Jon B. Cross

The photodissociation cage effect in van der Waals complexes: Fluorescence spectra of I2B(3Π0+u) from the hindered photodissociation of I2Ar at 488 nm

The B→X fluorescence from I2 produced in the hindered photodissociation of I2Ar at 488 nm has been resolved. The spectra show several vibrational progressions with low rotational energy, the most prominent of these is a (v′,0) progression extending from v′ = 49 to at least v′ = 23. The I2Ar complexes were excited and the I2 fluorescence was observed under collision‐free conditions in a supersonic free‐jet expansion. We attribute these observation to one‐atom photodissociation cage effect in the I2Ar comples, in which the molecular iodine, although excited more than 400 cm−1 above B state dissociation limit, is prevented from dissociating by energy transfer to the argon atom, resulting in the breaking of the van der Waals bond: I2Ar→I2(B)+Ar, leaving the iodine molecule in a bound level of the B electronic state, 386–1889 cm−1 below the dissociation limit. (AIP)

High repetition rate pulsed nozzle beam source

A high repetition rate piezoelectric pulsed molecular beam source is described. Reptition rates as high as 750 Hz have been obtained with gas pulse widths from 100 μs to 10 ms. The measured expansion characteristics are very nearly those of an ideal isentropic expansion. The valve is highly reliable: We have obtained 107 pulses without failure.

Multiproperty empirical anisotropic intermolecular potentials for ArSF6 and KrSF6

Crossed molecular beam measurements of the total differential cross sections (DCS) for the scattering of He and Ne by SF6 are reported. Using anisotropic MSV potential forms and the IOS approximation to do the scattering dynamics, we determine anisotropic intermolecular potentials by simultaneously fitting the DCS, viscosities, virial, and diffusion coefficients. Damping of the diffraction oscillations in the DCS determines the anisotropy of the minimum position rm of the HeSF6 interaction; damping of the rainbow determines the anisotropy of the well depth e of the NeSF6 interaction. Consistency between the systems verifies the accuracy of the resulting HeSF6 and NeSF6 potentials.

Vacuum ultraviolet radiation/atomic oxygen synergism in materials reactivity

Experimental results are presented which indicate that low fluxes of vacuum UV (VUV) radiation exert a pronounced influence on the atomic oxygen reactivity of such fluorocarbon and fluorocarbon spacecraft materials as the FEP Teflon and PCTFE that are under consideration for the Space Station Freedom. With simultaneous exposure to VUV fluxes comparable to those experienced in LEO, the reactivity of these materials becomes comparable to that of Kapton; VUV radiation has also been shown to increase the reactivity of Kapton with thermal-energy oxygen atoms. 8 refs.

EOIM-III Mass Spectrometry and Polymer Chemistry: STS 46, July-August 1992

The Evaluation of Oxygen Interactions with Materials III space-flight experiment was developed to obtain benchmark atomic-oxygen reactivity data and was conducted during Space Transportation System Mission 46. We present an overview of the flight experiment and the results of the Lyndon B. Johnson Space Center polymer chemistry and mass-spectrometer-carousel experiments. Mass-spectrometric measurements of gaseous products formed by O-atom reaction with 13C-labeled Kapton™ revealed CO, CO2, H2O, NO, and NCh. By operating the mass spectrometer to detect naturally occurring ionospheric species, we characterized the ambient ionosphere at various times during the flight experiment and detected the gaseous reaction products formed when ambient ions interacted with the 13C Kapton carousel sector. Direct comparison of the results of on-orbit O-atom exposures with those conducted in ground-based laboratory systems, which provide known O-atom fluences and translational energies, demonstrated the strong translational-energy dependence of O-atom reactions with a variety of polymers. A line-of-centers reactive scattering model was shown to provide a reasonably accurate description of the translational-energy dependence of polymer reactions with O atoms at high atom kinetic energies, and a Beckerle-Ceyer model provided an accurate description of O-atom reactivity over a three-order-of -magnitude range in translational energy and a four-order-of-magnitude range in reaction efficiency. Postflight studies of the polymer samples by x-ray photoelectron spectroscopy and infrared spectroscopy demonstrate that O-atom attack is confined to the near-surface region of the sample, that is, within 50 to 100 A of the surface.

