Electron Velocity-map Imaging Research Articles

Study of He*/Ne*+Ar, Kr, N2, H2, D2 Chemi-Ionization Reactions by Electron Velocity-Map Imaging.

The chemi-ionization of Ar, Kr, N2, H2, and D2 by Ne(3P2) and of Ar, Kr, and N2 by He(3S1) was studied by electron velocity map imaging (e-VMI) in a crossed molecular beam experiment. A curved magnetic hexapole was used to state-select the metastable species. Collision energies of 60 meV were obtained by individually controlling the beam velocities of both reactants. The chemi-ionization of atoms and molecules can proceed along different channels, among them Penning ionization and associative ionization. The evolution of the reaction is influenced by the internal redistribution of energy, which happens at the first reaction step that involves the emission of an electron. We designed and built an e-VMI spectrometer in order to investigate the electron kinetic energy distribution, which is related to the internal state distribution of the ionic reaction products. The analysis of the electron kinetic energy distributions allows an estimation of the ratio between the two-reaction channel Penning and associative ionization. In the molecular cases the vibrational or electronic excitation enhanced the conversion of internal energy into the translational energy of the forming ions, thus influencing the reaction outcome.

The role of the intermediate state in angle-resolved photoelectron studies using (2 + 1) resonance-enhanced multiphoton ionization of the chiral terpenes, α-pinene and 3-carene

Photoelectron angular distributions (PADs), ranging up to the maximum 6th order Legendre polynomial term set by the Yang theorem, have been recorded for the (2 + 1) resonance enhanced multiphoton ionization (REMPI) of two terpene isomers, 3-carene and α-pinene, employing femtosecond lasers and electron velocity map imaging detection. PAD measurements made with coincident photoion detection allow ion fragmentation effects to be assessed. Using circular polarization and enantiomerically pure samples the PAD measurements are extended to include chiral (odd) Legendre polynomial terms, and these are analysed and discussed as multiphoton photoelectron circular dichroism (MP-PECD). Comparisons are also made with single photon (synchrotron radiation) PECD of these compounds. Although for a given compound a common final cation state is reached, pronounced differences are observed between PECD and MP-PECD, and between the alternative identified REMPI intermediate states in the case of MP-PECD.

A co-axial velocity map imaging spectrometer for electrons

We present the design of an electron velocity map imaging (VMI) spectrometer where the ionizing laser source propagates along the symmetry axis of the spectrometer. The co-axial geometry is useful in a variety of experiments, because it provides a unique 2-dimensional projection of the 3-dimensional electron momentum distribution. Initial simulations show that this co-axial VMI can work with both high energy (more than 100 eV) and low energy (tens of eV) electrons. We demonstrate the performance of this co-axial VMI spectrometer at the Linac Coherent Light Source X-ray Free Electron Laser facility.

Ground and low-lying excited states of phenoxy, 1-naphthoxy, and 2-naphthoxy radicals via anion photoelectron spectroscopy.

We present the slow electron velocity map imaging spectroscopy of cryogenically cooled phenoxide, 1-naphthoxide, and 2-naphthoxide anions. The results allow us to examine the ground state and the lowest energy excited state in the corresponding neutral radicals. Care was taken to minimize autodetachment signals in the photoelectron spectra, allowing for more straightforward comparisons with Franck-Condon analyses. The ground states of these three aromatic oxide radicals all have the unpaired electron residing in a π orbital delocalized throughout the molecule. The electron affinity of 1-naphthoxy is measured to be 2.290(2) eV, while that of 2-naphthoxy is measured to be 2.404(2) eV, both of which are higher than that of the smaller phenoxy molecule at 2.253(1) eV. The first excited states have the unpaired electron residing in a more localized σ orbital, yielding measured term energies for the state of 1.237(2) eV in 1-naphthoxy and 1.068(1) eV in 2-naphthoxy, while that of phenoxy is lower at 0.952(1) eV. The calculated Franck-Condon spectra generally showed good agreement with the experimental spectra, yielding assignments of the more active vibrations in each electronic state. Significant autodetachment signals arising from dipole bound states near the ground states of all three radicals were observed in our efforts to avoid them, and comparably less autodetachment signals were observed near the excited states. Besides this type of non-Franck-Condon intensities in the photoelectron spectra, we also observed minor features arising due to vibronic coupling in the ground states of all three radicals.

Photoelectron spectroscopy of anthracene and fluoranthene radical anions.

We report the slow electron velocity map imaging spectroscopy of cryogenically cooled anthracene and fluoranthene radical anions, two similarly sized polycyclic aromatic hydrocarbon molecules. The results allow us to examine the lowest energy singlet and triplet states in the neutral molecules on equal footing from the anionic ground state. The analysis of the experimental spectra is aided by harmonic calculations and Franck-Condon simulations, which generally show good agreement with experimental values and spectra. The electron affinity of fluoranthene is measured to be 0.757(2) eV, which is larger than that of anthracene at 0.532(3) eV. The lowest energy triplet state in anthracene is observed at 1.872(3) eV above the singlet ground state, while that of fluoranthene is observed at 2.321(2) eV above its singlet ground state. Comparisons of experimental and calculated spectra show that in addition to the Franck-Condon active modes, there is a clear presence of vibrational modes that gain intensity via vibronic coupling in both the singlet and triplet states in both molecules. In addition, the triplet state generally exhibits increased vibronic coupling compared to the singlet state, with the fluoranthene triplet state exhibiting evidence of distortion from C2v symmetry.

