Fine structure excitation transfer between the lithiumD-lines by collisions with cesium atoms

Applying resonant Doppler-free 2-photon laser spectroscopy with thermionic diode detection, the cross sections for the excitation energy transfer of the collisional process⁷Li*(2P _1/2+Cs(6S _1/2)→⁷Li*(2P _3/2)+Cs(6S _1/2) have been measured. The experimental cross sections, σ^Li-Cs (1/2→3/2)=890 Ų and σ^Li-Cs (3/2→1/2)=430 Ų, are compared with theoretical data obtained by a sudden impact approximation approach taking into account the long-range interaction potentials only. The calculated cross sections show an excitation mixing process at large internuclear distances where Li-Cs dipole-dipole and dipole-quadrupole interaction forces are predominant.     

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.     

Cross sections for transitions between fine structure components of second group excited atoms induced by collisions with noble gas atoms

Cross sections of the transition ³ P ₁ ³ P ₀ for the atoms Zn, Cd, Hg upon collisions with inert gas atoms are calculated. It is shown that the transitions between fine structure components induced by nonadiabatic interaction are due to rotation of the molecule. A numerical estimation of the cross sections is performed.

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Zusammenfassung Es werden Wirkungsquerschnitte für den Übergang ³ P ₁ ³ P ₀ für die Atome Zn, Cd, Hg bei Stößen mit Edelgasatomen berechnet. Es wird gezeigt, daß die Übergänge zwischen Feinstrukturkomponenten, induziert durch nicht-adiabatische Wechselwirkung, durch Rotation des Moleküls bewirkt werden. Es wird eine numerische Abschätzung der Wirkungsquerschnitte durchgeführt.

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Résumé Calcul des sections efficaces pour la transition ³P₁ ³P₀ dans les atomes Zn, Cd, Hg par collision avec des atomes de gaz rare. On montre que les transitions entre les composantes de structure fine induites par interaction non adiabatique sont dues à la rotation de la molécule. Estimation numérique des sections efficaces.

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The authors are indebted to Dr. E. E. Nikitin for very valuable discussions.     

Electron transfer collisions between sulfur dioxide clusters and laser-excited Rydberg atoms

Electron attachment to SO₂ clusters is studied in a pulsed crossed beam apparatus, using laser-excited nf Rydberg atoms as a low energy electron source. The results are interpreted as an attachment to a dimer subcluster followed by a rapid impulsive dissociation of the nascent dimer anion. The remaining cluster anions possess a large amount of internal energy. At low principal quantum numbersn, the influence of the Rydberg ionic core leads to an important evaporation process interpreted with simple model calculations.     

Inelastic collisions of ultracold polar LiCs molecules with caesium atoms in an optical dipole trap

We investigate collisions of ultracold polar LiCs molecules and ultracold caesium atoms. LiCs molecules are formed in an optical dipole trap by photoassociation of caesium and lithium atoms via the B(1)Π excited state followed by spontaneous emission to the X(1)Σ(+) ground state and the lowest triplet state a(3)Σ(+). The molecules are then stored together with caesium atoms in the same optical trap. Rate coefficients for the loss of molecules induced by collisions with surrounding Cs atoms are measured for molecular ensembles produced via different photoassociation resonances. The results are analyzed in terms of the unitarity limit for the inelastic rates and predictions from the universal model of Idziaszek and Julienne (Phys. Rev. Lett., 2010, 104, 113202).     

Vibrattonal excitation of molecules by collisions with “hot” hydrogen atoms from laser photolysis

Excimer laser (ArF) photolysis of diatomic and triatomic hydrides produces hydrogen atoms with translational energies in excess of 15000 cm^?1 per atom. Subsequent collisions of these “hot” atoms with CO₂ and N₂O produces vibrationally excited molecules which can be detected by their characteristic infrared emission.     

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.     

