Distorted-wave calculations for the three dimensional chemical reaction H + H2(v ⩽ 2, j = 0) → H2(v′ ⩽ 2, j′, mj) + H

Calculations of the vibrational—rotational product state population distributions and differential cross sections for the chemical reaction H + H₂(v ? 2, j = 0) → H₂(v′ ? 2, j′, m_j) + H have been carried out on the Porter—Karplus potential energy surface. The vibrationally-adiabatic-distorted-wave (VADW) method has been used. The relative rotational product distributions, differential cross sections and the helicity m_j, dependences of these quantities for the v = 0 reaction agree well with accurate close-coupling results. The absolute integral cross sections are considerably smaller than the accurate quantum values, however. The calculations for the v = 1 reaction agree with the findings of previous sudden quantum, limited close-coupling and quasiclassical theoretical studies and experiments that product H₂(v′ = 1) is more likely to be produced than H₂(v′ = 0). For the reaction with v = 2, it is found that at high translational energies product H₂(v′ = 2) is favoured over H₂(v′ = 1) or H₂(v′ = 0). The VADW differential cross sections for the v = 1 and v = 2 reactions have a similar shape to those of the v = 0 reaction, with backward peaking when summed over all m_j states. The relative rotational distributions for the v = 2, j = 0 → v′ = 2, j and v = 1, j = 0 → v′ = 1, j reactions are also similar to those obtained for the v = 0, j = 0 → v′ = 0, j reaction, with low rotational excitation.     

Collisional metastability of high rotational states of CN(X2Σ+, = 0)

CN(X²Σ⁺, v′' = 0) high rotational states relax slowly via 300 K collisions with Ar and Kr. Relaxation decreases with increasing rotation, and the partially relaxed distributions are bimodal, with low N′' thermalized (300 K), and N′' = 80 unrelaxed after 1000 collisions. Relaxation by N₂, CO, and Xe is similar to Ar and Kr, but more efficient. He and NO remove many quanta in a single collision.     

N2 rotational energy distributions in cold,supersonic beams from electron excited fluorescence measurements of N+2

Ground-state rotational energy distributions of N₂ molecules produced in pure and He-seeded supersonic expansions have been determined by measurements of the N⁺₂ first negative band rotational line intensities produced by 800 eV electron impact on cooled pure and He-seeded N₂ supersonic beams. Sufficient spectral resolution was employed to resolve completely both P and R branches of the first negative bands. Rotational state distributions were obtained to much higher values of J than in previous investigations. The data show that at 800 eV, the electric dipole selection rule, |ΔJ| = 1, is consistent with the observed N⁺₂ emission bands and that the rotational energy distributions produced in the cooled, supersonic beam are non-Boltzmann with a large population in the first few rotational states followed by a long, high-energy fail to quite high J values.     

Determination of the absolute scattering cross section for the reaction D + H2(v=1) → HD+H at 0.33 eV

The reaction D+H₂(v=1) has been investigated in a crossed molecular beam experiment at the most probable collision energy of E₀=0.33 eV. Angular and time-of-flight distributions have been measured and the total absolute cross section has been determined to be σ_r(j_i=0, v = 1, E_c.m. = 0.33 eV) = 1.14 ± 0.50 Ų. This value, as well as the distributions, are in good agreement with the results of quasiclassical trajectory calculations (QCT) and the reactive infinite-order sudden approximation (RIOSA).     

Static characteristics of the ionization event in the He(23S)-HD collision system

Vibrational population factors for the nascent Penning ions HD⁺ (v′)(… He) and energy of the corresponding Penning electrons are calculated for the ionization event He(2³S)(SINGLEBOND)HD(v′ = 0) → [He … HD⁺(v′)] + e⁻ taking place at a range of the He*(SINGLEBOND)HD separations and orientations accessible by the system during thermal energy collisions. The vibrational population factors are obtained from the local widths of the He(2³S)(SINGLEBOND)HD(v′ = 0, N) state with respect to autoionization to HD⁺(… He) in its v′th vibrational level. The initial overall picture of the autoionization event is consistent with the He(2³S)(SINGLEBOND)H₂(v′ = 0) one. On the other hand, the vibrational population factors are different from the approximate average populations used in initial model theoretical considerations about the Penning processes in the system. Variation of the calculated considerations about the Penning processes in the system. Variation of the calculated quantities with changes in the He*(SINGLEBOND)HD separations and orientations is found to be smooth enough to guarantee that the present data might form a sound basis for construction of analytical representations of the corresponding 2D surfaces and for future study of the dynamics of the collision system. © 1996 John Wiley & Sons, Inc.     

