Friedrich Huisken

Infrared spectroscopy of small size‐selected water clusters

Infrared molecular beam depletion and fragment spectroscopy has been employed to study the absorption behavior of small water clusters [(H2O)n, n=2,3,4,5]. The spectral region between 3300 and 3800 cm−1 was covered with an injection‐seeded optical parametric oscillator. Size‐specific information has been obtained by dispersing the cluster beam with a secondary helium beam and measuring the depletion as a function of the scattering angle. Three absorption bands could be assigned to the water dimer (H2O)2, with the bonded OH stretch being localized at 3601 cm−1. For each of the larger water clusters (n=3,4,5), which have cyclic structures, two absorption bands could be identified, one belonging to the free OH stretch and the other being due to the excitation of the OH ring vibration. The measurements on free water clusters were complemented by studies on small water complexes formed on large argon clusters. The positions of the absorption bands observed in these spectra are close to those found for (H2O)n i...

Photoluminescence of size-separated silicon nanocrystals: Confirmation of quantum confinement

Silicon nanocrystals with diameters between 2.5 and 8 nm were prepared by pulsed CO2 laser pyrolysis of silane in a gas flow reactor and expanded through a conical nozzle into a high vacuum. Using a fast-spinning molecular-beam chopper, they were size-selectively deposited on dedicated quartz substrates. Finally, the photoluminescence of the silicon nanocrystals and their yield were measured as a function of their size. It was found that the photoluminescence follows very closely the quantum-confinement model. The yield shows a pronounced maximum for sizes between 3 and 4 nm.

Improved one-phonon confinement model for an accurate size determination of silicon nanocrystals

In this article, we show how the well-known one-phonon confinement model can be improved to determine the diameter of silicon nanocrystalline spheres from the optical phonon wave-number shift, even using a physical-meaning weighting function. We show that the fundamental parameter is the knowledge of the phonon dispersion. The accuracy of our approach is supported by experimental data obtained by selective UV Raman scattering on nanocrystalline silicon thin films produced by size-selected silicon cluster beam deposition.

Dissociation of small methanol clusters after excitation of the O–H stretch vibration at 2.7 μ

Molecular beam depletion spectroscopy has been employed to study the dissociation of small methanol clusters upon excitation of the O–H stretch vibration at 2.7 μ. The tunable infrared radiation has been obtained from a Nd:YAG laser pumped optical parametric oscillator. Pure methanol dimer spectra, without contamination from larger clusters, have been measured using the scattering selection technique or working at reduced temperatures. The dimer spectrum features two absorption peaks at 3574.4 and 3684.1 cm−1. The lower frequency peak is redshifted by 106.6 cm−1 from the monomer value and is assigned to the excitation of the proton donor in the hydrogen bonded complex. The proton acceptor peak is blueshifted by only 3.1 cm−1. Methanol trimer dissociation is observed at 3462 cm−1, whereas larger clusters absorb at still lower frequencies. Absolute photodissociation cross sections were measured for both methanol dimer bands. The integrated dissociation cross sections were 5.59(25)×10−21 cm2 /molecule and 7....

Vibrational spectroscopy of small water complexes embedded in large liquid helium clusters

Infrared molecular beam depletion spectroscopy has been employed to study the vibrational spectroscopy of water molecules and small water polymers [(H2O)n, n=2,3,4] embedded in large liquid helium clusters (HeN, 100≤〈N〉≤5000). The spectral region between 3500 and 3800 cm−1 was covered with an injection‐seeded optical parametric oscillator. The following vibrational bands could be located and investigated: for the monomer the ν3 asymmetric stretch, for the dimer the asymmetric stretch of the proton acceptor molecule and the free and bonded O–H stretches of the donor molecule, for the trimer both free and bonded O–H stretches, and for the tetramer the free O–H stretch. By comparison with the data on free gas phase water complexes, it was found that the helium host clusters induce only minor perturbations in form of small frequency redshifts and that they constitute an ideal nano‐matrix. Furthermore, it was found that both the water molecule and the dimer rotate almost freely in the host cluster and that the...

