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

Recombination of vibrationally cold N2+ ions with electrons

Recombination of vibrationally cold N2+ ions with electrons was studied in the temperature range of 140-250 K. A cryogenic stationary afterglow apparatus equipped with cavity ring-down spectrometer and microwave diagnostics was utilized to probe in situ the time evolutions of number densities of particular rotational and vibrational states of N2+ ions and of electrons. The obtained value of the recombination rate coefficient for the recombination of the vibrational ground state of N2+ with electrons is αv=0 = (2.95 ± 0.50) × 10-7(300/T)(0.28±0.07) cm3 s-1, while that for the first vibrationally excited state was inferred as αv=1 = (4 ± 4) × 10-8 cm3 s-1 at 250 K.

The vibrational dependence of dissociative recombination: rate constants for N2 (+)

Dissociative recombination rate constants are reported with electron temperature dependent uncertainties for the lowest 5 vibrational levels of the N2 (+) ground state. The rate constants are determined from ab initio calculations of potential curves, electronic widths, quantum defects, and cross sections. At 100 K electron temperature, the rate constants overlap with the exception of the third vibrational level. At and above 300 K, the rate constants for excited vibrational levels are significantly smaller than that for the ground level. It is shown that any experimentally determined total rate constant at 300 K electron temperature that is smaller than 2.0 × 10(-7) cm(3)/s is likely to be for ions that have a substantially excited vibrational population. Using the vibrational level specific rate constants, the total rate constant is in very good agreement with that for an excited vibrational distribution found in a storage ring experiment. It is also shown that a prior analysis of a laser induced fluorescence experiment is quantitatively flawed due to the need to account for reactions with unknown rate constants. Two prior calculations of the dissociative recombination rate constant are shown to be inconsistent with the cross sections upon which they are based. The rate constants calculated here contribute to the resolution of a 30 year old disagreement between modeled and observed N2 (+) ionospheric densities.

Role of excited core Rydberg states in dissociative recombination

Intermediate states formed during the dissociative recombination of molecular ions with electrons can play significant roles in determining the magnitude of the total rate coefficient. These resonances are Rydberg states of two types, that is, they can have the ground or excited states of the ion as a core. Those with the excited cores have a fundamentally different excitation mechanism than those with the ground state core. The importance of excited core states in dissociative recombination has received only limited attention in the literature. Theoretical calculations on the dissociative recombination of N2+ are reported which compare the two types of resonances. Potential curves, electronic widths, cross sections, and rate coefficients are calculated for dissociative recombination along the 2(1)Sigma(g)+ state, one of several routes for the dissociative recombination of N2+. The ground core resonances, in this example, are relatively unimportant compared to those with the excited core. Inclusion of the excited core resonances increases the rate coefficient by about a factor of 4 at room temperature, but the increase is not enough to establish 2(1)Sigma(g)+ as the dominant dissociative route.

Electron molecular ion recombination: product excitation and fragmentation

Electron-ion dissociative recombination is an important ionization loss process in any ionized gas containing molecular ions. This includes the interstellar medium, circumstellar shells, cometary comae, planetary ionospheres, fusion plasma boundaries, combustion flames, laser plasmas and chemical deposition and etching plasmas. In addition to controlling the ionization density, the process generates many radical species, which can contribute to a parallel neutral chemistry. Techniques used to obtain rate data and product information (flowing afterglows and storage rings) are discussed and recent data are reviewed including diatomic to polyatomic ions and cluster ions. The data are divided into rate coefficients and cross sections, including their temperature/energy dependencies, and quantitative identification of neutral reaction products. The latter involve both ground and electronically excited states and including vibrational excitation. The data from the different techniques are compared and trends in the data are examined. The reactions are considered in terms of the basic mechanisms (direct and indirect processes including tunneling) and recent theoretical developments are discussed. Finally, new techniques are mentioned (for product identification; electrostatic storage rings, including single and double rings; Coulomb explosion) and new ways forward are suggested.

Mechanisms of Electron-Ion Recombination of N2H+/N2D+ and HCO+/DCO+ Ions: Temperature Dependence and Isotopic Effect

The temperature dependencies of the rate coefficients, alpha(e), for electron-ion dissociative recombination (DR) of N2H+/N2D+ and HCO+/DCO+ ions with electrons have been measured over the range 100-500 K. Also, optical emissions have been detected at approximately 100 K from the N2(B3(pi)g) electronically excited products of N2H+/N2D+ recombination. The measurements were carried out using the classic FALP technique combined with an optical monochromator. For N2H+, there was no variation of alpha(e) with temperature above 200 K, with an average value of alpha(e)(N2H+) = 2.8 x 10(-7) cm3 s(-1). The temperature variation for T approximately 100-300 K observed for alpha(e)(HCO+) is similar to that of N2H+ ions for T approximately 300-500 K. The smaller rate coefficient measured for DCO+ and N2D+ ions shows the influence of an isotope effect. The substantial enhancement of the vibrational level, upsilon' = 6, from the N2B state for N2H+ recombination over N2D+ recombination is consistent with previous result at 300 K and implies the influence of a tunneling mechanism of DR.