Slow Electron Velocity-map Imaging Research Articles

Study of ArO− and ArO via Slow Photoelectron Velocity-Map Imaging Spectroscopy and Ab Initio Calculations

The high-resolution photoelectron spectrum of ArO(-) was obtained using slow electron velocity-map imaging (SEVI). The SEVI spectrum reveals well-resolved vibrational transitions between multiple electronic states of ArO(-) and ArO, both of which are open-shell species. These transitions occur within the broad envelope of previous lower resolution photoelectron spectra. Detailed assignments are made by comparison with theoretical simulations based on high level ab initio calculations and an atoms-in-molecule model that accounts for spin-orbit coupling in the anion and neutral. The adiabatic electron affinity of ArO is found to be 12481 +/- 2 cm(-1). Several ArO(-) and ArO vibrational frequencies and excited-state term energies are accurately determined from the analysis of the experimental spectra and are found to be in excellent agreement with the calculated values.

Slow photoelectron velocity-map imaging spectroscopy of C2N−, C4N−, and C6N−

High resolution photoelectron spectra of C(2)N(-), C(4)N(-), and C(6)N(-) anions are reported, obtained using slow electron velocity-map imaging. The spectra show well resolved transitions to the X (2)Pi neutral ground state of all three species and to the a (4)Sigma(-) excited state of C(2)N and C(4)N. This study yields the adiabatic electron affinity of C(2)N, C(4)N, and C(6)N, the spin-orbit splitting in the X (2)Pi state of each radical, and the term energy of the a (4)Sigma(-) state in C(2)N and C(4)N. Relatively little vibrational activity is observed, indicating small geometry changes upon photodetachment. This result, plus the observation of transitions to neutral quartet states, indicates that the C(2n)N(-) (n=1-3) anions all have linear (3)Sigma(-) ground states.

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.

Characterization of cyclic and linear C3H− and C3H via anion photoelectron spectroscopy

Anion photoelectron spectroscopy of C3H- and C3D- is performed using both field-free time-of-flight and slow electron velocity-map imaging. We observe and assign transitions originating from linear/bent (l-C3H) and cyclic (c-C3H) anionic isomers to the corresponding neutral ground states and low-lying excited states. Transitions within the cyclic and linear manifolds are distinguished by their photoelectron angular distributions and their intensity dependence on the neutral precursor. Using calculated values for the energetics of the neutral isomers [Ochsenfeld et al., J. Chem. Phys. 106, 4141 (1997)], which predict c-C3H to lie 74 meV lower than l-C3H, the experimental results establish c-C3H- as the anionic ground state and place it 229 meV below l-C3H-. Electron affinities of 1.999+/-0.003 and 1.997+/-0.005 eV are determined for C3H and C3D from the X 2B2<--X 1A1 photodetachment transition of c-C3H. Term energies for several low-lying states of c-C3H and l-C3H are also determined. Franck-Condon simulations are used to make vibrational assignments for the bands involving c-C3H. Simulations of the l-C3H bands were more complicated owing to large amplitude bending motion and, in the case of the neutral A 2Pi state, strong Renner-Teller coupling.

Nonadiabatic Interactions in the Cl + H 2 Reaction Probed by ClH 2 - and ClD 2 - Photoelectron Imaging

The degree of electronic and nuclear coupling in the Cl + H2 reaction has become a vexing problem in chemical dynamics. We report slow electron velocity-map imaging (SEVI) spectra of ClH2- and ClD2-. These spectra probe the reactant valley of the neutral reaction potential energy surface, where nonadiabatic transitions responsible for reactivity of the Cl excited spin-orbit state with H2 would occur. The SEVI spectra reveal progressions in low-frequency Cl.H2 bending and stretching modes, and are compared to simulations with and without nonadiabatic couplings between the Cl spin-orbit states. Although nonadiabatic effects are small, their inclusion improves agreement with experiment. This comparison validates the theoretical treatment, especially of the nonadiabatic effects, in this critical region of the Cl + H2 reaction, and suggests strongly that these effects are minor.

Applications of slow electron velocity map imaging to the study of spectroscopy and dynamics in small aromatic molecules

Slow electron velocity map imaging provides a means of performing relatively high resolution photoelectron spectroscopy while still maintaining many of the advantages of imaging techniques. Here, we describe its application to the spectroscopy and dynamics of some substituted toluene molecules and show it to be a versatile technique whose resolution can approach that of zero kinetic energy (ZEKE) photoelectron spectroscopy, and provides a good match to the bandwidth of transform limited 1 ps laser pulses. We provide a series of comparisons of the results obtained with different ionizing wavelengths and use these to help understand the advantages and limitations of the technique.

