Maria N. Sanchez Rayo

Electron-impact induced light emission from CF2Br2 and CF2H2

Abstract A study is presented of the optical emissions, lifetimes, emission thresholds and excitation functions in the visible and ultraviolet regions of the spectrum (λ = 200–600 nm) for atomic and molecular fragments derived from the electon-impact dissociation of CF 2 Br 2 and CF 2 H 2 . A range of products have been monitored including molecular fragments such as CH, CH + and CF which are expected and also CF 2 from CF 2 H 2 and some ion-pair states of molecular bromine from CF 2 Br 2 . In contrast to photodissociation products observed hitherto in the 11–35 eV region, the electron-dissociation investigations of these halomethanes are found to yield highly excited atoms and atomic ions. In some cases comparisons between appearance potentials and spectroscopic transitions, combined with published thermochemical data, have led to unique assignments of dissociation channels.

Time-dependent study of the dynamics of the collision-induced intramultiplet mixing of Ca(4s4p(3Pj)) by helium at 750 K following pulsed dye-laser generation of Ca (4s4p(3P1))

Abstract We present a kinetic study in the time-domain of the collisionally-induced intramultiplet mixing within Ca(4s4p(3PJ)), 1.888 eV above the 4s2(1S0) electronic ground state. Ca(4s4p(3P1)) was generated by the pulsed dye-laser excitation of calcium vapour at λ=657.3 nm (Ca(4s4p(3P1))←Ca(4s2(1S0)) at elevated temperature (T=750 K). The time-dependent evolution of the concentrations of the individual spin-orbit states, Ca(4 3P0,1,2), where the J=0–1 and 1–2 separations are 52.2 and 105.8 cm−1, were monitored in the presence of helium by laser-induced fluorescence fluorescence (LIF) of the 4s5s(3S1)-4s4p(3PJ) transitions using a second dye-laser, delayed by means of varying optical path length. The resonance transitions at λ=610.3, 612,2 and 616.2 nm (3S1-3P0,1,2, respectively) were normalized, to allow for variations in laser output, and monitored in the time regime 0–50 ns for pressures of He in the range 1–15 Torr, during which diffusion and spontaneous emission from Ca(3P1) could be totally neglected, the 3P0,2 states being, so-called, “reservoir states”. The rate equations for [Ca(3P0,1,2)] were solved analytically and then convoluted numerically with the digitized forms of the 1S-3P excitation laser and the LIF probe laser. Absolute rate data for the collisionally-induced spin-orbit processes in both directions, J=2-1, 1-0, 2-0, and the reverse, are reported. These results, determined from concentration profiles for Ca(3P0,1,2) determined in the time-domain for fixed pressures of He, are compared with analogous cross section data for Ca(3PJ)+He determined from molecular beams using J state selection optical pumping laser methods at a fixed time with varying low pressures of He and also with the results of recent quantum close-coupling calculations.

Preparation of CHF (1A′) by infrared multiphoton dissociation and reactions with alkenes

A study of the vibrational, and translational energies of the CHF (X1 A′) radical prepared by infrared multiphoton dissociation, IRMPD, is presented. The vibrational and rotational temperatures measured near the CO2 laser pulse peak depend on the delay, nature and pressure of the buffer gas. For pure precursor (20 mTorr) and at delays of 0.4 and 3 µs the measured rotational temperatures were 900 and 600 K, respectively. Vibrational temperatures at 3 µs delay for samples of pure precursor (20 m Torr) and of precursor with 1 Torr of Ar were 790 and 630 K.The transient migration method was applied to measure the diffusion coefficient, and hence the collisional diameter, of CHF (X1A′) in Ar. Attempts to measure the average translational energy of the CHF fragment by the same method at low pressures produced extremely low temperatures, and forced a re-examination of the validity of the technique. The average vibrational relaxation rate constant, as determined by the same method and confirmed by direct measurements, gives kVT 10–10 cm3 molecule–1 s –1.The bimolecular rate constants for removal of CHF (X1A′) with several alkenes are reported to be (/10–12 cm3 molecule–1 s–1) : kethene= 5.4 ± 0.3; kpropene= 13.0 ± 1.0; kbutene= 9.5 ± 2.0; kisobutene= 17.0 ± 2; kbutadiene= 22.0 ± 3.0. Some of the reactions are CO2 laser fluence dependent, and, in addition, butadiene gives a considerable luminescence. A method is suggested for extrapolation of the apparent kinetic constants to zero fluence to obtain meaningful results.

