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Bromine atom (Br) reactions lead to ozone depletion in the troposphere and stratosphere. Photodegradation of bromocarbons is one of the main sources of bromine atoms in the atmosphere. Here, we use high-level ab initio methods, including spin–orbit effects, to study the photodissociation of the CH2Br radical. All possible fragmentation pathways, namely CH2Br + hν → CH2 + Br, HCBr + H, and CBr + H2, have been analyzed. Potential-energy curves of the ground and several excited electronic states along the corresponding dissociating bond distance of each pathway have been calculated. Considering the actinic fluxes of solar irradiation in the troposphere and in the stratosphere in the relevant range of frequencies, it is found that the first five excited states of CH2Br can be accessed from the ground state. Analysis of the potential curves shows that the pathways producing CH2 + Br and HCBr + H can proceed through a fast direct dissociation mechanism, while the pathway leading to CBr + H2 involves much slower dissociation mechanisms like internal conversion between electronic states, predissociation, or tunneling through exit barriers. The main implications are that the two faster channels are predicted to be dominant, and the slower pathway is expected to be less relevant. The tropospheric and stratospheric solar actinic fluxes also allow for further dissociation of the HCBr and CBr fragments, generating additional Br atoms, provided that they survive possible collisions with other atmospheric reagents. Finally, we discuss the possible effect of each of the three CH2Br dissociation pathways on the depletion of atmospheric ozone. [ABSTRACT FROM AUTHOR]

Photodissociation of the CH2I radical and the CH2I+ cation is studied by means of high-level ab initio calculations, including spin–orbit effects. Potential-energy curves (PEC) along the dissociating bond distances involved in some fragmentation pathways of these species are computed for the ground and several excited electronic states. Based on the PECs obtained, the possible photodissociation mechanisms are analyzed and suggested. Significant differences are found between the fragmentation dynamics of the neutral radical and that of the cation. While a relatively simple dissociation dynamics is predicted for CH2I, more complex fragmentation mechanisms involving internal conversion and couplings between different excited electronic states are expected for CH2I+. The species studied here are relevant to atmospheric chemistry, and the present work can help to understand better how their photodissociation may affect chemical processes in the atmosphere. [ABSTRACT FROM AUTHOR]

Stark shift of the 3R1 Rydberg state of CH3I is measured for different Stark field intensities. Photodissociation products, generated after predissociation of the state, were detected when the molecules were excited with photons resonant with the energy difference between the ground and the shifted 3R1 state, allowing the shift to be quantified. Qualitative agreement has been found with a 1D model.

Femtosecond time-resolved velocity map imaging experiments are reported on several vibronic levels of the second absorption band (B-band) of CH3I, including vibrational excitation in the ν2 and ν3 modes of the bound 3R1(E) Rydberg state. Specific predissociation lifetimes have been determined for the 201 and 301 vibronic levels from measurements of time-resolved I*(2P1/2) and CH3 fragment images, parent decay, and photoelectron images obtained through both resonant and non-resonant multiphoton ionization. The results are compared with our previously reported predissociation lifetime measurements for the band origin 000 [Gitzinger et al., J. Chem. Phys. 132, 234313 (2010)]. The result, previously reported in the literature, where vibrational excitation to the C-I stretching mode (ν3) of the CH3I 3R1(E) Rydberg state yields a predissociation lifetime about four times slower than that corresponding to the vibrationless state, whereas predissociation is twice faster if the vibrational excitation is to the umbrella mode (ν2), is confirmed in the present experiments. In addition to the specific vibrational state lifetimes, which were found to be 0.85 ± 0.04 ps and 4.34 ± 0.13 ps for the 201 and 301 vibronic levels, respectively, the time evolution of the fragment anisotropy and the vibrational activity of the CH3 fragment are presented. Additional striking results found in the present work are the evidence of ground state I(2P3/2) fragment production when excitation is produced specifically to the 301 vibronic level, which is attributed to predissociation via the A-band 1Q1 potential energy surface, and the indication of a fast adiabatic photodissociation process through the repulsive A-band 3A1(4E) state, after direct absorption to this state, competing with absorption to the 301 vibronic level of the 3R1(E) Rydberg state of the B-band. [ABSTRACT FROM AUTHOR]

