Quenching Cross Sections Research Articles

Kinetic studies of Bi(6 4S [formula omitted]) by time-resolved atomic resonance fluorescence I: Reactions with C 2H 2 (+He), N 2O and fluorescence quenching of Bi(7 4P [formula omitted

We present a kinetic study of ground state atomic bismuth Bi(6 4S 3 2 ) generated by pulsed irradiation of Bi(CH 3) 3 in the presence of various gases and monitored with time-resolved atomic resonance fluorescence in the “single-shot” mode at λ = 306.8 nm (Bi(7 4P 1 2 ) → Bi(6 4S 1 2 ) + hv) following optical excitation. The third-order reaction Bi + C 2H 2 + He is studied in detail, including the effects of diffusion. The resulting third-order rate constant ( k 3 = (1.9 ± 0.3) × 10 −33 cm 6 molecule −2 s −1 (300 K)) is compared with previous data, in particular those from time-resolved resonance absorption measurements. The absolute second-order rate constant for the reaction between Bi(6 4S 3 2 ) and N 2O is also characterized ( k 2 = (2.1 ± 0.4) × 10 −16 cm 3 molecule −1 s −1 (300 K)) and compared with previous upper limits reported for this reaction. Finally, the technique is applied to the study of fluorescence quenching of Bi(7 4P 1 2 ) for which we report the following quenching cross sections σ 2 (Å 2) (errors, about 25%): N 2, 3.8; NO, 169; O 2, 570; N 2O, 164; CF 4, 12; C 2H 2, 97. These data are compared, where possible, with measurements of quenching cross sections derived from Stern—Volmer analyses on integrated plate intensities resulting from atomic emission following flash photolysis. The results from the two fundamentally different techniques are found to be in reasonable accord.

4D2→4F2excitation transfer in Li induced by collisions with Li, He, and Ar ground-state atoms

Excitation transfers between the $4^{2}D$ and $4^{2}F$ states of lithium are studied in a temperature-controlled heat-pipe oven. These transfers are induced by collisions with Li, He, and Ar ground-state atoms. Quenching cross section and radiative rates are also measured. For the excitation transfer process, the thermally averaged cross sections are, respectively, ${\ensuremath{\sigma}}^{\mathrm{Li}}=400$ ${\mathrm{\AA{}}}^{2}$, ${\ensuremath{\sigma}}^{\mathrm{He}}=120$ ${\mathrm{\AA{}}}^{2}$, and ${\ensuremath{\sigma}}^{\mathrm{Ar}}=190$ ${\mathrm{\AA{}}}^{2}$. These values agree with the experimental results of Gallagher in Na and the calculations of Gersten. Finally, the behavior of the $4^{2}D\ensuremath{-}4^{2}F$ two-level system in the $l$-mixing regime is investigated in the case of Ar collisions.

Radiative lifetimes and collisional quenching cross sections of selectively excited vibrational states of the B 2p 1Σ u+ state of H 2

Radiative lifetimes between 0.54 ns and 0.69 ns have been measured for the vibrational levels υ′ = 0...4 of the B 2p 1Σ u + state of H 2 after selective excitation by synchroton radiation. Quenching by collisions with H 2 ground state molecules has been observed with cross sections of 5 × 10 -15 cm 2.

Time-resolved laser induced fluorescence measurement of excited states relaxation constants: Application to the quenching of mercury atoms by NO molecules

A time-resolved laser induced fluorescence arrangement is described, which allows direct study of energy transfers and of relaxation processes. A first application to the direct measurement of quenching cross sections of mercury excited (6 3P1) and metastable (6 3P0) atoms by NO molecules is reported. For Hg(6 3P1) our result: σ=80±5 Å2 is in good agreement with previous works. On the contrary, our result for Hg(6 3P0): σ=85±5 Å2 strongly disagrees with previous ones. The reasons for this discrepancy are reviewed. We emphasize that our measurement is direct and that it has to do with a system in which only Hg and NO are present. On the contrary, in the previous works nitrogen was present, generally at pressures several orders of magnitude greater than NO pressures.

Classical models for electronic degrees of freedom: Quenching of Br*( 2P 1/2) by collision with H 2 in three dimensions

Three-dimensional classical trajectory calculations have been carried out for the quenching of Br*( 2P 1/2) by collision with ground-state H 2, using an approach that describes the electronic as well as heavy-particle (i.e. transition, rotation, vibration) degrees of freedom by classical mechanics. We find a sizeable quenching cross section (≈ 1 2 Å 2), with essentially all of the collision products having H 2 vibrationally excited to υ = 1, i.e. near-resonant EV transfer dominates the non-resonant ET,R processes. This is in agreement with experimental results.