Kinetic investigation of the collisional quenching of Ca(4 3Pj) by C4H10 at elevated temperatures (750–923 K). Time-resolved atomic emission and electronic energy transfer between Ca(4 3PJ) and CaH(X 2Σ+) observed by Molecular Chemiluminescence

A detailed investigation is presented of the collisional removal of electronically excited calcium atoms, Ca[4s4p(3PJ)], 1.888 eV above its 4s2(1S0) ground state, with n-butane at elevated temperatures in a slow-flow system, kinetically equivalent to a static system. Ca(4 3PJ) was generated by pulsed dye-laser excitation of calcium vapour at λ= 657.3 nm [Ca(4 3P1)â†� Ca(4 1S0)] and monitored by time-resolved atomic fluorescence at the resonance wavelength over timescales during which Boltzmann equilibration had taken place with the 3PJ spin–orbit manifold. Absolute second-order rate constants for the collisional removal of Ca(4 3PJ) by butane were measured over the temperature range 750–923 K, yielding the following Arrhenius form: kR=(3.5+3.6–1.8)× 10–9 exp(– 58.7 ± 5.0 kJ mol–1/RT) cm3 molecule–1 s–1, The results are compared with analogous data derived from our previous investigations on the collisional removal of Mg(3 3PJ) by various hydrocarbons. Molecular chemiluminescence for the system CaH(A 2Π–X 2Σ+) was also monitored in the time domain following excitation of Ca(4 3PJ) and shown to arise from electronic energy transfer from Ca(4 3PJ) to ground-state CaH(X 2Σ+) both from its bi-exponential time dependence and the observation of CaH(X 2Σ+), generated in the reaction, by laser-induced fluorescence (LIF). CaH(X 2Σ+) was also detected by LIF from the reaction of Ca(4 1S0) in the flow, and the source of this is discussed. These CaH(A–X) chemiluminescence profiles which provide a spectroscopic marker for Ca(4 3PJ) in this type of system, coupled with the LIF observations on CaH(X) and constituting, to the best of our knowledge, the first observation of its type, are compared with analogous observations reported previously where molecular chemiluminescence from CaO is employed as a spectroscopic marker for Ca(4 3PJ) with oxidising reactants.

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(43PJ) in the presence of CH3Cl

A kinetic study of the vibronic energy distribution in SrCl following the collision of the optically metastable, electronically excited strontium atom, Sr[(5s)1(5p)1, 3PJ], 1.807 eV above its (5s)2, 1S0 electronic ground state, in the presence of CH3Cl and CF3Cl is presented. This is investigated in the time-resolved mode following the generation of the excited atom by pulsed dye-laser excitation at elevated temperature from ground-state strontium vapour in the presence of reactants and an excess of helium buffer gas in a slow-flow system, kinetically equivalent to a static system. The decay of Sr(5 3PJ) is monitored by time-resolved atomic fluorescence at the resonance wavelength [λ= 689.3 nm, Sr(5 3P1)→ Sr(5 1S0)+hν], following rapid Boltzmann equilibrium within the 5 3PJ spin–orbit manifold using boxcar integration. Electronically excited SrCl in the A 2Π1/2, 3/2 and B 2Σ+ states, respectively 1.855 eV (179.0 kJ mol–1) and 1.948 eV (188.0 kJ mol–1) above the X 2Σ+ 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 Sr(5 3PJ). 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 within experimental error. SrCl(A 2Π) and SrCl(B 2Σ+) are thus shown to arise from direct production on the collision of Sr(5 3PJ) with CH3Cl and CF3Cl where these processes are energetically favourable. These results from measurements in the time-domain are considered with analogous data for Ca(4 3PJ) collisions involving F, Cl, Br and I atom abstraction reactions and with chemiluminescence from SrCl(A, B–X) systems in the single-collision condition following the reaction of Sr(5 3PJ) with various Cl-containing molecules.

