5 Time-Resolved Photoelectron Spectroscopy for Excited State Dynamics
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electrons (which are accessible using conventional laser sources), but also selectively non-bonding and core electrons, and have the potential to provide a
different view of the reaction dynamics.
The molecular ionization continuum provides a template for observing both excited state vibrational dynamics and evolving excited state electronic configurations.
The excited state vibrational dynamics are observed via Franck-Condon distributions in the photoelectron spectrum, while the electronic dynamics are manifested
through the cation states formed during photoionization (which have correspondingly different photoelectron energies), as well as the form of the photoelectron
angular distribution. These ideas are discussed in more detail below.
5.2.1 Photoelectron Spectra: Using the Cation to Map Excited
State Dynamics
The electronic states of the cation can provide a map of evolving electronic structures in the neutral state prior to ionization—in the independent electron approximation, emission of an outer electron occurs without simultaneous electronic reorganization of the ‘core’ (cation or neutral)—this is called the ‘molecular orbital’
or Koopmans’ picture [27–29]. Thus, it is possible to determine the most probable electronic state of the cation following single photon ionization of a specific
electronic state of the neutral molecule. The probabilities of partial ionization into
specific cation electronic states can differ drastically with respect to the molecular
orbital nature of the neutral electronic state being probed. For example, if an electronic configuration correlates, upon removal of a single active outer electron, to the
ground electronic configuration of the cation, then the photoionization probability
is generally higher than if it does not.
Figure 5.1 illustrates how the cation electronic structures can be used in (angleintegrated) time-resolved photoelectron spectroscopy to disentangle electronic dynamics from vibrational dynamics in ultrafast non-adiabatic processes. A BornOppenheimer, zeroth-order, optically bright state, S n , is prepared coherently with
a femtosecond pump pulse. According to the molecular orbital picture, it should
ionize into the continuum associated with the D 0 state of the cation (the ground
electronic state of the cation) in this example. This process gives rise to a photoelectron energy band ε 1 . Non-adiabatic coupling (promoted by vibrational modes
of appropriate symmetry) then transforms the zeroth-order optically bright state S n
into a lower lying zeroth-order optically dark state S n−1 . In this example, according
to the molecular orbital picture, this state should ionize into a different continuum
associated with an electronically excited state of the cation, D 1 , giving rise to a
photoelectron band ε 2 . Thus, for a sufficiently energetic probe pulse, (i.e., both ionization channels accessible energetically) the photoionisation channel will switch
from ε 1 to ε 2 during the non-adiabatic process. This simple picture illustrates how
the evolving excited-state electronic configuration can be monitored during a nonadiabatic process whilst following, simultaneously, the coupled nuclear dynamics
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electrons (which are accessible using conventional laser sources), but also selectively non-bonding and core electrons, and have the potential to provide a
different view of the reaction dynamics.
The molecular ionization continuum provides a template for observing both excited state vibrational dynamics and evolving excited state electronic configurations.
The excited state vibrational dynamics are observed via Franck-Condon distributions in the photoelectron spectrum, while the electronic dynamics are manifested
through the cation states formed during photoionization (which have correspondingly different photoelectron energies), as well as the form of the photoelectron
angular distribution. These ideas are discussed in more detail below.
5.2.1 Photoelectron Spectra: Using the Cation to Map Excited
State Dynamics
The electronic states of the cation can provide a map of evolving electronic structures in the neutral state prior to ionization—in the independent electron approximation, emission of an outer electron occurs without simultaneous electronic reorganization of the ‘core’ (cation or neutral)—this is called the ‘molecular orbital’
or Koopmans’ picture [27–29]. Thus, it is possible to determine the most probable electronic state of the cation following single photon ionization of a specific
electronic state of the neutral molecule. The probabilities of partial ionization into
specific cation electronic states can differ drastically with respect to the molecular
orbital nature of the neutral electronic state being probed. For example, if an electronic configuration correlates, upon removal of a single active outer electron, to the
ground electronic configuration of the cation, then the photoionization probability
is generally higher than if it does not.
Figure 5.1 illustrates how the cation electronic structures can be used in (angleintegrated) time-resolved photoelectron spectroscopy to disentangle electronic dynamics from vibrational dynamics in ultrafast non-adiabatic processes. A BornOppenheimer, zeroth-order, optically bright state, S n , is prepared coherently with
a femtosecond pump pulse. According to the molecular orbital picture, it should
ionize into the continuum associated with the D 0 state of the cation (the ground
electronic state of the cation) in this example. This process gives rise to a photoelectron energy band ε 1 . Non-adiabatic coupling (promoted by vibrational modes
of appropriate symmetry) then transforms the zeroth-order optically bright state S n
into a lower lying zeroth-order optically dark state S n−1 . In this example, according
to the molecular orbital picture, this state should ionize into a different continuum
associated with an electronically excited state of the cation, D 1 , giving rise to a
photoelectron band ε 2 . Thus, for a sufficiently energetic probe pulse, (i.e., both ionization channels accessible energetically) the photoionisation channel will switch
from ε 1 to ε 2 during the non-adiabatic process. This simple picture illustrates how
the evolving excited-state electronic configuration can be monitored during a nonadiabatic process whilst following, simultaneously, the coupled nuclear dynamics