Klaus Müller-Dethlefs

A new approach to vibrational spectroscopy of ion clusters: the “zero kinetic energy (ZEKE)” photoelectron spectrum of the phenol—water complex

Abstract The vibrational spectrum of the phenol—H2O cation complex is obtained by two-colour resonant pump—probe “zero kinetic energy (ZEKE)” spectroscopy. The ZEKE electron signal only appears when the total photon energy exactly matches a transition to a cluster ion state. The results are: an accurate value of the adiabatic ionisation energy, and a well resolved vibrational spectrum assigned to intermolecular modes. The ZEKE method provides a resolution (0.4 cm− close to laser bandwidths. Thus, vibrational frequencies of the cluster ion are obtained to a precision not available with any other method.

The non-resonant two-photon zero kinetic energy photoelectron spectrum of CS2

Abstract The high-resolution zero kinetic energy photoelectron spectrum of CS 2 is presented. Accurate values for the ionization potential and the spin—orbit splitting of the X + 2 Π g are obtained. Observation of the symmetry-forbidden excitations of the v 2 bending vibration yields accurate frequencies for this normal mode. The Renner—Teller splitting for the Δ u 3 2 and the Σ − u components of v + 2 in the upper 2 Π g 1 2 spin—orbit component has been resolved for the first time. The photoionization efficiency spectra show a strong counter-correlation of the CS + 2 and S + fragments.

The phenol dimer: Zero‐kinetic‐energy photoelectron and two‐color resonance‐enhanced multiphoton ionization spectroscopy

The two‐color, two‐photon (1+1’) resonance‐enhanced multiphoton ionization spectrum of the hydrogen‐bonded phenol dimer has been recorded in reasonable agreement with previously reported spectra. However, more features are obtained in this work and this has allowed a more detailed analysis of the spectrum. Five intermolecular vibrations (out of a possible six) are observed for the S1donor state, while only two modes are obtained for the S1acceptor state. Zero‐kinetic‐energy (ZEKE) photoelectron spectra were recorded via different intermediate vibronic states. The spectrum recorded via the vibrationless level of the S1donor state is rich in structure and indicates a large change in the geometry on ionization. Progressions in the intermolecular stretch mode and at least one other mode are obtained. ZEKE spectra were also recorded via a number of S1donor vibronic levels, and the S1acceptor vibrationless level. The lowest value measured for the ionization energy of the donor is 63 649±4 cm−1 (7.8915±0.0005 eV...

Basic principles of ZEKE spectroscopy. Optimized resolution and accurate ionization energy

Abstract Basic principles of the ZEKE (zero kinetic energy photoelectron) spectroscopic method are outlined. The experimental procedure takes advantage of specially tailored electric field ionization pulses. Two types of extraction pulses are considered: (i) linearly rising pulses (‘linear slope’) and (ii) pulses with a multi-step ‘staircase slope’. First, the dependence of the spectral ZEKE resolution on both linear slope pulses with different rise times and staircase slope pulses with varying step height is described. Secondly, the staircase slope pulse is used for an accurate determination of the ionization energy by measuring the ZEKE signals correlated to different field strengths simultaneously in one single scan. For the benzene molecule a predominantly diabatic ionization process (field shift equation: ΔE/cm−1 = (3.89±0.05) (F/V cm−1) 1 2 ) is observed.

Theory of rotational line strengths in coherent anti-stokes raman spectroscopy

The general expressions for rotationally resolved CARS transitions involving molecular electronic states of arbitrary electronic angular momentum and spin are derived. All possible types of resonances (triple, double, Raman) are considered and the explicit results are presented for Hunds coupling cases (a) and (b) and intermediate (a–b) coupling. The results given are applicable to arbitrary incident laser polarizations. Calculations of practical interest are performed for three diatomics, namely OH, C2 and CH, for the triple resonance. Complete sets of closed formulae are tabulated for Hunds cases (a) and (b) for 3Π ← 3Π and 2Σ ← 2Π type transitions. With minor modifications, these expressions are also applicable to resonant degenerate four wave mixing.

