Photodetachment spectroscopy of cluster anions. Photoelectron spectroscopy of H^–(NH₃)₁, H^–(NH₃)₂and the tetrahedral isomer of NH–4

Abstract

The dominant peaks in the photoelectron spectra of the gas-phase, negative cluster ions H-( NH3)1 and H-( NH3)* provide evidence for describing them as ion-molecule complexes comprised of intact hydride ions which are solvated by ammonia. Vertical detachment energies and approximate ionsingle-solvent dissociation energies are obtained. Other spectral features reveal the complexation-induced distortion of the ammonia solvent( s) by their hydride sub-ions. In the photoelectron spectra of H-(NH3), an additional peak appears which is small and unusually narrow and which does not shift upon deuteration. Evidence is presented for interpreting this peak as arising due to the photodetachment of a tetrahedral isomer of NH, . This previously unknown ammonium anion is not a cluster species, and it is described as a NH; ion core with two Rydberg-like electrons.The study of gas-phase cluster anions provides an avenue for addressing open questions in topics as diverse as ion solvation, excess electrons in fluids, ion-molecule reactions, ion-induced nucleation and electronic band structure in solids. In the past, experimental investigations of negative cluster ions have included thermo~hemical,'-~ kine ti^,^ elec-Tl eV= 1.60218 x low9 J. $The C/A peak intensity ratio increased steadily with decreasing photon energies.

Negative Ion Photoelectron Spectroscopy of NH2-(NH3)1 and NH2-(NH3)2: Gas Phase Basicities of Partially Solvated Anions

“…The above assignment of peaks B and C is supported by comparing the spacings between peaks A, B, and C with the stretching frequencies of the free NH3 m~lecule,~' by examining the isotope shifts in the ND~-(NH~)I spectrum, and by comparison to the photoelectron spectra of the H-(NH3), cluster ion series. 23 The spacing between peaks A and B in the NH2-(NH3)] photoelectron spectrum is 3570 f 120 cm-', where the cited error is a conservative estimate of the uncertainty in determining the difference between peak centers. The spacing between peaks B and C is 3000 f 150 cm-I.…”

“…The ion-solvent dissociation energies for H-(NH3)1 and H-(NH3)2 were extracted from the photoelectron spectra shown in Figure 3 as described in ref 23. In order to facilitate comparison between the NH2-(NH3)fl and H-(NH& cluster ion systems, the ion-solvent dissociation energies for both systems are collected together in Table 2.…”

Solvation effects on angular distributions in H−(NH3)n and NH2−(NH3)n photodetachment: Role of solute electronic structure

“…The ground-state band is labeled I, while the additional features, labeled IIa and IIb, respectively reflect activation of one and two NH 3 vibrational quanta, as first identified by the Bowen group 30 and analogous to the similar bands observed for OH − (NH 3 ) n , n = 1 and 2. 28,29,45 These features persist in the spectra of Figure 3 as n increases until clipped or suppressed at or near the energetic cutoff (hν = 3.49 eV).…”

“…The photoelectron energy spectra are consistent with those previously reported by the Bowen group for 488 nm photodetachment of H − (NH 3 ) 1-2 and NH 2 − (NH 3 ) 1-2 . [28][29][30] However, our spectra also include the previously unobserved NH 2 (A 2 A 1 ) ← NH 2 − (X 1 A 1 ) photodetachment transition, as well as signatures of autodetachment and photofragmentation in some of the larger clusters probed, suggesting the existence of metastable charge-transfer states analogous to those identified for the I − (NH 3 ) n , n = 4-15, cluster series. 31,32 We focus our discussion on the photoelectron angular distributions for the observed direct photodetachment transitions.…”

“…28,29 We adopt their assignment of the dominant peaks (I) to direct photodetachment from H − solvated by n ammonia molecules and the weaker feature (II) to photodetachment with activation of an asymmetric ammonia stretching mode. We do not observe the third peak identified by the Bowen group for n = 1 (with VDE = 0.472 eV) and assigned to direct photodetachment from the tetrahedral, double-Rydberg state described as (NH 4 + ) 2− .…”