Calculation to determine the mass of daughter ions in metastable decay

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Abstract

A new method is presented for determining the mass of products resulting from metastable decay in a reflectron time-of-flight mass-spectrometer. The validity of the calculation was confirmed through comparison to experimental values obtained while studying a water cluster and a methanol cluster system.

Introduction

The study of clusters has become a flourishing field providing new information between the realms of the gas and condensed phases. A plethora of new phenomena has been uncovered over the recent years as those interested look deeper into this medium with an increasing number and variety of methods. Cluster science continues to grow as a vehicle of study providing insight into the molecular world of nanoscale dimensions.

Previous studies of clusters, focusing on the study of unimolecular reactions in the form of metastable decay (MSD) during cluster fragmentation in the first field free region (FFR) of a reflectron time-of-flight mass spectrometer (R-TOFMS) (see Fig. 1), have provided information about the binding energy of clusters [1], [2], [3] and the kinetic energy released (KER) [1], [4], [5]. The metastable decay processes, which are observable in the time windows accessible by this technique, are ones which occur after neutral clusters are ionized in a Wiley-McLaren TOF lens assembly and thereafter accelerated into the first FFR. Ions possessing sufficient internal energy from the ionization process can undergo cluster rearrangement and/or a cluster fragmentation analogous to evaporation. These “hot” ion clusters, referred to as parent ions, which have a sufficient amount of energy to undergo fragmentation, lose neutral n-monomer units, where n is an integer, to become daughter ions. Rearrangement of the excited ion clusters can occur without fragmentation; however, daughter ions are not produced in this situation and therefore these are not included in the study of MSD.

Some examples of cluster systems that undergo the process of metastable decay are protonated water clusters, (H+(H2O)n) [6], protonated methanol clusters, (H+(CH3OH)n) [7], protonated ammonia clusters (H+(NH3)n) [8], and rare gas clusters such as Arn+ [9]. A number of properties of clusters can be determined from studying the unimolecular reaction processes such as the binding energy and KER as mentioned previously, and the rates of the unimolecular reaction [10]. However, according to theoretical predictions made by Klots considering metastable decay as an evaporative ensemble model [11] and experimental verifications [1], [8], [12], [13], [14], no cluster ion ensemble may be assigned a unique lifetime due to the fact that the reaction rates are not inherent to the ions, but are caused by a particular range of internal energies (temperatures) after the ionization event. In the experiments of Märk et al. [12], [13], [14], the voltage of the acceleration region was scanned. As the voltage of TOF1 was decreased, the rates of reaction of the metastable decay also decreased as large as a factor of 10, proving that the MSD did indeed occur over a statistical range as Klots predicted.

However, no theoretical treatment has been derived to determine the loss of mass due to the fragmentation of the parent ion clusters into daughter ion clusters. Previously, the only effective method of determining the mass of the daughter ions (md) was to perform a time intensive cutoff study using the reflectron as an energy analyzer. Presented herein is a method to determine md using the difference in time (Δt) between the arrival of the parent and daughter ions at the detector without the tedious process of performing a reflectron cutoff study.

Section snippets

Experimental

Water clusters were generated via supersonic expansion of room-temperature water vapor seeded in helium at a pressure between 1.7 and 2.4 bar. The molecular beam produced in this fashion was skimmed and then ionized between the TOF grids with femtosecond laser pulses. The methanol clusters were generated in a similar fashion.

Under typical operating conditions, a potential of 4600 V was applied to TOF1 whereas the potential applied to TOF2 was 2980 V. In the studies presented here, the

Results and discussion

The majority of detected ions are created in the ∼6.6 μm focal point of the laser positioned between the repeller plate (TOF1) and the accelerating plate (TOF2). The potential on these two plates is applied in order to accelerate the parent ions toward the FFR. Since all clusters are ionized in a very confined region within the potential, the charged clusters have a very narrow kinetic energy distribution (±1.1 V). Therefore, the velocity of each parent ion, once it is accelerated into the FFR,

Conclusions

Presented in this paper is a new, less demanding experimental method, to determine the mass of daughter ions that form when parent cluster ions fragment in an evaporative metastable decay process. This process occurs in the first FFR of a R-TOFMS. Previous studies of MSD have provided information concerning binding energy of clusters, kinetic energy released during fragmentation, and rates of decay that are not inherent to the ions, but are caused by a particular range of internal energies

Acknowledgements

The authors gratefully acknowledge Department of Energy support for this work under Grant DE-FG02-92ER14258. The authors would also like to thank Carisa Kelley, Kevin Riley, and Ryan Stairs for their invaluable assistance.

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