Wim J. van der Zande

Dissociative recombination and excitation of O2+: Cross sections, product yields and implications for studies of ionospheric airglows

We present experimental data on the dissociative recombination (DR) and the dissociative excitation (DE) of O2+ in its electronic and vibrational ground state using a heavy ion storage ring. The absolute DR cross section has been determined over an electron collision energy range from 1 meV to 3 eV. The thermal DR rate coefficient is derived; α(Te)=2.4×10−7(300/Te)0.70±0.01 cm3 s−1, for T>200 K. The threshold for DE was observed near its energetic threshold of 6.7 eV. The DE cross section curve has a maximum of 3×10−16 cm2 near 15 eV. We have determined the branching fractions to the different dissociation limits and present atomic quantum yields for the DR process between 0 to 300 meV collision energy. The quantum yield of O(1D) is found to be 1.17±0.05, largely independent of the electron energy. Arguments are presented that the branching fraction to O(3P)+O(1S) is negligible. The branching fraction to the O(1S)+O(1D) is smaller than 0.06 and varies strongly as a function of collision energy. The O(1S) ...

Effect of reagent rotation on product energy disposal in the light atom transfer reaction O(3P)+HCl(v=2,J=1,6,9)→OH(v’,N’)+Cl(2P)

A rovibronic‐state‐to‐rovibronic‐state experiment has been performed on the reaction O(3P)+HCl(v=2,J=1,6,9)→OH(v’,N’)+Cl(2P). The O(3P) atoms are produced with a known energy by photolysis of NO2. The HCl(v=2,J) molecules are prepared by IR excitation of thermal HCl using an optical parametric oscillator. All energetically accessible OH rovibrational product levels are probed by laser‐induced fluorescence for each prepared HCl rotational level. The OH(v’=0,N’) rotational distribution shows a dip at N’=11, the depth of which decreases with increasing HCl rotational excitation. The available energy of reaction is partitioned so that 40% appears as OH vibration (V’), 32% as OH rotation (R’), and 28% as product translation (T’). This energy partitioning does not change with HCl rotation, in contrast to the general expectation for light atom transfer reactions of approximate conservation of internal angular momentum (R→R’). A substantial vibrational inversion is observed, in agreement with the vibrational adia...

Predissociation of the d 1Πg Rydberg state in O2: Nature of the Rydberg–valence interactions

Predissociation of the higher vibrational states (v=4–8) of the d 1Πg Rydberg state is investigated experimentally using translational spectroscopy. These states predissociate to the O(1D)+O(1D) dissociation limit. Individual rotational levels of the v=4 state are observed for the first time, and the energies and rotational constants for both 16O2 and 18O2 isotopes are established. The observed predissociation behavior and natural linewidths are explained by invoking both Rydberg–valence and valence–valence interactions. The nearly diabatic nature of the 1 1Πg and 2 1Πg valence states is deduced from these observations. One of the‘‘diabatic’’ valence states is ‘‘bound,’’ and its perturbations on the v=1 to 3 of the Rydberg state explain observations reported from resonance enhanced multiphoton ionization experiments.

Precipitable Water Column Retrieval from GOME Data

We investigate the retrieval of terrestrial precipitable water columns using a new spectral fitting method applied to Global Ozone Monitoring Experiment (GOME) data. The method is an optical absorption spectroscopy technique and employs a new approach to the opacity sampling of absorption line spectra which we apply to a little-studied visible band between 585 and 600 nm. The GOME-retrieved columns are compared with data from the European Center for Medium-Range Weather Forecasts for different orbits and show good agreement. The new retrieval algorithm is sensitive to the temperature and pressure dependence of absorption lines in general and may be easily applied to spectra of trace gases other than water vapor.

Dissociation dynamics of the d 1Πg and C 3Πg v=0–2 vibrational states in O2

Using translational spectroscopy we have studied the d 1Πg and C 3 Πg Rydberg states of O2 . The dissociation of the vibrational levels v=0–2 to all energetically accessible dissociation limits has been followed. The dissociation pathways directly reflect the predissociation mechanisms involved. For the d 1 Πg Rydberg state competition between predissociation by a 3 Πg valence state, due to singlet–triplet mixing, and by a 1 Πg valence state has been observed. Using the Fermi golden rule the observed vibrationally dependent competition is reproduced, corroborating the positions of the lower 1 Πg and 3 Πg valence state curves and yielding various coupling strengths.Using translational spectroscopy we have studied the d 1Πg and C 3 Πg Rydberg states of O2 . The dissociation of the vibrational levels v=0–2 to all energetically accessible dissociation limits has been followed. The dissociation pathways directly reflect the predissociation mechanisms involved. For the d 1 Πg Rydberg state competition between predissociation by a 3 Πg valence state, due to singlet–triplet mixing, and by a 1 Πg valence state has been observed. Using the Fermi golden rule the observed vibrationally dependent competition is reproduced, corroborating the positions of the lower 1 Πg and 3 Πg valence state curves and yielding various coupling strengths.

