Stark manifolds of barium Rydberg states

In a CW laser-atomic beam experiment Stark manifolds in barium originating from the Rydberg states 6s40f ¹ F ₃, 6s40g ^1,3 G ₄,³ G ₅ and 6s40h ^1,3 H ₅ have been studied. Accurate quantum defect values for higher orbital angular momentum states (l=6, 7) have been determined. The Stark manifolds are also calculated by diagonalization of the energy matrix in the presence of an external electric field. Good agreement between experiment and calculations is obtained.     

A test of the accuracy of atomic-beam laser spectroscopy

To determine the accuracy to which hyperfine splittings can be measured using a cw dye laser with rf sideband tuning, a series of atomic beam experiments with⁵⁹Co was performed. One hyperfine splitting of the first excited metastable atomic state was measured and compared with the results for the same splitting from magnetic rf resonance experiments. The uncertainty with the methods applied was found to be about 0.05 to 1% of the experimental linewidth in general; it is ±20 kHz or ±0.05% of the linewidth in the present Co experiment. The hyperfine splitting constants of the 3d ⁸ 4p ² F _7/2 state were found to beA=419.3(9) MHz,B=?77(17) MHz.     

Study of nonradiative decay properties following resonant double excitation to the [1s2p]nln′l′ states of Ne atom using soft X-ray undulator radiation

We investigate the nonradiative decay properties of resonantly photoexcited [1s2p]nln′l′ (n,n′≧3) states of Ne atom, where square brackets indicate hole states, using soft X-ray undulator radiation. Partial ion-yield spectra for the Ne²⁺–Ne⁴⁺ ions were measured in the region of double excitation resonance by means of time-of-flight spectrometry. The Ne³⁺ partial ion-yield spectrum has been found to reveal resonant enhancements at the energy positions of doubly excited states, which implies the most plausible decay channels leading to the third step Auger processes. Energy diagram of the excited and relaxed states associated with the [1s2p](³P)3p² doubly excited state of the neutral Ne as well as the Ne ions was calculated based on the multiconfiguration Dirac–Fock (MCDF) method in order to interpret the possible nonradiative decay channels. The low-energy electron spectrum originated from the second/third step Auger processes was preliminarily measured to show a rich structure induced at the [1s2p](³P)3p² double excitation resonance.     

Quantum chemical ab initio calculations for excited states of F IV

Quantum chemical ab initio calculations have been performed for the ground state and for several excited states of the F³⁺ ion (F IV). Three levels of accuracy have been used: Frozen-core SCF calculations (FRC-SCF) to determine orbital energies ε _nl and quantum defects δ _l for excited Rydberg orbitalsnl; frozen-core SCF followed by CI calculations (FRC-CI) which account for multiplet splittings and configuration mixings, and multi-configuration coupled-electron-pair approximation (MC-CEPA) calculations which include dynamic correlation effects. The accuracy of the calculated excitation energies is in the order of 5000 cm^?1 at the FRC-CI level and in the order of 500 cm^?1 at the MC-CEPA level. This latter error amounts to about 0.1% for excitation energies in the range of 400000 to 600000 cm^?1. The MC-CEPA calculations have been performed for 17 experimentally known states and for 14 experimentally unknown states, in particular for the configurations 2s2p ² (² D)3s, 2s 2p ²(² S)3s, 2s ² 2p 4p, and 2s ² 2p 5p.     

Laser magnetic resonance spectra of the PH radical

The PH radical has been detected by laser magnetic resonance spectroscopy at 118.6 μm in the reaction products when hydrogen atoms were passed over red phosphorus. The spectra have been identified as the N = 4 → 5 rotational transition in the ground ³∑^? state and J = 4 → 5 transition in the a¹ Δ state. The hyperfine constants for the ¹Δ state are a_p = 775 MHz and a_H = 28 MHz.     

Two-photon absorption of dissociating TLI in strong laser fields

In a strong laser field (I=2 GW/cm²) TlI is dissociated by a two-photon process and the Tl fragment is detected state specifically. The yield of Tl 6p ² P _3/2 (Tl*) is measured as a function of the dissociation wavelength (480 nm–540 nm). If the dissociation wavelength is close to two-photon transitions of the Tl atom from 6p ² P _3/2 to np² P _1/2,3/2 or mf² F _5/2,7/2,n=10 ... 15,m=7 ... 12 dips in the yield are observed. These dips show a significant asymmetric broadening, compared to a free atomic transition and the observation is interpreted as an absorption process of the transient state of dissociating TlI during the laser pulse. By applying the Landau-Zener approximation for the potential crossing of dressed molecular states, we are able to describe the broadening and the power density dependence of the observations. Simulations show that the asymmetry is determined by the difference potential of the electronic states which are coupled by the laser field.     

