Fine Structure Branching Research Articles

Fine-structure branching ratios N(2D5/2)/N(2D3/2) above threshold N(2D5/2.3/2)+N(2D5/2.3/2) for 14N2 and 15N2

Accurate measurements of the product spin-orbit finne-structure branching ratios are important for understanding the detailed photodissociation dynamics of small molecules. In this study, the atomic spin-orbit fine-structure branching rar tio N(2D5/2)/N(2D3/2) to the dissociation channel N(2D5/2,3/2)+N(2D5/2,3/2) is measured for the c4′1Σu+(υ′=6) and b′1Σu+(υ′=21) states of 14N2, and the b′1Σu+(υ′=20) and b′1Σu+(υ′=21) states of 15N2 by using the vacuum ultraviolet (VUV)pump VUV-probe time-sliced velocity-mapped ion imaging setup. The measurements show that the fine-structure branching ratio N(2D5/2)/N(2D3/2) is independent of the rotational level of the parent molecule (14N2 or 15N2) in each vibronic state, while it does show dependence on the vibronic characteristics. It is ~1.35 for the c4′1Σu+(υ′=6) state of 14N2 and b′1Σu+(υ′=20) state of 15N2, which are both close to the dissociation threshold N(2D5/2,3/2)+N(2D5/2,3/2); while it becomes ~1.00 for the b′1Σu+(υ′=21) state of both 14N2 and 15N2, which are relatively far above the dissociation threshold. A possible change from a statistical process near the threshold to a diabatic process far above the threshold might have occurred to be responsible for the observed vibronic dependence of the branching ratio. Detailed informations on the potential energy curves and their mutual couplings near the dissociation threshold are highly desired for understanding the present experimental measurement.

Observation of rotationally dependent fine-structure branching ratios near the predissociation threshold N(2D5/2,3/2) + N(2D5/2,3/2) of 14N2.

Photofragment spin-orbit fine-structure branching ratios have long been predicted to depend on the rotational quantum number J' by theory near the dissociation thresholds of several diatomic molecules, while this has rarely been observed in any photodissociation experiments yet. Here, we measured the fine-structure branching ratios N(2D5/2)/N(2D3/2) produced in the N(2D5/2,3/2) + N(2D5/2,3/2) channel at the b'1Σu +(v = 20) state of 14N2 by using our vacuum ultraviolet (VUV)-pump-VUV-probe time-sliced velocity-mapped ion imaging setup. It is found that 14N2 almost exclusively dissociates into the spin-orbit channel N(2D5/2) + N(2D3/2) at low rotational levels and gradually approaches the statistical or diabatic limit by distributing all possible spin-orbit channels at higher rotational levels. The strongly rotationally dependent fine-structure branching ratios should be due to the increasing strength of nonadiabatic Coriolis interaction among various dissociative states in the so-called "recoupling zone" as J' increases. They are supposed to provide unprecedented information on the near threshold photodissociation dynamics of 14N2.

Photodissociation of S2 (X3Σg-, a1Δg, and b1Σg+) in the 320-205 nm Region.

Photodissociation of vibrationally and electronically excited sulfur dimer molecules (S2) has been studied in a combined experimental and computational quantum chemistry study in order to characterize bound-continuum transitions. Ab initio quantum chemistry calculations are carried out to predict the potential energy curves, spin–orbit coupling, transition moments, and bound-continuum spectra of S2 for comparison with the experimental data. The experiment uses velocity map imaging to measure S-atom production following S2 photoexcitation in the ultraviolet region (320–205 nm). A pulsed electric discharge in H2S produces ground-state S2 X3Σg–(v = 0–15) as well as electronically excited singlet sulfur and b1Σg+(v = 0, 1), and evidence is presented for the production and photodissociation of S2 a1Δg. In a previous paper, we reported threshold photodissociation of S2X3Σg–(v = 0) in the 282–266 nm region. In the present study, S(3PJ) fine structure branching and angular distributions for photodissociation of S2 (X3Σg–(v = 0), a1Δg and b1Σg+) via the B″3Πu, B3Σu– and 11Πu excited states are reported. In addition, photodissociation of the X3Σg–(v = 0) state of S2 to the second dissociation limit producing S(3P2) + S(1D) is characterized. The present results on S2 photodynamics are compared to those of the well-studied electronically isovalent O2 molecule.

