108
R. Spesyvtsev et al.
using VUV or x-ray probe pulses. Hemispherical electron energy analysers have
been employed in liquid jet experiments due to their efficient differential pumping [68]. Obtaining information about photoelectron angular distributions from ToF
measurements is more challenging, requiring ToF spectrometers with small solid
angles of collection (and so low collection efficiency) and repeated measurements
with different experimental geometries.
Velocity map imaging (VMI) allows both the photoelectron spectrum and the
photoelectron angular distribution to be measured simultaneously. VMI is based on
using static electric fields to project the 3D photoelectron momentum distribution
created in the molecule-laser interaction region onto a 2D photoelectron image in
the plane of the electron detector. The position sensitive electron detector usually
comprises a microchannel plate, a phosphor screen and a charge-coupled device.
The original 3D photoelectron momentum distribution can be reconstructed from
2D photoelectron image using an inversion algorithm such as an inverse Abel transform [70]. The energy resolution of a VMI apparatus depends on the electron detector resolution as well as on the focussing properties of the electrostatic lens. Energy
resolution as low as = 0.38 % can be achieved [71].
5.4 Applications
5.4.1 Internal Conversion and Intramolecular Vibrational Energy
Redistribution
Femtosecond TRPES has been exploited very successfully to observe ultrafast internal conversion (IC) in a number of polyatomic molecules. A classic example is
S 2 /S 1 IC in the linear polyene all-trans 2,4,6,8-decatetraene (DT) [72]. The energy
level scheme is presented in Fig. 5.2. The first optically allowed transition is to the
S 2 (1 1 B u ) state, which is a singly excited configuration, correlating electronically
with the D 0 (1 2 B g ) ground electronic state of the cation. The S 1 (2 1 A g ) state arises
from a configuration interaction between singly and doubly excited configurations
and correlates electronically with the D 1 (1 2 A u ) first electronically excited state of
the cation. The photoelectron spectra presented in Fig. 5.2 show a rapid shift in
electron distribution ε 1 (ionisation of S 2 ) to ε 2 (ionisation of S 1 ) with a ∼400 fs
timescale.
TRPES can also be exploited to follow intramolecular vibrational energy redistribution (IVR) in polyatomic molecules. An elegant example is restricted IVR in
toluene [73]. The pump pulse prepares a coherent superposition of the 6a 1 state,
corresponding to one quanta of vibrational excitation in the totally symmetric ring
breathing mode, and the 10b 1 6b 1 state, corresponding to one quanta in the CH 3
wagging mode (10b) and one quanta in the out-of-plane C–H bending mode (16b).
As a result of anharmonic coupling between these two states (a Fermi resonance)
population flows between them and this is observed as oscillations in the photoelectron spectrum (Fig. 5.3). The period of the oscillation is ∼ 6 ps and is inversely
proportional to the energy separation of the two states in the superposition.
R. Spesyvtsev et al.
using VUV or x-ray probe pulses. Hemispherical electron energy analysers have
been employed in liquid jet experiments due to their efficient differential pumping [68]. Obtaining information about photoelectron angular distributions from ToF
measurements is more challenging, requiring ToF spectrometers with small solid
angles of collection (and so low collection efficiency) and repeated measurements
with different experimental geometries.
Velocity map imaging (VMI) allows both the photoelectron spectrum and the
photoelectron angular distribution to be measured simultaneously. VMI is based on
using static electric fields to project the 3D photoelectron momentum distribution
created in the molecule-laser interaction region onto a 2D photoelectron image in
the plane of the electron detector. The position sensitive electron detector usually
comprises a microchannel plate, a phosphor screen and a charge-coupled device.
The original 3D photoelectron momentum distribution can be reconstructed from
2D photoelectron image using an inversion algorithm such as an inverse Abel transform [70]. The energy resolution of a VMI apparatus depends on the electron detector resolution as well as on the focussing properties of the electrostatic lens. Energy
resolution as low as = 0.38 % can be achieved [71].
5.4 Applications
5.4.1 Internal Conversion and Intramolecular Vibrational Energy
Redistribution
Femtosecond TRPES has been exploited very successfully to observe ultrafast internal conversion (IC) in a number of polyatomic molecules. A classic example is
S 2 /S 1 IC in the linear polyene all-trans 2,4,6,8-decatetraene (DT) [72]. The energy
level scheme is presented in Fig. 5.2. The first optically allowed transition is to the
S 2 (1 1 B u ) state, which is a singly excited configuration, correlating electronically
with the D 0 (1 2 B g ) ground electronic state of the cation. The S 1 (2 1 A g ) state arises
from a configuration interaction between singly and doubly excited configurations
and correlates electronically with the D 1 (1 2 A u ) first electronically excited state of
the cation. The photoelectron spectra presented in Fig. 5.2 show a rapid shift in
electron distribution ε 1 (ionisation of S 2 ) to ε 2 (ionisation of S 1 ) with a ∼400 fs
timescale.
TRPES can also be exploited to follow intramolecular vibrational energy redistribution (IVR) in polyatomic molecules. An elegant example is restricted IVR in
toluene [73]. The pump pulse prepares a coherent superposition of the 6a 1 state,
corresponding to one quanta of vibrational excitation in the totally symmetric ring
breathing mode, and the 10b 1 6b 1 state, corresponding to one quanta in the CH 3
wagging mode (10b) and one quanta in the out-of-plane C–H bending mode (16b).
As a result of anharmonic coupling between these two states (a Fermi resonance)
population flows between them and this is observed as oscillations in the photoelectron spectrum (Fig. 5.3). The period of the oscillation is ∼ 6 ps and is inversely
proportional to the energy separation of the two states in the superposition.