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.     

An optically pumped,highly polarized cesium beam for the study of spin-dependent electron scattering

We have set up an atomic beam of cesium for the study of spin-dependent electron-cesium scattering. The beam is produced by an effusive oven with continuous recirculation of the condensed metal. The beam is optically pumped by circularly polarized light from two laser diodes tuned to the 6² S _1/2(F=3)→6² P _3/2(F′=4) and 6² S _1/2(F=4)→6² P _3/2(F′=5) transitions, respectively. Nearly all atoms are transferred into theF=4,m _F =+4 orm _F =?4 Zeeman level of the ground state, depending on the sense of circular polarization of the pumping light. The population distribution in the optically pumped beam is analyzed in terms of them _J =?1/2 andm _J =+1/2 components with a Stern-Gerlach magnet. We find the atomic polarization to be very close to unity at a density of 8×10⁸ atoms/cm³ in the scattering center. The polarization decreases slightly with increasing density of the cesium beam due to radiation trapping. A spin flipper serves as a means of polarization reversal, introducing no systematic errors in the spin asymmetry measurements. Lock-in technique is used to stabilize the atomic beam polarization by detecting fluorescence light signals.     

The hyperfine muon-nucleus interaction andP-odd effects in the one-photon 2s → 1s transition of light muonic atoms

The weak interaction of neutral currents of the negative muon and a nucleus results inP-odd correlations in the one-photon 2s → 1s transition of light muonic atoms. If the standard electroweak model is used, the weak mixing of hyperfine components of the μ-atom 2s and 2p 1/2 orbits makes the main contribution to these correlations. A new mechanism of acting the hyperfine structure on theP-odd correlations is considered in the present work. The fact is that the hyperfine muon-nucleus interaction mixes the 2p 1/2 and 2p 3/2 orbits effectively in light muonic atoms whose nuclei have large quadrupole moments. As a result, two new sublevels arise for each of two valuesF _±=(I ± 1/2) of the total angular μ-atom momentum (I is the nuclear spin). The contributions of these sublevels to theP-odd correlations can be comparable with each other. The most strikes manifestation of the given effect occurs for berillium and boron, where the 2s and 2p 3/2 orbits may have close energies. Taking the above effect into account can lead to considerable modification of results against earlier calculations. In particular, theP-odd correlations can vanish and change sign. They can be additionally increased by a factor of several tens.     

An improved laser vaporization cluster source and time-of-flight mass spectrometer

The radiative lifetimes of the 7p ² P _{3/2, 1/2} states of silver and the hyperfine structure of the 7p ² P _3/2 state have been measured using pulsed laser excitation and direct observation of the induced fluorescence light decay. In order to excite this doublet, VUV radiation at 185 nm was applied, as generated by frequency tripling and anti-Stokes Raman shifting of the output of a Nd: YAG laser pumped dye laser. The lifetimes were found to be τ=255(20) ns and τ=285(25) ns for theJ=3/2 state andJ=1/2 state, respectively. The hyperfine structure of the 7p ² P _3/2 state was measured by the quantum beat method. The magnetic dipole interaction constant for the¹⁰⁷Ag isotope was found to bea=?4.5(2) MHZ.     

The collision cross sections for the fine-structure mixing of caesium 6P levels induced by collisions with potassium atoms

The cross section for the fine-structure excitation transfer Cs(6P _1/2) → Cs(6P _3/2), induced by collisions with the ground state potassium atoms, has been measured by resonant Doppler-free two-photon spectroscopy. The population densities of caesium 6P _J (J=1/2, 3/2) levels were probed by thermionic detection of the collisionally ionized caesium atoms from the Cs(6P _J ) → Cs(10S _1/2) excitation channel. The cross section for the transfer process at the temperatureT=503 K has been found to be σ(1/2 → 3/2)=45 Ų ± 20%. The result is compared with previously published experimental cross sections for fine-structure transfer in resonance states of other alkali elements perturbed by potassium and a thoeretical value of the Li(2P _J )-K system calculated in a simple approach.     

