Direct Spectroscopic Observation of Elementary Excitations in Superfluid He Droplets

Abstract

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...

Electronic spectroscopy of 9,10-dichloroanthracene inside helium droplets

“…However, the intensity pattern of the tiny peaks in Fig. 2(a) deviates drastically from what is expected for purely superfluid helium 32 and exhibits surprisingly sharp features indicative for localized helium atoms. For comparison, the electronic origin of bare AN is shown in Fig.…”

“…By means of the combined investigation of fluorescence excitation and dispersed emission spectra in addition with the saturation behavior, the fine structure could be assigned to a single peaked ZPL accompanied by a sharply structured PW. The spectral shape of the PW, which deviates drastically from the PW of superfluid helium, 32 provides a clear evidence for a non-superfluid helium solvation layer surrounding the dopant molecule. The spectral shape of the fine structure comprises a very high specificity for the dopant species as can be recognized by comparison with the corresponding fine structure measured for AN in helium droplets.…”

“…A sharply structured PW is in contrast to the well-known phonon spectrum of superfluid helium. 32 Sharp peaks in the PW reveal individual helium atoms localized at the surface of the dopant molecule, which by definition are non-superfluid. The challenge to be accepted is to calculate not only the appropriate dopant-He n complex, but in addition its spectroscopic response when inside a superfluid helium droplet.…”

Dynamics of solvation and desolvation of rubidium attached to He nanodroplets

“…While the superfluidity of He nanodroplets has been tested in numerous key experiments by probing stationary properties [8][9][10], the impact of the quantum nature of the droplets on their dynamic response to impulsive excitation or ionization is much less well established. As a prominent recent example, the rotational dynamics of various molecules embedded in He droplets induced by impulsive alignment was found to be significantly slowed down and rotational recurrences were completely absent [5].…”

Helium Droplets: An Apparatus to Study Ultra Cold Chemistry

“…One is the ordinary liquid ( 4 HeI) and the other is the superfluity ( 4 HeII) at different temperatures. When the temperature is below T =2.18 K (λ point), the 4 HeI turns into 4 HeII. In 1938, the ultra-cold superfluity helium has been discovered by Kapitza [1] and Allen et al [2] independently.…”