Excitation and decay processes in helium clusters studied by fluorescence spectroscopy

Joppien, M.; Müller, R.; Möller, Thomas

doi:10.1007/bf01429135

Cited by 15 publications

(33 citation statements)

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“…The Journal of Physical Chemistry A ARTICLE The internuclear separation in helium clusters is not constant and varies due to the decaying number density at the surface. The dashdotted line shows the average internuclear separation in bulk-liquid 4 He or in the center of large helium droplets which represents the lower limit. The wave function radii that belong to internuclear separations on the right-hand side of the dash-dotted line Figure 2 are those that can be expected in real helium clusters and droplets.…”

Section: ' Results and Discussionmentioning

confidence: 99%

“…Figure 5 shows, for a source temperature of 11 K, only sharp atomic resonance lines, for 10.6 K strongly rugged bands, and for 10.2 K fully developed bands. The shape and the intensity of the spectral features remain constant when the source temperature is further lowered to 9.0 K. Further lowering of the temperature causes the features to distort and to lose intensity, similar to large 4 He droplets that were produced by fragmentation from the liquid phase. Measurements of the droplet size by Harms et al using the cross beam deflection method reveal sizes that decrease slightly with decreasing source temperature.…”

Section: The Journal Ofmentioning

confidence: 94%

“…Midsized 4 He Clusters and Large 4 He Droplets. Figure 3 shows spectra of 4 He droplets containing 450À1800 atoms.…”

Section: The Journal Ofmentioning

confidence: 99%

“…We note that the intensity of the sharp 3p resonance line at 23.10 eV does not alter much when the stagnation temperature is lowered to 7.3 K. From 20 to 7.3 K the intensity is almost constant, whereas from T = 35 to 20 K the intensity decreases by about a factor of 4. At source conditions for T 0 < 13 K two effects have to be considered: first, the 4 He droplets produced are very large. Depending on the wavelength droplets containing more than 10 4 atoms are no longer optically thin, that is, the light transmitted through a droplet decays exponentially (due to Beer's law) because the droplets can have more than one excitation per synchrotron light The Journal of Physical Chemistry A ARTICLE pulse.…”

Section: The Journal Ofmentioning

confidence: 99%

“…11,13 The availability of this data allows a much better assignment of sizes than was possible in the early studies. Our method is to investigate both stable isotopes of helium, 3 He and 4 He. Clusters of the two isotopes differ in their particle density, liquid 3 He being about 75% the density of 4 He.…”

Section: ' Introductionmentioning

confidence: 99%

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Size and Isotope Effects of Helium Clusters and Droplets: Identification of Surface and Bulk-Volume Excitations

et al. 2011

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The helium atom is the simplest example of a many-electron atom. In light of the simplicity and model character of helium atoms one would naturally be interested to learn how the electronically excited states become modified when an excited helium atom is placed near one, two, or more helium atoms. A straightforward experiment to investigate this problem is to produce a beam of helium clusters of variable size and to photoexcite these clusters in the vacuum ultraviolet (VUV) spectral range using synchrotron radiation. Such an experiment probes a disordered arrangement of atoms because helium clusters and droplets in a free beam are always liquid. A molecular beam of clusters is optically thin and absorbs very little light, but helium clusters have a large fluorescence yield. Therefore, photofluorescence yield detection is the method of choice. The first experiments of 4 He clusters of variable size using a VUV fluorescence light detector identified a number of bands and it was found that the energies of these excitations, unlike the electronic excitations of heavy rare gas clusters, 1,2 did not fit to a Wannier-type excitonic series. 3 Subsequent experiments investigated fluorescence decay channels in different wavelength regions, 4,5 the effect of helium particle density, 6 fluorescence in cavities within large helium droplets, 7 and the possible Rydberg nature of the excited states of small helium clusters. 8 First quantum chemical calculations of an octahedral model cluster reported the energies of the n = 2, 3, and 4 states of the central atom as a function of the internuclear separation in the octahedron. 9 Electronic spectra simulations of a perturbed octahedron as well as an N = 25 atom-large cluster 10 were able to reproduce previously reported experimental data of small clusters. 6 Despite these efforts, we still have an incomplete picture of the electronically excited states of large helium clusters. The number of states increases dramatically with size making the computation and interpretation of the results laborious. The availability of experimental data that cover the entire size range of helium clusters and droplets is prerequisite as a benchmark for testing theoretical interpretation and likewise prerequisite in providing evidence for empirical interpretation. Furthermore, the comprehensive data set presented in this paper shows that each helium cluster size has a specific spectral fingerprint. A unique feature of helium clusters and droplets is the 6À7 Å thick surface region where the density drops smoothly from the bulk value to zero. 11,11,12 With regard to electronically excited states, such a surface layer represents a ABSTRACT: We report a comprehensive investigation of the electronically excited states of helium clusters and droplets of sizes ranging from a few to several 10 7 atoms using time-resolved fluorescence excitation spectroscopy and quantum chemical ab initio calculations. We employ various approaches for our analysis considering the lifetime-dependence of the fluorescence intens...

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Section: ' Results and Discussionmentioning

confidence: 99%

Section: The Journal Ofmentioning

confidence: 94%

“…Midsized 4 He Clusters and Large 4 He Droplets. Figure 3 shows spectra of 4 He droplets containing 450À1800 atoms.…”

Section: The Journal Ofmentioning

confidence: 99%

Section: The Journal Ofmentioning

confidence: 99%