Daniel M. Neumark

Molecular beam studies of the F+H2 reaction

The dynamics of the F+H2 reaction have been investigated in a high resolution crossed molecular beam study. Differential cross sections and kinetic energy distributions were obtained for each HF vibrational state. The v=1 and v=2 states were predominantly backward scattered, but substantial forward scattering was observed for HF (v=3) over the range of collision energies accessible in our apparatus, from 0.7 to 3.4 kcal/mol. The results strongly suggest that dynamical resonances play a significant role in the reaction dynamics of F+H2 and that resonance effects are most prominent in the v=3 product channel. Quantal reactive scattering calculations on F+H2 predict that the v=2 channel should be most strongly affected by resonances. This discrepancy is attributed to inadequacies in the potential energy surface used in the calculations, and several modifications to the surface are proposed based on the experimental results. Other features of the reaction are also discussed, including the integrated partial c...

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.

Anion spectroscopy of uracil, thymine and the amino-oxo and amino-hydroxy tautomers of cytosine and their water clusters

Abstract In this work we investigate different forms of electron binding in the mass-selected and cooled nucleobases uracil, thymine and cytosine and their water clusters. In photodetachment–photoelectron spectra of the pyrimidine nucleobases, sharp structures were found at 86±8 meV (uracil), 62±8 meV (thymine) and 85±8 meV (cytosine), which are due to photodetachment of dipole-bound states. The photodetachment angle dependence of these states shows mostly p-wave detachment, which confirms the predicted predominant s-character of the electronic wave function of dipole-bound states. This anisotropy of electron emission and their sharp photodetachment structures can be taken for dipole-bound state recognition. Water attachment to the nucleobases results in positive valence-bound electron affinity, s-wave detachment and broad spectra, implying that the electron now is trapped inside the π * LUMO of the nucleobases, stabilized by the water dipole. The solvent shifts in dependence on water aggregation are linear and allow by extrapolation an estimation of the monomer electron affinities. All three pyrimidine nucleobases are estimated to have a very similar valence-bound electron affinity in the range of 0–200 meV. In nucleobase·(H 2 O) n clusters, due to the large total dipole moment, dipole-bound states also exist. Resonant excitation of these dipole-bound states with a photon of 1064 nm wavelength causes dissociation of the anion cluster, leading to monomer anions in their dipole-bound state. These monomer anions can be photodetached by a second IR photon. Whereas, for uracil and thymine, one dipole-bound state is detected, for cytosine we find two dipole-bound states (85±8, 230±8 meV) which are attributed to the dipole-bound states of the simultaneously present amino-hydroxy and amino-oxo cytosine tautomers. We also give a possible explanation why the formation of the dipole-bound state of the amino-oxo tautomer at 230 meV is improbable in the supersonic expansion.

Molecular beam studies of the F+D2 and F+HD reactions

The F+D2 and F+HD reactions were investigated in a high resolution crossed molecular beams experiment at several collision energies. The DF product from both reactions was predominantly backward scattered although some forward scattered DF(v=4) was observed at the highest energy studied. The HF angular distributions from F+HD were quite different, showing considerable forward scattered (v=3) and no other identifiable structure. These results disagree with classical trajectory studies, which predict only small variations in the product angular distributions among F+H2 and its isotopic variants. They agree, however, with the predicted dependence of dynamical resonance effects on isotopic substitution. The results therefore support the conclusions drawn in the previous paper regarding the role of dynamical resonances in the F+H2 reaction.

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...

SLOW-ELECTRON VELOCITY-MAP IMAGING OF NEGATIVE IONS: APPLICATIONS TO SPECTROSCOPY AND DYNAMICS

Anion photoelectron spectroscopy (PES) has become one of the most versatile techniques in chemical physics. This article briefly reviews the history of anion PES and some of its applications. It describes efforts to improve the resolution of this technique, including anion zero electron kinetic energy (ZEKE) and the recently developed method of slow electron velocity-map imaging (SEVI). Applications of SEVI to studies of vibronic coupling in open-shell systems and the spectroscopy of prereactive van der Waals complexes are then discussed.

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.

High resolution photodetachment spectroscopy of negative ions via slow photoelectron imaging.

A technique for high resolution anion photodetachment spectroscopy is presented that combines velocity map imaging and anion threshold photodetachment. This method, slow electron velocity-map imaging, provides spectral line widths of better than 1 meV. Spectra over a substantial range of electron kinetic energies are recorded in a single image, providing a dramatic reduction of data acquisition time compared to other techniques with comparable resolution. We apply this technique to atomic iodine and the van der Waals cluster I.CO2 as test systems, and then to the prereactive Cl.D2 complex where partially resolved structure assigned to hindered rotor motion is observed.

Attosecond band-gap dynamics in silicon

Electron transfer from valence to conduction band states in semiconductors is the basis of modern electronics. Here, attosecond extreme ultraviolet (XUV) spectroscopy is used to resolve this process in silicon in real time. Electrons injected into the conduction band by few-cycle laser pulses alter the silicon XUV absorption spectrum in sharp steps synchronized with the laser electric field oscillations. The observed ~450-attosecond step rise time provides an upper limit for the carrier-induced band-gap reduction and the electron-electron scattering time in the conduction band. This electronic response is separated from the subsequent band-gap modifications due to lattice motion, which occurs on a time scale of 60 ± 10 femtoseconds, characteristic of the fastest optical phonon. Quantum dynamical simulations interpret the carrier injection step as light-field–induced electron tunneling. Excited electrons in semiconducting silicon are tracked on a time scale faster than the lattice vibrations. [Also see Perspective by Spielmann] Watching electrons dart through silicon The ultimate speed limit in electronic circuitry is set by the motion of the electrons themselves. Schultze et al. applied attosecond spectroscopy to glimpse this motion in a sample of silicon, the semiconducting building block of modern integrated circuits (see the Perspective by Spielmann). The technique distinguished the electron dynamics—which proceed faster than a quadrillionth of a second after laser excitation—from the comparatively slower lattice motion of the silicon atomic nuclei. Science, this issue p. 1348; see also p. 1293