State-resolved quantum yields of the reactions of metastable Ca(3P, 1D) with NO2 and N2O

Absolute chemiluminescence and quenching (beam attenuation) cross sections were measured for the title reactions in a beam/gas experiment. In the Ca* + NO2 system, both quantities are about twice as large as in the N2O reaction. The chemiluminescence quantum yields, corrected for infrared emission from the A1Σ+ and A′ 1Π states, are of similar magnitude YCL(Ca* + N2O) = 7.2 ± 1.1% and YCL(Ca* + N2O) = 8.0 ± 0.9%. The quenching cross sections are στ(Ca* + N2O) = 153 ± 32 Å2 and στ(Ca* + N2O) = 73.2 ± 5.7 Å2, consistent with a long-range harpooning process in the former and an electron transfer at closer range for the latter system. The partial resolution of CL spectra was aided by lifetime-separated spectroscopy. The principal emitter in both systems is the long-lived CaO(A′ 1Π), next to minor yields of CaO in the short-lived A1Σ+, B1Π, and "orange and green arc band" states. The results are discussed in light of adiabatic correlations. The arc states are argued to arise in nearly coplanar collisions from the rearrangement of an excited intermediate. An information theoretical analysis reveals considerable dynamical bias in the form of non-linear surprisal plots.

Radiative lifetimes ofS1andP1Rydberg levels of He

Radiative lifetimes of four $^{1}S$ levels ($n=6\ensuremath{-}9$) and seven $^{1}P$ levels ($n=7\ensuremath{-}13$) of He I have been measured using the high-frequency deflection technique. The $^{1}S$ lifetimes, with estimated limiting errors of about 3-7%, agree well for $n=6\ensuremath{-}8$ with theoretical lifetimes based on transition probabilities computed from central-field wave functions with exchange and configuration interaction. An apparent discrepancy between theory and experiment in the case of the $9^{1}S$ measurement is largely removed when blackbody-radiation-induced emission and absorption processes are included in the theoretical lifetime. Radiative lifetimes for the $^{1}P$ levels were determined under conditions where, in favorable cases ($n=8\ensuremath{-}12$), the zero-pressure lifetimes differed by no more than 10-20% from the values measured at low pressures. In these cases, the estimated accuracy of the measurements is comparable to that of theory, and the agreement is deemed within the combined uncertainties. Quenching cross sections for the $n^{1}S$ levels are also reported.

Vibrational relaxation of CO+(A 2Πi, v′=1) and CO+(B 2Σ+, v′=1) ions in the collision with CO molecules

The vibrationally selected CO+(A 2Πi, v′=0,1) and CO+(B 2Σ+, v′=0,1) ions were generated by the photoionization of CO using Ne and He resonance lamps, respectively. The quenching cross sections of the excited ions were found to be very large and exceed gas kinetic values in the B 2Σ+ state. In both excited states it has been observed that the vibrationally excited v′=1 ion relaxes to v′=0 level through the collision with the parent CO molecules. The branching ratio of quenching to relaxation in the v′=1 ions was explained by a cascading relaxation mechanism in which all the vibrational levels underlying the excited v′=1 level contribute to the energy transfer.

Vibrational-rotational deexcitation of HF in collision with He

State-to-state cross sections are reported for vibrational-rotational transitions for HF in collisions with He, at collisional energies of 0.5 and 1.0 eV. These were computed within the infinite-order sudden (IOS) approximation using adiabatic, distorted-wave techniques. Values are tabulated for the vibrational-rotational deexcitation sequences (v, j) → (v–1, 0), with v=1, 2, 3, 4 and j=0 – 40. These quenching cross sections can be used in conjunction with IOS factorization formulas to compute VRT cross sections for final rotational states other than jf=0. In addition to IOS results, vibrational quenching cross sections were computed using the much more simple breathing-sphere technique. The breathing-sphere results compare favorably to the more accurate IOS results, particularly as to energy dependence. This suggests a simple method of utilizing known quenching cross sections to predict values for different vibrational levels and/or collisional energies.

Trajectory-surface-hopping study of Na(3p 2P) +H2 → Na(3s 2S)+H2(v′, j′, θ)

Trajectory-surface-hopping calculations involving the three lowest-energy 2A′ potential surfaces are reported for Na(3p 2P) collisions with H2(v=0, low j) at 0.9 kcal/mol relative translational energy. In addition to the total quenching cross section, we report distributions of final translational energy, final vibrational and rotational quantum numbers, internal energy, scattering angle, and collision time for the quenching collisions. We also report the opacity function, the correlation of scattering angle with impact parameter, and separate product translational spectra for the forward and backward scattered halves of the quenched ensemble. These results provide a detailed picture of the chemical dynamics of a typical quenching system proceeding through a quasibound intermediate configuration with large ionic character.

