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G.M. Roberts and V.G. Stavros
1 ππ ∗ state) [25, 62]. Using a temporally delayed fs probe pulse centered at 243 nm,
the appearance timescale for any photodissociated H-atoms was subsequently monitored through (2 + 1) REMPI, enabling the detection of H + with TR-MS. The
results of these experiments are presented in Fig. 6.5(b). For all pump-probe delays,
t, there is a constant (time-independent) one-color background signal, generated
from the 243 nm probe via the process depicted in the left panel of Fig. 6.5(c). At
t < 0 ps (i.e. pumping with 243 nm and probing with 250 nm), some H + signal is generated through an inefficient two-color ‘probe-pump’ process (Fig. 6.5(c),
middle panel). At t = 0 ps delay, when the fs pump and probe pulses are temporally overlapped, a sharp spike in the H + signal is observed (due to a combination
of multiphoton absorption processes [25]), which then decays within the temporal
resolution of the experiment (∼ 160 fs). Finally, when t > 0 ps, a notable rise in
signal is observed, relative to negative pump-probe delays, correlating to the appearance of photodissociated H-atoms from pyrrole at 250 nm (Fig. 6.5(c), right panel).
This signal rise exhibits a bi-exponential profile with two associated time constants,
τ 1 and τ 2 , which take values of 110 ± 80 fs and 1.1 ± 0.5 ps, respectively. With
reference to Fig. 6.5(a), the ultrafast time constant, τ 1 (110 fs), is ascribed to the
timescale for direct 1 πσ ∗ mediated N–H bond fission, proceeding via non-adiabatic
passage through the 1 πσ ∗ /S 0 CI, yielding H-atoms with large amounts of KE (defined by TKER max ) in coincidence with pyrrolyl (C 4 H 4 N) radical co-fragments in
their electronic ground state. Alternatively, as population reaches the 1 πσ ∗ /S 0 CI
it may evolve adiabatically, traversing the upper cone of the CI and then returning to subsequently undergo non-adiabatic coupling back into vibrationally excited
ground state species. Radloff and co-workers [25] postulated that the slower τ 2 timeconstant (1.1 ps) originates from this initial adiabatic behavior around 1 πσ ∗ /S 0 CI,
resulting in statistical elimination of H-atoms with less KE from vibrationally hot S 0
species, after some degree of intramolecular vibrational energy redistribution (IVR)
in S 0 .
6.4.2 Comparing Dynamics in Simple Azoles
Upon exchanging one of the ring C–H bonds in pyrrole for an N atom, the simple azole isomers imidazole and pyrazole can be formed (structures shown in
Fig. 6.6), both of which belong to the lower C s symmetry group, transforming the 1 πσ ∗ (A ) ← S 0 (A ) transition into an electric dipole allowed process
(A ⊗ a ⊗ A ⊇ A ). Imidazole is a common subunit of photostable biomolecules,
most notably in the amino acid histidine and the purine derived DNA bases adenine
and guanine. In contrast, naturally occurring pyrazole derivatives are rare [64], particularly in biomolecules which are inherently required to be photostable. In light of
this, TR-VMI experiments have begun to investigate whether there are any notable
differences in the UV photochemistry of these two simple azole isomers [26], particularly along their 1 πσ ∗ states, despite their qualitatively similar electronic structure
[63]—Fig. 6.6 presents schematic potential energy profiles along their N–H bond
coordinates.
G.M. Roberts and V.G. Stavros
1 ππ ∗ state) [25, 62]. Using a temporally delayed fs probe pulse centered at 243 nm,
the appearance timescale for any photodissociated H-atoms was subsequently monitored through (2 + 1) REMPI, enabling the detection of H + with TR-MS. The
results of these experiments are presented in Fig. 6.5(b). For all pump-probe delays,
t, there is a constant (time-independent) one-color background signal, generated
from the 243 nm probe via the process depicted in the left panel of Fig. 6.5(c). At
t < 0 ps (i.e. pumping with 243 nm and probing with 250 nm), some H + signal is generated through an inefficient two-color ‘probe-pump’ process (Fig. 6.5(c),
middle panel). At t = 0 ps delay, when the fs pump and probe pulses are temporally overlapped, a sharp spike in the H + signal is observed (due to a combination
of multiphoton absorption processes [25]), which then decays within the temporal
resolution of the experiment (∼ 160 fs). Finally, when t > 0 ps, a notable rise in
signal is observed, relative to negative pump-probe delays, correlating to the appearance of photodissociated H-atoms from pyrrole at 250 nm (Fig. 6.5(c), right panel).
This signal rise exhibits a bi-exponential profile with two associated time constants,
τ 1 and τ 2 , which take values of 110 ± 80 fs and 1.1 ± 0.5 ps, respectively. With
reference to Fig. 6.5(a), the ultrafast time constant, τ 1 (110 fs), is ascribed to the
timescale for direct 1 πσ ∗ mediated N–H bond fission, proceeding via non-adiabatic
passage through the 1 πσ ∗ /S 0 CI, yielding H-atoms with large amounts of KE (defined by TKER max ) in coincidence with pyrrolyl (C 4 H 4 N) radical co-fragments in
their electronic ground state. Alternatively, as population reaches the 1 πσ ∗ /S 0 CI
it may evolve adiabatically, traversing the upper cone of the CI and then returning to subsequently undergo non-adiabatic coupling back into vibrationally excited
ground state species. Radloff and co-workers [25] postulated that the slower τ 2 timeconstant (1.1 ps) originates from this initial adiabatic behavior around 1 πσ ∗ /S 0 CI,
resulting in statistical elimination of H-atoms with less KE from vibrationally hot S 0
species, after some degree of intramolecular vibrational energy redistribution (IVR)
in S 0 .
6.4.2 Comparing Dynamics in Simple Azoles
Upon exchanging one of the ring C–H bonds in pyrrole for an N atom, the simple azole isomers imidazole and pyrazole can be formed (structures shown in
Fig. 6.6), both of which belong to the lower C s symmetry group, transforming the 1 πσ ∗ (A ) ← S 0 (A ) transition into an electric dipole allowed process
(A ⊗ a ⊗ A ⊇ A ). Imidazole is a common subunit of photostable biomolecules,
most notably in the amino acid histidine and the purine derived DNA bases adenine
and guanine. In contrast, naturally occurring pyrazole derivatives are rare [64], particularly in biomolecules which are inherently required to be photostable. In light of
this, TR-VMI experiments have begun to investigate whether there are any notable
differences in the UV photochemistry of these two simple azole isomers [26], particularly along their 1 πσ ∗ states, despite their qualitatively similar electronic structure
[63]—Fig. 6.6 presents schematic potential energy profiles along their N–H bond
coordinates.