Kinetic investigation of the time-resolved atomic fluorescence Ca(43P1→41S0) and molecular chemiluminescence CaF(A2Π→X2Σ+) following the pulse-dye laser generation of Ca(43PJ) in the presence of CH3F

Abstract A time-resolved investigation is presented of the collisional behaviour of the optically metastable, electronically excited Ca(4s4p(3PJ)), 1.888 eV above its 4s2(1S0) electronic ground state, with the molecule CH3F. Ca(4s4p(3P1)) was generated by the pulsed dye-laser excitation of calcium vapour at an elevated temperature in the presence of CH3F and excess helium buffer gas and the resulting atomic fluorescence at the resonance wavelength (λ=657.3 nm, Ca(4s4p(3P1))→Ca(4s2(1S0)+hv)) was monitored, following rapid Boltzmann equilibration within the Ca(43PJ) spin-orbit manifold, using boxcar integration. The electronically excited atom exhibited exponential decay profiles which were characterized quantitatively. Time-resolved chemiluminescence was also monitored under identical conditions for the CaF(A2Π→X2Σ+) system, principally via the (0,0) transition, on account of Franck—Condon limitations. This molecular emission exhibits decay profiles which are also exponential in form, characterized by decay coefficients which are equal in magnitude to those from Ca(43PJ), and arising from chemiluminescence following the direct reaction Chemiluminescence from CaF(B2Σ+) via the B2Σ+−X2Σ+ system, which is also thermochemically accessible on collision between Ca(43P)+CH3F, was not detected. The laser-induced fluorescence spectrum, CaF(A2Π 1 2 , 3 2 −X2Σ+), following excitation of the X2Σ+ ground state, where the A2Π 1 2 , 3 2 spin-orbit components were, of course, resolved, was also monitored following the reaction of Ca(43PJ)+CH3F. However, these spin-orbit components could not be resolved in chemiluminescence on account of intensity restrictions at the optical resolution required. These results for reaction leading to CaF(A2Π) with the subsequent chemiluminescence are compared with analogous collisional processes for Ca(43PJ) involving oxygen, hydrogen, chlorine and bromine atom abstraction processes in the time-domain, and halogen atom abstraction studied in molecular beams.

Removal rates for the collisional quenching of various vibronic levels of ground state NCO by simple molecules (N2,O2, NO, CO2,N2O, and SO2)

The collisional behavior of NCO[X(0,n,0)] in specific vibronic states in the gas phase has been investigated in the time-domain by laser-induced fluorescence (LIF) on transitions within the system NCO(A 2Σ+–X 2Π). The NCO radical was generated by the infrared multiphoton dissociation (IRMPD) of phenyl isocyanate (PhNCO) by means of a TEA-CO2 laser operating on the 9R24 line at λ=9.25 μm with subsequent monitoring of the vibronic levels of the X state, characterized by Renner–Teller coupling, in the presence of N2, O2, NO, CO2, N2O, SO2, and PhNCO itself. The states probed were as follows: (0010)2Π3/2, (0010)2Π1/2, (0100)μ2Σ+, (0120)2Δ5/2, (0120)2Δ3/2, (0210)μ2Π3/2,1/2, (0230)2Φ7/2, and (0230)2Φ5/2. Various pairs of spin–orbit states were found to be tightly coupled kinetically. Thus, the time-evolution of the pairs of vibronic states (0010)2Π3/2 and (0010)2Π1/2; (0120)2Δ5/2 and (0120)2Δ3/2; (0230)2Φ7/2 and (0230)2Φ5/2 were found to be equal, yielding an effective local equilibrium within these spin–orb...

