ROTATIONAL SPECTRA OF CARBON MONOXIDE SOLVATED WITH HELIUM
ROTATIONAL SPECTRA OF CARBON MONOXIDE SOLVATED WITH HELIUM ATOMS Leonid Surin, Thomas Giesen, and Stephan Schlemmer I. Physikalisches Institut, University of Cologne, 50937 Cologne, Germany; Alexey Potapov and Boris Dumesh Institute of Spectroscopy RAS, 142190 Troitsk, Moscow region, Russia 64 th International Symposium on Molecular Spectroscopy Columbus, Ohio, June 22– 26, 2009
He. N–(MOLECULE) CLUSTERS Studies of helium nanodroplets (103 -104 helium atoms) with embedded chromophore molecule provided insight into the superfluid properties of helium clusters with a characteristic size of several nanometers S. Grebenev, J. P. Toennies, and A. F. Vilesov, Science 279, 2083 (1998) See also: F. Stienkemeier and K. K. Lehmann, J. Phys. B: At. Mol. Opt. Phys. 39, R 127 (2006) A fascinating fundamental question: how many helium atoms are required for the onset of superfluidity?
MW AND IR STUDIES OF He. N–(MOLECULE), N ≈ 2 - 80 He. N–OCS and He. N–N 2 O (IR and MW) He. N–CO 2 and He. N–CO (IR+MW) He. N–HCCCN (MW) J. Tang, Y. Xu, A. R. W. Mc. Kellar, and W. Jäger. Science 297, 2030 (2002). A. R. W. Mc. Kellar, Y. Xu, and W. Jäger. Phys. Rev. Lett. 97, 183401 (2006). Y. Xu, W. Jäger, J. Tang, and A. R. W. Mc. Kellar. Phys. Rev. Lett. 91, 163401 (2003). J. Tang, A. R. W. Mc. Kellar, F. Mezzacapo, and S. Moroni. Phys. Rev. Lett. 92, 145503 (2004). J. Tang and A. R. W. Mc. Kellar. J. Chem. Phys. 119, 754 (2003). W. Topic, W. Jäger, N. Blinov, P. -N. Roy, M. Botti, S. Moroni, J. Chem. Phys. 125, 144310 (2006).
ROTATIONAL CONSTANT B VERSUS CLUSTER SIZE N observed: an increase of the effective rotational constant B with increasing cluster size at a certain number of He atoms; interpreted: as decoupling of part of the helium density from the rotational motion of the chromophore molecule; proposed (by analogy with “macroscopic” experiment): that decoupling is a sign of superfluidy at the microscopic level; corroborated: by theory [e. g. S. Moroni, A. Sarsa, S. Fantoni, K. E. Schmidt, and S. Baroni. Phys. Rev. Lett. 90 143401 (2003); F. Paesani, Y. Kwon, and K. B. Whaley. Phys. Rev. Lett. 94 153401 (2005) ; it now appears: onset of superfluidity may take place in clusters with as few as 6 to 10 helium atoms, depending strongly on the probe molecule.
SPECIFICITY OF He. N – CO binding energy between He and CO (9 K) is very close to the chemical potential of liquid helium (7. 4 K) CO is therefore a more subtle probe of the surrounding He density than other, more strongly interacting molecules
IR STUDY OF He. N – CO • infrared spectra of He. N–CO complexes with N up to about 20 have been observed in the 2145 cm-1 region of the C–O stretch vibration using a tunable diode laser spectrometer; • two series of J = 1 ← 0 transitions were observed, each correlating smoothly with the known a-type (K = 0 - 0) and b-type (K = 1 - 0) J = 1 ← 0 lines of the binary complex, He–CO; it has not been possible to separate the effects of vibrational shifts and rotational dynamics on the line positions J. Tang and A. R. W. Mc. Kellar. J. Chem. Phys. 119, 754 (2003).
IR STUDY OF CO IN He NANODROPLETS J = 1 ← 0, v. CO = 1 ← 0 IR spectrum of CO and its isotopologues: 12 C 16 O, 13 C 16 O, 12 C 18 O, 13 C 18 O. the additional information from four isotopic species of CO was used to separate the effects of vibration and rotation K. von Haeften, S. Rudolph, I. Simanovski, M. Havenith, R. E. Zillich, and K. B. Whaley. Phys. Rev. B 73, 054502 (2006). R. E. Zillich, K. B. Whaley, and K. von Haeften. J. Chem. Phys. 128, 094303 (2008).
