Quenching Cross Sections Research Articles

Temperature dependence of the Cd*(5 3P 1) quenching cross sections for collision with H 2 or D 2

The quenching cross sections for collision of Cd(5 3P 1) with H 2 or D 2 are measured in the range 430<T<900 K. They show a threshold behaviour, increasing by one order of magnitude in the studied temperature range, and a marked isotopic effect, being about two times greater for H 2 than for D 2.

Change In Total Angular Momentum Within The Z3F0 Titanium Excited StateInduced By Collisions With Hydrogen And Nitrogen Molecules

Collisions that produce a change in the total angular‐momentum and quenching processes involving the triplet Z3F0 titanium excited level and diatomic molecules have been studied by laser perturbation and time‐resolved spectroscopy. The thermally averaged total angular‐momentum changing cross sections in units of urn:x-wiley:02786273:media:lc030758:lc030758-math-0002 , respectively, for nitrogen and hydrogen colliding partners.Large quenching cross sections are obtained, as , and in units of 10-16 cm2. These results are discussed in relation with previous measurements dealing with titanium‐noble‐gas collisions, and with available published results.

Laser-induced fluorescence studies of methoxy radicals

Laser-induced fluorescence measurements have been performed on methoxy radicals in a discharge flow at room temperature. Rotationally resolved excitation scans in the /anti/A/sup 2/A/sub 1/-/anti/X/sup 2/E system showed features which can be qualitatively reproduced by spectrum simulations but not quantitatively matched on a line-by-line basis. Fluorescence scans showed distinct band intensity patterns and vibrational relaxation. A lifetime of 2.1 /plus minus/ 0.2 ..mu..s for the vibrationless level was affirmed and quenching cross sections for CO/sub 2/, O/sub 2/, N/sub 2/O, and CO were measured. A more precise value for the electronic origin, 31 620 cm/sup /minus/1/, was determined from excitation scans and spectrum simulations.

Formation of NH(A 3Π) in the quenching of NH(c 1Π) by Xe, NO, O 2 and Kr

The reactions of NH(c 1Π) have been investigated by using a pulsed-laser technique. The cross sections for the collision-induced intersystem crossing to produce NH(A 3Π) as well as the overall quenching cross sections are determined. Efficient production of NH (A) is recognized for Xe, NO and O 2. The efficiencies of the intersystem crossing by Xe, NO, O 2, Kr and N 2O have been measured to be 0.32, 0.44, 0.50, 0.03 and <0.03, respectively. These results are compared with those of previous studies. As for NO and N 2O, experiments have been performed not only under thermalized conditions but also under translationally hot conditions of NH(c 1Π).

Total Cross Sections for Collisional Quenching of H(2s) Atom in Molecular Targets

Total cross sections σ t for collisional quenching of H(2s) atoms in H 2 , N 2 , O 2 , CO, CO 2 and CH 4 molecular targets have been measured in the projectile energy range from 0.2 to 3.5 keV. A H(2s) atom beam was produced by using H + –K near-resonant charge transfer collisions, and σ t 's were determined with a beam attenuation method. It is demonstrated that, in the lower energy region (<0.5 keV) where the 2s-2p de-excitation process dominates, the long-range interaction due to polarization of a target molecule is more important than that due to its permanent quadrupole moment M 2 . This result is not consistent with theoretical treatments where M 2 is assumed to be more effective than the polarization. In the higher energy region (\({\gtrsim}0.5\) keV), the σ t curves behave in complex manners due to opening of other quenching processes such as electron-loss and electron-capture processes.

Quenching of the translationally hot and thermalized NH(c 1Π) radicals by HN 3

The cross section for the quenching of NH(c 1Π, ν = 0) by HN 3 was measured by using a pulsed laser technique. A single rotational level of NH(c 1Π, ν = 0) was formed by exciting NH(a 1Δ, ν = 0) with a frequency doubled dye laser. NH(a 1Δ) was produced by photolyzing HN 3 with a XeCl excimer laser. The time profiles of the NH(c-a) fluorescence were measured at various pressures of HN 3. Experiments were performed both in the presence and in the absence of He buffer gas. In the absence of He, the NH radicals were found to be translationally hot; the average velocity was 3800±600 m s −1. The quenching cross sections for the translationally hot and thermalized NH(c) radicals by HN 3 were determined to be (28±5) × 10 −16 and (85±3) × 10 −16 cm 2, respectively. No rotational level dependence could be observed in the quenching of the hot NH(c) radicals.