Communication: direct measurements of nascent O((3)P0,1,2) fine-structure distributions and branching ratios of correlated spin-orbit resolved product channels CO(ã(3)Π; v) + O((3)P0,1,2) and CO(X̃(1)Σ(+); v) + O((3)P0,1,2) in VUV photodissociation of CO2.

We present a generally applicable experimental method for the direct measurement of nascent spin-orbit state distributions of atomic photofragments based on the detection of vacuum ultraviolet (VUV)-excited autoionizing-Rydberg (VUV-EAR) states. The incorporation of this VUV-EAR method in the application of the newly established VUV-VUV laser velocity-map-imaging-photoion (VMI-PI) apparatus has made possible the branching ratio measurement for correlated spin-orbit state resolved product channels, CO(ã(3)Π; v) + O((3)P0,1,2) and CO(X̃(1)Σ(+); v) + O((3)P0,1,2), formed by VUV photoexcitation of CO2 to the 4s(10 (1)) Rydberg state at 97,955.7 cm(-1). The total kinetic energy release (TKER) spectra obtained from the O(+) VMI-PI images of O((3)P0,1,2) reveal the formation of correlated CO(ã(3)Π; v = 0-2) with well-resolved v = 0-2 vibrational bands. This observation shows that the dissociation of CO2 to form the spin-allowed CO(ã(3)Π; v = 0-2) + O((3)P0,1,2) channel has no potential energy barrier. The TKER spectra for the spin-forbidden CO(X̃(1)Σ(+); v) + O((3)P0,1,2) channel were found to exhibit broad profiles, indicative of the formation of a broad range of rovibrational states of CO(X̃(1)Σ(+)) with significant vibrational populations for v = 18-26. While the VMI-PI images for the CO(ã(3)Π; v = 0-2) + O((3)P0,1,2) channel are anisotropic, indicating that the predissociation of CO2 4s(10 (1)) occurs via a near linear configuration in a time scale shorter than the rotational period, the angular distributions for the CO(X̃(1)Σ(+); v) + O((3)P0,1,2) channel are close to isotropic, revealing a slower predissociation process, which possibly occurs on a triplet surface via an intersystem crossing mechanism.

Experimental and theoretical investigation of correlated fine structure branching ratios arising from state-selected predissociation of BrO (A2Π3/2)

We present results for the v'-dependent predissociation dynamics of the BrO (A(2)Π3/2) state using velocity map ion imaging. Correlated fine structure branching ratios, Br((2)P(J)) + O((3)P(J)), have been measured for v' = 5-16 states. The experimental branching ratios are non-statistical and strongly dependent on the initial vibronic state. The current measurements represent an extensive dataset containing rich information about the predissociation dynamics of this system and should provide a stringent test for modern theory. New high level ab initio excited state potentials are presented and have been optimized using experimental v'-dependent predissociation lifetimes and calculated coupling constants. Comparisons between the experimental branching ratios and the predictions based on diabatic and adiabatic limiting models are presented. We find that the adiabatic model is most consistent with the observed trends in the correlated branching ratios, in contrast to previous studies on the related ClO system.

Angular momentum polarisation in the O(1D) products of O2 photolysis via the B state

The first fully allowed spectroscopic transition in O2 is the transition comprising the well-known Schumann–Runge bands and continuum. We report a velocity-map imaging study in which the O(1 D) angular momentum polarisation and O(3 P) spin–orbit branching ratios arising from this process have been measured. We show that direct 157 nm excitation into the Schumann–Runge continuum leads to extremely strong angular momentum polarisation in the O(1D) product. Comparison with previous studies indicates that this is a general feature of dissociation via the B state. The fine structure branching ratios in the co-fragment O(3P J=2,1,0) were measured to be 88.5 ± 1.6 : 9.7 ± 1.4 : 1.9 ± 0.4. Based on a consideration of the Massey parameter for the system, the data were modelled using theoretical calculations based on adiabatic and diabatic models of the dissociation. While both models were able to describe some aspects of the dissociation accurately, neither was able to predict both the fine structure branching ratios of the O(3P) products and the O(1D2) alignment. We have also investigated O(1D2) alignment arising from 203.8 and 205.5 nm photodissociation via the state of O2 vibrationally excited to v=6–11. As in the 157 nm photodissciation of vibrationally ground state O2, strong polarisation of the O(1D2) photofragments is observed.

