Neutral cluster mass spectrometry

Abstract

The application of mass spectrometry (particularly time of flight mass spectrometry-TOFMS) to the study of neutral van der Waals and metal containing covalent clusters is discussed. In order to apply TOFMS to the study of neutral clusters, the clusters are ionized by single photon excitation or by resonance enhanced multiphoton excitation, at or near their ionization thresholds. With these techniques, neutral clusters can be accessed and explored to determine their size, structures, dynamics, reactions, ion chemistry, catalytic behavior, and electronic, vibrational, and rotational energy levels. The histories of specific central cluster species are presented to acquaint the reader with the development of neutral cluster studies since the 1980s. Clearly only the highlights (as we see them) of neutral cluster science can be mentioned and referenced in this overview, the intent of which is to explain the roots of the field, present its evolution, and to proffer at least a few of its continuing, developing directions. 3 In order for neutral clusters to be studied through mass spectrometry, the neutral clusters must first be ionized. The generation of neutral clusters in the gas phase, where they can be mass analyzed, almost always yields a rich distribution of cluster sizes and often structures. If mass spectrometry is to yield information about the properties and distribution of neutral clusters, then the neutral clusters must be gently ionized, near threshold, so that fragmentation, significant rearrangement, and internal chemistry can be kept to an absolute minimum. Threshold ionization of neutral clusters is most efficiently and effectively accomplished with a tunable ultraviolet or vacuum ultraviolet (VUV) laser, as most molecular systems have ionization energies between 6 and 16 eV. This can be accomplished by single photon ionization or a resonant one (or more) plus one (or more) photon ionization scheme. Multiphoton, non-resonant laser ionization can cause extensive fragmentation and changes in neutral cluster distribution and structure. Given this caution, we can divide neutral clusters most studied with mass spectrometry techniques into three broad categories: small inorganic molecule clusters, such as (H 2 O) n , (HCOOH) n , (SO 2) n , (NH 3)n, (CO 2) n , (CO) n etc.; organic molecule clusters, such as, (CH 3 OH) n , (C 6 H 6) n , (RCOOH) n , (R'OR) n , (C 6 H 5 NH 2 (N 2) n , etc.; and inorganic, metal clusters, such as, M m X n (X=O, S, N, C, Cl, H,…), M m , M m M' n X y ,… This range of systems includes both van der Waals and covalent clusters. 1 With this collection of clusters, one can study solvent effects, intra-cluster chemical reactions, intra-cluster energy and structure dynamics, cluster fragmentation, and electron distribution for neutral van der Waals molecular clusters, and cluster stability, catalytic reactions, and general cluster chemistry with neutral covalent clusters. All these possible insights depend on mass spectrometry of these clusters, but of course not entirely on its own. First,...

Neutral Au1-Doped Cluster Catalysts AuTi2O3–6 for CO Oxidation by O2

“…In this case, the reactivity study of neutral HMOCs is crucial to have a comprehensive understanding of heterogeneous catalysis. However, it is experimentally challenging to study neutral HMOCs because it is difficult to ionize and detect neutral clusters without fragmentation . Thus, the studies on neutral HMOCs are scarce and the available examples are VCoO 4 and CeAlO 4 clusters, each of which can oxidize one CO molecule.…”

CO oxidation by neutral gold-vanadium oxide clusters

“…Theoretical calculations have been performed to obtain the structures of the AuVO . 2 were calculated to be in the range of 9.12−9.90 eV, thus these neutral clusters could be detected by single photon ionization (118 nm, 10.5 eV) in the experiments [27]. Note that all of these low-lying cluster isomers (IS01−IS06) could be populated in the cluster source.…”

“…After single photon ionization of the clusters by the VUV laser, the cluster ions passed through the reflector of the TOF mass spectrometer and then were detected by a dual microchannel plate detector. The temperature of the reactant gases (10% CO or pure O 2 ) and the fast flow reactor was around 298 K. Note that most of the metal oxide clusters have ionization energies below 10.5 eV [27], and thus can be ionized and detected. The signals from the detector were recorded with a digital oscilloscope.…”

Catalytic conversion of NO and CO into N2 and CO2 by rhodium–aluminum oxides in the gas phase

“…The signals from the detector were recorded with a digital oscilloscope. Note that most of the metal oxides have ionization energies below 10.5 eV and the VUV photoionization is widely considered as a soft ionization process, 28 and thus, the neutral metal oxide species can be gently ionized and generally detected without fragmentation or isomerization. 29 The details of the experimental setup can be found in our previous work.…”