Table of contents

Through solving the three-dimensional (3D) time-dependent Schrödinger equation, we have investigated how an intense, circularly/elliptically-polarized, few-cycle pulse rotates the electronic wave packets of a dissociated diatomic molecule. When such rotating wave packets from one atomic core sweep quickly through the other nucleus, recombination occurs and generates a supercontinuum radiation spectrum. The numerical solution of the Maxwell equation for the 3D propagation of such supercontinuum radiation in realistic molecular gases has confirmed the generation of a single attosecond (as) pulse, with a duration of ∼186 as. Moreover, the proposed scheme can push the extreme ultraviolet (EUV) pulse generation down to the sub-100-attosecond regime. As an example, we have shown a single EUV pulse of ∼90 as, using an elliptical driving pulse.

Spectroscopic measurements on the B ¹Σ⁺_u (v'= 0–2) ← X ¹Σ⁺_g (v'' = 0) Lyman bands in H₂ and the B ¹Σ⁺ (v'= 0–2) ← X ¹Σ⁺ (v'' = 0) Lyman bands in HD are presented using a narrow bandwidth vacuum ultraviolet laser system combined with an accurate frequency calibration. These measurements complete the recently published data on the Lyman frequency transitions of Philip et al (2004 Can. J. Chem.82 713–22).

A completely dense femtosecond optical supercontinuum is observed for the first time. It does not exhibit the oscillation-like frequency structure of the well-known optical supercontinuum generated by self-phase modulation. The newly observed optical supercontinuum is attributed to bremsstrahlung radiation from electrons that tunnel the bound potential under a strong optical field. The basic properties of the dense optical supercontinuum are studied.

This article provides a review of recent work in the field of helium nanodroplet spectroscopy with an emphasis on the dynamical aspects of the interactions between molecules in helium as well as their interaction with this unique quantum solvent. Emphasis is placed on experimental methods and studies introducing recent new approaches, in particular including time-resolved techniques. Corresponding theoretical results on the energetics and dynamics of helium droplets are also discussed.

This paper studies the effects of the incoherent pumping field and the spontaneously generated coherence (SGC) on the phase control of group velocity. The effects of a relative phase between probe and coupling fields on the absorption and the dispersion are then discussed. It is shown that the phase dependence of the group velocity not only depends on the existence of the SGC, but also depends on the existence of the incoherent pump field. We show that for the weak probe field, and in the presence of SGC, the existence of the incoherent pump field is a necessary condition for the phase control of the dispersion, the absorption and the group index.

The interaction at mean distance (a few tens up to a few hundreds of a₀), i.e. in the van der Waals interaction range, between metastable nitrogen molecules, N₂* (A³Σ_u⁺), and the slit edges of a micro-slit copper grating depends on both the molecular orientation and the internuclear distance in the molecule. Such an interaction is able to induce rotational and vibrational transitions. Endo-energetic transitions (v → v + 1, v ranging from 5 to 10) are observed by means of a time-of-flight technique combined with an angular distribution measurement. By setting the grating plane at an angle with respect to the incident direction, different from that imposed by ideally planar slit walls, it is shown that the angular distribution of the inelastic process reveals a departure of the surface from an ideal plane. Assuming a regular evolution of the tangent plane along the surface profile, a mean wall profile can be derived from this distribution.

We discuss the properties of the lifetime or the time-delay matrix Q(E) for multichannel scattering, which is related to the scattering matrix S(E) by Q = iℏS(dS†/dE). For two overlapping resonances occurring at energies E_ν with widths Γ_ν(ν = 1, 2), with an energy-independent background, only two eigenvalues of Q(E) are proved to be different from zero and to show typical avoided-crossing behaviour. These eigenvalues are expressible in terms of the four resonance parameters (E_ν, Γ_ν) and a parameter representing the strength of the interaction of the resonances. An example of the strong and weak interaction in an overlapping double resonance is presented for the positronium negative ion. When more than two resonances overlap (ν = 1, ..., N), no simple representation of each eigenvalue has been found. However, the formula for the trace of the Q-matrix leads to the expression δ(E) = −∑arctan[(Γ_ν/2)/(E − E_ν)] + δ_b(E) for the eigenphase sum δ(E) and the background eigenphase sum δ_b(E), in agreement with the known form of the state density. The formulae presented in this paper are useful in a parameter fitting of overlapping resonances.

