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Nonadiabatic electronic interactions in the ion-pair states of NeICl (1991)

Stephenson, Thomas A. ; Hong, Yujian ; Lester, Marsha I.

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 94 (1991), S. 4171-4181

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: Nonadiabatic interactions in the NeICl van der Waals complex have been explored in the lowest energy triad of ICl ion-pair states (∼39 000 cm−1). Dispersed fluorescence measurements reveal emission characteristic of multiple ion-pair electronic states, with the relative contributions from the E(0+), β(1), and D'(2) states changing with the initial ICl vibrational excitation (vICl). Emission directly from NeICl (vICl=0) complexes indicates that the initially prepared NeICl levels have mixed electronic character and that the ICl electronic parentage changes with the initial van der Waals vibrational level selected. NeICl complexes prepared with 1–4 quanta of ICl stretch undergo rapid vibrational predissociation with a strong propensity for ΔvICl=−1 relaxation. The electronic state(s) populated in the ICl fragments differ from the mixed electronic character of the initially prepared level, demonstrating that vibrational predissociation is accompanied by nonadiabatic electronic state changing processes. The observed final state selectivity may be attributed to the relative strength of the nonadiabatic couplings between the initial NeICl bound state and the final ICl states or a momentum gap rationale based on the overlap between the NeICl bound state wave function and the highly oscillatory continuum wave function of the separating fragments.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.460650

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Fragment energy and vector correlations in the overtone-pumped dissociation of HN3 X˜ 1A' (1991)

Casassa, Michael P. ; Foy, Bernard R. ; Stephenson, John C. ; [et al.]

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 94 (1991), S. 250-261

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: NH stretching overtone and combination states in HN3 X˜ 1A' were excited by IR–visible double resonance pumping and by direct overtone pumping in the range 6ν1 (17 670 cm−1) to 7ν1 (20 070 cm−1). NH fragments in the a 1Δ and X 3Σ− states were detected by laser induced fluorescence with sub-Doppler resolution to determine branching ratios, correlated fragment rotational state and kinetic energy distributions, and fragment vector correlations. The spin-forbidden triplet channel was accessible to all states excited, while the threshold for the singlet channel was determined to lie in the range 18 190 to 18 755 cm−1. The measured energy release places limits on the HN–NN bond energy, and the heights of barriers to reaction. The barrier in the singlet exit channel is at least 540 cm−1. The singlet channel accessed by 7ν1 dissociation is characterized by a Boltzmann-like NH rotational distribution (〈J NH〉≈3.5), highly excited N2 rotations (〈JN2〉 ≥ 20), and total translational energy release peaked away from zero (〈ET〉≈1350 cm−1). Vector correlations and Λ-doublet propensities indicate that nonplanar dissociation processes influence the NH rotations, but become less important for higher NH rotational states. The principal correlations are a strong positive recoil anisotropy (β≈0.6), a weak positive v–J correlation (βvJ≈0.17), and a JNH-dependent Λ-doublet propensity. A model using parent vibrational motion projected onto fragment rotation is suggested to explain these observations. The triplet channel exhibits similar NH and N2 rotational state distributions, with most of the available energy (substantially greater than in the singlet channel) appearing as fragment kinetic energy.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.460392

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Subpicosecond transient infrared spectroscopy of adsorbates. Vibrational dynamics of CO/Pt(111) (1991)

Beckerle, J. D. ; Cavanagh, R. R. ; Casassa, M. P. ; [et al.]

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 95 (1991), S. 5403-5418

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: The vibrational dynamics of excited CO layers on Pt(111) were studied using infrared (IR) pump–probe methods. Resonant IR pulses of 0.7 ps duration strongly pumped the absorption line (ν≈2106 cm−1 ) of top-site CO. Weak probe pulses delayed a time tD after the pump were reflected from the CO-covered Pt(111) surface, and dispersed in a monochromator to determine the absorption spectrum of the vibrationally excited CO band, with time resolution 〈1 ps and monochromator resolution 〈1 cm−1. Transient spectra were obtained as a function of CO coverage, surface temperature, and laser fluence. Complex spectra for tD〈0 show features characteristic of a perturbed free induction decay, which are expected based on multiple-level density-matrix models. For tD≥0, the CO/Pt absorption exhibits a shift to lower frequency and an asymmetric broadening which are strongly dependent on fluence (1.3–15 mJ/cm2 ). Spectra return to equilibrium (unexcited) values within a few picoseconds. These transient spectral shifts and the time scale for relaxation do not depend (within experimental error) on coverage for 0.1≤aitch-thetaCO≤0.5 ML or on temperature for 150≤Ts≤300 K. A model for coupled anharmonic oscillators qualitatively explains the tD〉0 spectra in terms of a population-dependent decrease in frequency of the one-phonon band, as opposed to a transition involving a true CO(v=2) two-phonon bound state. The rapid relaxation time and its insensitivity to Ts and aitch-thetaCO are consistent with electron–hole pair generation as the dominant decay mechanism.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.461657

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Laboratory experiments on turbulent mixing across sheared density interfaces (1991)

Stephenson, Philip ; Fernando, Harindra J. S.

New York, NY : American Institute of Physics (AIP)

Physics of Fluids 3 (1991), S. 1461-1461

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ISSN: 1089-7666

Source: AIP Digital Archive

Topics: Physics

Notes: An experimental study was carried out to investigate turbulent mixing and entrainment across a density interface subjected to velocity shear. The flow configuration consisted of a salinity (stably) stratified two-fluid system with a driven upper turbulent layer and a quiescent lower layer. The experiments were performed in an Odell–Kovasznay tank and the mean flow in the upper layer was generated by using a conventional disk pump. The velocity and salinity measurements were made using a laser-Doppler anemometer and conductivity probes, respectively, and (quantitative) flow visualization was performed using the laser-induced fluorescence LIF technique. The refractive indices of upper and lower layers were matched, using salt and alcohol, to facilitate the use of laser-based flow diagnostic techniques. The measurements show that the rms velocity fluctuation u in bulk of the mixed layer scales well with the mean velocity jump Δu across the interface. The Thorpe, buoyancy, overturning, and integral length scales, as well as the maximum Thorpe displacement in the mixed layer, were also found to be proportional to the depth h of the upper mixed layer.The structure of the entrainment interface was found to depend strongly on the bulk Richardson number Ri (=Δb h/u2), where Δb is the buoyancy jump across the interfacial layer. At lower Ri, the entrainment occurred rapidly, as in a nonstratified fluid, but as Ri increases, the entrainment rate becomes a strong function of Ri: under the latter conditions, the interfacial wave breaking and Kelvin–Helmholtz instabilities were common features. At still higher Ri, the entrainment rate becomes vanishingly small and the interfacial mixing events were found to be controlled by the molecular diffusive effects. The measurement of the interfacial-layer thickness using LIF shows that it is much thinner than that measured using less-accurate techniques such as traversing probes. The nondimensional rms amplitude of the interfacial distortions at moderate and high Ri was found to be a strong function of Ri. The interfacial instabilities cause the formation of isolated mixing patches within the interface, which, when collapsed, form horizontal intrusions. The experimental measurements were in agreement with theoretical formulations based on scaling arguments.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.857995

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