Ion imaging studies of Cl(2P3/2) fragments arising in the visible photolysis of BrCl: Measurement of orientation, alignment, and alignment-free anisotropy parameters

Abstract

The photodissociation dynamics of jet-cooled BrCl molecules have been investigated at four different wavelengths in the range 425–485 nm by high-resolution velocity map ion imaging. Four images of the Cl(2P3/2) 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 left circularly polarized on successive laser shots, thereby ensuring automatic mutual self-normalization. Appropriate linear combinations of these images allow quantification of the angular momentum alignment of the Cl(2P3/2o) fragments [i.e., the correlation between their recoil velocity (v) and their electronic angular momentum (J)] in terms of the alignment anisotropy parameters s2, α2, η2, and γ2, and determination of the “alignment-free” recoil anisotropy parameter, β0, as a function of parent excitation wavelength. Both incoherent and coherent contributions to the alignment are identified, with both simultaneous parallel and perpendicular excitations to the B 3Π(0+) and C 1Π(1) states and excitations to the Ω=±1 components of the C state contributing to the latter. The deduced values of the alignment-free β 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 absorption spectrum that are currently available, and with determinations of the orientation parameter γ1′ obtained by fitting the difference image obtained when using left and right circularly polarized radiation to probe the ground state Cl atoms arising in the 480.63 nm photodissociation of BrCl when the photolysis laser radiation is polarized linearly at 45° to the detection axis.

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

“…In practice, this means measuring as complete as possible a set of anisotropy parameters for the photofragment Cl( 2 P 0 3/2 ) atoms because these parameters are constructed from simple combinations of the dynamical functions. [11,16]. As explained by Smolin et al [16], the uncertainties are large because rapid and substantial hyperfine depolarization of the electronic angular momentum polarization of the Cl atoms degrades the signature of the anisotropy parameters in the images.…”

“…Values of all except one of the rank K = 0, 1, 2 and 3 Cl( 2 P 0 3/2 ) angular momentum anisotropy parameters for the 467-nm, one-photon photodissociation of BrCl, previously determined by velocity map imaging experiments [11,16] 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 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 …”

“…The broad absorption spectrum of BrCl in the visible region of the spectrum was deconvoluted into 3 The experimental measurements culminated in successful evaluation of 12 of the 13 electronic angular momentum anisotropy parameters for the Cl( 2 P 0 3/2 ) photofragment. Values for the rank K = 0 anisotropy parameter (β), four of the five rank K = 2 alignment parameters (α 2 , s 2 , γ 2 , and η 2 , but not γ ′ 2 ) and all the rank K = 1 and 3 orientation parameters (α K , γ K , γ ′ K and η K ) were reported previously for 467-nm photodissociation of BrCl [11,16]. These parameters describe the contributions to the angular momentum polarization from coherent and incoherent mechanisms involving parallel (∆Ω = 0) and perpendicular (∆Ω = ±1) photoexcitations to dissociative states.…”

“…The photodissociation dynamics of BrCl at a variety of visible and near-UV wavelengths have been the subject of extensive study in the Bristol laboratory using a high resolution photofragment velocity map imaging spectrometer [11,16,19,20]. Wavepacket calculations of the dissociation dynamics following photoexcitation to the A 3 Π 1 , B 3 Π 0+ and C 1 Π 1 states, with allowance for non-adiabatic dynamics at the avoided crossing between the B 3 Π 0+ state and a further Ω = 0 + state, were used to optimize the shapes of the potential energy curves, as shown in figure I [20], and successfully reproduced all wavelength-dependent measurements of branching ratios and photofragment translational anisotropies.…”

“…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.…”