Due in part to their prevalence in nature and applicability in light harvesting metal complexes, there has been considerable interest in investigating the molecular dynamics of organic molecules that may absorb photons in the visible and ultraviolet
range.20,184,185 Due to its aromatic nature and the wide prevalence of the phenyl sub-unit
in nature, benzene has been the subject of considerable interest for several decades. While initial PE studies of benzene focussed upon the assignment and ordering of the
molecular orbitals,186-188 the dynamics of the S1 excited state were subsequently
investigated by picosecond189 and femtosecond190,191 TRPES. TRPES was also applied
to investigate internal conversion from the S2 state.192
The Fielding group have recently applied gas phase TRPES to neutral benzene molecules in a molecular beam in order to investigate the ultrafast dynamics at the onset
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promoted the molecule to the S1 excited state, which was then probed at various wavelengths centred between 235 and 260 nm. While it was found that a large proportion of the excited states rapidly internally convert to the S0 state, a significant
proportion undergoes intersystem crossing to the T1 state on an ultrafast timescale.193
Additionally a small proportion of molecules were found to oscillate between the S1 and T2 states. These results indicate that the ultrafast decay of the S1 excited state previously
attributed to internal conversion191 is actually a result of competition between internal
conversion and intersystem crossing. Theoretical investigations suggest that the unexpectedly high rate of intersystem crossing is due to the accessing of a vibrational
mode favourable to intersystem crossing.194 The S2 excited state of benzene has recently
been probed through TRPES by Shen et al.,195 who produced the excited state through
two photon absorption of 400 nm and probed at 267 nm. Two lifetimes were observed: a sub 100-femtosecond lifetime assigned to the S1←S2 transition and a 5 picosecond lifetime assigned to the loss of the S1 state. The authors note that the second lifetime is shorter than previous observations, which is ascribed to the opening of an additional decay channel; intersystem crossing between the vibrationally excited S1 state and the
T3 state.195
In addition to their work on the excited states of benzene, the Fielding group have recently employed TRPES to determine the dynamics for the first two excited
states (S1 and S2) of styrene.196 It was found that, while both excited states decay via
internal conversion through conical intersections, the rate of internal conversion from S1 to S0 was five times slower if it was preceded by internal conversion from an S2 state produced with excess vibrational energy. Nunn et al. concluded that the conical intersection between the S1 and S0 states did not occur on the intial geometry relaxation coordinate following photoexcitation and was therefore barrier activated, whereas the
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converse was true for the S1←S2 transition.196
The internal conversion from the S2 state
therefore produces an S1 excited state population that is geometrically ‘further’ from the
S0←S1 conical intersection, resulting in the observed increase in lifetime.
TRPES has been extensively employed to investigate the excited state dynamics of indole, which is of particular interest due to its presence in the amino acid
tryptophan.197-200 The dynamics of the mixed Rydberg-valence 3s/πσ* state produced by
photoexcitation at 267 nm and 258 nm have been well characterised.198 In this, it was
found that population existed on the excited state surface at large N-H separations for long timescales before the excited state decays by dissociation or internal conversion. This was linked to the rapid evolution of the mixed Rydberg-valence excited state character as the along the N-H separation and indicates the importance of careful consideration of mixed Rydberg-valence states in the assignment of excited state dynamics. As the excitation energy is decreased, however, the involvement of this state
in the excited state dynamics becomes negligible.199
The excited state dynamics of aniline have been the subject of considerable interest, due to numerous studies drawing opposing conclusions. Wren et al. have
reported the gas phase PE spectrum of the anilinide anion,201 in which the authors note
the clear progression of a vibrational mode. Spesyvtsev et al. have observed the excited state dynamics of aniline in the gas phase through TRPES following excitation at 269-
238 nm.202 In this, it was concluded that the S2(3s/πσ*) excited state was produced
