Fine Structure Branching Ratios Research Articles

Photodissociation processes in carbon monoxide at 193 nm

The photodissociation of carbon monoxide at 193 nm has been investigated using resonance enhanced multiphoton ionization detection of atomic oxygen and carbon fragments. The results of these experiments indicate a quadratic photodissociation process in CO at 12.8 eV. In addition, the nascent fine structure branching ratios of atomic oxygen 2p4 (3P2,1,0) produced in the 193 nm photodissociation of CO are reported using both multiphoton laser induced fluorescence and ionization detection. Analysis of the experimental data and computer generated simulations indicate that the distributions are not statistical.

Penning ionization and photoionization electron spectrometry of hydrogen bromide

An electron spectrometric study has been performed on HBr using metastable helium and neon atoms as well as helium resonance photons. High resolution electron spectra were obtained for a mixed He(21S, 23S) beam, a pure He(23S) beam, a mixed Ne(3s3P2,3P0) beam, and for HeI VUV light. From the comparison of vibrational populations of HBr+ (X, v′) and HBr+ (A, v′), formed by either He* and Ne* Penning ionization (PI) or HeI photoionization, we conclude that HBr+ (X) formation by PI exhibits only little perturbation of HBr potentials, whereas HBr+ (A) formation by PI exhibits substantial bond stretching of HBr due to metastable atom attack preferably from the H end. For He(21S) + HBr theX- andA-state vibrational peak shapes are substantially broader than for the He(23S) + HBr case pointing to an additional, charge exchanged interaction (He+ + HBr−) in the entrance channel of the former system which is also responsible for a broad feature found at lower electron energies in the He(21S, 23S) induced PI electron spectra. For the first time, we have detected the low energy electrons in both the He(21S) + HBr and He(23S) + HBr spectra, associated with the major mechanism for the formation of Br+ ions: energy transfer to repulsive HBr** Rydberg states, dissociating to H(1s) and autoionizing Br** atoms. The HBr+ (X)2II3/2:2II1/2 fine structure branching ratios vary significantly with the ionizing agent in a similar way as for the isoelectronic, atomic target case krypton.

Fine structure branching ratios and Doppler spectroscopy of chlorine atoms from the photodissociation of alkyl chlorides and chlorofluoromethanes at 157 and 193 nm

Alkyl chlorides (R=CH3, C2H5, C3H7, and C4H9), chloromethanes (CHnCl4−n), and chlorofluoromethanes (CFnCl4–n) are photodissociated at 157 and 193 nm. The chlorine atom photofragments are detected by a resonance enhanced multiphoton ionization technique. The branching ratios of the Cl photofragments [Cl*(3p2P1/2)]/[Cl(3p2P3/2)] are almost identical (0.23±0.03) for alkyl monochlorides at 157 and 193 nm and for CH2Cl2 and CHCl3 at 157 nm, while the ratios are rather small (0.10±0.02) for CH2Cl2 and CHCl3 at 193 nm. No discernible isotope effects on the branching ratios were observed when D atoms were substituted for H atoms in chloromethanes. For CCl4, CF3Cl, CF2Cl2, CFCl3 at 157 and 193 nm, the ratios are small (<0.05) irrespective of the number of Cl atoms in the parent molecules. The Doppler profiles of the chlorine photofragments have been obtained and it is found that (a) CF3Cl undergoes a perpendicular optical transition at 157 nm and (b) for halomethanes containing more than two Cl atoms, the Cl photofragment has a Boltzmann distribution in translational energy and an isotropic angular distribution.

Fine-structure branching in the near-threshold photodissociation of NaK(X 1 Sigma +-B 1 Pi ).

We have measured the ${\mathit{X}}^{1}$${\mathrm{\ensuremath{\Sigma}}}^{+}$--B $^{1}\mathrm{\ensuremath{\Pi}}$ bound-free molecular photodissociation profile of NaK under molecular-beam conditions. The dissociation results in excited ${\mathrm{K}}^{\mathrm{*}}$(4 $^{2}$P). We have measured the ${\mathit{D}}_{1}$/${\mathit{D}}_{2}$ fine-structure branching ratio; this ratio is a strong function of wavelength ranging from near zero to ${\mathit{D}}_{1}$/${\mathit{D}}_{2}$\ensuremath{\simeq}25% as the laser is tuned through the bound-free absorption profile.

Fine structure branching ratios of the O(3P j) atomic fragments from photodissociation of oxygen molecules at 157 and 193 nm

Direct photodissociation and predissociation of molecular oxygen from the B 3∑−u state have been studied by photoexcitation at 157 and 193 nm, respectively. The fine structure branching ratios and Doppler profiles of O(3Pj) photofragments were measured by a resonance-enhanced multiphoton ionization technique. The branching ratios of O(3Pj) j=2,1,0 are 0.74 ±0.03:0.21 ±0.02:0.042±0.004 for photodissociation at 157 nm and 0.47±0.05:0.31 ±0.04:0.22±0.44 at 193 nm. Both distributions are not statistical. The Doppler profiles of O(3Pj) from photodissociation of O2 at 157 nm are consistent with formation of O(1D)+O(3Pj).

