Roberto Fernandez Martinez

Ultrafast dynamics of aniline in the 294-234 nm excitation range: The role of the πσ* state

The ultrafast relaxation of jet-cooled aniline was followed by time-resolved ionization, after excitation in the 294-234 interval. The studied range of energy covers the absorption of the two bright ππ∗ excitations, S(1) and S(3), and the almost dark S(2) (πσ∗) state. The employed probe wavelengths permit to identify different ultrafast time constants related with the coupling of the involved electronic surfaces. A τ(1) = 165 ± 30 fs lifetime is attributed to dynamics along the S(2) (πσ∗) repulsive surface. Other relaxation channels as the S(1)→S(0) and S(3)→S(1) internal conversion are also identified and characterized. The work provides a general view of the photophysics of aniline, particularly regarding the role of the πσ∗ state. This state appears as minor dissipation process due to the ineffective coupling with the bright S(1) and S(3) states, being the S(1)→S(0) internal conversion the main non-radiative process in the full studied energy range. Additionally, the influence of the off-resonance adiabatic excitation of higher energy electronic states, particularly S(3), is also observed and discussed.

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.

Evaluation of the computational methods for electron-impact total ionization cross sections: Fluoromethanes as benchmarks

The experimental electron-impact total ionization cross sections (TICSs, ICSs) of CF4, CHF3, CH2F2, and CH3F fluoromethanes reported so far and a new set of data obtained with a linear double focusing time-of-flight mass spectrometer have been compared with the ab initio and (semi)empirical based ICS available methods. TICSs computational methods include: two approximations of the binary-encounter dipole (BED) referred to hereafter as Kim (Kim-BEB) and Khare (Khare-BEB) methods, the Deutsch and Mark (DM) formalism, also requiring atomic and molecular ab initio information, the modified additivity rule (MAR), and the Harland and Vallance (HV) methods, both based on semiempirical or empirical correlations. The molecular ab initio information required by the Kim, Khare, and DM methods has been computed at a variety of quantum chemistry levels, with and without electron correlation and a comprehensive series of basis sets. The general conclusions are summarized as follows: the Kim method yields TICS in excell...

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.

Electron-impact dissociative ionization of fluoromethanes CHF3 and CF4

The electron-impact dissociative ionization of fluoromethanes CHF3 and CF4 has been investigated by probing their supersonic expansions with a beam of electrons of selected kinetic energy within a time-of-flight mass spectrometer system. Partial (absolute) ionization cross sections of all the nascent ions arising from electron impact, namely CF3+, CHF2+, CF2+, CHF+, CF+, HF+, F+, CH+, C+, and H+ from CHF3 and CF2+, CF22+, CF3+, CF32+, CF+, C+, and F+ from CF4, were measured for electron incident energies up to 100 eV (nominal). The kinetic energy distributions of the nascent ions were also characterized and the information that they supply combined with the appearance potentials (APs) provides an improved identification of the molecular dissociative ionization channels. The correlation between the channel AP and the character and energies of the precursor molecular orbitals is discussed.

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.

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.

The electron-impact dissociative ionization of CCl2F2

An investigation of the formation channels and properties of ion fragments following electron-impact dissociative ionization of the CCl2F2 molecule using electron kinetic energies in the 0–100 eV range is reported. Measurements of ion appearance potentials (APs) and nascent translational energy distributions were made on a supersonic expansion of CCl2F2 in a time-of-flight mass spectrometer. A discussion of the correlation between the channel APs, the precursor bond characters as calculated from the population analysis, and the low-resolution photoelectron spectrum of the CCl2F2 molecule is presented.

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.