The time-resolved photodissociation dynamics of CH3I in the A-band has been studied theoretically using a wave packet model including four degrees of freedom, namely the C-I dissociation coordinate, the I-CH3 bending mode, the CH3 umbrella mode, and the C-H symmetric stretch mode. Clocking times and final product state distributions of the different dissociation (nonadiabatic) channels yielding spin-orbit ground and excited states of the I fragment and vibrationless and vibrationally excited (symmetric stretch ν1 and umbrella ν2 modes) CH3 fragments have been obtained and compared with the results of femtosecond velocity map imaging experiments. The wave packet calculations are able to reproduce with very good agreement the experimental reaction times for the CH3(ν1, ν2)+I*(2P1/2) dissociation channels with ν1 = 0 and ν2 = 0,1,2, and also for the channel CH3(ν1 = 0, ν2 = 0)+I(2P3/2). However, the model fails to predict the experimental clocking times for the CH3(ν1, ν2)+I(2P3/2) channels with (ν1, ν2) = (0, 1), (0, 2), and (1, 0), that is, when the CH3 fragment produced along with spin-orbit ground state I atoms is vibrationally excited. These results are similar to those previously obtained with a three-dimensional wave packet model, whose validity is discussed in the light of the results of the four-dimensional treatment. Possible explanations for the disagreements found between theory and experiment are also discussed. [ABSTRACT FROM AUTHOR]

The photodissociation of CH3I in the second absorption band (the B-band) has been studied at the wavelength 199.11 nm, coincident with the 301 3R1(E)←X(1A1) CH3I vibronic transition, using a combination of slice imaging and resonance enhanced multiphoton ionization detection of the CH3 fragment. The kinetic energy and angular distributions of the recoiling CH3 fragment confirm a major predissociation dynamics channel as a result of the interaction between the bound 3R1 Rydberg state and the repulsive 3A1(E) state - ascribed to the A-band - yielding CH3 fragments in correlation with spin-orbit excited I*(2P1/2) atoms. In addition, first evidence of a non-negligible population of ground state I(2P3/2) atoms in the CH3 fragment slice images, suggests a secondary predissociation mechanism via interaction between the 3R1 Rydberg state and the repulsive A-band 1Q1 state. [ABSTRACT FROM AUTHOR]

The photodissociation of acetaldehyde in the radical channel has been studied at wavelengths between 315 and 325 nm using the velocity-map imaging technique. Upon one-photon absorption at 315 nm, the molecule is excited to the first singlet excited state S1, which, in turn, undergoes intersystem crossing to the first excited triplet state T1. On the triplet surface, the molecule dissociates into CH3 and HCO radicals with large kinetic energy release (KER), in accordance with the well characterized exit barrier on T1. However, at longer wavelengths (>320 nm), which correspond to excitation energies just below the triplet barrier, a sudden change in KER is observed. At these photolysis wavelengths, there is not enough energy to surpass the exit barrier on the triplet state, which leaves the possibility of unimolecular dissociation on S0 after internal conversion from S1. We have characterized the fragments’ KER at these wavelengths, as well as determined the energy partitioning for the radical fragments. A new accurate estimate of the barrier height on T1 is presented. [ABSTRACT FROM AUTHOR]

A femtosecond pump-probe experiment, coupled with velocity map ion imaging, is reported on the second absorption band (B-band) of CH3I. The measurements provide a detailed picture of real-time B-band predissociation in the band origin at 201.2 nm. Several new data are reported. (i) A value of 1.5±0.1 ps has been obtained for the lifetime of the excited state, consistent within errors with the only other direct measurement of this quantity [A. P. Baronavski and J. C. Owrutsky, J. Chem. Phys. 108, 3445 (1998)]. (ii) It has been possible to measure the angular character of the transition directly through the observation of fragments appearing early with respect to both predissociation lifetime and molecular rotation. (iii) Vibrational activity in CH3 has been found, both in the umbrella (ν2) and the symmetric stretch (ν1) modes, with estimates of relative populations. All these findings constitute a challenge and a test for much-wanted high level ab initio and dynamics calculations in this energy region. [ABSTRACT FROM AUTHOR]

