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Three Dimensional Coherent Photoemission Spectroscopy

Wang, Cong (2017) Three Dimensional Coherent Photoemission Spectroscopy. Doctoral Dissertation, University of Pittsburgh. (Unpublished)

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We integrate interferometric time-delay scanning with angle/momentum resolved photoemission spectroscopy to enable three-dimensional coherent electronic spectroscopy. We apply the method to study optical response of clean and alkali atom modified metal surfaces. The interferometric measurements probe the coherent optical response, which are embodied in coherent polarization oscillations. By Fourier transforming the coherent response, we obtain 2D electronic spectra, which map the dynamical relationship between the coherent polarization in the sample and the multiphoton photoemission spectrum. Also, we apply Optical Bloch Equations as a framework for simulating the coherent polarization dynamics in multiphoton photoemission processes.

With wide-range-wavelength-tunable ultrashort pulse laser, we have found several interesting phenomena. Though bound states like excitons cannot exist in metals due to the strong screening of Coulomb interaction, we find the first experimental evidence of a transient exciton, which exists on Ag(111) surface before screening fully established. Furthermore, the interferometric measurements show that an internal polarization field established by the two-photon resonance between the Shockley (SS) and image potential (IP) surface states dominates the surface response, rather than the external laser field.

On Cu(111), we observe abnormal line shapes of SS state, when laser spectrum overlaps with interband transitions from the top d-bands to the sp-band crossing with the Fermi level. This interband transition creates a polarization field, which builds up with time to sufficient strength to co-excite photoelectrons with external laser field. The same interband polarization causes distortions of the IP state on Cu(100) when the IP state can mediate the multiphoton photoemission process. The energy dependence of hole lifetimes causes the asymmetry of IP state line shape.

Lastly, we model coherent surface processes in multi-electron transitions that are excited in three-photon photoemission via the valence ns state of chemisorbed alkali atoms on Cu(111) surface. The two-photon excitation of the ns state causes coherent beating at fractional photon frequencies that results from screening-induced multi-electron transitions. The simulation reproduces the experiment in a model where the Coulomb interaction creates local polarization fields associated with transition from the SS state to both the ns state and the Fermi level.


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Item Type: University of Pittsburgh ETD
Status: Unpublished
CreatorsEmailPitt UsernameORCID
Wang, Congcow9@pitt.educow9
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Committee ChairPetek,
Committee MemberSnoke,
Committee MemberBoyanovsky,
Committee MemberHan,
Committee MemberGarrett-Roe,
Date: 2 July 2017
Date Type: Publication
Defense Date: 31 March 2017
Approval Date: 2 July 2017
Submission Date: 10 April 2017
Access Restriction: 1 year -- Restrict access to University of Pittsburgh for a period of 1 year.
Number of Pages: 164
Institution: University of Pittsburgh
Schools and Programs: Dietrich School of Arts and Sciences > Physics
Degree: PhD - Doctor of Philosophy
Thesis Type: Doctoral Dissertation
Refereed: Yes
Uncontrolled Keywords: Three Dimensional Coherent Photoemission Spectroscopy
Date Deposited: 02 Jul 2017 19:27
Last Modified: 02 Jul 2018 05:15


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