ChemInform Abstract: FLUCTUATIONS OF CROSS SECTIONS IN CHEMICAL PROCESSES: INFRARED PHOTOQUENCHING OF FLUORESCENT STATES IN THIOPHOSGENE

Pronounced fluctuations have been observed in the cross section for the quenching of the fluorescence of the different members of the vibrational manifold belonging to the A 1A2 state of thiophosgene. This phenomenon is reminiscent of fluctuations that occur for nuclear reactions in regions of high level density, and that can be interpreted in terms of reduced transition amplitudes that are approximately normally distributed. The analysis of the fluctuations gives an effective number of independent reaction channels; in general, the greater the number of channels, the narrower is the range of the fluctuations. A knowledge of the number of independently contributing channels can be useful in the interpretation of experimental results, and sometimes lends important insight into reaction processes. The quenching cross sections for thiophosgene were therefore examined to see whether an analysis of their fluctuations would be useful. It was found necessary to develop a form of analysis that circumvents a seri...

Collisional quenching of indium (5p2 4P1/2) and indium (6p 2P) by Ar, H2, D2, N2, and CH4

Collisional quenching of In(5p2 4P1/2) and In(6p 2P) by Ar, H2, D2, N2, and CH4 has been measured by laser pumping of the 5p2 4P1/2 level. The quenching cross sections for 5p2 4P1/2 range from 1.6×10−16 cm2 for Ar to 37×10−16 cm2 for H2. The cross sections for quenching the 6p 2P state range from 0.96×10−16 cm2 for Ar to 18×10−16 cm2 for H2. The radiative lifetimes of the 5p2 4P1/2 and 6p 2P state were found to be 950 and 104 ns. Population ratios measured under steady-state conditions show that much of the excited-state population resides in the 6s 2S1/2 level after excitation of the 5p2 4P1/2 or 5d 2D levels.

Rotational relaxation of the A1π(ν′ = 13, J′ = 13+) state of 13C16O by various gases

Rotational relaxation of the A1π (ν′ = 13, J′ = 13+) state of 13C16O was studied in the presence of H2, D2, He, Ne, Ar, Xe, N2, CO, 13C16O, and 12C18O. The rotational relaxation cross sections Σσ13 → J′, in cm2 × 1016 and in the above order, were: 19 ± 1, 27 ± 1, 23 ± 1, 32 ± 1, 55 ± 2, 14 ± 1, 43 ± 3, 69 ± 5, 23 ± 1, and 100 ± 15. The rotational relaxation probabilities per gas-kinetic collision correlate with the square root of the reduced mass, except for Xe and the CO isotopic molecules. The latter molecules have several electronic states accessible at this energy. Their rotational relaxation cross sections depend on the magnitude of their respective electronic quenching cross sections, in a manner indicating preemption by fast electronic–electronic energy transfer.

Fluctuations of cross sections in chemical processes: Infrared photoquenching of fluorescent states in thiophosgene

Pronounced fluctuations have been observed in the cross section for the quenching of the fluorescence of the different members of the vibrational manifold belonging to the à 1A2 state of thiophosgene. This phenomenon is reminiscent of fluctuations that occur for nuclear reactions in regions of high level density, and that can be interpreted in terms of reduced transition amplitudes that are approximately normally distributed. The analysis of the fluctuations gives an effective number of independent reaction channels; in general, the greater the number of channels, the narrower is the range of the fluctuations. A knowledge of the number of independently contributing channels can be useful in the interpretation of experimental results, and sometimes lends important insight into reaction processes. The quenching cross sections for thiophosgene were therefore examined to see whether an analysis of their fluctuations would be useful. It was found necessary to develop a form of analysis that circumvents a serious obstacle in the chemical data, namely, the existence of significant correlations among the cross sections. The resulting formalism is quite simple to apply however, and, after suitable generalization, should be widely applicable to chemical experiments in which resolved cross sections that exhibit fluctuations are measured. Application of this formalism to the thiophosgene quenching cross sections shows that two hypotheses of the quenching mechanism must be rejected, but provides no reason to reject a third, that quenching is via absorption by states in a ‘‘bottleneck’’ region that is reached by rapid photon transitions from the fluorescent target states, which transitions cannot be statistically distributed. The analysis also reveals the existence of an important, experimentally determined constraint that must still be accounted for by any successful hypothesis, viz. the number of channels increases approximately as the square root of the peak power of the quenching infrared radiation.

