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The absorption spectra of the electronic S 1 √ S 0 transition of glyoxal molecules (C 2 H 2 O 2 ) embedded in He droplets (ഠ5500 atoms) show well-resolved vibronic bands with a width , 0.5 cm 21 . The phonon wings at higher frequencies have distinct gaps amounting to DE 8.1 K followed by a small maximum at 14.8 K. The phonon wing shape agrees with a theoretical simulation based on the dispersion curve of elementary excitations in bulk He II, providing the first evidence for superfluidity in the finite-sized He droplets. [S0031-9007(96)00383-3] PACS numbers: 67.40.Yv, 33.20.Kf, 67.40.Db A number of recent theoretical studies predict that 4 He clusters with more than about 64 atoms are superfluid with a transition temperature which is somewhat lower than the bulk l-point temperature T l 2.2 K [1-3]. So far, however, there is no direct experimental evidence for superfluidity in these nanosize liquid particles. Recently it has been possible to observe a very sharp rovibrational spectrum of single SF 6 molecules located in the interior of He droplets ͑N . 1000 atoms͒ which were produced in free jet expansions [4,5]. From this the rotational temperature was found to be T 0.37 6 0.05 K [5] in good agreement with theoretical predictions [6]. In addition, infrared spectra of SF 6 dimers indicate that these larger entities can also rotate freely in the He droplets [7].In the bulk the most direct evidence for superfluidity comes from neutron diffraction experiments which indicate sharp elementary excitations with a dispersion characterized by a maximum at E max 13.7 K ͑Q max 1.10 Å 21 ͒ called a maxon and the well known roton minimum at E rot 8.65 K ͑Q rot 1.91 Å 21 ͒ [8]. As first pointed out by Landau the sharp excitations at the roton minimum enable the fluid to flow unhindered at velocities below about 58 m͞s which is the most prominent manifestation of superfluidity. This critical velocity could so far only be observed for negative ions which were found to move without friction in liquid helium at P 25 atm and T 0.4 K [9]. Two-phonon Raman spectroscopy in bulk helium also has been shown to provide information on the elementary excitations [10]. In the quest for more direct spectroscopic probes of superfluidity several groups have developed sophisticated techniques to levitate atoms in liquid helium [11][12][13][14]. Up to now only broad lines of several cm 21 width could be observed. A recent study of the electronic spectra of Na 2 attached to the surface of He droplets reveals vibronic bands consisting of a sharp zero phonon line (ZPL) and an intense broad phonon wing (PW) [15]. In this system multiphonon processes appear to dominate the spectra and conceal the elementary excitations of the droplet.To circumvent these difficulties we have undertaken the first spectroscopic experiments with a simple organic molecule. Glyoxal (C 2 H 2 O 2 ) was chosen since its visible spectroscopy has been studied both as a free molecule and in cryomatrices [16,17]. Compared to the alkali metals glyoxal is readily solvated by helium...

Single OCS molecules have been embedded in large 4He droplets (N̄=1×103–8×103 atoms) and their infrared spectra in the vicinity of the ν3-fundamental at 2062 cm−1 have been studied using coaxial laser depletion spectroscopy. Sharp lines corresponding to the P- and R-branches with a linewidth of 160 MHz or greater are observed. From the line intensities a droplet temperature of 0.37±0.02 K is obtained and from the line positions the rotational constants B for the ground and excited states and an average centrifugal distortion constant D are determined. The former are about a factor of 2.8 smaller and the latter four orders-of-magnitude larger than for the free molecule. The decrease in B is attributed to an attached nonsuperfluid component which has the same effect as a ring of about six 4He atoms in an equatorial plane around the waist of the OCS molecule, which is carried along in the end-over-end rotation. The widths of the individual lines show an interesting asymmetry which is oppositely sloped for the P- and R-branches and increases with the rotational quantum number. Several possible explanations for the line shapes and widths are presented.

Small van der Waals clusters of sulfur hexafluoride (SF6) and mixed SF6-rare gas clusters were prepared inside large droplets of helium-4, with each droplet consisting of about 4000 helium atoms. A diode laser was used to measure the high-resolution infrared spectra of these clusters in the vicinity of the nu3 vibrational mode. In all cases rotational structure was observed, indicating that the embedded species rotate nearly freely, although they had been cooled to a temperature of 0.37 kelvin. The results indicate that helium droplets are probably superfluid and thereby provide a uniquely cold yet gentle matrix for high-resolution spectroscopy.

Negative symptoms in schizophrenia have been grouped into the 2 factors of apathy and diminished expression, which might be caused by separable pathophysiological mechanisms. Recently, it has been proposed that apathy could be due to dysfunctional integration of reward and effort during decision making. We asked whether apathy in particular is associated with stronger devaluation ("discounting") of monetary rewards that require physical effort. Thirty-one patients with schizophrenia and 20 healthy control participants performed a computerized effort discounting task in which they could choose to exert physical effort on a handgrip to obtain monetary rewards. This procedure yields an individual measure for the strength of effort discounting. The degree of effort discounting was strongly correlated with apathy, but not with diminished expression. Importantly, the association between apathy and effort discounting was not driven by cognitive ability, antipsychotic medication, or other clinical and demographic variables. This study provides the first evidence for a highly specific association of apathy with effort-based decision making in patients with schizophrenia. Within a translational framework, the present effort discounting task could provide a bridge between apathy as a psychopathological phenomenon and established behavioral tasks to address similar states in animals.

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The laser-induced fluorescence spectra of single tetracene (C 18 H 12 ) and pentacene (C 24 H 14 ) molecules embedded in liquid He N droplets (N ˜= 10 4 ) show sharp zero phonon lines (ZPL) (δν e 0.2 cm -1 ), accompanied by weaker phonon wings (PW) on the blue side. The ZPL of tetracene is anomalously split into a doublet with a separation of 1.1 cm -1 , whereas for pentacene, the ZPL is not split. Hole-burning measurements with two pulsed dye lasers and lifetime measurements indicate that inside He droplets the ground and excited states of tetracene are each split into two levels. The splitting is attributed either to the occupation of two nearly equivalent sites by localized helium atoms or to a tunneling of one or two localized helium atoms through the barrier in the double-well potential on the surface of the tetracene molecule.