Search

Experiments on the unimolecular decomposition of ion clusters of the type Ar n -(CH 3 ) 2 O + to form Ar n -CH 3 OCH 2 + and Ar n -CHO + , show the total product ion intensity to be sensitive to the stagnation pressure behind the expansion nozzle. At suffiently low pressures it is possible to form Ar n -(CH 3 ) 2 O + clusters alone with no evidence of any product ions. As the nozzle pressure is increased so the total product ion intensity is observed to increase also. These results are interpreted in terms of the argon component of tile cluster undergoing a transition from a liquid-like to a solid-like state.

A Monte Carlo technique has been used to calculate the relative population densities of rotational and vibrational energy states in non-reactive molecules during the thermal decomposition of O 2 . The results show that non-equilibrium behaviour can lead to extensive population depletion in both the vibrational and rotational energy states.

A Monte Carlo solution to the ro-vibrational master equation for the low pressure unimolecular decomposition of a polyatomic molecule presented recently has been extended to include the simulation of pressure dependent behaviour. The calculations concentrate on two molecular properties which can influence the pressure dependence of a unimolecular reaction, namely adiabatic rotations and the random lifetime assumption. The results from the calculations involving adiabatic rotations show a qualitative agreement with those presented by other authors. However, an examination of the form taken by the random lifetime assumption in RRKM theory reveals that the use of the microcanonical rate constant to give an average lifetime for an activated molecule. does not provide an adequate description of the entire distribution of lifetimes. In particular, calculated results in the fall-off region are found to be sensitive to details of the short lifetime tail of the distribution.

The Monte Carlo method has been used to provide a numerical solution to the ro-vibrational master equation for the low pressure unimolecular decomposition of a polyatomic molecule. This type of solution is made possible through the use of a simple exponential transition probability function, that represents the efficiency with which energy transfer takes place between the reactant molecule and an unspecified heat bath gas. The Monte Carlo technique is used to generate random variables that are distributed in a manner prescribed by the transition probability function. In the case of the present simulation, these variables correspond to random energy jumps induced in the molecule through single collision events. In order to account for the energy dependence of the vibrational state densities, we have proposed that vibrational relaxation in the polyatomic takes place from a single vibrational mode. Under equilibrium conditions we are able to show that with this assumption, the Monte Carlo model is capable of reproducing molecular quantities, such as the average vibrational energy per molecule and the vibrational specific heat, that compare favourable with the corresponding values calculated from equilibrium statistical mechanics. The model has been applied to a study of the low pressure unimolecular decomposition of a series of polyatomics. For three of the molecules, CH 4 , CD 4 , and C 2 H 6 the agreement between the calculated and the high temperature experimental rate constants is very good. The calculations indicate that a significant proportion of the molecules that dissociate are rotationally as well as vibrationally excited. Very few of the reactive molecules have a vibrational energy content equal to or greater than E 0 , the dissociation energy. The extent of rotational excitation is found to be temperature dependent.

A combined molecular beam—mass spectrometer apparatus has been used to measure the average kinetic energy Et released during the unimolecular decomposition of CO2 ion clusters in the field-free region of the mass spectrometer. For the reaction (CO2), n→ (CO2n−1+CO2, the measured value for Et is found to increase as n increases from 3 to 13.

Observations on metastable peaks resulting from the unimolecular decomposition of ion clusters show that intensity variations as a function of cluster size can reveal the presence of stable cluster configurations. This technique has been used to confirm that (H2O)21H+ and (D2O)21D+ are stable ion clusters, and the method also provides evidence to suggest that Ar19+ is a particularly stable species.

Results are presented of a series of calculations in which an attempt has been made to reproduce experimental peak profiles that result from the collision-induced dissociation of polyatomic ions. The calculations show that the peak profiles are insensitive to the details of the collisional energy transfer distribution, but that the shape of the peaks is influenced by changes in the kinetic energy release distribution. For two collision-induced fission reactions, those of the ethanol and propanol ions, good agreement between the experimental and calculated peak profiles is obtained using a quasi-equilibrium theory (QET) kinetic energy release distribution. In the case of the acetone ion decomposition, however, alternative forms of the kinetic energy release distribution have to be considered, in order for the calculated result to reproduce the experimental peak profile.

Ion clusters of the type Ar/sub n/ x X/sup +/ and (CO/sub 2/)/sub n/ x X/sup +/, where X is one of four isotopic variants of methanol, have been formed by electron impact following the adiabatic expansion of an inert gas/methanol mixture. In the case of argon clusters the product ions Ar/sub n/ x CH/sub 2/OH/sup +/ and Ar/sub n/ x COH/sup +/ (or their isotopic equivalents) are observed. For carbon dioxide clusters the only product ion is (CO/sub 2/) x CH/sub 2/OH/sup +/. A consideration of the available experimental data on the fragmentation pattern of methanol ions, and in particular results from charge-transfer and photoelectron spectroscopy experiments, supports the proposal that cluster-bound methanol ions are excited by charge transfer from the inert gas cluster as opposed to an individual inert gas atom or the ionizing electron. From a knowledge of the internal energy of the methanol ions a lower limit to the vibrational predissociation time has been calculated as 10/sup -12/ s.

The electron impact ionization of hydrogen-bonded clusters of water and ammonia results in the formation of stable doubly charged ions only when the cluster size exceeds a critical value. The observed minimum cluster size for water is (H 2 O) 35 H 2 2+ and for ammonia is (NH 3 ) 51 H 2 + . From a knowledge of the relative hydrogen-bond strenghts, it has been possible to account for the size difference between these ions and to qualitatively discuss the locations of the positive charges within each cluster.