The photodetachment of H⁻ near an interface has been investigated by the closed orbit theory. It is found that the elastic interface has significant influence on the photodetachment process. An analytical formula of the cross section of photodetachment is derived. It is found that the cross section consists of a smooth background and sinusoidal oscillation, which is quite similar to that of electric field.

We investigate relativistic and correlation effects in electron impact excitation of singly ionized oxygen using the Breit–Pauli R-matrix method. The intermediate coupling close-coupling calculations are carried out using a 16-level target representation dominated by the electronic configurations 1s²2s²2p³, 1s²2s2p⁴, 1s²2s²2p²3s. Resonance structures are delineated in detail to ascertain the effect on averaged collision strengths. Convergence of the partial wave summation is ensured for non-dipole transitions in the R-matrix calculations. The present results differ significantly from the similar Breit–Pauli R-matrix calculations by McLaughlin and Bell (1998 J. Phys. B: At. Mol. Opt. Phys.31 4317–29), but are essentially in agreement with the LS coupling results of Pradhan (1976a J. Phys. B: At. Mol. Opt. Phys.9 433–43, 1976b Mon. Not. R. Astron. Soc.177 31–8). A comprehensive study of the detailed energy behaviour of all forbidden transitions among the five levels of the ground configuration, i.e. 2s2p³(⁴S^o_3/2, ²D^o_5/2,3/2, ²P^o_3/2,1/2) shows that the finestructure collision strengths do not significantly depart from the values obtained from a purely transformation, and relativistic effects are therefore small. We find that the Maxwellian-averaged effective collision strengths for the ten transitions also differ from the previous work, most likely due to more extensive delineation of resonances in the present work. However, the differences are largely systematic and therefore the OII line intensity ratios are not significantly affected. We also obtain an excellent agreement between the present-calculated cross sections for the ⁴S^o − ²D^o transition and the experimental merged beam measurements.

Energy differential cross sections from the triple photoionization of lithium are calculated using the non-perturbative time-dependent close-coupling method. Collision probabilities are found by projection of the time-evolved nine-dimensional coordinate space wavefunction onto a fully antisymmetric product of spatial and spin functions representing three outgoing Coulomb waves. We then derive a differential cross section that is a function of the energy sharing between all three ejected electrons moving in the Coulomb field of the nucleus; i.e. four-body Coulomb break-up. At an incident photon energy of 300 eV, the energy differential cross section for lithium is bowl shaped with magnitudes generally below 10⁻³ barns/(eV)²; certainly a challenge for future advanced light source experiments.

In 1967 Griem gave a general formula for the electron-impact broadening of radio recombination lines, and an approximate expression for the width of the Δn = 1 lines. Here his general formula is calculated numerically for higher values of Δn, and an approximate expression is fitted to the results for Δn up to 70. It is shown that, for the lines with different n and Δn near a given frequency ν, the calculated widths increase monotonically up to n > 300. This result does not agree with the maximum found near n ≈ 200 by Bell et al (2000 Publ. Astron. Soc. Pac.112 1236) for the astronomical sources W51 and Orion A at 6 GHz. Thus this mystery is not resolved by the present calculations.

The triple differential cross section (TDCS) of the He²⁺(1s⁻²) and Ne²⁺(2s⁻²) states has been studied under unequal energy sharing conditions and perpendicular geometry, for a ratio of about 3 between the energies of the two ejected electrons. The dynamical quantities which govern the photodouble ionization (PDI) process, i.e. the squared moduli of the gerade and ungerade complex amplitudes and the cosine of their relative phase, have been extracted from the experimental data. The results from the two targets have been compared between themselves as well as with the theoretical predictions of the SC3 and convergent close coupling (CCC) calculations. This work represents a joint experimental and theoretical approach to the investigation of PDI of atomic systems with more than two electrons.

We report the experimental observation of the amplified spontaneous emission (ASE) generated at the 4P_3/2–4S_1/2 transition of potassium atoms following the excitation of the 4S_1/2–6S_1/2 two-photon transition. We propose that the amplification of the observed radiation is inversionless, due to driving fields on neighbouring transitions that are not imposed externally, but are internally generated in the medium. A theoretical model based on the density operator formalism is presented, which confirms that the radiation observed in the 4P_3/2–4S_1/2 transition is generated without population inversion for a broad range of the parameters used. It seems that a self-generated ladder scheme is the dominant mechanism, but other schemes are also examined and compared with the experimental results in order to assess their impact on the generation of the inversionless 4P_3/2–4S_1/2 emission.