Quenching of two-photon-excited H(3s, 3d) and O(3p 3P 2,1,0) atoms by rare gases and small molecules

Measurements of the quenching rate coefficients for hydrogen atoms in the 3s 2S and 3d 2D states and of oxygen atoms in the 3p 3P state by rare gases and several combustion-relevant collision partners are presented. The excited atomic states are prepared by two-photon laser excitation. Quenching cross sections for most collision partners are found to be larger than classical collision cross sections; H atoms are mostly quenched faster than O atoms. For the quenching of H and O atoms by the rare gases, the rate coefficients increase significantly with the mass of the rare gas atom.

Intermultiplet and angular momentum transfers of excited sodium atoms in collisions with molecules. I. Experiment

The deexcitation processes of moderately excited sodium atoms have been studied in a pulsed crossed-beam experiment. The time resolution of the sodium beam fluorescence allows the simultaneous observation of intermultiplet and angular momentum transfers. The absolute calibration procedure is described and results are compared with those obtained with different experimental techniques. Partial quenching cross sections of the Na(4P) and Na(4D) states by He, Ar, H2, N2, O2, N2O, and SF6 are reported.

A comparison of two classical trajectory surface hopping methods for Na(2P)+H2,D2 quenching

Trajectory surface hopping calculations were performed on optimized diatomics-in-molecules surfaces to study Na(2P) collisions with H2 and D2 molecules (v=0, j=1) at four different translational energies (0.039, 0.062, 0.101, and 0.140 eV). Two methods were used to predict surface hopping: (1) transformation of the multidimensional surface intersection to a local one-dimensional curve crossing and calculation of the Landau–Zener transition probability, and (2) integration of the coefficients of the adiabatic electronic states to determine transition probability. For all initial conditions used in this work, we found that method (2) gave significantly larger quenching cross sections. Also in this paper we present results that show nonadiabatic coupling terms calculated by the diatomics-in-molecules method are in good agreement with ab initio values.

LIF study of CH A 2Δ collision dynamics in a low pressure oxyacetylene flame

Steady-state linear laser induced fluorescence (LIF) has been used to investigate internal energy redistribution rates in A 2 Δ v′ = 0 CH in a low pressure oxyacetylene flame. By obtaining rotationally resolved spectra as a function of pressure the branching ratio, defined as the ratio of rotational transfer ( R) out of the laser excited state divided by the electronic quenching rate ( Q), was measured for a variety of flame conditions. For a stoichiometric 1600K flame and K′ = 6 excitation R Q = 3.6 ± 0.5 . The branching ratio was also found to increase with K′ and decrease with equivalence ratio. In addition, for K′ = 6 excitation, a value of the electronic quenching cross section of 5.4 ± 3.6 Å 2 was obtained.

Branching ratios for chemical reaction in collisional quenching of excited 5s1 2S1/2 gallium atoms

We report cross sections for fluorescence quenching and branching ratios for chemical reaction in collisional deactivation of Ga(5s1 2S1/2) by added gases at room temperature following resonance 4p1 2P○1/2 →5s1 2S1/2 excitation at 403.30 nm with a pulsed laser. Total quenching cross sections are obtained from Stern–Volmer analysis of fluorescence lifetimes. Chemical contributions to fluorescence quenching are investigated by a pump and probe technique involving saturation of the resonance transition by a pump laser pulse to produce a known fraction of gallium atoms in the excited state, and measurement of the depletion of the gallium atom concentration due to chemical reaction in the excited state. Relative concentration measurements in the presence and absence of the saturating pump pulse are by resonance fluorescence excitation by a probe laser pulse suitably delayed relative to the pump pulse. An analysis of the experiment in terms of rate equations shows how branching ratios for chemical reaction may be obtained from the depletion measurements. Fluorescence quenching cross sections (Å2) are large for N2O (99±10), CO2 (100±20), CH4 (55±6), C2H6 (77±20), C3H8 (100±20), n-C4H10 (130±20), and C2H4 (75±20), moderately large for CO (11±4) and N2 (8±2), and very small for CF4 (&amp;lt;0.3) and H2 (&amp;lt;0.05). Among the efficient quenchers only C2H4 showed no detectable contribution from chemical quenching. Branching ratios for chemical removal of Ga(5 2S1/2) are N2O (0.96+0.04−0.1), CO2 (0.55±0.1), CH4 (0.27±0.07), C2H6 (0.33±0.1), C3H8 (0.26±0.1), and C2H4 (0.0+0.05−0.0). Results for H2 and the alkane hydrocarbons are discussed with reference to simple concepts of orbital interactions in the entrance channel.

