Formation and distribution of neutral vanadium, niobium, and tantalum oxide clusters: Single photon ionization at 26.5eV

Abstract

Neutral vanadium, niobium, and tantalum oxide clusters are studied by single photon ionization employing a 26.5 eV/photon soft x-ray laser. During the ionization process the metal oxide clusters are almost free of fragmentation. The most stable neutral clusters of vanadium, niobium, and tantalum oxides are of the general form (MO2)0,1(M2O5)y. M2O5 is identified as a basic building unit for these three neutral metal oxide species. Each cluster family (Mm, m=1,...,9) displays at least one oxygen deficient and/or oxygen rich cluster stoichiometry in addition to the above most stable species. For tantalum and niobium families with even m, oxygen deficient clusters have the general formula (MO2)2(M2O5)y. For vanadium oxide clusters, oxygen deficient clusters are detected for all cluster families Vm (m=1,[ellipsis (horizontal)],9), with stable structures (VO2)x(V2O5)y. Oxygen rich metal oxide clusters with high ionization energies (IE>10.5 eV, 118 nm photon) are detected with general formulas expressed as (MO2)2 (M2O5)y O1,2,3. Oxygen rich clusters, in general, have up to three attached hydrogen atoms, such as VO3H1,2, V2O5H1,2, Nb2O5H1,2, etc.

Single photon ionization of hydrogen bonded clusters with a soft x-ray laser: (HCOOH)x and (HCOOH)y(H2O)z

“…EI causes substantial fragmentation, and neutral parent cluster information is thereby lost. Our soft x-ray laser ͑26.5 eV photons͒ has proven itself in the past to be a gentle, single photon ionization source [3][4][5][6] and we confirm that to be the case as well in the present study of formic acid.…”

“…The metastable dissociation process of losing one dimer to obtain reaction ͑5b͒ may also be identified as a process of losing two monomers; however, it has been demonstrated in our previous work [3][4][5][6] that the loss of only one neutral molecule from a metastable cluster ion by unimolecular dissociation can occur on a microsecond time scale using single photon ionization of our 26.5 eV laser. So in this case, losing one dimer unit, not two monomers, from the parent ion is probably correct.…”

“…The experimental apparatus and soft x-ray laser have been described in previous publications from this laboratory [3][4][5][6] and therefore only a general outline of the experimental scheme will be presented in this report. A tabletop soft x-ray laser ͑26.5 eV photons͒ is used as the single photon ionization source.…”

“…We have learned from studies of H 2 O, CH 3 OH, NH 3 , 3 SO 2 , 4 CO 2 , 5 and metal oxide clusters, 6 as well as this study of HCOOH, that 26.5 eV photons ionize van der Waals and hydrogen bonded clusters such that very little energy above the vertical ionization energy ͑VIE͒ remains in the cluster. Cluster fragmentation in the ionization region of the mass spectrometer ͑less than 1 s following ionization͒ is driven in most instances by the energy difference between cluster VIE and adiabatic ionization energy ͑AIE͒ or the energy difference between VIE and the barrier to the proton transfer/ dissociation reaction following ionization.…”

“…Similar results, with regard to soft x-ray laser single photon ionization, are found for other van der Waals and hydrogen bonded clusters, as well as metal and metal oxide clusters. [3][4][5][6] The neutral cluster distribution ͑HCOOH͒ n+1 , as observed through ͑HCOOH͒ n H + ions, shows an anomalous relative intensity peak ͑magic number͒ at n =5 ͑for certain conditions͒. For the neutral HCOOH / H 2 O cluster series, the protonated mixed cluster series ͑HCOOH͒ n ͑H 2 O͒ m H + for n =1-8 and m =0-4 is observed.…”