A. Friedmann

Electronic spectra of N+2–(He)n (n=1, 2, 3)

Ionic clusters of nitrogen and helium have been investigated by recording their electronic spectra in the near UV. Structured bands belonging to 14N+2 –He, 14N+2–(He)2, 14N+2–(He)3, and 15N+2 –He have been measured between 390 and 392 nm close to the N+2 B 2Σ+u←X 2Σ+g band origin. Spectra were obtained by exciting mass selected cluster cations with tunable laser radiation and recording the photodissociation cross section as a function of wavelength. The data support the hypothesis that the He– – –N+2 interaction potential has only a small barrier to internal rotation in both the X and B electronic states. A lower estimate for the dissociation energy of the N+2 –He cluster of 101 cm−1 is inferred.

The B←X electronic spectra of N+2–Nen (1≤n≤8)

Spectra of 14N+2–20Ne, 14N+2–22Ne, and 15N+2–20Ne have been recorded in the region of the B 2Σ+u←X 2Σ+g origin transition of N+2. Measurements are made by mass selecting cooled ionic complexes and photodissociating them whilst monitoring the N+2 fragment ion intensity as the laser wavelength is scanned. Various bands are assigned to transitions involving the stretching and bending motions of the Ne...Ne+2 bond with their structure and spacings consistent with transitions between quasilinear geometries in the X and the B states. Spectra of complexes with up to eight neon atoms attached to a 14N+2 core have also been measured. Evidence from shifts of the band origins and analysis of the vibrational frequencies of N+2–Ne2 and N+2–Ne3 suggest a structure where the Ne ligands are sited at one end of the N+2 chromophore.

Photoinitiated charge transfer in N2O+–Ar

Vibrationally structured electronic transitions of N2O+–Ar have been observed by measuring the wavelength‐dependent yields of the photodissociationreactions to yield N2O+ or Ar+. There appear to be four structured overlapping electronic band systems which are distinguished by vibrational spacings and by their propensity towards production of either N2O+ or Ar+. Variations in the Ar+/N2O+ photoproduct ratio with wavelength are explained as due to vibrational predissociation on different potential‐energy surfaces correlating with either Ar+ or N2O+ products. The first band system, observed exclusively at the N2O mass, has its origin close to 445 nm, corresponding approximately to the difference in the energies of N2O+[X  2Π3/2]+Ar[1 S 0] and N2O[1Σ+]+Ar+[2 P 3/2] and is assigned as an intracluster charge‐transfer transition. Two strong band systems situated to higher energy are assigned as transitions to the two additional electronic states which are expected to correlate with 2 P 3/2 and 2 P 1/2 Ar+ and N2O[1Σ+] products. While excitation of these two bands results almost exclusively in Ar+ production, a fourth weaker band near 342 nm leads to N2O+ and appears likely to be a transition to a state correlating with an excited vibronic state of N2O+[A  2Σ+(1,0,0)]+Ar[1 S 0]. The different band systems exhibit extensive vibrational progressions involving the deformation of the bond between the N2O and the Ar. The shift in the onset of the first charge transfer from the difference in the Ar and N2O ionization potentials combined with the appearance energy for Ar+ production allow tentative estimates of 690 and 1340 cm−1 to be made for the dissociation energies of the lowest and first excited states of N2O+–Ar.

Vibrational predissociation lifetime of N2+-He (X, υ=1)

The B←X spectrum of N+2–He exhibits a hot band which arises from transitions in complexes which have one quantum of the N–N stretching vibration. By measuring the intensity of this peak relative to that of the origin peak as function of the time between ion preparation and laser interrogation we have determined the vibrational predissociation lifetime of the N+2–He complex to be 220±30 μs.

Metastable decay of N+2Hen (υ = 1) (1 < n⩽6) clusters

Abstract Laser and mass spectroscopic techniques have been combined to explore the fragmentation process of metastable N + 2 He n ionic clusters. It has been found that clusters possessing a single quantum of the N⋯N stretching vibration decay on the 100 μm time scale. The rate for the relaxation of the N⋯N stretching vibration in larger N + 2 He n complexes has been determined by measuring the relative intensities of hot bands involving the N⋯N stretch vibration in the B ← X spectra and comparing with the hot-band intensity for the N + 2 He complex for which a lifetime for the υ = 1 state of 220 ± 30 μm has been previously measured. While for the smaller complexes ( n = 1–3) the relaxation rate is proportional to ligand number there appears to be some saturation for the larger species ( n = 4–6). The roughly linear increase in dissociation rate with ligand number and the ionic fragment size distribution are consistent with a two-step fragmentation process whereby a single He atom collides inelastically with the vibrationally excited core with the recoiling N + 2 undergoing intracluster collisions with the other ligands leading to further He evaporations.

