Nis Bjerre

The triplet 3s,3d complex of HD

High‐resolution laser photofragment spectroscopy of the 3s,3d←2p c 3Π−u transitions in HD has been performed in a fast mass and state‐selected beam. Transition wave numbers and rotational energy levels are given for v=0–3 of the h3∑+g, g 3∑+g, i 3Πg, and j 3Δg states. The data are represented by an effective Hamiltonian which accounts for the rotational breakdown of the Born–Oppenheimer approximation (L‐uncoupling). The model parameters indicate a vibrational Born–Oppenheimer breakdown in the i 3Πg state. Good agreement with the most accurate ab initio calculations is obtained.

FEMTOSECOND PHOTODISSOCIATION DYNAMICS OF I2 STUDIED BY ION IMAGING

Ion imaging is employed to analyze the fragments from timed Coulomb explosion studies of femtosecond (fs) molecular dynamics. The technique provides high detection efficiency and direct recording of the two-dimensional velocity of all ionized fragments. We illustrate the approach by studying photodissociation of iodine initiated by a weak femtosecond pump pulse and probed by an intense, delayed femtosecond pulse. The ion images, recorded as a function of the pump-probe delay, reveal the evolution of the internuclear separation of the dissociating I2 molecule. The experimental results are in good agreement with quantum mechanical wave packet simulations. We discuss the perspectives for extending the studies to photochemical reactions of small polyatomic molecules.

Lifetime measurements of He2- by means of a heavy-ion storage ring

The lifetime of the metastable autodetaching 4Pig state of the He2- ion has been measured by a time-of-flight technique to be 135+or-15 mu s, using a heavy-ion storage ring. The lifetime is nearly a factor of three shorter than the only available theoretical prediction. It is proposed that the lifetime should be assigned to the 4Pig- Lambda doubling component.

Spin—orbit coupling in H3

Abstract The fine-structure splittings of the 3d 2 E″ and 2 A′ 1 states of H 3 are measured using fast neutral-beam laser spectroscopy. The splittings are analyzed in terms of the rotational mixing of the E″ and A′ 1 states to yield a value for the spin—orbit coupling constant of H 3 very close to those of the H and Li atoms. Contrary to previously published values of 40–90 μs, but in agreement with a recent measurement by Helm and co-workers, we find a lifetime of less than 2.1 μs for the long-lived 2p 2 A″ 2 N = K = 0 state. The spin—orbit contribution to predissociation of this state is discussed.

Barrier tunneling in the He2 c 3Σ+g state

High resolution fast beam photofragment spectroscopy on the a 3Σ+u has been used to measure level positions and widths of the c 3Σ+g in the positive energy region near the top of its barrier. Tunneling through the barrier produces energetic neutral fragments that diverge from the beam and are detected. The information is used to determine the shape and height of the barrier to spectroscopic accuracy.

Fine structure of the lowest triplet states in He2

Laser driven transitions from the metastable a 3Σ+u state to the predissociating level v’=4 of the electronic state c 3Σ+g of He2 are investigated by fast beam laser spectroscopy. For transitions to the rotational levels N’=8, 10, and 12 of the c 3Σ+g state, the fine structure is resolved and the fine structure parameters of both electronic states can be determined. The fine structure in the rotational levels N‘=7, 9, and 11 of v‘=0 in the a 3Σ+u state is measured to 20 kHz precision (3σ) using laser–radio frequency double‐resonance spectroscopy. These measurements are complementary to the previous radio frequency (rf) measurements for the levels N‘=1, 3, and 5 by Lichten et al. The fine structure parameters are compared to theoretical calculations and simple models for the electronic interactions and the couplings between different electronic states.

ESR study of the librations of O2 defects in NaClO3 and KClO4 single crystals

The ESR spectra of O2 defects in UV‐irradiated single crystals of NaClO3 and KClO4 were measured at 9 and 35 GHz in the temperature range 2.7–23 K. The O2 defect in NaClO3 has a g‐tensor close to that of the free molecule and exhibits well‐resolved 16O2–18O2 isotope shifts. Model calculations reproducing these shifts reveal that the discrepancy between the spin Hamiltonians of the trapped and the free molecule mainly results from the angular librations of the trapped molecule. The marked temperature dependence of the spin Hamiltonian of an O2 defect in KClO4 is accounted for in terms of fast transitions between the librational states.

Observation of population trapping in a two-photon resonant three-level atom

Abstract We report narrow structures observed in the population of the intermediate level of a two-photon transition in a fast beam of metastable 20 Ne ∗ atoms. Steady-state theory provides a description of two coherent processes leading to population trapping. Both processes proceed via the ordinary two-photon coherence. One is then completed with a reversed-time ordering of the last two interactions, leading to a dispersive variation of the population of the real intermediate level, the other creates a hole in this population. A narrow Doppler profile enhances the visibility of these processes.

Photolysis of NaClO3 and KClO3 at 26 K studied by optical and ESR spectroscopy

Two types of defects are formed when crystalline NaClO3 and KClO3 are photolyzed at 26 K with monochromatic light of a wavelength corresponding to the fundamental absorption. One type of defects absorbs around 380 nm and has an ESR spectrum characteristic of trapped O2. The other type of defects absorbs around 270 and 580 nm and has no ESR spectrum. The two types of defects are interconverted reversibly by bleaching within the respective absorption bands. Chemical analysis and isotopic labeling indicates that the defects consist of fragments of a single chlorate ion. The 380 nm absorption is assigned to charge transfer in the complex [ClO−,O2]; the 270 and 580 nm absorptions are assigned to [Cl−,O3]. Both the optical spectra and the mechanism of photolysis are represented by a model based on the enthalpies of formation of various configurations of one chlorine and three oxygen atoms.

Dissociative recombination for vibrationally excited ions: the effect of laser photodissociation on the initial vibrational distribution

We report on measurements of dissociative recombination of ions populated in a range of initial vibrational levels with free electrons in the energy range of 0 - 20 eV. A photodis-sociation technique was applied to modify the vibrational distribution, and the influence on the dissociative-recombination (DR) spectrum was studied. To estimate the vibrational distribution in the experiment, the results of a simple model calculation describing a part of the measured spectrum are presented. From the comparison between the experimental data and the model calculation, we conclude that when laser photodissociation is not applied, our result is consistent with a Franck - Condon vibrational distribution from the ion source. When laser photodissociation is applied, a dramatic change of the DR spectrum is observed, which is consistent with the expected effect, based on theoretical photodissociation cross sections.