J. C. Gomet

Laser induced fluorescence and vacuum ultraviolet spectroscopic studies of H‐atom production in the dissociative recombination of some protonated ions

The flowing afterglow technique, coupled with laser induced fluorescence (LIF) and vacuum ultraviolet (vuv) absorption spectroscopy, has been used to determine the fractional H‐atom contributions, fH, to the product distributions for the dissociative recombination of a series of protonated ions (N2H+, HCO+, HCO+2, N2OH+, OCSH+, H2CN+, H3O+, H3S+, NH+4, and CH+5 ) with electrons. The measurements were made at 300 K in two separate ways in two laboratories by (i) directly determining the H‐atom number density using vuv absorption spectroscopy at the Lα (121.6 nm) wavelength and (ii) converting the H atoms to OH radicals using the reaction H+NO2→OH+NO followed by LIF to determine the OH number density. The agreement between the two techniques is excellent and values of fH varying from ∼0.2 (for OCSH+ ) to 1.2 (for CH+5 ) have been obtained showing that in some of the cases recombination can lead to the ejection of two separate H atoms. Comparison of the oxygen/sulphur analogs, HCO+2/OCSH+ and H3O+/H3S+ showe...

The dissociative recombination of N2+(v=0,1) as a source of metastable atoms in planetary atmospheres

Abstract The yield of metastable nitrogen atoms in dissociative recombination of N2+ (v = 0, 1)ions has been tudied for different experimental conditions. In a first experiment, the branching ratio for N(2D) production was directly measured as being higher than 1.85; for N2+ (v = 0) this implies that 2D + 2D is the main reaction channel; for N2+ (v = 1) a minor channel could be 2P + 2D, 2P being then quenched toward 2D by electrons. In a second experiment, at higher electron densities, the influence of superelastic collisions was studied; a steady state analysis yields the quenching rate coefficient k4, of 2D towards 4S equal to 2.4 × 10−10 cm3 s−1for Te = 3900 K and shows that 2D + 2D is always the major channel of the reaction for N2+ (v = 1), 2D + 2P being a minor channel. All these results are in good agreement with thermospheric models but imply that N2+ dissociative recombination is a less important source for nitrogen escape of Mars.

The yield of oxygen and hydrogen atoms through dissociative recombination of H2O+ ions with electrons

The branching ratios of the dissociative recombination of excited H2O+ ions with electrons were studied using a plasma flow tube experiment. The total oxygen atom channel accounts for 45% of the recombination with more than 24% in O+H+H. The remaining channel is OH+H for 55%. These results are discussed in comparison with the few available theories and with photodissociation data.

Measurements of C3H+3, C5H+3, C6H+6, C7H+5, and C10H+8 dissociative recombination rate coefficients

The rate coefficients α for the dissociative recombination (DR) of the aromatic cyclic ions C6H6+ and C10H8+ have been measured in a flowing afterglow experiment at room temperature using Xe+ and Kr+ as precursor ions. C+ was also used as a precursor ion and this allowed the DR rate coefficients for C3H3+, C5H3+, and C7H5+ to be determined. The values obtained (in units of 10−6 cm3u2009s−1) at 300 K are, respectively, α(C3H3+)=0.7, α(C5H3+)=0.9, α(C6H6+)=1.0, α(C7H5+)=0.7, α(C10H8+)=0.3 with an uncertainty estimated to be ∼30%. A discussion is given concerning the relationship between the DR rate and the structural complexity of the ions.

A further study of HCO+ dissociative recombination

The rate coefficient for the dissociative recombination of HCO+ has been measured using a new flowing afterglow technique which employs a movable Langmuir probe to measure electron density and a movable mass spectrometer to measure ion density, both as a function of distance along the flow. A value of 2.2×10−7 cm3u2009s−1 has been found at 300 K. An analysis of the excitation state of the ions indicates that more than 93% are in the v=0 state while the rest have ∼0.1 eV of internal energy. A discussion of recent theoretical controversy concerning this ion is given.

Further measurements of the H+3(v=0,1,2) dissociative recombination rate coefficient

A new flowing afterglow apparatus that utilizes a Langmuir probe/mass spectrometer to monitor both electron and ion decay in a hydrogen plasma has been used to measure the dissociative recombination rate coefficient of H+3 at two different electron temperatures. At 300 K a rate coefficient of 1.5×10−7 cm3u2009s−1 was found for H+3 ions with a low degree of vibrational excitation (v≤2). The rate coefficient for ground state ions H+3(v=0) was measured as 1.1×10−7 cm3u2009s−1 at 650 K. A discussion is given of the excitation states of H+3 ions in the afterglow in the light of slow deexcitation rates for low vibrational states. A new model for the recombination of H+3 is presented.

The yield of metastable atoms through dissociative recombination of O+2 ions with electrons

The yield of metastable oxygen atoms through dissociative recombination of O+2 ions with electrons has been studied using a plasma flow tube experiment. For O+2 with high vibrational excitation (around v=9) it was found that half of the oxygen atoms are formed on the O(1D) state and that the branching ratio toward O(1S) is large (∼0.4). Using Xe+ instead of Ar+ as precursor ions, it was shown that the O(1S) yield is much less for ions with low vibrational excitation. However, the present experimental results are not compatible with the extremely low theoretical value of this yield which was reported recently for O+2 (v=0).

Flowing Afterglow Langmuir Probe measurement of the N+2(v=0) dissociative recombination rate coefficient

A measurement of the dissociative recombination coefficient α is presented for reactions of electrons with ground state N+2(Xu20092Σ+g,v=0) ions at 300 K. The measurement has been made under truly thermalized conditions using a Flowing Afterglow Langmuir Probe (FALP). The rate constant was determined to be α[N+2(v=0)]=2.6×10−7 cm3u2009s−1 which is in good agreement with previous data except for the latest merged beam experiment.

Observation of the fourth positive system of CO in dissociative recombination of vibrationally excited CO+2

Abstract The CO fourth positive system obtained by dissociative recombination of CO 2 + (X 2 π g ) ions has been observed under various conditions of CO 2 + vibrational excitation. It is shown that the CO(A 1 π) vibrational distribution is directly linked to CO + 2 excitation. Implications for planetary airglows are discussed.

Dissociative recombination of HCS+ and H3S+ ions studied in a flowing afterglow apparatus

Abstract The rate coefficients α for dissociative recombination of HCS+ and H3S+ ions have been studied in a flowing afterglow experiment at room temperature. HCS+ ions were produced via two different routes. The value of the rate coefficient α (HCS+ was measured to be 5.8 × 10−7 and 7.9 × 10−7 cm3 s−1 when using respectively OCS + H2 or H2S as the reactant neutrals. This value is one order of magnitude larger than previous experimental and theoretical studies. The value obtained for α (H3S+) was 5.2 × 10−7 cm3 s−1, in reasonable agreement with FALP results. The vibrational populations of the ions could not be determined in the present experiment so that these measurements could relate to a mixture of ions with different vibrational excitation. We argue that the high HCS+/CS ratio observed in interstellar clouds result from a low branching ratio towards the channel H + CS in HCS+ dissociative recombination.