Discrete UV absorption by N+3 (N2)n clusters

Abstract Photoabsorptions by N + 3 (N 2 ) n complexes have been observed near 282 nm by detecting both N + and N + 3 photofragments. The absorptions occur near the recently characterized A 3 Π←X 3 ∑ − g transition of the N + 3 cation, implying that the larger complexes essentially consist of an N + 3 core surrounded by electrostatically bound N 2 ligands. Two mechanisms for the photodissociation are proposed, one involving coupling of the A state of the N + 3 chromophore to a dissociative 3 Π surface to produce N + fragments, the other beginning with A→X internal conversion followed by a series of N 2 ligand evaporations to leave only N + 3 .

Photodissociation spectroscopy of the [OCS⋅C2H2]+ cluster ion

The potential‐energy surface features involved in the [OCS+C2H2]+ reaction system allow a metastable enroute to reaction to be trapped. The absolute photodissociation spectrum of this trapped metastable [OCS⋅C2H2]+ cluster ion has been measured in the wavelength region between 375 and 735 nm using a coaxial‐laser–triple‐quadrupole ion‐beam apparatus. The photodissociation spectrum consists of at least three broad overlapping profiles, and OCS+, C2H+2, and C2H2S+ are the observed ionic photoproducts. The spectrum is interpreted in terms of transitions to excited charge‐transfer states which are repulsive and to a dissociative transition which is localized on the perturbed OCS+ moiety. The C2H+2 fragment becomes detectable within sensitivity limits at 1.69 eV, yielding an upper limit estimate of 1.46 eV (+0.15/−0.23 eV) for the cluster bond dissociation energy. The C2H2S+ product is only observable at photon energies greater than 2.74 eV and is considered to be the product of a photoinitiated intracluster i...

Spectroscopy and dynamics of ionic complexes: N2+-He and N2O+-Ar

This paper describes investigations of fundamental photophysical processes in the small inorganic ionic complexes N2+-He and N2O+-Ar. Studies were carried out by observing the wavelength dependent photofragment yield of mass selected ionic species in a guided ion beam apparatus. Spectra of the N2+-He complex exhibit several bands in the near UV that correspond with those of the N2+ chromophore. By measuring the relative intensities of the B

Electronic spectroscopy of ionic clusters

IMP X origin and 111 hot bands as a function of ion flight time from ion source to the laser, a vibrational predissociation lifetime of 220 +/- 30 microsecond(s) has been determined for the (v equals 1) state of the N2+-He complex. For the N2O+-Ar complex, vibrationally structured electronic bands were observed which arise from charge transfer type transitions. In the ground state of the complex the charge is localized principally on N2O. Lying above this ground state by the difference in the N2O and Ar ionization potentials are several excited states where the charge is localized on Ar. Photodissociation to produce both N2O+ and Ar+ fragments occurs. While the former fragment probably arises via vibrational predissociation from vibrationally excited levels in the ground state, the appearance of Ar+ fragments is evidence for rapid vibrational predissociation on the excited state surface. Dissociation energies (D0 values) of 690 cm-1 for the X state of the N2O+-Ar complex and 1340 cm-1 for the lowest charge transfer state dissociation energy were inferred from the appearance thresholds for N2O+ and Ar+ fragments. Various possible dissociation routes are discussed.

A 311u +- X 32g- Electronic Spectrum of N3

The electronic transitions of the ionic clusters N2+-Hen (n equals 1,2,3), N2+-Ne, and N2O+-Ne have been detected. The cluster ions, produced by electron impact of a pulsed, seeded supersonic expansion, are mass-selected and injected into an octopole where the electronic transition is induced by a tunable, pulsed laser. The electronic spectra are recorded by detecting the yield of fragment ions, produced by vibrational predissociation within the octopole, in a further quadrupole mass-selector, as a function of the laser wavelength. The spectra show rotational and/or vibrational structure, which has been analyzed to provide information on the structure, binding energies, and vibrational frequencies of these species.