Energy-resolved positron-molecule annihilation

136 Pages·2005·1.52 MB·English

UNIVERSITY OF CALIFORNIA, SAN DIEGO
Energy-resolved positron-molecule annihilation:
Vibrational Feshbach resonances and bound states
A dissertation submitted in partial satisfaction of the requirements for
the degree of Doctor of Philosophy in Physics
by
Levi Daniel Barnes
Committee in charge:
Clifford M. Surko, Chair
Lu Sham
Dmitri Basov
Charles Perrin
Robert Continetti
2005
Copyright
Levi Daniel Barnes, 2005
All rights reserved.
The dissertation of Levi Daniel Barnes is approved and it is acceptable in
quality and form for publication on microfilm.
Chairman
University of California, San Diego
2005
iii
iv
Contents
Signature Page . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii
Table of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi
Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xii
Vita, Publications and Fields of Study . . . . . . . . . . . . . . . . . . xiii
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvi
1 Introduction 1
1.1 Early positron research. . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Technological applications of positrons . . . . . . . . . . . . . . . 2
1.3 Interaction of positrons with gas phase atoms and molecules . . . 3
1.4 Outline of the dissertation . . . . . . . . . . . . . . . . . . . . . . 4
2 Previous Positron Annihilation Measurements 7
2.1 Challenges in measuring direct annihilation . . . . . . . . . . . . 7
2.2 The meaning of Z . . . . . . . . . . . . . . . . . . . . . . . . . 8
eff
2.3 Early measurements of Z . . . . . . . . . . . . . . . . . . . . . 8
eff
2.4 Models of positron annihilation . . . . . . . . . . . . . . . . . . . 9
2.5 Further investigation of positron annihilation rates . . . . . . . . 10
2.6 Measurements of Z using Penning traps . . . . . . . . . . . . . 11
eff
2.7 Fluorine substitution and its effect on Z . . . . . . . . . . . . . 12
eff
2.8 Doppler-broadening studies of positron annihilation. . . . . . . . 14
3 Description of the Experiment 17
3.1 Source and moderator . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2 Buffer gas trapping and cooling . . . . . . . . . . . . . . . . . . . 19
3.3 Positron beam production and characterization . . . . . . . . . . 22
3.4 Test gas delivery and measurement . . . . . . . . . . . . . . . . . 26
3.5 Adjusting beam energy. . . . . . . . . . . . . . . . . . . . . . . . 27
v
3.6 Radiation detection. . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.7 Reducing background counts . . . . . . . . . . . . . . . . . . . . 29
3.8 Computation of Z . . . . . . . . . . . . . . . . . . . . . . . . . 31
eff
3.9 Verifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.10 Error analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.10.1 Error in the magnitude of Z . . . . . . . . . . . . . . . 32
eff
3.10.2 Error in positron energy . . . . . . . . . . . . . . . . . . . 33
4 Vibrational Feshbach Resonances in the Z Spectra for Alkane
eff
Molecules 35
4.1 Energy-resolvedZ forbutane: anexampleofresonantenhance-
eff
ment of annihilation . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.2 Comparison with capture resonances in electron scattering . . . . 39
4.3 The Feshbach resonance picture for positron annihilation . . . . 41
4.4 Feshbach resonance analysis of the butane spectrum . . . . . . . 42
4.5 Increasing binding energy for large alkanes . . . . . . . . . . . . . 46
4.6 Additional deuterated alkanes . . . . . . . . . . . . . . . . . . . . 51
4.7 Shape of the CH stretch resonance peak . . . . . . . . . . . . . . 53
4.8 Factors affecting the magnitude of resonance features in Z . . 55
eff
4.8.1 Estimating Γ . . . . . . . . . . . . . . . . . . . . . . . . 56
cap
4.9 Z below 50 meV . . . . . . . . . . . . . . . . . . . . . . . . . . 61
eff
4.10 Effects of fluorine substitution on Z . . . . . . . . . . . . . . . 63
eff
4.10.1 Energy-resolved Z spectra for fluoro-alkanes . . . . . . 64
eff
4.10.2 Doppler-broadening of annihilation radiation for fluoro-
alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
4.11 Effects of double and triple bonds on Z . . . . . . . . . . . . . 67
eff
4.12 Molecular isomers . . . . . . . . . . . . . . . . . . . . . . . . . . 69
4.13 Z for ring hydrocarbons . . . . . . . . . . . . . . . . . . . . . . 69
eff
5 Annihilation for Smaller Molecules 75
5.1 Z for methane . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
eff
5.2 Z at very low positron energy. . . . . . . . . . . . . . . . . . . 77
eff
5.3 Fluoromethanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
5.4 Ammonia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5.5 Vibrational modes of the positron-molecule complex . . . . . . . 83