Population and deactivation of lowest lying barium levels by collisions with He,Ar, Xe and Ba ground state atoms

Excitation transfer between the barium low lying excited states 6s6p ³ P ₁ ⁰ , 6s5d ¹ D ₂ and 6s5d ³ D _J by collisions with He,Ar,Xe and Ba has been investigated. The population densities in all levels involved were probed by absorption or by fluorescence usingcw lasers. The depopulation cross sections of the Ba³ P ₁ ⁰ state by collisions with noble gases were found to be σ_He(³ P ₁ ⁰ )=5.5·10^?16 cm², σ_Ar(³ P ₁ ⁰ )=4.6·10^?16 cm², and σ_Xe(³ P ₁ ⁰ )=1.7·10^?16 cm². For Ar, the collisional depopulation of the³ P ₁ ⁰ level is exclusively due to the transition to the¹ D ₂ state. Under the assumption that the³ D _J metastable states are populated collisionally by¹ D ₂ →³ D _J transfer only, we have deduced the upper limit for the corresponding cross section σ ₁₃ ^Ar =1.5·10^?18 cm². From the Ba¹ D ₂ and Ba³ D _J steady-state diffusion distributions, collisional relaxation rates of the¹ D ₂ and³ D _J levels were evaluated. The collisional relaxation rates by Ar and Ba yielded total cross sections for the depopulation of metastable levels: σ_Ar(¹ D ₂)=1.5·10^?17 cm², σ_Ba(¹ D ₂)?1·10^?13 cm², σ_Ar(³ D _J)=7·10^?21 cm², and σ_Ba(³ D _J)=1·10^?15 cm². Furthermore, it was found that the main contribution of the collisional depopulation of the¹ D ₂ state by Ar is related to back transfer to the³ P _J ⁰ state, whereas the deactivation of the³ D _J metastable state is due to back transfer to the¹ D ₂ state. Taking into account other cross sections reported in literature we can conclude that collisional deactivation of both metastable levels by Ba ground state atoms can be attributed to their mutual collisional mixing.     

Nichtadiabatische Übergänge bei Stößen zwischen Atomen und Molekülen. Desaktivierung von Br(42P1/2)- und J(52P1/2)-Atomen durch zweiatomige Moleküle (N2, CO)

The desactivation of excited ² P _1/2-states of halogene atoms (Br, J) by collisions with diatomic molecules has been studied. The non-adiabatic transition to the ground state ² P _3/2 takes place in the “quasi crossing point” of the vibrational terms and is connected with an excitation of the molecular vibration.     

Population transfer between the fine-structure levels 5s5p 3 P 1 and 5s5p 3 P 0 of114Cd atoms by collisions with molecular gases

Fine-structure mixing of excited¹¹⁴Cd 5³ P _J atoms induced by collisions with various molecular gases (N₂, H₂, D₂, CO, CH₄, C₂H₆) has been investigated by a combined method of absorption and fluorescence spectroscopy. After pulsed optical excitation of the Cd(5³ P ₁) level, the time dependence of the population densities has been measured simultaneously both for the 5³ P ₁ state and for the collisionally populated 5³ P ₀ state. By analyzing the signal curves at different molecular gas pressures cross sections for collisional transfer between the¹¹⁴Cd 5³ P ₁ and 5³ P ₀ levels as well as the quenching cross sections for the 5³ P levels have been obtained.     

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.     

Analysis of splitting patterns from Stern-Gerlach magnetic deflection of supersonic molecular beams: application toM J -state-resolved deflection ofJ=2 atoms

Measurements ofM _J -state resolved Stern-Gerlach deflection patterns for the³ P ₂ states of noble gas metastable atoms in supersonic beams are analyzed using a modification of the method originally worked out by Otto Stern. Velocity distribution breadth and beam collimation required to resolve theM _J states are explored, and the modeling is improved by including variation in the field gradient along the deflected atomic trajectories.     

Investigations of radiative lifetimes in the 3p5p configuration of neutral silicon

Lifetimes for the³ S ₁,³ P _{0, 1, 2} and³ D _{1, 2, 3} states in the 3s ²3p5p configuration of silicon have been determined using stepwise dye laser excitation and time resolved detection. A comparison is made with theoretical values, calculated using multi-configuration Hartree-Fock wavefunctions. Laser-evaporation was used to produce free silicon atoms by focusing a Nd: YAG laser on a rotating silicon target.     

