104
R. Spesyvtsev et al.
such, the evolution of the photoelectron angular distribution, which reflects the allowed symmetries of the partial waves, will reflect the evolution of the molecular
electronic symmetry.
Since the symmetry of the free electron wavefunction determines the form of
the PAD, the shape of the PAD will reflect (i) the electronic symmetry of the neutral molecule and (ii) the symmetries of the contributing molecular frame transition
dipole moment components. Since the relative contributions of the molecular frame
transition dipole moments are determined geometrically by the orientation of the
molecule relative to the ionizing laser field polarization, the form of the laboratory
frame PAD will reflect the distribution of molecular axis in the laboratory frame,
and so will reflect the rotational dynamics of the molecule [44–50].
5.3 The Experimental Toolkit for TRPES
Virtually all polyatomic molecules have strong absorption bands in the ultraviolet (200–350 nm) and ionisation potentials in the vacuum ultraviolet (VUV)
(< 150 nm). The most common form of TRPES employs a UV pump pulse to access
lower lying electronically excited states and a UV probe pulse to ionise the molecule
from these excited states and generate photoelectrons with electron kinetic energies
in the range 0–3 eV. Generally, it is not possible to ionise a molecule that has returned to its electronic ground state using a UV probe pulse, since the molecule
will have considerable excess vibrational energy and there is a υ = 0 propensity
for photoionisation. To photoionise molecules that have returned to the electronic
ground-state by non-radiative decay from electronically excited states, requires a
VUV probe pulse to access higher vibrational states in the ionisation continuum.
Extreme ultraviolet (XUV) probe pulses allow the additional possibility of ionising
core electrons; photoelectrons generated by photoionisation of the valence electrons
will have high electron kinetic energies (just a few eV less than the XUV probe
pulse).
5.3.1 Femtosecond Light Sources
Most TRPES experiments employ commercial femtosecond laser systems that are
tuneable in the UV. Femtosecond laser technology is a research field in itself, and
there are many excellent reviews available, see for example [51, 52]. The most common scheme for the production of femtosecond UV pulses is based on a Ti:Sapphire
(Ti:S) laser operating at around 800 nm. A bandwidth of around 100 nm can be
achieved, enabling the production of pulses as short as 10 fs. The Ti:S crystal is
usually pumped using a continuous laser with an output wavelength around 525 nm.
The femtosecond pulses from the Ti:S oscillator are used to seed a chirped pulse
regenerative amplifier; before amplification, the seeding pulses are stretched to a
R. Spesyvtsev et al.
such, the evolution of the photoelectron angular distribution, which reflects the allowed symmetries of the partial waves, will reflect the evolution of the molecular
electronic symmetry.
Since the symmetry of the free electron wavefunction determines the form of
the PAD, the shape of the PAD will reflect (i) the electronic symmetry of the neutral molecule and (ii) the symmetries of the contributing molecular frame transition
dipole moment components. Since the relative contributions of the molecular frame
transition dipole moments are determined geometrically by the orientation of the
molecule relative to the ionizing laser field polarization, the form of the laboratory
frame PAD will reflect the distribution of molecular axis in the laboratory frame,
and so will reflect the rotational dynamics of the molecule [44–50].
5.3 The Experimental Toolkit for TRPES
Virtually all polyatomic molecules have strong absorption bands in the ultraviolet (200–350 nm) and ionisation potentials in the vacuum ultraviolet (VUV)
(< 150 nm). The most common form of TRPES employs a UV pump pulse to access
lower lying electronically excited states and a UV probe pulse to ionise the molecule
from these excited states and generate photoelectrons with electron kinetic energies
in the range 0–3 eV. Generally, it is not possible to ionise a molecule that has returned to its electronic ground state using a UV probe pulse, since the molecule
will have considerable excess vibrational energy and there is a υ = 0 propensity
for photoionisation. To photoionise molecules that have returned to the electronic
ground-state by non-radiative decay from electronically excited states, requires a
VUV probe pulse to access higher vibrational states in the ionisation continuum.
Extreme ultraviolet (XUV) probe pulses allow the additional possibility of ionising
core electrons; photoelectrons generated by photoionisation of the valence electrons
will have high electron kinetic energies (just a few eV less than the XUV probe
pulse).
5.3.1 Femtosecond Light Sources
Most TRPES experiments employ commercial femtosecond laser systems that are
tuneable in the UV. Femtosecond laser technology is a research field in itself, and
there are many excellent reviews available, see for example [51, 52]. The most common scheme for the production of femtosecond UV pulses is based on a Ti:Sapphire
(Ti:S) laser operating at around 800 nm. A bandwidth of around 100 nm can be
achieved, enabling the production of pulses as short as 10 fs. The Ti:S crystal is
usually pumped using a continuous laser with an output wavelength around 525 nm.
The femtosecond pulses from the Ti:S oscillator are used to seed a chirped pulse
regenerative amplifier; before amplification, the seeding pulses are stretched to a