Rotational distribution of H2+ ions produced by 90 eV electron impact

The rotational temperature for the vibrational levels v = 7 to 11 of H₂⁺ ions formed by 90 eV electron impact on H₂ is determined. This is achieved by photodissociating the ions in a fast beam, combined with translational spectroscopy using a time- and position-sensitive detector. The temperatures were found to decrease from 127 K for v = 7 to 110 K for v = 11. A model that conserves the angular momentum N during the ionization process can explain these results. The inclusion of the N-dependence of the photodissociation cross section is compulsary to obtain accurate temperatures. This is demonstrated in a separate experiment, in which we looked at the minimum near 575 nm of the photodissociation cross section for H₂⁺ ions in v = 9 and different N. Due to the rotational stretching of the molecule, this minimum shifts to a larger wavelength for increasing N, in accordance with our experimental findings.     

Analysis of anomalous vibrational and rotational distributions in the CN(B2Σ+) state produced in the reaction of Ar(3P0,2) metastables with BrCN

The CN(B²Σ⁺ - X²Σ⁺) tail band emission system for μ′ = 11–20 resulting from the energy transfer reaction Ar(³P_0,2) + BrCN in a flowing afterglow apparatus was measured. The vibrational and rotational distributions were determined as a function of argon pressure. Numerous perturbed rotational lines were observed; analysis of the dependences of these lines on argon pressure, with the aid of experimental information already published, led to the following assignments as to the origins of the perturbations: For μ′ = 11, N′ = 20 and μ′ = 13, N′ = 9, the perturbing state is a ⁴Σ⁺; for μ′ = 12, N′ = 10 and 14, μ′ = 14, N′ = 7 and 10, and μ′ = 17, N′ ≈ 17–19 the perturbing state is A ²Π_i. The perturbed rotational line, μ′ = 11, N′ = 20, is found to be the primary source of intensity in the μ′ =11 vibrational band, but in all other cases the perturbed rotational lines do not significantly aid in the populating of the vibrational state. The anomalously high vibrational populations found in the tail band emission system (μ′ = 12, 14, 17 and 18), as well as the significantly high rotational excitations observed in the μ′ = 12–20 vibrational bands, apparently arise directly from the reaction intermediate.     

Detailed electron spectrometric study of the ionization of H2 by He* (21 S, 23 S)

The energy spectra of electrons released in thermal energy (≈ 50 meV) ionizing collisions of He*(2¹ S, 2³ S) with H₂ have been measured with high resolution and low background. Based on a detailed data analysis, we report accurate H ₂ ⁺ (v′) vibrational populationsP(v′) for both He*(2¹ S)+H₂(v′=0–10) and He*(2³ S)+H₂(v′=0–15) and the spectral shapeS(ε) for the individual vibrational peaks. The vibrational populationsP(v′) are quite similar to the Franck-Condon factorsf _v ′0 for unperturbed H₂(v″=0)→H ₂ ⁺ (v′) transitions, but, more in detail, the ratiosP(v′)/f _v ′0 show a characteristically differentv′-dependence for He*(2³ S), He*(2¹ S), and HeI_α(58.4 nm) ionization. The vibrational level separations in the He*(2¹ S, 2³ S)+H₂ spectra agree with those in the HeI photoelectron spectrum to within 1–2 meV. The spectral shapesS(ε) are characteristically different for He*(2¹ S)+H₂ and He*(2³ S)+H₂, reflecting the respective differences in the entrance channel potentials, as determined previously in ab initio calculations and from scattering experiments.     

The total ionisation cross section and the large angle differential cross section for the system He(21S, 23S) + Ar,N2

We have measured the total ionisation cross section Q_ion(g) and the large angle differential cross section σ(θ, g) for the system He(2¹S, 2³S)+ Ar, N₂ at energies 0.05 < E_c.m. (eV) < 6. This energy range is covered by applying two different discharge sources for the production of metastable atoms. In the atomic beam the He(2³S) level is most abundant with relative populations C = 0.91±0.01 and C = 0.96±0.01 for thermal energy range and the superthermal energy range, respectively. A quench lamp is used for the quenching of the (2¹S) level population. In the thermal energy range, σ(θ, g) and Q_ion(g) are in fair agreement with experimental results of other authors and with calculated cross sections based on the optical potential given by Siska. In the superthermal energy range, the He(2³S)+Ar optical potential is modified to describe our experimental data. The slope of the repulsive branch of the real potential is increased for r < 2.85 Å; in the imaginary potential a saturation to a constant (or even decreasing) value for internuclear distances less than 2.5 Å is introduced.     

