J. T. Snodgrass

Dipole bound, nucleic acid base anions studied via negative ion photoelectron spectroscopy

The anions of the nucleic acid bases, uracil and thymine, were studied by negative ion photoelectron spectroscopy. Both monomer anions exhibit spectroscopic signatures that are indicative of dipole bound excess electrons. The adiabatic electron affinities of these molecules were found to be 93±7 meV for uracil and 69±7 meV for thymine. No conventional (valence) anions of these molecules were observed.

Photoelectron spectra of the alkali metal cluster anions: Na−n=2−5, K−n=2−7, Rb−n=2−3, and Cs−n=2−3

Obtention des spectres de photoelectrons a 2,540 eV sur faisceaux supersoniques. Deduction des energies de detachement vertical et des affinites electroniques adiabatiques, dont on analyse les variations avec n

Photoelectron spectroscopy of the negative ion SeO

We have recorded the photoelectron spectrum of SeO− using a newly constructed negative ion photoelectron spectrometer. The adiabatic electron affinity of SeO is determined to be 1.456±0.020 eV. Values of ν00(a 1Δ–X 3Σ−0+) and ΔG1/2(a 1Δ) are found to be 5530±200 and 916±35 cm−1, respectively, in substantial accord with previous measurements. The negative ion parameters determined in this work are: B‘e(SeO−) =0.4246±0.0050 cm−1 which leads to  r’e(SeO−)=1.726±0.010 A, ω‘e(SeO−)=730±25 cm−1, ω’e x‘e(SeO−)=2±4 cm−1, and  D0(SeO−)=3.84±0.09 eV. In addition, the spectroscopic parameters of SeO− are compared with those of the electronically analogous negative ions: O−2, SO−, and S−2.

Negative ion photoelectron spectroscopy of the negative cluster ion H−(NH3)1

The negative ion photoelectron spectrum of the negative cluster ion H− (NH3)1 has been recorded with 2.540 eV photons. This negative cluster ion was prepared in a supersonic nozzle‐ion source involving the injection of electrons into an expanding jet. While the spectrum is dominated by a broadened peak centered at 1.430±0.019 eV, there is also a small feature centered at 0.997±0.031 eV. Our interpretation of this spectrum is that the main peak contains the origin of the photodetachment transition, and that the smaller one is due primarily to the excitation of a stretching mode in the ammonia solvent during photodetachment. An upper limit to the dissociation energy of H−(NH3)1 into H− and NH3 is found to be 0.36 eV. This result is in good agreement with calculations by Kalcher, Squires, and Schleyer. The separation between the main and the small peaks is 3490±130 cm−1, and the symmetric stretching frequency of free NH3 is 3506 cm−1. The small peak also shifts appropriately upon deuteration in support of th...

Photodissociation dynamics of Ar3

Abstract Mass selected Ar3+ clusters are photodissociated by a polarized argon-ion laser beam at 514, 488 and 458 nm. Only Ar+/2Ar products are observed. Laser polarization dependence of the product laboratory kinetic energy peak shapes indicates that a “pure” parallel transition is responsible for the photodissociation. Analysis indicates it is almost certainly a 2Σu+ ← 2Σg+ transition resulting in Ar+ (2P 1 2 ). The product kinetic energy distribution is strongly bimodal with approximately 75% of the products at high kinetic energy (near the energy conservation limit) and 25% with near zero kinetic energy. A model is developed that explains this unexpected (and dynamically informative) result.

Photoelectron spectroscopy of the negative cluster ions NO−(N2O)n=1,2

We have recorded the photoelectron (photodetachment) spectra of the gas‐phase negative cluster ions NO−(N2 O)1 and NO−(N2 O)2 using 2.540 eV photons. Both spectra exhibit structured photoelectron spectral patterns which strongly resemble that of free NO−, but which are shifted to successively lower electron kinetic energies with their individual peaks broadened. Each of these spectra is interpreted in terms of a largely intact NO−subion which is solvated and stabilized by nitrous oxide. For both NO−(N2 O)1 and NO−(N2 O)2, the ion–solvent dissociation energies for the loss of single N2 O solvent molecules were determined to be ∼0.2 eV. Electron affinities were also determined and found to increase with cluster size. The localization of the cluster ion’s excess negative charge onto its nitric oxide rather than its nitrous oxide subunit is discussed in terms of kinetic factors and a possible barrier between the two forms of the solvated ion.

Negative ion photoelectron spectroscopy of N2O− and (N2O)−2

Abstract We have recorded the photoelectron spectra of the gas phase negative ions N 2 O − and (N 2 O) 2 − both of which were prepared in a nozzle ion source. The shift between the maxima of the two spectra is interpreted in terms of the dissociation energy of the dimer ion.

Negative ion photoelectron spectroscopy of P2

Address of Investigators: Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218.

Anion solvation at the microscopic level: Photoelectron spectroscopy of the solvated anion clusters, NO−(Y)n, where Y=Ar, Kr, Xe, N2O, H2S, NH3, H2O, and C2H4(OH)2

The negative ion photoelectron spectra of the gas-phase, ion-neutral complexes; NO−(Ar)n=1–14, NO−(Kr)1, NO−(Xe)n=1–4, NO−(N2O)n=3–5, NO−(H2S)1, NO−(NH3)1, and NO−(EG)1 [EG=ethylene glycol] are reported herein, building on our previous photoelectron studies of NO−(N2O)1,2 and NO−(H2O)1,2. Anion solvation energetic and structural implications are explored as a function of cluster size in several of these and as a result of varying the nature of the solvent in others. Analysis of these spectra yields adiabatic electron affinities, total stabilization (solvation) energies, and stepwise stabilization (solvation) energies for each of the species studied. An examination of NO−(Ar)n=1–14 energetics as a function of cluster size reveals that its first solvation shell closes at n=12, with an icosahedral structure there strongly implied. This result is analogous to that previously found in our study of O−(Ar)n. Inspection of stepwise stabilization energy size dependencies, however, suggests drastically different st...

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

The dominant peaks in the photoelectron spectra of the gas-phase, negative cluster ions H–(NH3)1 and H–(NH3)2 provide evidence for describing them as ion-molecule complexes comprised of intact hydride ions which are solvated by ammonia. Vertical detachment energies ad approximate ion–single-solvent dissociation energies are obtained. Other spectral features reveal the complexation-induced distrotion of the ammonia solvent(s) by their hydride sub-ions. In the photoelectron spectra of H–(NH3)1 and 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–4. This previously unknown ammonium anion is not a cluster species, and it is described as a NH+4 ion core with two Rydberg-like electrons.