Absolute electron-impact total ionization cross sections of chlorofluoromethanes.

An experimental study is reported on the electron-impact total ionization cross sections (TICSs) of CCl4, CCl3F, CCl2F2, and CClF3 molecules. The kinetic energy of the colliding electrons was in the 10-85 eV range. TICSs were obtained as the sum of the partial ionization cross sections of all fragment ions, measured and identified in a linear double focusing time-of-flight mass spectrometer. The resulting TICS profiles--as a function of the electron-impact energy--have been compared both with those computed by ab initio and (semi)empirical methods and with the available experimental data. The computational methods used include the binary-encounter-Bethe (BEB) modified to include atoms with principal quantum numbers n> or =3, the Deutsch and Märk (DM) formalism, and the modified additivity rule (MAR). It is concluded that both modified BEB and DM methods fit the experimental TICS for (CF4), CClF3, CCl2F2, CCl3F, and CCl4 to a high accuracy, in contrast with the poor accord of the MAR method. A discussion on the factors influencing the discrepancies of the fittings is presented.

Direct measurements of removal rates of electronically excited NCO(Ã2Σ+(0,0,0)) by simple molecules

Abstract Direct measurements of absolute rate constants are reported for the collisional removal at room temperature of NCO ( A 2 Σ + (0,0,0)) by O 2 ( 3 Σ g ), N 2 ( 1 Σ + g ), NO ( 2 Π), CO 2 ( 1 Σ + g ), N 2 O ( 1 Σ + g ), SO 2 ( 1 A ) and the photochemical precursor itself, phenyl isocyanate, PhNCO. NCO ( X 2 Π 3 2 (0,0,0)) was prepared by infrared multiphoton dissociation (IRMPD) of PhNCO followed by tuneable dye laser pumping at the transition NCO ( A 2 Σ + (0,0,0)) ← X 2 Π 3 2 (0,0,0)) coupled with laser-induced fluorescence (LIF) monitoring in the time-domain. Removal was found to be very efficient where the rate constants with the above gases were measured as 2.05±0.2, 15.2±0.2, 32.0±3.0, 8.52±0.4, 6.46±0.2, 27.1±1.0 and 51.8±5.0, respectively, in units of 10−11 cm3 molecule−1 s−1 (at a laser fluence of 43 J/cm2). An upper limit of the rate constant for the physical quenching of NCO(A) by Ar is also reported as (ϵ/k) 1 2 ) of the collision partners, consequently yielding a pair well depth ( (ϵ/k) 1 2 ) of 94±15 K 1 2 for NCO ( A 2 Σ(0,0, 0)) .

Direct measurements of removal rates of CFCl(Ã1A″(0, 1, 0)) by simple alkenes

Abstract Direct measurements of rate constants for the collisional removal of CFCl(A 1 A″(0, 1, 0)) by ethylene (C 2 H 4 ), propene (C 3 H 6 ), 1-butene (1-C 4 H 8 ), i -butene ( i -C 4 H 8 ) and 1,3-butadiene (C 4 H 6 ) are reported. CFCl(A 1 A″(0, n , 0)) was prepared by infrared multiple photon dissociation (IRMPD) of CHFCl 2 followed by tunable dye laser pumping at the transition CFCL( A 1 A ″(0, 1, 0) ← X 1 A ″(0, 1, 0)) . Removal rates at threshold CO 2 -laser fluence for the gases ethylene, propene, 1-butene, 1,3-butadiene and i -butene were measured to be 1.4 ± 0.15, 3.7 ± 0.7, 2.2 ± 0.2, 4.0 ± 0.8 and 4.1 ± 0.4 in 10 −11 cm 3 molecule −1 s −1 units. An upper limit of the rate constant for physical quenching of CFCl(A) by Ar is also reported as −13 cm 3 molecule −1 s −1 . The rate date are compared with those for reactions of other carbenes and the appropriate Parmenter-Seaver plot is presented.