Tandem time-of-flight experiment for low energy collision studies

Abstract

We present an experiment adapted to collisional studies of cluster ions based on a laser vaporization setup coupled to a supersonic expansion. The ions are selected in a first time-of-flight, slowed down to the desired energy, and collided in an octopolar guide. The parent and fragment ions are then reaccelerated in order to be mass analyzed in a reflectron time-of-flight. An original method for the extraction of the ion that uses a double voltage pulse, is proposed. The experiment has been applied to collisions of hydrated cobalt ions. An absolute cross section of 17 A2 for the loss of one water molecule by Co(H2O)2+ in collision with neon at a center-of-mass energy of 10 eV, has been determined, with an accuracy of 10%. The threshold for this reaction has been measured at 1.5 eV and is in good agreement with the existing literature (Dalleska et al. J. Am. Chem. Soc. 1994, 116, 3519). Ions that cannot be formed by conventional ligand exchange methods, can also be studied. As an example, the threshold for dehydration of the Co2(H2O)+ ion has been measured at 1.5 +/- 0.2 eV.

New device to study unimolecular cluster nucleation

“…Plates n°2 to 10 of the apparatus are used to slow down the ions. An exponential decelerating potential is applied (V i =V 0 (1-e -i/Γ ) for the plate i) in order to minimize spatial defocusing [11]. The exponential shape of the potential is obtained by connecting the plates with appropriate resistors.…”

“…Nevertheless, low energy collisions of mass selected clusters with atoms or molecules have already been carried out: ref. [11] deals with the fragmentation of M(H 2 0) 2 + (M=Fe, Co, Au), in [12] the collision Ar + n=2-23 + 36 Ar is studied, in [13] the reaction of H + (H 2 O) n=2-6 with acetone is investigated and in [14] the authors measure the cross section for incorporation of ND 3 in H + (NH 3 ) n=3-9 . In these experiments quite small clusters are produced in abundance in a supersonic expansion.…”

“…In these experiments quite small clusters are produced in abundance in a supersonic expansion. In reference [11] mass selection is made by a TOF and the energy spread is reduced by setting particular temporal shapes to the extracting acceleration voltages.…”

“…Unfortunately, this is at the cost of a severe decrease in the intensity of mass-selected clusters. In references [11,12,13] the authors use the so-called guided ion beam (GIB) method. The mass selection of both reactant and products is done by quadrupole mass filters.…”

“…Without any additional device, the kinetic energy spread is about 10 eV [11]. In order to reduce it to a few eV, Kawai [12] and Orii [13] use a Bessel box energy analyzer.…”