Dissociative recombination of NO+: Dynamics of the X 1Σ+ and a 3Σ+ electronic states

We have studied the dissociation dynamics of NO+ ions in their ground, X 1Σ+, and first excited metastable, a 3Σ+ states, induced by the capture of electrons of variable collision energy in the dissociative recombination (DR) process. The branching over the different dissociation channels has been measured in a merged-beam experiment on the heavy-ion storage ring, CRYRING. In accord with previous observations, NO+ (X 1Σ+,v=0) ions dissociate dominantly to the N(2D)+O(3P) product limit at 0 and 1.2 eV collision energies. In contrast to earlier reports, the spin-forbidden N(4S)+O(1D) dissociation limit contributes 0(±2)% at 0 eV. At 5.6 eV a new channel coupled to the production of ground-state atoms becomes more important, but no increase in the production of ground-state product atoms was observed. All observed branching fractions compare very favorably with predictions from a simple statistical model, which is based on the multiplicity of each dissociation limit in combination with spin conservation during the dissociation and the initial electron capture. We also report the distribution of fragment pairs from the DR reaction involving the metastable a 3Σ+ state. This state is found to dissociate to nearly all of the energetically allowed product pairs. The lifetime of the a 3Σ+ state is found to be 730(±50) ms, in agreement with earlier, sometimes indirect, observations. The experimental observations have been complemented with ab initio calculations on the different radiative decay processes both for the X 1Σ+ and the a 3Σ+ states. It is found that vibrational relaxation via infrared radiation is faster for NO+ (a 3Σ+,v>0) ions than the electronic decay of these metastable-state ions to the ground state.We have studied the dissociation dynamics of NO+ ions in their ground, X 1Σ+, and first excited metastable, a 3Σ+ states, induced by the capture of electrons of variable collision energy in the dissociative recombination (DR) process. The branching over the different dissociation channels has been measured in a merged-beam experiment on the heavy-ion storage ring, CRYRING. In accord with previous observations, NO+ (X 1Σ+,v=0) ions dissociate dominantly to the N(2D)+O(3P) product limit at 0 and 1.2 eV collision energies. In contrast to earlier reports, the spin-forbidden N(4S)+O(1D) dissociation limit contributes 0(±2)% at 0 eV. At 5.6 eV a new channel coupled to the production of ground-state atoms becomes more important, but no increase in the production of ground-state product atoms was observed. All observed branching fractions compare very favorably with predictions from a simple statistical model, which is based on the multiplicity of each dissociation limit in combination with spin conservation duri...

Branching ratios and lifetimes for the dissociative decay of triplet H2

Molecular hydrogen is selectively excited to low vibrational levels in the triplet 3sσ and 3dλ states. We have observed the decay to both the b 3Σ+u and the c 3Π+u state for all excited levels. The dissociative final states have been investigated simultaneously using translational spectroscopy with a time and position sensitive detector. In this way the branching ratios were determined which depend on the mixing between the n=3 states. For some excited states we also measured the radiative lifetime. The experimental results are compared to a model which, in its most elaborate form, contains some transition dipole moments that depend on the internuclear coordinate. Even though the absolute values of the dipole moments are varied to fit the experiment, overall quantitative agreement cannot be obtained.

The ionospheric oxygen Green airglow: Electron temperature dependence and aeronomical implications

The laboratory measurement of processes in- volved in terrestrial airglows is essential in developing di- agnostic tools of the dynamics and photochemistry of the upper atmosphere. Dissociative electron recombination of O + in the ionospheric F-region is expected to produce both O( 1 D) and O( 1 S) which are the sources of the 630.0 nm red airglow and the 557.7 nm green airglow lines, respectively. We present both theoretical and experimental evidence, the latter from a heavy ion storage ring technique, that the O( 1 S) quantum yield from O + (v = 0) is a strong function of the electron temperature due to a molecular resonance phenomenon. At present the O + (v = 0) theoretical and laboratory recombination data cannot explain rocket obser- vations of the ionospheric green and red airglows (Takahashi et al. 1990; Sobral et al. 1992).

Competition between photodetachment and photodissociation in O-2

Using fast beam photofragment spectroscopy, we have studied the competition between photodissociation and photodetachment in O2−. The photodissociation fraction is ∼10% and increases somewhat between 285 and 266 nm. High resolution photofragment spectroscopy shows that the O2− beam is composed of v=0 ions only. The photofragment angular distribution has an anisotropy parameter β=1.9, indicating that the process of dissociation has a parallel character, attributed to the A 2Πu←X 2Πg transition. A detailed analysis of the kinetic energy release of the photofragments shows the structure related to the distribution over the six fine-structure dissociation limits O− (2P3/2,1/2)+O (3P2,1,0). The results are compared with an analytical diabatic dissociation model, taking into account the long range charge–quadrupole interactions; the A 2Πu is described in a Born–Oppenheimer electrostatic aproximation and projected onto the different fine-structure states at large internuclear separation using analytical expressi...

A new DOAS parameterization for retrieval of trace gases with highly‐structured absorption spectra

Differential Optical Absorption Spectroscopy (DOAS) has proven to be an extremely useful technique for retrieving trace gas columns from atmospheric measurements of reflected solar irradiance. We show that the use of DOAS is strongly constrained by the complexity of the absorption spectral structure that may be sampled by individual pixels in a detector array (for example, in the vibration-rotation bands of many species in the visible and infrared). To this end we introduce a Spectral Structure Parameterization (SSP) modification to DOAS which depends primarily on structure in the absorption cross-section of the atmospheric species intended for retrieval. We show that DOAS-SSP may be used to determine which absorption spectra are suitable, in terms of complexity of absorption structure, for use in DOAS retrievals as well as how to extend DOAS trace gas retrieval to those species with highly-structured absorption spectra.