Singlet–triplet anticrossing spectra of the helium isotope ³He have been measured for the first time. These anticrossing spectra differ strongly from those of the isotope ⁴He due to the hyperfine interaction. We exploited this difference for distinguishing between the excitation of target atoms and the formation of excited projectile atoms by electron capture in experimental investigations on the symmetric collision system He⁺–He.

To study the possible variation of the fine structure constant, α, in laboratory experiments, we propose using narrow lines that correspond to atomic transitions between close lying, long-lived atomic states of different configurations. The small transition frequency, coupled with differences in the electron structure ensures a strong enhancement of the relative frequency change compared to a possible change in α. The small linewidth makes it possible to perform very accurate measurements. These two features, when combined, make the measurements very sensitive to any variation of α.

We present experimental values of the electron impact coherence parameters (EICP) and reduced Stokes parameters for excitation of 5¹P₁ state of cadmium atoms. The results have been obtained using electron–photon coincidence technique for incident electron energies 80 eV and 60 eV and electron scattering angles in the range of 5° to 50°. We also present an additional set of data for electron energy 100 eV and scattering angle 50° which complements our previous results. All the experimental values are compared with theoretical relativistic distorted-wave approximation (RDWA) calculations. The first Born approximation (FBA) predictions of the alignment angle are also presented. The theoretical results are in good qualitative agreement with the experimental data.

We have measured momentum-space images of low-energy electrons generated by the interaction of short intense laser pulses with argon atoms at high intensities. We have done this over a wavelength range from 400 to 800 nm. The spectra show considerable structure in both the energy and angular distributions of the electrons. Some, but not all, energy features can be identified as multi-photon resonances. The angular structure shows a regularity which transcends the resonant structure and may be due instead to diffraction. The complexity of the results defies easy model-dependent interpretations and invites full solutions to Schrödinger's equation for these systems.

We propose a method for simulating the behaviour of small clusters of particles that explicitly accounts for all mean-field and binary-correlation effects. Our approach leads to a set of variational equations that can be used to study both the dynamics and thermodynamics of these clusters. As an illustration of this method, we explore the BCS–BEC crossover in the simple model of four fermions, interacting with finite-range potentials, in a harmonic potential. We find, in the crossover regime, that the particles prefer to occupy two distinct pair states as opposed to the one assumed by BCS theory.

We present a scheme for symmetric multiparty quantum state sharing of an arbitrary m-qubit state with m Greenberger–Horne–Zeilinger states following some ideas from the controlled teleportation (2005 Phys. Rev. A 72 02338). The sender Alice performs m Bell-state measurements on her 2m particles and the controllers need only take some single-photon product measurements on their photons independently, not multipartite entanglement measurements, which makes this scheme more convenient than the latter. Also it does not require the parties to perform a controlled-NOT gate on the photons for reconstructing the unknown m-qubit state and it is an optimal one as its efficiency for qubits approaches 100% in principle.

It is theoretically shown that the tunnelling dynamics of light between two sinusoidally curved coupled optical waveguides of different sizes exactly mimics the population dynamics of a two-level system with nonzero diagonal dipole matrix elements subjected to an applied sinusoidal electric field, thus realizing an optical analogue of a two-level dipolar molecule in an external ac field. Some characteristic dynamical features, including multi-photon transitions, sinusoidal population oscillations built up in a series of stair steps or square-wave population oscillations, are reproduced in the optical waveguide system. Design parameters for their observation are given with reference to lithium-niobate optical waveguides.

We derive a two-band finite-dimensional model for the description of the condensate tunnelling in an accelerating optical lattice, taking into account the fine Bloch band structure. The model reveals a very strong dependence of the final band populations on the initial populations and phases. Most importantly, additionally to the known asymmetric dependence on the nonlinearity, there is also a notable asymmetry in the sensitivity of the tunnelling probability to the nonlinearity-induced initial population of the Bloch band to which the tunnelling takes place. This fact can explain the experimentally observed unexpected independence of the upper-to-lower tunnelling probability on the nonlinearity. Finally, we compare the predictions of the two-band model with that of the well-known nonlinear Landau–Zener model and find disagreement when the two bands are initially populated. The disagreement can be qualitative and reveals itself even for a negligible nonlinearity. However, the two models agree remarkably well if just one band is populated initially.

