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((2)P3/2) and spin-orbit excited I*((2)P1/2) 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.

Highlights

  • CH2I2 - CH2I + I D0 = 2.155 Æ 0.008 eV (1)CH2I2 - CH2I + I* D0 = 3.098 Æ 0.008 eV (2)The bond dissociation energies have been derived from the results of this work

  • EAVL can be partitioned into internal excitation of the CH2I radical EINT, spin–orbit excitation of the iodine atom ESO and relative translation ET, which is derived from the images

  • We consider the implications for kinetics studies which use photolysis of CH2I2 as a source of CH2I radicals

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Summary

Introduction

The bond dissociation energies have been derived from the results of this work (vide infra). The electronic ground state geometry of CH2I2 was optimized at the explicitly correlated coupled cluster with single, double and perturbative triple excitations [CCSD(T)-F12b] level of theory,[31,32] using the correlation consistent cc-pVQZ-F12 basis sets for carbon and hydrogen,[33] and the cc-pVQZ-PP-F12 basis set paired with the small-core relativistic pseudopotential (PP) ECP28MDF for iodine.[34,35] The density fitting of the Fock and exchange matrices used the cc-pVQZ/JKFit and def2-QZVPP/ JKFit auxiliary basis sets,[36,37] while the aug-cc-pwCV5Z/MP2Fit and cc-pVQZ-PP-F12/MP2Fit sets were used in the density fitting of the remaining two-electron integrals.[34,38] The manyelectron integrals arising in explicit correlation theory were accurately and efficiently approximated using the resolution-ofthe-identity method with cc-pVQZ-F12/OptRI and cc-pVQZ-PPF12/OptRI used in the complementary auxiliary basis set (CABS) approach.[34,39,40] The geminal Slater exponent was set to 1.0 a0À1 in all cases Such combinations of basis sets and pseudopotentials will be referred to as cc-pVnZ-F12 . All calculations were performed using the MOLPRO 2012.1 quantum chemistry package.[47,48]

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