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11

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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12

Digitale Medien

Vibrational branching ratios from the dissociation of the NeIBr van der Waals molecule (1992)

Walter, Sarah A. ; Stephenson, Thomas A.

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

The Journal of Chemical Physics 96 (1992), S. 3536-3541

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ISSN: 1089-7690

Quelle: AIP Digital Archive

Thema: Physik , Chemie und Pharmazie

Notizen: The degree of vibrational excitation in the IBr fragment from the vibrational predissociation of NeIBr (A 3Π1) has been measured using two-color pump–probe laser-induced fluorescence spectroscopy. We find that for the lowest initial vibrational states examined, Δv=−1 dissociation pathways dominate the dynamics, while this channel is closed for v≥17. From this result, the A state binding energy (D0) of the complex is determined to be 67±4 cm−1, while that in the X electronic state is found to be 73±4 cm−1. The X state binding energy is identical to that for NeI2 and NeBr2, suggesting that the potential energy surface for NeIBr can be constructed from a summation of atom–atom pair potentials; we present such a model potential energy surface. The variations in the vibrational branching ratios, when combined with the trends in the predissociation rates, point to the importance of fragment rotational excitation in the dynamics of the dissociation.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.461908

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13

Digitale Medien

Dissociation lifetimes and level mixing in overtone-excited HN3(X˜ 1A') (1989)

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

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

The Journal of Chemical Physics 90 (1989), S. 7037-7045

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ISSN: 1089-7690

Quelle: AIP Digital Archive

Thema: Physik , Chemie und Pharmazie

Notizen: Vibrational overtone photodissociation is used to examine the spectroscopy and vibrational predissociation lifetimes of HN3 in its ground electronic state. Direct overtone pumping of the N–H stretching levels 5νNH and 6νNH prepares molecules in selected states (v,J,K) near 15 100 and 17 700 cm−1 of vibrational energy; spin-forbidden NH(X 3 Σ−) dissociation fragments are detected by laser-induced fluorescence. Photodissociation spectra of beam-cooled HN3 display mixing of individual rotational levels of the nνNH vibrations with several background states, with derived coupling matrix elements in the range 0.01–0.1 cm−1. Vibrational predissociation lifetimes of mixed components of 5νNH are state specific, with variations of a factor of 2 for only 0.1 cm−1 energy differences. Average lifetimes for low J, K are 210 ns for 5νNH and 0.95 ns for 6νNH. The ratio of decay rates for the two overtone levels, k(6νNH)/k(5νNH)=220, is much greater than predicted by statistical theory, which gives a ratio of 4.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.456230

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14

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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15

Digitale Medien

Nascent product states in the photoinitiated reaction of O3 and H2O (1992)

Sauder, Deborah G. ; Stephenson, John C. ; King, David S. ; [weitere]

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

The Journal of Chemical Physics 97 (1992), S. 952-961

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ISSN: 1089-7690

Quelle: AIP Digital Archive

Thema: Physik , Chemie und Pharmazie

Notizen: The rotational, vibrational and fine-structure state distributions for the reaction 16O(1D)+ H218O →16OH +18OH, triggered by 266 nm photolysis of O3, have been measured under conditions where less than 1% of the nascent fragments experienced collisions prior to detection. The distributions are qualitatively different than those reported earlier for 266 nm photolysis, which were evidently affected by collisions. The rotational and vibrational state distributions are similar to recent 248 nm photolysis experiments, but with differences attributed to collisional and/or energetic effects in those experiments. The "new'' 16OH is formed with vibrational populations in the ratio 0.39(v=0):0.29(v=1):0.3(v≥2). Gaussian rotational energy distributions peaked near N=12 give average rotational energies of 〈Erot〉 = 3440 and 2780 cm−1 for 16OH v=0 and v=1, respectively. The "old'' 18OH is much colder with vibrational populations 0.94(v=0):0.06(v=1) and a 18OH v=0 Gaussian rotational energy distribution characterized by 〈Erot〉 = 1920 cm−1.There is negligible rotational alignment of the 16OH fragments [βμJ' = β20(02) = 5A(0)2/4 = 0.06± 0.09], which is significantly less than expected for fragment rotations aligned with respect to the O+H2O relative velocity vector. The spin-orbit propensities deviate slightly from the statistical expectation and are characterized by [F2,N]/[F1,N]=(0.89±0.06) ×N/(N + 1). The Λ-doublet distributions for 18OH (all N) and 16OH (low-to-moderate N) fragments conform to an unconstrained-dynamics prior distribution. A slight propensity for Π(A') in excess of this expectation is seen for the 16OH (high N) fragments. These new results are discussed in terms of possible insertion and abstraction mechanisms for the reaction.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.463198

