M.N. Sánchez Rayo

Nascent kinetic energy distributions of the ions produced by electron-impact on the CH3F molecule

Abstract The nascent translational energy distributions of ions produced by electron-impact on CH 3 F, prepared in a skimmed supersonic expansion, have been investigated by their correlation with the time-of-flight mass-spectrometry band profiles. The electron energies used in the collisions range from the ionisation threshold to 100 eV. CH 3 F + and other ion fragments close in mass are characterized by low average kinetic energies. Medium-sized mass ion fragments, have kinetic energy distributions with average values of ca. 1 eV, whilst atomic ions have the highest average energies. The set of ion nascent energy distributions produced by electron-impact on CH 3 F, at several electron energies, are reported.

Removal rates of CHF (Ã 1A″ (0, 0, 0)) by alkenes

Abstract Absolute removal rates of CHF (A 1 A″ (0, 0, 0)) by ethene (C 2 H 4 ), propene (C 3 H 6 ), 1-butene (1-C 4 H 8 ), isobutene( i -C 4 H 8 ), 1,3-butadiene (C 4 H 6 ), difluoromethane (CH 2 F 2 ), nitric oxide (NO) and argon (Ar) have been measured at room temperature. CHF in the A 1 A″ state was produced by infrared multiphoton dissociation of CH 2 F 2 forming the CHF (X 1 A′) state and further pumping to the A 1 A″ state by absorption of a visible dye laser pulse. Removal processes were found to be second order with the following rate constants in units of 10 −10 cm 3 molecule −1 s −1 : k (C 2 H 4 ) = 0.9 ± 0.2. k (C 3 H 6 ) = 1.0 ± 0.2; k (1−1C 4 H 8 ) = 1.1 ± 0.2; k ( i -C 4 H 8 ) = 1.1 ± 0.2; k (C 4 H 6 ) = 1.0 ± 0.2; k (Ar) = 0.27 ± 0.02; k (NO) = 0.8 ± 0.1; k (CH 2 F 2 ) = 1.3 ± 0.1. The Parmenter—Seaver correlation for collisional removal of A 1 A″ CHF is discussed.

Kinetic investigation of the molecular chemiluminescence SrBr(A2II, B2Σ+ → X2Σ+) and the time-resolved atomic fluorescence Sr(53P1 → 51S0) following the pulsed dye-laser generation of Sr(53PJ in the presence of CH3Br and CF3Br

A kinetic study is presented of the vibronic energy distribution in SrBr following the reactions of the optically metastable, electronically excited strontium atom, Sr(5s5p(3PJ)), 1.807 eV above its 5s2(1S0 electronic ground state, in the presence of CH3Br and CF3Br. Sr(53P1) was generated by pulsed dye-laser excitation of ground-state strontium vapour at λ=689.3 nm (Sr(53P1←51S0)) at elevated temperature (850 K) in the presence of excess helium buffer gas in a slow flow system, kinetically equivalent to a static system. The decay of Sr(53PJ) is then monitored by time-resolved atomic fluorescence from Sr(53P1) at the resonance wavelength following rapid Boltzmann equilibration within the 3PJ spin-orbit manifold using boxcar integration. Time-resolved molecular chemiluminescence to the ground state was also observed from electronically excited strontium bromide and investigated in detail via the SrBr(A2Πsol:32 → X2Σ+ (Δν=0, λ≈667 nm) and SrBr(B2Σ+ → X2Σ+) (Δν=0, λ≈651 nm) systems. The A2Π (177.6 kJ mol−1 and B2Σ+ (183.6 kJ mol−1) states of SrBr are both energetically accessible on collision between Sr(3P) and CH3Br and CF3Br. Both the atomic and molecular (A,B,–X) chemiluminescence emissions are shown to be exponential in form with both reactants and characterized by first-order decay coefficients which are found to be equal under identical conditions within experimental error. SrBr(A2Π) and SrBr(B2Σ+ are thus shown to arise from direct reaction with both CH3Br and CF3Br. The results obtained here in the time domain are compared with analogous chemiluminescence studies on Sr(3P) with halides, including bromides, using molecular beams and with previous time-resolved measurements on SrCl(A3II, B2Σ+ X2Σ+) that we have reported. The results are also compared with those from a series of investigations we have presented from time-domain investigations of molecular emissions from CaF, Cl, Br, I (A,B–X) arising from the collisions of Ca(43PJ) with appropriate halides.

