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Notes: We report the effect of static electric fields on the collisional ionization of highly excited sodium atoms by HBr. The binding energy dependence of the collisional ionization cross section is measured at zero field and in static electric fields up to that point at which the atom field ionizes. The applied electric field lowers the ionization threshold of the atom from its zero field value. Therefore an atom near the ionization threshold in an electric field is of smaller size than a free field atom with the same binding energy. Thus measuring the binding energy dependence of the cross section at different values of the electric field allows us to study the effects of the physical size of the atom on the cross section. The effect of the electric field was to lower the measured ionization cross section. However, the binding energy dependence of the cross section remains unchanged at the level of our measurement accuracy. The measured cross sections are larger for larger atoms, exhibit a drop with increasing binding energy characteristic of rotational to electronic excitation transfer, and are of order 10−12–10−11 cm2. A simple calculation based on dipole (J→ J−1) excitation transfer from the molecule to the atom predicts, with good agreement, the binding energy dependence of the cross section. The electric field dependence of the data however, is not shown in the theory.

Notes: Single photon double ionization of CS2 has been investigated, using mass spectrometric coincidence techniques: both metastable and dissociative CS++2 states have been observed; three different dissociation pathways of CS++2 have been demonstrated, including one bond (S++CS+) and two bonds (S++C+S+ and S++C++S) breakings; simulation of the observed dissociation provided some insight into the dissociation mechanisms. Monochromatized synchrotron radiation enabled us to measure the excitation spectra of these relatively intense processes, in the 25–75 eV photon energy range. Our results provide an approach to the spectroscopy of the doubly charged CS++2 ion; comparison with a SCF-LCAO-MO calculation leads to a tentative assignment of the observed states.

Notes: The integral equations used in the microscopic theory of the electric double layer are extended to the case of an impermeable interface separating two conducting media (ionic solutions or plasma). This system is a model for an ideally polarizable interface. Exact relations are given for the contact values of the one particle density function, and also for the pair correlation functions. We solve numerically the Poisson–Boltzmann (PB), the hypernetted chain (HNC), and mean spherical (MSA) approximations, and compare the results to the exact solution of the one component plasma in two dimensions.

Notes: This paper reports the results of an extensive study of the internal energy-transfer processes that occur in benzene–argon collisions. We used laser-induced fluorescence and information theory for determining the energy-transfer rates between internal states of benzene in the ground electronic state (1A1g). The method provides an estimate for the rate of rotational relaxation. It gives a measure of the fraction of molecules that absorb the laser radiation at a frequency near the center of the ν18 absorption band of benzene. The use of information theory gives estimates for all of the vibrational energy transfer rates. These fit the experimental data reasonably well. However, some of the data do deviate from the information theory model. This suggests that the statistical assumptions of the model are not sufficiently restrictive. One such restriction may be in the number of vibration quanta changing per collision..

Notes: In a recent paper (Az I), well-structured T=300 K resonance Raman (RR) profiles for the 1400, 1260, 900, and 2×825 cm−1 lines of azulene in CS2 and for the 825 cm−1 line of azulene in methanol were reported. Previously developed transform techniques were used to (1) compute RR profile line shapes directly from measured optical absorption spectra, and (2) extract ratios of Stokes loss parameters from the line shape scale factors. The transform analysis indicated that (1) our model assumptions (adiabatic and Condon approximations, harmonic phonons, atomic equilibrium position shifts, and small vibrational frequency shifts upon excitation to a single electronic state) are basically correct allowing forminor modifications, and (2) any deviations from these assumptions are likely to be larger for the 900 cm−1 mode and smaller for the 1400 and 1260 cm−1 modes. In this paper (Az II), we report model calculations of the optical absorption spectra, RR profile line shapes, and relative RR intensities. In these calculations, we use a recently proposed nonzero temperature multimode time-correlator modeling procedure. Compared with the conventional sum-over-states method, our time-correlator modeling procedure is superior in that (1) our optical absorption spectra and RR profiles computed via fast Fourier transform techniques have a practically unlimited spectral range and (2) the computing times are short for nonzero temperature multimode calculations. In our basic model, we adopt the assumptions of Az I and use seven azulene modes to obtain simultaneous good fits of the well-structured RR profile line shapes and optical absorption spectra. However, we find that the basic model does not account for the intensity of the 900 cm−1 Raman line relative to that of the 1400 cm−1 line, even though the individual profile line shape fits for these modes are very good. The basic model is therefore modified to allow mixing of the normal coordinates of these two modes. By introducing a single, relatively small mode-mixing parameter, we obtain a good fit of the relative RR intensities in addition to simultaneous detailed fits of the optical absorption spectra and RR profile line shapes. In an alternate approach, we modify our basic model and find that the inclusion of two relatively small non-Condon parameters, instead of one mode mixing parameter, can also produce simultaneous detailed fits of all of our optical absorption and RR data.A comparison of the two modified models solely on the basis of simplicity favors the mode-mixing model, since only one extra parameter is required to modify our basic model.

Notes: The positron-annihilation spectrum for C8K obtained from the Doppler broadening of the 511 keV annihilation radiation is composed of two Gaussian bands: one due to annihilation with electrons in the interlayer region, the other mainly due to annihilation with electrons in the graphitic layers. Upon introduction of hydrogen to C8K, both bands sharpened, which indicates a conversion of hydrogen atoms to hybride ions through charge transfer from the graphitic layers and the formation of positron–hydride ion complexes in the interlayer region. (AIP)

Notes: The results of picosecond time-resolved measurements of intramolecular vibrational-energy redistribution (IVR) in jet-cooled t-stilbene are presented. The results show that the changes in the nature of IVR that were found to occur with increasing energy in anthracene also occur in t-stilbene. In particular, at intermediate energies a number of different excitations give rise to phase-shifted quantum beats in fluorescence decays, indicating restricted IVR in the molecule. At higher energies decay behavior characteristic of dissipative IVR is observed. These results are compared with those of anthracene and are discussed in terms of molecular symmetry and vibrational density of states. The results suggest the generality of the conclusions about large molecule IVR that have been stated in the other papers of this series.

Notes: The dynamics of CO3− photodissociation is studied with a new photodissociation spectrometer that allows kinetic energy-resolved detection of parent ions and photofragments. Kinetic energy release distributions, photodissociation spectra, and the dependence of the photofragment intensity on the laser power and background pressure are presented. Photodissociation of CO3− in the energy range 1.95–2.2 eV leads to CO2+O− fragments, and is found to occur by two distinct mechanisms. These mechanisms involve three electronic states that correlate with CO2+O−—the 2B1 ground state, a 2A1 weakly bound state, and a repulsive 2B2 state. The first mechanism begins with a low cross section 2A1 ← 2B1 transition that gives structure to the spectra. From this intermediate state, a second photon carries the ion to the 2B2 state. Dissociation to the observed photofragments occurs rapidly on the repulsive surface. In this two photon mechanism, at least 20% of the available energy is disposed of in relative translation of photofragments. The second mechanism is also initiated by the 2A1 ← 2B1 transition. Deexcitation of the 2A1 bound state by internal conversion, however, leads to high lying vibrational levels of the ground 2B1 state. These vibrational levels are found to have an enhanced collision-induced dissociation cross section.