Peter Erman

The Influence of Collisional Transfers and Perturbations on Measured A and B State Lifetimes in CN

Lifetimes of a number of the red and violet CN bands have been studied at high spectral resolution using the High Frequency Deflection technique. While the result for the violet bands essentially supports earlier results, it is found that the lifetimes as well as the intensities of the red bands are strongly perturbed by processes which are suggested to be collisional transfers from X levels to near-lying A levels. These effects remain if additional catalyser gases are added to the ordinary C2N2 gas target and the increase of the A-X lifetimes and band-intensities depends on the polarizibility of the added gases. Simple models are presented for the time-resolved case as well as the steady state case from which the coefficient for the transfer rate is found to be of the order 10-10-10-11 cm3/s, mol. The new oscillator strengths for the red system corrected for the transfer effects are notably higher than earlier estimates and the astrophysical consequences of this are briefly discussed.

High Resolution Measurements of Atomic and Molecular Lifetimes Using the High Frequency Deflection Technique

The basic principles of the High Frequency Deflection (HFD) technique for the determination of atomic and molecular lifetimes as well as the first experimental tests were presented a couple of years ago. In principle this technique is a further refinement of the conventional multichannel delayed coincidence technique with periodic electron excitation of free molecules. The differences are mainly that the HFD technique utilizes a continuous electron beam from a high-power gun operating in the kilovolt range and that the periodic excitation is accomplished by sweeping the beam at an optimal rate, i.e. with a period equal to 3-10 times the lifetime to be studied. With these two refinements the HFD technique enables lifetime measurements in all kinds of atoms, molecules, ions and ion-molecules with several orders of magnitude higher efficiency than other methods permit, thus opening possibilities for high resolution work in the absence of Doppler broadening. Thus a number of lifetimes have been determined at a spectral resolution of 0.1 A FWHM or better, which is 20-50 times higher a resolution than what is normally achieved in lifetime measurements. The variable sweep frequency and the multichannel registration make possible measurements over a large lifetime range (up to about 104 ns) which in turn offers unique possibilities for analysing multi-exponential decay curves. With a time scale of arbitrary precision given by the sweep frequency, lifetime measurements have so far been performed with a total error down to 0.5% in favourable cases. The HFD technique is now currently in use at this laboratory and several hundreds of lifetimes have been studied and reported in a number of publications. In this paper we shall discuss more in detail the technique itself, its properties and possibilities compared to other methods for lifetime measurements. A review of some of the experimental results obtained so far, with particular emphasis on abundance determinations of atoms and molecules of fundamental astrophysical interest, is also included as well as a critical discussion of given error limits in lifetime measurements in general.

Direct Determination of the Ionization-potential of Co By Resonantly Enhanced Multiphoton Ionization Mass-spectroscopy

Direct determination of the ionization potential of CO by resonantly enhanced multiphoton ionization mass spectroscopy

Correlation between perturbation and collisional transfers in theA, B, C andb states of CO as revealed by high resolution lifetime measurements

Lifetimes and intensities of theA-X, B-A, C-A andb-a systems in CO have been measured at high spectral resolution using the High Frequency Deflection technique. Of special interest are the pressure effects observed in theA-X andb-a transitions, both as a function of the partial pressure of CO and as a function of added catalyst gas such as argon, helium and CO2. The nature of these pressure effects are examined with the conclusion that resonance induced collision exchange takes place between theA orb states and other states that strongly perturb them. In particular, the collision exchange cross section estimated for theA state is unusually large, being the order of 10−7 to 10−8 cm3/s, mol. In contrast, theB andC (Rydberg) states, which are not known to be highly perturbed, showed no evidence of collision induced exchange. In addition, calculations on Franck-Condon factors and ¯r centroids have been carried out for each of the bands studied. Transition moments and oscillator strengths are also extracted from these calculations and from the new data presented in this paper, and considerable discrepancies to earlier values in the literature are found in some cases.

Photoionization and photodissociation of nitric oxide in the range 9–35 eV

Photoionization and photodissociation of NO has been studied using 9–35 eV synchrotron light and detection of fluorescence as well as of mass selected NO+, N+, O+, and O− ions. Rydberg series converging to all known states in NO+ below 24 eV have been analyzed and classified and several of them are found to be predissociated by various NO+ states as well as by NO ion pair states. The complex structure in the 950–1200 A excitation region is suggested to originate from interaction between a ‘‘new’’ NO valence state at T0=77 470 cm−1 and high vibrational levels of Rydberg series converging to the NO+ ground state.

Lifetimes of Excited Levels in Some Important Ion-Molecules, Part III: CS+, COS+

Lifetimes of the recently discovered B-A system in CS+ and of the A - system in COS+ have been studied using the High Frequency Deflection Technique. The measured CS+ lifetime (425 ns) supports the identification of the B-A system by comparisons with CO+. Different lifetimes are found for the (0, 0, 0) A 2Π3/2 and A 2ΠA1/2 levels in COS+ (105 ± 3 ns and 77 ± 3 ns respectively), which shows that the earlier discovered predissociation is different for the two levels.

Predissociation effects in the A 2Δ and B 2Σ− states of CD

Lifetimes have been measured for a large number of rotational levels of the A 2Δ and B 2Σ− states in CD using the high frequency deflection technique. Four B‐state levels are found to be influenced by predissociation by rotation which demands a lower value of the CD dissociation energy. The B state predissociations have also been studied using ab initio calculations. The A state levels above the dissociation limit are found to be weakly predissociated through interaction with the ground state continuum. The results are compared with earlier lifetime investigations of CH.

An energy resolved electron?ion coincidence study near the S 2p thresholds of the SF6 molecule

The fragmentation dynamics of the SF6 molecule following the excitations of S 2p electrons into unoccupied molecular orbitals has been studied using the energy-resolved electron–ion coincidence technique. Fragmentation patterns were found to depend on the particular excitation and on the electronic state of the molecular ion. The spectator resonant Auger decay at the 2p → 6a1g resonance induces changes in the ion distributions as compared to direct photoionization. Furthermore, coincidence spectra related to the same Auger structure display different ion abundances at the 2t2g and 4eg shape resonances. Differences were also found in the Auger decay spectra. These findings give further support for the previously suggested many-electron character of the 4eg shape resonance.

Time resolved studies of collisional transfer and radiative decay of the CS A 1Π state

Abstract The lifetimes of the ν′ = 0–5 levels originating from the CS A 1 Π state have been studied at high spectral resolution using the high frequency deflection technique. In addition to the well-known effects of triplet—singlet perturbations on the lifetimes, a strong sensitivity to large collisional transfers from more distant triplet levels have been revealed. The latter causes long-lived components in the decay curves and large increases of the AX band intensities when foreign catalyser gases are added.

Direct Measurement of the N2+ C State Predissociation Probability

The lifetimes of the vibrational ν = 1-5 levels of the N2+ C state have been directly measured at high resolution using the High Frequency Deflection technique. For the ν = 2 level, a radiative lifetime τ = 78.9 ± 3.0 ns is obtained, while the lifetimes drop sharply to a constant value of 4-5 ns for the ν ≥ 3 levels. Since the ν ≥ 3 levels are all above the lowest dissociation limit, the measurements confirm that these levels are predissociated and at a rate of about 2 × 108 s-1. The lifetime variation suggests that the predissociation mainly takes place through vibrational coupling with the B state.