tokens: excited state along the HT pathway. In the previous He i photoelectron study (Reutt et al. 1986 ), the IE values for the formation of the ( X 2 Π u and states were determined as 11.403 and 16.297 eV, respectively. Taking into account that the IE(N 2 ) = 15.581 eV, the first excited electronic state is expected to become accessible when E cm is greater than 0.72 eV. Compared with the distinct increase of σ HT ( v + = 0) at E cm = 0.70–1.00 eV, the σ CT ( v + = 0) only exhibits a slight decline at the same E cm range, indicating that the accessibility to the excited state only enhances the HT channel. While the σ CT ( v + = 0) curve of Figure 2 (a) is consistent with a long-range CT mechanism, the σ HT ( v + = 0) may involve an efficient coupling between kinetic energy and internal vibrational energy, enhancing the formation of N 2 H + in shorter ranges. We note that the thermal motion of neutral reactant molecules C 2 H 2 at room temperature T = 298 K can affect the kinetic energy resolution (Δ E cm ) for a given E cm value. Chantry has shown that the Δ E cm value or the Doppler width caused by the thermal motion of the target gas alone is equal to (11.1 γkTE cm ) 1/2 , where γ = m + /( m + + M ) (Chantry 1971 ). The estimated thermal broadening for the present reaction system is about one order of magnitude smaller than the E cm when E cm ≥ 1.00 eV. But the thermal broadening becomes comparable with E cm when 0.10 eV ≤ E cm 1.00 eV, and even larger than the E cm when E cm 0.10 eV. Because of the relatively large Doppler width at E cm 0.10 eV, the measured σ CT ( v + = 0) values at low E cm values shown in Figure 2 (a) could be distorted. Vibrational-state dependence. In order to investigate the effect of vibrational excitation on the chemical reactivity of ; v + ) toward C 2 H 2 , we have compared in Figures 3 (a) and (b) the respective σ CT ( v + ) and σ HT ( v + ) curves obtained at E cm = 0.03–10.00 eV with the reactant PFI-PIs prepared in the v + = 0, 1, and 2 vibrational states. As shown in the figures, after taking into account the experimental uncertainties, the σ CT ( v + ) and σ HT ( v + ) curves for v + = 1 and 2 vibrational states are found to be nearly in agreement with the corresponding σ CT ( v + = 0) and σ HT ( v + = 0) curves. Thus, we conclude that both the CT (reaction ( 2.1 )) and the HT (reaction ( 2.2 )) channels have only marginal vibrational effects. Nevertheless, the measured CT and HT cross sections seem to show weak vibrational inhibition at low E cm values and weak vibrational enhancement at high E cm values. In the range of E cm = 0.03–0.60 eV, the ( v + = 1) state shows the lowest CT reactivity. For E cm > 0.60 eV, the CT cross section is higher for reactant ion in the v + = 1 and 2 states compared to that in the v + = 0 state. For the convenience of utilizing the results obtained here for atmospheric modelings, we have listed in Table 1 all integral cross sections plotted in Figures 2 , 3 (a), and 3 (b). Figure
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