Collisional quenching and energy transfer in NS B 2Π

Total collisional removal rate constants kd for the B 2Π excited electronic state of the NS free radical are measured for several collider molecules. For the lowest vibrational level (v′=0), kd is the electronic quenching rate constant; and for the vibrationally excited v′=1 and v′=6 levels, kd is the sum of those for electronic quenching and vibrational relaxation. Nitrogen sulfide free radicals are produced in a discharge flow reactor and the B 2Π state is prepared and monitored by laser-induced fluorescence. Measurements are made for nine different collision partners: He, N2, O2, SF6, N2O, H2, CH4, CO2, and NH3. Except for NH3, the thermally averaged quenching cross sections at room temperature are less than 10 Å2; this is a small value for electronic quenching of open shell diatomic radicals. For v′=6, kd is smaller than for v′=1 for N2, SF6, N2O, and CO2, but larger for H2, O2, and He. The vibrational relaxation pathway from v′=1 to v′=0 constitutes about 25% of the total v′=1 collisional removal for O2, N2O, and SF6.

Quenching of metastable hydrogen atoms by low energy collisions with hydrogen molecules: comparison of experiments with theory

The experimental velocity dependence of the quenching cross section of metastable H(2S) atoms in low energy collisions with hydrogen molecules is compared with theoretical calculations using the formalism of Gersten. This formalism takes into account possible changes of the molecular rotational energy and is in good agreement with experiments at low energies where other theories fail. A dependence of this cross section on the temperature of the molecular gas is predicted which is due to the temperature dependent population of the molecular rotational levels.

Bound-free fluorescence of rare gas hydrides

Bound-free fluorescence of HeH, NeH, ArH, KrH and XeH is reported. The excimers are formed via a photochemical reaction between H 2(C 1Π u, B 1Σ u +) and rare gases following excitation with synchrotron radiation. Broad fluorescence continua in the near UV are observed and assigned to the rare gas hydride transition B 2π→ X 2Σ +. In the case of NeH, the A 2Σ + state also contributes to the fluorescence spectrum. Experimentally derived binding energies for the B state and bound-free spectra of HeH and ArH are in good agreement with recent ab initio calculations. Radiative lifetimes, quenching cross sections and formation cross sections have also been determined.

Absolute Density Measurement of OH Radicals in an Atmospheric Pressure Flame Using Time-Resolved Laser-Induced Fluorescence

We determined the absolute density of OH radicals in a propane-air flame using laser-induced fluorescence (LIF). To obtain the quantum efficiency of the fluorescence, the quenching lifetime of the A2Σ+ (υ'=0) level in the flame was measured with a time-correlated single-photon counting method using a picosecond dye laser. The efficiency of the fluorescence detection system was calibrated by a Rayleigh scattering measurement. The measured quenching rate, 5.6×108 s-1, was in agreement with a previous value deduced from the quenching cross sections at a high temperature. The number density obtained in the present work is consistent with the results of the absorption measurement.

Collisional quenching of metastable ions by diatomic molecules

The deexcitation (quenching) of He+ ions in the 2s metastable state by collision with H2, N2, O2, CO, and NO molecules is studied using the time dependent perturbation theory. Velocity dependence of quenching cross section is presented. The results are compared with the available experimental data of Prior and Wang.

