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Digitale Medien

Unimolecular dynamics following vibrational overtone excitation of HN3 v1=5 and v1=6: HN3(X˜;v,J,K)→HN(X 3Σ−;v,J,Ω)+N2 (X 1Σ+g) (1988)

Foy, B. R. ; Casassa, M. P. ; Stephenson, J. C. ; [weitere]

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

The Journal of Chemical Physics 89 (1988), S. 608-609

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ISSN: 1089-7690

Quelle: AIP Digital Archive

Thema: Physik , Chemie und Pharmazie

Notizen: Excitation of the NH-stretch overtone transitions of HN3 to v1=5 and 6 resulted in predissociation to HN(X) and N2(X) with lifetimes of 80+60−30 and ≤3 ns, respectively. Following excitation of either overtone, the HN fragments were formed predominantly in the symmetric F1, F3 spin–rotation states, with less than 4% population in the antisymmetric F2 levels. Fragment Doppler profiles confirmed that most of the available energy (〉96%) went into translational motion.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.455454

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2

Digitale Medien

Picosecond study of the population lifetime of CO(v=1) chemisorbed on SiO2-supported rhodium particles (1988)

Heilweil, E. J. ; Cavanagh, R. R. ; Stephenson, J. C.

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

The Journal of Chemical Physics 89 (1988), S. 5342-5343

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ISSN: 1089-7690

Quelle: AIP Digital Archive

Thema: Physik , Chemie und Pharmazie

Notizen: Infrared pump–probe characterization of the excited state lifetimes reveals that CO bound to isolated metal sites (T1=140±20 ps) persists longer than the signal observed for CO bound to (approximate)35 A(ring) diameter metal particles (≤18 ps), suggesting paticipation of electron–hole excitations in the larger metal particles.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.455626

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3

Digitale Medien

CO(v=1) population lifetimes of metal–carbonyl cluster compounds in dilute CHCl3 solution (1988)

Heilweil, E. J. ; Cavanagh, R. R. ; Stephenson, J. C.

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

The Journal of Chemical Physics 89 (1988), S. 230-239

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ISSN: 1089-7690

Quelle: AIP Digital Archive

Thema: Physik , Chemie und Pharmazie

Notizen: Tunable infrared picosecond pulses in the 5 μ region have been used for time-resolved pump–probe measurements of the population relaxation lifetime (T1) of CO(v=1) stretching vibrations in a series of metal–carbonyl cluster compounds in room temperature chloroform solution. T1 was the same for symmetric (ν=2084 cm−1, 90±10 ps) and antisymmetric (2014 cm−1, 87±10 ps) modes of the dicarbonyl Rh(CO)2(C5H7O2); T1 was the same for the B1 (2092 cm−1 , 710±130 ps) and B2 (2036 cm−1, 750±90 ps) modes of Rh2(CO)4Cl2. Similarly long T1 times were found for Rh4(CO)12 (2075 cm−1, 610±65 ps) and Rh6(CO)16 (2077 cm−1, 700±100 ps). The molecule Co4(CO)12 has also been compared to the corresponding rhodium analog and it exhibits an initially fast relaxation of 47±5 ps followed by a slower 396±70 ps decay. The transient response of the more complex systems to the single frequency experiment is found to be sensitive to frequency and can exhibit bleaching, absorption, and a combination of these effects. Such behavior is attributed to overlap of the IR pulse with v=1 to v=2 and higher transitions in the M4(CO)12 (M=Rh or Co) and Rh6(CO)16 molecules. The long CO(v=1) T1 values for the metal cluster molecules suggest relaxation via multiquantum transfer of vibrational energy to adjacent M–C stretch and M–C–O bend vibrations; energy transfer to vibrational or electronic states of the central metal core seems unimportant in determining T1 for these systems.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.455510

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4

Digitale Medien

Energetics and spin- and Λ-doublet selectivity in the infrared multiphoton dissociation DN3→DN(X 3Σ−, a 1Δ)+N2(X 1Σ+g): Experiment (1988)

Stephenson, John C. ; Casassa, Michael P. ; King, David S.