Spectroscopic analysis of a collisional complex. Redistribution calculation for the K(5P-4S) doublet perturbed by neon

The close-coupled theory of collisions in a radiative field is applied to the calculation of the absorption profile for the K(5P-4S) doublet broadened by collisions with Ne. Using both quantal and semiclassical approaches allows quantitative results to be obtained and also points out the more important physical aspects. The theory gives simultaneously the absorption coefficient, the fine-structure branching ratio and the polarisation redistribution from the line centre, where the impact limit is valid, to the far spectral wings. The analysis of the influence of non-adiabatic couplings shows that the total absorption coefficient is rather insensitive to these couplings but that they play an important role in the fine-structure branching ratio and in the polarisation redistribution. The authors also find a strong dependence of the polarisation redistribution with the curvature of the trajectories.

Non-adiabatic effects on oxygen atom fine structure populations in the predissociation of the A 2Σ + state of OH

Multichannel scattering calculations are used to evaluate the role of non-adiabatic interactions and interference effect on the predissociation linewidths of several vibrational levels of the A 2Σ + state of OH due to coupling to three different repulsive electronic states. Oxygen atom fine structure branching ratios are calculated as a possible probe of the predissociation pathway. Non-adiabatic effects are found to be very strong for the lower vibrational predissociative A 2Σ + levels and to remain non-negligible even for the higher vibrational states of A 2Σ +.

Nonadiabatic theory of fine-structure branching cross sections for Na-He, Na-Ne, and Na-Ar optical collisions

The nonadiabatic close-coupled theory of atomic collisions in a radiation field is generalized to include electron spin and is used to consider the weak-field Na--rare-gas (RG) optical collision Na${(}^{2}$${S}_{1/2}$)+RG+nh\ensuremath{\nu} \ensuremath{\mu}Na${(}^{2}$${\mathrm{P}}_{\mathrm{j}}$)+RG+(n-1)h\ensuremath{\nu}. The effects of detuning and incident energy on the branching into the atomic Na 3${p}^{2}$${P}_{3/2}$ and 3p $^{2}P_{1/2}$ states are examined. The cross sections \ensuremath{\sigma}(j) are found to have a strong asymmetry between red and blue detuning as well as a complex threshold and resonance structure dependence on energy. A partial cross-section analysis of \ensuremath{\sigma}(j) shows a significant difference between contributions from states of e and f molecular parity. The theoretically calculated detuning dependence of the branching ratio into each fine-structure state is in good agreement with available experimental data for Na-Ar, Na-Ne, and Na-He, as well as the total absorption coefficient for the production of Na 3p atoms. The fine-structure branching ratio for thermal energy collisions shows considerable variation with a rare-gas collision partner, due to the different interaction potentials. For sufficiently high collision energy, the branching approaches a recoil limit which is independent of collision partner.

Electronic Energy Partitioning in Photodissociation

Despite the apparent simplicity of photodissociation in diatomic molecules, some of the essential physics of this process is not understood when there is fine structure in the atomic photofragments. Previous theories cannot treat the branching ratios and angular distributions of the individual fine structure sublevels. We have developed a complete quantum mechanical theory of the effects of nonadiabatic couplings and of electronic angular momentum on the fine structure branching ratios, angular distributions, and polarization in diatomic photodissociation. When the photofragments separate with large relative kinetic energy, simple limiting expressions can be obtained for branching ratios and the symmetry parameters which characterize fragment angular distributions and polarized fluorescence from excited fragments. Information about the symmetry of the molecular states involved in the optical transition which dissociates the molecule may be deduced from fine structure branching ratios and asymmetry parameters in the high energy limit. At low relative kinetic energies where non-adiabatic couplings are crucial, cross sections and asymmetry parameters exhibit interesting behavior which intimately reflect the shape of the dissociative molecular surfaces. We employ the example of sodium hydride photodissociation to produce P2 excited sodium atoms as a model system because of the availability of ab initio potential curves and oscillator strength matrix elements. The low energy photodissociation cross section and angular distributions are shown to exhibit resonances which arise in part due to non-adiabatic spin–orbitand Coriolis couplings. Their energy dependence can therefore be utilized to probe the nature of potential curves which are not directly pumped in optical absorption processes and may therefore provide a unique spectroscopic means for measuring properties of these “dark” states.

Inner-shell photoemission studies of lithium and sodium vapour

The complete electron spectra caused by photoionisation of Li and Na vapour in the 1s and 2p shells respectively, have been investigated in the region from 5 eV above threshold up to about twice the binding energy of the ionised electron. Even though the contribution of molecules to the vapour pressure was only of the order of a few per cent, molecular features are found in the experimental spectra with intensities comparable with the atomic structures. The interpretation of all features observed in the spectrum is given. In specific energy regions, where the molecular contributions do not disturb the atomic ones, a detailed study of the fine-structure branching ratio and of the satellite structure accompanying these ionisation processes has been made. The results of this investigation are compared with other experimental and theoretical data.