246
M.E. Vaida and T.M. Bernhardt
(Fig. 10.7c) an interpretation of the measured reaction times of the methyl iodide
surface reaction will be proposed in the following discussion.
First, the peak structure with the rise and decay time constants of 90 fs, apparent
in all observed transients (cf. Fig. 10.9a–c), will be discussed. In accordance with
the gas-phase interpretation, the peak structure is assigned to the ionization of the
excited methyl iodide before complete decomposition according to the schema:
CH 3 I ads
266 nm
−→ CH 3 I
∗‡
ads → CH 3 I
+∗ 2×333 nm
−→ CH
+
3 + I
(10.1)
The peak structure of the methyl ion signal therefore reflects the passage of the
wave packet initially launched on the 3 Q 0+ state through the transition state FranckCondon region of the two-photon ionization (cf. Fig. 10.7c). The methyl iodide
cation thus generated immediately decomposes leading to the observed methyl ion
signal
In the direct gas-phase dissociation the CH 3 I ∗‡ transition state was detected instantaneously after excitation and decayed faster than 50 fs [58, 60, 102]. In the
case of methyl iodide adsorbed on MgO, however, a 90 fs rise is observed with a
maximum signal around 130 fs followed by a 90 fs decay. This indicates that the
presence of the magnesia surface has a fundamental impact on the dynamics of the
transition state: (1) It influences the excited electronic state structure of methyl iodide so that the optimal Franck-Condon window for CH 3 I ∗‡ detection is reached
only after 130 fs. (2) The process of methyl iodide dissociation is considerably delayed by the presence of the surface. This indicates a strong inelastic interaction of
the CH 3 I ∗‡ transition state with the surface prior to decomposition. (3) If the methyl
iodide molecules would be adsorbed with the iodine facing the magnesia surface
[70, 71], the dissociation of CH 3 I ∗‡ would lead to the prompt ejection of the light
methyl fragments resulting in a time constant for the CH 3 I ∗‡ decay identical to the
gas-phase [78–80]. Thus, the observed dynamics support the adsorption structure
with the methyl heading toward the surface. Further evidence in this respect is provided by the measured dynamics of the methyl fragments emerging from the A-band
dissociation, which will be presented in the following.
The second part of the discussion will be concerned with the delayed exponential
rise that is particularly apparent at high probe laser flux (see Fig. 10.9c). It is assigned, again in accordance with the gas-phase interpretation, to the (2 + 1) REMPI
detection of methyl radicals emerging from the A-band dissociation of the adsorbed
methyl iodide molecules according to the reaction schema (cf. Fig. 10.7c):
CH 3 I ads
266 nm
−→ CH 3 I
∗‡
ads → CH 3 I + I/I
∗ (2+1)×333 nm
−→
CH
+
3 + I/I
∗
(10.2)
The initial delay of t 0 = 170 fs (cf. Fig. 10.9c) of this part of the methyl ion
signal reflects the liberation of the methyl fragments from the molecular force field
and from the force field of the magnesia surface. Gas-phase investigations showed
a 120 fs coherent delay in the methyl fragment appearance [59, 60]. Because of the
considerably prolonged initial delay observed here, the direct ejection of methyl as
in the free molecule can be ruled out. Instead, the data are again in favor of a CH 3 I
adsorption structure with the methyl facing the magnesia substrate (cf. Fig. 10.3a).
M.E. Vaida and T.M. Bernhardt
(Fig. 10.7c) an interpretation of the measured reaction times of the methyl iodide
surface reaction will be proposed in the following discussion.
First, the peak structure with the rise and decay time constants of 90 fs, apparent
in all observed transients (cf. Fig. 10.9a–c), will be discussed. In accordance with
the gas-phase interpretation, the peak structure is assigned to the ionization of the
excited methyl iodide before complete decomposition according to the schema:
CH 3 I ads
266 nm
−→ CH 3 I
∗‡
ads → CH 3 I
+∗ 2×333 nm
−→ CH
+
3 + I
(10.1)
The peak structure of the methyl ion signal therefore reflects the passage of the
wave packet initially launched on the 3 Q 0+ state through the transition state FranckCondon region of the two-photon ionization (cf. Fig. 10.7c). The methyl iodide
cation thus generated immediately decomposes leading to the observed methyl ion
signal
In the direct gas-phase dissociation the CH 3 I ∗‡ transition state was detected instantaneously after excitation and decayed faster than 50 fs [58, 60, 102]. In the
case of methyl iodide adsorbed on MgO, however, a 90 fs rise is observed with a
maximum signal around 130 fs followed by a 90 fs decay. This indicates that the
presence of the magnesia surface has a fundamental impact on the dynamics of the
transition state: (1) It influences the excited electronic state structure of methyl iodide so that the optimal Franck-Condon window for CH 3 I ∗‡ detection is reached
only after 130 fs. (2) The process of methyl iodide dissociation is considerably delayed by the presence of the surface. This indicates a strong inelastic interaction of
the CH 3 I ∗‡ transition state with the surface prior to decomposition. (3) If the methyl
iodide molecules would be adsorbed with the iodine facing the magnesia surface
[70, 71], the dissociation of CH 3 I ∗‡ would lead to the prompt ejection of the light
methyl fragments resulting in a time constant for the CH 3 I ∗‡ decay identical to the
gas-phase [78–80]. Thus, the observed dynamics support the adsorption structure
with the methyl heading toward the surface. Further evidence in this respect is provided by the measured dynamics of the methyl fragments emerging from the A-band
dissociation, which will be presented in the following.
The second part of the discussion will be concerned with the delayed exponential
rise that is particularly apparent at high probe laser flux (see Fig. 10.9c). It is assigned, again in accordance with the gas-phase interpretation, to the (2 + 1) REMPI
detection of methyl radicals emerging from the A-band dissociation of the adsorbed
methyl iodide molecules according to the reaction schema (cf. Fig. 10.7c):
CH 3 I ads
266 nm
−→ CH 3 I
∗‡
ads → CH 3 I + I/I
∗ (2+1)×333 nm
−→
CH
+
3 + I/I
∗
(10.2)
The initial delay of t 0 = 170 fs (cf. Fig. 10.9c) of this part of the methyl ion
signal reflects the liberation of the methyl fragments from the molecular force field
and from the force field of the magnesia surface. Gas-phase investigations showed
a 120 fs coherent delay in the methyl fragment appearance [59, 60]. Because of the
considerably prolonged initial delay observed here, the direct ejection of methyl as
in the free molecule can be ruled out. Instead, the data are again in favor of a CH 3 I
adsorption structure with the methyl facing the magnesia substrate (cf. Fig. 10.3a).