁴N¹⁵N and ¹⁵N₂. Above about 5 keV, the relative 2N emission rates from the various vibrational levels of the C state are the same as those observed for ? 2 keV Ne⁺, or > / 90 eV electron-impact. These limiting distributions are compared to those predicted for a Franck-Condon excitation of the C state, modified by configuration interaction. The weakening in 2N emission at the vibrational levels ν′ > / 3 is ascribed to spontaneous C-state predissociation. The data fully confirm recent reports that this predissociation extends over a wide range of ν′ and that it is subject to a strong isotope effect. The ratios of the rates of C-state predissociation to 2N emission are obtained for the levels ν′ = 3 to 8 of each nitrogen isotope. By means of these data it is shown that near-resonant charge transfer dominates the distribution of vibrational excitation probabilities only at energies below about 10 eV. A comparison is made of absolute cross-sections for C-state emission with those for N⁺ and N⁺₂ production in He⁺/¹⁴N₂ collisions at energies between 5.5 eV and 25 keV.     

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.     

Hindered rotation of the ammonium ion in (NH4)2SnCl6

The hindered rotational energy levels of the ammonium ion in (NH₄)₂SnCl₆ have been computed. These energy levels are used to interpret the librational frequencies, tunneling frequencies, and rotational heat capacity of the ammonium ion.