100
R. Spesyvtsev et al.
The coupling of vibrational and electronic degrees of freedom described above
is a breakdown of the Born-Oppenheimer approximation (BOA). The BOA is based
on the assumption that, as a result of the large difference in mass between electrons
and nuclei, their motions can be considered separately, to a first approximation.
Non-adiabatic coupling of electronic and nuclear motion often leads to complex,
broadened absorption spectra as a result of the high density of vibrational states and
strong variation of transition dipole moment with nuclear coordinate. However, photoelectron spectroscopy is sensitive to both electronic configuration and vibrational
composition and, as a result, time-resolved photoelectron spectroscopy (TRPES) is
an extraordinarily powerful tool for unravelling the dynamical details of ultrafast
non-adiabatic processes.
TRPES has been the subject of a number of excellent reviews [10–26]; this chapter focuses on providing a brief overview of the fundamental molecular physics
behind TRPES, a brief description of the experimental toolkit and some examples
of applications of TRPES—some classic and some more recent.
5.2 Probing Non-adiabatic Dynamics Using Time-Resolved
Photoelectron Spectroscopy
In a TRPES experiment, a molecule is promoted to an excited electronic state with
an ultrashort pump pulse. A probe pulse then ionises the evolving excited state of the
molecule, generating free electrons, and the electron kinetic energy and/or angular
distribution of these free electrons is measured as a function of the time delay between the pump and probe laser pulses. TRPES has several practical and conceptual
advantages over other pump-probe methods.
(i) Ionization is always an allowed process because the selection rules are relaxed
as a result of the range of possible symmetries of the outgoing electron—there
are no optically dark states in photoionization.
(ii) Very detailed information can be obtained by differential analysis of the outgoing photoelectron in terms of its kinetic energy and angular distribution; these
measurements are made simultaneously in time-resolved photoelectron imaging.
(iii) Charged particle detection is extremely sensitive.
(iv) Higher order (multiphoton) processes, which can be difficult to identify in
energy-integrated femtosecond experiments, are identified more easily in a
photoelectron spectrum.
(v) Photoelectron-photoion coincidence measurements reveal the mass of the carrier of the photoelectron spectrum, making it possible to disentangle reaction
pathways in fragmentation experiments, studies of cluster solvation effects as
a function of cluster size, and studies of scalar and vector correlations in photodissociation dynamics.
(vi) As free-electron laser light sources become available, tuning the probe photon wavelength will allow ionization of not only valence and inner valence
R. Spesyvtsev et al.
The coupling of vibrational and electronic degrees of freedom described above
is a breakdown of the Born-Oppenheimer approximation (BOA). The BOA is based
on the assumption that, as a result of the large difference in mass between electrons
and nuclei, their motions can be considered separately, to a first approximation.
Non-adiabatic coupling of electronic and nuclear motion often leads to complex,
broadened absorption spectra as a result of the high density of vibrational states and
strong variation of transition dipole moment with nuclear coordinate. However, photoelectron spectroscopy is sensitive to both electronic configuration and vibrational
composition and, as a result, time-resolved photoelectron spectroscopy (TRPES) is
an extraordinarily powerful tool for unravelling the dynamical details of ultrafast
non-adiabatic processes.
TRPES has been the subject of a number of excellent reviews [10–26]; this chapter focuses on providing a brief overview of the fundamental molecular physics
behind TRPES, a brief description of the experimental toolkit and some examples
of applications of TRPES—some classic and some more recent.
5.2 Probing Non-adiabatic Dynamics Using Time-Resolved
Photoelectron Spectroscopy
In a TRPES experiment, a molecule is promoted to an excited electronic state with
an ultrashort pump pulse. A probe pulse then ionises the evolving excited state of the
molecule, generating free electrons, and the electron kinetic energy and/or angular
distribution of these free electrons is measured as a function of the time delay between the pump and probe laser pulses. TRPES has several practical and conceptual
advantages over other pump-probe methods.
(i) Ionization is always an allowed process because the selection rules are relaxed
as a result of the range of possible symmetries of the outgoing electron—there
are no optically dark states in photoionization.
(ii) Very detailed information can be obtained by differential analysis of the outgoing photoelectron in terms of its kinetic energy and angular distribution; these
measurements are made simultaneously in time-resolved photoelectron imaging.
(iii) Charged particle detection is extremely sensitive.
(iv) Higher order (multiphoton) processes, which can be difficult to identify in
energy-integrated femtosecond experiments, are identified more easily in a
photoelectron spectrum.
(v) Photoelectron-photoion coincidence measurements reveal the mass of the carrier of the photoelectron spectrum, making it possible to disentangle reaction
pathways in fragmentation experiments, studies of cluster solvation effects as
a function of cluster size, and studies of scalar and vector correlations in photodissociation dynamics.
(vi) As free-electron laser light sources become available, tuning the probe photon wavelength will allow ionization of not only valence and inner valence