The photodissociation of methyl iodide at different wavelengths in the red edge of the A-band (286–333 nm) has been studied using a combination of slice imaging and resonance enhanced multiphoton ionization detection of the methyl fragment in the vibrational ground state (ν=0). The kinetic energy distributions (KED) of the produced CH3(ν=0) fragments show a vibrational structure, both in the I(2P3/2) and I*(2P1/2) channels, due to the contribution to the overall process of initial vibrational excitation in the ν3(C–I) mode of the parent CH3I. The structures observed in the KEDs shift toward upper vibrational excited levels of CH3I when the photolysis wavelength is increased. The I(2P3/2)/I*(2P1/2) branching ratios, photofragment anisotropies, and the contribution of vibrational excitation of the parent CH3I are explained in terms of the contribution of the three excited surfaces involved in the photodissociation process, 3Q0, 1Q1, and 3Q1, as well as the probability of nonadiabatic curve crossing 1Q1←3Q0. The experimental results are compared with multisurface wave packet calculations carried out using the available ab initio potential energy surfaces, transition moments, and nonadiabatic couplings, employing a reduced dimensionality (pseudotriatomic) model. A general qualitative good agreement has been found between theory and experiment, the most important discrepancies being in the I(2P3/2)/[I(2P3/2)+I*(2P1/2)] branching ratios. Inaccuracies of the available potential energy surfaces are the main reason for the discrepancies. [ABSTRACT FROM AUTHOR]

A nonresonant femtosecond laser pulse centered at 802 nm is used to probe the real time photodissociation dynamics of CH3I in the A-band at 267 nm. Using multiphoton ionization with this probe laser pulse and velocity map ion imaging of CH3+, we have followed the time evolution of the translational energy and spatial anisotropy of the CH3 fragment, which in turn has permitted to image the C–I bond breaking from the initial Franck–Condon region up to the final products along the reaction coordinate. Given the temporal width of our pump and probe laser pulses (∼80 fs), a mechanism is proposed by which transient species are probed by simultaneous absorption of pump and probe laser pulses through intermediate Rydberg and ionic states of CH3I while the pump and probe pulses overlap in time. This study shows how the combination of femtosecond multiphoton ionization and ion imaging techniques provides an ideal tool to resolve in time the different stages of the bond breaking event in a polyatomic molecule. [ABSTRACT FROM AUTHOR]

The CH3(X 2A1)+SH(X 2Π) channel of the photodissociation of CH3SH has been investigated at several wavelengths in the first 1 1A″←X 1A′ and second 2 1A″←X 1A′ absorption bands by means of velocity map imaging of the CH3 fragment. A fast highly anisotropic (β=-1±0.1) CH3(X 2A1) signal has been observed in the images at all the photolysis wavelengths studied, which is consistent with a direct dissociation process from an electronically excited state by cleavage of the C–S bond in the parent molecule. From the analysis of the CH3 images, vibrational populations of the SH(X 2Π) counterfragment have been extracted. In the second absorption band, the SH fragment is formed with an inverted vibrational distribution as a consequence of the forces acting in the crossing from the bound 2 1A″ second excited state to the unbound 1 1A″ first excited state. The internal energy of the SH radical increases as the photolysis wavelength decreases. In the case of photodissociation via the first excited state, the direct production of CH3 leaves the SH counterfragment with little internal excitation. Moreover, at the longer photolysis wavelengths corresponding to excitation to the 1 1A″ state, a slower anisotropic CH3 channel has been observed (β=-0.8±0.1) consistent with a two step photodissociation process, where the first step corresponds to the production of CH3S(X 2E) radicals via cleavage of the S–H bond in CH3SH, followed by photodissociation of the nascent CH3S radicals yielding CH3(X 2A1)+S(X 3P0,1,2). [ABSTRACT FROM AUTHOR]

The reaction times of several well-defined channels of the C–I bond rupture of methyl iodide from the A band, which involves nonadiabatic dynamics yielding ground state I(2P3/2) and spin-orbit excited I*(2P1/2) and ground and vibrationally excited CH3 fragments, have been measured by a combination of a femtosecond laser pump-probe scheme and velocity map imaging techniques using resonant detection of ground state CH3 fragments. The reaction times found for the different channels studied are directly related with the nonadiabatic nature of this multidimensional photodissociation reaction. [ABSTRACT FROM AUTHOR]