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

The Journal of Chemical Physics 89 (1988), S. 1378-1387

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ISSN: 1089-7690

Quelle: AIP Digital Archive

Thema: Physik , Chemie und Pharmazie

Notizen: Multiphoton vibrational excitation of deuterated hydrazoic acid, DN3, by a CO2 laser (I=10 GW/cm2) leads to dissociation forming DN in both X 3Σ− (spin forbidden) and a 1Δ (spin allowed) electronic states. Under collisionless conditions, the nascent DN fragments were probed via laser induced fluorescence, to determine initial product state distributions. The DN(X 3Σ−) molecules are formed predominantly in the symmetric F1 and F3 spin–rotation states with little population (≤6%) in the antisymmetric F2 levels. There is no significant population (〈3%) in excited DN(3Σ−) vibrational levels. The distribution of rotational states is Boltzmann-like, characterized by a rotational "temperature'' of about 920 K for the F1, F3 states and 500 K for F2 levels. Doppler profiles showed a large kinetic energy release of about 10 100 cm−1 total in the triplet channel. The DN(1Δ) products are formed preferentially in the symmetric Δ(A'), e-labeled lambda doublet levels: Δ(A')/Δ(A‘)=1.44. The DN(1Δ) is formed with no vibrational excitation (〈2%); the rotational states are populated Boltzmann-like with a rotational "temperature'' of 425 K. Doppler profiles give a total kinetic energy of about 1500 cm−1 in this channel. These observations give information about the distribution of energy in the reactant, the location of the barriers to dissociation, and the geometry of the transition states. Alexander, Werner, and Dagdigian (accompanying article) show that the observed DN(3Σ−) spin- and DN(1Δ) Λ-doublet selectivities reflect the symmetry properties of a planar transition state and that the low degree of DN(3Σ−) rotational and vibrational excitation is also expected from the transition state geometry.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.455137

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5

Digitale Medien

Time- and state-resolved measurements of nitric oxide dimer infrared photodissociation (1988)

Casassa, Michael P. ; Stephenson, John C. ; King, David S.

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

The Journal of Chemical Physics 89 (1988), S. 1966-1976

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ISSN: 1089-7690

Quelle: AIP Digital Archive

Thema: Physik , Chemie und Pharmazie

Notizen: Picosecond and nanosecond lasers and pulsed molecular beam techniques have been used to measure the infrared photodissociation spectra, the product state distributions, and the predissociation lifetimes of vibrationally excited nitric oxide dimer (NO)2 . Results for the ν1 (v=1) symmetric NO stretching mode and the ν4 (v=1) antisymmetric NO stretching mode are presented. Predissociation lifetimes are determined by time-resolved laser induced fluorescence probing of the NO monomer product appearance rate. A dramatic mode dependence of the predissociation lifetimes is observed with the higher energy ν1 mode decaying in approximately 1 ns, and the lower energy ν4 mode decaying in approximately 40 ps. The mode dependence is independent of which product state is probed. The product state distributions show that 75% to 80% of the available energy is channeled into relative translational energy of the fragments for both modes. Rotational state distributions are Boltzmann-like with temperatures ranging from 71 to 112 K depending on both the initially excited mode and on the NO product spin–orbit state. Predissociation from ν1 produces NO fragments in the 2Π1/2 and 2 Π3/2 states with equal probability. Predissociation from ν4 exhibits a propensity for producing the lower energy 2 Π1/2 spin–orbit state. The observations are discussed in terms of various vibrational predissociation mechanisms, including vibrational potential coupling and electronically nonadiabatic predissociation.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.455694

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6

Digitale Medien

Field-induced magnetic anisotropy in a dilute dispersion of γ Fe2O3 particles (1988)

Potter, D. K. ; Stephenson, A.