Avoided level crossings in bound Rydberg states of barium in an electric field

In a CW laser experiment on bound Rydberg states of barium electric-field-induced level crossings were studied. A regular type of avoided crossing of 6snf ³ F ₂ and 6s (n+4)s ³ S ₁ levels (n≈60,M=0) was observed under circumstances where the minimum separation was larger than the linewidth involved. This type of crossing could be reproduced using a diagonalisation procedure of the total energy matrix in the presence of the field. However, in the case of avoided crossings of the 6snf ³ F ₂ and 6s(n+4)p ¹ P ₁ sublevels, where the minimum separation was smaller than the linewidth of the³ F ₂ level, an interference-like profile was recorded. An important feature in the explanation of this interference-like profile turned out to be the broadening of the³ F ₂ level by small field inhomogeneities.     

The rotational spectrum of the hydrogen-bonded heterodimer H2O…HF in the frequency range 40–80 GHz

The μ_a R-branch rotational spectra of the ground and first excited states of the three lowest vibrational modes of the H₂O…HF heterodimer have been observed in the frequency range 40–80 GHz. Coriolis perturbations between the ground vibrational state (υ = 0) and the first excited state of the out-of-plane bending vibration (υ_β(0) = 1) show that for a given J the K₋₁ = 2 levels of υ_β(0) = 1 lie approximately 3 cm⁻¹ above the K₋₁ = 3 levels of υ = 0. The vibrational separation between these two states is estimated to be 70±3 cm⁻¹. This value is consistent with those determined by other methods and reinforces the conclusion that ν_β(0) is governed by a double-minimum potential energy function with the quantitative form previously published. A perturbation is also observed in the first excited state of the hydrogen-bond stretching vibration υ_σ = 1. This manifests itself as a large, negative centrifugal distortion constant D_JK = −8.5 MHz compared with 2 MHz in the other vibrational states.     

Hyperfine structure quantum beat spectroscopy of Tl statesn 2 P 3/2 (n=10−13) with pulsed two-photon excitation

By quantum beat spectroscopy with pulsed two-photon excitation the hyperfine constants of the Thallium states 6s ² np ² P _3/2 (n=10?13) are determined: 2A(10P _3/2)=77.73 (74) MHz, 2A(11P _3/2)=48.82 (49) MHz, 2A(12P _3/2)=32.34 (45) MHz and 2A(13P _3/2)=23.08 (35) MHz.     

Collisional and unimolecular ionisation of monatomic halogen cations

Charge-stripping processes of monatomic halogen cations F⁺, Cl⁺, Br⁺ and I⁺ having keV translational energies were investigated, and found to closely resemble those of the isoelectronic dications of the noble gases. In the absence of collision gas, an ionisation process is still observed which cannot be attributed to charge-stripping collisions of very high cross-section with background gas molecules, nor to surface interactions, nor to field ionisation. The observed time-dependence of the yield from this collision-free process is, however, also difficult to reconcile with an interpretation in terms of autoionisation of some highly excited metastable state. In addition, the appearance energy of the states involved was found to lie within 1– eV of the ionisation onset. A mechanism was proposed which can account qualitatively for these observations. The crucial step in this proposal involves excitation of Rydberg states by ambient (300 K) black-body radiation, a phenomenon well established for neutral atoms.     

Time development of resonant two-photon autoionisation of H2

Total two-photon autoionisation of H₂ through the lowest autoionising state of ¹ Σg (1σ²_u symmetry has been studied as a function of time for three resonant cases [corresponding to (ν = 0,j = 1), (ν = 1,j = 1) and (ν = 2,j = 1) levels of the B″¹ Σ_u intermediate state]. A structure due to ac Stark splitting has been obtained in the ionisation profile. Effects of intensity and detuning on the ionisation profile have also been studied.     

Linewidth dependence of quasi-stationary states in large molecules

We study the quasi-stationary states, or eigenstates, of the decay matrix for a system described by one single vibronic state |s〉 coupled to a manifold {l}, both |s〉 and {l} levels being coupled to separate continua which endow them with finite linewidths γ_s, γ_l. When all the zero-order states have the same linewidth (not necessarily zero) the quasi-stationary states are identical to the molecular eigenstates; when the difference between the linewidths γ_s ? γ_l is large enough, the zero-order sets (s, {l}) are completely decoupled, a result which has been established for the two-level system. We indicate some consequences of collisional broadening involving pressure effects in the gas phase.     

Natural radiative lifetimes in the2 S 1/2 and2 D 3/2, 5/2 sequences of neutral copper

Natural radiative lifetimes have been measured of the 3d ¹⁰ ns ² S _1/2 (n=6?10) and of the 3d ¹⁰ nd ² D _{3/2, 5/2} (n=5?9) sequences by using two step excitation of copper atoms in an atomic beam. The states investigated were populated by the light from two pulsed dye lasers pumped by the same Nd:YAG laser. The lifetimes of the² D sequence increase regularly with increasing principal quantum number. This is not the case for the² S series, because of configuration interaction in the² P series. In addition the lifetime of the 3d ¹⁰5p ² P _3/2 state has been measured together with its branching ratio.     

