Shaun A. Carl

KU Leuven

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Jozef Peeters

KU Leuven

Minh Tho Nguyen

Van Lang University

Fernando Castaño

AMS (Austria)

David Husain

University of Cambridge

Marı́a N. Sánchez Rayo

University of the Basque Country

Rehab M. I. Elsamra

Alexandria University

Bart Dils

Royal Belgian Institute for Space Aeronomy

Hue Minh Thi Nguyen

Hanoi National University of Education

Thanh Lâm Nguyễn

University of Florida

John N. Crowley

Max Planck Institute for Chemistry

Cooperative Institutions

KU Leuven

Friedrich-Alexander-Universität Erlangen-Nürnberg

Max Planck Society

Max Planck Institute for Chemistry

University of Cambridge

University of the Basque Country

Vietnam National University, Hanoi

Forschungszentrum Jülich

University of Leeds

University of Edinburgh

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Rate Constant and Mechanism of the Reaction between Cl and CH₃OCl at 295 K

The Journal of Physical Chemistry (1996)

Shaun A. Carl M. Roehl Rolf Müller G. K. Moortgat John N. Crowley

The reaction between Cl atoms and CH3OCl was investigated at 295 K in both air and N2 bath gases at total pressures between 100 and 750 Torr by the relative rate method. The rate constant of the title reaction was found to be a factor 1.07 ± 0.02 (2σ) greater than that of Cl + C2H6 at room temperature and independent of pressure between 100 and 750 Torr. This yields a rate constant of (6.1 ± 0.6) × 10-11 cm3 molecule-1 s-1. The products of the reaction were detected by FTIR and UV absorption spectroscopy. Analysis of Cl2 and HCl products allowed branching ratios of 0.2 ± 0.1 for HCl + CH2OCl formation and 0.8 ± 0.2 for Cl2 + CH3O formation to be determined. The high rate constant implies that reaction with Cl atoms is an important loss process for CH3OCl in the polar stratosphere.

Torr

Reaction rate

10.1021/jp9611075

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Citations (14)

Kinetics of O(1D) + H2O and O(1D) + H2: absolute rate coefficients and O(3P) yields between 227 and 453 K

Physical Chemistry Chemical Physics (2010)

Stijn Vranckx Jozef Peeters Shaun A. Carl

The rate coefficients for the crucial atmospheric reactions of O((1)D) with H(2)O and H(2), k(1) and k(2), were measured over a wide temperature range using O((1)D) detection based on the chemiluminescence reaction of O((1)D) with C(2)H. Analyzing the decays of the chemiluminescence intensities yielded a value for k(1)(T) of (1.70 x 10(-10)exp[36 K/T]) cm(3) s(-1). Multiplying or dividing k(1)(T) by a factor f(T) = 1.04 exp(5.59(|1 K/T- 1/287|)), gives the 95% confidence limits; our new determination, in good agreement with previous studies, further reduces the uncertainty in k(1). An extended study of k(2) yielded a temperature independent rate constant of (1.35 +/- 0.05) x 10(-10) cm(3) s(-1). This precise value, based on an extended set of determinations with very low scatter, is significantly larger than the current recommendations, as were two other recent k(2) determinations. Secondly, the fractions of O((1)D) quenched to O((3)P) by H(2)O and H(2), k(1b)/k(1) and k(2b)/k(2), were precisely determined from fits to chemiluminescence decays. A temperature-independent value for k(1b)/k(1) of 0.010 +/- 0.003 was found. For the quenching fraction k(2b)/k(2) a value of 0.007 +/- 0.007 was obtained at room temperature. Both determinations are significantly smaller than values and upper limits from previous studies.

Atmospheric temperature range

10.1039/b923959f

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Citations (32)

Kinetic Investigation of the Time‐Resolved Atomic Fluorescence Ca(4³P₁ → 4¹S₀) and Molecular Chemiluminescence CaI(A²Π, B²Σ⁺ → X²Σ⁺) Following the Pulse‐Dye Laser Generation of Ca(4³P_J) in the Presence of CH₃I

Berichte der Bunsengesellschaft für physikalische Chemie (1991)

