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Time-resolved '''two-photon photoelectron''' ('''2PPE''') spectroscopy is a [[time-resolved spectroscopy]] technique which is used to study [[electronic structure]] and electronic excitations at [[surface science|surfaces]]. The technique utilizes femtosecond to picosecond [[laser pulse]]s in order to first [[photoexcitation|photoexcite]] an electron. After a time delay, the excited electron is [[photoemission|photoemitted]] into a [[Free electron model|free electron]] state by a second pulse, as shown in Figure 1. The [[kinetic energy]] and the emission angle of the photoelectron are measured in an electron analyzer. To facilitate investigations on the population and relaxation pathways of the excitation, this measurement is performed at different time delays.
[[File:Time-Resolved Two Photon Photoemission Schematic.png|thumb|A lower energy pump pulse photoexcites an electron in a [[ground state]] or [[HOMO and LUMO|HOMO]] into a higher lying [[excited state]]. After a time delay, a second, higher energy pulse photoemits the excited electron into free electron states above the [[vacuum level]].]]Time-resolved '''two-photon photoelectron''' ('''2PPE''') spectroscopy is a [[time-resolved spectroscopy]] technique which is used to study [[electronic structure]] and electronic excitations at [[surface science|surfaces]].<ref name="Weinelt2002">{{cite journal|last1=Weinelt|first1=Martin|title=Time-resolved two-photon photoemission from metal surfaces|journal=Journal of Physics: Condensed Matter|volume=14|issue=43|year=2002|pages=R1099–R1141|issn=0953-8984|doi=10.1088/0953-8984/14/43/202}}</ref> The technique utilizes femtosecond to picosecond [[laser pulse]]s in order to first [[photoexcitation|photoexcite]] an electron. After a time delay, the excited electron is [[photoemission|photoemitted]] into a [[Free electron model|free electron]] state by a second pulse. The [[kinetic energy]] and the emission angle of the photoelectron are measured in an [[electron energy analyzer]]. To facilitate investigations on the population and relaxation pathways of the excitation, this measurement is performed at different time delays.


This technique has been used for many different types of materials to study a variety of exotic electron behaviors, including image potential states at metal surfaces,<ref>Fauster, Thomas, and Wulf Steinmann. "Two-photon photoemission spectroscopy of image states." Electromagnetic Waves: Recent Developments in Research 2 (1995): 347-411.</ref><ref>Weinelt, Martin. "Time-resolved two-photon photoemission from metal surfaces." Journal of Physics: Condensed Matter 14.43 (2002): R1099.</ref> and even electron dynamics at [[molecular]] interfaces.<ref>Zhu, X-Y. "Electron transfer at molecule-metal interfaces: a two-photon photoemission study." Annual Review of Physical Chemistry 53.1 (2002): 221-247.</ref>
This technique has been used for many different types of materials to study a variety of exotic electron behaviors, including image potential states at metal surfaces,<ref>Fauster, Thomas, and Wulf Steinmann. "Two-photon photoemission spectroscopy of image states." Electromagnetic Waves: Recent Developments in Research 2 (1995): 347-411.</ref><ref>Weinelt, Martin. "Time-resolved two-photon photoemission from metal surfaces." Journal of Physics: Condensed Matter 14.43 (2002): R1099.</ref> and even electron dynamics at [[molecular]] interfaces.<ref>Zhu, X-Y. "Electron transfer at molecule-metal interfaces: a two-photon photoemission study." Annual Review of Physical Chemistry 53.1 (2002): 221-247.</ref>
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where the E<sub>B</sub> is the binding energy of the initial state, E<sub>kin</sub> is the kinetic energy of the photoemitted electron, Φ is the [[work function]] of the material in question, and E<sub>pump</sub>, E<sub>probe</sub> are the [[photon energy|photon energies]] of the laser pulses, respectively. Without a time delay, this [[equation]] is exact. However, as the delay between the [[Pump–probe spectroscopy|pump and probe]] pulses increases, the excited electron may relax in an energy. Hence the energy of the photoemitted electron is lowered. With large enough time delay between the two pulses, the electron will relax all the way back to its original state. The timescales at which the electronic relaxation occurs, as well as the relaxation mechanism (either via [[vibronic coupling]] or electronic [[coupling (physics)|coupling]]) is of interest for applications of functional devices such as [[solar cells]] and [[light-emitting diode]]s.
where the E<sub>B</sub> is the binding energy of the initial state, E<sub>kin</sub> is the kinetic energy of the photoemitted electron, Φ is the [[work function]] of the material in question, and E<sub>pump</sub>, E<sub>probe</sub> are the [[photon energy|photon energies]] of the laser pulses, respectively. Without a time delay, this [[equation]] is exact. However, as the delay between the [[Pump–probe spectroscopy|pump and probe]] pulses increases, the excited electron may relax in an energy. Hence the energy of the photoemitted electron is lowered. With large enough time delay between the two pulses, the electron will relax all the way back to its original state. The timescales at which the electronic relaxation occurs, as well as the relaxation mechanism (either via [[vibronic coupling]] or electronic [[coupling (physics)|coupling]]) is of interest for applications of functional devices such as [[solar cells]] and [[light-emitting diode]]s.


== Experimental setup ==
== Experimental configuration ==
⚫
[[File:Schematic 2PPE-setup pdf.pdf|thumb|Setup (schematic) for two-photon photoemission experiments|thumb|A laser pulse is first split using a [[beam splitter]] into two different laser lines. One laser line is used to create its second harmonic, giving it a higher photon energy which will serve as the probe pulse. The other laser line passes through a delay stage, which allows the experimenter to vary the delay between the laser pulses impinging on the sample.]]