Correlated fine structure branching ratios arising from state-selected predissociation of ClO (A2Π3/2)

We have extended our investigation of the v'-dependent predissociation dynamics of the ClO A(2)Pi(3/2) state using velocity-map ion imaging. Correlated fine-structure branching ratios are reported for v' = 0-5. The measured branching ratios are non-statistical and are qualitatively inconsistent with adiabatic dissociation dynamics. The coupling constants between the A(2)Pi(3/2) state and several dissociative excited state potentials have been optimized, as have the locations of the crossing points, based on comparison to previously reported v'-dependent predissociation rates. Using these optimized potentials we have modeled the branching ratios in the diabatic limit but the lack of agreement with experiments suggests the importance of exit channel coupling. Coupled channel calculations including 9 coupled potentials provide modest improvement with experiment but differences remain.

Photodissociation of vibrationally excited OH/OD radicals

This paper describes a joint experimental and theoretical study of the photodissociation of vibrationally excited hydroxyl radicals. OH and OD radicals produced in a pulsed electric discharge supersonic beam are state-selected and focused by a hexapole and then photo-dissociated by a single laser tuned to various H/D or O atom (2 + 1) resonance enhanced multiphoton ionization (REMPI) wavelengths between 243 nm and 200 nm. The angle velocity distributions of the resulting O+ and D+ photofragment ions were recorded using velocity map imaging. Photodissociation to the O(3PJ) + H(2S) limit is shown to take place by one-photon excitation to the repulsive 1 2Σ− state. The experimental data shows that vibrationally excited OH/OD which are formed in the discharge are dissociated, and a vibrational temperature of ≈2000 K was estimated for the beam source. An analysis in the high-energy recoil sudden limit is used to predict the O(3PJ) fine structure branching ratios and alignment information in the molecular and laboratory velocity frame of the imaging experiment. The measured and predicted fine structure branching ratios and alignment parameters agree well at all dissociation wavelengths, supporting the model for photodissociation in the sudden limit regime. Several aspects of the experiment such as OH pre-alignment and orientation, ion-recoil, and Doppler-free imaging are discussed.

Photodissociation dynamics of OCS and CS 2 adsorbed on water ice films at 193 nm

Photodissociation dynamics of OCS and CS 2 adsorbed on water ice films at 90 K was investigated at 193 nm by measuring the time-of-flight spectra of photodissociated S( 1D) and S( 3P) atoms. The translational energy distributions are well represented by a combination of Gaussian and Maxwell–Boltzmann (M–B) distributions corresponding to the fast and slow time-of-flight components, respectively. Formation of the sulfur dimer on the ice surface is proposed as a secondary photoproduct. The fine-structure branching ratios of S( 3P J ) from OCS and CS 2 adsorbed on H 2O or D 2O water ice films were also determined.

Vibrational state-dependent predissociation dynamics of ClO (A2Π3/2): Insight from correlated fine structure branching ratios

We have studied the v'-dependent predissociation dynamics of the ClO A (2)Pi(3/2) state using velocity-map ion-imaging. Experimental final correlated state branching ratios, i.e. Cl((2)P(J=3/2,1/2)) + O((3)P(J=2,1,0)) channels, have been measured for v' = 6-11. We find that the branching ratios are highly variable and depend strongly on v', providing a window into the v'-dependent predissociation mechanism. A comparison of the experimental results with the recent model of Lane et al. (I. C. Lane, W. H. Howie and A. J. Orr-Ewing, Phys. Chem. Chem. Phys., 1999, 1, 3087) in both the diabatic and adiabatic limits suggests that the dynamics are closer to the diabatic limit. The overall Cl((2)P(J)) branching ratios are in good agreement with the diabatic model results. There are significant differences, however, between theory and experiment at the correlated state level, demonstrating the sensitivity of correlated measurements to the role of the exit channel coupling in the predissociation dynamics. The results highlight the need for more sophisticated quantum dynamical calculations to describe the correlated fine structure branching ratios in this system.