F. J. Basterrechea Fernando Beitia Fernando Castaño Marı́a N. Sánchez Rayo Shaun A. Carl

Abstract The collisional behaviour of the optically metastable, electronically excited calcium atom, Ca(4s4p( 3 P 1 )), 1.888 eV above its 4s 2 ( 1 S 0 ) electronic ground state, is investigated in the time‐resolved mode following its generation by pulsed dye‐laser excitation at elevated temperature from calcium vapour in the presence of CH 3 I and excess helium buffer gas in a slow flow system, kinetically equivalent to a static system. The removal of Ca(4 3 P 1 ) is monitored by time‐resolved atomic fluorescence at the resonance wavelength ( λ = 657.3 nm, Ca(4 3 P 1 ) → Ca(4 1 S 0 ) + hv ), following rapid Boltzmann equilibration within the 4 3 P 1 spin‐orbit manifold using boxcar integration. Electronically excited CaI in the A 2 Π and B 2 Σ + states, respectively 1.9372 (186.9 kJ mol −1 ) and 1.9466 eV (187.8 kJ mol −1 ) above the X 2 Σ + ground state, was also monitored by time‐resolved molecular chemiluminescence of the A, BX systems under identical conditions to those employed for characterising the decay profiles of Ca(4 3 P 1 ). Both atomic and molecular chemiluminescence emissions showed exponential decay profiles, characterised by first‐order decay coefficients which were found to be equal under the same experimental conditions. CaI(A 2 II) and CaI(B 2 Σ + ) are thus shown to arise from direct production on the collision of Ca(4 3 P 1 ) with CH 3 I where these processes are energetically favourable: As with complementary measurements that have been reported on the collisional behaviour of Ca(4 3 P J ) with CH 3 I under single‐collision conditions in molecular beams, where the AX and BX chemiluminescence of CaI was recorded, it was not possible to resolve the AX and BX profiles individually in view of spectroscopic overlap between these two close lying electronic states where vibrational population in higher levels of the A and B states at these elevated temperatures (900 K) also complicates the chemiluminescence spectra, as well as the spin‐orbit components in CaI(A 2 II 1/2, 3/2 ). Time‐resolved chemiluminescence from CaI(A, BX) is hence employed the to monitor the global emission from these two electronically excited states. Thus, the A, BX emission can be used as an overall spectroscopic marker for Ca(4 3 P J ) in the time‐domain and can be used to estimate a reaction rate constant for the overall collisional removal of Ca(4 3 P J ) by CH 3 I of ca. 2 · 10 −11 cm 3 molecule −1 s −1 at T = 900 K though this must be viewed with caution on account of reaction of ground state Ca(4 1 S 0 ) with CH 3 I in the flow system prior to dye laser excitation. Broad band chemiluminescence is recorded for the Ca(4 3 P 1 )CH 3 I system as well as laser‐induced fluorescence by excitation of the X 2 Σ + state which is also generated on reaction of the excited atom. The principal conclusion of the present investigation in the time‐domain is the direct production of CaI(A 2 Π, B 2 Σ + ) on the collision of Ca(4 3 P 1 ) with CH 3 I, in accord with complementary observations that have been made hitherto under single‐collision conditions and which are compared with the present investigation. The present results for CaI(A 2 Π, B 2 Σ + ) are further compared with chemiluminescence measurements for analogous collisional processes for Ca(4 3 P 1 ) involving O, H, F, Cl and Br‐atom abstraction in the time‐domain and under molecular beam conditions where the complementary comparison is possible.

Metastability

10.1002/bbpc.19910951205

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INFLUENCE OF STELLAR FLARES ON THE CHEMICAL COMPOSITION OF EXOPLANETS AND SPECTRA

The Astrophysical Journal (2016)

Olivia Vénot M. Rocchetto Shaun A. Carl A. Hashim L. Decin

More than 3000 exoplanets have been detected so far, and more and more spectroscopic observations of exoplanets are performed. Future instruments are eagerly awaited as they will be able to provide spectroscopic data with a greater accuracy and sensitivity than what is currently available. An important aspect to consider is temporal stellar atmospheric disturbances that can influence the planetary composition, and hence spectra, and potentially can lead to incorrect assumptions about the steady-state atmospheric composition of the planet. We focus on perturbations that come from the host star in the form of flare events that significantly increase the photon flux impingement on the exoplanet atmosphere. In some cases, and particularly for M stars, this sudden increase may last for several hours. We aim at answering the question to what extent a stellar flare is able to modify the chemical composition of the planetary atmosphere and, therefore influence the resulting spectra. We use a 1D thermo-photochemical model to study the neutral atmospheric composition of two hypothetic planets located around the star AD Leo. This active star has already been observed during a flare. We use the spectroscopic data from this flare event to simulate the evolution of the chemical composition of the atmospheres of the two hypothetic planets. We compute synthetic spectra to evaluate the implications for observations. The increase of the incoming photon flux affects the chemical abundances of some important species down to altitudes associated with an atmospheric pressure of 1 bar, that can lead to variations in planetary spectra if performed during transit.

