Abstract:We have measured level to level differential cross sections for rotational level changing collisions in ground state Na2 with Ar: Na2 (v\=0, ji=7)+Ar→Na2 (v\=0, jf)+Ar. Measurements range in Δj(jf−ji) from −4 to 80 and in Θ to π. A seeded supersonic beam source produces rotationally cold Na2 with a narrow velocity distribution; the levels are isolated and the differential cross sections are measured with cw dye lasers, using a new technique, angular distributions using the Doppler shift. The differential cross… Show more
“…The singularity is caused by simultaneous zeros of B' and the square root in the denominator of (10a) both of which behave as lfx for x--*0. This feature of a rapid decrease of the cross section immediately beyond elastic scattering into the superelastic regime at all angles tends to be obscured by low resolving power in translation energy loss experiments, but is qualitatively confirmed by the observations of Pritchard and Kinsey [6] on the 7--.9 and 7--.3 rotational transitions of Ar-Na 2. In the inelastic regime u*<l, B>0, at increasing the other extrema B~: of B(XI) enter and leave the potentially contributing B range, see Fig.…”
Section: Ill Discussionsupporting
confidence: 62%
“…A resolving power of about 5 % has been sufficient to resolve the structure of bulge scattering well, but, for non-hydrogenic molecules, is not enough to distinguish individual rotational transitions. Enormously more selective pump-and-probe laser techniques have also been used [4,6]. They are capable of labelling single j levels and, probing for f, resolve level-to-level transitions.…”
Section: Introductionmentioning
confidence: 99%
“…The observation of the bulge effect or orientational rainbows in collisions of K+N2, C160, C180 [1,2], Na2+He, Ne, Ar [3][4][5][6], K+O2 [7], and probably also of D2+CO [8] has indicated that this strong, classical feature of rotationally inelastic scattering is a general, direct and sensitive probe to the anisotropy of the repulsive intermolecular potential. Its essence is in short: Let atoms and unoriented diatomic (or linear) molecules collide with momentum of relative motion p and magnitude of molecular angular momentum j sufficiently well defined.…”
Rotationally inelastic atom-diatom collisions at wide angles and energies large compared to the attractive potential well depth are approximately described by classical scattering of structureless particles and rigid rotor molecules interacting by a rigid potential shell of C~v symmetry. The double differential cross section for deflection and rotational energy transfer is analytically related to the geometry of the shell. In continuation of previous work the influence of molecular rotation before collision is considered. It is found: The conspicuous structure of bulge scattering or orientational rainbows in this cross section persists, but is modified by initial rotation. The modifications are angle dependent. The (classical) information of bulge scattering on the anisotropy of the potential is increasingly averaged out by higher initial rotation. In experiments exploiting this new probe to the anisotropy of the repulsive molecular interaction initial rotation should be kept as low as possible.
“…The singularity is caused by simultaneous zeros of B' and the square root in the denominator of (10a) both of which behave as lfx for x--*0. This feature of a rapid decrease of the cross section immediately beyond elastic scattering into the superelastic regime at all angles tends to be obscured by low resolving power in translation energy loss experiments, but is qualitatively confirmed by the observations of Pritchard and Kinsey [6] on the 7--.9 and 7--.3 rotational transitions of Ar-Na 2. In the inelastic regime u*<l, B>0, at increasing the other extrema B~: of B(XI) enter and leave the potentially contributing B range, see Fig.…”
Section: Ill Discussionsupporting
confidence: 62%
“…A resolving power of about 5 % has been sufficient to resolve the structure of bulge scattering well, but, for non-hydrogenic molecules, is not enough to distinguish individual rotational transitions. Enormously more selective pump-and-probe laser techniques have also been used [4,6]. They are capable of labelling single j levels and, probing for f, resolve level-to-level transitions.…”
Section: Introductionmentioning
confidence: 99%
“…The observation of the bulge effect or orientational rainbows in collisions of K+N2, C160, C180 [1,2], Na2+He, Ne, Ar [3][4][5][6], K+O2 [7], and probably also of D2+CO [8] has indicated that this strong, classical feature of rotationally inelastic scattering is a general, direct and sensitive probe to the anisotropy of the repulsive intermolecular potential. Its essence is in short: Let atoms and unoriented diatomic (or linear) molecules collide with momentum of relative motion p and magnitude of molecular angular momentum j sufficiently well defined.…”
Rotationally inelastic atom-diatom collisions at wide angles and energies large compared to the attractive potential well depth are approximately described by classical scattering of structureless particles and rigid rotor molecules interacting by a rigid potential shell of C~v symmetry. The double differential cross section for deflection and rotational energy transfer is analytically related to the geometry of the shell. In continuation of previous work the influence of molecular rotation before collision is considered. It is found: The conspicuous structure of bulge scattering or orientational rainbows in this cross section persists, but is modified by initial rotation. The modifications are angle dependent. The (classical) information of bulge scattering on the anisotropy of the potential is increasingly averaged out by higher initial rotation. In experiments exploiting this new probe to the anisotropy of the repulsive molecular interaction initial rotation should be kept as low as possible.
