Don W. Arnold

Vibrationally resolved spectra of C2–C11 by anion photoelectron spectroscopy

Anion photoelectron spectroscopy has been employed to obtain vibrationally resolved spectra of the carbon molecules C2–C11. The spectra of C−2–C−9 are dominated by linear anion to linear neutral photodetachment transitions. Linear to linear transitions contribute to the C−11 spectrum, as well. From these spectra, vibrational frequencies and electron affinities are determined for the linear isomers of C2–C9 and C11. The term value is also obtained for the first excited electronic state of linear C4. The spectra of C−10 and C−11 show evidence for transitions involving cyclic anions and/or neutrals. Similar types of transitions are identified in the spectra of other smaller molecules, specifically C−6, C−8, and to a lesser extent C−5.

The Transition State of the F + H2 Reaction

The transition state region of the F + H2 reaction has been studied by photoelectron spectroscopy of FH2–. New para and normal FH2–photoelectron spectra have been measured in refined experiments and are compared here with exact three-dimensional quantum reactive scattering simulations that use an accurate new ab initio potential energy surface for F + H2. The detailed agreement that is obtained between this fully ab initio theory and experiment is unprecedented for the F + H2 reaction and suggests that the transition state region of the F + H2 potential energy surface has finally been understood quantitatively.

Photoelectron spectroscopy of CN−, NCO−, and NCS−

The 266 nm photoelectron spectra of CN−, NCO−, and NCS− have been recorded with a pulsed time‐of‐flight photoelectron spectrometer. The photoelectron spectrum of CN− has also been recorded at 213 nm revealing transitions to the A 2Π state as well as the ground X 2Σ+ state of the CN radical. The following adiabatic electron affinities (EAs) are determined: EA(CN)=3.862±0.004 eV, EA(NCO)=3.609±0.005 eV, and EA(NCS)=3.537±0.005 eV. The adiabatic electron affinity of cyanide is in disagreement with the currently accepted literature value. Our measurement of the electron affinity of NCS confirms recent theoretical estimates that dispute the literature experimental value. By Franck–Condon analysis of the vibrational progressions observed in each spectrum, the change in bond lengths between anion and neutral are also determined. For NCO− this yields R0(C–N)=1.17±0.01 A and R0(C–O)=1.26±0.01 A, and for CN− the equilibrium bond length is found to be Re(C–N)=1.177±0.004 A. The gas phase fundamental for CN− is deter...

Examination of the 2A’2 and 2E‘ states of NO3 by ultraviolet photoelectron spectroscopy of NO−3

The photoelectron spectrum of the NO−3 anion has been obtained at 266 and at 213 nm. The 266 nm spectrum probes the 2A’2 ground state of NO3. The 213 nm spectrum represents the first observation of the 2E‘ lowest‐lying excited state of NO3. The 2A2 band shows vibrational progressions in the ν1 symmetric stretch and the ν4 degenerate in‐plane bend of NO3. Our analysis of this band indicates that the NO3 ground state has a D3h equilibrium geometry and is vibronically coupled to the 2E’ second excited state via the ν4 mode. We also obtain the electron affinity of NO3, 3.937±0.014 eV, and the heat of formation of NO3 at 298 K, 0.777±0.027 eV (17.9±0.6 kcal/mol). The 2E‘ state of NO3 lies 0.868±0.014 eV above the ground state. The 2E‘ band shows complex and extensive vibrational structure. Several possible assignments of this structure are discussed.

Experimental and theoretical studies of the F+H2 transition state region via photoelectron spectroscopy of FH−2

The transition state region of the F+H2 reaction is studied by photoelectron spectroscopy of FH2−. The photoelectron spectra consist of overlapping electronic bands with different angular distributions. The ground state band shows partially resolved features which differ depending on whether the anion is made from normal or para hydrogen. This dependence on the anion nuclear spin statistics implies that these features are due to progressions in bending levels of the neutral FH2 complex. In order to confirm this, and to determine the sensitivity of the photoelectron spectrum to the bend potential near the F+H2 transition state, three‐dimensional simulations of the FH2− photoelectron spectrum were performed assuming various potential energy surfaces for the F+H2 reaction. We found that the London–Eyring–Polanyi–Sato surface proposed by Takayanagi and Sato gave better agreement than either the T5a or 5SEC surfaces. From the higher energy band, we can extract information on the F+H2 excited electronic states,...

