Chapter 5
Time-Resolved Photoelectron Spectroscopy
for Excited State Dynamics
Roman Spesyvtsev, Jonathan G. Underwood, and Helen H. Fielding
Abstract This chapter provides an overview of time-resolved photoelectron spectroscopy (TRPES) for unravelling excited state dynamics in polyatomic molecules.
It begins with a brief introduction to the basics of nonadiabatic dynamics in polyatomic molecules. This is followed by a description of the principles behind TRPES
and a discussion of the roles of the ionisation continuum. We then describe the experimental toolkit, from light and molecule sources to photoelectron spectrometers.
Finally, we describe several examples where TRPES has been employed to unravel
non-adiabatic dynamics in polyatomic molecules.
5.1 Introduction
When a molecule absorbs a femtosecond pulse of UV light, it is promoted to an excited electronic state in which the nuclei are no longer in their equilibrium positions.
The resulting excess vibrational energy can be redistributed within the molecule in
various ways: it may undergo a photochemical reaction on the excited state (such
as isomerisation, proton-transfer or electron-transfer) before relaxing back to the
ground-state, or it may undergo electronic relaxation from the initially populated
excited state to another state of the same multiplicity (internal conversion) or to one
of different multiplicity (intersystem crossing) [1–8]. These non-radiative processes
often occur at, or near to, molecular configurations where two or more electronic
states are degenerate, known as conical intersections [9]. Conical intersections provide very efficient ‘photochemical funnels’ for the ultrafast (sub-picosecond) nonradiative decay processes that underpin the photochemistry of almost all polyatomic
molecules. Such non-radiative dynamics are key to many photobiological functions,
such as vision and photosynthesis, and underlie many concepts in active molecular
electronics.
R. Spesyvtsev · H.H. Fielding (B)
Department of Chemistry, University College London, London, UK
e-mail: h.h.fielding@ucl.ac.uk
J.G. Underwood
Department of Physics and Astronomy, University College London, London, UK
R. de Nalda, L. Bañares (eds.), Ultrafast Phenomena in Molecular Sciences,
Springer Series in Chemical Physics 107, DOI 10.1007/978-3-319-02051-8_5,
© Springer International Publishing Switzerland 2014
99
Time-Resolved Photoelectron Spectroscopy
for Excited State Dynamics
Roman Spesyvtsev, Jonathan G. Underwood, and Helen H. Fielding
Abstract This chapter provides an overview of time-resolved photoelectron spectroscopy (TRPES) for unravelling excited state dynamics in polyatomic molecules.
It begins with a brief introduction to the basics of nonadiabatic dynamics in polyatomic molecules. This is followed by a description of the principles behind TRPES
and a discussion of the roles of the ionisation continuum. We then describe the experimental toolkit, from light and molecule sources to photoelectron spectrometers.
Finally, we describe several examples where TRPES has been employed to unravel
non-adiabatic dynamics in polyatomic molecules.
5.1 Introduction
When a molecule absorbs a femtosecond pulse of UV light, it is promoted to an excited electronic state in which the nuclei are no longer in their equilibrium positions.
The resulting excess vibrational energy can be redistributed within the molecule in
various ways: it may undergo a photochemical reaction on the excited state (such
as isomerisation, proton-transfer or electron-transfer) before relaxing back to the
ground-state, or it may undergo electronic relaxation from the initially populated
excited state to another state of the same multiplicity (internal conversion) or to one
of different multiplicity (intersystem crossing) [1–8]. These non-radiative processes
often occur at, or near to, molecular configurations where two or more electronic
states are degenerate, known as conical intersections [9]. Conical intersections provide very efficient ‘photochemical funnels’ for the ultrafast (sub-picosecond) nonradiative decay processes that underpin the photochemistry of almost all polyatomic
molecules. Such non-radiative dynamics are key to many photobiological functions,
such as vision and photosynthesis, and underlie many concepts in active molecular
electronics.
R. Spesyvtsev · H.H. Fielding (B)
Department of Chemistry, University College London, London, UK
e-mail: h.h.fielding@ucl.ac.uk
J.G. Underwood
Department of Physics and Astronomy, University College London, London, UK
R. de Nalda, L. Bañares (eds.), Ultrafast Phenomena in Molecular Sciences,
Springer Series in Chemical Physics 107, DOI 10.1007/978-3-319-02051-8_5,
© Springer International Publishing Switzerland 2014
99