Oxygen Interactions with Materials III— Mission and Induced Environments

The Evaluation of Oxygen Interactions with Materials III (EOIM-IH) flight experiment was developed to obtain benchmark atomic-oxygen-material reactivity data. The experiment was conducted during Space Shuttle mission 46, July 31 to August 7, 1992. Quantitative interpretation of the materials reactivity measurements requires a complete and accurate definition of the space environment exposure, including the thermal history of the payload, the solar ultraviolet exposure, the atomic-oxygen fluence, and any spacecraft outgassing and contamination effects. The thermal history of the payload was measured using 11 thermocouple sensors placed behind selected samples and on the EOIM-III payload structure. The solar ultraviolet exposure history of the EOIM-IH payload was determined by analysis of the as-flown orbit and vehicle attitude combined with daily average solar ultraviolet and vacuum ultraviolet fluxes. The atomic-oxygen fluence was assessed in three ways. First, the O-atom fluence was calculated using a program that incorporates the MSIS-86 atmospheric model, the as-flown Space Shuttle trajectory, and solar activity parameters. Second, it was estimated directly from Kapton film erosion. Third, ambient O-atom measurements were made using the quadrupole mass spectrometer on the EOIM-III payload. As of this writing, our best estimate of the O-atom fluence is (2.3 ± 0.3) X1020 atoms/cm2. Finally, results of postflight surface analysis of selected samples by x-ray photoelectron spectroscopy indicate low levels of molecular contamination on the payload surface.

The dynamics of infrared photodissociation of van der Waals molecules containing ethylene: An experimental study

Infrared (∼950 cm−1) predissociation of ethylene clusters has been studied using a crossed laser beam–molecular beam apparatus equipped with a moveable detector. van der Waals molecules undergo dissociation following absorption of a single infrared photon. Angular distributions, obtained for product molecules (C2H4)n, n=1–3, all show nearly exponentially decreasing product flux with increasing scattering angle. A product flux contour map has been generated for the photolysis reaction (C2H4)2 → C2H4+C2H4. Two isotropic center‐of‐mass distribution functions yield excellent agreement with experimental results. One is a function of reaction kinetic energy E, with P(E)=exp(−E/80 cm−1); the other is a function of product velocity (momentum) u, with U(u) =u exp(−u/9×103 cm/s). The latter distribution is characteristic of a dissociation pathway with a barrier in the exit channel. Such a barrier could result from centrigufal effects. It is argued that isotropic product scattering can be consistent with a direct di...

Atomic oxygen-MoS2 chemical interactions

Abstract The present study shows that an an O-atom translation energy of 1.5 eV, SO 2 is generated and outgases from an anhydrous MoS 2 surface with an initial reactivity nearly 50% that of kapton. The reaction of atomic oxygen with MoS 2 has little or no translational energy barrier, i.e. thermally generated atomic oxygen reacts as readily as that having 1.5 eV of translational energy. For MoS 2 films sputter-deposited at 50–70 °C, friction measurements showed a high initial friction coefficient (up to 0.25) for MoS 2 surfaces exposed to atomic oxygen, which dropped to the normal low values after several cycles of operation in air and ultrahigh vacuum. For MoS 2 films deposited at 200 °C, the friction coefficient was not affected by the O-atom exposure.

Flight mass-spectrometer calibration in a high-velocity atomic-oxygen beam

Calibration and characterization of the quadrupole mass-spectrometer component of the Evaluation of Oxygen Interactions with Materials III space-flight experiment are reported in this paper. A high-velocity atom beam system was used to characterize the response of the flight mass spectrometer to high-velocity oxygen atoms (0.8 to 2.5 eV). The response factor based on oxygen atom flux in the high-velocity beam was found to be logarithmically dependent on the exposure history of the instrument, i.e., the calibration factor was logarithmically dependent on atomic oxygen fluence. This dependence was independent of the background pressure over the range between 10~ and 10~ Torn Subsequent contamination of the instrument restored the instrument sensitivity to the original value before exposure to atomic oxygen. Carbon dioxide, carbon monoxide, and water were observed in the mass spectrometer whenever high-velocity oxygen atoms were present. The intensity of reaction products caused by interaction of atomic oxygen with contaminated surfaces within the instrument decreases with increasing atomic oxygen fluence, whereas C>2 resulting from recombination of atomic oxygen on surfaces increases with fluence.

A molecular beam study of the trapping desorption of I2 from a LiF(001) surface

Abstract Rotational, vibrational and translational Boltzmann distributions of I 2 desorbing from LiF(001) were measured using laser-induced fluorescence. The I 2 rotational temperature is lower than the surface temperature ( T S ) at T S > 300 K while the vibrational and translational temperatures are ≈ T S . An upper limit on the I 2 -LiF(001) potential well depth was found to be 0.45 ± 0.03 eV.