Electron-impact excitation of the transition 1s 22s 22p 5(2 P 3 2/0 −2 P 1 2/0 ) of Mg IV

Effective collision strengths for electron-impact excitation of the ground state fine structure transition² P ₃ ^2/0 ?² P ₁ ^2/0 have been calculated by using theR-matrix method. Twelve lowest target states, represented by configuration interaction wavefunctions are included in the scattering calculations.M1 andE2 transition probabilities are also calculated by employing the Breit-Pauli Hamiltonian.     

Neutron diffraction structure determination of the high-temperature form of lithium tritantalate,HLiTa3O8

The crystal structure of HLiTa₃O₈ has been reexamined by electron and neutron diffraction techniques. Neutron Weissenberg and electron diffraction photographs show that the space group of the compound isPmmn and notPmma as determined previously by X-ray diffraction techniques. There are eight molecules in the unit cell of lattice parametersa = 16.718(2)A?,b = 7.696(1)A?,c = 8.931(1)A?. These values show that thebaxis of the new cell is doubled with respect to the parameter measured by X-rays. The structural refinement was based on 1074 independent reflections measured on a single crystal with a four-circle neutron diffractometer. The positions of all atoms, including the lithium atoms have been determined. The finalR andwRfactors were 0.036 and 0.035, respectively. The eight lithium cations occupy two sets of4fpositions(x,¼, z)of thePmmn space group. The ordering of four lithium ions over two sets of possible positions (4j) of space groupPmma is responsible for the doubling of theb axis. The other four Li⁺ occupy two sets of positions (2d) of space groupPmma. All lithium ions are surrounded by 12 oxygen atoms arranged as cuboctahedra. The large thermal vibrations found for the lithium atoms and the ionic conductivity of HLiTa₃O₈ at high temperatures are consistent with weak LiO bonding.     

Displacement of metal atoms from salts by hydrogen and the role of this reaction in chain processes

The interaction of hydrogen atoms with a variety of alkali metal and alkaline-earth metal salts results not only in the recombination of these atoms but also in the displacement, into the gas phase, of free radicals (CaCl^·(A ¹ P _1/2, B ² S ⁺) and CaF^·(A ² P)) and metal atoms, including their excited species, which are detected spectroscopically. Transmission spectra indicate that the NaCl surface undergoes metallization when treated with a high-frequency discharge and a rarefied hydrogen flame. Combustion is affected by the gas-phase hydrogen atoms involved in the chain reaction and by the varying composition and properties of the surface. The concentration of Na atoms over the NaCl surface at 770 K is 10⁹?10¹¹ cm^?3 in a stream of H atoms at 1 Torr and in the 2H₂ + O₂ flame at 4 Torr. The concentration of sodium atoms in the ² P _3/2 and ² P _1/2 excited states is ～5 × 10⁶?5 × 10⁸ cm^?3. The role of the discovered reactions in combustion, pyrolysis, and plasma chemistry is discussed.     

Accurate comparison of Hel,NeI photoionization and He(23, 1 S), Ne(3s 3 P s,3 P 0) penning ionization of argon atoms and dimers

Using crossed beams and mass spectrometric ion detection, we have investigated the ionization of argon atoms and dimers in a skimmed supersonic beam by HeI (58.4 nm) and NeI (73.6, 74.4 nm) photons and by He(2^3,1 S) and state selected Ne(3s ³ P ₂,³ P ₀) metastable atoms. The cross section ratioq ₂₂/q ₁ (i.e. the cross sectionq ₂₂ for formation of Ar ₂ ⁺ ions from Ar₂ divided by the total ionization cross sectionq ₁ for Ar atoms), arbitrarily normalized to 1 for HeI impact, is found to vary weakly as follows: HeI/NeI/He(2^{3, 1} S)/Ne(³ P ₀)Ne(³ P ₂)=1/1.136(9)/0.893(4)/1.034(12)/0.985(9). The results are qualitatively interpreted using available information on the intermolecular potentials and the two different ionization processes. The observation thatq ₂₂/q ₁ is 5% larger for Ne(³ P ₀) than for Ne(³ P ₂) is attributed to anomalies in the respective branching ratios for formation of the Ar+(² P _3/2)/Ar⁺(² P _1/2) ion states in conjunction with differences in the stability of the formed Ar-Ar⁺(² P _3/2) and Ar-Ar⁺(² P _1/2) molecular ions.     