Measurements of the electronic excitation in atomic collisions of potassium with the rare gases

Excitation cross sections for the potassium 4²P_{$\text{1}\text{2}$} and 4²P_{$\text{3}\text{3}$} states produced in collisions with rare gases are reported. They have been measured by the lightly yield as a function of the energy from 100 to 2500 eV. In addition to this, a spectral analysis from 3300 to 8600 Å of the light is given for some energies. This spectral analysis gives experimental evidence that the excited and ionized states of both potassium and the rare gases are involved in the collision process.     

Vibrational excitation of SF6 in collisions with He atoms

Vibrational excitation of SF₆ molecules in collisions with He atoms is studied using a vibrational close-coupling, rotational infinite-order-sudden method of calculation. Integral and differential cross sections for excitation of the triply degenerate ν₆ and ν₅ vibrational modes of SF₆ are reported for thermal collisional energies. Excitation of the ν₆ mode is found to be particularly efficieny. The cross sections are much larger than those calculated previously for the excitation of the bending mode in the He + CO₂ system. The differential cross sections are backward peaked.     

Cadmium (5 1 P 1 → 5 3 P J) excitation transfer and 5 3 P J intramultiplet relaxation by collisions with molecular gases

Collisional 5 ¹ P ₁ → 5 ³ P _J spin changing fine structure transfer as well as 5 ³ P _J intramultiplet mixing induced by various molecular gases (H₂, D₂, N₂, CO, CO₂, CH₄, C₂H₆, C₃H₈, C₂H₄) have been investigated using a combined method of fluorescence and absorption spectroscopy. After pulsed optical excitation of the Cd(5 ¹ P ₁) level the time dependence of the population densities has been measured both for the Cd(5 ¹ P ₁) level as well as the three collisionally populated Cd(5 ³ P _J) levels. By analyzing the signal curves at different molecular gas pressures not only the ratios of 5 ¹ P ₁ → 5 ³ P _J population transfer cross sections but also the Cd(5 ¹ P _J), Cd(5 ³ P _J) quenching cross sections and the Cd(5 ³ P _J) intramultiplet population transfer cross sections have been obtained.     

Transitions between the 2s and 2p states of atomic hydrogen in collisions with slow noble gas atoms

The thermal decomposition of diatomic molecules in a light inert gas was examined within the scope of the diffusion theory with allowance for interaction between vibration and rotation. It was shown that molecular rotation changes the value of the dissociation constant within an order of magnitude and appreciably affects its temperature dependence. The results, obtained by computer, can be applied directly to the dissociation of I₂, Br₂, and Cl₂.In conclusion, we express our gratitude to A. I. Vol'pert and L. N. Stesik for the possibility of carrying out the computer calculation.     

Dissociative ion-pair formation in collisions of fast potassium atoms with benzene and fluorobenzene

Using a crossed molecular-beam technique, electron transfer is studied in collisions of fast potassium atoms with benzene and fluorobenzene molecules. The negative fragment ions formed in the collision region are extracted by a pulsed voltage, and their masses and energy release distributions analyzed by a time-of-flight (TOF) method. The dominant fragment from C6H5F is F-. The phenyl ring fragments CH- and C2H- are also observed but with lower intensities. In the case of C6H6 the dominant anionic specie is C2H-. The kinetic-energy release distributions of the various fragments are derived from the width and shapes of the TOF peaks in the spectrum.     

Ionizing collisions of potassium atoms with molecules: Excitation energy of the product molecular ion

For processes of the type K + M

K⁺ + M^? at eV collision energies, measurements are reported of the population of excited states of the product negative ions. For M = Br₂ and SF₆ only a small excitation (0.5 – 3 eV) is found. For M = O₂, the products can be excited up to higher energies (? 5 eV); this is ascribed to a second electronic channel of the process.     

