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Register a new userDynamics of gas phase Ne$^*$ + NH$_3$ and Ne$^*$ + ND$_3$ Penning ionization at low temperatures
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Added by
Krzysztof Jachymski
Publication date
2014
fields
Physics
and research's language is
English
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Two isotopic chemical reactions, $mathrm{Ne}^*$ + NH$_3$, and $mathrm{Ne}^*$ + ND$_3$, have been studied at low collision energies by means of a merged beams technique. Partial cross sections have been recorded for the two reactive channels, namely $mathrm{Ne}^*$ + NH$_3$ $rightarrow$ Ne + NH$_3^+$ + $e^-$, and $mathrm{Ne}^*$ + NH$_3$ $rightarrow$ Ne + NH$_2^+$ + H + $e^-$, by detecting the NH$_3^+$ and NH$_2^+$ product ions, respectively. The cross sections for both reactions were found to increase with decreasing collision energy, $E_{coll}$, in the range 8 $mu$eV$<E_{coll}<$ 20 meV. The measured rate constant exhibits a curvature in a log(k)-log($E_{coll}$) plot from which it is concluded that the Langevin capture model does not properly describe the $mathrm{Ne}^*$ + NH$_3$ reaction in the entire range of collision energies covered here. Calculations based on multichannel quantum defect theory were performed to reproduce and interpret the experimental results. Good agreement was obtained by including long range van der Waals interactions combined with a 6-12 Lennard-Jones potential. The branching ratio between the two reactive channels, $Gamma = frac{[NH_2^+]}{[NH_2^+]+[NH_3^+]}$, is relatively constant, $Gammaapprox 0.3$, in the entire collision energy range studied here. Possible reasons for this observation are discussed and rationalised in terms of relative time scales of the reactant approach and the molecular rotation. Isotopic differences between the $mathrm{Ne}^*$ + NH$_3$ and $mathrm{Ne}^*$ + ND$_3$ reactions are small, as suggested by nearly equal branching ratios and cross sections for the two reactions.
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We present state-selective measurements on the NH$_2^{+}$ + H$^{+}$ and NH$^{+}$ + H$^{+}$ + H dissociation channels following single-photon double ionization at 61.5 eV of neutral NH$_{3}$, where the two photoelectrons and two cations are measured in coincidence using 3-D momentum imaging. Three dication electronic states are identified to contribute to the NH$_2^{+}$ + H$^{+}$ dissociation channel, where the excitation in one of the three states undergoes intersystem crossing prior to dissociation, producing a cold NH$_2^+$ fragment. In contrast, the other two states directly dissociate, producing a ro-vibrationally excited NH$_2^+$ fragment with roughly 1 eV of internal energy. The NH$^{+}$ + H$^{+}$ + H channel is fed by direct dissociation from three intermediate dication states, one of which is shared with the NH$_2^{+}$ + H$^{+}$ channel. We find evidence of autoionization contributing to each of the double ionization channels. The distributions of the relative emission angle between the two photoelectrons, as well as the relative angle between the recoil axis of the molecular breakup and the polarization vector of the ionizing field, are also presented to provide insight on both the photoionization and photodissociation mechanisms for the different dication states.
We report measurements on the H$^{+}$ + H$^{+}$ fragmentation channel following direct single-photon double ionization of neutral NH$_{3}$ at 61.5 eV, where the two photoelectrons and two protons are measured in coincidence using 3-D momentum imaging. We identify four dication electronic states that contribute to H$^{+}$ + H$^{+}$ dissociation, based on our multireference configuration-interaction calculations of the dication potential energy surfaces. The extracted branching ratios between these four dication electronic states are presented. Of the four dication electronic states, three dissociate in a concerted process, while the fourth undergoes a sequential fragmentation mechanism. We find evidence that the neutral NH fragment or intermediate NH$^+$ ion is markedly ro-vibrationally excited. We also identify differences in the relative emission angle between the two photoelectrons as a function of their energy sharing for the four different dication states, which bare some similarities to previous observations made on atomic targets.
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