Study of halogen–carbon dioxide clusters and the fluoroformyloxyl radical by photodetachment of X−(CO2) (X=I,Cl,Br) and FCO−2

Photoelectron spectra have been measured for the anions X−(CO2), with X=I, Br, Cl, and F. The vibrationally resolved spectra show that I−(CO2), Br−(CO2), and Cl−(CO2) are primarily electrostatically bound clusters, although the charge‐quadrupole interaction is strong enough to distort the CO2 molecule by as much as 10° [in Cl−(CO2)]. Ab initio calculations and electrostatic models are used to describe the geometry and bonding of these clusters. The photoelectron spectrum of FCO−2 is qualitatively different and shows transitions to both the X 2B2 ground and the A 2A2 first excited electronic states of the covalently bound FCO2 radical. The previously unobserved A 2A2 state is measured to lie 0.579 eV above the ground state. Vibrational frequencies are assigned with the assistance of ab initio calculations. The FCO2 heat of formation is determined to be ΔfH0298(FCO2)=−85.2±2.8 kcal/mol. While both FCO−2 and FCO2 are more strongly bound than the other halide–CO2 clusters, the C–F bonds are very weak relat...

Study of low‐lying electronic states of ozone by anion photoelectron spectroscopy of O−3

The low‐lying electronic states of ozone are studied using anion photoelectron spectroscopy of O−3. The spectra show photodetachment transitions from O−3 to the X 1A1 ground state and to the five lowest lying electronic states of the ozone molecule, namely the 3A2, 3B2, 1A2, 3B1, and 1B1 states. The geometry of the ozonide anion determined from a Franck–Condon analysis of the O3 X 1A1 ground state spectrum agrees reasonably well with previous work. The excited state spectra are dominated by bending vibrational progressions which, for some states, extend well above the dissociation asymptote without noticeable lifetime broadening effects. Preliminary assignments are based upon photoelectron angular distributions and comparison with ab initio calculations. None of the excited states observed lies below the ground state dissociation limit of O3 as suggested by previous experimental and theoretical results.

Study of HCO2 and DCO2 by negative ion photoelectron spectroscopy

Photoelectron spectra of HCO−2 and DCO−2 at 299 nm, 266 nm, and 213 nm are reported. Photodetachment accesses the 2A1, 2B2, and 2A2 states of the formlyoxyl radical, HCO2. The 2A1 state is assigned as the HCO2 ground state, although it is nearly degenerate with the 2B2 state (T0=0.027 eV), and the 2A2 state lies at T0=0.536 eV. The electron affinity of HCO2 is 3.498±0.015 eV. The spectra show partially resolved vibrational features, primarily involving progressions in the CO2 bending mode. The irregular appearance of the spectra in some regions suggests vibronic coupling between the 2A1 and 2B2 states. The possible role of the HCO2 radical as an intermediate in the OH+CO→H+CO2 reaction and in H+CO2 inelastic scattering is discussed.

Examination of the Br+HI, Cl+HI, and F+HI hydrogen abstraction reactions by photoelectron spectroscopy of BrHI−, ClHI−, and FHI−

The photoelectron spectra of the ions BrHI−, ClHI−, and FHI−, along with their deuterated counterparts, are presented. These spectra provide information on the transition state region of the potential energy surfaces describing the exothermic neutral reactions X+HI→HX+I(X=Br, Cl, F). Vibrational structure is observed in the BrHI− and ClHI− spectra that corresponds to hydrogen atom motion in the dissociating neutral complex. Transitions to electronically excited potential energy surfaces that correlate to HX+I(2P3/2,2P1/2) products are also observed. A one‐dimensional analysis is used to understand the appearance of each spectrum. The BrHI− spectrum is compared to a two‐dimensional simulation performed using time‐dependent wave packet propagation on a model Br+HI potential energy surface.

Study of I−(CO2)n, Br−(CO2)n, and I−(N2O)n clusters by anion photoelectron spectroscopy

Photoelectron spectra of the I−(CO2)n=1–13, I−(N2O)n=1–12, and Br−(CO2)n=1–11 clusters are presented. The spectra provide information about the stepwise solvation of the bromide and iodide anions and about the size of the first solvation shells in these clusters. The data suggest that significantly different solute–solvent interactions exist in the three sets of clusters studied here. The X−(CO2)n spectra exhibit resolved progressions which are assigned to in‐phase CO2 solvent bending vibrations in the neutral clusters. These vibrations are excited by photodetachment of anion clusters in which the CO2 molecules are distorted from linearity by a charge‐quadrupole interaction. The I−(N2O)n spectra do not show any vibrational structure, presumably because the weaker ion–solvent interactions are insufficient to distort the N2O molecules.