Gert von Helden

Phase shift cavity ring down absorption spectroscopy

Abstract Cavity ring down absorption spectroscopy with a continuous light source is used to measure the transition frequencies and absolute absorption coefficient of the weak b1Σg+(v′ = 2) ← X 3Σg−(v″ = 0) transition of 18O2. The absorption spectrum is extracted from a measurement of the magnitude of the phase shift that an intensity modulated continuous light beam experiences upon passing through an unstabilized optical cavity.

Infrared spectroscopy of jet-cooled neutral and ionized aniline-Ar

We report the infrared (IR) absorption spectrum of the jet-cooled neutral aniline–Ar Van der Waals complex together with that of the aniline–Ar cation in the 350–1700 cm−1 range. The spectra are measured using mass-selective ion detection in two different IR–ultraviolet double-resonance excitation schemes, using a free-electron laser as a source of widely tunable, intense IR radiation. A comparison with calculated IR spectra of the bare neutral aniline and of the cation of aniline allows for an unambiguous assignment of all the observed modes. The dissociation limit of the neutral aniline–Ar complex is bracketed between 273 and 329 cm−1, significantly lower than previously estimated.

Infrared spectroscopy of gas-phase zirconium oxide clusters

Abstract Gas-phase zirconium oxide clusters are resonantly excited using IR radiation from a free electron laser. At sufficiently large excitation fluences, clusters with a composition ZrnO2n−1 are observed to thermally emit electrons. Scanning the IR wavelength while monitoring the ion yield gives IR spectra of the clusters. For all clusters, resonances are observed between 600 and 700 cm−1 and the spectra are almost independent of cluster size.

Cavity ring down spectroscopy on solid C60

The light absorption of a solid sample in the 8.5 μm region is measured via cavity ring down (CRD) absorption spectroscopy, using a free electron laser (FEL) as a source of widely tunable infrared (IR) radiation. A 3 mm thick zinc-selenide (ZnSe) window is used as a substrate for a 20–30 nm thick C60 film. On top of the structureless absorption due to ZnSe (<340 ppm), one of the well-known fundamental IR absorption lines of C60 is measured with monolayer sensitivity.

Mass selective infrared spectroscopy using a free electron laser

Abstract Mass selective gas-phase infrared spectroscopy using IR/VUV double resonance ionization is demonstrated. A free electron laser (FEL), tunable over the 100–2000 cm−1 range, is used as the IR excitation source while VUV light, produced by a F2 laser, is used for subsequent ionization. Spectra are obtained by monitoring the ion of interest as a function of IR wavelength. It is shown that these spectra can give the same information as direct IR absorption spectra. The sensitivity of the method is many orders of magnitude higher than that of conventional IR absorption spectroscopy. First results on para amino benzoic acid (PABA) are presented.

A study on the structure and vibrations of diphenylamine by resonance-enhanced multiphoton ionization spectroscopy and ab initio calculations

Laser-desorption jet-cooling has been applied in combination with mass-selective gas-phase spectroscopic techniques to study the structure and low-frequency vibrations of diphenylamine ~DPA!. Two-color ~1118! resonance-enhanced multiphoton ionization has been used to measure the vibrationally resolved excitation spectrum of the S 1 S 0 transition in the 305‐309 nm region. Ion-dip measurements have been performed to determine the vibrational structure in the electronic ground state. The electronic spectra of DPA are dominated by long progressions in low-frequency vibrations involving the motion of the phenyl rings as a whole. For the interpretation of the experimental data ab initio calculations have been performed at the Hartree‐Fock level for the S 0 -state and using single-excitation configuration interaction for the S 1 -state. The DPA molecule is found to change from a pyramidal geometry around the N-atom with unequal torsional angles of the phenyl groups in the S0-state to a planar geometry with equal torsional angles in the S1-state. The two most prominent vibrational motions are the in-phase wagging and the in-phase torsion of the phenyl rings. In addition, the resonance-enhanced multiphoton ionization spectra of the S1 S0 transition in the DPA-Ar, DPA-Kr, and DPA-Xe van der Waals complexes have been measured. From these spectra it is inferred that there is a coupling between the van der Waals modes and the low-frequency intra-molecular modes of DPA.

Confining CO molecules in stable orbits

A scheme is presented for the confinement of neutral molecules in stable orbits on the basis of the linear Stark effect in a properly designed electrostatic trap. Based on Stark shift measurements in electric fields up to 140 kV/cm, an electrode configuration which is, under realistic experimental conditions, capable of confining metastable CO(a31-1) molecules with velocities up to 22 m/s is described. Metastable CO molecules can be laser-prepared inside the trap and their trajectories can be directly visualized via the spatially resolved detection of their fluorescence. (~) 1997 Elsevier Science B.V.

Gas-phase infrared spectroscopy on the lowest triplet state of the pyrazine-argon complex

Abstract The infrared (IR) absorption spectrum of the jet-cooled pyrazine–argon Van der Waals complex in its lowest triplet state is measured. Vibrational excitation of the laser-prepared triplet state by IR absorption causes dissociation of the complex, which is monitored via the depletion of the pyrazine–argon ion signal. A total of 10 vibrational transitions are detected in the 500–1500 cm −1 region. Comparison of these data with ab initio calculations might help to elucidate the nature of the distortion of the lowest triplet state by other electronically excited states.

Compressibility of CO intercalated C60 crystals

Abstract We report the pressure dependence of the cubic lattice parameter of (CO)xC60 at different temperatures, as determined by X-ray powder diffraction. In the measured range of pressures from 0.2 to 2 GPa, the volume is found to depend linearly on the pressure. The extrapolated volume bulk modulus of the ordered phase at room temperature is found to be K 0 =−V d P/ d V=31.5(5) GPa. The bulk modulus at T=150 K is found as K0=26.9(5) GPa. The greater stiffness at higher temperature is explained by the entropic contribution of the CO molecules to the bulk modulus.