Electron collisions and internal excitation in stored molecular ion beams [Elektronische Ressource] / presented by Henrik Buhr

Sujets

• Physik

Informations

• Publié par: ruprecht-karls-universitat_heidelberg
• Publié le: 01 janvier 2006
• Nombre de lectures: 23
• Langue: Deutsch
• Poids de l'ouvrage: 2 Mo

Extrait

Dissertation
submitted to the
Combined Faculities for the Natural Sciences and for Mathematics
of the Ruperto-Carola University of Heidelberg, Germany
for the degree of
Doctor of Natural Sciences
presented by
Dipl.-Phys. Henrik Buhr
born in Heidelberg
Oral examination: July 26th, 2006Electron collisions and internal excitation in
stored molecular ion beams
Referees:
Prof. Dr. Andreas Wolf
Prof. Dr. Peter SchmelcherKurzfassung
Elektronenstoße und interne Anregung schneller gespeicherter Molekulionen¨ ¨
In Speicherring-Experimenten untersuchen wir Stoße¨ von Elektronen mit Molekulionen,¨ deren
+innere Anregung und, wie sich diese beiden Aspekte gegenseitig beeinflussen. Fur¨ He wer-
2
den besonders dissoziative Rekombination und superelastische Stoße¨ betrachtet. Der DR-
Ratenkoeffizient hangt¨ sehr stark von der Vibrationsanregung des Molekulions¨ vor dem Stoß
ab. Deshalb ist man durch die Messung des DR-Ratenkoeffizienten in der Lage, sehr kleine
+Ve randerungen¨ in der mittleren Vibrationsanregung von He durch Stoße¨ mit Elektronen oder
2
Restgas zu beobachten, wovon in dieser Arbeit Gebrauch gemacht wird.
+Der DR-Ratenkoeffizient von HD weist im Bereich niedriger Elektronenenergie zahlreiche
Resonanzen aus dem indirekten DR Prozess auf, die durch das Zusammenspiel von Rotation-
sleveln im Molekulion¨ und in Rydberg-Zustanden¨ des neutralen Molekuls¨ entstehen. Mit einem
hier neu entwickelten Verfahren zur hochauflosenden¨ Elektronenstoßspektroskopie konnen¨ die
Resonanzen im DR-Ratenkoeffizienten mit hoher Energieselektivitt gemessen werden. Dies er-
laubt den detailierten Vergleich mit den Ergebnissen einer MQDT-Rechnung und den Versuch,
die Resonanzen einzelnen intermediaren¨ Rydberg-Zustanden¨ zuzuordnen.
Abstract
Electron collisions and internal excitation in stored molecular ion beams
In storage ring experiments the role, which the initial internal excitation of a molecular ion
can play in electron collisions, and the effect of these collisions on the internal excitation are
investigated. Dissociative recombination (DR) and inelastic and super-elastic collisions are
+studied in the system of He . The DR rate coefficient at low energies depends strongly on the
2
initial vibrational excitation in this system. Therefore changes in the DR rate coefficient are a
+very sensitive probe for changes in the vibrational excitation in He , which is used to investigate
2
the effects of collisions with electrons and residual gas species.
+The low-energy DR of HD is rich with resonances from the indirect DR process, when cer-
tain initial rotational levels in the molecular ion are coupled to levels in neutral Rydberg states
lying below the ion state. Using new procedures for high-resolution electron-ion collision spec-
troscopy developed here, these resonances in the DR cross section can be measured with high
energy sensitivity. This allows a detailed comparison with results of a MQDT calculation in an
effort to assign some or all of the resonances to certain intermediate Rydberg levels.Contents
1 Introduction 1
2 Electron-molecular ion reactions at small energies 5
2.1 Born-Oppenheimer approximation and potential energy surfaces . . . . . . . . 5
2.2 Molecule-electron reactions . ........................... 6
2.2.1 Dissociative recombination . ....................... 6
2.2.2 More electron-molecular ion reactions . . ................ 8
2.3 Collisions of fast molecular ions with residual gas species . . . ......... 10
