Rotational Study of Carbon Monoxide Solvated with Helium Atoms L. A. Surin, 1,2 A. V. Potapov, 1,2 B. S. Dumesh, 2 S. Schlemmer, 1 Y. Xu, 3 P. L. Raston, 3 and W. Ja ¨ger 3 1 I. Physikalisches Institut, University of Cologne, 50937 Cologne, Germany 2 Institute of Spectroscopy of RAS, 142190 Troitsk, Moscow Region, Russia 3 Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2 (Received 7 July 2008; published 3 December 2008) High resolution microwave and millimeter-wave spectra of He N -CO clusters with N up to 10, produced in a molecular expansion, were observed. Two series of J ¼ 1–0 transitions were detected, which correspond to the a-type and b-type J ¼ 1–0 transitions of He 1 -CO. The B rotational constant initially decreases with N and reaches a minimum at N ¼ 3. Its subsequent rise indicates the transition from a molecular complex to a quantum solvated system already for N ¼ 4. For N  6, the B value becomes larger than that of He 1 -CO, indicating an almost free rotation of CO within the helium environment. DOI: 10.1103/PhysRevLett.101.233401 PACS numbers: 36.40.Mr, 61.46.w, 67.90.+z Studies of helium (He) nanodroplets, which consist of 10 3 –10 4 He atoms, have provided insight into the super- fluid properties of He clusters with a characteristic size of several nanometers [1]. A fascinating fundamental ques- tion arose from these nanodroplet studies [2,3]: how many He atoms are required for the onset of superfluidity? To answer this question, small He N -molecule clusters with N  2–80 were recently explored using high resolution microwave (MW) and infrared (IR) spectroscopy [4–18]. Thus far, the He N -OCS [4–8] and He N -N 2 O [9–11] sys- tems were studied by both IR and MW spectroscopy, while He N -CO 2 [12–14] and He N -CO [15–17] were probed by IR and He N -HCCCN [18] by MW spectroscopy. A rather amazing effect has been observed for such small clusters, namely, an increase of the effective rota- tional constant B (proportional to the inverse moment of inertia) with increasing cluster size at a certain N. This nonclassical behavior has been interpreted as decoupling of part of the He density from the rotational motion of the molecule. It was proposed [4] that this decoupling is a sign of superfluidity at the microscopic level [4], in analogy to the ‘‘macroscopic’’ Andronikashvili experiment [19]. This was later corroborated by theory [10,20,21], and it now appears that the onset of superfluidity occurs in clusters with as few as 6 to 10 He atoms, depending strongly on the probe molecule. Carbon monoxide, CO, is of special interest as a probe molecule. The He-CO dimer [22,23] has a binding energy (9 K) that is very close to the chemical potential of liquid He (7.5 K); CO is therefore a rather subtle probe of the surrounding He density. In the IR study of He N -CO [15– 17], two series of Rð0Þ transitions (a transition in which the rotational quantum number J changes from 0 in the vibra- tional ground state to 1 in the first excited vibrational state of CO) were observed. They were denoted as a- and b-types in analogy with He-CO, in which the a-type tran- sition corresponds to end-over-end rotation of the whole complex, and the b-type corresponds to nearly free rotation of CO within the complex [22,23]. Theoretical simulations of the IR results [24,25] showed that the existence of two series at small N is likely due to a larger asymmetry of the cluster in this size regime and that the b-type series even- tually disappears because of increasing cylindrical sym- metry in larger clusters. At the low jet temperature ( < 1K) only the lowest rotational level, J ¼ 0, is significantly populated and mainly Rð0Þ transitions could be detected in the IR spectra of He N -CO [15]. These data alone are insufficient to clearly separate the rotational frequency of the clusters from the shift of the fundamental vibration of CO. IR and theoretical studies of CO in He nanodroplets have also been reported [26]. The observed line broadening was attributed to coupling to phonons and droplet size distribution effects which are absent in the smaller He N -CO clusters reported here. In that work, four isotopic species of CO were investigated in an effort to approxi- mately separate the effects of vibration and rotation. However, a similar analysis applied to He N -CO clusters gave scattered and inconsistent band shifts and rotational constants [17]. In this Letter, we present MW and millimeter-wave (MMW) measurements of pure rotational transitions of He N -CO clusters in the range from N ¼ 2 to 10. One goal is to elucidate the effect of a rather isotropic and shallow He-molecule potential on the spectroscopic prop- erties of He N -molecule clusters. The results will allow us to unambiguously separate the effects of rotation and vibrational shift on the spectra in order to detect possible nonclassical behavior of spectroscopic parameters. Three different experimental techniques were used: a molecular beam Fourier transform microwave (FTMW) spectrometer [27] for detection of a-type transitions of clusters with N ¼ 2–7 at 14–24 GHz; a MMW OROTRON intracavity jet spectrometer [28] for detection of b-type transitions with N ¼ 2–10 at 114–150 GHz; and a MW-MMW double resonance (DR) technique combining the OROTRON spectrometer with a MW pump source [29] for detection of a-type transitions with N ¼ 7–8 at PRL 101, 233401 (2008) PHYSICAL REVIEW LETTERS week ending 5 DECEMBER 2008 0031-9007= 08=101(23)=233401(4) 233401-1 Ó 2008 The American Physical Society