Electron-impact-induced light emission from CF2Cl2

A study is presented of the optical emissions, lifetimes, emission thresholds and relative emission cross sections, in the wavelength region ( lambda =200-600 nm) for molecular fragments derived from electron impact dissociation of CCl2F2. In addition to observations for previously reported molecular fragments including CCl, CCl+, CF, etc. some others have been identified here, including CCIF(A) and CF2(A(0, n, O))(n=5,6). In contrast to 11-35 eV photodissociation processes, the partners in the electron dissociation of these carbenes are highly excited atoms, including atomic ions. In some cases comparisons between appearance potentials and spectroscopic transitions combined with published thermochemical data led to unique assignments of dissociation into specific channels.

Analysis of the optical emission ( lambda =400-900 nm) of fragments produced by electron impact on CF2Br2

A study of the optical emission in the 400-900 nm region from fragments produced by electron impact on the CF2Br2 molecule is reported. The emission is dominated by atomic Br I and Br II transitions and the cross sections of the most intense transitions have been determined from threshold up to 500 eV. Lifetimes of the electronically-excited atomic states have also been measured, the result being that, on the whole, ionic excited fragments have shorter lifetimes than neutral fragments. Time decay of a broad-band emission in the 400-440 nm region yields bi-exponential decays that fit to the same short and large lifetimes at any wavelength and are attributed to CFBr(A-X) and CF2(A-X) transitions. It has been shown that Br* I species appear as a result of a collision primary process while Br* II are formed by secondary processes. Excitation thresholds and cross sections for the more significant, emissions of Br* I and Br* II have also been determined in the range from 50-500 eV and are discussed. The applications of Bethe-Born theory in the form of Fano plots shows that formation of most Br* I and Br* II excited states proceed via optically-allowed transitions.

Collisionally induced intramultiplet mixing of metastable states by Ne, Kr and Xe atoms

Abstract An investigation is presented of the spin–orbit mixing within the Sr [5 s5p ( 3 P 0,1,2 )] manifold on collision with Ne, Kr and Xe at elevated temperatures following pulsed dye-laser generation of Sr ( 3 P 1 ) . The population profiles of all three states were monitored individually by laser-induced fluorescence and fitted to a kinetic model involving six collisional rates connected by detailed balance. The model also includes mixing by Sr ( 1 S 0 ) itself. The resulting rate constants, together with those for Sr ( 1 S 0 ) , Ar and He reported previously, are considered using a standard model employing the potential wells of the reactants and also a collision-complex model.

Collisionally induced intramultiplet mixing of Sr(53PJ) metastable states by He, Ar and Sr ground state atoms

Abstract Intramultiplet collisionally induced mixing within the Sr[5s5p(3P0,1,2)] manifold is investigated by time-resolved laser-induced fluorescence (LIF) following the pulsed dye-laser generation of Sr(3P1) of the electronic ground state {Sr[5s5p(3P0,1,2)]←Sr[5s2(1S0)], λ=689.26 nm} at elevated temperatures. The population profiles of the three spin–orbit states were individually monitored by LIF as well as that of Sr(3P1) by spontaneous emission at the resonance wavelength. A kinetic model is employed that enables the process of spontaneous emission from Sr(3P1) to be isolated initially and characterised by experiment. Particular emphasis is placed on the modelling procedure itself in which the separate kinetic component due to spontaneous emission and the positions of the maxima in the 3P0 and 3P2 population profiles constitute severe constraints on the model. The collisional components within the model are reduced to three rate constants where pairs of J states are connected in this context by detailed balance. Thus k10 and k12, and, by detailed balance, k01 and k21 are quantified at various temperatures and pressures to yield the absolute value of these collision properties for Sr(3PJ) with He and Ar. Rate data for collisionally induced intramultiplet mixing in Sr(3PJ) by Sr(1S0) itself is also reported found to proceed at close to unit collisional efficiencies in all cases. Thus, at elevated temperatures, variations in atomic profiles are dominated by the differing vapour pressures of atomic strontium. Estimates of the activation energies associated with k10 and k12 for the noble gases observed against this large competing background are found to be of the order of the spin–orbit splittings. The model overall is found to be insensitive to k02 and k20 whose magnitudes are small by comparison with those for the collisional rate data connecting adjacent J states. Whilst collisional processes for He are some two orders of magnitude more efficient than those for Ar, all of these are seen to be ‘adiabatic’, in contrast with the gas kinetic rate constants of ground Sr, considered to be ‘sudden’ in character. The results are compared with analogous data derived by atomic resonance absorption spectroscopy following pulsed generation of Sr(3P1) and are considered in the context of theoretical calculations employing quantum close coupling calculations.

