Near-UV photodissociation dynamics of CH2I2

Abstract

The near-UV photodissociation dynamics of CH₂I₂ has been investigated using a combination of velocity-map (slice) ion imaging and ab initio calculations characterizing the excited states. Ground state I(²P_3/2) and spin–orbit excited I*(²P_1/2) atoms were probed using 2 + 1 resonance-enhanced multiphoton ionization (REMPI) or with single-photon VUV ionization. Two-color ion images were recorded at pump wavelengths of 355 nm, 266 nm and 248 nm, and one-color ion images at the REMPI wavelengths of ∼304 nm and ∼280 nm. Analysis of the ion images shows that, regardless of iodine spin–orbit state, ∼20% of the available energy is partitioned into translation E_T at all excitation wavelengths indicating that the CH₂I co-fragment is formed highly internally excited. The translational energy distributions comprise a slow, “statistical” component that peaks near zero and faster components that peak away from zero. The slow component makes an increasingly large contribution to the distribution as the excitation wavelength is decreased. The C–I bond dissociation energy of D₀ = 2.155 ± 0.008 eV is obtained from the trend in the E_T release of the faster components with increasing excitation energy. The I and I* ion images are anisotropic, indicating prompt dissociation, and are characterized by β parameters that become increasingly positive with increasing E_T. The decrease in β at lower translational energies can be attributed to deviation from axial recoil. MRCI calculations including spin–orbit coupling have been performed to identify the overlapping features in the absorption spectrum and characterize one-dimensional cuts through the electronically excited potential energy surfaces. The excited states are of significantly mixed singlet and triplet character. At longer wavelengths, excitation directly accesses repulsive states primarily of B₁ symmetry, consistent with the observed 〈β〉, while shorter wavelengths accesses bound states, also of B₁ symmetry that are crossed by repulsive states.