Laser fluorescence study of the S( 1D) + CD 4 reaction: determination of the SD product internal state distribution

The dynamics of the S( 1D) + CD 4 → SD + CD 3 reaction has been studied in a photolysis-probe experiment. S( 1D) reagent was prepared by laser photolysis of CS 2, and the SD(X 2Π) product was detected by laser fluorescence excitation. The nascent rotational/fine-structure state distribution of the SD(X 2Π) product was determined. The experimentally determined rotational state distribution within each fine-structure manifold agrees well with the predictions of a statistical ‘prior’ distribution, while the F 1 to F 2 fine-structure branching ratio is higher than predicted statistically. The product state distribution is consistent with reaction through the formation and decay of an energized CD 3SD complex.

Nonadiabatic transitions in the exit channel of atom-molecule collisions: Fine-structure branching in Na+N2

We study Na+N(2) collisions by laser excitation of the collision complex in a differential scattering experiment. The measured relative population of the Na(3p) fine-structure levels reflects the nonadiabatic transitions occurring in the exit channel of the collision. Theoretical results obtained with a classical-path formalism and accurate quantum chemical data for NaN(2) are found to be in good agreement. The presence of a conical intersection for the T-shaped geometry has a profound influence on the observed fine-structure branching.

Photodissociation of O2 in the Herzberg continuum. II. Calculation of fragment polarization and angular distribution

Parallel and perpendicular components of the Herzberg I, II, and III transitions contribute to the photodissociation of O2 in the Herzberg continuum. The photodissociation dynamics determines the O(3Pj), j=0,1, and 2 atomic fine-structure branching ratios and angular distributions, which were determined in ion imaging experiments at λ=236, 226, and 204 nm by Buijsse et al. [J. Chem. Phys. 108, 7229 (1998)]. In the preceding paper we presented potential energy curves for all eight ungerade O2 states that correlate with the O(3P)+O(3P) dissociation limit, and the R-dependent spin–orbit and the nonadiabatic radial derivative couplings between these states. Here, we employ these potentials and couplings in a semiclassical calculation of the fine-structure branching ratios, atomic polarizations, and fine-structure resolved anisotropy parameters. We discuss the adiabaticity of the dissociation by comparing the results with adiabatic and diabatic models. The O(3Pj) 2+1 REMPI detection scheme used in the experiment is sensitive to the polarization of the atomic fragments. We predict an important effect of the polarization on the anisotropy of the j=1 and j=2 ion images at low energies (λ>236 nm). The agreement between the semiclassical calculations and experiment is reasonable, possible explanations for the remaining differences are discussed.

Photodissociation of NaK: Ab initio spin-orbit interactions of the Na (3 2S) K (4 2PJ) manifold

The relevant interstate b{sup 3}II, A{sup 1}{Sigma}{sup +}, c{sup 3}{Sigma}{sup +}, and B{sup 1}II spin-orbit induced matrix elements, arising from the Ma (3{sup 2}S) K (4{sup 2}P{sub j}) manifold are treated within the full microscopic Breit-Pauli approximation based on ab initio configuration interaction (CI) wave functions. The determination of these couplings as a function of the internuclear distance of NaK should permit a full treatment of the fine-structure branching ratio K{asterisk}(4{sup 2}P{sub 1/2}(D{sub 1}))/K{asterisk}(4{sup 2}P{sub 3/2}(D{sub 2})) in manifold-meditated photodissociation and in the treatment of interstate perturbations.

Photodissociation of NaK: Ab initio spin-orbit interactions of the Na (32S) K (42PJ) manifold

The relevant interstate b3Π, A1Σ+, c3Σ+, and B1Π spin-orbit induced matrix elements, arising from the Na (3 2S) K (4 2PJ) manifold are treated within the full microscopic Breit–Pauli approximation based on ab initio configuration interaction (CI) wave functions. The determination of these couplings as a function of the internuclear distance of NaK should permit a full treatment of the fine-structure branching ratio K*(4 2P1/2(D1))/K*(4 2P3/2(D2)) in manifold-meditated photodissociation and in the treatment of interstate perturbations. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 693–697, 1999

A theoretical analysis of the state-specific decomposition of OH(A 2Σ+,v′,N′,F1/F2) levels, including the effects of spin–orbit and Coriolis interactions

The state-specific decomposition OH(A 2Σ+,v′,N′,F1/F2)→O(3PJ)+H(2S) is investigated using multichannel scattering theory based on potential energy curves, spin–orbit couplings, and Coriolis couplings, obtained from multireference configuration interaction wave functions. The fine-structure branching fractions of the O(3PJ) fragment are determined and compared with the results of frequently used approximate models. The predissociation rates of the individual OH(A 2Σ+,v′,N′,F1/F2) levels are also computed and compared with the results of recent experiments.

