Vibrationally Mediated Photodissociation of CH₃I [v₁ = 1] at 277.5 nm: The Vibrationally Adiabatic Process

Abstract

From the photofragment translational spectra of C-H symmetric stretch excited CH3I [v1 = 1, v2 = 0] photodissociatioin at 277.5 nm, the vibrational distribution of photofragments CH3 (v1 = 0, v2 = 0), (0,1), (1,0), (1,1) in the I* channel are measured to be 0.02, 0.02, 0.47, 0.25, and those of CH3 (1,0), (1,1) in the I channel are 0.04, 0.05, respectively. It shows that most of the dissociated CH3I [1,0] retain the C-H symmetric stretch vibration v1 = 1 in the photofragments CH3, and the vibrational distribution in umbrella bending mode is not seriously affected by the original C-H symmetric stretch excitation. The photodissociation of CH3I [1,0] mainly follows the vibrationally adiabatic process. The original vibrational excitation [v1 = 1] of CH3I is quite like a spectator, and the intramolecular vibrational-energy redistribution (IVR) does not play obvious part during photodissociation.

Stretching vibration is a spectator in nucleophilic substitution

“…The fraction of excited molecules was probed by a scheme adapted from Hu et al ( 39 ). At first, the infrared excitation pulse was passed through the molecular beam entrained with CH 3 I.…”

New insights into the photodissociation of methyl iodide at 193 nm: stereodynamics and product branching ratios

“…The effect of the initial vibrational excitation in the parent CH 3 I molecule was studied by the same authors 35 in a vibrationally mediated photodissociation experiment at 277.5 nm, where the CH 3 vibrational distribution produced in the photodissociation of CH 3 I(ν=0) and CH 3 I(ν 1 =1) molecules were compared. In addition, the TOF spectra measured at different pump and probe laser polarization configurations showed that the CH 3 fragments pro- duced in correlation with ground state I atoms are generated significantly hotter than those produced in the CH 3 (ν)+I * channel.…”

“…34,35 The CH 3 (ν 2 ) activity in the CH 3 +I channel has been traditionally assigned to the change in the equilibrium geometry of the umbrella bending angle occurring in the 1 Q 1 ← 3 Q 0 curve crossing. In the center of the Aband, the absorption is produced mainly to the 3 Q 0 state, which due to the similar CH 3 I equilibrium geometry with the ground state, preserves adiabatically the vibrational content of the parent molecule.…”

“…The authors concluded that the direct 1 Q 1 ' 3 Q 0 curve crossing induces larger vibrational activity in the CH 3 fragment than the inverse 3 Q 0 ' 1 Q 1 curve crossing. The effect of the initial vibrational excitation in the parent CH 3 I molecule was studied by the same authors 35 in a vibrationally mediated photodissociation experiment at 277.5 nm, where the CH 3 vibrational distribution produced in the photodissociation of CH 3 I(n = 0) and CH 3 I(n 1 = 1) molecules is compared. The results showed that more than 90% of the C-H symmetric stretch vibration of the parent CH 3 I is retained in the CH 3 fragment.…”

“…In the center of the A-band, the absorption is produced mainly to the 3 Q 0 state, which due to the similar CH 3 I equilibrium geometry with the ground state, preserves adiabatically the vibrational content of the parent molecule. 34,35 The CH 3 (n 2 ) activity in the CH 3 + I channel has been traditionally assigned to the change in the equilibrium geometry of the umbrella bending angle occurring in the 1 Q 1 ' 3 Q 0 curve crossing. A similar argument stands for the dissociation at the B-band origin: the 3 R 1 Rydberg and ground X ˜1A 1 states present similar equilibrium geometries in the Franck-Condon region and, since the crossing with the A-band 3 A 1 (E) state is produced near the equilibrium distance, the initial vibrational excitation is preserved in the dissociation process.…”