Molecular beam scattering studies of the reaction D + H2 (v = 0) and D + H2 (v = 1) → HD + H

The reaction D + H₂ → HD + H has been investigated in two molecular beam scattering experiments. Angular and time-of-flight distributions have been measured for the initial vibrational ground state (v = 0) at a most probable collision energy of E_cm = 1.5 eV and for the first vibrational excited state (v = 1) at E_cm = 0.28 eV with the same apparatus. Results for the ground-state experiment are compared with quasiclassical trajectory calculations(QCT) on the LSTH-hypersurface transformed into the laboratory system and averaged over the apparatus distributions. The agreement isquite satisfactory. At this high collision energy the HD products are no longer scattered in a backward direction but in a wide angular region concentrated about θ = 90° in the center-of-mass system. The absolute reactive cross section has been determined and the agreement with the theoretical value from QCT calculations is within the experimental error. The high sensitivity of the experiment to different properties of the doubly differential cross section has also been demonstrated. A preliminary evaluation of the experiment with initial vibrational excitation (v = 1) shows that the HD-product molecules are preferably backward scattered and the change of internal energy is small supporting the concept of a reaction which is adiabatic with respect to the internal degrees of freedom.     

Fully converged sudden rotation total integral cross sections for 4He + H2 (ν = 0, j = 0) → 4He + H2 (ν′ = 1, j′)

Total integral cross sections for ⁴He + H₂ (ν = 0, j = 0) → ⁴He + H₂ (ν′ = 1, j′ = 0, 2) have been calculated in the total energy range 1.2 to 5.5 eV, according to a quantal sudden approximation for the H₂ rotational degrees of freedom and a close coupling expansion of the vibrational degree of freedom. Convergence of the above cross sections is investigated by employing four vibration basis sets in the close coupling calculations, i.e., ν = 0,1, ν = 0,1, 2, ν = 0, 1, 2, 3 and ν = 0, 1, 2, 3, 4. Between 4.2 and 5.5 eV calculations were done with three vibration basis sets; ν = 0.–4, ν = 0–5, and ν = 0–6. It is found that at least four vibrational basis functions are needed to converge (to within 5–10%) these cross sections in the above energy range. Comparison of breathing sphere calculations and summed sudden rotation results shows good agreement for the (weakly anisotropic) Mies-Krauss potential. However, as expected the former results underestimate the vibrational 0 → 1 total integral cross sections.     

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.     

Relaxation and spectroscopy of CaCl in seeded supersonic beams

The rotational and vibrational relaxation of a calcium-monochloride beam, seeded in He, Ar and Kr has been investigated by laser-induced fluorescence. The rotational distributions cannot be described by a single temperature. A two-parameter formula is given which represents the measured distributions and which is characterized by local temperatures from 30 to 200 K depending on the rotational states and the expansion conditions with source temperatures of T₀ = 1320 K and T₀ = 1520 K. The effective cooling increases from He to Ar as carrier gases. In contrast, the vibrational distributions could be represented by Boltzmann distributions with temperatures of 1400 to 589 K. The order of vibrational cooling goes from Ar < He < Kr. Parts of the CaCl X-B spectrum have been recorded with high resolution. The attempt to identify the measured lines reveals that the best molecular constants for this system do not reproduce all lines of Ca³⁵Cl and especially not those of Ca³⁷Cl within the needed accuracy. The spin-rotational constant of the B²Σ state is determined to γ₀₀ = ?0.06513 (9) with a 2σ uncertainty.     

Millimeter wave spectrum of gaseous bismuth monofluoride (BiF)

Rotational transitions of gaseous bismuth monofluoride (BiF) have been observed for the first time. The molecules were produced in a double oven system and subsequently led into the absorption cell at a temperature of 700 °C. Transitions (F′,J + 1,v) ← (F,J,v) with J = 4,6 and 7 and v = 0–6 in the O⁺ electronic ground state have been measured in the 65–110 GHz region. The hyperfine structure due to the Bi nucleus is completely resolved. Analysis of the spectrum yields the following rotational and hyperfine constants: Y₀₁ = 6894.89594(84) MHz, Y₁₁ = ?45.0474(11) MHz, Y₂₁ = 81.69(45) kHz, Y₃₁ = 0.177(51) kHz, Y₀₂ = ?5.5440(73) kHz, eq_eQ(Bi) = ?1150.28(12) MHz, eq_IQ(Bi) = 4.20(12) MHz, c_Bi = ?30.0(5) kHz. In the Dunham constants Y_lm contribution due to interaction with 1⁺-level is included. The equilibrium distance, potential and vibrational constants are derived.     

