Cross sections for the quenching of the excited strontium state (51P1) measured in CO/N2O flames

The quantum efficiency of fluorescence, Y, of the 4607.33 Å Sr line $\text{(}{}^1\text{P}{}_1\text{?}{}^1\text{S}{}_0\text{transition}\text{)}$ was measured in four pre-mixed, laminar, shielded CO/N₂O flames of about 2700 K, with different quantitative compositions at 1 atm. From these data, the specific quenching cross sections for O₂, CO₂, CO and N₂ were found to be (152±20 Ų), (30±5 Ų), (49±8 Ų) and (16±3 Ų), respectively.     

Deexcitation of Cd 53P1 atoms in collisions with ground state atoms and molecules

Collisions of excited Cd 5³P₁ atoms were investigated using atomic fluorescence spectroscopy. Cadmium vapor, together with a quenching gas, was irradiated in a quartz fluorescence vessel with Cd 3261 Å resonance radiation and the intensity of the resulting resonance fluorescence was monitored in relation to the gas pressures. The experiments yielded the following cross sections Q₁₀ (in A²) for collisional transfer 5³P₁→5³P₀: CdAr=2×10^?3, CdN₂=8.0, CdH₂=7.0, CdCO=15.6. The cross sections Q for collisional deexcitation to the ground state (quenching) in A² are CdN₂ = 2.6×10^?2, CdH₂ = 11.0, CdCO = 3.4, CdCO₂ = 26.     

Broadening of a two-photon resonance in a zinc atom in collisions with CO<Subscript>2</Subscript>, CO,and NO molecules

The broadening of a two-photon resonance is studied experimentally at the 4s¹S₀?6s³S₁ transition in a zinc atom upon absorption of two waves with a small detuning from an intermediate state in collisions with CO₂, CO, and NO molecules. The measured absolute values of broadening cross sections greatly exceed gas-kinetic cross sections and are (9.4±2.4, 6.5±1.6, and 3.9±1.0)×10^{? 14}cm² for CO², CO, and NO, respectively. Anomalously large rate constants and cross sections obtained in experiments are explained by the efficient resonance quenching of the excited states of zinc atoms in collisions with molecules accompanied by transfer of the energy of excited atoms to vibrational-rotational degrees of freedom of molecules.     

Quenching cross sections of the B3Π+(Ou+) v′ = 14 level of I2 by H2, CO,and CH4

In this work the deexcitation of the B³Π⁺(O_u⁺), v′ = 14 level of I₂ after pulsed laser excitation has been studied. The quenching cross sections by collisions with I₂, H₂, CO, and CH₄ have been measured. The experimental results are 190 ± 14, 2.5 ± 0.3, 15.1 ± 0.4, and 18.0 ± 0.6 Ų, respectively. These values are compatible (within 30%) with the semiempirical scaling law of proportionality with the product of polarizability and the square root of reduced mass.     

Excitation transfer and quenching induced in inelastic collisions of cadmium 51P1 atoms

The quenching of excited Cd(5¹P₁) atoms induced in collisions with Ar, N₂, H₂, CO, and CO₂ has been investigated using methods of fluorescence spectroscopy. Mixtures of Cd vapor and quenching gases contained in a quartz cell were irradiated with 2288 Å resonance radiation and the fluorescence spectrum consisting of the 2288 Å direct component, as well as of the 3261 Å sensitized component, was monitored in relation to the gas density. The experiments yielded the following cross sections Q_tot for the overall collisional deexcitation of the ¹P₁ state, as well as the cross sections Q₁₃ for ¹P₁→³P_J excitation transfer (in Ų).For argon: Q₁₃ < 2 × 10^-2, for N₂: Q_tot =48.5, Q₁₃ =48.0; for H₂: Q_tot = 19, Q₁₃ = 7.2; for CO: Q_tot = 110, Q₁₃ = 15; for CO₂: Q_tot = 121, Q₁₃ < 6 × 10^-3.     

