Clocking of electronically and vibrationally state-resolved channels of the fast photodissociation of CH 3 I in the A-band is reexamined in a combined experimental and theoretical study. Experimentally, a femtosecond pump-probe scheme is employed in the modality of resonant probing by resonance enhanced multiphoton ionization (REMPI) of the methyl fragment in different vibrational states and detection through fragment velocity map ion imaging (VMI) as a function of the time delay. We revisit excitation to the center of the A-band at 268 nm and report new results for excitation to the blue of the band center at 243 nm. Theoretically, two approaches have been employed to shed light into the observations: first, a reduced dimensionality 4D non-adiabatic wavepacket calculation using the potential energy surfaces by Xie et al. [J. Phys. Chem. A, 104, 1099(2000]; and second, a full dimension 9D trajectory surface-hopping calculation on the same potential energy surfaces, including the quantization of vibrational states of the methyl product.In addition, high level ab initio electronic structure calculations have been carried out to describe the CH 3 3p z Rydberg state involved in the (2+1) REMPI probing process, as a function of the carbon-iodine (C-I) distance. A general qualitative agreement is obtained between experiment and theory, but the effect of methyl vibrational excitation in the umbrella mode on the clocking times is not well reproduced. The theoretical results reveal that no significant effect on the state-resolved appearance times is exerted by the non-adiabatic crossing through the conical intersection present in the first absorption band. The vibrationally-state resolved clocking times observed experimentally can be rationalized when the (2+1) REMPI probing process is considered. None of the other probing methods applied thus far, i.e multiphoton ionization photoelectron spectroscopy, soft Xray inner-shell photoelectron spectroscopy, VUV single-photon ionization and XUV core-to-valence transient absorption spectroscopy, have been able to provide quantum state-resolved (vibrational) clocking times. More experiments would be needed to disentangle the fine details in the clocking times and dissociation dynamics arising from the detection of specific quantum-states of the molecular fragments.
A comparative study of the ultrafast photodissociation dynamics of the dihalomethanes CH2ICl and CH2BrI has been carried out at 268 nm, around the maximum of the first absorption band, employing femtosecond velocity map ion imaging in conjunction with high level ab initio electronic structure calculations and full dimension on-the-fly trajectory calculations including surface hopping. Total translational energy distributions and angular distributions of the iodine fragments as well as reaction times for the C-I bond cleavage are presented and discussed along with the computed absorption spectra, potential energy curves and trajectories. The revealed dynamics is mainly governed by absorption to the 5A' state for CH2BrI while two dissociation pathways, through the 4A' or 5A' states, are in competition for CH2lCI. An anchor effect due to the substituent halogen atom (Br or Cl), which implies significant rotational motion of the dissociating molecule, characterizes the photodissociation in both dihalomethanes and leads to a remarkable rotational energy of the radical co-fragment. This energy flux into the internal degrees of freedom of the molecules is the main key factor governing the real time reaction dynamics.
The time-resolved photodynamics of the methyl iodide cation (CH3I+) are investigated by means of femtosecond XUV–IR pump–probe spectroscopy. A time-delay-compensated XUV monochromator is employed to isolate a specific harmonic, the 9th harmonic of the fundamental 800 nm (13.95 eV, 88.89 nm), which is used as a pump pulse to prepare the cation in several electronic states. A time-delayed IR probe pulse is used to probe the dissociative dynamics on the first excited A ̃ A 1 2 state potential energy surface. Photoelectrons and photofragment ions— C H 3 + and I+—are detected by velocity map imaging. The experimental results are complemented with high level ab initio calculations for the potential energy curves of the electronic states of CH3I+ as well as with full dimension on-the-fly trajectory calculations on the first electronically excited state A ̃ A 1 2 , considering the presence of the IR pulse. The C H 3 + and I+ pump–probe transients reflect the role of the IR pulse in controlling the photodynamics of CH3I+ in the A ̃ A 1 2 state, mainly through the coupling to the ground state X ̃ E 3 / 2 , 1 / 2 2 and to the excited B ̃ E 2 state manifold. Oscillatory features are observed and attributed to a vibrational wave packet prepared in the A ̃ A 1 2 state. The IR probe pulse induces a coupling between electronic states leading to a slow depletion of C H 3 + fragments after the cation is transferred to the ground X ̃ E 3 / 2 , 1 / 2 2 states and an enhancement of I+ fragments by absorption of IR photons yielding dissociative photoionization.
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