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Instrumentation for particle-beams research using laser-excited merging beams. Final report, 25 June 1983-24 December 1984

R. H. Neynaber, Suo Tang

Published Feb 21, 1985• 0 citations

Abstract

A summary of research performed under AFOSR Grant No. 83-0242 is given. The review describes molecular-beam studies of ion-pair production and measurements of the fraction of excited Na atoms, F*, in a composite beam of ground-state and excited Na atoms. Molecular beam techniques will be employed to study two-body reactions that will result in an intense, collimated beam of Li atoms at an energy of several hundred keV. Generating such a beam by charge transfer of a Li/sup +/ beam is not efficient at this energy. It appears that it can most efficiently be produced by forming a Li/sup -/ beam and stripping away valence electrons in a suitable gas. Plans are to study reactions associated with the production of Li/sup -/ beam which is formed when a Li/sup +/ beam interacts with a vapor of ground and excited (resonance state) Na atoms. One of the reactions (after Li/sup +/ has been converted to Li) is Li + Na yield Li/sup -/ + Na(+). This will be examined by merging a Li beam with a beam of Na atoms excited with a single frequency dye laser. The instrumentation required to complete this study consists of some laser and optical components,more » some vacuum equipment, and a laboratory computer system for data acquisition and analysis. This apparatus will supplement as well as rejuvenate an existing merging-beam apparatus.« less

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Physical Review A•Mar 12, 2021•17 citations

A summary of research performed under AFOSR Grant No. 83-0242 is given. The review describes molecular-beam studies of ion-pair production and measurements of the fraction of excited Na atoms, F*, in a composite beam of ground-state and excited Na atoms. Molecular beam techniques will be employed to study two-body reactions that will result in an intense, collimated beam of Li atoms at an energy of several hundred keV. Generating such a beam by charge transfer of a Li/sup +/ beam is not efficient at this energy. It appears that it can most efficiently be produced by forming a Li/sup -/ beam and stripping away valence electrons in a suitable gas. Plans are to study reactions associated with the production of Li/sup -/ beam which is formed when a Li/sup +/ beam interacts with a vapor of ground and excited (resonance state) Na atoms. One of the reactions (after Li/sup +/ has been converted to Li) is Li + Na yield Li/sup -/ + Na(+). This will be examined by merging a Li beam with a beam of Na atoms excited with a single frequency dye laser. The instrumentation required to complete this study consists of some laser and optical components,more » some vacuum equipment, and a laboratory computer system for data acquisition and analysis. This apparatus will supplement as well as rejuvenate an existing merging-beam apparatus.« less

A summary of research performed under AFOSR Grant No. 83-0242 is given. The review describes molecular-beam studies of ion-pair production and measurements of the fraction of excited Na atoms, f*, in a composite beam of ground-state and excited Na atoms. The actual ion-pair production reactions studied are the following: Na + Br yield Na(+) + B(-), Na + I2 yield Na(+) + I2(-), Na(Na*) + Na yield Na(+) + Na(-), and Li + Na yield Li(-) + Na(+). The reaction Na + I2 yield Na(+) + I2(-) was used to measure f*. A value of about 6% was obtained. The work was performed using instrumentation provided by the grant.

In studying ionization reactions in a single dense effusive beam of laser-excited Na(3p) atoms, we observe production of both Na{sub 2}{sup +} and Na{sup +} ions. The Na{sup +} results from the familiar associative ionization reaction of two Na(3p) atoms and the Na{sup +} ions under our conditions result mostly from the collisional dissociation of the Na{sub 2}{sup +} by reaction with a third excited Na(3p) atom, rather than from photo- dissociation or photoionization. A CW, single-frequency dye laser tuned to 589 nm resonantly excites the Na(3s) atoms to the 3p state. The atomic beam Na(3s) density is determined from low-intensity 589 absorption measurements. Na{sup +} ion yield depends on the cube of the Na beam density, suggesting successive collisions of three Na(3p) atoms: we believe the Na{sub 2}{sup +} ions polarize the third Na(3p) atom, resulting in a R{sup -4} orbiting collision interaction. A large collisional dissociation cross-section of approximately (4{plus_minus}2)x10{sup -13} cm{sup 2} results, for oven temperatures in the 500-600K range.

An investigation of the 'Hornbeck-Molnar ionization' process between laser-excited Cs and ground state Na atoms has been performed in a crossed beam apparatus. Caesium atoms in the 8d, 10s, 9d, 11s and 10d levels respectively have been obtained by two-step resonant CW laser excitation. The relative collisional cross sections and the dissociation energy of the NaCs + molecular ion have been obtained from quantitative analysis of the produced atomic and molecular ions. The experimental results are compared with the theoretical values calculated using a model developed by Janev and Mihajlov (1980).

Merging two subfields, collisions and spectroscopy, this book presents the latest research proposals, innovative experiments and fresh ideas. Topics considered include radiative collision induced electron continuum-continuum scattering; models of laser-induced chemical reactions; experimental studies of laser-induced ionizing collisions; laser photodissociation of Na/sub 2/ + ions produced by laser resonant ionization of Na vapor; steady-state electron-atom collision in a laser field; conditions for the observation of the predicted splitting of the laser assisted electron impact of the differential cross section of atoms; transitions between resonances; some proposals for experiments; above-threshold multiphoton ionization; observability of optical collisions in the frequency and the time domain; laser switched collisions - a diagrammatical approach; polarization of atomic fluorescence excited by molecular photodissociation; multiphoton selective excitation; on the role of chaotic dynamics with many basins of attraction; predissociation in a strong electromagnetic field; theory of double resonance; and coherent excitation of polyatomic molecules up to highly excited vibrational states by laser light.

The present paper reports on a merged-beam experiment on mutual neutralization between

{\mathrm{Na}}^{+}

and ${\mathrm{D}}^{\ensuremath{-}}$. For this experiment, we have used the DESIREE ion-beams storage-ring facility. The reaction products are detected using a position- and time-sensitive detector, which ideally allows for determination of the population of each individual quantum state in the final atomic systems. Here, the

4s, 3d

, and

4p

final states in Na are observed and in all cases the D atom is in its ground state

1s ^{2}S

. The respective branching fractions of the states populated in Na are determined by fitting results from a Monte Carlo simulation of the experiment to the measured data. The center-of-mass collision energy is controlled using a set of biased drift tubes, and the branching fractions are measured for energies between 80 meV and 393 meV. The resulting branching fractions are found to agree qualitatively with the only available theoretical calculations for this particular system, which are based on a multichannel Landau-Zener approach using dynamic couplings determined with a linear combination of atomic orbitals model.