The absorption cross sections of N2, O2, CO,NO, CO2, N2O,CH4, C2H4, C2H6 and C4H10 from 180 to 700Å

The absorption cross sections of N₂, O₂, CO, NO, CO₂, N₂O, CH₄, C₂H₄, C₂H₆, C₄H₁₀ have been measured photoelectrically in the 180–700 Å region using synchrotron radiation. The absorption cross sections in the region λ ≥ 500 Å was found to be structureless and to increase monotonically with wavelength for all gases. The positions of the structure observed in the 520–720 Å region for N₂, O₂, CO₂ and N₂O are consistent with the various Rydberg series reported by previous authors.     

Electronic-excitation transfer collisions in flames—II temperature dependence of the quenching of Na(32P)-doublet by H2 and O2

Effective cross sections for quenching of the Na(3²P)-doublet by H₂ and O₂ molecules have been measured in flames in the temperature range 1500–2500 K. The H₂-cross section decreases from (9.3±1.0) Ų at 1500 K to (6.8±1.0) Ų at 2500 K. The O₂-cross section decreases from (39±2) Ų at 1720 K to (31±2) Ų at 2500 K. A critical comparison of the flame values with previous literature data on the H₂-cross section at lower temperatures shows that it decreases systematically when the temperature rises from about 400 to 2500 K.     

Application of atom-photon coincidence techniques to the determination of the differential cross section for excitation ofK(42 P) in low-energy K-N2, CO collisions

The differential cross section for the processK(4² S)+N₂, CO→K(4² P)+ N₂, CO is presented for reduced scattering angles τ<2·10⁴ eV deg. The inelastic process is identified by determining the time-correlation between the inelastically scattered potassium atom and the emittedK(4² P→4² S)photon. The performance of different coincidence techniques is compared. From the differential cross sections values for the position of the curve crossing responsible for the excitation process are derived (R _c=2.77, 2.48 Å andV(R _c )=0.55, 1.00 eV for K-N₂, CO respectively). The values indicate that the lowest ionic intermediate state of the form K⁺-N ₂ ^? , CO^? is responsible for the excitation process.     

Electronic-excitation transfer collisions in flames—V. Cross sections for quenching and doublet-mixing of K(42P)-doublet by N2, O2, H2 and H2O

Weighted average cross sections for quenching of the K(4²P)-doublet by N₂, H₂, O₂ and H₂O, measured in flames, show no significant temperature dependence in the range from 1500 to 2500K. Doublet mixing cross sections for K(4²P$\text{3}\text{2}$?4²P$\text{1}\text{2}$) transitions were measured at 1720K for N₂, O₂, H₂O. The ratios of both mixing cross sections were measured independently and were found to agree with the detailed balance condition within 2 per cent. It is shown that an ionic intermediate-state model cannot explain the large magnitude of N_2? mixing cross sections.     

Excitation transfer and quenching induced in inelastic collisions of Zn 41P1 and 43P1 atoms

Quenching of excited Zn(4¹P₁) and Zn(4³P₁) atoms by collisions with Ar, N₂, H₂, CO and CO₂ has been investigated using methods of fluorescence spectroscopy. Mixtures of Zn vapor and quenching gases topped-up with Ar to maintain constant pressure were irradiated in a quartz cell with 2139 or 3076 Å resonance radiation which excited the 4¹P₁ or the 4³P₁ state, respectively. The resulting resonance fluorescence and sensitized fluorescence were monitored in relation to the gas density. Intensity measurements yielded the following cross sections Q_tot for the overall collisional deexcitation of the ¹P₁ state, Q₃ for quenching of the ³P₁ state, and Q₁₃ for ¹P₁→³P_J excitation transfer.For N₂: Q_tot = 26, Q₃ = 4.4, Q₁₃ = 5.8; for H₂: Q_tot = 12, Q₃ = 21, Q₁₃ < 5 × 10^-3; for CO: Q_tot = 28, Q₃ = 14, Q₁₃ = 17; for CO₂: Q_tot = 76, Q₃ = 23. All values are in Ų.     