“…Previous work (15) indicated that the rotational-state distributions have a very weak dependence on ⌬v; one might reasonably expect to observe larger changes in the DCS, a more sensitive probe of the dynamics, for such large differences in the amount of energy transferred into vibration (recall that the energy of a quantum of D 2 vibration is approximately equal to the height of the reaction barrier on the minimum energy path), but this is not what we observe. Hints of this counterintuitive behavior were also found by Serri et al (24,25) when they measured DCSs for Na 2 ϩ Ar inelastic scattering and found that the peak scattering angle shifted by only 10°b etween ⌬v ϭ 0 and ⌬v ϭ 1 at large ⌬j. The vibrational spacing of Na 2 is small, and its interaction with Ar is almost totally repulsive.…”
We have measured differential cross sections (DCSs) for the vibrationally inelastic scattering process H ؉ o-D2(v ؍ 0, j ؍ 0,2) 3 H ؉ o-D2(v ؍ 1-4, j even). Several different collision energies and nearly the entire range of populated product quantum states are studied. The products are dominantly forward-scattered in all cases. This behavior is the opposite of what is predicted by the conventional textbook mechanism, in which collisions at small impact parameters compress the bond and cause the products to recoil in the backward direction. Recent quasiclassical trajectory (QCT) calculations examining only the o-D2(v ؍ 3, j) products suggest that vibrationally inelastic scattering is the result of a frustrated reaction in which the DOD bond is stretched, but not broken, during the collision. These QCT calculations provide a qualitative explanation for the observed forward-scattering, but they do not agree with experiments at the lowest values of j. The present work shows that quantum mechanical calculations agree closely with experiments and expands upon previous results to show that forward-scattering is universally observed in vibrationally inelastic H ؉ D2 collisions over a broad range of conditions. fully quantum calculations ͉ ion imaging ͉ reaction dynamics ͉ vibrationally inelastic scattering T he hydrogen exchange reaction, H ϩ H 2 3 H 2 ϩ H, as the prototypic model system in the study of reaction dynamics, has garnered much attention from experimentalists and theoreticians alike. Dozens of studies over the past few decades have compared measurements of integral and differential cross sections (ICSs and DCSs) for reactive scattering with the results of quasiclassical trajectory (QCT) and quantum mechanical (QM) calculations; several excellent reviews have been published recently (1-3). Despite a few remaining slight discrepancies that possibly result from errors in these challenging experiments (4, 5), the agreement between recent experiments and calculations is nearly perfect (6-9), suggesting that reactive scattering in this fundamental reaction is well understood. In contrast to the wealth of information available for the reactive channel, only a handful of studies have focused on the vibrationally inelastic scattering channel (1, 10-16), and only one of these presented rotational-state-selected DCSs (16).For the H ϩ D 2 isotopic variant, 1 quantum of D 2 vibration is roughly equivalent in energy to the reaction barrier to form HD on the minimum energy path (Ϸ0.4 eV), and the nuclear motion involved in D 2 vibration is qualitatively similar to the lengthening of the DOD bond that occurs near the transition state in a reactive collision. Indeed, inelastic scattering trajectories may recross the reaction barrier multiple times (1,13,16,17). Reactive collisions are dominated by direct recoil at small impact parameters (1, 18, 19), but nonreactive collisions comprise a much larger range of impact parameters and may sample quite different regions of the potential energy surface (PES). Studying inel...
“…The most extensive diatomic data are probably those of Na 2 that include the entire rare gas series of collision partners. − To bring emphasis to the generally unrecognized existence of common rotational distributions that occur in many diatomic systems, we display in Figure some of the Na 2 data 24 plotted in a form that reveals the close similarity of the rate constant distributions among the entire set of rare gas collision partners. 9 Rate constants for state-to-state rotational energy transfer from J = 4, J = 6 and J = 38 of A(Σ) Na 2 in collision with He, Ne, Ar, Kr, or Xe plotted against Δ J .…”
To provide data for the complete series of rare gases, relative cross sections are obtained for the crossed
molecular beam state-to-state rotationally and rovibrationally inelastic scattering of S1 (1Au) glyoxal (CHO−CHO) in its 00, K‘ = 0 states by Ne, Ar, and Xe. When added to cross sections from a new analysis of Kr
data and to published data for H2 and He, sets of cross sections for the entire rare gas series are available that
show the competition among more than 25 rotational and rovibrational channels. The latter all involve Δυ7‘
= +1 where ν7‘ = 233 cm-1 is the lowest frequency mode. Despite large variations in the collisional kinematics
and in the interaction potential energy surfaces, the competition among rotationally inelastic channels is
essentially identical for the gases Ne, Ar, Kr, and Xe. In turn, those cases differ from H2 and He solely by
the fact that orbital angular momentum constraints with the light gases limit scattering to only those states
with ΔK ≲ 15. In contrast, the competition between rotational and rovibrational scattering changes with the
collision partner to the extent that state-to-state resolution of rovibrational scattering is not possible for Ar,
Kr, and Xe. Previous theoretical predictions for Ar inelastic scattering are consistent with earlier arguments
that this competition is dominated by kinematic factors rather than by variations in the interaction potential.
The relative cross sections are obtained from experiments in which a laser prepares S1 glyoxal in the 00
K‘
= 0 state with J‘ ≈ 0−10. Dispersed S1−S0 fluorescence is used to monitor the inelastic scattering to more
than 25 destination states with ΔK‘ resolution. Inelastic cross sections are extracted by computer simulation
of the fluorescence spectra.
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