A guided-ion beam study of the hydrogen atom transfer reaction of state-selected N+2 with H2 at collision energies ranging from subthermal to 2 eV (c.m.)

Abstract

This article presents detailed internal and kinetic energy dependent cross sections and reaction rates for the hydrogen atom transfer processes N+2(X 2Σ+g, v+=0–4, J+=2)+H2→N2H++H, which were obtained under single-collision conditions in a guided-ion beam/scattering gas experiment. Preparation of ions in specific states relied on single-color excitation within a resonantly enhanced (2+1) multiphoton ionization scheme. The translational energy of the ions, Elab, was varied from 0.1 eV to approximately 30 eV. A small activation barrier impedes the reaction. Vibronic state preparation of the nitrogen ion is influential on the nature of the energy surface—N+2+H2 or H+2+N2—along which the H atom transfer proceeds. Calculations of model potential energy surfaces suggest that the reaction pathway must involve several exoergic and endoergic channels which open successively as the collision energy increases. A purely collision determined cross section—as would be evidenced by the E−1/2 dependence formulated in the Langevin–Gioumousis–Stevenson model—is observed only within a narrow window of kinetic energies.

CRITICAL REVIEW OF N, N + , N + 2 , N ++ , And N ++ 2 MAIN PRODUCTION PROCESSES AND REACTIONS OF RELEVANCE TO TITAN'S ATMOSPHERE

“…At 150 K, it is about 1.3 × 10 −9 cm 3 s −1 , which is our recommended value for Titan's ionosphere. Knott et al (1995) measured the cross section as a function of collision energy and found two regimes: it follows an E −0.5 CM law for E CM = 8-90 meV and an E −0.33 CM law for E CM = 90 meV to 2 eV. These authors did not observe a variation of the rate constant with the vibrational level of N + 2 .…”

“…For ground-state reactants, the reaction proceeds at thermal energies with the rate predicted by the Langevin classical ion-molecule capture theory (1.56 × 10 −9 cm 3 s −1 ). Vibrational excitation of the N + 2 appears to have only minor effects on the reaction rates (de Gouw et al 1995;Knott et al 1995). As far as the electronic excitation is concerned, the reaction rate for N + 2 in the first electronically excited A 2 Π u state is 50% lower with respect to the X state (Uiterwaal et al 1995).…”

“…Given the above, the recommended rate constant values are the Langevin N + 2 + H 2 300 K 1.8 × 10 −9 (±30%) SIFT (Tichy et al 1979) N + 2 + H 2 300 K 2.1 × 10 −9 (±20%) SIFT (Smith et al 1978 (Tosi et al 1992) N + 2 + H 2 300 K 1.72 × 10 −9 (±30%) for H 2 GIB (Schultz & Armentrout 1992) 1.27 × 10 −9 (±30%) for HD 1.15 × 10 −9 (±30%) for D 2 N + 2 + H 2 300 K 1.71 × 10 −9 (±5%) for (X, v = 0) SIFT-LIF (de Gouw et al 1995) 1.69 × 10 −9 (±3%) for (X, v = 1) 1.80 × 10 −9 (±6%) for (X, v = 2) 1.68 × 10 −9 (±3%) for (X, v = 3) 1.47 × 10 −9 (±5%) for (X, v = 4) N + 2 + H 2 300 K 1.1 × 10 −9 (±20%) for (X, v = 0-4, J = 2) a GIB (Knott et al 1995 (Warneck 1972) N + 2 + CH 4 300 K 1.06 × 10 −9 (±5%) IonTrap (Li & Harrison 1978) N + 2 + CH 4 300 K 1.30 × 10 −9 (±30%) SIFT (Tichy et al 1979) N + 2 + CH 4 298 K 9.80 × 10 −10 (±10%) ICR (Huntress et al 1980) N + 2 + CH 4 300 K 1.00 × 10 −9 (±20%) SIFT (Smith et al 1978) N + 2 + CH 4 70 K 1.20 × 10 −9 (±30%) CRESU (Rowe et al 1989 (Praxmarer et al 1998) N + + N 2 300 K 1.8 × 10 −10 (± 40%) b FA (Fehsenfeld et al 1974) N + + N 2 300 K 3.3 × 10 −10 (±25%) b ICR (Anicich et al 1977) N + + N 2 300 K 1.6 × 10 −10 (±20%) b,c GIB (Freysinger et al 1994) N + + N 2 300 K 1.9 × 10 −10 (± 20%) b,c GIB-TOF (Glosik et al 2000) N + + H 2 300 K 5.6 × 10 −10 (± 30%) FA (Fehsenfeld et al 1967) N + + H 2 298 K 4.8 × 10 −10 (± 10%) ICR (Kim et al 1975) N + + H 2 300 K 6.4 × 10 −10 (± 20%) SIFT (Adams & Smith 1976) N + + H 2 300 K 6.2 × 10 −10 (± 30%) SIFT (Tichy et al 1979) N + + H 2 300 K 4.8 × 10 −10 (± 20%) SIFT (Smith et al 1978) N + + H 2 300 K 3.7 × 10 −10 (±20%) for H 2 SIFT ( 298 K 1.35 × 10 −9 (±10%) ICR (Anicich et al 1977) N + ( 3 P ) + CH 4 300 K 1.1 × 10 −9 (±30%) SIFT (Tichy et al 1979) N + ( 3 P ) + CH 4 300 K 9.4 × 10 −10 (±20%) SIFT N + ( 3 P ) + CH 4 300 K 1.20 × 10 −9 (±10%) DT (Dheandhanoo et al 1984) N + ( 3 P ) + CH 4 8 K 8.2 × 10 −10 (±35%) CRESU N + ( 3 P ) + CH 4 300 K 1.15 × 10 −9 (±15%) Compilation (Anicich 1993) N + ( 3 P ) + CH 4 298 K 1.1 × 10 −9 (± 20%) SIFT (Anicich et al 2004) N + ( 3 P ) + CH 4 0.2-2 eV 1.5 × 10 −9 (±25%) a GIB Reactive collisions between N + 2 and H...…”

Theoretical investigations of the N2H2+ cation and of its reactivity

“…11 This diagram shows the high density of exiting channels, which may complicate the studies for this system. Experimentally, the following channels have been investigated: 6,10,[12][13][14][15][16] …”

“…6,10,15,16,29 Absolute cross sections are available for processes ͑1͒-͑3͒ from thermal to high collision energies. The final products may carry some internal energy or not corresponding to their electronic and/or vibrational and/or rotational excitation during these reactions.…”