Gregory I. Gellene

Stability of the ammonium and methylammonium radicals from neutralized ion‐beam spectroscopy

The stability of the ammonium radical (NH4) was determined from measurements of the kinetic energy released in its fragmentation products following formation in a fast electron capture process: NH4++Na → NH4*+Na+. Scattering profiles for heavy (NH3) and light (H) dissociation products were obtained from beam measurements with 5–16 keV NH4+ ions. The existence of a predissociative barrier in the radical is inferred from edge structure and scattering continua in H atom profiles. The radical is bound with respect to a potential minimum but all of the isotopic species NH4, NH3D, NH2D2, and NHD3 undergo rapid loss of H atoms and have ground states lying above their dissociation limits. The radical ND4 has unusual stability with its ground state lying close to or below its dissociation limit. Dissociative lifetimes for stable and unstable states of ND4 differ by at least two orders of magnitude. The possible significance of these observations on the interpretation of optical transitions involving the ground sta...

Experimental evidence for metastable states of D3O and its monohydrate by neutralized ion beam spectroscopy

The oxonium radical (H3O) has been generated in its ground state by neutralizing a fast beam of ions in the near resonant electron transfer reaction H3O++K(g)→H3O*+K+. Analysis of neutral beam scattering profiles and collisionally reionized mass spectra indicate that the fully deuterated species (D3O) can be formed in a distribution of dissociative and metastable states (τ>0.6 μs). Thermalization of the precursor D3O+, prior to electron transfer, is required for production of metastable D3O. Neither H3O nor D2HO is observed in metastable states. These isotope effects support earlier theoretical predictions of a shallow local minimum on the oxonium potential surface. The ionization potential of D3O is calculated to be 4.3±0.1 eV. Some spectroscopic implications for this radical are discussed. The oxonium monohydrates (H3O⋅H2O) are also observed to exist in metastable states for several H/D isotopic variants. The ionization potential of D3O⋅D2O is estimated to be ≥3.4 eV.

Experimental observations of excited dissociative and metastable states of H3 in neutralized ion beams

Electron transfer reactions for a fast beam of H3+ ions with Mg and K atoms have been investigated by neutral beam scattering techniques. Reactions with Mg and K targets form H3 molecules in the dissociative 2p 2E′ ground state and predissociative 2s 2A1′ and 2p 2A2″ excited states, respectively. Fragmentation energies, obtained from beam scattering measurements, allow the scaling of these electronic states of H3 with respect to their dissociation products. A metastable form of H3 observed in the H3+/K reaction is identified as the nonpredissociating, nonrotating molecule in the 2p 2A2″ electronic state. The cross section for the state‐to‐state process H3+(X 1A1′, N=1, K=0)+K(g)→H3*(2p 2A2″, N=K=0)+K+ for a 6 keV ion beam is 7.0±1.0 A2. Total ion beam attenuation cross sections for the species H3+, H2D+, D2H+, and D3+ with K targets are in the relative order 1.0, 0.59, 0.58, 0.53. The higher cross section observed for the H3+/K reaction is partially accounted for by an usually high cross section for the n...

A study of the target gas dependence on the collisional ionization mass spectra of some metastable and hypervalent molecules

Collisional ionization mass spectra of a fast beam of neutral molecules with ionization potentials in the range 4–5 eV have been obtained with a series of target gases, NO2, O2, N2 and He. Neutral beams were generated in a near resonant electron transfer process X+ +K(g) → X* + K+ where X+ is He2+, H3+, ND4+ or N2D7+. The efficiency of collisional ionization of metastable He2* and the ground state radicals ND4 and N2D7 on target gas decreases in the order NO2 > O2 > N2 > He. Of the four neutral species studied, only in the case of H3* is a change in rotational state of the projectile required for ionization. This restriction may explain the lack of an apparent target gas dependence on the efficiency of ionizing H3*. The technique is extended to a study of the CD5 radical where no evidence for metastability is observed. An upper limit of 10−8 s is set on the lifetime of this species.