State resolved rotational excitation in D2+H2 collisions

In a crossed molecular beam experiment time‐of‐flight distributions of ortho D2 molecules scattered from normal H2 (nH2) and para H2 (pH2) have been measured in a center‐of‐mass angular range of 75° to 180°. The collision energies were 84.1 and 87.2 meV, respectively. In all spectra the rotational excitation of D2 from j=0 to j=2 has been resolved. With pH2 as secondary beam the same transition could also be observed for H2. The measurements show that the probability for rotational excitation of D2 depends on whether the scattering partner H2 is rotating (nH2) or not (pH2). In the first case the cross sections are larger by a factor of approximately 2. The reason for this behavior is the presence of an additional interaction term which is at long range distances, identical to the quadrupole–quadrupole interaction and which is absent if H2 is in the j=0 state. The experimentally derived differential cross sections for the rotational excitation of D2 and H2 are compared with theoretical results obtained by ...

Differential cross sections for the j=0→1 rotational excitation in HD–Ne collisions and their relevance to the anisotropic interaction

In a crossed molecular beam experiment, time‐of‐flight distributions of HD‐molecules scattered from Ne at an energy of 31.5 meV have been measured using the pseudorandom chopper method. Each time‐of‐flight spectrum shows a clearly resolved inelastic peak due to rotational excitation of HD from j=0 to j=1. With the aid of these spectra, together with additional measurements of the total differential cross section, the angular dependence of the differential cross section for the excitation j=0→1 is derived over a large angular range (20° to 120°). The sensitivity of the data to the interaction potential is carefully studied and the measured inelastic differential cross sections are compared with calculations based on interaction potentials recently proposed for this system.

Vibrational frequency shift of HF in helium clusters: Quantum simulation and experiment

We report accurate variational and diffusion quantum Monte Carlo calculations for the size dependence of the vibrational frequency shift of HF molecules embedded in helium clusters with up to n=198 helium atoms. The frequency shift exhibits a strong initial size dependence and saturates at a redshift of about 2.7 ± 0.1 cm−1 for clusters with over 100 atoms. This value is in good agreement with our experimental redshift of 2.65 ± 0.15 cm−1 for clusters with over 1000 atoms. The helium cluster is found to undergo significant structural changes upon embedding of HF. The density in the nearest neighbor shell exceeds the bulk helium density by a factor of two. A second nearest neighbor density maximum and a peripheral density plateau very close to the bulk helium value is found. In spite of the anisotropic interaction between HF and helium all clusters have almost perfectly spherical helium density profiles and indicate close to free rotor behavior of HF inside the cluster. The cluster size dependence of the r...

The anisotropic interaction potential of D2Ne from state‐to‐state differential cross sections for rotational excitation

Differential cross sections for the rotational excitation from j=0 to j=2 of D2 scattered by Ne have been measured at an energy of E=84.9 meV. The experiments have been performed in a crossed nozzle beam apparatus with time‐of‐flight analysis of the scattered particles using the pseudorandom chopper method. A detailed analysis of the experimental data which are peaked in the backward direction showed that they are mainly sensitive to the repulsive part of the pure anisotropic potential. From a combined analysis of the state‐to‐state differential cross sections of the j=0 to j=0 and the j=0 to j=2 transition of D2+Ne and the j=0 to j=1 transition of HD+Ne previously measured, the complete potential energy surface for the hydrogen–neon system is obtained using the coupled states method. The anisotropic contribution varies from 37% of the isotropic part in the repulsive region (2.4 A) to 12% in the attractive region (3.5 A). The results differ from the other potential models derived for this system from calc...

Formation of Polycyclic Aromatic Hydrocarbons and Carbonaceous Solids in Gas-Phase Condensation Experiments

Carbonaceous grains represent a major component of cosmic dust. In order to understand their formation pathways, they have been prepared in the laboratory by gas-phase condensation reactions such as laser pyrolysis and laser ablation. Our studies demonstrate that the temperature in the condensation zone determines the formation pathway of carbonaceous particles. At temperatures lower than 1700 K, the condensation by-products are mainly polycyclic aromatic hydrocarbons (PAHs) that are also the precursors or building blocks for the condensing soot grains. The low-temperature condensates contain PAH mixtures that are mainly composed of volatile three to five ring systems. At condensation temperatures higher than 3500 K, fullerene-like carbon grains and fullerene compounds are formed. Fullerene fragments or complete fullerenes equip the nucleating particles. Fullerenes can be identified as soluble components. Consequently, condensation products in cool and hot astrophysical environments such as cool and hot asymptotic giant branch stars or Wolf-Rayet stars should be different and should have distinct spectral properties.