Flare

10.3847/0004-637x/830/2/77

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Citations (29)

Early Aggregation Steps in α-Synuclein as Measured by FCS and FRET: Evidence for a Contagious Conformational Change

Biophysical Journal (2010)

Sangeeta Nath Jessika Meuvis Jelle Hendrix Shaun A. Carl Yves Engelborghs

Fluorescence Correlation Spectroscopy

10.1016/j.bpj.2009.12.4290

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Citations (148)

ELECTRONIC-ENERGY DISTRIBUTION IN SRF BY INVESTIGATION OF THE MOLECULAR CHEMILUMINESCENCE SRF(A(2)PI(1/2,3,2)B(2)SIGMA(+)-]X(2)SIGMA(+)) AND THE ATOMIC RESONANCE FLUORESCENCE SR(5(3)P(1)-5(1)S(0)) IN THE TIME-DOMAIN FOLLOWING THE PULSED DYE-LASER GENERATION OF SR(5(3)P(J)) IN THE PRESENCE OF CH3F

Journal of the Chemical Society. Faraday Transactions II (1995)

S. Antrobus Shaun A. Carl David Husian Jie Lei Fernando Castaño

Sigma

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Experimental and theoretical study of the reaction of the ethynyl radical with nitrous oxide, C2H + N2O

Physical Chemistry Chemical Physics (2012)

Vinh Sơn Nguyễn Rehab M. I. Elsamra Jozef Peeters Shaun A. Carl Minh Tho Nguyen

We investigated the rate constants and reaction mechanism of the gas phase reaction between the ethynyl radical and nitrous oxide (C2H + N2O) using both experimental methods and electronic structure calculations. A pulsed-laser photolysis/chemiluminescence technique was used to determine the absolute rate coefficient over the temperature range 570 K to 836 K. In this experimental temperature range, the measured temperature dependence of the overall rate constants can be expressed as: k(T) (C2H + N2O) = 2.93 × 10−11 exp((−4000 ± 1100) K/T) cm3 s−1 (95% statistical confidence). Portions of the C2H + N2O potential energy surface (PES), containing low-energy pathways, were constructed using the composite G3B3 method. A multi-step reaction route leading to the products HCCO + N2 is clearly preferred. The high selectivity between product channels favouring N2 formation occurs very early. The pathway corresponds to the addition of the terminal C atom of C2H to the terminal N atom of N2O. Refined calculations using the coupled-cluster theory whose electronic energies were extrapolated to the complete basis set limit CCSD(T)/CBS led to an energy barrier of 6.0 kcal mol−1 for the entrance channel. The overall rate constant was also determined by application of transition-state theory and Rice–Ramsperger–Kassel–Marcus (RRKM) statistical analyses to the PES. The computed rate constants have similar temperature dependence to the experimental values, though were somewhat lower.

Nitrous oxide

Potential energy surface

Transition state theory

Atmospheric temperature range

10.1039/c2cp40367f

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Citations (9)

Kinetic investigation of the time-resolved atomic fluorescence Ca(4³P₁→ 4¹S₀) and molecular chemiluminescence CaCl(A²Π, B²Σ⁺→ X²Σ⁺) following the pulsed dye-laser generation of Ca(4³P_J) in the presence of CH₃Cl

Journal of the Chemical Society Faraday Transactions (1991)

Fernando Beitia Fernando Castaño Marı́a N. Sánchez Rayo Shaun A. Carl David Husain