5.6 Z at higher positron energies . . . . . . . . . . . . . . . . . . . 86
eff
5.7 Z for noble gases . . . . . . . . . . . . . . . . . . . . . . . . . . 87
eff
vi
6 Progress in the Theory of Positron Annihilation on Molecules 89
6.1 Basic vibrational Feshbach resonance theory . . . . . . . . . . . . 89
6.2 Static calculations . . . . . . . . . . . . . . . . . . . . . . . . . . 90
6.2.1 Configuration interaction ground state calculations . . . . 90
6.2.2 Density functional models . . . . . . . . . . . . . . . . . . 92
6.2.3 Schwinger multichannel method. . . . . . . . . . . . . . . 92
6.3 Incorporating nuclear motion . . . . . . . . . . . . . . . . . . . . 94
6.4 Direct annihilation near the positronium threshold . . . . . . . . 97
7 Additional Observations 101
7.1 Annihilation for very large alkanes–second positron bound state? 101
7.2 Incorporating positron escape by
de-excitation of thermally excited vibrations . . . . . . . . . . . . 104
8 Summary and Concluding Remarks 107
8.1 Summary of results . . . . . . . . . . . . . . . . . . . . . . . . . . 107
8.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
References 112
vii
viii
List of Figures
2.1 Z compared with Z for thermal positron distributions . . . . . 13
eff
3.1 Drawing of the positron source and moderator . . . . . . . . . . 18
3.2 Drawing of the buffer-gas trap. . . . . . . . . . . . . . . . . . . . 20
3.3 Extracting a beam of cold positrons . . . . . . . . . . . . . . . . 23
3.4 Analyzing the energy distribution of the positron beam . . . . . 24
3.5 Drawing of gas region, detector and shielding . . . . . . . . . . . 25
3.6 Detection efficiency of CsI detector . . . . . . . . . . . . . . . . . 28
4.1 Z for butane (C H ) . . . . . . . . . . . . . . . . . . . . . . . 36
eff 4 10
4.2 Pressure dependence of the annihilation signal . . . . . . . . . . 37
4.3 Z for butane-d (C D ) . . . . . . . . . . . . . . . . . . . . . 38
eff 10 4 10
4.4 FeshbachresonanceindissociativeattachmentofCH Ibyelectron
3
impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
4.5 Potential energy space picture of a Feshbach resonance . . . . . . 43
4.6 ComparisonoftheZ spectrumandvibrationalmodesforbutane 45
eff
4.7 Z for the alkane molecules I . . . . . . . . . . . . . . . . . . . . 47
eff
4.7 Z for the alkane molecules II . . . . . . . . . . . . . . . . . . . 48
eff
4.7 Z for the alkane molecules III . . . . . . . . . . . . . . . . . . . 49
eff
4.8 Positron affinity for the alkane molecules . . . . . . . . . . . . . . 50
4.9 Z for nonane-d (C D ) . . . . . . . . . . . . . . . . . . . . . 52
eff 20 9 20
4.10 Comparison of the shape of the resonance peaks for propane
(C H ) and heptane (C H ) . . . . . . . . . . . . . . . . . . . . 54
3 8 7 16
4.11 Comparison of the Z and IR absorption spectra for propane . 59
eff
4.12 Comparison of energy-resolved Z to Z for thermal distribu-
eff eff
tions of positrons . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
4.13 Z for fluorohexane (C H F) and fluorononane (C H F) . . . 65
eff 6 13 9 19
4.14 Infrared absorption spectra for hexane and 1-fluorohexane . . . . 66
4.15 Z for ethane, ethylene and acetylene . . . . . . . . . . . . . . . 68
eff
4.16 Z for 2-methylbutane (isopentane) and pentane . . . . . . . . 70
eff
ix
4.17 Z for benzene (C H ) and cyclohexane (C H ). . . . . . . . . 71
eff 6 6 6 12
4.18 Computed infrared spectrum for hexane, cyclohexane and benzene 73
4.19 Three-dimensional rendering of benzene, cyclohexane and hexane
molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
5.1 Z for methane (CH ) . . . . . . . . . . . . . . . . . . . . . . . 76
eff 4
5.2 Z for methane (CH ) and carbon tetrafluoride (CF ) . . . . . 78
eff 4 4
5.3 Z for fluorine-substituted methane molecules . . . . . . . . . . 80
eff
5.4 Z for ammonia (NH ) . . . . . . . . . . . . . . . . . . . . . . . 82
eff 3
5.5 Z spectrum for ethane (C H ). . . . . . . . . . . . . . . . . . . 84
eff 2 6
5.6 Z and IR absorption for methyl fluoride (CH F) . . . . . . . . 85
eff 3
5.7 Energy-resolved Z for argon and xenon. . . . . . . . . . . . . . 88
eff
6.1 Density of vibrational modes in the alkanes . . . . . . . . . . . . 91
6.2 Z for acetylene as a function of CH bond length (calculation) . 93
eff
6.3 Z forethyleneasafunctionincidentpositronenergy(calculation) 95
eff
6.4 Z for Kr (calculation) . . . . . . . . . . . . . . . . . . . . . . 96
eff 2
6.5 Annihilationcrosssectionnearpositroniumthreshold(calculation) 98
7.1 Z for dodecane (C H ) . . . . . . . . . . . . . . . . . . . . . 102
eff 12 26
7.2 Z for tetradecane (C H ) . . . . . . . . . . . . . . . . . . . . 103
eff 14 30
7.3 Magnitude of resonances in alkanes compared with positron bind-
ing energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
x