Volume 214, Issue 9 - 9

We report the spectroscopic detection of vibrationally excited molecular hydrogen using 2+1 resonantly enhanced multiphoton ionization (REMPI) via the I 1 Π g (v´ = 0) - X 1 ∑ + g (v" = 3) band ca. 198 nm. Vibrationally excited H 2 was produced by passing room-temperature hydrogen over a hot ion gauge filament in a high-vacuum chamber. The internal energy distributions were characterized spectroscopically by use of the EF 1 ∑ + g -X 1 ∑ + g 2+1 REMPI detection scheme. We have identified band origins for the S, Q, R, and P rotational branches of the I-X (0,3) band, as well as isolated lines corresponding to two-photon transitions into other nearby H 2 gerade states, including EF 1 ∑ + g (v´ = 2, 3, 4), GK 1 ∑ + g (v´ = 1), and J 1 Δ g (v´ = 0). We propose the I-X transition as a suitable candidate for the determination of the rotational anisotropy of vibrationally excited ground-state H 2 molecules. We support this contention with a calculation of the line strength moments and sensitivities to the second- (quadrupolar) and fourth-rank (hexadecapolar) moments of the rotational angular momentum distributions, which is compared against the well-established Q-branch members of the EF 1 ∑ + g -X 1 ∑ + g two-photon transition.

The effects of coherent radiation on a general molecular system are studied by evaluating the influence functional of the field. It is shown that the quantized radiation field is equivalent to a classical driving term plus a dissipative term equivalent to that of the vacuum state. It is thus concluded that the dissipative character of quantized radiation cannot be controlled by adjusting the intensity of the beam.

Standard heats of formation of SF, FSO, FSO 2 and HSO radicals have been calculated with the hybrid B3LYP density functional using extended basis sets and with CBS-4M, CBS-q, CBS-Q and CBS-QB3 model chemistries. The values computed from bond dissociation enthalpies and atomization energies are typically 2-3 kcal mol -1 higher than those obtained from a large number of isodesmic reactions. Application of isodesmic reactions calculations with energies computed at the CBS-q level of theory yielded our best values of 0.2±1.0, -69.9±2.0, -96.2±3.0, and -4.4±1.5 kcal mol -1 for SF, FSO, FSO 2 and HSO respectively. The values for the FSO and FSO 2 reduce markedly the literature uncertainties while the values for the SF and HSO are in very good agreement with previous ab initio estimates.

The results of dielectric relaxation experiments on benzonitrile, propylene carbonate and butylene carbonate, performed between 228.15 K and 338.15 K in the frequency range 0.1 ≤ ν /GHz ≤ 89 are presented. The spectra of the three liquids can be formally fitted with two relaxation processes. The long relaxation time, τ 1 ( T ), is equally well described by the Vogel-Fulcher-Tammann and the mode-coupling theory. However, the parameters differ from literature data for propylene carbonate based on the frequency of maximum dielectric loss. This low-frequency dispersion step is attributed to the essentially isotropic rotational diffusion of the molecular dipole vector with the carbonates exhibiting some inter-molecular association. For butylene carbonate the fast relaxation process, τ 2 ≈ 2 ps, partly arises from the rotation of the ethyl side chain. However, for all three liquids the observed high-frequency contribution in the spectra reflects the transition from molecular dynamics governed by free rotation to long-time rotational diffusion.

We report on the fabrication of periodic nanostructures on selected materials by short-pulse uv-laser ablation. Temporal studies reveal details of the ablation process with sub-picosecond time resolution. Biochemical applications of the nanostructures for planar waveguide technology are presented.

In this paper, we present an extension of our phenotype algorithm for global cluster geometry optimization from atomic clusters to the more difficult molecular clusters. A successful application to the benchmark case of TIP4P water clusters shows that the new method is at least as good as one of the best methods in the literature so far. We point out why the TIP4P water cluster system starts to become very complicated beyond twenty water molecules and follow the traces of a beginning structural transition from clusters with all water molecules at the surface to clathrate-like cages with a truly interior water molecule.

We present the application of classical molecular dynamics methods to the simulation of femtosecond time-resolved experiments. Choosing I 2 in Ar as an example, we consider two cases of laser excitation processes: i) a 496 nm excitation from the ground electronic state to the excited B-state prepares a wave packet within the dissociation continuum. Collisions between dissociating molecules and Ar atoms lead to caging of I 2 ; ii) a 620 nm pulse creates a coherent superposition of bound vibrational states and the interaction with Ar leads to a dephasing of the wave packet. We calculate pump-probe signals which are in excellent agreement with experiment. The purely classical simulation gives valuable insight into the dynamical processes which are reflected in the time-resolved signals.

Guided-Ion Beam (GIB) measurements of the Ar + + Ar symmetric charge-transfer (SCT) system are presented for ion energies ranging from 0.2 to 300 eV. Two methods are applied to distinguish primary and secondary ions: (i) based on isotopic-labeling, (ii) based on significant laboratory velocity differences. The absolute cross sections measured with these methods are in excellent agreement at energies above 1 eV. The experimental results are compared with semi-classical calculations performed with various published Ar 2 + potentials. The calculations including spin-orbit effects lie within 10% of the isotope-selected and attenuation measurements at all investigated ion energies. The present results lie significantly above the simple Rapp and Francis model [1]. Important errors in the latter approach are pointed out and a correct one-electron model is proposed. First measurements of the differential cross section at 0.5 eV collision energy are briefly mentioned.