6 Biomolecules, Photostability and 1 πσ ∗ States
123
by ultrafast dissociation timescales of < 100 fs (see for example [20]). Alternatively, dissociating flux can evolve adiabatically around CI2 (process 2, solid blue
arrow), and generate AX radicals in an electronically excited state, AX ∗ , together
with H [19, 24]. However, in scenarios where the dissociative flux does not have
enough energy to access the higher energy dissociation asymptote (e.g. initial photopreparation of wavepacket b in Fig. 6.2), some fraction may return towards CI2
(dashed blue arrow) and non-adiabatically couple back onto S 0 , forming thermally
(vibrationally) hot S 0 molecules [25]. In the gas phase, these vibrationally hot S 0
species may ultimately undergo statistical unimolecular dissociation to generate Hatoms with low amounts of KE on a timescale as fast as picoseconds [25, 26].
A third scenario that can occur involves excited state flux tunneling through an
energy barrier to access the 1 πσ ∗ surface (see for example [27–29]). Wavepacket
b in Fig. 6.2 correlates to flux which has been photo-prepared on 1 ππ ∗ below the
energy of CI1. Depending on the size of the barrier area under CI1, the wave-like
nature of the flux (in this instance the dissociating H-atom) may enable tunneling under CI1 (process 3, dashed red arrow) and coupling onto the 1 πσ ∗ state at extended
AX–H distances (H tunneling is revisited in Sect. 6.4.3). Timescales for H-atom
elimination dynamics mediated through tunneling are inherently linked to the size
of the barrier and can range up to nanoseconds [29], but none-the-less, still generate
high KE H-atoms as a result of subsequent dissociation on the 1 πσ ∗ surface.
In the following section we introduce experimental techniques which have been
used to identify the participation of these processes in 1 πσ ∗ based dynamics, with
particular focus on methods which monitor the appearance of H-atom photoproducts.
6.3 Experimental Detection of 1 πσ ∗ Mediated Dynamics
A number of spectroscopic techniques in the gas phase have been utilized to characterize 1 πσ ∗ mediated dynamics in heteroaromatic chromophores, from both a
frequency- and time-domain perspective. Frequency-domain measurements can offer precise information regarding the energetics of the dissociation process and the
vibrational motions involved, while time-domain measurements are often able to observe direct participation of these states and their ultrafast temporal evolution. The
complementary information yielded from both domains has proved highly fruitful in
understanding the role of 1 πσ ∗ states in the excited state dynamics of a broad range
of biomolecules and their subunits (see for example [18, 20, 27, 29–33]). Whilst
this chapter focuses on time-domain experiments, we briefly list both approaches
for completeness. High resolution frequency-resolved studies have been dominated
by photofragment translational spectroscopies, in particular multi-mass ion imaging
[34, 35], H (Rydberg) atom photofragment translational spectroscopy [18, 36] and
velocity map ion imaging [23, 37]. Time-resolved techniques have included timeresolved photoelectron spectroscopy [31, 38, 39], time-resolved mass spectrometry
(TR-MS) [25, 27, 40, 41] and time-resolved velocity map ion imaging (TR-VMI)
123
by ultrafast dissociation timescales of < 100 fs (see for example [20]). Alternatively, dissociating flux can evolve adiabatically around CI2 (process 2, solid blue
arrow), and generate AX radicals in an electronically excited state, AX ∗ , together
with H [19, 24]. However, in scenarios where the dissociative flux does not have
enough energy to access the higher energy dissociation asymptote (e.g. initial photopreparation of wavepacket b in Fig. 6.2), some fraction may return towards CI2
(dashed blue arrow) and non-adiabatically couple back onto S 0 , forming thermally
(vibrationally) hot S 0 molecules [25]. In the gas phase, these vibrationally hot S 0
species may ultimately undergo statistical unimolecular dissociation to generate Hatoms with low amounts of KE on a timescale as fast as picoseconds [25, 26].
A third scenario that can occur involves excited state flux tunneling through an
energy barrier to access the 1 πσ ∗ surface (see for example [27–29]). Wavepacket
b in Fig. 6.2 correlates to flux which has been photo-prepared on 1 ππ ∗ below the
energy of CI1. Depending on the size of the barrier area under CI1, the wave-like
nature of the flux (in this instance the dissociating H-atom) may enable tunneling under CI1 (process 3, dashed red arrow) and coupling onto the 1 πσ ∗ state at extended
AX–H distances (H tunneling is revisited in Sect. 6.4.3). Timescales for H-atom
elimination dynamics mediated through tunneling are inherently linked to the size
of the barrier and can range up to nanoseconds [29], but none-the-less, still generate
high KE H-atoms as a result of subsequent dissociation on the 1 πσ ∗ surface.
In the following section we introduce experimental techniques which have been
used to identify the participation of these processes in 1 πσ ∗ based dynamics, with
particular focus on methods which monitor the appearance of H-atom photoproducts.
6.3 Experimental Detection of 1 πσ ∗ Mediated Dynamics
A number of spectroscopic techniques in the gas phase have been utilized to characterize 1 πσ ∗ mediated dynamics in heteroaromatic chromophores, from both a
frequency- and time-domain perspective. Frequency-domain measurements can offer precise information regarding the energetics of the dissociation process and the
vibrational motions involved, while time-domain measurements are often able to observe direct participation of these states and their ultrafast temporal evolution. The
complementary information yielded from both domains has proved highly fruitful in
understanding the role of 1 πσ ∗ states in the excited state dynamics of a broad range
of biomolecules and their subunits (see for example [18, 20, 27, 29–33]). Whilst
this chapter focuses on time-domain experiments, we briefly list both approaches
for completeness. High resolution frequency-resolved studies have been dominated
by photofragment translational spectroscopies, in particular multi-mass ion imaging
[34, 35], H (Rydberg) atom photofragment translational spectroscopy [18, 36] and
velocity map ion imaging [23, 37]. Time-resolved techniques have included timeresolved photoelectron spectroscopy [31, 38, 39], time-resolved mass spectrometry
(TR-MS) [25, 27, 40, 41] and time-resolved velocity map ion imaging (TR-VMI)