R-3 Repository :: Browsing by Author "Stebbings, Ronald F."

Browsing by Author "Stebbings, Ronald F."

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Adiabetic electric field ionization of Sodium Rydberg atoms
(1982) McMillian, Gary Benton; Stebbings, Ronald F.; Dunning, F. Barry; Lane, Neal F.
Utilizing the technique of high resolution selective field ionization it is possible to observe ionization structure as narrow as 2 V/cm for Na(nS,nD) atoms. Field ionization structure this narrow is characteristic of atoms evolving predominantly adiabatically in a rapidly increasing electric field. By increasing the slew rate of the applied field it is possible to induce departures from the adiabatic path at specific avoided level crossings. These departures result in multiple ionization thresholds which may be explained by arguments based on a simple quantum mechanical model of adiabatic field ionization.
Chemical abundances in planetary nebulae in NGG 6822
(1984) Kirsch, Russell Gary; Dufour, Reginald J.; Reiff, Patricia H.; Stebbings, Ronald F.; O'Dell, Charles R.
The results of an investigation of the optical spectra of five planetary nebulae and two H II regions are presented. The nebulae, located in NGC 6822, a Local Group Irregular Galaxy at a distance of .557 Mpcs, were examined using the Richey-Cretien spectrograph and the Silicon Intensified (SIT) Vidicon, on the 4-meter telescope at the Cerro Tololo Interamerican Observatory. From the measurement of selected emission line strengths in the XX37 - XX71 spectral ranges, the relative abundances of He, N, , Ne, S are calculated with respect to H (Log (H) = 12.) for each nebula. Electron densities are derived for the planetary nebulae from the ratio of the Cs IlU line intensities, I(6716)/I(6731), while their temperatures are obtained from theiatio of the Co IIlH line intensities, 1(4363), 1(4959), 1(57). The derived chemical abundances are examined in the context of possible Nitrogen enrichment of the Interstellar Medium and the possible age of the galaxy. Type I planetaries exhibited a He and N enrichment of factors of up to 2 and 31, respectively. The high N enrichment indicates it is a product of Primary Nucleosynthesis, and Type I planetaries are probably significant in N enrichment of the ISM. Type II planetaries exhibit a He and N enrichment of a lesser magnitude than Type I; factors of 1.4 and 5, respectively. This lower enrichment indicates N is a product of Secondary Nucleosynthesis. The age of NGC 6822 is probably similar to that of the Milky Way Galaxy, about 1^1 years, whereas the Magellanic Clouds are younger, a few 1^9 years.
Chemiionization reactions involving rare gas metastable atoms
(1975) West, William Philip; Stebbings, Ronald F.
Measurements of the branching ratios for chemiionization of Ar, Kr, Xe, N2, CO^2, NO, CO, H2, C2, and N2 in collisions with rare gas metastable atoms are described. In addition measurements of absolute total ionization cross sections are discussed. A detailed analysis of the experimental procedure and the possible sources of error are given. The results are compared with previous experimental work. For the particular case of He(2^1S, 2^3S)-^2 collisions, a qualitative description of the processes involved which fits the observations is presented.
Classical study of collisions of Rydberg atoms with polar molecules
(1982) Preston, Steven Craig; Lane, Neal F.; Trammell, George T.; Stebbings, Ronald F.
Cross sections for state changing collisions of Rydberg atoms with polar molecules have been calculated using a classical trajectory Monte-Carlo technique. Although the dynamical problem was restricted to two dimensions, useful insights into the full three dimensional system were obtained. For example it was found that at thermal velocities the Rydberg electron can collide with the molecule many times during a angle trajectory. This implies that the impulse approximation, which is commonly used for many Rydberg collision problems, is not valid for this problem, since it cannot account for this effect. Finally, a finite effective range for the dipole interaction was observed, which can be used to make the calculation of three dimensional cross sections more tractable. Without these insights, the three dimensional calculation would have been much more difficult and costly.
Collisions involving XE(nf) Rydberg atoms: mℓ [enclosed by vertical lines]--changing and the influence of an external electric field
(1982) Slusher, Morgan Paul; Dunning, F. Barry; Stebbings, Ronald F.; Corcoran, Marjorie D.
Thermal energy collisions between xenon Rydberg atoms in n = 31, l > 3 states and CO2 are reported. The rate constant is derived for those collisions that result in a change in m^l. The value so obtained is similar to that obtained previously for Xe(nf) - CO2 collisions that result in a change in l. It is argued that these observations imply that collisions result in simultaneous changes in l and m^l. In addition, the influence of an applied electric field on state-changing in collisions between laser-excited Xe(nf) atoms and xenon target gas is described. The presence of the field- results in marked changes in both the rate constant for collisional state-changing and in the final state distribution. The changes can be accounted for by use of energy transfer arguments based on the "essentially-free" electron model, although the degree of spatial overlap between wavefunctions associated with the initial and final states may also be important.