High resolution study of photoabsorption photodissociation and fluorescence in H2

Extensive excitation fluorescence spectra for both the molecule and the atomic fragment are presented with the absorption spectrum in the 765–850 Å spectral range with a resolution varying from 0.07 to 0.05Å. A review is made of the previously published results on photodissociation cross sections, photodissociation rates, predissociation probabilities. New results on the predissociation of then p σ Σ⁺ _u levels are presented. The competitions between the various decay processes: radiation, predissociation and autoionisation, are discussed.     

Resonant and Off-resonance ionization of laser-excited sodium vapor

Electrons produced in a sodium effusive beam irradiated by pulsed laser light have been energy-analyzed. For resonant excitation of the atoms to the 3p state, we have studied associative ionization of excited Na(3p) (AI), Penning ionization (PI) and photoionization of highly-excited Na(n l),n l=3d, 4p, 5s, 4d/4f, by measurements of electron energy spectra. Our interpretation of the spectra is supported by a numerical analysis of the data which leads to estimates of the cross sections compatible with values previously measured in other experiments. Off-resonance ionization is considered at a particular wavelength. It is presumed to involve laser-excited dimers Na ₂ ^** . Spectra show that highly-excited atoms are produced by photodissociation of Na ₂ ^** or from Na ₂ ^** -Na collisional energy transfer, and that these atoms support the dominant off-resonance ionization channel in our laser excitation conditions.     

Determination of radiative lifetimes in neutral nitrogen using short laser pulses from a distributed feedback dye laser

Radiative lifetimes were determined for two short-lived states in neutral nitrogen. Following photo-dissociation and two-photon excitation employing the same UV source, excitation to higher states was performed with a distributed feedback dye laser (DFDL). The lifetimes were found to be τ(2p ²4d ⁴ D _7/2)=17(3) ns and τ(2p ²5s ⁴ P _5/2)=22(3) ns.     

Accurate time-resolved laser spectroscopy on lithium atoms

The lifetime of the lithium 2p ² P states has been measured with high accuracy using the delayed coincidence technique with a continuous mode-locked dye laser as the source of the excitation light. The value 27.22 (0.20) ns was obtained. In addition, the hyperfine structure of the⁷Li 2p ² P _3/2 state, which can normally scarcely be resolved, has been studied by recording the slow quantum beats at zero field and the very fast beats in the Paschen-Back regime. New accurate values for the²³Na 3p ² P _3/2 state hyperfine structure constants are also presented.     

Short-time quantum dynamics of the driven rigid rotor

Stark splitting of the barium 6snp ¹ P ₁ states with principal quantum numbers ranging from 8 to 15 and of the 5d7p ¹ P ₁ perturber level was measured in an atomic beam via pulsed laser excitation with time- and wave length-resolved fluorescence detection at low electric field strengths. Tensor polarizabilities were derived from the quantum beat signals observed in pure homogeneous electric fields. Strong variations of the tensor polarizabilities of the levels in the 6snp ¹ P ₁ series were found.     

Proton-impact excitation of HeI triplet levels

We investigated the excitation of the λ(1s3d ³ D?1s2p ³ P)=588 nm line of atomic helium by proton and deuteron impact for projectile energies 10 keV≦E _p≦25 keV. In apparent contradiction to Wigner's spin conservation rule, the emission cross section does not vanish. By measuring the intensity of the impact radiation as a function of homogeneous magnetic and electric fields applied to the collision volume, it has been shown thatp- andd-impact excitation of the 1s3d ³ D level of HeI proceeds via 1snl states withl≧3, which populate the 3³ D states by cascade decays. The well-known strong singlet-triplet mixing of these 1snl states enables a population of triplet states in accord with Wigner's rule. Accordingly, we determine the excitation cross section of the 1s4f multiplet from the measured emission cross section of the 588 nm line. The field-dependent signals give evidence that predominantly substates with |m _L|≦1 are excited.     

Microwave ionization and excitation of Ba Rydberg atoms

We have investigated ionization and excitation of the Ba 6s n s ¹ S ₀ and 6s n d ^1,3 D ₂ series in strong microwave fields. The observed microwave ionization threshold fields, scaling as 0.28n ^?5, and the state mixing fields cannot be completely explained in terms of a single cycle Landau-Zener model. However, by taking into account multiphoton resonant transitions driven by many cycles of the microwave field we have been able to interpet the data. In particular, multi-photon transitions have been found to be responsible for apparent resonance structures and for the unexpectedly low mixing fields. Not surprisingly, doubly excited valence states introduce irregularities into both the microwave ionization and the state mixing field values.