Fine Structure Branching Ratios 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.

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.

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

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.

The 248 nm photodissociation of KI: Determination of the branching ratio of K(4 2P J) doublets in the presence of Ar, H2, and N2

By means of a three-level kinetic model developed previously, the nascent fine-structure branching ratios of K 4 2PJ doublets following photodissociation of KI at 248 nm can be determined accurately in the presence of foreign gases. With the forward and reverse Ar-induced collisional fine-structure mixing cross sections of 15 and 30 Å2 reported by Lijnse, and our measurement of Ar quenching cross section 0.81±0.08 Å2, the nascent branching ratio of the K 4 2P3/2 component is determined to be 0.611±0.002. Analogously, with the N2-induced collisional mixing cross sections of 100 and 190 Å2 reported by Lijnse and Hornman, and our measurement of N2 quenching cross section 18±2 Å2, the branching ratio of K 4 2P3/2 is determined to be 0.608±0.002. The agreement between these values confirms reliability of the kinetic model. However, a lack of confirmity is found in the presence of H2 quencher, using the collisional mixing cross sections of 53 and 75 Å2 reported by McGills and Krause, and our quenching cross section measurement of 2.8±0.3 Å2. The discrepancy is caused by the unsuitable adoption of mixing cross sections. Using the average branching ratio 0.610 determined from the cases of Ar and N2, the H2-induced collisional mixing cross sections can be evaluated to be 51±4 and 90±7 Å2. The obtained quenching cross sections by collisions with foreign gases are consistent with those reported elsewhere except for the case of Ar. The discrepancy of our Ar measurement of 0.81±0.08 Å2 from a reported value <0.07 Å2 does not cause a significant difference in the branching ratio determination; it is because the magnitude of the mixing cross sections are more than ten times larger than the relevant quenching cross sections, such that the dependence on the collisional quenching becomes insignificant.

Photodissociation of hydrogen chloride at 157 and 193 nm: Angular distributions of hydrogen atoms and fine-structure branching ratios of chlorine atoms in the 2P j levels

The H, D, and Cl atoms from the photodissociation of HCl and DCl at 157 and 193 nm were detected by laser-induced fluorescence (LIF) in the vacuum ultraviolet region. The Doppler profiles of the H and D resonance lines at 121.6 nm in the LIF spectra indicate that the absorption of HCl and DCl at 157 nm is a mixture of perpendicular and parallel transition (A 1Π, a 3Π0+←X 1Σ+), while that at 193 nm results almost solely from the A 1Π←X 1Σ+ transition. The fine-structure branching ratios, are measured for the chlorine atom by LIF. Combined with the previous results, the branching ratios [Cl*(2P1/2)]/[Cl(2P3/2)] are 0.81±0.09 at 157 nm, 0.50±0.05 at 193 nm for HCl, and 0.23±0.05 at 193 nm for DCl. These results suggest that the nonadiabatic couplings during the breakup of HCl (DCl) in the excited states play an important role in determination of the fine-structure branching ratios.

Erratum: Fine structure branching ratios and Doppler spectroscopy of chlorine atoms from the photodissociation of alkyl chlorides and chlorofluoromethanes at 157 and 193 nm [J. Chem. Phys. 94, 2669 (1991)

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Fine-structure branching ratios and Doppler profiles of Cl(2P j) photofragments from photodissociation of the chlorine molecule near and in the ultraviolet region

Photofragment chlorine atoms in the 2Pj states from Cl2 at 266–500 nm are measured by a resonance-enhanced multiphoton ionization technique. The measured branching ratio of [Cl*(2P1/2)]/[Cl(2P3/2)] increases monotonically with dissociation wavelengths from (9.5±1.8)×10−3 at 308 nm to (4.7±0.5)×10−1 at 475 nm. At 375 nm≤λ≤475 nm, Doppler profiles of the chlorine atoms indicate adiabatic formation of two Cl(2P3/2) from the 1Π(1u) state of Cl2 and Cl(2P3/2)+Cl*(2P1/2) from the B 3Π(0+u) state. On the other hand, at a shorter wavelength (308 nm), Cl*(2P1/2) is generated from the 1Π(1u) state through nonadiabatic transitions during the dissociation. The relative contribution of the 1Π(1u)←X 1Σ(0+g) and B 3Π(0+u)←X 1Σ(0+g) transitions in the photoabsorption is estimated from the obtained branching ratios at 375 nm≤λ≤475 nm.

Fine structure branching ratios and Doppler spectra of O(3P j) produced by the reaction of H+O2→OH+O

O(2p3Pj) atoms produced from the reaction between H(1s2S) and O2(X3Σ−g) at collision energies (Ecoll) of 2.5 and 2.1 eV are detected by a resonance enhanced multiphoton ionization technique. From the Doppler profile measurements of O(3Pj), the average kinetic energy released to the products is estimated. The results suggest that the average fraction of the kinetic energy disposal at Ecoll =2.1 eV is larger than that at Ecoll =2.5 eV. The obtained branching ratios of O(3Pj) j=2, 1, and 0 at Ecoll =2.5 eV are (0.70±0.02), (0.21±0.02), and (0.09±0.02), and those at Ecoll =2.1 eV are (0.63±0.02), (0.25±0.04), and (0.12±0.02), respectively, with error bars of ±σ. The results suggest that the nonadiabatic couplings at the last stage of the reaction are important for the determination of the ratios.