Application of the pople-santry-segal CNDO method to the cyclopropylcarbinyl and cyclobutyl cation and to bicyclobutane

Mise au point. Theorie des effets de couplage vibronique multimodes. Probleme a 2 etats. Couplage vibronique mettant en jeu des modes et des etats degeneres. Effets du couplage vibronique multimodes en spectroscopie. Comportement statistique des niveaux d'energie vibroniques. Intersections coniques et evolution temporelle de la fluorescence

Multimode molecular dynamics beyond the Born-Oppenheimer approximation

We describe the design, construction, and operation of a new type of microwave spectrograph which allows the measurement of the resonant transitions of transient or otherwise short‐lived species. The spectrograph is composed of three parts: a Fabry–Perot cavity, a pulsed supersonic nozzle as a source for the sample, and the pulsed microwave Fourier transform method. Following a detailed discussion of the three above components in the spectrograph, the operation of the entire system is described and several examples are given.

Fabry–Perot cavity pulsed Fourier transform microwave spectrometer with a pulsed nozzle particle source

The development of wave optics for light brought many new insights into our understanding of physics, driven by fundamental experiments like the ones by Young, Fizeau, Michelson-Morley and others. Quantum mechanics, and especially the de Broglie’s postulate relating the momentum p of a particle to the wave vector k of an matter wave: k = 2 λ = p/ℏ, suggested that wave optical experiments should be also possible with massive particles (see table 1), and over the last 40 years electron and neutron interferometers have demonstrated many fundamental aspects of quantum mechanics [1].

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Optics and interferometry with atoms and molecules

Hartree‐Fock perturbation theory of magnetic susceptibility and magnetic shielding is developed using a basis set of gauge invariant atomic orbitals. The theory is used to calculate magnetic shielding and spin‐rotation constants associated with the nuclei in LiH and HF giving results in good agreement with experimental values.

Molecular Orbital Theory of Magnetic Shielding and Magnetic Susceptibility

The HeI2 van der Waals complex was prepared from a dilute mixture of iodine in helium at a pressure of 100 atm by supersonic expansion through a nozzle into a vacuum. Laser‐induced fluorescence excitation spectra were recorded for the B←X transition of HeI2 as well as corresponding spectra for the He2I2 and I2 molecules in the expanding gas. I2 was found to be cooled by the expansion to a rotational temperature of 0.4 K, and a vibrational temperature of 50 K. Similarly, cold internal temperatures were attained by the van der Waals complexes. Evidence was found for vibrational predissociation of the HeI2 complex in both the X and B electronic states. The vibrational predissociation rate was found to depend weakly upon the degree of excitation of the I–I stretching mode, ν1. For ν1=1 in the X state the predissociation rate was found to be greater than 5×106 s−1. In the B state the vibrational predissociation rate is ∼5×1010 s−1 for ν1=27, decreasing to <5×109 s−1 for ν1⩽7. The small (3.4–4.0 cm−1) blue shifts of the vibronic bands of the HeI2spectrum relative to corresponding bands of I2 indicate (1) the van der Waals complex is slightly more strongly bound in the X state than it is in the B state, and (2) the I–I bonding in both the X and B states of iodine is largely unaffected by the formation of the van der Waals bond with helium.

The fluorescence excitation spectrum of the HeI2 van der Waals complex

The fluorescence excitation spectrum of NO2 was measured in the region 5708–6708 A using a tuneable dye laser as an excitation source. The NO2 was cooled to a rotational temperature of ∼3 K by expansion with argon as a carrier gas through a supersonic nozzle. This cooling drastically reduced the rotational structure and thereby permitted a clear separation and analysis of 140 vibronic bands found in this 1000 A region of the spectrum. The results indicate that most of the fluorescence in this region is due to a 2B2 electronic state, and that this state is so heavily perturbed by high lying levels of the ground electronic state that a substantial fraction of all ground state levels of appropriate vibronic symmetry in this region have appreciable fluorescence intensity.

The fluorescence excitation spectrum of rotationally cooled NO2

Measurement of fast desorption kinetics of D2, from tungsten by laser induced thermal desorption

A laser heating technique for studying fast surface processes has been applied in an initial study to the thermal desorption of D 2 from a polycrystalline tungsten sample. This technique is a means for measuring surface reactions at rates, concentrations, and temperatures that approach conditions of technical interest, but with the high degree of definition and control made possible with an ultrahigh vacuum apparatus. The method is analogous to the fast temperature jump method used for studying reactions in condensed phases, and can sort out elementary processes that have differing activation energies. The variation of total flux desorbed with maximum surface temperature reached and initial surface coverage serves, with the aid of a model kinetic rate expression, to determine the desorption rate parameters. It is shown that the desorption of D 2 from W at rates of 5 × 10 7 monolayers/sec is governed by the same kinetics as obtained by extrapolating previous measurements made at a rate about 10 5 times slower. The surface is subjected to a sufficiently fast and large temperature rise to desorb surface atoms or molecules in a time short compared to the range of flight times to a mass spectrometer detector. In this way the velocity distribution of the desorbing species may be determined. This along with the surface temperature history gives additional information on the reaction rate model and also whether the species are emerging in translational thermal equilibrium with the surface. In the present experiments a significant number of desorbatedesorbate collisions occur. Corrections are made for the collision effects in the interpretation of the data. It is shown how modifications of the technique can be made to substantially eliminate these effects. The present conditions were laser pulse width of 3 × 10 −8 sec and surface temperature rise of 300 to 3000 K.

Lennard Wharton

Papers

The fluorescence excitation spectrum of the HeI2 van der Waals complex

The fluorescence excitation spectrum of rotationally cooled NO2

Measurement of fast desorption kinetics of D2, from tungsten by laser induced thermal desorption

Measurement of fast desorption kinetics of D2 from tungsten by laser induced thermal desorption

Observation of Direct Inelastic Scattering in the Presence of Trapping-Desorption Scattering: Xe on Pt(111)