Threshold photoionization and ZEKE spectroscopy: a historical perspective

Abstract The new ZEKE spectroscopy is described, and its development is presented and related to the older threshold electron spectroscopy. This is compared to the field of photoelectron spectroscopy. Results from the different techniques are compared.

Zero kinetic energy photoelectron (ZEKE) spectroscopy of the heterotrimer phenol-water-argon: Interaction between a hydrogen bond and a van der Waals bond

Abstract The heterotrimer phenol -water- Ar, a complex containing two different types of intermolecular bonds — a van der Waals bond and a hydrogen bond — has been studied in a supersonic jet with various spectroscopic techniques. The two-photon, two-color (1 + 1′) resonance-enhanced multiphoton ionization (REMPI) spectrum of the S1 state shows striking differences compared to the spectrum of the corresponding complex without water. From the zero kinetic energy photoelectron (ZEKE) spectrum an accurate ionization energy and the frequencies of all three van der Waals vibrations of the ionic ground state have been obtained. Comparison of the ZEKE and REMPI spectra of phenol-water-Ar with the corresponding spectra of phenol-Ar and phenol-water indicates that the stronger hydrogen bond noticeably influences the weaker van der Waals bond, while vice versa the hydrogen bond is nearly not affected by the additional van der Waals bond. Sharp steps in the photoionization efficiency (PIE) spectra of phenol-water-Ar and the fragment complex phenol-water provide an upper limit for the dissociation energy of the van der Waals bond in the ionic state, and from this value upper limits for the binding energies in both neutral states (S0, S1) have been derived. For comparison, REMPI and PIE spectra have also been recorded for phenol -water-Ne. Finally, the first mass-analyzed threshold ionization spectrum of a hydrogen-bonded complex, namely phenol-water, has been recorded in order to demonstrate that this technique can also be utilized for such type of complexes.

Applications of ZEKE spectroscopy

Abstract Typical applications of high resolution Zero kinetic energy (ZEKE) photoelectron spectroscopy to molecules and clusters are reviewed. The high resolution of ZEKE spectroscopy compared to conventional photoelectron spectroscopy allows the rotational structure of larger molecular cations, for instance the benzene cation, to be obtained. The dynamic Jahn-Teller effect and the shape of the benzene cation is apparent from these rotationally resolved ZEKE spectra. For molecular clusters, the phenol-water and phenol-methanol complexes show dense, clearly resolved intermolecular vibrations.

Angular distribution of near-zero kinetic energy photoelectrons from the lowest rotational states of the NO A 2Σ+ state

Abstract We report the first rotationally resolved angular distributions of near-zero kinetic energy photoelectrons produced from ionization of the lowest rotational states ( N A =0, 1, 2 or 3) of the nitric oxide A 2 Σ + (ν A =0) state. The angular distributions, for the ion in the X + 1 Σ + (ν + =0, N + =0, 1, 2 or 3) state, agree well with a theoretical model proposed by McKoy and co-workers, with the exception of the ionizing N + = 0← N A =1 rotational transition. The latter transition exhibits an isotropic angular distribution, which seems to indicate some hitherto unobserved threshold dynamics.

ZEKE spectroscopy of hydrogen-bonded phenol complexes

Abstract The recently-developed technique of zero-kinetic-energy photoelectron spectroscopy (ZEKE Spectroscopy) has been applied to a series of hydrogen-bonded complexes: phenol-water, phenol-methanol and phenol-ethanol. These resonant (1+1′) ZEKE spectra easily resolve intermolecular vibrational frequencies as well as a number of intramolecular (phenol-localized) ones. Accurate ionization energies are derived for these species and by comparison with the ionization energy of phenol, the change in hydrogen-bond strength between the neutral and corresponding ionic state for each complex is derived. By exciting the molecule through different intermediate vibrational levels of the S1 state, the Franck-Condon (FC) activity in the ionization step is changed leading to the observation of all six intermolecular modes for each complex.