+2.4 Franck-Condon suppressed direct DR in He .................. 10
2
+2.4.1 Previous studies of the DR of He .................... 1
2
+2.4.2 The latest theoretical treatments of He DR............... 13
2
2.4.3 Experimental goals . ........................... 14
+2.5 HD , the indirect process and low energy rotational resonances......... 15
+2.5.1 Earlier experimental results on the low-energy DR of HD ....... 15
+2.5.2 Other electron collisions studied with HD ............... 17
2.5.3 A simple model to predict the positions of resonances . ......... 18
3 Experimental method 27
3.1 Ion production, acceleration and storage . . . . . ................ 27
3.2 Vibrational excitation diagnostics . . ....................... 29
3.2.1 Principles of foil-induced Coulomb Explosion Imaging ......... 29
3.2.2 The CEI Setup . . . ........................... 31
3.3 Standard collision experiments at the TSR . . . . ................ 3
3.3.1 Ion beam preparation and temperature of a merged electron beam . . . 33
3.3.2 Detectors at the TSR electron cooler . . . ................ 34
3.3.3 Energy dependent collision measurements with the electron cooler . . . 35
3.4 A new arrangement for high resolution collision experiments . ......... 37
IContents
3.4.1 The ultra-cold electron beam device . . ................. 37
3.4.2 A beamline for advanced molecular breakup imaging (BAMBI) . . . . 39
3.4.3 New fragment detector station . . . . . ................. 40
3.4.4 The new data acquisition system . . . . ................. 42
+4 Franck-Condon suppressed dissociative recombination in He 45
2
+4.1 Vibrational excitation as a function of time in He – CEI results . . ...... 45
2
3 4 +4.1.1 The infrared-active, heteronuclear He He ............... 46
+
44.1.2 The homonuclear He .......................... 47
2
4.2 Absolute DR rate coefficients at high electron energy . . . . .......... 49
4.2.1 A method for an absolute normalization without measuring the ion current 50
+4.2.2 Absolute rate coefficients of both He species . . . . .......... 53
2
4.2.3 Comparison to other experiments and to theory . . . .......... 57
4.3 Rovibrational cooling by electron collisions . . ................. 57
4 +4.3.1 Time dependence of the DR rate coefficient of He .......... 57
2
v4.3.2 Vibrational state dependent DR rate coefficients α (0eV ) and SEC inDR
+
4He ................................... 59
2
3 4 +4.3.3 Time dependent DR rate coefficient of He He at 0 eV – rotational SEC 62
4.4 Rovibrational excitation by electron collisions . ................. 63
4.5 Ro e by residual gas collisions . . . . . . .......... 6
+4.6 Energy dependent DR rate coefficients of He ................. 68
2
+
44.6.1 Time dependence of α (E ) in He ................. 68DR d
2
+4.6.2 High energy structures in the DR rate coefficient of He ........ 70
2
4.7 Summary . . . . . . ............................... 72
+5 Rotational resonances in the recombination of HD 75
5.1 Measurements with two electron beam devices ................. 75
5.1.1 Ion beam dragging with one and two electron beams .......... 75
5.1.2 Stability of the relative electron beam velocity . . . . .......... 7
5.2 Improved energy resolution with electron target and photocathode . ...... 79
5.3 Comparison to MQDT . . . . . . ........................ 83
+5.4 Time dependence in HD DR .......................... 85
5.5 Summary . . . . . . ............................... 90
6 Summary and outlook 91
IIContents
+6.1 He as a probe for rovibrational excitation . . . . ................ 91
2
+6.2 Improvement of the energy resolution seen with DR of HD .......... 92
6.3 Opportunities with the new system . ....................... 92
6.4 Future experiments . . . . . ........................... 93
References 95
IIIIV