Collisions of highly excited xenon atoms
(1976) Foltz, Greg William; Stebbings, Ronald F.
The collisional ionization of xenon high Rydberg atoms by various target gases is investigated. Ground state xenon atoms are excited by an electron impact-photoexcitation method to the selected high Rydberg states where n has a value between 25 and 4. Absolute rate constants are measured for the ionization of these atoms in collision under conditions of zero electric field and small electric fields. The experimental method is discussed and the results are compared with other experiments and with theory.
Collisions of highly excited xenon atoms with electron attaching targets
(1978) Hildebrandt, George Flavius; Stebbings, Ronald F.
This thesis reports absolute rate constants for the ionization of xenon atoms in well defined high Rydberg states by collision with a variety of electron attaching targets. Theoretical models view such collisions as being dominated by the interaction between the Rydberg electron and the attaching molecule, with the xenon ion core playing only a minor role. It is thus predicted that if the ion products of a collision between a Rydberg atom and an attaching molecule are identical to those resulting from a collision of a free electron with the same molecule, then the rate constant for collisional ionization of the high Rydberg atom colliding with an attaching molecule is the same as the rate constant for free electron attachment to the same molecule, providing the free electron has the same energy as the Rydberg electron, A time of flight technique has been employed to identify the charged products of the observed reactions in order to verify the applicability of the theory. The present results are compared with free electron attachment data, and the results in many cases confirm the theory.
Collisions of Xenon(nf) Rydberg atoms with electron attaching targets
(1984) Zollars, Byron George; Stebbings, Ronald F.; Dunning, F. Barry; Dodds, Stanley A.
The destruction of Xenon(nf) Rydberg atoms in collisions with 1,1,1-C2C13JF3 and C5F8 takes place by transfer of the Rydberg electron to the target, resulting in a positive and negative ion. Rate constants for this reaction have been measured for Xe(nf) over the range 26 < n < 43, and the negative ion products have been identified. The cross sections derived from these measurements are in good agreement with recent free electron attachment cross sections.
Cross sections for double ionization of atomic oxygen by electron impact
(1981) Ziegler, Daniel L.; Stebbings, Ronald F.; Dunning, F. Barry; Brooks, Phillip R.
Laboratory measurements of the cross sections for double ionization of atomic oxygen by electrons are presented for collision energies from threshold to 4eV. This process displays a maximum cross section of about .5 A^2 at a collision energy of approximately 2eV. These absolute cross sections were obtained from measurements of the ratio of the cross sections for double and single ionization, coupled with absolute cross sections for single ionization previously measured by other investigators. The configuration and operation of the apparatus for these measurements are described. Procedures used to assess the effects of oxygen excited states on the measurements are reported.
Differential change exchange cross sections for collisions of HE++ ions with H atoms
(1983) Day, Amelia; Lane, Neal F.; Stebbings, Ronald F.; Chamberlain, Joseph W.
Differential charge exchange cross sections are presented for ^4He^++ incident on an H(ls) atom for projectile energies of 3, 8, and 2 keV. A molecular state basis and straight line impact parameter formalism are used in the semiclassical eikonal approximation. Calculations were made with a ten state basis modified by Bates and McCarroll plane-wave translational factors and with an unmodified four state basis. Only cross sections corresponding to electron transfer into He+ (n = 2) states are included. Large and small angle oscillations are examined in terms of a diffractive scattering process. These results are consistent with other theoretical calculations.
Differential cross sections for neutral-neutral and charge-transfer collisions
(1985) Schafer, David Allen; Stebbings, Ronald F.; Dunning, F. Barry; Mutchler, Gordon S.
Absolute differential cross sections for neutral-neutral and charge—transfer collisions involving fast oxygen are presented. Neutral-neutral collisions are measured over the energy range 5-5eV, and between angles of .1 degrees and 5. degrees, while charge transfer measurements are presented for 15eV oxygen ions over the same angular range. A position sensitive detector is used to detect both the primary and scattered particles, yielding absolute differential cross sections. Target chambers with small ratios of length to exit aperture diameter are used and the end effects due to gas streaming out of the aperture are estimated by a Monte Carlo computer simulation. The excited state composition of the oxygen ion beam is considered, and differential cross sections for charge transfer are presented for both ground and excited-state oxygen ions.
Investigation of experimental problems encountered in measuring electron transfer cross sections using a merged beam apparatus
(1978) Cogan, John D.; Stebbings, Ronald F.