Geminate recombination processes induced by rare gas collisions with predissociating NaI molecules prepared by femtosecond excitation

NaI molecules predissociate after excitation with ultrashort pulses at a peak wavelength of 320 nm. In rare gas environments at higher pressures the process of geminate recombination is likely to occur. We studied the dynamics of these processes under various pressure conditions and for different collision partners. The calculations were performed within a three-dimensional statistical model which simulates the NaI wave-packet dynamics with classical trajectories and the collisions via an instantaneous hard-sphere scattering. Received: 4 May 1998 / Accepted: 27 July 1998 / Published online: 28 September 1998     

On the electron transfer in low-energy collisions of the fully-ionized muonic boron with helium atoms

In connection with recent experiments on the observation of the metastable 2s state of muonic boron (μB)⁴⁺ formed in helium with a small admixture of diborane (B₂H₆) we estimated the cross-section of the electron transfer from helium to the muonic ion. The collisions energy T was considered to lie between the thermal energy and 1 a. u. The muonic ion (μB)⁴⁺ was treated as a heavy beryllium isotope Be4+. It was found that the main reaction responsible for the electron transfer at the energies specified above was: Be⁴⁺ + He → Be³⁺(1s) + He²⁺ + e^-. . Its mechanism is the Auger effect in the two-electron quasi-molecule (Be-He)⁴⁺. In our estimation of its cross-section we used adiabatic quasi-molecular terms obtained by diagonalizing the electronic Hamiltonian on a basis of several diabatic states constructed from two-centre orbitals. The electronic wavefunctions built with this way were employed to calculate Auger effect rates. The outgoing electron was treated to be free. The relative motion of the nuclei was considered classically. It was found that in the region 0.001 ≤ T ≤ 0.01 (in atomic units) the cross-section was a decreasing function of T and it was small (～ 10^-2 Ų). Then it starts to increase and mounts to typical atomic values (1 Ų) at T = 1 ÷ 2. The comparison of the reaction rate constant estimated on the basis of the obtained cross-section with experimental results establishes the upper limit of the energy of muonic boron during the 2s state lifetime (≈ 40 ns). It proves to be about 25 eV. This limit is in agreement with the typical value of 1 eV resulting from the energy transfer in the muon capture by the diborane molecule and the recoil energy acquired in the cascade. However, it does not exclude the possibility of an additional acceleration of muonic boron during its formation in the molecule.     

Dynamics of energy transfer in collisions of O(3P) atoms with a 1-decanethiol self-assembled monolayer surface

Chemical dynamics simulations are reported of energy transfer in collisions of O(3P) atoms with a 300 K 1-decanethiol self-assembled monolayer (H-SAM) surface. The simulations are performed with a nonreactive potential energy surface, developed from PMP2/aug-cc-pVTZ calculations of the O(3P) + H-SAM intermolecular potential, and the simulation results represent the energy transfer dynamics in the absence of O(3P) reaction. Collisions energies E(i) of 0.12, 2.30, 11.2, 75.0, and 120.5 kcal/mol and incident angles theta(i) of 15, 30, 45, 60, and 75 degrees were considered in the study (theta(i) = 0 degrees is the surface normal). The translational energy distribution of the scattered O(3P) atoms, P(E(f)), may be deconvoluted into Boltzmann and non-Boltzmann components, with the former fraction identified as f(B). The trajectories are also analyzed in terms of three types; that is, direct scattering from and physisorption on the top of the H-SAM and penetration of the H-SAM. There are three energy regimes in the scattering dynamics. For the low E(i) values of 0.12 and 2.30 kcal/mol, physisorption is important and both f(B) and the average final translational energy of the scattered O(3P) atom, E(f), are nearly independent of the incident angle. The dynamics is much different for hyperthermal energies of 75.0 and 120.5 kcal/mol, where penetration of the surface is important. For hyperthermal collisions, the penetration probability decreases as theta(i) is increased, with a significant transition between theta(i) of 60 and 75 degrees . Hyperthermal penetration occurs upon initial surface impact and is more probable if the impinging O(3P) atom may move down a channel between the chains. For E(i) = 120.5 kcal/mol, 90% of the trajectories penetrate at theta(i) = 15 degrees , while only 3% penetrate at theta(i) = 75 degrees. For the former theta(i), the energy transfer to the surface is efficient with E(f) = 4.04 kcal/mol, but for the latter theta(i), E(f) = 85.3 kcal/mol! Particularly interesting penetrating trajectories are those in which O(3P) is trapped in the H-SAM for times exceeding 60 ps, linger near the Au substrate, and strike the Au substrate and scatter directly. For E(i) = 11.2 kcal/mol, there is a transition between the scattering dynamics for the low and hyperthermal collision energies. Additional detail in the energy transfer dynamics is obtained from the final polar and azimuthal angles, the residence time on/in the H-SAM, the minimum height with respect to the Au substrate, and the number of inner turning points in the O-atom's velocity. Calculated values of E(f) vs the final polar angle, theta(f), are in qualitative agreement with experiment. The O(3P) + H-SAM nonreactive energy transfer dynamics, for E(i) of 11.2 kcal/mol and lower, are very similar to previously reported Ne + H-SAM simulations.     