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16

Digitale Medien

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

Quelle: AIP Digital Archive

Thema: Physik , Chemie und Pharmazie

Notizen: 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.

Materialart: Digitale Medien

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

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17

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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18

Digitale Medien

Picosecond measurement of substrate-to-adsorbate energy transfer: The frustrated translation of CO/Pt(111) (1993)

Germer, T. A. ; Stephenson, J. C. ; Heilweil, E. J. ; [weitere]

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

The Journal of Chemical Physics 98 (1993), S. 9986-9994

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ISSN: 1089-7690

Quelle: AIP Digital Archive

Thema: Physik , Chemie und Pharmazie

Notizen: The transient infrared response of CO/Pt(111) following picosecond visible excitation is reported. A spectrally broad decrease in reflectivity correlates with heating of the Pt lattice, and an observed shift in the CO(v=0→1) transition is interpreted as heating of the 60 cm−1 in-plane frustrated translational mode. A phenomenological three temperature model that assumes the adsorbate vibrational temperature Tads exclusively couples to either the electronic temperature Te (with a time constant τe) or to the lattice temperature Tlat (with a time constant τlat) describes the temporal response of the adsorbate vibrations. The lattice phonon temperature Tlat(z,t) and measured temperature dependence of the optical constants predict the observed spectrally broad reflectivity change. Density matrix methods model the infrared response of the transiently heated molecule. Limits of τe=2±1 ps or τlat〈1 ps are established by comparison of predicted spectra and the data.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.464324

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19

Digitale Medien

The extraction of signal to noise values in x-ray absorption spectroscopy : Proceedings of the 4th international conference on synchrotron radiation instrumentation (1992)

Dent, A. J. ; Stephenson, P. C. ; Greaves, G. N.

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

Review of Scientific Instruments 63 (1992), S. 856-858

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ISSN: 1089-7623

Quelle: AIP Digital Archive

Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik

Notizen: The assessment of signal to noise (S/N) values in x-ray absorption spectroscopy is important for a number of reasons. Two methods will be described that remove the signal to give the noise. The first uses polynomials and the second Fourier filtering. Having extracted the noise the signal can then be used to calculate the S/N ratio. The method described operates on a background subtracted spectrum and is quick so it can be done on line in an experimental situation.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.1142627

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20

Digitale Medien

Studies of nanoscale dynamics using coherent x rays from an x-ray free electron laser (abstract) (invited) : Papers from the 12th National Synchrotron Radiation Instrumentation Conference (2002)

Stephenson, G. Brian

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

Review of Scientific Instruments 73 (2002), S. 1367-1367

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ISSN: 1089-7623

Quelle: AIP Digital Archive

Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik

Notizen: This talk will describe proposed studies of atomic- and nanoscale dynamics in condensed matter which take advantage of the high coherent x-ray flux to be produced by future x-ray free-electron laser (FEL) sources, such as the Linac Coherent Light Source at Stanford. In particular, I will focus on the current status and future prospects for photon correlation spectroscopy using coherent x-rays (XPCS), and the use of the ultrashort pulse structure of the x-ray FEL to observe dynamics into the femtosecond range. © 2002 American Institute of Physics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.1448148

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