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

Photolysis of a 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 (parallel to), a perpendicular transition (perpendicular to), or a mixed transition (parallel to,perpendicular to) 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 less than or equal to 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))(parallel to); photofragment angular momentum distributions arising from pure perpendicular transitions require only two parameters, a(0)((2))(perpendicular to) and a(2)((2))(perpendicular to); 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))(parallel to,perpendicular to)] and Re[a(1)((2))(parallel to,perpendicular to)] 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))(parallel to) and a(0)((k))(perpendicular to) describe the photofragment m-state distribution along the recoil direction; the a(2)((k))(perpendicular to) 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))(parallel to,perpendicular to) are related to the asymptotic phase difference associated with the interfering channels, and are thus sensitive to the shapes of the dissociative surfaces. (C) 1999 American Institute of Physics. [S0021-9606(99)01607-4].