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.     

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.     

Observation of collision-induced amplified emission in Na-noble gas system

A stimulated resonance emission and stimulated Raman scattering signals have been generated in an atomic sodium and rare gas mixture by a laser tuned to the 3² S_1/2-3² P_3/2 transition. The intensity dependence of these effects on the buffer gas pressures is obtained in the experiment. We demonstrate good qualitative agreement between experimental observations and a simple theoretical model. A new method using stimulated emission is applied to measure the cross section for the J mixing of 3P states of sodium due to collisions with noble gas atoms. Good agreement between our results and the measurements of Pitre and Krause is obtained for Ar and Kr perturbers     

Monte Carlo Simulation of Radiation Trapping in Hg–Ar Fluorescent Discharge Lamps

The line spectra of emitted resonance radiation from mercury and the effective decay rates of the Hg 6³P₁ and 6¹P₁ levels in mercury–argon discharges are simulated by a Monte Carlo method. The hyperfine splitting, the natural isotopic composition, collisional transfer of excitation, foreign gas collisions and quenching are considered to describe in detail the 253.7 nm and 184.9 nm lines. The calculations are performed for Hg vapor densities corresponding to coldest spot temperatures of 5–100°C, and discharge parameters typical for fluorescent lamp operation. The densities of the Hg 6³P₁ and 6¹P₁ levels are consistently estimated by means of a set of balance equations for the Hg 6³P₀, 6³P₁, 6³P₂, and 6¹P₁ excited states. The resulting uv radiation output of the discharge is then estimated for a tube radius of 18 mm, argon pressure of 400 Pa, discharge current 0.4 A, and wall temperatures of 20–80°C. The results obtained show a good agreement as compared with published experimental data.     

Absolute detection of metastable rare gas atoms by a cw laser photoionization method

A novel, accurate method for the absolute detection of metastable rare gas atoms is described and demonstrated. It involves a direct in situ determination of the electron emission coefficient γ for impact of the respective metastable atom on a conducting surface. γ is reliably obtained by a cw two-photon ionization — depletion technique: the reduction ΔI _S in electron current from the detector surface due to efficient photoionization removal of the metastable flux is compared with the photoelectron current ΔI _P (γ = ΔI _S/ΔI _P). The principle of the method, possible realization schemes for the different metastable rare gas atoms and the apparatus are described in detail. The method has been applied so far to metastable Ne* (3s ³ P ₂), Ar* (4s ³ P ₂), and Kr* (5s ³ P ₂) atoms, and corresponding results for γ, obtained with five different chemically clean, polycrystalline surface materials and at two surface temperatures (300 K, 360 K) are reported. Whereas for Ne*, the value of γ (≈0.35) showed only a rather weak dependence on the surface material and temperature (as also found for a mixed He* (2³ S, 2¹ S) beam), strong variations in γ, especially at 300 K, were detected for Ar* and Kr* (values between 0.25 and 0.003). Some applications of the described method, especially with regard to the determination of absolute reaction cross sections involving metastable rare gas atoms, are discussed.     

Vibrational relaxation and quenching of CO(A1Π, ν = 0–8) in collisions with rare gas atoms

Steady state fluorescence experiments yielded effective cross sections for the vibrational relaxation and quenching in collisions of electronically excited CO molecules, in single vibrational levels ν = 0 to 8 of the A¹Π state, with rare gas atoms. Vibrational relaxation was found to proceed not only by Δν = 1 but also by Δν = 2 and Δν = 3 processes. The relaxation cross sections for the Δν = 1 processes decrease with increasing vibrational quantum number ν. The quenching data give strong evidence for collision induced intersystem crossing to nearby triplet states.     

Dynamics of the collision induced rotational excitation of diatomic molecules in a Π(-state: Classical trajectory studies for the Li2(B1 Πu) - Noble gas system

Rotationally inelastic collisions of noble gas atoms and electronically excited Li₂(B¹ Π_u) have been studied theoretically in the classic approximation. We propose a simple potential model for the interaction between the colliding particles. Cross sections are calculated by a Monte-Carlo average over the classical trajectories. The comparison with results from laser induced fluorescence experiments is satisfactory. The dynamics of the angular momentum transfer, in particular, an orientation effect of the π-electron distribution is investigated.     

