Photofragment angular momentum distributions in the molecular frame: Determination and interpretation

摘要

Photolysis of molecule typically yields open-shell photofragments having angular momenta. A procedure is described for the measurement of the photofragment angular momentum distribution in terms of polarization parameters a_q~((k)) (p) which are expressed in the molecular frame and which may be related to the transition dipole matrix elements. The index (p) indicates either a parallel transition (||), a perpendicular transition (perpendicular), or a mixed transition (||, perpendicular) having both parallel and perpendicular character. This procedure has the advantage that it decouples the angular momentum distributions in the molecular frame from the photofragment angular distributions in the laboratory frame, which gives new insight into the photodissociation dynamics. For cases in which k <= 2 and with linearly polarized photolysis light, the photofragment angular momentum distribution arising from pure parallel transitions can be described with only one parameter, a_0~((2))(||); photofragment angular momentum distributions arising from pure perpendicular transitions require only two parameters, a_0~((2))(perpendicular) and a_2~((2))(perpendicular); photofragment angular momentum distributions arising from mixed transitions, having both parallel and perpendicular character, can be described with five parameters: the two (coherent) interference terms Im[a_1~((1))(||, perpendicular)] and Re[a_1~((2))(||, perpendicular)] in addition to the three incoherent terms mentioned above. We describe procedures for the measurement of the complete angular momentum distribution of state-selected photofragments using laser detection (such as REMPI) and some form of laboratory velocity selection (such as time-of-flight mass spectrometry, Doppler spectroscopy, or ion imaging). The laser-detection probability of a single photofragment is presented in the form I = 1 + f[#theta#_#epsilon#, #THETA#, #PHI#, #beta#, a_q~((k))(p)], where #theta#_#epsilon# is the angle between the recoil direction and the photolysis polarization, #THETA# and #PHI# are the spherical polar angles describing the orientation of the probe polarization about the recoil direction, and #beta# is the spatial anisotropy parameter. The physical significance of the a_q~((k))(p) is discussed; in particular, the a_0~((k))(||) and a_0~((k))(perpendicular) describe the photofragment m-state distribution along the recoil direction; the a_2~((k))(perpendicular) describe how broken cylindrical symmetry in the parent molecule is reflected in the photofragment angular momentum distribution in a plane perpendicular to the recoil direction; and the a_1~((k))(||, perpendicular) are related to the asymptotic phase difference associated with the interfering channels, and are thus sensitive to the shapes of the dissociative surfaces.