Absolute photoabsorption cross sections for H2O and D2O from λ 180–790 Å

Absolute photoabsorption cross sections for H₂O and D₂O have been measured photoelectrically from λλ 180 to 790 Å using synchrotron radiation. The cross sections increase smoothly with wavelength to ～λ610 Å, with both H₂O and D₂O displaying a broad absorption band extending above a nearly linear background from λλ 400 to 490 Å. The continuum has a maximum of ～ 22.5 Mb at λ 640 Å. Above λ 615 Å, superimposed on the continuum, a diffuse structure appears which is similar to the vibrational structure of the ²B₂ states of H₂O⁺ and D₂O⁺ as observed in photoelectron spectra. The structure is believed to arise from excitation of a 1b₂ electron to the vibrational levels of a Rydberg orbital with n¹ ≈ 2.64.     

Elektronennachlieferung in Niederdruckentladungen durch metastabile Stickstoffmoleküle und deren Stoßlöschung durch Fremdgase

Oscilloscopic studies of pre-breakdown discharge currents in pure nitrogen and mixtures of nitrogen with other gases show that metastable N₂ molecules influence the discharge by releasing secondary electrons when they collide with the electrodes. By this slow process an after current is produced, which lasts a time of milliseconds. It was found that the diffusion coefficient of the metastable N₂ molecule is 167 cm² s^?1 ±15% at 1 Torr and 20°C, its radiative life time is larger than 40 ms, and its cross section for deexciting collisions with ground state N₂ molecules is smaller than 7 · 10^?22 cm². There is strong evidence that the metastables are in theA ³Σ _u ⁺ state. From measurements in gas mixtures cross sections for deexciting collisions with other molecules are derived. The results are for O₂ 1.9 · 10^?17 cm², for CO 1.5 · 10^?17 cm², for H₂ 2.5 · 10^?20 cm², and for CO₂ smaller than 5 · 10^?20 cm².     

Adsorption of CO on the (110) plane of tungsten electron impact,thermal desorption,and work function measurements

The adsorption of CO on the (110) plane of tungsten has been studied using electron impact desorption, thermal desorption, and work function measurements in a single apparatus combining these various techniques. It is concluded that a single molecular adsorption state exists at 20–250 K (virgin-CO). At 300–400 K, 60% of the low temperature layer desorbs, the remainder converting principally to a beta-1 state, which has very small electron impact cross section; in addition to beta-1 an O⁺ yielding state, which we call beta-precursor is formed. The beta-1 state is stable to 900 K, where some desorption and conversion of the remaineder to a beta-2 state occurs. The O⁺ yielding state decays with increasing T and is gone at 800 K. Readsorption on beta-1 leads to two types of adsorption states called alpha and gamma, which seem to be site specific. Electron impact desorption yields mostly CO⁺ and CO for virgin, O⁺ for beta-precursor, and CO⁺ and CO for the readsorption states. There is no isotopic mixing in virgin or in readsorbed CO, nor does readsorbed CO exchange with beta-1 or beta precursor. There is complete isotopic mixing in beta desorption. In addition, massive EID creates another state, characterized by a large dipole moment, also yielding O⁺ in EID. This state can be converted to beta-1 by heating to 400 K. The total disappearance cross sections for the various states are virgin-CO5 × 10^?17cm²; γ-CO 1.6 × 10^?16cm²; α-CO 5 × 10^?17cm²; β-precursor 6 × 10^?18cm²and 1.2 × 10^?19cm²; EID induced state 8 × 10^?18cm². In addition, cross sections for ion production are determined and found to be several orders of magnitude less than total disappearance cross sections. These results, and Leed and coverage data obtained in parallel investigations are used to formulate models of the various adsorption states. It is concluded that virgin and readsorbed CO are molecular and beta-precursor and beta dissociated, although strong interactions between C and O remain. The electron impact desorption of physisorbed CO was investigated and found to yield C⁺, O⁺, and neutral CO, but very little CO⁺. These results suggest primary dissociation of CO by electron impact, and desorption of neutral physisorbed CO by the energetic fragments. Physisorbed CO⁺, although undoubtedly created, lies on the attractive part of its potential curve relative to the surface, and thus does not desorb as CO⁺.     

