FU Berlin Digitale Dissertation
Axel Scholz : Realtime Studies at Sodium-Ammonia-Clusters Echtzeit-Untersuchungen an Natrium-Ammoniak-Clustern
|Abstract| |Table of Contents| |More Information|

Abstract To measure transient absorption and ionization of Na(Ammonia)n-clusters different femtosecond spectrometer in the visible (390-410 nm, 500-600 nm) and near infrared (1200-2000 nm) spectral range were realized. Therefor the wavelength of an amplified Ti:Sapp.-lasersystem is converted with different three wave interaction technics (SHG, SFG, OPG/OPA) in nonlinear crystals. The clusters are produced in a pick-up source, where a neat sodium beam is crossed with a supersonic ammonia beam. In the cross section the heteroclusters are formed. The clusters are excited with a pump-pulse and, after a certain delay, ionized with a probe-pulse. The mass separation and detection is obtained with a time of flight mass spectrometer. By varying the delay time, the lifetimes of the prepared states can be observed. With this experimental setup it was possible to determine and study the lifetimes of the A-state of Na(NH3)n to a size of n = 20. The lifetimes decrease drastically with increasing the clustersize, Na(NH3) has a lifetime of 1,1 ns, while the lifetime of Na(NH3)8 is only 150 fs. The lifetimes for 10 < n < 20 has 120 fs and does not change further. From n = 3 to n = 4 an increase in the lifetimes shows the closing of the first solvation shell at n = 4 with a higher geometry. In Na(NH3)3 the coupling of the electronic clusterabsorption and the vibrational absorption can be seen in the pump-probe signal. Excitation in the spectral overlap of both absorptions show a double exponential decay, 1 ps corresponding to the electronic excitation, 18 ps the vibrational absorption. We assume the internal conversion from the A => X state with a reduced Franck-Condon overlap to the ionic state, to be responsible for the decay of the Pump-Probe signal. Deuteration of the clusters leads to an extension of the lifetime. The isotopic effect has its maximum at n = 2 with a factor of about 75 and decreases with increasing n. This effect can be understood with the "Energy Gap Law", that describes the decrease of the transition probability with the increase of the vibrational quanta. Beside the one photon excitation, there occurs also a resonant two photon excitation for small cluster (n = 1, n = 3) with a subsequent fragmentation of the cluster by severing one NH3, ND3 respectively.

Table of Contents Download the whole PhDthesis as a zip-tar file or as zip-File For download in PDF format click the chapter title Titelseite und Literaturverzeichnis 1 Einleitung 13 2 Solvatisierte Elektronen, solvatisierte Ionen, Stand der Forschung 17 2.1 Solvatisierte Elektronen in der Lösung 18 2.2 Solvatation in Clustern 19 3 Dreiwellen-Mischung, Grundlagen der Frequenzkonversion 31 3.1 Nichtlineare Wechselwirkung von Licht mit Materie 32 3.2 Optik nichtlinearer Kristalle 37 4 Meßprinzip, Pump-Probe Spektroskopie 45 4.1 Spektroskopische Meß methoden 45 4.2 Pump-Probe Signalauswertung 49 5 Experimenteller Aufbau 53 5.1 Das Lasersystem 56 5.2 Die optischen Aufbauten 63 5.3 Die Molekularstrahlapparatur 83 5.4 Steuerung des Experiments, Datenaufnahme 90 6 Experimentelle Ergebnisse, Dynamik des A-Zustands 93 6.1 Abhängigkeit der Lebensdauer des A-Zustands von der Clustergröße 96 6.2 Abhängigkeit der Lebensdauer von der Anregungsenergie 100 6.3 Isotopeneffekte in der Lebensdauer 109 6.4 Zusammenfassung und Diskussion 121 7 Resonante Zweiphotonen-Anregung kleiner Na(Ammoniak)n-Cluster 129 7.1 Zweiphotonen-Anregung von Na(NH3) 129 7.2 Zweiphotonen-Anregung von Na(Ammoniak)3 133 7.3 Zusammenfassung 140 8 Zusammenfassung und Ausblick 143 Anhang A 147 A.1 Beta-Barium-Borat (BBO) 147 A.2 Lithium-Triborat (LBO) 148

More Information:
Online available:	http://www.diss.fu-berlin.de/1999/5/indexe.html
Language of PhDThesis:	german
Keywords:	Clusterphysics, fs-Spectroscopy, Femto-Chemistry, OPG/OPA, SFG
DNB-Sachgruppe:	29 Physik, Astronomie
Classification PACS:	31.70.Hq, 36.40.Qv, 39.10.+j, 39.30.+w, 42.65.+k, 42.65.Yj, 42.79.Nr
Date of disputation:	25-Nov-1998
PhDThesis from:	Fachbereich Physik, Freie Universität Berlin
First Referee:	Prof. Dr. Ingolf V. Hertel
Second Referee:	Prof. Dr. Dietmar Stehlik
Contact (Author):	ascholz@mbi-berlin.de
Contact (Advisor):	hertel@mbi-berlin.de
Date created:	30-Jan-1999
Date available:	10-Feb-1999

 

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