Atomic fluorine emission cross sections observed in the 600-900 nm region following 0-500 eV electron impact on fluoromethanes (CF4, CHF3, CH2F2 and CH3F)

Absolute emission cross sections (em) of F I atom electronic transitions in the 600-900 nm region, following the electron impact on CHxF4-x (x = 0-3) molecules and determined by the optical method, are reported. The range of electron energies studied varies from threshold to 500 eV. In the electron impact on CF4 at pressures higher than 2 mTorr, a continuum background band in the 500-700 nm region is observed and has been assigned to the CF3* (2A2´´1A1´ and/or 1E´1A1´) transitions. Continuum bands of the same features were also found in collisional processes on CHF3 and CH2F2, although their maxima are shifted to the blue with respect to that of CF4 and have been tentatively assigned to the F2* ion pairs. For low F content fluoromethanes high electronic excited states (3d) of atomic F are observed and may be due to redistribution of the excess electron energy over that for C-F bond dissociation.

The collisional quenching of I(52P12) by the molecules H2O, CH3OH, C2H5OH, HCOOH and CH3COOH by time-resolved emission (I(52P12) → I(52P32) + hν, λ = 1.315 μm) following pulsed-laser photolysis

Abstract The collisional quenching of electronically excited iodine atoms, I(5p 5 ( 2 P 1 2 )), by the molecules H 2 O, CH 3 OH, C 2 H 5 OH, HCOOH and CH 3 COOH was investigated directly in the time domain. I( 2 P 1 2 ) was generated by the pulsed excimer laser photolysis of C 3 F 7 I at λ = 248 nm and monitored by time-resolved emission at λ = 1.315 μm (I( 2 P 1 2 → I( 2 P 3 2 ) + h ν). Absolute second-order quenching rate constants ( k Q ), including those for the collisional removal of the excited atom by the dimers (HCOOH) 2 and (CH 3 COOH) 2 , were obtained for room temperature. The following results were obtained: k Q per cm 3 mol −1 s −1 H 2 O (2.1 ± 0.2) × 10 −12 CH 3 OH (9.4 ± 0.9) × 10 −13 C 2 H 5 OH (14.6 ± 1.3) × 10 −13 HCOOH (3.5 ± 0.3) × 10 −13 (HCOOH) 2 (3.5 ± 0.4) × 10 −13 CH 3 COOH (5.4 ± 0.5) × 10 −13 (CH 3 COOH) 2 (5.4 ± 0.6) × 10 −13 These results indicate that the main route for removal is the electronic energy transfer in the iodine atom to the second vibrational level in the OH stretching mode in each case, via long-range attractive forces, probably involving dipole-quadrupole interaction on collision.

Analysis of the optical emission following electron impact on CF

An experimental study of the visible and ultraviolet emission features following pulsed electron impact on molecules is presented. The spectra in the 200-600 nm region, for electron-impact energies of 100 eV, show a number of narrow lines superimposed on two very broad bands centred at 300 and 470 nm thought to originate from the and ion-pair emissions. The atomic lines arise from and fragments and the lifetimes of the 14 more intense fragments have been measured to be in the 10-30 ns range. Atomic and molecular electronically excited emission thresholds and relative cross sections have been studied for electron energies of up to 500 eV. Application of the Bethe-Born theory under Fanos characteristic parameters analysis shows that formation of most and species originates via optically allowed transitions.