Fluorescence quenching of tertiary amines by halocarbons

Abstract The quenching of fluorescence of a variety of tertiary amines by halocarbons has been investigated in both vapour and solution phases. In the vapour phase, quenching cross sections do not correlate well with a parameter based upon well-depth for collision complexes, but reasonable correlation between rate constant and free energy change for charge-transfer complex formation is obtained. The quenching by halocarbons is very efficient. In solution phase, evidence of static quenching is presented.

Quenching of the delayed fluorescence by charge carriers in crystals of anthracene—TCNB CT complex

The decrease in delayed fluorescence intensity in anthracene—tetracyanobenzene upon application of an electric field is investigated. It is shown, that photogenerated free carriers are responsible for the observed quenching and that the entity quenched is an intermediate state between a triplet-pair state and a fluorescent singlet state. A model is presented that identifies this state as a separated charge pair. This model can explain the main feature of the observations, namely, the large quenching cross section, which is of the order of cross sections typical of carrier—carrier interactions.

Measurements of lifetimes and quenching cross sections of the A 0u+state of130Te2by laser-induced fluorescence. Preliminary results

To study the modifications of lifetimes near perturbed levels observed for the A 0u+ state of 130Te2 the authors have made preliminary measurements concerning lifetimes of the unperturbed levels v'=16 (J=53, 87, 103) and v'=17 (J=103) at zero pressure and quenching cross sections by laser-induced fluorescence.

Rates of collision-induced electronic relaxation of single rotational levels of SO2 (Ã 1A2): Quenching mechanism by collision complex formation

The electronic quenching cross sections (σQ,M) of SO2 excited to single rotational levels (SRL’s) of the ‘‘B’’ band (J′=9, K′=9) and the ‘‘E’’ band (J′=6, K′=6) were measured from the shortening of the fluorescence decay times by the addition of a variety of nonpolar and polar collision partners (M) at room temperature. σQ,M for the B band SRL varied from 32 Å2 for M=He to 863 Å2 for M=CH3NO2. A similar variation of σQ,M for the E band SRL was also observed. Eighteen observed values of σQ,M are compared to the values calculated from various theoretical models of electronic quenching including collision complex formation. Temperature dependence of the electronic quenching cross section for the collision complex is presented.

Stereochemical effects in the quenching of Na*(3 2P) by CO: Crossed beam experiment and ab initio CI potential energy surfaces

Electronic to vibration-rotational-translational energy transfer in the quenching of Na(3 2P3/2) by CO has been studied with state of the art crossed atomic, molecular, and laser beam techniques at 0.16 eV initial kinetic energy, and by ab initio CI calculations for the potential energy surfaces involved in the process. Double differential quenching cross sections are measured as a function of scattering angle and energy transferred to the molecule. A pronounced structure in the energy transfer spectra as well as a partial backward scattering is attributed to two different mechanisms, a ’’direct’’ one and one which proceeds through ’’complex’’ formation. The observations are explained by the calculated potential energy surfaces (PES) for the first excited states à 2A′(Ã′ 2A″) and the ground state X̃ 2A′ which exhibit two crossing seams below the 2.1 eV excitation energy: (i) one for colinear approach of Na* on the carbon side of CO with its lowest energy 1.06 eV at Rc(Na–CO)= 5.5a0, rc(C–O) = 2.35a0 responsible for the direct process and (ii) one for colinear approach of Na* on the oxygen end of CO with 1.28 eV at Rc= 4.9a0 and rc= 2.47a0, allowing the quenching after ’’complex’’ formation. The angularly integrated cross sections are maximal (27 Å2/eV) at an energy transferred to the molecule equivalent to five vibrational quanta. Comparison with bulk data suggests strong rotational excitation (two vibrational quanta in the average) as can be rationalized from the anisotropy of the X̃ 2A′ PES near the crossing region. Total quenching cross sections and their temperature dependence can be explained by the absorbing sphere model using the calculated location and energy of the crossing seams.

Quenching of Na(3p2P) atoms by N2 and CO molecules at low temperatures

Quenching cross sections of Na(3 2P) by N2 and CO molecules at temperatures below 400 K have been measured by studying the dependence of the 3 2P lifetime on gas pressure. An original pulsed laser configuration has been employed to excite Na atoms. Lifetimes have been measured by the delayed coincidence method. From comparison of the present result with previous ones a linear dependence of quenching cross section on reciprocal mean kinetic energy can be inferred.