A multi-plate velocity-map imaging design for high-resolution photoelectron spectroscopy.

A velocity map imaging (VMI) setup consisting of multiple electrodes with three adjustable voltage parameters, designed for slow electron velocity map imaging applications, is presented. The motivations for this design are discussed in terms of parameters that influence the VMI resolution and functionality. Particularly, this VMI has two tunable potentials used to adjust for optimal focus, yielding good VMI focus across a relatively large energy range. It also allows for larger interaction volumes without significant sacrifice to the resolution via a smaller electric gradient at the interaction region. All the electrodes in this VMI have the same dimensions for practicality and flexibility, allowing for relatively easy modifications to suit different experimental needs. We have coupled this VMI to a cryogenic ion trap mass spectrometer that has a flexible source design. The performance is demonstrated with the photoelectron spectra of S- and CS2-. The latter has a long vibrational progression in the ground state, and the temperature dependence of the vibronic features is probed by changing the temperature of the ion trap.

Photoelectron Velocity Map Imaging Spectroscopy of the Beryllium Sulfide Anion, BeS.

Slow electron velocity map imaging (SEVI) spectroscopy was used to examine the BeS- anion to neutral ground-state transition, X 2Σ+ → X 1Σ+. Rotational constants, vibrational intervals, and the electron binding energy of BeS- were determined. Partially resolved rotational contours were seen due to the relatively small moment of inertia of beryllium sulfide. Upon analysis of the rotational contours, it was found that changes in the molecular rotational angular momentum, ΔN = -1, -2, -3, and -4, facilitated photodetachment at near-threshold photon energies. The electron affinity of BeS was found to be 2.3346(2) eV. SEVI spectra recorded using photon energies near the threshold for Δv = -1 processes exhibited features that were associated with a dipole-bound state (DBS) of BeS-. Autodetachment spectroscopy was used to probe this state, and rotationally resolved data were obtained for the DBS 2Σ+, v' = 0 - X 2Σ+, v″ = 0 transition. Analysis of this structure provided the rotational constants for BeS- X, v″ = 0, and the electron binding energy of the DBS. Electronic structure calculations, performed at the RCCSD(T) and MRCI levels of theory, gave predictions that were in good agreement with the experimental observations.

Electron velocity map imaging and theoretical study on CuXH (X = O and S) anions

Vibrationally resolved photoelectron spectra of CuOH− and CuSH− have been determined via velocity map imaging method to investigate the transitions of X1A′←X2A′ at 532nm. Adiabatic detachment energies of CuOH− and CuSH− are assigned to 0.995(12) and 1.098(12) eV, respectively. Combined theoretical calculations with Franck-Condon simulations, it allows extracting the vibrational frequencies in neutral, which yields 629(32) cm−1 with CuO stretching mode and 387(24) cm−1 with CuS stretching mode for CuXH (X=O and S). Parallel transition properties of photoelectron angular distributions (PADs) for both species are correlated to the photodetachment of SOMO orbitals, which mainly involved in the Cu atom s orbital and partial s orbital in other atoms. Based on chemical bonding analyses (Wiberg, NAO, Mayer, NRT, and ELF), it is suggested that a trend is observed with a subtle variation of covalent component from weak covalent behavior between CuO in CuOH–1/0 to stronger covalent bonding between CuS in CuSH–1/0 (especially for non-ignorable covalent component in CuSH species) though ionic bonding dominates both in CuO and CuS bonds for the two systems.

Anion photoelectron spectroscopy of deprotonated ortho-, meta-, and para-methylphenol.

The anion photoelectron spectra of ortho-, meta-, and para-methylphenoxide, as well as methyl deprotonated meta-methylphenol, were measured. Using the Slow Electron Velocity Map Imaging technique, the Electron Affinities (EAs) of the o-, m-, and p-methylphenoxyl radicals were measured as follows: 2.1991±0.0014, 2.2177±0.0014, and 2.1199±0.0014 eV, respectively. The EA of m-methylenephenol was also obtained, 1.024±0.008 eV. In all four cases, the dominant vibrational progressions observed are due to several ring distortion vibrational normal modes that were activated upon photodetachment, leading to vibrational progressions spaced by ∼500 cm-1. Using the methylphenol O-H bond dissociation energies reported by King et al. and revised by Karsili et al., a thermodynamic cycle was constructed and the acidities of the methylphenol isomers were determined as follows: ΔacidH298K0=348.39±0.25, 348.82±0.25, 350.08±0.25, and 349.60±0.25 kcal/mol for cis-ortho-, trans-ortho-, m-, and p-methylphenol, respectively. The excitation energies for the ground doublet state to the lowest excited doublet state electronic transition in o-, m-, and p-methylphenoxyl were also measured as follows: 1.029±0.009, 0.962±0.002, and 1.029±0.009 eV, respectively. In the photoelectron spectra of the neutral excited states, C-O stretching modes were excited in addition to ring distortion modes. Electron autodetachment was observed in the cases of both m- and p-methylphenoxide, with the para isomer showing a lower photon energy onset for this phenomenon.