We have measured hyperfine structure in the 6P_1/2 state of ¹³³Cs using a frequency-stabilized diode laser and acousto-optic modulators that produce calibrated frequency offsets. We check for errors due to radiation-pressure and optical-pumping effects by repeating the measurements at different values of pump power in a saturated-absorption spectrometer. We obtain a precise value of the magnetic-dipole constant, A = 291.9135(15) MHz, representing an improvement of more than an order of magnitude.

Single ionization cross sections for He²⁺ and O⁶⁺ impact on Na*(3p) in the 2–10 keV amu⁻¹ energy range have been measured using recoil-ion momentum spectroscopy of cold Na atoms. The present results are compared with our previous data on Na(3s). It is found that ionization of these weakly bound alkali systems yields larger cross sections than predicted from scaling laws derived from H and He targets. Also information on the ejected electron momenta could be obtained from the recoil spectra.

In order to specify the mechanisms involved in a xenon dielectric barrier excimer lamp, an experimental work was undertaken using two different set-ups. One study, performed under selective multiphotonic excitation, is presented here and the other, of a dielectric barrier micro-discharge, will be published in another paper. In this work, excitation of the two lower atomic states of the 5p⁵6s configuration was performed: the Xe5p⁵6s(³P₂) metastable state and the molecular g states correlated to it or to the Xe5p⁵6s(³P₁) resonant state were excited by absorption of two photons. A spectroscopic and kinetic study was performed for the first time, comparing the radiation characteristics consecutive to initially populating either the resonant Xe5p⁵6s(³P₁) or the metastable Xe5p⁵6s(³P₂) states. A first continuum issuing from 1_u state and broader than the 0⁺_u one is observed. The transfer from the 0⁺_u(³P₁) state to a dissociative state correlated to Xe5p⁵6s(³P₂) is clearly shown. The sum of the transfer and relaxation rate constants was measured: k_t + k_v = (1.6 ± 0.5) × 10⁶ Torr⁻¹ s⁻¹.

The high-resolution absolute photoabsorption cross sections with an absolute photon energy scale for Ne 1s2p/1s2s double excitation in the energy region of 900–940 eV have been measured using a multi-electrode ionization chamber and monochromatized synchrotron radiation. The Ne [1s2p](^3,1P)nln'l' doubly excited states observed in the energy range of 900–920 eV are analysed with the aid of theoretical calculations based on the multiconfiguration Dirac–Fock method. The second strongest peak in this energy region is assigned to be [1s2p](³P)3p4p.

A relativistic and non-Markovian study is carried out on the magnetic-dipole emission of a hydrogen-like atom of large atomic number Z without perturbation approximation. The correlation spectra are derived analytically and the corresponding correlation functions are calculated numerically. The Markovian approximation is seen to be satisfied with more accuracy as compared with the case of electric-dipole emission while the relativistic corrections are quite larger. In transition 2S_1/2 to 1S_1/2, the relativistic values even become several times of the non-relativistic values. As a comparison with the magnetic-dipole emission, the electric-quadrupole emission in the case of the 2D_3/2 to 1S_1/2 transition is also studied.

Transition properties of the Kα and Kβ x-ray from CuXX through CuXXVIII have been calculated by using relativistic configuration interaction (RCI) and multi-configuration Dirac–Fock (MCDF) method in the active interaction approach. The contributions from Breit interaction, quantum electrodynamics (QED) corrections and nuclear mass corrections to the initial and final levels have been taken into account. Through the use of the active space method, the calculated values are compared with the other available data on He-like and Be-like copper and are found to be in very good agreement with them. In this paper, we give accurate transition properties from CuXX through CuXXVIII. These data provide reference value for level lifetime, charge state distribution and average charge of copper plasma.

We present a nonrelativistic calculation of the rotation-vibration levels of the molecular ions H⁺₂, D⁺₂ and HD⁺, relying on the diagonalization of the exact three-body Hamiltonian in a variational basis. The J = 2 levels are obtained with a very high accuracy of 10⁻¹⁴ au (for most levels) representing an improvement by five orders of magnitude over previous calculations. The accuracy is also improved for the J = 1 levels of H⁺₂ and D⁺₂ with respect to earlier works. Moreover, we have computed the sensitivities of the energy levels with respect to the mass ratios, allowing these levels to be used for metrological purposes.