Ion imaging studies of CI(~2P_(3/2) fragments arising in the visible photolysis of BrCl:measurement of orientation,alignment,and alignment-free anisotropy parameters

摘要

The photodissociation dynamics of jet-cooled BrCl molecules have been investigated at four different wavelengths in the range 425-482 nm by high-resolution velocity map ionmimaging.Four images of the Cl(~2P_(3/2) atomic fragments are recorded at each photolysis wavelength with the probe laser polarization,respectively,linearly aligned and vertical(i.e.,perpendicular to the detection axis),right circularly polarized,horizontally linearly polarized(i.e.,parallel to the detection axis) and ledt circularly polarized on successive laser shots,therby ensuring automatic mutual self-normalization.Appropriate linear combinations of these images allow quantification of the angular momentum alignment of the Cl 9~2P_(3/2~deg) fragments [i.e.,the correlation between their recoil velocity (v) and their electronciangular momentum (J)] in terms of the alignment anisotropy parameters s2,a2,eta2,and gamma2,and determination of the "alignment-free" recoil anisotropy parameter,beta_0,as a function of parent excitation wavelength.Both incoherent and coherent cotributions to the alignment ae identified,with both simultaneous parallel and perpendicular excitations to the B~3II(0~+) and C~1II(1) states and excitations to the OMEGA=+-1 components of the C state contributing to the latter.The deduced values of the alignment-free beta parameters indicate (wavelength dependent)contributions from both parallel and perpendicular parent absorptions in this wavelength range.Such a conclusion accords with approximate deconvolutions of the parent absorptin spectrum that are currently available,and with determinations of the orientation.parameter gamma'_1 obtained by ritting the difference image obtained when using left and right circularly polarized radiation to probe the ground state Clatoms arising in the 480.63 nm photodissociation of BrCl when the photolysis laser radiation ispolarized linearly at 45deg to the detection axis.