$X^{+} (X=Ar, Kr, N_{2}) + H_{2}O$ charge-transfer luminescence: Vibrationally-selected $H_{2}O^{+} \tilde{A} - \tilde{X}$ emissions
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Date
1994
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Ohio State University
Abstract
Exothermic low-energy (1-20 $eV^{+}$ c.m.) charge-transfer reactions are primarily governed by the energy resonance criterion. They thus offer a convenient means of producing ions in a narrow energy range of rovibronic states. A further constraint on the probability of populating near-resonant charge-transfer states is the necessity of a favourable Frank-Condon overlap between the vibrational wavefunctions of the reactants and the products. Rotational energy transfer is insignificant due to the long-range nature of charge-transfer process. Consequently, if electronically excited states are accessed in near-resonant charge-transfer collisions, omission spectra can be observed involving only few excited state vibrational levels with near-thermal rotational distributions. We have studied charge-transfer luminescence from following systems: \begin{eqnarray} \begin{array}{ll} Ar^{+}(^{2}P_{3/2}) + H_{2}O &\rightarrow Ar + H_{2}O^{+} (\widetilde{X}\ ^{2}B_{1}) + 3.14eV \\ Kr^{+}(^{2}P_{1/2}) + H_{2}O &\rightarrow Kr + H_{2}O^{-} (\widetilde{X}\ ^{2}B_{1}) + 2.04 eV \\ N_{2} + H_{2}O &\rightarrow N_{2} + H_{2}O^{+} (\widetilde{X}\ ^{2}B_{1}) + 2.096 eV \end{array} \end{eqnarray} The luminescence is produced by propagating a mass and energy-selected ion beam through a collision cell that is fiber-optically coupled to an optical multichannel analyzer. The 0.5 nm (fwhm) resolution spectra show that the reaction exothermicity is almost exclusively partitioned to internal modes of $H_{2}O^{+}$, yielding $H_{2}O^{+} \widetilde{A}^{2}A_{1}$ sate products in few bending vibrational levels at energies given by the respective exothermicities. In some cases emissions from previously not observed high K levels (up to $K = 6$) are identified that are related to dynamical effects. The most recent $Kr^{1} + H_{2}O$ luminescence measurements are carried out with a very sensitive liquid-nitrogen cooled CCD detector providing greater than 40% quantum efficiency in the 600-800 nm range. We plan measurements at considerably higher resolution and hope to determine hitherto unknown spectroscopic constants in order to support the numerous computational studies of the $H_{2}O^{+} \bar{A}^{2}A_{1} = \widetilde{X}^{2}B_{1}$ Renner-Teller system.
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Author Institution: Philips Laboratory, PL/WSSI