Laserspectroscopic investigations of the lithium-D-lines in magnetic fields

Laser-atomic-beam investigations of the lithiumD ₁- andD ₂-line in magnetic fields were performed using cw-laser excitation and fluorescence detection. For both isotopes⁶Li and⁷Li, the hyperfine splittings of the ground level 2² S _1/2 and the upper level of theD ₁-line, 2² P _1/2, as well as the isotopic shifts of theD ₁- and theD ₂-line were determined from the registrations without field. In magnetic fields, Zeeman- and Paschen-Back-effects of the lines were studied. Using the Zeeman pattern of theD ₁-line for a calibration of the field strength, values for the hyperfine constantsA andB of the 2² P _3/2-level of⁷Li could be derived from the Zeeman pattern of theD ₂-line.     

Polarization and velocity dependence of associative ionization in Na(3p)+Na(3p) collisions

We have excited Na atoms of two counterrunning thermal beams by means of linearly polarized laser light and have investigated associative ionization processes. To this end we measured the total ionization signal as a function of the angle ? between light polarization and the relative collision velocity. Velocities of the excited atoms were selected by exploiting the Doppler effect. We found an increasing polarization dependence at increasing collision velocities. At all velocities the preparation of the collision partners in the |M _j |=1/2 sublevel of the Na² P _3/2 state is most efficient in producing ionization.     

Velocity dependence of impact line-broadening studied by resonant doppler-free two-photon laser spectroscopy

The velocity dependence of the collison broadening of the⁴⁰Ca 4s ^{2 1} S ₀→4s5p ¹ P ₁ →4s17d ¹ D ₂ transition by neon atoms has been studied. The experiment was performed exciting one velocity group of Ca atoms within the Doppler profile of the first transition and probing this group with a counterpropagating laser beam inducing the second transition. Using thermionic detection we have measured broadening rates for relative velocities between Ca and Ne atoms which corresponds to a temperature range of about 850 K (cell temperature) to 3000 K. The measured line-width was found to have very weak temperature dependence. This is attributed to the weak velocity dependence of polarization broadening of the final level which was found to be the major contribution to the broadening of the two-photon line. For large relative velocities significant asymmetries in the measured line profiles due to velocity changing collisions had to be taken into account.     

Collisional depolarization of excited114Cd 5s5p 3 P 1 atoms by collisions with molecular gases

Depolarization of excited¹¹⁴Cd 5s5p ³ P ₁ atoms induced by collisions with various molecular gases (N₂, H₂, D₂, CO, CO₂, CH₄, C₂H₆, C₂H₄) has been investigated using polarized fluorescence spectroscopy. After pulsed optical excitation of the Cd 5³ P ₁ level with appropriately polarized light the temporal behaviour of Zeeman quantum beats has been observed showing the influence of collisional destruction of orientation and alignment. By analyzing the signal curves at different molecular gas pressures the corresponding depolarization cross sections for¹¹⁴Cd atoms in the 5³ P ₁ state have been obtained. With regard to a test of a nuclear spin decoupling model for the collisions the cross sections were compared with previously measured hyperfine structure transfer cross sections of¹¹³Cd 5s5p ³ P ₁ atoms.     

Kinetic study of electronically excited silicon atoms,Si(3p2(1S0)), by time-resolved attenuation of atomic resonance radiation

The collisional behaviour of electronically excited silicon atoms in the 3p²(¹S₀) state, 1.909 eV above the 3p²(³P₀) ground state, is investigated by time-resolved attenuation of atomic resonance radiation at λ = 390.53 nm (4s(¹P^o₁)←3p² (¹S₀)). The optically metastable Si(3¹S₀) atoms were generated by the repetitive pulsed irradiation of SiCl₄ and their decay monitored in the presence of added gases. Absolute quenching rate constants (k_Q, cm³ molecule^?1 s^?1, 300 K) are reported for the following collision partners: He (?1.3 × 10^?15), SiCl₄ ((9.1 ± 1.4) × 10^?11), O₂ ((1.5 ± 0.2) × 10^?11) and N₂O ((4.3 ± 0.4) × 10^?11). The results for O₂ and N₂O are compared with analogous data reported hitherto for Si(3p²(³P_J)) and with those for the other np²(¹S₀) states of the group IV atoms C, Ge, Sn and Pb. The rate data for the silicon atoms are considered in terms of the nature of the potential surfaces arising from symmetry arguments based on the weak spin orbit coupling approximation.