Study On The Product Polarization And Related Dynamics In The Photolysis Of O₂ Molecule Via The Schumann-runge Continuum

Investigation on the photodissociation of small gas phase molecules has already got deeply into some chain reactions or other complex reactions in which molecules,radicals,and especially the polarization of photofragments provide profound insights into complex photodissociation dynamics in important fields such as environmental chemistry and atmospheric chemistry.Using tunable vacuum ultraviolet(VUV)source combined with time-sliced ion velocity imaging experimental apparatus,and(2+1)REMPI scheme,we investigated photofragment spatial anisotropy distribution and atomic polarization information in the 152-158 nm photodissociation of O2 via the Schumann-Runge(SR)continuum.Based on the polarization theory of Rakitzis and Zare,we have derived the mathematical expressions for the angular distributions of atomic polarization for four different combinations of linearly polarized photolysis and probe light(HH,HV,VH and VV)in the laboratory frame.The defined polarization parameters aq(k)(p)are apply to deal with the experimentally data and obtain some important results.Raw images of the product fragments O(1D2),O(3P2)and O(3P1)in the 152-158 nm photodissociation of O2 via the SR continuum are collected by using different pump-probe laser polarization configurations or combined with different detection transitions.It is found by total kinetic energy release distributions extracted from O2 photolysis in this wavelength range show that there are two dissociation channels,corresponding to the slow channel OO(1D2)+O(3PJ)and the fast channel O(3PJ)+O(3PJ).The slow channel products occurs via direct dissociation transition of B-?u-?X3?g-state,while the O(3PJ)+O(3PJ)channel is the predissociation channel with relatively small quantum yield but not negligible.A 'split flap'phenomenon is observed in the raw sliced velocity map images of O(1D2),indicating that there is an obvious anisotropic spatial distribution in the rotational angular momentum of the product.The polarization parameters aq(k)p)and the spatial anisotropy parameter ? for the corresponding products are extracted by fitting the angular momentum distributions to the different images of products.The population of each magnetic sublevel mJ can be measured by the parameters.The angular distribution of these products indicates that a pure parallel transition corresponding to the slow channel and a mixed transition corresponding to the fast channel.The results show that the experimental mJ state populations of the photofragments O(1D2)which are concentrated in m0,revealing that the polarization of the O(1D2)products about the molecular axis is orbital aligned.The angular momentum alignment of the products in the slow channel is almost not strongly dependent on the wavelength of photolysis laser,revealing the same dependence between the mJ state populations of these photofragments with the wavelength.The dissociation process of slow channel is inferred to be mainly adiabatic according to the relationship between the alignment of O(3P2)and O(1D2).Polarization of the angular momentum of the photofragments O(3P1)in the fast channel decreases slightly with the increase of dissociation wavelength and shows an isotropic distribution at 156.65 nm.The angular distribution and atomic polarization of other fragments are not sensitive to changes in dissociation wavelength and pump-probe polarization configurations.The polarization information in these two cases can provide experimental references basic for further discussion of the incoherent induced polarization.