C–Br bond rupture in 193 nm photodissociation of vinyl bromide

Photofragment ion imaging has been applied to 193 nm photodissociation of vinyl bromide to measure the speed and angular distributions of Br atoms. Br atoms were observed in both spin–orbit states ( 2P J ; J=1/2 or 3/2) with the branching ratio, [Br*]/[Br], of 0.06±0.03. Both Br( 2P J ) distributions were dominated by anisotropic high translational energy components, which are ascribed to C–Br bond rupture via surface crossing between the optically-excited 1(π, π*) state and 1(n(Br) or π(Br), σ*(C–Br)) repulsive state(s). The anisotropy parameters of the high translational energy components, 1.3 (Br) and 1.2 (Br*), provide the direction of the transition dipole moment for the π* ← π transition to be 26±3° from the CC bond axis, in good agreement with ab initio calculations by Yamashita. Despite that the available energy is 10.5 kcal mol −1 smaller in the Br* channel, the peak energies of P( E T) in the Br* and Br channels were quite similar, indicating that the fine structure branching occurs in the molecular region. The low translational energy components of Br and Br* observed are ascribed to the dissociation of VBr from the ground state, although contribution from the secondary dissociation of C 2H 2Br radical is also suggested for the Br channel. The anisotropy of the low energy component implies that the lifetime of the ground state VBr is shorter than its rotational period (4.7 μs).

C−Cl Bond Rupture in Ultraviolet Photodissociation of Vinyl Chloride

Cl atomic elimination in ultraviolet photodissociation of vinyl chloride at 193 and 210 nm has been studied by photofragment ion imaging. The translational energy and angular distributions of the Cl fragments were measured for the two spin−orbit states of 2PJ (J = 1/2, 3/2). The translational energy distributions of both of the 2PJ fragments consist of two components; one is a Gaussian-like translational energy distribution with high anisotropy, β = 1.0 ± 0.2, while the other is a statistical distribution with low anisotropy, β = 0.3 ± 0.2. These features were almost identical for the two photoexcitation wavelengths. The anisotropy parameter, β, of the Gaussian translational energy component is in good agreement with the value predicted for the π* ← π transition by ab initio calculations. The high translational energy component of the Cl fragment is assigned to a prompt dissociation induced by the electronic relaxation from the (π,π*) to (π(Cl),σ*(C−Cl)) repulsive state, while the low translational energy component is assigned to a slow dissociation induced by the relaxation to the ground state. The [Cl*]/[Cl] fine structure branching has been found to be almost identical for the two dissociation pathways despite the large difference of the recoil velocities of the Cl atoms, suggesting an important role of the motion along the coordinate perpendicular to the C−Cl stretching in the nonadiabatic transition.

Product fine structure state populations and nonadiabatic dissociation dynamics: Cl*(2P1/2)/Cl(2P3/2) branching ratio in the visible and near ultraviolet photodissociation of nitrosyl chloride

Chlorine atoms in their 2Pj states were observed using resonance-enhanced multiphoton ionization and time-of-flight techniques, following the photodissociation of nitrosyl chloride at preselected wavelengths (i.e., 600, 562, 478, and 355 nm). The fine structure branching ratio and the spatial anisotropy parameter were determined. Combining these observations with results from previous studies, a complete picture about product fine structure state populations is obtained. The observed fine structure populations are explained with the aid of a new correlation diagram in the Cs point group. The adiabatic, nonadiabatic, and diabatic dissociation dynamics involving several avoided crossings at large interfragment separations are discussed.

Electronic control of the spin–orbit branching ratio in the photodissociation and predissociation of HCl

The fine structure branching ratio of chlorine atoms produced in the photodissociation and predissociation of HCl was measured for excitation energies between 51 800 and 83 800 cm−1. The branching ratio between 60 000 and 70 000 cm−1 is in good agreement with the calculations of Alexander et al. [J. Chem. Phys. 99, 1752 (1993)], converging to the statistical limit at high energy. Predissociation of electronically excited bound states display a surprisingly rich behavior, demonstrating that the nature of the predissociating continuum state has a large effect on the atom fragment branching ratio.