10 Surface-Aligned Femtochemistry: Dynamics on Oxide Surfaces
245
pump photon (Fig. 10.9d), while the detection of the methyl signal through ionization involves two probe photons at 130 fs time delay (Fig. 10.9e) and three probe
photons at 2500 fs delay time (Fig. 10.9f) [28].
The fitting of a ‘rise and decay’—in conjunction with a ‘delayed exponential
rise’-model to the transient in Fig. 10.9c resulted in similar rise and decay time constants for the peak as in Figs. 10.9a and 10.9b. For the exponential rise at longer
delay times, a time constant of τ 3 = 680 ± 50 fs with an initial coherent delay of
t 0 = 170 fs was obtained from the fitting procedure. The two separate components of the fit, peak and delayed rise, respectively, are indicated by the dashed
lines in Fig. 10.9c. Identical fitting parameters with adjusted amplitudes also fitted
the transient CD
+
3 signal obtained at lower probe laser power (see dashed curves
in Fig. 10.9b). For the interpretation of these transient data the gas phase dissociation dynamics reported in the literature in conjunction with the respective potential
energy curves displayed in Fig. 10.7c will be considered.
First gas-phase time-resolved investigations of CH 3 I photodissociation dynamics
via A-band employing fs pump-probe technique in conjunction with mass spectrometry were performed by Zewail and coworkers [58, 102]. The transient data obtained
in these experiments indeed exhibited a similar appearance of a peak structure followed by an exponential rise as the transients shown in Fig. 10.9, however, with
completely different time constants [58]. In the gas-phase investigations, the peak
structure was attributed to the dynamics of the dissociating excited transition state of
methyl iodide, CH 3 I ∗‡ , which was directly ionized by the probe pulse immediately
after A-band excitation. Subsequent, rapid decomposition of the CH 3 I +∗ ion leads
to the observed CH
+
3 fragment. The escape of the wavepacket from the CH 3 I ∗‡ transition state Franck-Condon region was determined to be about 50 fs or even faster
[58, 62, 102].
In contrast, the delayed rise of the transient signal was attributed to the direct
REMPI detection of the emerging neutral fragments (I [58, 102] or CH 3 [59, 60])
reflecting the dynamics of their liberation from the force field of each other in coherent motion along the dissociative potential energy surfaces (cf. Fig. 10.7c). A dissociation time of 120 fs could be established for the CH 3 + I ∗ channel by monitoring
the I + signal intensity by means of (2 + 1) REMPI detection at 304 nm [58].
Recent gas-phase investigations performed by Bañares and coworkers employing fs-laser pump-probe spectroscopy in conjunction with velocity map imaging
have monitored several well defined channels of the C–I bond breakage of the
CH 3 I molecule via A-band excitation by detecting the methyl radical via (2 + 1)
REMPI at 333.5 nm [59, 60]. Bañares and coworkers, found that the CH 3 I dissociation yielding vibrationless methyl radicals as well as ground and spin-orbit excited
iodine atoms occurs in 40 fs and 80 fs, respectively. The CH 3 I dissociation yielding
the iodine fragments in the ground state and vibrationally excited methyl radicals
(CH 3 (1 1
1 )) occurs in 135 fs.
Because of the similar appearance of the measured methyl transient signals (cf.
Fig. 10.9a–c), our interpretation is based on the gas-phase explanation of the molecular dynamics of transition state and emerging fragments, respectively. On the basis
of this assignment and by considering the potential energy diagram of methyl iodide
245
pump photon (Fig. 10.9d), while the detection of the methyl signal through ionization involves two probe photons at 130 fs time delay (Fig. 10.9e) and three probe
photons at 2500 fs delay time (Fig. 10.9f) [28].
The fitting of a ‘rise and decay’—in conjunction with a ‘delayed exponential
rise’-model to the transient in Fig. 10.9c resulted in similar rise and decay time constants for the peak as in Figs. 10.9a and 10.9b. For the exponential rise at longer
delay times, a time constant of τ 3 = 680 ± 50 fs with an initial coherent delay of
t 0 = 170 fs was obtained from the fitting procedure. The two separate components of the fit, peak and delayed rise, respectively, are indicated by the dashed
lines in Fig. 10.9c. Identical fitting parameters with adjusted amplitudes also fitted
the transient CD
+
3 signal obtained at lower probe laser power (see dashed curves
in Fig. 10.9b). For the interpretation of these transient data the gas phase dissociation dynamics reported in the literature in conjunction with the respective potential
energy curves displayed in Fig. 10.7c will be considered.
First gas-phase time-resolved investigations of CH 3 I photodissociation dynamics
via A-band employing fs pump-probe technique in conjunction with mass spectrometry were performed by Zewail and coworkers [58, 102]. The transient data obtained
in these experiments indeed exhibited a similar appearance of a peak structure followed by an exponential rise as the transients shown in Fig. 10.9, however, with
completely different time constants [58]. In the gas-phase investigations, the peak
structure was attributed to the dynamics of the dissociating excited transition state of
methyl iodide, CH 3 I ∗‡ , which was directly ionized by the probe pulse immediately
after A-band excitation. Subsequent, rapid decomposition of the CH 3 I +∗ ion leads
to the observed CH
+
3 fragment. The escape of the wavepacket from the CH 3 I ∗‡ transition state Franck-Condon region was determined to be about 50 fs or even faster
[58, 62, 102].
In contrast, the delayed rise of the transient signal was attributed to the direct
REMPI detection of the emerging neutral fragments (I [58, 102] or CH 3 [59, 60])
reflecting the dynamics of their liberation from the force field of each other in coherent motion along the dissociative potential energy surfaces (cf. Fig. 10.7c). A dissociation time of 120 fs could be established for the CH 3 + I ∗ channel by monitoring
the I + signal intensity by means of (2 + 1) REMPI detection at 304 nm [58].
Recent gas-phase investigations performed by Bañares and coworkers employing fs-laser pump-probe spectroscopy in conjunction with velocity map imaging
have monitored several well defined channels of the C–I bond breakage of the
CH 3 I molecule via A-band excitation by detecting the methyl radical via (2 + 1)
REMPI at 333.5 nm [59, 60]. Bañares and coworkers, found that the CH 3 I dissociation yielding vibrationless methyl radicals as well as ground and spin-orbit excited
iodine atoms occurs in 40 fs and 80 fs, respectively. The CH 3 I dissociation yielding
the iodine fragments in the ground state and vibrationally excited methyl radicals
(CH 3 (1 1
1 )) occurs in 135 fs.
Because of the similar appearance of the measured methyl transient signals (cf.
Fig. 10.9a–c), our interpretation is based on the gas-phase explanation of the molecular dynamics of transition state and emerging fragments, respectively. On the basis
of this assignment and by considering the potential energy diagram of methyl iodide