was used for mass analysis of the ions. Andersen and co-workers found that
[AMP–H]
À anions dissociate with two different time constants of 95 ns and 2.4 μs,
with branching ratios of 54 % and 46 %, respectively [31]. The 2.4-μs time constant is
likely associated with statistical dissociation, although it is significantly shorter than
that measured at ELISA (16 μs) [17]. It is difficult to explain this behaviour unless the
internal energy distributions of the ion populations are different in the two
experiments. Also, the 94-ns decay is hard to reconcile with the results from the
ELISA experiment, where decay was observed to be predominantly on the microsecond time scale. The authors exclude the possibility of two-photon absorption based
on measurements of the yield of photoneutrals as a function of laser power. However,
such a conclusion is difficult to draw if there are too few ions in the interaction region.
Indeed, Fayeton and co-workers [29] found no such short lifetime component for the
similar [dAMP–H]
À anion using 263-nm light. Instead, they found that all dissociation took place with a time constant of 1 μs, which is not substantially different than
the 2.4-μs constant measured by Andersen and co-workers but is significantly shorter
than that obtained from the ELISA experiment. In the case of [AMP+H]
+ cations,
Andersen and co-workers [31] found two short dissociation times of 85 ns and 640 ns,
which would both be too short to be measured in the delayed dissociation experiment
at ELISA and are therefore in accordance with the conclusion from the beamdepletion results [17]. On the other hand, these times are much longer than expected
for dissociation from electronically excited states as well as after IC but prior to IVR.
However, we caution that this result does not exclude prompt bond ruptures since the
ionic fragment and the neutral fragment may stick together as a long-lived, ionmolecule encounter complex. Such complexes are often formed and may dissociate
on time scales of up to tens of nanoseconds [54]. The only way to fully exclude
prompt fragmentation would be to perform pump-probe experiments on the same
time scale as the initial energy conversion processes (i.e., the 10
À13 to 10
À11 s time
scale). More work is clearly needed to fully understand the different results obtained
from various experimental setups. In this regard, storage-ring experiments with
shorter revolution times of about five microseconds would be valuable.
Brøndsted Nielsen and co-workers [19] also performed lifetime experiments on
singly deprotonated DNA homopolymers of adenine, dA 2 , dA 3 , and dA 4 (all with one
deprotonated phosphate group) and on doubly deprotonated dA 4 (two deprotonated
phosphate groups). Decay curves obtained after photoexcitation at 210 nm and
250 nm are shown in Fig. 10.4. The decay behaviour of dA 2 and dA 3 is ascribed to
one-photon absorption, while that of dA 4 is due to two-photon absorption. Not
surprisingly, the decay is faster at the higher photon energy, being complete within
half a millisecond at 210 nm and about 2.5 ms at 250 nm for the dA 2 anion. It is
noteworthy that, even with 210-nm light (5.9 eV) where the photon energy is above
the adiabatic electron detachment energies (5.72 eV and 5.62 eV for dA 2 and dA 3
monoanions, respectively [55]), delayed dissociation is still observed. This indicates
that internal conversion back to the ground state is quick enough to compete with the
crossing from the electronically excited state to a state in the electron detachment
continuum, where an electron is liberated. Unfortunately, the measurement did not
provide direct information on the branching ratio between the two deexcitation
10 UV Photophysics of DNA and RNA Nucleotides In Vacuo: Dissociation. . .
191
[AMP–H]
À anions dissociate with two different time constants of 95 ns and 2.4 μs,
with branching ratios of 54 % and 46 %, respectively [31]. The 2.4-μs time constant is
likely associated with statistical dissociation, although it is significantly shorter than
that measured at ELISA (16 μs) [17]. It is difficult to explain this behaviour unless the
internal energy distributions of the ion populations are different in the two
experiments. Also, the 94-ns decay is hard to reconcile with the results from the
ELISA experiment, where decay was observed to be predominantly on the microsecond time scale. The authors exclude the possibility of two-photon absorption based
on measurements of the yield of photoneutrals as a function of laser power. However,
such a conclusion is difficult to draw if there are too few ions in the interaction region.
Indeed, Fayeton and co-workers [29] found no such short lifetime component for the
similar [dAMP–H]
À anion using 263-nm light. Instead, they found that all dissociation took place with a time constant of 1 μs, which is not substantially different than
the 2.4-μs constant measured by Andersen and co-workers but is significantly shorter
than that obtained from the ELISA experiment. In the case of [AMP+H]
+ cations,
Andersen and co-workers [31] found two short dissociation times of 85 ns and 640 ns,
which would both be too short to be measured in the delayed dissociation experiment
at ELISA and are therefore in accordance with the conclusion from the beamdepletion results [17]. On the other hand, these times are much longer than expected
for dissociation from electronically excited states as well as after IC but prior to IVR.
However, we caution that this result does not exclude prompt bond ruptures since the
ionic fragment and the neutral fragment may stick together as a long-lived, ionmolecule encounter complex. Such complexes are often formed and may dissociate
on time scales of up to tens of nanoseconds [54]. The only way to fully exclude
prompt fragmentation would be to perform pump-probe experiments on the same
time scale as the initial energy conversion processes (i.e., the 10
À13 to 10
À11 s time
scale). More work is clearly needed to fully understand the different results obtained
from various experimental setups. In this regard, storage-ring experiments with
shorter revolution times of about five microseconds would be valuable.
Brøndsted Nielsen and co-workers [19] also performed lifetime experiments on
singly deprotonated DNA homopolymers of adenine, dA 2 , dA 3 , and dA 4 (all with one
deprotonated phosphate group) and on doubly deprotonated dA 4 (two deprotonated
phosphate groups). Decay curves obtained after photoexcitation at 210 nm and
250 nm are shown in Fig. 10.4. The decay behaviour of dA 2 and dA 3 is ascribed to
one-photon absorption, while that of dA 4 is due to two-photon absorption. Not
surprisingly, the decay is faster at the higher photon energy, being complete within
half a millisecond at 210 nm and about 2.5 ms at 250 nm for the dA 2 anion. It is
noteworthy that, even with 210-nm light (5.9 eV) where the photon energy is above
the adiabatic electron detachment energies (5.72 eV and 5.62 eV for dA 2 and dA 3
monoanions, respectively [55]), delayed dissociation is still observed. This indicates
that internal conversion back to the ground state is quick enough to compete with the
crossing from the electronically excited state to a state in the electron detachment
continuum, where an electron is liberated. Unfortunately, the measurement did not
provide direct information on the branching ratio between the two deexcitation
10 UV Photophysics of DNA and RNA Nucleotides In Vacuo: Dissociation. . .
191