Cleanthes A. Nicolaides

Delay in Photoemission

Defining Time-Zero When a high-energy photon hits an atom and is absorbed, the result can be the excitation and emission of an electron. This photoemission, or photoelectric effect, is generally assumed to occur instantaneously, and represents the definition of “time-zero” in clocking such ultrafast events. Schultze et al. (p. 1658, see the cover; see the Perspective by van der Hart) use ultrafast spectroscopy, with light pulses on the time scale of several tens of attoseconds, to test this assumption directly. They excite neon atoms with 100 eV photons and find that there is a small (20-attosecond) time delay between the emission of electrons from the 2s and 2p orbitals of the atoms. These results should have implications in modeling electron dynamics occurring on ultrafast time scales. Ultrafast metrology reveals a 20-attosecond delay between photoemission from different electronic orbitals in neon atoms. Photoemission from atoms is assumed to occur instantly in response to incident radiation and provides the basis for setting the zero of time in clocking atomic-scale electron motion. We used attosecond metrology to reveal a delay of 21±5 attoseconds in the emission of electrons liberated from the 2p orbitals of neon atoms with respect to those released from the 2s orbital by the same 100–electron volt light pulse. Small differences in the timing of photoemission from different quantum states provide a probe for modeling many-electron dynamics. Theoretical models refined with the help of attosecond timing metrology may provide insight into electron correlations and allow the setting of the zero of time in atomic-scale chronoscopy with a precision of a few attoseconds.

The variational calculation of energies and widths of resonances

Abstract It is shown that the “difficulties” which Bransden recently associated with the variational calculation of resonance parameters are nonexistent, provided the norm of the Gamow functions is defined appropriately and consistently.

Energy generation from volcanic ground states. Application to cold He22

The lifetime of the ν=0 vibrational level of the volcanic He22+1Σg+ state, which autodissociates to He+ + He+ fragments, has been calculated to be extremely long: 220 min. This finding, together with the high energy content of He22+ and its small mass, offers the possibility of new physical and chemical reactions for the release of propulsive energies in the range 230-1000 kcal/mol, with specific impulses much larger than that of existing liquid hydrogen plus oxygen fuels. The observation of He22+ via charge-stripping mass spectroscopy has been reported in the literature. Based on computed data, we propose alternative methods for its production involving collisional and radiative processes.

Efficient LaF3 :Nd3+-based vacuum-ultraviolet laser at 172 nm

Vacuum-ultraviolet (VUV) laser radiation at 172 nm has been obtained from a solid-state LaF3:Nd3+-based laser pumped by a pulsed-discharge molecular F2 laser at 157 nm. The maximum slope efficiency of the solid-state laser described in this experiment was 21% (14% conversion efficiency), and the maximum output energy at 172 nm was 0.4 mJ for a nonoptimized optical cavity. This finding introduces serious prospects for realizing versions of active-medium-plus-source tunable VUV laser devices.

Potential energy surfaces for the photodissociation H2O→O(1Dg + H2(X1Σ+g)

Abstract The minimum energy pathways for symmetrical dissociation of water into O( 1 D g + H 2 ( X 1 Σ + g ) are calculated by the MRD Cl technique for various excited states of H 2 O and possible mechanism for the photodissociation are discussed.

Nonorthonormal CI for molecular excited states. I. The sudden polarization effect in 90° twisted ethylene

The accurate and efficient calculation of properties of excited states, especially those involved in near‐degeneracies and valence‐Rydberg mixings, may depend crucially on the zeroth order orbitals employed for the description of a few important configurations. When self‐consistent field orbitals specific for the states of interest are employed, one becomes involved with nonorthonormal basis sets, which circumstance has conceptual as well as computational implications. In this work, we have developed a nonorthonormal configuration interaction (NONCI) method and have applied it to the calculation of the ‘‘sudden polarization’’ effect in the zwitterionic excited states of ethylene, in the spirit of a state‐specific theory. A very small NONCI, (6×6), yields similar results to those from previous large CI calculations. In particular, the sudden rise of the dipole moment as a function of the pyramidalization angle of the CH2 group of the 90° twisted Z state is predicted while it is shown that the inclusion of ...

A non-van der Waals minimum of the He(1S) + H2(B 1Σu+) excited surface

Abstract We have carried out MRD CI calculations for the first excited state of HeH 2 ( 1 A′) at geometries which have He( 1 S) approaching the center of mass of H 2 (B 1 Σ u + ) at 45°. The calculated energies yield a barrier of 0.24 eV and a minimum of 1.5 eV. This minimum occurs near an avoided crossing with the ground-energy surface which is not found for collinear or perpendicular geometries. These results suggest an explanation for the electronic quenching of HD(B 1 Σ u + ) in the presence of He observed by Moore and co-workers.

Approach to the calculation of the important many-body effects on photoabsorption oscillator strengths

Abstract By a combination of selection rules for dipole transitions and notions from variation-perturbation theory, we develop a first-order theory of oscillator strengths (FOTOS) whereby the important correlations for an accurate treatment of electric dipole processes in atoms and molecules can be easily understood, formally derived and calculated. FOTOS is applied here to certain one- and two-electron transitions in Li I, N I and N II. The calculation of the corresponding oscillator strengths present considerable theoretical problems. Our results resolve the existing discrepancy between lifetime experiments and previous many-body calculations.

Adiabatic calculations of the 2Σ+g excited states of He+2

Abstract The first six 2 Σ + g adiabatic potential energy curves of He + 2 are calculated with the MRD CI method employing configuration selection ( T = 10 μ hartree) and perturbative energy corrections and using two basis sets differing in the number of diffuse functions. The non-adiabatic matrix elements at the numerous avoided crossings are also calculated and approximate diabatic curves are constructed. Various aspects of the results are discussed.

Theoretical dipole transition moments for transitions between bound electronic states and non-adiabatic coupling matrix elements between 2Σ+ of HeH

Dipole transition moments and radial non-adiabatic coupling matrix elements between electronic states of HeH are presented. The estimated radiative lifetimes of the excited states of HeH are in good agreement with the available experimental data.