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Author(s): H. Wabnitz [1]; L. Bittner [1]; A. R. B. de Castro [2]; R. Döhrmann [1]; P. Gürtler [1]; T. Laarmann [1]; W. Laasch [1]; J. Schulz [1]; A. Swiderski [...]

The caging effect of the host environment on photochemical reactions of molecular oxygen is investigated using monochromatic synchrotron radiation and spectrally resolved fluorescence. Oxygen doped clusters are formed by coexpansion of argon and oxygen, by pickup of molecular oxygen or by multiple pickup of argon and oxygen by neon clusters. Sequential pickup provides radially ordered core-shell structures in which a central oxygen molecule is surrounded by argon layers of variable thickness inside large neon clusters. Pure argon and core-shell argon-neon clusters excited with ∼12 eV monochromatic synchrotron radiation show strong fluorescence in the vacuum ultraviolet (vuv) spectral range. When the clusters are doped with O2, fluorescence in the visible (vis) spectral range is observed and the vuv radiation is found to be quenched. Energy-resolved vis fluorescence spectra show the 2 1Σ+→1 1Σ+(ArO(1S)→ArO(1D)) transition from argon oxide as well as the vibrational progression A ′3Δu(ν′=0)→X 3Σg-(ν″) of O2 indicating that molecular oxygen dissociates and occasionally recombines depending on the experimental conditions. Both the emission from ArO and O2 as well the vuv quenching by oxygen are found to depend on the excitation energy, providing evidence that the energy transfer from the photoexcited cluster to the embedded oxygen proceeds via the O2+ ground state. The O2+ decays via dissociative recombination and either reacts with Ar resulting in electronically excited ArO or it recombines to O2 within the Ar cage. Variation of the Ar layer thickness in O2–Ar–Ne core-shell clusters shows that a stable cage is formed by two solvation layers. [ABSTRACT FROM AUTHOR]

Intense femtosecond laser pulses, judiciously tailored in an adaptive, optimal control feedback loop were used to break preferentially the acyl-N (“peptide”) bond of Ac-Phe-NHMe that may be regarded as a dipeptide model. We show that coherent excitation of complex wave packets in the strong-field regime allows to cleave strong backbone bonds in the molecular system preferentially, while keeping other more labile bonds intact. These results show the potential of pulse shaping as a powerful complementary analytical tool for protein sequencing of large biopolymers in addition to the well-known mass spectrometry and chemical analysis. [ABSTRACT FROM AUTHOR]

The fragmentation dynamics of C60 irradiated with intense femtosecond laser pulses is studied with one-color pump-probe spectroscopy. Small neutral fragments (C, C2, and C3) are formed by an 800-nm pump pulse which are then postionized by a delayed probe pulse. The respective ion signals detected by the time-of-flight mass spectrometry dramatically increase on a time scale of 10–20 ps. [ABSTRACT FROM AUTHOR]

In this paper we report on the isotopic effect on the cage-induced excited-state quenching inside small Ar[sub m] clusters (m<10[sup 2]) solvated in large Ne[sub N] clusters (N≈7.5×10[sup 3]). Excited OH(A)/OD(A) fragments are produced by photodissociation of H[sub 2]O and D[sub 2]O molecules and the quenching agents are correspondingly H or D atoms. The decrease of the fluorescence yield with the size of the cluster m>m[sub 0] is observed in both cases and it is attributed to the formation of the cage of argon atoms around the doped molecule. Interestingly, more atoms are needed to induce the fluorescence quenching of OD[sup *](A) fragments, m[sub 0]=21±3, compared to the electronically excited state quenching of OH[sup *](A) molecules, 11±2. A diffusion model containing two free parameters, the quenching cross section σ[sub q] and the number of argon atoms forming the cage m[sub 0], explains the effect in terms of the residence time of the hydrogen atom inside the cage. We suggest that the melting of the doped rare gas clusters is responsible for the different predissociation dynamics. The quenching cross section obtained from the experimental data is in good agreement with former experiments. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Energy relaxation processes of photo-excited Kr[sub &50macr;] clusters covered with a shell of Ar atoms (up to 40), which are embedded inside large Ne[sub &7500macr;] clusters are investigated with energy resolved fluorescence spectroscopy. In the energy range of the characteristic Ne cluster absorption (16.5-18 eV) a strong energy transfer to the embedded Kr cluster is observed, which results in the desorption of electronically excited Kr[sup *] atoms. Kr[sup *] atoms move through the Ne cluster, desorb and emit visible and near-infrared light in the vacuum (5p → 5s). By coating the Kr clusters with Ar atoms, the Kr lines disappear and 4p → 4s transitions of Ar[sup *] become dominant. Additionally, new emission bands occur, which are assigned to transitions of perturbed atomic Kr 5p-states inside Ne clusters. Due to the interaction of electronically excited Kr[sup *] atoms with neutral Ar atoms in the surrounding shell, several excited Kr states namely 5p [½][sub 0] and 5p [3/2][sub 2] decay nonradiatively. This is in agreement with the well-known "energy-gap law." The results give experimental evidence that clusters with a multishell structure can be prepared by a sequential pick-up technique. This allows the preparation of Kr clusters embedded inside Ne clusters and coated with a shell of Ar atoms. Such clusters cannot be prepared with conventional coexpansion techniques. [ABSTRACT FROM AUTHOR]