Slow electron velocity-map imaging spectroscopy of the C4H− and C4D− anions

High resolution photodetachment spectra of C4H- and C4D- obtained via slow electron velocity-map imaging (SEVI) are presented. The spectra reveal closely spaced transitions to the neutral 2Sigma+ and 2Pi states which can be distinguished based on the corresponding photoelectron angular distributions. The C4H ground state is confirmed as the X2Sigma+ state, with the excited A2Pi state lying only 213 cm(-1) higher (201 cm(-1) for C4D). The electron affinities (EAs) are slightly revised to EA (C4H)=28,497+/-8 cm(-1) and EA (C4D)=28,478+/-10 cm(-1). Progressions in low frequency bending vibrations are observed in both states, yielding experimental frequencies of nu7=179(169) cm(-1) and nu6=408(392) cm(-1) for the X2Sigma+ state of C4H (C4D), and nu7=220(215)cm(-1) and nu6=446(437) cm(-1) for the A2Pi state.

Vibronic structure in C2H and C2D from anion slow electron velocity-map imaging spectroscopy

The C2H and C2D radicals are investigated by slow electron velocity-map imaging (SEVI) of the corresponding anions. This technique offers considerably higher resolution (<0.5 meV) than photoelectron spectroscopy. As a result, SEVI spectra of the two isotopomers yield improved electron affinities and reveal many new structures that are particularly sensitive to vibronic coupling between the ground 2Sigma+ and low-lying excited 2Pi states. These structures, which encompass more than 5000 cm(-1) of internal excitation, are assigned with the aid of previous experimental and theoretical work. We also show that SEVI can be applied to photodetachment transitions resulting in ejection of an electron with orbital angular momentum l=1, a p wave, in contrast to anion zero-electron kinetic energy spectroscopy which is restricted to s-wave detachment.

Slow electron velocity-map imaging spectroscopy of the 1-propynyl radical

High resolution photoelectron spectra of the 1-propynyl and 1-propynyl-d(3) anions acquired with slow electron velocity-map imaging are presented. The electron affinity is determined to be 2.7355+/-0.0010 eV for the 1-propynyl radical and 2.7300+/-0.0010 eV for 1-propynyl-d(3). Several vibronic transitions are observed and assigned using the isotopic shifts and results from ab initio calculations. Good agreement between experimental spectra and calculations suggests a C(3v) geometry for the 1-propynyl radical. No evidence is found for strong vibronic coupling between the ground electronic state and the low-lying first excited state.

Slow electron velocity-map imaging photoelectron spectra of the methoxide anion

High resolution anion photodetachment spectra are presented for the methoxide anion and its fully deuterated counterpart. The spectra were obtained with slow electron velocity-map imaging. Improved electron affinities are determined for CH3O as 1.5690+/-0.0019 eV and for CD3O as 1.5546+/-0.0019 eV. The spectra resolve many features associated with spin-orbit and vibronic coupling that were not seen in previous photodetachment studies. Photoelectron angular distributions taken as a function of detachment wavelength for the ground vibronic state transitions are recorded and are consistent with the removal of a nonbonding, p-type electron localized on the oxygen atom. Several hot bands and sequence bands are observed for the first time, providing insight into the vibrational structure of the methoxide anion. The results are compared to recent calculations of the anion photoelectron spectra that incorporate bilinear coupling terms among the methoxy vibrational modes and are found to be in reasonable agreement.

A new action photoelectron spectroscopy for anions

We present a new experimental approach, in which anion photodetachment spectroscopy is recorded with electrons of fixed kinetic energy. This approach circumvents some shortcomings of the zero electron kinetic energy method. Our method is based on a modified magnetic bottle photoelectron spectrometer (MBPES). A tunable laser is used to detach electrons from mass selected anions, drifting collinearly with the 40 cm MBPES drift tube. To avoid Doppler broadening, a low voltage pulse removes the velocity component of anions from the detached electrons. Spectra are recorded by collecting the wavelength dependence of electron-signal at a predetermined TOF window, corresponding to a specific electron-kinetic energy. We call this approach PEACE, denoting photoelectron action spectroscopy at constant kinetic energy. Our best resolution is 0.65 meV for 1.5 meV electrons. We present a PEACE spectrum of HgCl(-) together with the corresponding simulated theoretical spectrum. The method is similar in resolution and data collection rates to the slow electron velocity map imaging technique recently introduced by Neumark and co-workers.

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.