Stratospheric condensation nuclei variations may relate to solar activity

HSO3Cl: a prototype molecule for studying OH-stretching overtone induced photodissociation

Vibrationally induced photodissociation in sulfurochloridic acid (HSO₃Cl) is found to be a viable process to form SO₃and HCl from excitations of the OH-stretching overtone starting atν_OH= 4.

Competitive reaction pathways in vibrationally induced photodissociation of H2SO4

Vibrationally induced photodissociation of sulfuric acid into H2O + SO3 is investigated based on reactive molecular dynamics (MD) simulations. Multisurface adiabatic reactive MD simulations allow us to follow both, H-transfer and water elimination after excitation of the ν9 OH-stretching mode. Analysis of several thousand trajectories finds that the H2O and SO3 fragments have distinct final state distributions with respect to translational, rotational, and vibrational degrees of freedom. Rotational distributions peak at quantum numbers j ≤ 5 for water and j ≈ 60 for SO3. The final state distributions should be useful in identifying products in forthcoming experiments. Based on the MD trajectories, a kinetic scheme has been developed which is able to explain most of the trajectory data and suggests that IVR is very rapid. Typical lifetimes of the excited complex range from several 10 picoseconds to hundreds of nanoseconds, depending on the excitation level. Including temperature and pressure profiles characteristic for the stratosphere in the kinetic model shows that excitations higher than ν9 = 4 can significantly contribute to the photolysis rate. This extends and specifies earlier work in that multi-level modeling is required to understand the significance of vibrationally induced decomposition pathways of sulfuric acid in the middle atmosphere.

Vibrationally induced dissociation of sulfuric acid (H2SO4)

One of the important reactive steps in Earth's atmosphere is the decomposition of H(2)SO(4) to H(2)O and SO(3). However, because the UV spectrum of H(2)SO(4) was not found up to 140 nm, alternative mechanisms, including vibrationally induced dissociation, were proposed. Using adiabatic reactive molecular dynamics (ARMD) simulations with validated force fields for the product and educt channels, it is shown through explicit atomistic simulation that by exciting the ν(9) (OH-stretching-) mode, photodissociation can occur on the picosecond time scale. With the potential energy surfaces used in the present work, ν(9) = 4 is sufficient for this process. From a statistically significant number of trajectories (several thousands), vibrationally induced dissociation times are found to follow Gamma-distributions with most likely reaction times between 40 and 200 ps by depositing energies ranging from 40 to 60 kcal/mol, corresponding to 4 and 6 vibrational quanta in the OH stretching vibration. Because ARMD simulations allow multiple and long-time simulations, both nonstatistical, impulsive H-transfer and statistical, IVR-regimes of the decomposition reaction can be discussed in detail at an atomistic level.