Loss of electrons by fast H− ions in gases

The characteristic angles and cross sections for a (\mathop 1^- 0) + (\mathop 1^- 1)(\mathop 1\limits^ - 0) + (\mathop 1\limits^ - 1) reaction in which H⁻ atoms with an energy of 0.1–10 000 MeV lose electrons when scattered on C, N, or O atoms were calculated for the relativistic case of the Bethe scattering theory. The cross sections for N₂, O₂, and CO₂ molecular targets were obtained using the additivity rule. The results presented are compared with known experimental and theoretical data.     

A quantum-chemical study of intermediates of the 1O2 photogeneration sensitized by buckminsterfullerene and accompanying photochemical reactions

The intermediates of hypothetical photochemical reactions that accompany the quenching of the ³C ₆₀ ^* triplet state by triplet oxygen are studied by the (U)PBE0 quantum-chemical method. The diradical C₆₀-O-O formed from ³O₂ and photoexcited buckminsterfullerene ³C ₆₀ ^* is characterized by a negative binding energy ?1.11 eV (with respect to C₆₀ and ³O₂), the singlet-triplet splitting ΔE _ST of 0.07 eV, and the dipole moment of 3.2 D at the equilibrium internuclear separations 1.522 Å (CO) and 1.294 Å (OO). Its decay produces ¹O₂. The formation of a dioxetane circle lowers the energy by 0.8 eV. The ground-state energy of diketone C₅₈(C=O)₂ is 2.0 eV lower than the energy of C₆₀-O-O. The metastable centrosymmetric diradical C₆₀-C₆₀, formed upon ineffective light absorption by clusters (C₆₀)_N, has a single interpolyhedral C-C bond (1.657 Å). Its triplet state T ₁ lies 0.16 eV higher than the S ₁ singlet. The S ₁ → S ₀ relaxation leads to the formation of a stable C₆₀-C₆₀ dimer with a shorter (1.584 Å) bis-single exothermic (+0.24 eV) bond of polyhedra. The photoexcited C₆₀-C₆₀ dimer is able to form isomeric metastable diradicals C₆₀-C₆₀-O-O.     

Precision neutron total cross section measurements on gold and cobalt in the 40 μeV-5 meV range

The slow neutron absorption cross sections of gold and cobalt have been accurately redetermined by transmission measurements in the neutron wavelength range 4 to 47 Å. For the range 4 to 7.6 Å a new time-of-flight spectrometer at the FRM reactor was used which involves a system of three synchronized choppers and a 150 m long guide tube as flight path. Utilizing the high wavelength resolution of the spectrometer, the time-of-flight wavelength scale could be accurately calibrated (±2 · 10^?3 Å) by means of various Bragg cutoff breaks observed in transmission on polycrystalline samples. Supplementary transmission measurements on gold and cobalt were performed in the range 11–47 Å using the time-of-flight spectrometer for ultracold neutrons at the FRM reactor. The absorption cross sections were evaluated considering corrections for incoherent and inelastic scattering and for the slight deviation from 1/v of the absorption of gold due to the 4.9 eV resonance level. For the absorption at subthermal energies we obtainσ _a /gl=(54.35±0.06) b/Å for Au, (20.66±0.04) b/Å for Co. Evaluation of the absorption cross sections at 2200 m/s neutron velocity gives (98.68±0.12) b for Au, (37.15±0.08) b for Co.     