Time-resolved two-photon [[photoemission spectroscopy|photoelectron spectroscopy]] usually employs a combination of ultrafast [[optical technology]] as well as ultrahigh vacuum components. The main optical component is an ultrafast (femtosecond) laser system which generates pulses in the near infrared. [[Nonlinear optics]] are used to generate photon energies in the visible and ultraviolet spectral range. Typically, ultraviolet radiation is required to photoemit electrons. In order to allow for [[time-resolved spectroscopy|time-resolved]] experiments, a fine adjustment delay stage must be employed in order to manipulate the [[time]] delay between the pump and the probe pulse. These components can be noted in Figure 2.
Time-resolved two-photon [[photoemission spectroscopy|photoelectron spectroscopy]] usually employs a combination of ultrafast [[optical technology]] as well as ultrahigh vacuum components. The main optical component is an ultrafast (femtosecond) laser system which generates pulses in the near infrared. [[Nonlinear optics]] are used to generate photon energies in the visible and ultraviolet spectral range. Typically, ultraviolet radiation is required to photoemit electrons. In order to allow for [[time-resolved spectroscopy|time-resolved]] experiments, a fine adjustment delay stage must be employed in order to manipulate the [[time]] delay between the pump and the probe pulse.

[[File:Time-Resolved Two Photon Photoemission Schematic.png|thumb|Figure 1. First, a lower energy pulse, the pump pulse, photoexcites an electron in a [[ground state]] or HOMO into a higher lying [[excited state]]. After some time delay, a second, higher energy pulse photoemits the excited electron into free electron states above the [[vacuum level]].]]

⚫
[[File:Schematic 2PPE-setup pdf.pdf|thumb|Setup (schematic) for two-photon photoemission experiments|thumb|Figure 2. A laser pulse is first split using a [[beam splitter]] into two different laser lines. One laser line is used to create its second harmonic, giving it a higher photon energy which will serve as the probe pulse. The other laser line passes through a delay stage, which allows the experimenter to vary the delay between the laser pulses impinging on the sample.]]


==See also==
==See also==
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==References==
==References==
{{reflist}}
Weinelt, Martin. "Time-resolved two-photon photoemission from metal surfaces." Journal of Physics: Condensed Matter 14.43 (2002): R1099. [http://iopscience.iop.org/article/10.1088/0953-8984/14/43/202]

<references />


[[Category:Emission spectroscopy]]
[[Category:Emission spectroscopy]]

Revision as of 13:51, 27 April 2019

A lower energy pump pulse photoexcites an electron in a ground state or HOMO into a higher lying excited state. After a time delay, a second, higher energy pulse photoemits the excited electron into free electron states above the vacuum level.

Time-resolved two-photon photoelectron (2PPE) spectroscopy is a time-resolved spectroscopy technique which is used to study electronic structure and electronic excitations at surfaces.[1] The technique utilizes femtosecond to picosecond laser pulses in order to first photoexcite an electron. After a time delay, the excited electron is photoemitted into a free electron state by a second pulse. The kinetic energy and the emission angle of the photoelectron are measured in an electron energy analyzer. To facilitate investigations on the population and relaxation pathways of the excitation, this measurement is performed at different time delays.

This technique has been used for many different types of materials to study a variety of exotic electron behaviors, including image potential states at metal surfaces,[2][3] and even electron dynamics at molecular interfaces.[4]

Basic physics

The final kinetic energy of the electron can be modeled by

where the EB is the binding energy of the initial state, Ekin is the kinetic energy of the photoemitted electron, Φ is the work function of the material in question, and Epump, Eprobe are the photon energies of the laser pulses, respectively. Without a time delay, this equation is exact. However, as the delay between the pump and probe pulses increases, the excited electron may relax in an energy. Hence the energy of the photoemitted electron is lowered. With large enough time delay between the two pulses, the electron will relax all the way back to its original state. The timescales at which the electronic relaxation occurs, as well as the relaxation mechanism (either via vibronic coupling or electronic coupling) is of interest for applications of functional devices such as solar cells and light-emitting diodes.

Experimental configuration

A laser pulse is first split using a beam splitter into two different laser lines. One laser line is used to create its second harmonic, giving it a higher photon energy which will serve as the probe pulse. The other laser line passes through a delay stage, which allows the experimenter to vary the delay between the laser pulses impinging on the sample.

Time-resolved two-photon photoelectron spectroscopy usually employs a combination of ultrafast optical technology as well as ultrahigh vacuum components. The main optical component is an ultrafast (femtosecond) laser system which generates pulses in the near infrared. Nonlinear optics are used to generate photon energies in the visible and ultraviolet spectral range. Typically, ultraviolet radiation is required to photoemit electrons. In order to allow for time-resolved experiments, a fine adjustment delay stage must be employed in order to manipulate the time delay between the pump and the probe pulse.

See also

References

  1. ^ Weinelt, Martin (2002). "Time-resolved two-photon photoemission from metal surfaces". Journal of Physics: Condensed Matter. 14 (43): R1099–R1141. doi:10.1088/0953-8984/14/43/202. ISSN 0953-8984.
  2. ^ Fauster, Thomas, and Wulf Steinmann. "Two-photon photoemission spectroscopy of image states." Electromagnetic Waves: Recent Developments in Research 2 (1995): 347-411.
  3. ^ Weinelt, Martin. "Time-resolved two-photon photoemission from metal surfaces." Journal of Physics: Condensed Matter 14.43 (2002): R1099.
  4. ^ Zhu, X-Y. "Electron transfer at molecule-metal interfaces: a two-photon photoemission study." Annual Review of Physical Chemistry 53.1 (2002): 221-247.
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