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G.M. Roberts and V.G. Stavros
formation of an electrostatic lens, which causes ionized photofragments with the
same velocity vectors to be focused onto the same position of the 2-D detector
plane, regardless of their initial position within the ionization volume of the focused
probe laser. As a result, carefully designed VMI arrangements, in combination with
high-resolution frequency-domain studies, can deliver an energy resolution down to
= 0.38 % [49]. Although this energy resolution is not achievable in ultrafast
TR-VMI experiments, due to the inherently broad bandwidth of fs pulses (typically
hundreds of cm −1 ), coupling ultrafast pump-probe spectroscopy with VMI still vitally affords temporal and energy information (as well as photofragment angular recoil information—vide infra), regarding photodissociation mechanisms, unlike TRMS. For completeness, we highlight that, very recently, magnetic-bottle analyzers
(which are traditionally used to perform photoelectron spectroscopy [38]) have been
adapted to perform time-resolved photofragment translational spectroscopy measurements on H + [50, 51], also delivering energy and time information about 1 πσ ∗
dynamics.
At each pump-probe time delay, t, the desired 1-D TKER distribution can
be recovered from a measured 2-D H + velocity map image using image reconstruction methods [52–54]. Reconstruction algorithms return the 1-D radial (r)
spectrum, which may then be converted into KE space using an appropriate Jacobian (r 2 ∝ KE) and a KE calibration factor. The latter is obtained by measuring
photofragments from a well characterized dissociation event (e.g. ionizing H-atoms
from photolyzed HBr [55]). The measured KE of the photofragments may finally be
converted into the desired TKER scale according to:
TKER = KE
m p
m p − m f
(6.2)
where m p is the mass of the parent molecule.
When considering 1 πσ ∗ mediated H-atom elimination dynamics, the different
signal features in the derived TKER distribution can be related back to different
photodissociation mechanisms. We consider this with respect to the schematic potentials in Fig. 6.4(a). Dynamics proceeding along dissociative 1 πσ ∗ states typically
generate features with large TKERs [18]. For a parent species, AX–H, in its zero
point vibrational level prior to photoexcitation, partitioning of the available energy
into TKER during this dissociation process can be understood according to:
TKER = hν pu − D 0 (AX–H) − E elec − E vib
(6.3)
where hν pu is the pump photon energy, D 0 (AX–H) is the AX–H bond strength, and
E elec and E vib are the electronic and vibrational energies of the radical co-fragments,
AX. The maximum TKER, TKER max , after AX–H dissociation along the 1 πσ ∗
surface corresponds to a scenario where radical co-fragments are formed with zero
internal energy (E elec = 0 and E vib = 0), following non-adiabatic dynamics through
the 1 πσ ∗ /S 0 CI. In this case, Eq. (6.3) simplifies to:
TKER max = hν pu − D 0 (AX–H)
(6.4)
G.M. Roberts and V.G. Stavros
formation of an electrostatic lens, which causes ionized photofragments with the
same velocity vectors to be focused onto the same position of the 2-D detector
plane, regardless of their initial position within the ionization volume of the focused
probe laser. As a result, carefully designed VMI arrangements, in combination with
high-resolution frequency-domain studies, can deliver an energy resolution down to
= 0.38 % [49]. Although this energy resolution is not achievable in ultrafast
TR-VMI experiments, due to the inherently broad bandwidth of fs pulses (typically
hundreds of cm −1 ), coupling ultrafast pump-probe spectroscopy with VMI still vitally affords temporal and energy information (as well as photofragment angular recoil information—vide infra), regarding photodissociation mechanisms, unlike TRMS. For completeness, we highlight that, very recently, magnetic-bottle analyzers
(which are traditionally used to perform photoelectron spectroscopy [38]) have been
adapted to perform time-resolved photofragment translational spectroscopy measurements on H + [50, 51], also delivering energy and time information about 1 πσ ∗
dynamics.
At each pump-probe time delay, t, the desired 1-D TKER distribution can
be recovered from a measured 2-D H + velocity map image using image reconstruction methods [52–54]. Reconstruction algorithms return the 1-D radial (r)
spectrum, which may then be converted into KE space using an appropriate Jacobian (r 2 ∝ KE) and a KE calibration factor. The latter is obtained by measuring
photofragments from a well characterized dissociation event (e.g. ionizing H-atoms
from photolyzed HBr [55]). The measured KE of the photofragments may finally be
converted into the desired TKER scale according to:
TKER = KE
m p
m p − m f
(6.2)
where m p is the mass of the parent molecule.
When considering 1 πσ ∗ mediated H-atom elimination dynamics, the different
signal features in the derived TKER distribution can be related back to different
photodissociation mechanisms. We consider this with respect to the schematic potentials in Fig. 6.4(a). Dynamics proceeding along dissociative 1 πσ ∗ states typically
generate features with large TKERs [18]. For a parent species, AX–H, in its zero
point vibrational level prior to photoexcitation, partitioning of the available energy
into TKER during this dissociation process can be understood according to:
TKER = hν pu − D 0 (AX–H) − E elec − E vib
(6.3)
where hν pu is the pump photon energy, D 0 (AX–H) is the AX–H bond strength, and
E elec and E vib are the electronic and vibrational energies of the radical co-fragments,
AX. The maximum TKER, TKER max , after AX–H dissociation along the 1 πσ ∗
surface corresponds to a scenario where radical co-fragments are formed with zero
internal energy (E elec = 0 and E vib = 0), following non-adiabatic dynamics through
the 1 πσ ∗ /S 0 CI. In this case, Eq. (6.3) simplifies to:
TKER max = hν pu − D 0 (AX–H)
(6.4)