Ions produced by crossed-beam collisions of pulsed monoenergetic electrons and supersonic expansion molecules have been analysed by time-of-flight mass spectroscopy (TOF-MS) in order to determine the appearance potentials, absolute total, dissociative and parent ionization cross sections and nascent ion kinetic energy distributions. The electron impact study was conducted at incident electron energies up to 100 eV on the parent CH 2 F 2 molecule (and Ar/CH 2 F 2 mixtures), a fluoromethane where the CH 2 F + ion is produced at higher yields (1:18) than the parent molecule ion, CH 2 F + 2 . TOF-MS band profiles analysis has enabled us to determine the ions' nascent kinetic energy distributions, information that combined with the dissociative ionization appearance potentials, calculated molecular orbital energies and orbital bond characters, leads to improved identification of the electron impact dissociative channels.

A study of the vibrational, rotational and translational energies of the CHF (z 'A') radical prepared by infrared multiphoton dissociation, IRMPD, is presented. The vibrational and rotational temperatures measured near the CO, 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 ps the measured rotational temperatures were 900 and 600 K, respectively. Vibrational temperatures at 3 ps delay for samples of pure precursor (20 mTorr) 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 (R 'A') 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 k , , < cm3 molecule-' s -' , while the rotational relaxation rate constant isk,, > lo-'' cm3 molecule-' s-'.The bimolecular rate constants for removal of CHF (z 'A') with several alkenes are reported to be (/10-l2 cm3 kbutadiene = 22.0 3.0. Some of the reactions are C 0 2 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. molecule-' S -' ) :kethene = 5.4 f 0.3; kpropene = 13.0 f 1.

The experimental electron-impact total ionization cross sections ͑TICSs, ICSs͒ of CF 4 , CHF 3 , CH 2 F 2 , and CH 3 F 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 ͑semi͒empirical based ICS available methods. TICSs computational methods include: two approximations of the binary-encounter dipole ͑BED͒ referred to hereafter as Kim ͑Kim-BEB͒ and Khare ͑Khare-BEB͒ methods, the Deutsch and Märk ͑DM͒ formalism, also requiring atomic and molecular ab initio information, the modified additivity rule ͑MAR͒, and the Harland and Vallance ͑HV͒ 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 excellent agreement with the experimental method; the Khare method provides TICS very close to that of Kim at low electron-impact energies ͑Ͻ100 eV͒, but their Mott and Bethe contributions are noticeably different; in the Kim and Khare approximations the electron correlation methods improve the fittings to the experimental profiles in contrast with the large basis sets, that leads to poorer results; the DM formalism yields TICS profiles with shapes similar to the experimental and the BEB methods, but consistently lower and with the profiles maxima shifted towards lower incident electron energies; the MAR method supplies very good ICS profiles, between those of BEB and DM methods; finally, the empirical HV method provides rather poor fittings concomitant with the simplicity and the few empirical parameters used.

No abstract

Velocity and kinetic energy distributions ͑VDs, KEDs͒ of metal ions generated by nanosecond ͑ns͒ pulsed laser ablation under high vacuum have been determined using an electrostatic analyzer plus time-of-flight coupled system. A number of metal and alloy targets, principally involving Fe and Ni, have been studied at different laser fluences. At low fluence, the ion distributions have been shown to fit single Maxwell-Boltzmann-Coulomb ͑MBC͒ distributions; for medium and higher fluences, each ion distribution is found to comprise that of the surviving "precursor" ion, itself, overlapped with sidebands which arise from ion-electron recombination and/or ionization. The so-called surviving "precursor" ion of a distribution is that which underwent no change of charge. The Coulomb velocities of the surviving "precursor" ions and those of the ion products resulting from ion-electron collisions have been compared. Ion velocities are correlated with the local electric field resulting from ejection of the photoelectrons following laser ablation. Under identical conditions of laser fluence, the ions are seen to experience an electrical field nearly independent of their charge. The transit times of ions in the plasma have been estimated to be of the order of 1 ps. An overall quantitative mechanism for metal ablation on this basis is presented, including the ejection time for photoelectrons and differences in ion distributions resulting from employing laser pulses in the nanosecond ͑ns͒ and femtosecond ͑fs͒ regimes.

No abstract

A time-resolved investigation is presented of the electronic energy distribution in SrI following the collision of the optically metastable strontium atom, Sr [5s5p(3PJ)], with the molecule CF3I. Sr[5s5p(3PJ)], 1.807 eV above its 5s2(1S0) electronic ground state, was generated by pulsed dye-laser excitation of ground state strontium vapour to the Sr(53P1) state at , λ =689.3 nm {Sr(53P1←51S0)} at elevated temperature (840 K) in the presence of excess helium buffer gas in which rapid Boltzmann equilibration within the 53PJ spin-orbit manifold takes place. Time resolved atomic emission from Sr(53P1→51S0) at the resonance transition and the molecular chemiluminescence from SrI(A2∏1,2,3/2,B2∑+→X2∑+) resulting from reaction of the excited atom with CF3I were recorded and shown to be exponential in character. SrI in the A2∏1/2,3/2 (172.5, 175.4 kJ mol-1) and B2∑+ (177.3 kJ mol-1) states are energetically accessible on collision by direct-I-atomic abstraction between Sr(3P) and CF3I. The first-order decay coefficients for the atomic and molecular emissions are found to be equal under identical conditions and hence SrI(A2∏1/2,3/2, B2∑+) are shown to arise from direct I- atom abstraction reactions. The molecular systems recorded were SrI (A2∏1/2→X2∑+, Δv=0, λ=694 nm), SrI(A2∏3/2→X2∑+, Δv=0, λ=677 nm) and SrI(B2∑+→X2∑+) (Δv=0, λ=674 nm), dominated by the Δv=0 sequences on account of Franck-Condon considerations. The combination of integrated m61ecular and atomic intensity measurements yields estimates of the branching ratios into the specific electronic states, A1/2, A3/2 and B, arising from Sr(53PJ)+CF3I which are found to be as follows: A1/2,1.2 × 10-2; A3/2, 6.7 × 10-3; B, 5.1 × 10-3 yielding ∑SrI(A1/2+A3/2+B)=2.4 × 10-2. As only the X, A and B states SrI are accessible on reaction, assuming that the removal of Sr(53PJ) occurs totally by chemical removal, this yields an upper limit for the branching ratio into the ground state of ca. 98%. The present results are compared with previous time-resolved measurements on excited states of strontium halides that we have reported on various halogenated species resulting from reactions of Sr(53PJ), together with analogous chemiluminescence studies on Sr(3PJ) and Ca(43PJ) from molecular beam measurements.

No abstract