[S.l.] : American Institute of Physics (AIP)

Journal of Applied Physics 63 (1988), S. 1691-1693

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ISSN: 1089-7550

Quelle: AIP Digital Archive

Thema: Physik

Notizen: The application of an alternating field (AF) of 60-mT peak to an isotropic sample containing dilutely dispersed particles of γFe2O3 from magnetic tape causes it to become magnetically anisotropic. The anisotropy of initial susceptibility so produced can be described by an oblate ellipsoid of revolution with the unique axis aligned with the AF direction. A consequence of this effect is that an isothermal remanent magnetization (IRM) is then more easily acquired if the direct field is applied perpendicular to the previously applied AF direction than if it is applied parallel to this direction. The anisotropy of IRM acquisition at direct fields between 20 and 40 mT is an order of magnitude greater than the anisotropy of initial susceptibility and is 40% of the maximum possible value.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.339903

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7

Digitale Medien

Determination of the cubic to hexagonal fraction in GaN nucleation layers using grazing incidence x-ray scattering (1998)

Munkholm, A. ; Thompson, C. ; Foster, C. M. ; [weitere]

Woodbury, NY : American Institute of Physics (AIP)

Applied Physics Letters 72 (1998), S. 2972-2974

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ISSN: 1077-3118

Quelle: AIP Digital Archive

Thema: Physik

Notizen: Recent electron diffraction and microscopy studies of GaN nucleation layers have shown that faults in the stacking of the close-packed planes result in the coexistence of cubic and hexagonal phases within the layers. Using grazing incidence x-ray scattering, we have quantified the proportion of the cubic and hexagonal phases throughout the nucleation layer. We compare the structure of a 20 nm nucleation layer grown on sapphire by atmospheric pressure metal-organic chemical vapor deposition at 525 °C to that of an identical layer heated to 1060 °C. The fractions of cubic and hexagonal phases in the layers are determined by a comparison of the scattering data with a Hendricks–Teller model. High temperature exposure results in a decrease of the cubic fraction from 0.56 to 0.17. The good agreement with the Hendricks–Teller model indicates that the positions of the stacking faults are uncorrelated. © 1998 American Institute of Physics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.121511

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8

Digitale Medien

Contrast reversal in scanning capacitance microscopy imaging (1998)

Stephenson, Robert ; Verhulst, Anne ; De Wolf, Peter ; [weitere]

Woodbury, NY : American Institute of Physics (AIP)

Applied Physics Letters 73 (1998), S. 2597-2599

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ISSN: 1077-3118

Quelle: AIP Digital Archive

Thema: Physik

Notizen: We have investigated the quantification properties of scanning capacitance microscopy (SCM) by using two dedicated test structures and highlight the response of SCM to changes in dopant density. Our results indicate that contrast reversal occurs and that the SCM output is not always a monotonically increasing signal with decreasing dopant density. Two epitaxially grown staircase structures covering the doping ranges 1014–1020 cm−3 p type and 5×1014–5×1019 cm−3 n type were produced for this study as the turning point in the response function typically occurs at a doping level of around 1017 cm−3. Through the use of a simple simulation model we see that contrast reversal is expected due to a relative shift between the dC/dV curves for different doping levels. The onset of contrast reversal can be adjusted by changing the dc sample bias leading to a shift in the operating position of the SCM, and the significance of this point will be discussed here. © 1998 American Institute of Physics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.122517

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9

Digitale Medien

Internal surfaces of porous media studied by nuclear magnetic resonance cryoporometry (1998)

Allen, S. G. ; Stephenson, P. C. L. ; Strange, J. H.

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

The Journal of Chemical Physics 108 (1998), S. 8195-8198

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ISSN: 1089-7690

Quelle: AIP Digital Archive

Thema: Physik , Chemie und Pharmazie

Notizen: The filling processes of water and cyclohexane in porous silica (with a characteristic pore size of 60 Å) are investigated using the nuclear magnetic resonance (NMR) technique of cryoporometry. In this technique, the liquid was frozen in the pores before the temperature was raised gradually; melting the smallest particles first and then particles of increased size. The volume of the molten liquid present was measured using the height of a T2 spin echo. The experiments were performed with filling fractions ranging from 10% to 100%. The results showed distinctly different behaviors of the fluids, which depended on the surface adhesion. It was found that water (a fluid which wets the pore surface) forms small puddles—much smaller than the smallest pore size—at low filling fractions. These puddles grow in size as more water is added until all the pore volume is filled. Cyclohexane (a non-wetting fluid) on the other hand, does not form small puddles but completely fills the pores with a preference for the smaller pores. Water is found to give more accurate results for the pore size distribution than cyclohexane, in 60 Å silica. © 1998 American Institute of Physics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.476175

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