Femtosecond Photoelectron Spectroscopy PDF Download
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Author: Benjamin Jefferys Greenblatt
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Languages : en
Pages : 484
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Author: Benjamin Jefferys Greenblatt
Publisher:
ISBN:
Category :
Languages : en
Pages : 484
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Author: Oksana Graefe
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Category :
Languages : en
Pages : 171
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Author: Olof Johan Johansson
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Category :
Languages : en
Pages : 219
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An experimental apparatus has been constructed to investigate ionisation mechanisms of complex molecules and nanoparticles after femtosecond and/or picosecond laser excitation. The photoproducts are detected by time-of-flight mass spectrometry and velocity-map imaging (VMI) photoelectron spectroscopy. Test measurements on C60 and Xe have successfully reproduced previously published work indicating that the setup is working in a satisfactory manner. New detailed investigations of mass spectra and angle resolved photoelectron spectra (PES) have been carried out as a function of laser intensity, wavelength and pulse duration for C60 and C70, providing new insights into the electronic structure and ionisation mechanisms of these molecules. For 400 nm, 130 fs laser excitation, an isotropic contribution from thermally emitted electrons is found. A series of peaks are seen superimposed on the thermal background with binding energies in agreement with the recently discovered superatom molecular orbitals (SAMOs) of C60 [Feng et.al. Science 320 (2008) p. 359]. Furthermore, the angular dependence of the peak in the PES corresponding to the s-SAMO is in agreement with this assignment. To confirm the assignment of the other observed peaks it is concluded that the measured photoelectron angular distributions (PADs) need to be compared to calculated angular distributions. Measurements have also been made with the same wavelength but with a pulse duration of about 5 ps. Mass spectra, PES and PADs for these measurements show that the main ionisation mechanism for these laser conditions is delayed (thermionic) ionisation. For 800 nm, 130 and 180 fs laser excitation, thermally emitted electrons are observed. In contrast to the 400 nm measurements, the PADs show an asymmetry with higher apparent temperatures along the laser polarisation direction. Measurements were also made for longer pulse durations (1.0 - 3.8 ps). For pulse durations above 1 ps the asymmetry is gradually reduced while the delayed ionisation component in the mass spectrum increases with increasing pulse duration. The asymmetry is compared to calculations made assuming a field-assisted thermal electron emission. Similarly to the 400 nm experiments, a series of peaks are seen superimposed on the thermal background. PADs are presented for these peaks. PADs for peaks with the same binding energy as peaks seen in the 400 nm experiments follow the same trend. Isotropic PADs after ns laser excitation are also presented confirming delayed ionisation for these pulse durations.
Author: Matthew Parkin Blackwell
Publisher:
ISBN:
Category :
Languages : en
Pages : 204
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Author:
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Category :
Languages : en
Pages :
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A new experimental technique for the time-resolved study of anion reactions is presented. Using femtosecond laser pulses, which provide extremely fast ([approx] 100 fs) time resolution, in conjunction with photoelectron spectroscopy, which reveals differences between anion and neutral potential energy surfaces, a complex anion reaction can be followed from its inception through the formation of asymptotic products. Experimental data can be modeled quantitatively using established theoretical approaches, allowing for the refinement of potential energy surfaces as well as dynamical models. After a brief overview, a detailed account of the construction of the experimental apparatus is presented. Documentation of the data acquisition program is contained in the Appendix. The first experimental demonstration of the technique is then presented for I[sub 2][sup -] photodissociation, modeled using a simulation program which is also detailed in the Appendix. The investigation of I[sub 2][sup -] photodissociation in several size-selected I[sub 2][sup -](Ar)[sub n] (n= 6-20) and I[sub 2][sup -](CO[sub 2])[sub n] (n= 4-16) clusters forms the heart of the dissertation. In a series of chapters, the numerous effects of solvation on this fundamental bond-breaking reaction are explored, the most notable of which is the recombination of I[sub 2][sup -] on the ground[tilde X]([sup 2][Sigma][sub u][sup+]) state in sufficiently large clusters. Recombination and trapping of I[sub 2][sup -] on the excited[tilde A]([sup 2][Pi][sub 3/2,g]) state is also observed in both types of clusters. The studies have revealed electronic state transitions, the first step in recombination, on a[approx]500 fs to[approx]10 ps timescale. Accompanying the changes in electronic state is solvent reorganization, which occurs on a similar timescale. Over longer periods ([approx]1 ps to & gt;200 ps), energy is transferred from vibrationally excite d I[sub 2][sup -] to modes of the solvent, which in turn leads to solvent evaporation. These effects become more important as cluster size increases. In addition, differences in timescale and mechanism are observed between clusters of Ar, which binds to I[sup -] and I[sub 2][sup -] rather weakly, and CO[sub 2], whose large quadruple moment allows substantially stronger binding to these anions.
Author: Anja Röder
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Category :
Languages : en
Pages :
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Author: Peter Šiffalovič
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Category :
Languages : de
Pages : 104
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Author: Carolyn Patricia Schick
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Category :
Languages : en
Pages : 300
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Author: Annette Pietzsch
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ISBN: 9783899637991
Category :
Languages : en
Pages : 89
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Author: Martin Thomas Zanni
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ISBN:
Category :
Languages : en
Pages : 546
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