following excitation at all wavelengths, which would then decay through two possible channels: ultrafast internal conversion to the S1(ππ*) state via a conical intersection and subsequent relaxation to the electronic ground state by a far slower process, and a non- radiative channel that appeared to involve motion along the N-H stretch coordinate on the πσ* dissociative potential energy surface. Although the precise nature of this
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channel was not able to be discerned by this study, the timescales recorded were extremely similar to lifetimes reported by Livingstone et al. in their recently reported
TRPES investigation into the excited state dynamics of indole and 5-hydroxyindole.197
The Townsend group assigned these lifetimes to motion on the dissociative πσ* potential energy surface, leading Spesyvtsev et al. to suggest that similar motion may
occur in aniline.202
However, these observations appear to conflict with the observations of Ashfold
and co-workers203 and Stavros and co-workers,204 who employed H atom Rydberg
tagging and time resolved H atom imaging, respectively. These works inferred the presence of a S1(ππ*)/S2(3s/πσ*) coupling interaction and so the reverse of the non- adiabatic coupling observed by the Fielding group. Townsend and co-workers have recently performed TRPES studies on aniline following photoexcitation at 250 nm, N,N- dimethylaniline and 3,5-dimethylaniline in order to investigate the internal molecular
coordinates involved in the excited state coupling.205,206 In this, it was found that both
excited states were initially excited and that the S2(3s/πσ*) population may decay by
internal conversion to the S1(ππ*) or via a postulated dissociation mechanism along the N-H/Me stretch coordinate. The aromatic ring system was linked to the internal
conversion between the S2(3s/πσ*) and S1(ππ*) excited states, as methylation of the
aromatic ring results in only the direct dissociative mechanism being observed.190
Time-resolved PE spectroscopy has also been exploited to investigate the ring
opening reaction of 1,3-cyclohexadiene following UV excitation.207 Excitation of 1,3-
cyclohexadiene at 270 nm produces an excited state population on an extremely repulsive section of the 1B excited state surface, which induces ultrafast internal conversion via a conical intersection to the 2A state. This state then decays back to the ground 1A state via a second conical intersection. At this point, the system branches
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into two populations: the system may undergo a ring-opening reaction to form 1,3,5- hexatriene or return to the initial 1,3-cyclohexadiene strucutre. The branching ratio of this process has been the subject of some controversy, with gas and liquid phase
experiments yielding differing branching ratios.208,209 Adachi et al. employed gas phase
TRPES to promote 1,3-cyclohexadiene into the excited state and subsequently probe
the relaxation dynamics.207 The probe pulse used is a 13.6 eV (90 nm), 40 fs pulse,
which is sufficient to ionise the molecule from the ground state and so allows for observation of complete ground state recovery. It was determined that 30% of the initial excited state population undergoes the ring opening reaction, in good accord with experimental observations in the liquid phase and theoretical calculations for the gas phase. This study is a clear example of the utility of TRPES where the ground state recovery may be probed to comprehensively determine the fate of an excited state population.
Finally, TRPES has recently been used to explore the deactivation dynamics of
the ππ*(V) state of ethene at sub-20 fs resolution.210 The system is promoted into the
desried excited state by an ultrafast pump pulse at 159 nm and the photoelectron detached by an ultrafast probe pulse at 198 nm. The resultant TRPE spectrum is presented in Figure 1.6.
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Figure 1.6 TRPES of ethene, following excitation to the ππ*(V) state. Reproduced with permission from Kobayashi et al.210 Copyright 2015 American Chemical Society
A clear downward trend in the photoelectron kinetic energy distribution can be observed, occuring on a sub-20 fs timescale. This was ascribed to a combination of a twist around the C–C bond and a pyrimidalization motion. Through careful examination of the TRPES, a partial wave packet revival at 3 eV at 18 fs after the initial photoexcitation can be observed. This was attributed to recovery of an equivalent
geometry to the Franck-Condon geometry through a 180o twist of the C–C bond. The
unprecedented time-resolution of this work provides a fantastic example of the application of TRPES to exploring ultrafast molecular dynamics.