Attempts to measure the cross section for resonant charge transfer between hydrogen atoms and protons in the .1 eV to TOO eV energy range by the merged beams technique have been hampered by experimental problems. In particular, measured cross sections have been smaller than would be expected from theoretical treatments and other experiments. In addition, measurements at individual collision energies have not been reproducible. An investigation has revealed that these problems are caused by previously undiscovered physical effects. Correction of these difficulties has apparently paved the way for successful measurements of the charge transfer cross sections.
Penning ionization of atomic oxygen by helium (2¹S, 2³S) metastable atoms
(1974) Cook, Thomas Bratton; Stebbings, Ronald F.
Penning ionization of atomic oxygen is one of the important deexcitation processes for helium triplet metastable atoms in the atmosphere. Measurements of absolute Penning ionization cross sections for the reactions He(21S, 23S) + (3P) -> + + He + e are reported. Calculations of the atmospheric He(23S) concentration are discussed and the results of one such calculation, as recently reported in the literature, are presented.
Resonant charge transfer in hydrogen and helium at low collision energies
(1982) Newman, James Hansen; Stebbings, Ronald F.; Lane, Neal F.; Walters, G. King
Absolute charge-transfer cross-sections for collisions of protons with hydrogen and deuterium atoms have been measured within the energy range .1 eV to ISO eV using the merging-beams technique. The results are in excellent agreement with a fully quantum mechanical treatment of this reaction. In addition, cross sections for resonant charge transfer of helium ions and neutrals have been measured within the energy range 4 meV to 1 eV. These data show good agreement with the available theory. The experimental technique and apparatus are reviewed briefly and a discussion of the use and implementation of a CAMAC based microprocessor as an aid greatly facilitating experimental diagnostics is included.
State mixing in collisions of laser excited Rydberg atoms of xenon with ammonia
(1978) Kellert, Forrest Graham; Stebbings, Ronald F.
Evidence for the state mixing collision is given, where the principal quantum number n ranges' from 26 to 4. The laser induced Xe(nf) atoms transfer their excitation to one or more Xe(^nf) reservoirs during collisions with ammonia. Measurements of the collisional ionization in a 3.7 v/cm field reveal Xe+ ion signals at times which are long compared to the radiative lifetime of the laser induced state. From this data, the absolute collisional ionization rate constants for the Xe(j^nf) reservoirs are obtained. Simultaneous measurements of the number of Xe(nf) atoms excited by the laser allows one to derive absolute rate constants for the depopulation of Xe(nf) atoms due to state mixing and direct collisional ionization. A serial model for reservoir formation is used in conjunction with a computer data fitting routine to derive the above rate constants.
State-changing collisions between Xenon Rydberg atoms and neutral targets
(1982) Higgs, Charles; Stebbings, Ronald F.; Dunning, F. Barry; Walters, G. King
Thermal collisions between laser excited Xe(nf) Rydberg atoms and a variety of neutral atomic and molecular targets have been investigated. In Xe(nf)-HF collisions, n-changing, 4-changing and collisional ionization were observed. For n = 26 and 27, absolute collisional cross sections for these processes have been determined using time-resolved selective field ionization. In addition, the first evidence of molecular rotational excitation through the transfer of energy of electronic excitation from a Rydberg atom is presented. In Xe(nf) - Xe, Kr, Ar, CO2, CO and N2 collisions the dominant mechanism was observed to be 4-changing: no evidence of n-changing or collisional ionization was apparent. Absolute collisional cross sections have been determined for a range of n between 24 and 37. Comparison of these cross sections with those obtained using a theoretical model in which effects due to the Rydberg core are ignored appears to lead to satisfactory results only for Xe, Kr and CO2, whereas when core effects are included satisfactory agreement between experiment and theory is found in all cases.
State-changing in Rb(Ns, Np, Nd)-Xenon collisions
(1984) Goeller, Lawrence Nelson; Stebbings, Ronald F.; Dunning, F. Barry; Mutchler, Gordon S.
A crossed atomic beam/pulsed laser technique and electric field ionization are used to study Rb(nS,nP,nD)-Xe collisions for 27 =< n =< 7. Parent nS, nP, nD atoms are most often destroyed by mixing to the states of high-1 that lie nearest in energy to the parent level. The 1-mixed population is monitored at several specific times after laser fire and the data are fit to a. rate equation to determine the rate constants for state changing. At higher values of n, the data approach agreement with theoretical models involving the Born Approximation and information determined from free-electron scattering cross-sections. Discrepancies between theory and experiment at lower n can be partially explained by considering the relatively large energy separations between the initial and final states, although it may be necessary to consider the relation of initial- and final-state wavefunctions as well. Present results are compared to other experimental data.

Browsing by Author "Stebbings, Ronald F."

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