Two-photon photoelectron spectroscopy

[1][2] The technique utilizes femtosecond to picosecond laser pulses in order to first photoexcite an electron.

To facilitate investigations on the population and relaxation pathways of the excitation, this measurement is performed at different time delays.

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.

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 .
Setup (schematic) for two-photon photoemission experiments
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.