We present a kinetic study of the collisional behaviour of the optically metastable, electronically excited Ca[4s4p(3PJ)], 1.888 eV above its 4s2(1S0) electronic ground state, with molecular CH3Cl. Ca[4s4p(3P1)] was generated by pulsed dye-laser excitation of calcium vapour at elevated temperatures (900 K) and in the presence of CH3Cl and an excess of helium buffer gas at λ= 657.3 nm [Ca(4s4p(3P1)]â†� Ca[4s2(1S0)] in a slow-flow system, kinetically equivalent to a static system. Atomic fluorescence at the resonance wavelength following rapid Boltzmann equilibration within the Ca(4 3PJ) spin–orbit manifold was monitored using boxcar integration and exhibited exponential decay profiles which were characterised. Time-resolved chemiluminescence was also monitored under identical conditions for both the CaCl(A 2Π1/2, 3/2→ X 2Σ+) and CaCl(B 2Σ+→ X 2Σ+) systems, principally via the (0,0) transitions, on account of Franck–Condon limitations for which the X 2Σ+, A 2Π1/2, 3/2(2.000 eV) and B 2Σ+(2.089 eV) states are characterised by internuclear separations which are close in magnitude. These emissions also exhibit single-exponential decay profiles which are equal in magnitude to those from Ca(4 3PJ). In the case of the A–X chemiluminescence it was possible under some conditions to resolve the spin–orbit components of the (0, 0) transitions for A 2Π1/2 and A 2Π3/2→ X 2Σ+, whose exponential decay coefficients were also found to be equal in magnitude and thus indicating that these also are in equilibrium during the kinetic decays. The exponential form and equality in the first-order decay coefficients for the atomic and both A–X and B–X chemiluminescence profiles are in accord with a mechanism involving the production of these electronically excited states by direct reaction: Ca(4 3PJ)+ CH3Cl → CaCl(A 2Π1/2, 3/2v′⩽ 9)+ CH3(k2)(2), Ca(4 3PJ)+ CH3Cl → CaCl(B 2Σ+v′⩽ 7)+ CH3(k3)(3) Generation of the A 2Π and B 2Σ+ states from collision between Ca(4 3PJ) and CaCl(X 2Σ+), which is detected by laser-induced fluorescence, could only involve higher vibrational levels of the ground state, namely, v 3 and v 5, respectively. It is concluded that these (E, V–E) processes, involving the steady concentrations of CaCl(X 2Σ+) from the reaction of Ca(4 1S0)+ CH3Cl in the flow, would make only a small contribution to the generation of the A and B states, characterised by the same time dependences as observed in chemiluminescence, but whose contribution cannot be totally ruled out; a similar mechanism from CaCl(X 2Σ+) generated in the time domain from the reaction between Ca(4 3PJ) and CH3Cl would yield double-exponential molecular chemiluminescence decay profiles, which are not observed. The results for reactive channels into specific electronic states involving halogen abstraction are compared with analogous processes for Ca(4 3PJ) with O-atom and H-atom abstraction in the time domain, and halogen atom abstraction studied in molecular beams.

Metastability

Exponential decay

10.1039/ft9918702413

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Citations (10)

Rate Coefficients for Reaction of OH with Acetone between 202 and 395 K

The Journal of Physical Chemistry A (2000)

M. Wollenhaupt Shaun A. Carl A. Horowitz John N. Crowley

The kinetics of the title reaction were investigated between 202 and 395 K and at 20, 50, and 100 Torr of Ar or N2 bath gas using pulsed laser photolysis (PLP) generation of OH combined with both resonance fluorescence (RF) and laser-induced fluorescence (LIF) detection. OH was generated either by the sequential 439 nm, two-photon dissociation of NO2 in the presence of H2, or by HONO photolysis at 351 nm. The accuracy of the rate constants obtained was enhanced by optical absorption measurements of acetone concentrations both before and after the photolysis reactor. The temperature dependence is not described by a simple Arrhenius expression but by k1 (202−395 K) = 8.8 × 10-12 exp(−1320/T) + 1.7 × 10-14 exp(423/T) cm3 s-1, indicating that a simple H atom abstraction may not be the only reaction mechanism. The estimated total error (95% confidence) associated with the rate coefficient derived from this expression is estimated as 5% and is independent of temperature. The curvature in the Arrhenius plot results in a significantly larger rate coefficient at low temperatures than obtained by extrapolation of the previous measurement and implies greater significance for the reaction with OH as a sink for acetone in the upper troposphere than presently assumed.

Arrhenius plot

Reaction rate

Flash photolysis

10.1021/jp993738f

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Citations (158)

Kinetic Investigation of Chlorine Atom-Abstraction Reactions by Ground-State Atomic Rubidium, Rb(5²S_½), by Time-resolved Laser-induced Fluorescence [Rb(5²P_3/2 – 5²S_½); λ = 780 nm] Following Pulsed Irradiation

Zeitschrift für Physikalische Chemie (1998)

Shaun A. Carl K. M. Nalin De Silva D. Husain

Rubidium

10.1524/zpch.1998.203.part_1_2.113

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Citations (3)