Spin—orbit relaxation of Pb(3P01) by atomic hydrogen

Lead atoms in 7 ³P⁰₁ state were generated by 405.8 nm laser irradiation of metastable Pb(³P₂) in a series of premixed H₂/N₂/O₂ flames. Fluorescence was observed from both the ³P⁰₁ and ³P⁰₀ states. The ratio of the intensities of fluorescence from the two states varied with flame parameters in a manner which indicated that spin—orbit relaxation of Pb(³P⁰₁) occurred rapidly in collisions with atomic hydrogen. The rate constant for this process is estimated as 3 × 10^?10 cm³ molecule^?1 s^?1 at 1800 K.     

The effect of hyperfine structure on the collisional relaxation of the thallium atom excited by laser beam

We present a theoretical analysis of the transient population inversions which appear in the optically pumped thallium (6² P _1/2, 6² P _3/2, 7² S _1/2) atom. We account for the hyperfine structure and we assume that the pumping occurs in the presence of buffer gas. Numerically computed time dependencies of population differences between the Zeeman sublevels of hyperfine levels of the 7² S _1/2 and 6² P _{1/2, 3/2} states are given. It is demonstrated how the described effects depend on the strength of the atom-field coupling and buffer gas pressure. The role of resonant and nonresonant pumping is explained.     

The electron paramagnetic resonance spectrum of NF(Δ) in the gas phase

EPR transitions of NF in its ³Σ⁻ ground state and for the J = 2 level of its ¹Δ first excited state have been detected in the reaction between hydrogen atoms and NF₂ in the gas phase. Second order effects have been detected in the nuclear hyperfine interaction; comparison of the hyperfine splittings is of some interest.     

Radiative lifetimes of the lowest 3P1 metastable states of Ca and Sr

The lifetimes of the lowest ³P₁ states of Ca and Sr have been measured in atomic beam experiments. These measurements have been made by observing the exponential decay of the fluorescence emitted from excited states populated either by a discharge or a dye-laser pumping. In both cases, the velocity distributions of the radiating atoms were also measured by a time-of-flight technique. Our results show that the lifetime measurements using the discharge excitation are hampered by a systematic error introduced probably by cascade transitions that repopulate the upper energy levels of the transitions of interest. The radiative lifetimes of the 4s4p ³P₁ state of Ca and 5s5p ³P₁ state of Sr are determined to be 0.34 ± 0.02 ms and 21.3 ± 0.5 μs, respectively.     

Collision cross sections for excitation energy transfer in Na* (3P 1/2)+K(4S 1/2)⇔Na*(3P 3/2)+K(4S 1/2) processes

The cross sections for the excitation energy transfer between the 3² P _J states of sodium atoms by collisions with ground-state potassium atoms have been measured by resonant Doppler-free two-photon spectroscopy, where the population densities of directly pumped and collisionally excited Na(3P _J )(J=1/2, 3/2) levels were probed by counter-propagating Na(3P _J ) → Na(4D _{3/2, 5/2}) excitation and detected with the thermionic diode. Cross sections of σ(3P _1/2 → 3P _3/2)=190 Ų±20% and σ(3P _3/2 → 3P _1/2)=100 Ų±20% were found. The theoretical calculations taking into account the long-range interaction termsR ^?6,R ^?8 andR ^?10 yield a value σ(3P _1/2 → 3P _3/2)=165 Ų. On the basis of these long-range interaction potentials the differential cross section has been calculated and compared with recently published experimental data. Very good agreement between the theoretical and experimental data was found.     

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.     

Symmetry constraints in energy transfer between state-selected Ar*(3P2, 3P0) metastable atoms and ground state H atoms

The excitation-transfer reaction in thermal energy collisions of state-selected metastable Ar*(³P₂) and Ar*(³P₀) atoms with ground state H atoms, giving excited H*(n = 2) atoms, has been studied with the stationary afterglow technique. The rate constant for the excitation of H atoms by Ar*(³P₂) has been found to be more than one order of magnitude larger than in excitation by Ar*(³P₀). This difference in the reactivity of two metastable species is explained to be a consequence of the attractive nature of the D(²II) and E(²Σ⁺) potentials that develop from the Ar*(³P₂)+H entrance channel and which give curve crossing with the B(²II) and C(²Σ⁺ potentials, respectively, leading to the Ar+H*(n=2) exit channel, whereas only a repulsive ⁴II (Ω=$\text{1}\text{2}$) potential develops from the Ar*(³P₀+H entrance channel.     

