Stereodynamics Of The Small Molecular Photolysis Reaction

This PhD dissertation presents experimental studies on the state-to-state stereodynamics of OCS and HN3 molecules through photodissociation in the ultraviolet region using the time-slice velocity ion imaging technique,aiming for a more in-depth understanding of the molecular elementary reaction and to search for the experimental evidence of geometric phase effect(GP effect)of gas-phase reactions.The major findings are summarized as follows:(1)Photodissociation dynamics of OCS in the deep ultraviolet(DUV)regionPhotodissociation dynamics of OCS in the deep ultraviolet region is investigated using time-slice ion velocity map imaging technique.The measured total kinetic energy release spectra from photodissociation of OCS at?210 nm show three dissociation channels to the fragment S(1D2),corresponding to low,medium and high kinetic energy release(ET),respectively.The high ET channel is found to be a new dissociation channel opening with photolysis wavelength at-210 nm.We have deduced the mathematical expressions for atomic angular polarization equation that are applicable to the present experimental conditions.Using these expressions,the full set of polarization parameters,aq(k)[p),as well as anisotropic parameter,p,for the photofragmented S(1D2)are accurately determined from the experimental images.Based on the determined polarization parameters and a reconstructed potential energy surfaces of OCS,we attribute the three dissociation channels to:the low ET component arises from a non-adiabatic transition from the repulsive A(21 A')state to the electronic ground state X(1 1A');the medium ET component arises from a simultaneous excitation to two repulsive excited states;the high ET component arises from the intersystem crossing from the triplet c(23A")state to the repulsive A(21 A')state.The present study shows that,due to the strong spin-orbit coupling between the triplet c(23A")state and the repulsive A(21A')state,a direct excitation to c(23A")significantly contributes to the photodissociation dynamics of OCS in the deep-UV region.It is also found that,since 5.90 eV(210 nm)is nearly the exact vertical excitation energy for c(23A")from the ground state,an excitation energy of ? ? 210 nm is needed to open the high-ET channel.We have also measured the photodissociation dynamics of OCS at?207 nm and determined the full set of anisotropy and polarization parameters.However,the interpreted polarization parameters for 207 nm photodissociation are substantially different from the 210 nm results,specifically for the high ET component.This indicates a different photoexcitation mechanism.The calculated PESs leads to an explanation for the different high ET components at two wavelengths that,207 nm excitation can reach the vertical excitation region for both A(21A')and c(23A")state,but at 210 nm,the vertical excitation of A(21A')state is not accessible.(2)Photodissociation dynamics of HN3 in the ultraviolet(UV)region(a)The adiabatic dynamics of HN3 via NH(a1?)+ N2(X1?+)channelBy employing the technique of time-slice velocity ion imaging in combination with REMPI,we have experimentally investigated the adiabatic dynamics of the NH(a1 ?)+N2(X2?+)dissociation channel of HN3 molecules.We detected the NH(a1?)product images from photodissociation of HN3 at?285 nm,265 nm and 260 nm,respectively.The resulting anisotropic images at all three wavelengths suggest a fast direct dissociation mechanism for this channel.The anisotropic parameter,?,of the product increases from negative to positive,with the increasing of the J value of 1NH,indicating a significant change of the photoexcitation character from a perpendicular transition to a parallel transition.From the obtained REMPI spectra at 265 nm and 260 nm,the produced 1NH is found to be populated at low-J rotational levels,with a small amount of vibrational excitation.In contrast,from the experimentally measured kinetic energy release spectrum,the co-fragment N2(v = 0)is found to be highly rotationally excited and exhibited a bimodal distribution.The results indicates that,during photoexcitation in the 285-260 nm region,the parent HN3 molecule undergoes a strong out-of-plane twisting,and the complexity of the multidimensional potential energy surface will affect the product's internal energy distributions.(b)The non-adiabatic dynamics of HN3 via NH(a1?)+ N2(X1?+)channelIn the photodissociation of HN3 at 285 nm,we observed a 'splitting' phenomenon in the experimental images of NH(a1?),which cannot be explained by classical theories.To get a physical insight into this novel phenomenon,we re-calculated the PESs of HN3 at the CASPT2//CAS(16,13)/6-31 G(d)level,from which a conical intersection between So and S1 are theoretically predicted.We found that the 'splitting' phenomenon can only be induced by the interference between two dissociation channels along So and Si state surfaces,respectively,which split from the conical intersection and combine at the dissociation product limit.We have developed a theoretical model based on the Young's double slit' theory,using which the experimental results are well reproduced.This further allows us to determine the phase difference of the molecular wavefunctions for the two dissociation channels to be by odd integer times of ?.This result provides an unambiguous experimental identification of molecular geometric effect,i.e.,the nuclear wavefunction stems from a sign reverse after encircling a conical intersection.