A study of energy relaxation processes of H[SUB2]O-doped rare gas clusters excited with vacuum ultraviolet synchrotron radiation is presented. An enhanced quenching of electronically excited fragment OH[SUP*](A) is observed in Ar[SUBm]Ne[SUBN] clusters (m<10[SUP2], N=7.5x10[SUP3]). The doping of interior of small argon clusters has been achieved by using a "sequential pick-up" technique (Ne[SUBN] + H[SUB2]O +mAr). Due to a low temperature of the Ne host cluster the Ar atoms are fixed around the H[SUB2]O molecule prohibiting structural rearrangements. A strong decrease of the OH[SUP*](A) fluorescence yield has been observed for re>m[SUB1] = 12. This effect is attributed to a formation of the second (m[SUB2] = 54) and higher shells of Ar atoms around the water molecule. The principal contribution from a noncomplete second shell (m[SUP*][SUB2] = 32) after a closure of 20 triangular windows in the first shell is suggested. Due to a finite size of the cluster matrix and fast sample renewal, the cage exit and reentry processes can be investigated. [ABSTRACT FROM AUTHOR]

The charge localization process in Xe[sub N][sup +] cluster ions (N=40–20 000) is investigated with fluorescence spectroscopy methods. New discrete and continuous luminescence bands in the visible and near infrared spectral range are observed and are assigned to radiative transitions of ionic dimers, trimers and tetramers inside Xe clusters. The bands are related to the 5p[sup 5] [sup 2]P[sub 1/2]→5p[sup 5] [sup 2]P[sub 3/2] transition of electronically excited atomic Xe ions and the 6p→6s transitions of electronically excited Xe neutrals. The dependence of the size of the ionic centers on the Xe cluster size is discussed. In large clusters discrete lines are due to embedded dimer emission and they are identified as 2(1/2)[sub u]→1(3/2)[sub g] transitions between different vibrational levels. Line positions are blue-shifted by 30 (±1) meV with respect to free molecular dimer ions. The energy shift is due to the interaction of the surrounding neutral Xe cluster atoms with the embedded ionic Xe molecules. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Electronic structure and energy transfer in H[sub 2]O doped He[sub N] clusters (Napprox. 10[sup 4]) is studied with photoexcitation in the spectral range of 140–40 nm (9–30 eV). The reaction dynamics is investigated by fluorescence of neutral OH[sup *] and H[sup *] and ionic H[sub 2]O[sup +[sup *]] fragments. The rotational temperature of embedded water molecules has been estimated from the 124 nm line shape (3pa[sub 1] C [sup 1]B[sub 1]←1b[sub 1] X˜ [sup 1]A[sub 1] transition). Two different temperatures (T1≤5 K and T2approx. 30 K) have been found. We propose that the lower temperature (T[sub 1]) is due to completely thermalized water molecules trapped inside helium clusters, while the warmer molecules (T[sub 2]) are formed if they are first captured by helium clusters but then leave the clusters again. Predissociation of H[sub 2]O with excitation below the ionization limit (λ[sub exc]>100 nm) is found to be unaffected by the cluster environment. On the other hand, the ionization (λ[sub exc]<100 nm) seems to be suppressed inside helium clusters in favor of the fragmentation into neutral products. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]