An experimental study of the bridged and classical structures of the ethyl radical and cation by neutralized ion‐beam spectroscopy

The states of ethyl radicals formed in electron capture reactions of a fast beam of C2H5+ with a series of metal target atoms (K, Na, Ca, Mg, and Zn) have been investigated by neutral beam scattering techniques. Reactions of C2H5+ with Mg or Zn atoms lead to formation of ground state radicals and an excited state about 0.5 eV above the dissociation limit of C2H4+H. Analysis of branching ratios for these processes over a range of Mg atom densities shows that the ion beam is a mixture of two isomers in the ratio of about 7.3/1.0 for beams generated either by electron impact or chemical ionization methods. From neutral‐precursor ion relationships we propose that the major and minor components of the ion beam are the bridged and classical C2H5+ structures, respectively. Analysis of neutral beam profiles in experiments with Na or K targets indicate the formation of a dissociative state of the classical radical lying about 4.2 eV above the ground state. An upper limit of 2.2 eV to the barrier to 1, 2 hydrogen m...

Formation of CHn molecules in excited electronic states by electron capture collisions of CH+n ions with metal atoms

Neutralized ion beam experiments with 5 keV CH+n ions have provided information on the nature of the electronic state of the neutrals formed by electron capture from metal atom targets (Na, K, Mg, and Zn). A general trend from formation of excited dissociative states toward stable electronic states is noted as the ionization potential of the target metal is increased from 4.4 eV (K) to 9.6 eV (Zn). Within experimental uncertainties fragmentation energies of the dissociating state formed in electron capture collisions of CH+4 or CH+3 with Na, K, or Mg atoms are independent of the ionization potential of the target. Results of our measurements show that CH4 and CH3 are formed exclusively in these reactions in the 3T2 and 2A1′ states, respectively. The dissociating state of CH2 formed by electron transfer to CH+2 from Na or K atoms is identified as the nonoptical 3B2 state which is known only from theoretical calculations. Analysis of neutralized CH+ beam profiles indicates that CH+ ions formed by electron i...

Electronic energy transfer in near‐resonant electron capture collisions of H+2 with metal atoms: Radiative and nonradiative transitions

Modes of energy disposal in electron capture of H+2 with metal atoms (Cs, K, Mg, and Zn) for ion velocities in the range 3–7×107 cm/s are examined using combined optical and beam scattering techniques. Radiative and nonradiative transitions are observed for processes occurring under near resonant conditions. The following branching sequences are identified: Branching ratios are dependent on the vibrational state and the nuclear separation (Franck–Condon factors) of the H+2 ion at the time of electron capture. The branching ratio decreases for the (triplet)/(singlet) formation for H2 produced from reactions of vibrationally relaxed H+2 ion with K or Cs. Under conditions of H+2 ion relaxation, the kinetic energy of scattered atomic hydrogen following radiative decay from 3Σ+g state of H2 increases, implying a shift in the 3Σ+g→3Σ+u continuum toward longer wavelengths. The results also show that, at these velocities, the reations occur under near‐resonant conditions with vertical transitions.

Energy partitioning in He2+/K collisions

Final state distributions and state‐specific reaction cross sections for 1.5–6 keV He2+/K collisions have been determined from data acquired using both neutralized ion beam spectroscopy and direct observation of luminescence from decay of excited collision products. While several minor reaction channels can be identified, it is found that near‐resonant processes yielding He2(a 3Σu+) and He2(A 1Σu+) are predominant. These major channels, both of which have cross sections in excess of 100 A2, lead to substantial production of energetic metastable molecules and VUV continuum radiation extending from about 750 to 850 A.

Observations of Excited Metastable and Radiative States of He2, Ne2, and Ar2 by Neutralized Ion-Beam Spectroscopy

Rare gas dimers formed when a fast beam of X 2 + (X = He, Ne, Ar) is neutralized in the reaction