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

Abstract

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 aq(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 (⊥), or a mixed transition (∥,⊥) 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, a0(2)(∥); photofragment angular momentum distributions arising from pure perpendicular transitions require only two parameters, a0(2)(⊥) and a2(2)(⊥); 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[a1(1)(∥,⊥)] and Re[a1(2)(∥,⊥)] 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[θε,Θ,Φ,β,aq(k)(p)], where θε is the angle between the recoil direction and the photolysis polarization, Θ and Φ are the spherical polar angles describing the orientation of the probe polarization about the recoil direction, and β is the spatial anisotropy parameter. The physical significance of the aq(k)(p) is discussed; in particular, the a0(k)(∥) and a0(k)(⊥) describe the photofragment m-state distribution along the recoil direction; the a2(k)(⊥) 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 a1(k)(∥,⊥) are related to the asymptotic phase difference associated with the interfering channels, and are thus sensitive to the shapes of the dissociative surfaces.

Determination of all scattering phases and amplitudes in the photodissociation of a diatomic molecule

“…For most of the studies undertaken to date [6,7,9,10,12,14,15], the number of interacting molecular quantum states was limited to 2-3. In these cases, the relationship between the dynamical functions and the T-matrix amplitudes and phases is straightforward and the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 F o r P e e r R e v i e w O n l y corresponding amplitudes and phases can be relatively easily determined from experimental data; see, for example, the review paper [25].…”

“…Several experimental studies have reported measurements of such orientation and alignment parameters (generically referred to hereafter as anisotropy parameters) for photodissociation of a variety of diatomic systems (see, for example, [6][7][8][9][10][11][12][13][14][15][16]), and results have been used to infer certain non-adiabatic crossing probabilities and phase shifts [10], as well as the shapes of molecular potential energy curves [17]. Theoretical analysis by Balint-Kurti and coworkers [2,3] pointed the way to the determination of all amplitudes and phases of T-matrix elements, and illustrated the connection to anisotropy parameters by using wavepacket propagation to compute the photodissociation dynamics of HF and HCl.…”

The photodissociation of CH3I in the red edge of the A-band: Comparison between slice imaging experiments and multisurface wave packet calculations

“…1 have been fitted to the commonly used expression for one-photon dissociation and ͑2+1͒ REMPI detection processes: [63][64][65][66] …”

Frequency modulated circular dichroism spectroscopy: application to ICN photolysis

“…In (a) and (b), filled circles represent data recorded in geometry (1), while open circles represent data recorded in geometry (2), as shown in figure 1. In (c), open circles represent data recorded in geometry (3), while filled circles represent data recorded in geometry (4). In all cases solid lines are the best simultaneous fit to all data, as described in the text.…”

“…This has been driven by the F o r P e e r R e v i e w O n l y 2 realization that these vector properties contain information not just on the magnitudes of different photofragment pathways, but also on their relative phases [3][4][5]. The vast majority of the experimental studies have investigated the orientation of the electronic angular momenta of the atomic products from the dissociation of diatomics, or in fewer cases, triatomics.…”