Wing profile measurements of the resonance emission lines of Na (5890/96Å) and Sr (4607Å) in flames

The blue and red wing intensity distributions of the Na 5890Åand 5896Åresonance lines were measured in emission in a pre-mixed, laminar, shielded H₂/O₂ flame at 1 atm with Ar or N₂ as diluent gases (T≈2000 K). The wavelength range scanned amounted to about 20Åfrom line center. In addition, we measured the wing intensity distributions of the resonance line of Sr (4607Å) in CO/N₂O flames at 1 atm (T≈2800 K) in a wavelength range of about 10Åfrom line center. The scanning monochromator used in these emission experiments had a spectral bandwidth of 0.30±0.04Å.For the resonance lines of Na and of Sr, the wavelength dependence of each wing was derived from 25 repeated scans and found to differ from theoretical predictions based on binary quasistatic theory.The red wing of the D₁ line of Na was investigated for the occurrence of satellites. We discovered a structure resembling a “smoothed” satellite at about 12Åfrom line centre. A satellite-like structure was also found in the blue wing of the resonance emission line of Sr at about 5Åfrom line center.     

Differential elastic and quenching cross sections for Ar*(3P) and CO2(X1 ∑ g +)

The energy dependence of the differential scattering of metastable Ar*(³P) by ground-state CO₂(X¹ ∑ _g ⁺) has been studied at relative kinetic energies from 58 to 126 meV over an angular range of 5–160° c.m. using crossed molecular beams. The position and curvature of rainbow maxima, which are observed at each energy, are used to obtain parameters for a Lennard-Jones (12, 6) spherically symmetric potential. The position of the minimum, r _m = 5·02 ± 0·65 Å, is identical to that for K + CO₂ and the well depth, ε = 16·3 ± 0·8 meV, is about 10 per cent greater. The scattered intensity shows a distinct fall-off on the dark side of the rainbow compared to that expected for elastically scattered Ar*. This depletion, caused primarily by the quenching of Ar*, is analysed in terms of the optical-shadow model to determine the energy dependence of the observed quenching cross section, which is predicted to have a maximum of 67 Ų at 193 meV.     

Molekularstrahlmessungen von Stoßquerschnitten für Übergänge zwischen definierten Rotationszuständen zwei-atomiger Moleküle

Inelastic collision cross sections for transitions between specified rotational states designated by (J, M) have been measured in a molecular beam apparatus. With an electrostatic four pole field molecules in a specified rotational state are separated out of a molecular beam and focussed into a gas filled scattering chamber. Molecules which have been scattered by less than 1/2° are then collected in a second four pole field, located directly behind the scattering chamber, and are analyzed for their rotational state. From a comparison of the measured pressure dependence with calculated curves a determination of inelastic collision cross sections for specified quantum jumps is possible. Measured inelastic scattering cross sections for the transitions (2,0→3,0) are reported for the gases He, Ne, Ar, Kr, CH₄, SF₆, H₂, O₂, Air, N₂O, H₂O, CF₂Cl₂. The values range between about 5 and 100 Ų in the order indicated. The scattering gases NH₃ und ND₃ yielded larger cross sections of about 600 Ų and, in addition, the transitions (3,0)→(2,0),(1,0)→(2,0), (2,0)→(1,0) and (3,0)→(1,0) were observed. Total cross sections for the same gases were also measured with the apparatus.     

Dynamical treatment of binding,polarization and recoil in asymmetric ion-atom collisions

The cross sections for quenching the lowestn ² P states of the alkali atoms Li, Na, K., and Rb by the inert gases He, Ne, Ar, Kr, and Xe are presented for 5 eV≦E _c.m.≦ 100 eV. These cross sections are derived from the corresponding cross sections for collisional excitation by applying the principle of microreversibility. Upper estimates for the quenching cross sections at thermal energies are given; in all studied cases the quenching cross sections are <8·10^?3Ų. These new upper limits are in most cases much lower than those obtained from other methods previously.