Dissociative excitation of water by metastable argon

The reaction Ar(²P_2,0) + H₂O → Ar + H + OH(A²Σ⁺)was studied in crossed molecular beams by observing the luminescence from OH(A²Σ⁺). No significant dependence of the spectrum on collision energy was found over the 22–52 meV region. Spectral simulation was used to obtain the OH(A) vibrational distribution and rotational temperature, assuming a Boltzmann rotational distribution. Since predissociation is known to strongly affect the rovibrational distribution, the individual rotational state lifetimes were included in the simulation program and were used to obtain the average vibrational state lifetimes. Excellent agreement with experiment was obtained for vibrational population ratios N₀/N₁/N₂ of 1.00/ 0.40/0.013 and a rotational temperature of 4000 K. Correction for the different average vibrational lifetimes gave formation rate ratios P₀/P₁/P₂ of 1.00/0.49/0.25. The differences between these results and those from flowing afterglow studies on the same system are discussed. Three reaction mechanisms are considered, and the vibrational prior distributions are calculated from a simple density-of-states model. Only fair agreement with experiment is obtained. The best agreement for the mechanisms giving OH(A) in two 2-body dissociation steps is obtained by assuming 1.0 eV of internal energy remains in the second step. The OH(A) vibrational population distribution of the present work is similar to that found in the photolysis of H₂O at 122 nm, where there is 1.10 eV of excess internal energy.     

C2 and CN formation by optical pumping of CO/Ar and Co/N2/Ar mixtures at room temperature

A continuous wave carbon monoxide laser is used to excite the vibrational mode of CO in CO/Ar and CO/N₂/Ar mixtures flowing through a gas absorption cell. High steady-state excitation of the CO vibrational mode (0.3 eV/molecule) is achieved, while a translational—rotational temperature near 300 K is maintained by the steady flow of cold gas into the cell. These non-equilibrium conditions result in extreme vibration—vibration pumping, population high-lying vibrational quantum levels (to V = 42) of CO. N₂ can also be pumped by vibrational energy transfer from CO. Under these conditions, C₂ and CN molecules are formed, and are observed to fluoresce on various electronic band transitions, notably C₂ Swan (A ³Π_g—X ³Π_u) and CN violet (B ²Σ⁺—X²Σ⁺).     

Semiclassical calculation of the rate constant for the process N2(v = 1) + N2(v = 0) → 2N2(v = 0) + 2330.7 cm−1 at low temperatures

Previous semiclassical calculations of the rate constant for vibrational deactivation of N₂(v = 1) colliding with N₂(v = 0) have been extended to temperatures below 200 K. The sensitivity to changes in the short range potential has also been investigated.     

Hyperfine structure of N2 (B 3Π g andA 3Σ u + ) from LIF measurements on a beam of metastable N2 molecules

Crossing an intense beam of nitrogen molecules in the metastable N₂(A) state with the beam from a CW dye laser, laser-induced fluorescence was observed in the first positive system of N₂,B ³Π _g ?A ³Σ _u ⁺ . About 300 lines of the (10, 6) band were studied at sub-Doppler resolution (15 MHz FWHM). From the well-resolved hyperfine structure of the lines, the hyperfine splittings of both the upper and the lower state were derived for a range of rotational quantum numbers up toJ=12. Using multiple independent determinations of each splitting via lines belonging to different branches, the hfs could be measured with an accuracy of about 2 MHz. Fitting known theoretical expressions for the hyperfine energies to the data, the following nuclear coupling constants were obtained (in MHz): For theA state,v=6: α=12.86, β=?11.40,e ² q ₀ Q=?2.5. For theB state,v=10:K ₁₁=86.46,D ₁₁=12.67,D _1?1=?44.64,G ₁₁=69.18,Q ₁₁=0.64,Q _1?1=1.38. The hfs is mostly due to nuclear magnetic dipole interactions. For theA state the results are essentially in agreement with hfs constants derived from RF resonance experiments, but are superior as regards the data fit over the entireJ range covered. For theB state, the results are new and are interpreted in terms of a simple LCAO model. The Fermi contact coupling constant is in good agreement with unpublished SCF results by V. Staemmler. The striking dependences of the hfs splitting on the fine structure levels, Λ sublevels and onJ are explained both quantitatively and in terms of vector models.     

Sensitization of the OH(A 2Σ+ → X 2Πi) emission by Hg(6 3P0) metastables

The electronic energy transfer process Hg(6 ³P₀) + OH(X²Π_i, υ = 0,K) → Hg(6 ¹S₀) + OH(A ²Σ⁺, υ,K) has been studied by the sensitized fluorescence method. A rather broad spectrum of rotational population, N_υ′K′, was obtained under conditions of minimum relaxation, which illustrates the non-resonant and non-optical nature of this energy transfer process. The fractions of the exoergicity, above electronic excitation of OH(A ²Σ⁺, υ = 0, K = 0), going into vibrational, rotational and translational excitation are 0.11, 0.31, and 0.58, respectively. A statistical mode of energy partitioning, such as would result from long-lived complex formation, seems to account well for these observations.