Atish D. Sen

Association reactions at low pressure. III. The C2H+2/C2H2 system

The association reactions, C4H2(+) + C2H2 and C4H3(+) + C2H2 have been examined at pressures between 8 x 10(-8) and 1 x 10(-4) Torr at 298 K in an ion cyclotron resonance mass spectrometer. Association occurred via two different mechanisms. At pressures below approximately 2 x 10(-6) Torr, the association was bimolecular having rate coefficients k2 = 2.7 x 10(-10) cm3 s-1 and 2.0 x 10(-10) cm3 s-1 for C4H2+ and C4H3+, respectively. At pressures above approximately 2 x 10(-6) Torr, termolecular association was observed with rate coefficients, k3 = 5.7 x 10(-23) cm6 s-1 and 1.3 x 10(-23) cm6 s-1 for C4H2+ and C4H3+, respectively, when M = C2H2. The termolecular rate constants with N2, Ar, Ne, and He as the third body, M, are also reported. We propose that the low pressure bimolecular association process was the result of radiative stabilization of the complex and the termolecular association process was the result of collisional stabilization. Elementary rate coefficients were obtained and the lifetime of the collision complex was > or = 57 microseconds for (C6H4+)* and > or = 18 microseconds for (C6H5+)*. At pressures below 1 x 10(-6) Torr, approximately 11% of the (C6H4+)* were stabilized by photon emission and the remaining approximately 89% reverted back to reactants, while approximately 24% of the (C6H5+)* were stabilized by photon emission and the remaining approximately 76% reverted back to reactants. The ionic products of the C2H2(+) + C2H2 reaction, C4H2+ and C4H3+, were found to be formed with enough internal energy that they did not react by the radiative association channel until relaxed by several nonreactive collisions with the bath gas.

Association reactions at low pressure. IV, The HC3N+/HC3N system

The reactions between HC3N+ and HC3N, and between HC5N+ and HC3N have been examined at pressures from 1×10−7 to 1×10−3 Torr by ion cyclotron resonance mass spectrometry. The reaction between HC3N+ and HC3N has both a bimolecular reaction path and a termolecular reaction path. The overall bimolecular reaction rate coefficient was found to be 1.3×10−9 cm3 s−1. The primary product, HC5N+, represents 90% of the product ions, while the minor products HC6N+2 and H2C6N+2 each represent 5%. The termolecular association rate coefficient was 3.7×10−24 cm6 s−1 with He as the third body. From double resonance experiments the mean lifetime of the collision complex was determined to be 180 μs. HC5N+ was found to react with HC3N and form the adduct ion H2C8N+2 through both bimolecular and termolecular channels. The bimolecular rate coefficient was 5.0×10−10 cm3 s−1 and the termolecular rate coefficient was observed to be 1.2×10−22 cm6 s−1 with HC3N as the third body. With He as the stabilizing molecule, the termolecular...

Lifetime measurement of a collision complex using ion cyclotron double resonance. H2C6N+2

In the ion–molecule reaction between HC3N+ and HC3N, the lifetime of the collision complex (H2C6N+2)* was long enough that ion cyclotron double resonance techniques could be used to probe the distribution of the lifetimes of the collision complex. The mean lifetime of the collision complex at room temperature was measured as 180 μs with a distribution ranging from 60 to 260 μs as measured at the half‐heights in the distribution. Lifetimes of this magnitude with respect to unimolecular dissociation allow for some stabilization of the collision complex by the slower process of infrared photon emission.

Association reactions at low pressure. V. The CH+3/HCN system. A final word?

The reaction of the methyl cation with hydrogen cyanide is revisited. We have confidence that we have resolved a long standing apparent contradiction of experimental results. A literature history is presented along with one new experiment and a re‐examination of an old experiment. In this present work it is shown that all of the previous studies had made consistent observations. Yet, each of the previous studies failed to observe all of the information present. The methyl cation does react with HCN by radiative association, a fact which had been in doubt. The product ions formed in the two‐body and three‐body processes react differently with HCN. The collisionally stabilized association product formed by a three‐body mechanism, does not react with HCN and is readily detected in the experiments. The radiatively stabilized association product, formed by a slow two‐body reaction, is not detected because it reacts with HCN by a fast proton transfer reaction forming the protonated HCN ion. Previous studies eit...

Dynamics of the dissociative and nondissociative scattering of hyperthermal CS2+ from a self-assembled fluoroalkyl monolayer surface on gold substrate

Dissociative and nondissociative scattering of low energy CS2+ ions from a self-assembled monolayer surface of fluorinated alkylthiol [CF3(CF2)9CH2CH2SH] on vapor deposited gold has been studied using a modified crossed-beam instrument. Dissociation of CS2+ ions begins at ∼30 eV ion kinetic energy, much higher than the thermochemical threshold of 4.7 eV for the lowest energy dissociation channel forming S+. This product channel is dominant up to the ion energy of ∼50 eV, the highest energy accessible by this instrument. Both inelastically scattered parent ions and product ions leave the surface with very low kinetic energies, demonstrating that most of the ions’ kinetic energy is taken up by the surface rather than transferred into internal modes of recoiling ions. The scattered ion intensity maximum is found between the specular angle and the surface parallel. At all energies studied, primary ion intensity remains higher than that of fragment ions. Also, the intensity of S+ fragment ions is higher than t...

A comparison of an experimental unimolecular lifetime distribution with Rice-Ramsperger-Kassel-Marcus theory

The ion–molecule association system (CH+3/CH3CN) has been reexamined by the ion cyclotron double resonance technique. An experimental distribution of lifetimes has been measured for the collision complex (CH3CNCH+3)* formed in the association reaction between CH+3 and CH3CN. The experimental mean lifetime of the association complex formed within the ICR cell was 140 μs. A theoretical examination of the distribution of complex lifetimes using an RRKM model was also undertaken. The matrix of lifetimes for the various values of the total energy of the system (E) and the total angular momentum of the system (J) was obtained. This information was used to visualize the canonical ensemble of collision complexes in the ICR experiment in terms of their lifetimes. Once the distribution of lifetimes predicted by the model was modified to conform to experimental constraints, it was found to give a good approximation of the lifetime distribution determined experimentally. As a result of the new measurements of the com...

Sulfur and nitrogen reactions for cometary comae ion chemistry

The low pressure reactions of sulfur dioxide, carbon disulfide, and hydrazine with H2O+ and H3O+ were studied by the ion cyclotron resonance technique. These reactions are potentially important for sulphur chemistry in cometary comae. Rate coefficients and branching ratios of product channels are presented.