Electron transfer processes in potassium collisions with 5-fluorouracil and 5-chlorouracil

Electron transfer to uracil (U), 5-chlorouracil (5-ClU) and 5-fluorouracil (5-FU) yielding anion formation has been investigated in 30-100 eV potassium-molecule collisions. The rich fragmentation patterns of all three molecules suggest that electron transfer in collisions with electronegative neutrals may cause efficient damage to RNA. The main ring fragment anion in all the mass spectra was NCO(-) while the production of X(-) (X = F, Cl) was a strong decomposition of the halouracil temporary negative ions. Cl(-) was the most intense fragment anion in the 5-chlorouracil measurements, whereas NCO(-) production dominated in the U and 5-FU data. Arguments based on energetics and vibrational dynamics have been proposed to explain these differences. Electronic coupling between dipole- and valence-bound states may play a particularly important role in the fragmentation pathways of the 5-ClU parent anion. The stabilizing influence of the potassium cation following electron transfer (ionic scattering) on the observed fragmentation patterns is discussed, notably in the context of comparisons with free electron attachment processes.     

Comparison of the fragmentation pattern induced by collisions, laser excitation and electron capture. Influence of the initial excitation

Collision-induced dissociation, laser-induced dissociation and electron-capture dissociation are compared on a singly and doubly protonated pentapeptide. The dissociation spectrum depends on the excitation mechanism and on the charge state of the peptide. The comparison of these results with the conformations obtained from Monte Carlo simulations suggests that the de-excitation mechanism following a laser or an electron-capture excitation is related to the initial geometry of the peptide.     

Electron transfer mechanism and photochemistry of ferrioxalate induced by excitation in the charge transfer band

The photoredox reaction of ferrioxalate after 266/267 nm excitation in the charge transfer band has been studied by means of ultrafast extended X-ray absorption fine structure (EXAFS) analysis, optical transient spectroscopy, and quantum chemistry calculations. The Fe-O bond length changes combined with the transient spectra and kinetics have been measured and in combination with ultrahigh frequency density functional theory (UHF/DFT) calculations are used to determine the photochemical mechanism for the Fe(III) to Fe(II) redox reaction. The present data and the results obtained with 266/267 nm excitations strongly suggest that the primary reaction is the dissociation of the Fe-O bond before intramolecular electron transfer occurs. Low quantum yield electron photodetachment from ferrioxalate has also been observed.     

Nitrogen rotational excitation by collisions with argon-observations and comprison with theory

Measurements of the distribution of rotational states in a nitrogen molecular beam before and after deflection by collision with rare gas atoms are interpreted in terms of the intermolecular potential. A potential of the form V(R, θ) = 4?(1 + b₂P₂(cosθ)| R^*12 ? 4θ [1 + α₂P₂(cosθ)]/R^*6 is used with semiclassical theory using the sudden approximation to fit the observations. As α₂ is known, as well as ? and the size parameter, σ, the objective is to find b₂. However, the calculated state distributions are found to systematically vary from those measured, irrespective of the value of b₂. This result is tentatively explained as due to the inadequacies of the 12-6 potential.