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

The total quenching cross sections for strontium (5s6p 1p 1) collisions with the rare gases are determined by pulsed laser, time-resolved fluorescence measurements to be 80, 40, 73, 101, 140 Å 2 for He, Ne, Ar, Kr and Xe, respectively. For collisions with Kr, all of the near-resonant states, 4d5p 3F, 5s6p 3P and 4d5p 1D 2, 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.

Quenching of 6s6d and 6s7d Hg atoms by Hg *-Hg, Hg *-Ar and Hg *-N 2 collisions

The radiative decays and collisional quenching of Hg 6 1 D 2, 6 3 D 2, 7 1 D 2 and 7 3 D 2 atoms were investigated by time-resolved methods. Hg vapour, pure or mixed with a buffer gas in a fluorescence cell, was irradiated with u.v. light pulses from a frequency-doubled N 2 laser-pumped dye laser, populating selectively each state in turn by 2-photon absorption from the 6 1 S 0 ground state. Fluorescent components arising from the decays to the 6 1 P 1 or 6 3 S 0 state were resolved with a spectrometer and their time evolutions recorded with a transient digitizer or time-to-amplitude converter and pulse-height analyzer, yielding the respective decay rates. The quenching cross sections were obtained from variations of measured decay rates with Hg or buffer-gas density.

Collisional quenching of excited vinylidene (3B2) radicals

Rate constants for the removal of excited-state vinylidene D/sub 2/CC (/sup 3/B/sub 2/) in the presence of He, Ar, N/sub 2/, H/sub 2/, CO, and CH/sub 4/ are reported at room temperature. The excited vinylidene radicals are generated via vacuum-ultraviolet photolysis of mixtures of C/sub 2/D/sub 3/Cl in the quencher gases through the reaction sequence C/sub 2/D/sub 3/Cl..-->..C/sub 2/D/sub 3/Cl*; C/sub 2/D/sub 3/Cl*..-->..D/sub 2/CC(/sup 3/B/sub 2/) + DCl. Excited triplet vinylidene radicals are then collisionally quenched to the singlet ground state which undergoes rapid isomerization to acetylene. Rate constant for removal of D/sub 2/C (/sup 3/B/sub 2/) by quenchers are determined by observation of the time-resolved absorption of either D/sub 2/CC(/sup 3/B/sub 2/) at 137 nm or product C/sub 2/D/sub 2/ at 151 nm. The observed quenching rate constants for the deuterated species are k/sub He/ = (1.9 +/- 0.3) x 10/sup -15/, k/sub Ar/ = (6.64 +/- 0.3) x 10/sup -15/, k/sub N/sub 2// = (8.5 +/- 0.2) x 10/sup -15/, k/sub H/sub 2// = (33.8 +/- 6.1) x 10/sup -15/, k/sub CO/ = (28.0 +/- 8.0) x 10/sup -15/, and k/sub CH/sub 4// = (28.0 +/- 8.0) x 10/sup -15/ cm/sup 3/ molecule/sup -1/ s/sup -1/. No evidencemore » for chemical reaction could be observed. The derived quenching cross sections are correlated with the potential well depth model. This correlation is valuable in predicting nonreactive collisional cross sections for additional gases.« less

Time resolved study of the luminescence produced by the pulse radiolysis of D2O vapor

D2O vapor was irradiated using a 0.53 MeV pulsed electron beam and the 200–900 nm wavelength range was examined for luminescence emission from excited fragments. Spectral lines of the OD (C 2∑+→A 2∑+) and the OD (A 2∑+→X 2Π) transitions were observed, as were the Balmer transitions of deuterium between n=6→n=2 and n=3→n=2, and the oxygen transitions O+(3p 4D0→3s 4P),O(3p 5P→3s 5S0), and O(3p 3P→3s 3S0). Values were obtained for the natural lifetimes, quenching rate constants, and quenching cross sections at 298 K for the majority of the above excited species. The natural lifetimes and quenching data obtained for the transitions of excited OD and oxygen species were, as expected, similar to the values obtained for the corresponding species in H2O vapor. However, Balmer D transitions exhibited shorter natural lifetimes and smaller quenching cross sections than the corresponding Balmer H transitions. The differences in quenching rate were much larger than the effect of isotopic substitution on collision frequency. Both lifetime and quenching differences were attributed to greater population of the p sublevels in excited D as compared with H.