Photoionization of epichlorohydrin enantiomers and clusters studied with circularly polarized vacuum ultraviolet radiation

The photoionization of enantiomerically pure epichlorohydrin (C(3)H(5)OCl) has been studied using linearly and circularly polarized vacuum ultraviolet synchrotron radiation. The threshold photoelectron spectrum was recorded and the first three bands assigned using molecular orbital calculations for the expected conformers, although uncertain experimental conformer populations and an anticipated breakdown in Koopmans' theorem leave some ambiguity. Measurements of the photoelectron circular dichroism (PECD) were obtained across a range of photon energies for each of these bands, using electron velocity map imaging to record the angular distributions, during which a record PECD chiral asymmetry factor of 32% was observed. A comparison with calculated PECD curves clarifies the assignment achieved using ionization energies alone and further suggests a likely relative population of the conformers. Threshold photoelectron-photoion coincidence methods were used to study the ionic fragmentation of epichlorohydrin. Fragment ion appearance energies show nonstatistical behavior with clear indications that the cationic epoxide ring is unstable and lower energy decay channels proceeding via ring breaking are generally open. Extensive neutral homochiral clusters of epichlorohydrin may be formed in supersonic molecular beam expansions seeded in Ar. Electron angular distribution measurements made in coincidence with dimer and trimer ions are used to effect an examination of the PECD associated with ionization of size-selected neutral cluster species, and these results differ clearly from PECD of the neutral monomer. The shifted ionization thresholds of the n-mers (n = 2, ..., 7) are shown to follow a simple linear relationship, but under intense beam expansion conditions the monomer deviates from this relationship, and the monomer electron spectra tail to below the expected monomer adiabatic ionization potential (IP). PECD measurements made in coincidence with monomer ions obtained under different beam expansion conditions were used to identify unambiguously a contribution from dissociative photoionization of larger clusters to the monomer parent mass ion yield above and below its adiabatic IP.

Vibronic Analysis of the S1−S0 Transition of Phenylacetylene Using Photoelectron Imaging and Spectral Intensities Derived from Electronic Structure Calculations

The vibrational structure of the S(1)-S(0) electronic band of phenylacetylene has been recorded by 1 + 1 resonance-enhanced multiphoton ionization, accompanied by slow electron velocity map imaging photoelectron spectroscopy at each resonant vibrational band. Assignments of the S(1) vibrations (up to 2000 cm(-1) above the band origin) are based upon the relative intensities of the vibronic bands calculated by complete second-order vibronic coupling, vibration-rotation (Coriolis and Birss) coupling calculations, and the vibrational structure of the S(1) resonant photoelectron spectra. Although this is an allowed electronic transition, the relative intensities of the a(1) bands are often largely determined by vibronic coupling rather than simple Franck-Condon factors, and second-order coupling is substantial. Nonsymmetric vibrations have intensities obtained through either vibronic or Coriolis coupling, and the calculations have been instrumental in discriminating between alternate possibilities in the assignments. Strong vibronic effects are expected to be present in the spectra of most monosubstituted benzenes, and the calculations presented here show that theoretical treatments based upon electronic structure calculations will generally be useful in the analysis of their spectra.

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.

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.

Two-dimensional momentum imaging of Rydberg states using half-cycle pulse ionization and velocity map imaging

The influence of the half-cycle pulse (HCP) kick on the asymptotic velocity of the ejected electron has been studied for excited xenon atoms (n{sup *}=34) in the presence of a static electric field (220 V cm{sup -1}). We find that the HCP does not change the momentum distribution perpendicular to the kick direction. Therefore half-cycle pulse ionization and electron velocity map imaging can be used to obtain two-dimensional momentum distributions of atomic Rydberg states. Semiclassical and quantum mechanical calculations complement the experimental results.

Rydberg state ionization by half-cycle-pulse excitation: strong kicks create slow electrons.

The asymptotic velocity distribution of electrons ionized in half-cycle-pulse excitation of high Rydberg states (n=34), placed in a static electric field, is studied using electron velocity-map imaging. At weak half-cycle pulse strengths, the electrons escape over the saddle point in the potential. For strong half-cycle pulses, the electrons are emitted in the direction of the field kick. The much slower and less intense half cycle of opposite polarity, which necessarily follows the main half-cycle pulse, strongly affects the momentum distribution and reduces the excess energy of the electrons significantly.