Diffusion of resonance radiation in atomic vapor imaging

The effect of resonance radiation diffusion due to radiation trapping has been studied in an atomic vapor imaging filter. Using a cesium resonance fluorescence imaging monochromator, the spatial distortions due to resonance radiation trapping by Cs atoms in the pumping/imaging region have been investigated. It was found that the spatial distortions were dependent on the number density of the Cs atoms, as well as the irradiance of the signal photons at 852.12 nm (6²S_1/2→6²P^o_3/2). The pump laser (917.23 nm, 6²P^o_3/2→6²D_5/2) did not influence the radiation diffusion to the same degree as the signal beam. It is shown that there is a compromise between maximum optical density and spatial resolution. The number density of the Cs resonance fluorescence imaging monochromator was optimized to obtain the highest spatial resolution and signal-to-background ratio.     

Fluorescence of Xe2 molecules in the vacuum-ultraviolet

Fluorescence of the Xe₂ continuum between 1460 and 1800 Å has been observed during excitation of Xe₂ molecules with CO fourth positive bands from a microwave discharge lamp. Fluorescence spectra at different Xe and added gas pressures show that in collisions with Xe or Kr atoms, the excited Xe₂ diatoms are efficiently relaxed to low vibrational levels of the ¹Σ_u⁺ and ³Σ_u⁺ potentials without being quenched noticeably. With Kr the rate constant for relaxation out of the primarily excited high vibrational levels is found to be about 3×10^?11 cm³ molecule^?1 s^?1.     

Near-resonant collisional energy transfer in Sr(5s6p 1p1-rare-gas systems

The total quenching cross sections for strontium (5s6p ¹p₁) collisions with the rare gases are determined by pulsed laser, time-resolved fluorescence measurements to be 80, 40, 73, 101, 140 Ų for He, Ne, Ar, Kr and Xe, respectively. For collisions with Kr, all of the near-resonant states, 4d5p ³F, 5s6p ³P and 4d5p ¹D₂, are observed to be excited. The cross sections are interpreted in terms of a curve crossing mechanism. The product branching can be explained in terms of strong coupling of the initial state with the near-resonant states, weighted by energy gap considerations.     

Isotope shifts and hyperfine structures investigation of doubly excited levels in SrI

We measured isotope shifts and hyperfine structure of visible transitions of stable strontium isotopes by means of Doppler-free saturated absorption spectroscopy. In particular, we investigated transitions between excited states where the upper level involves two excited electrons. We report hyperfine coupling constants for the levels 5p4d¹D₂, 5p4d³F₂, 5s4d³D₁, 5s6s³S₁, 5s5p³P₁ and, for some of the studied transitions, we separate the specific mass and volume contribution to the isotope shifts.     

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.     

The state dependence of the interaction of metastable rare gas atoms Rg*(ms3 P 2,3 P 0) (Rg=Ne,Ar, Kr,Xe) with ground state sodium atoms

Using crossed beams of metastable rare gas atoms Rg*(ms³ P ₂,³ P ₀) (Rg=Ne, Ar, Kr, Xe) and ground state sodium atoms Na(3s ² S _1/2), we have measured the energy spectra of electrons released in the respective Penning ionization processes at thermal collision energies. For Rg*(³ P ₂)+Na(3s), the spectra are quite similar for the different rare gases, both in width and shape; they reflect attractive interactions in the entrance channel with well depthsD* _e [meV] decreasing slowly from Rg=Ne to Xe as follows: 676(18); 602(23); 565(26); 555(30). For Rg*(³ P ₀)+Na(3s), the spectra vary strongly with the rare gas, indicating a change in the character of the interaction from van der Waals type attraction (Ne) to chemical binding for Kr and Xe with well depthsD* _e [meV] of: 51(19); 107(25); 432(30); 530(50). These findings are explained through model calculations of the respective potential curves, in which the exchange and the spin orbit interaction in the excited rare gas and the molecular interaction between the two valences-electrons in terms of suitably chosen singlet and triplet potentials are taken into account. These calculations also explain qualitatively the experimental finding that the ratiosq ₂/q ₀ of the ionization cross sections for Rg*(³ P ₂)+Na and Rg*(³ P ₀)+Na vary strongly with the rare gas from Ne to Xe as follows: 15.8(3.2); 2.6(4); 1.4(2); 1.6(4).