IR STUDY OF He. N– 12 C 16 O, – 13 C 16 O, – 12 C 18 O, – 13 C 18 O (N ≤ 20) even with data on the J = 1 – 0 transitions from four CO isotopologues, a clear and consistent separation of vibration and rotation could not be achieved for He. N–CO clusters in the size range N ≤ 20 A. R. W. Mc. Kellar. J. Chem. Phys. 121, 6868 (2004). A. R. W. Mc. Kellar. J. Chem. Phys. 125, 164328 (2006).
MOTIVATION OF ROTATIONAL STUDY OF He. N–CO Direct measurements of pure rotational transitions of He. N–CO allow an unambiguous separation of the effects of rotation and of vibrational shift in order to detect possible non-classical behavior of spectroscopic parameters
First MW and MMW results on He. N– 12 C 16 O J=1← 0 Two series: a-type and b-type transitions in cooperation with W. Jäger’s group, Edmontom
J = 1 – 0 a-TYPE AND b-TYPE TRANSITIONS OF THE He. N–CO CLUSTERS Calculated frequencies from the CBS+corr potential: T. Škrbić, S. Moroni, and S. Baroni. J. Phys. Chem. A 111, 7640 (2007). The dashed line indicates the 2 B value inferred from the helium nanodroplet experiments: K. von Haeften, S. Rudolph, I. Simanovski, M. Havenith, R. E. Zillich, K. B. Whaley. Phys. Rev. B 73, 054502 (2006). L. A. Surin, A. V. Potapov, B. S. Dumesh, S. Schlemmer, Y. Xu, P. L. Raston, and W. Jäger. Phys. Rev. Lett. 101, 233401 (2008).
BYNARY COMPLEX He– CO J sym J=1– 0 b-type J=1– 0 a-type J=1– 0 vbend He–CO A. R. W. Mc. Kellar, Yu. Xu, W. Jäger, and C. Bissonnette. J. Chem. Phys. 110, 10766 (1999). L. A. Surin, D. A. Roth, I. Pak, B. S. Dumesh, F. Lewen, and G. Winnewisser. J. Chem. Phys. 112, 4064 (2000).
OROTRON JET SPECTROMETER JET <0. 1% CO in He backing pressure 10 -80 bar nozzle cooled to temperatures as low as – 80°C Detection of b-type transitions with N = 2 -10 at 114 -150 GHz
He 10 –CO: RECORD EXAMPLE
J = 1 – 0 b-TYPE TRANSITIONS OF THE He. N– 12 C 16 O, 13 C 16 O, 12 C 18 O, 13 C 18 O CLUSTERS Existence of b-type series asymmetry in the classical distribution of He atoms around the molecular axis large positive shifts of the b-type series in the range N = 1 – 6 increase in the effective anisotropy of the potential energy surface experienced by the CO in the clusters
J = 1 – 0 b-TYPE TRANSITIONS OF THE He. N– 12 C 16 O, 13 C 16 O, 12 C 18 O, 13 C 18 O CLUSTERS Atomic binding energy EN = EN – EN-1 as a function of the cluster size in He. N-CO. = 7. 4 K P. Cazzato, S. Paolini, S. Moroni, and S. Baroni J. Chem. Phys. 120, 9071 (2004)
J = 1 – 0 b-TYPE TRANSITIONS OF THE He. N– 12 C 16 O, 13 C 16 O, 12 C 18 O, 13 C 18 O CLUSTERS N = 7 splitting interactions with cluster vibrational modes?
ISOTOPIC SHIFT IN He. N–CO Differences between b-type R(0) line positions for He. N-13 C 16/He. N-12 C 18 O and He. N-12 C 16 O The b-series differences tend towards zero with increasing N these transitions increasingly reflect He atom motions rather than CO rotation the “instability” at N = 7 causes the larger, seemingly random isotope shifts
MW-MMW DOUBLE RESONANCE DR technique combines the OROTRON spectrometer with a MW pump source for detection of a-type transitions with N = 7 -9… at 23 -43… GHz
MW-MMW DOUBLE RESONANCE: (He 8–CO) Pumping: R(0) a-type at 32053. 5 MHz (MW synthesizer) Probing: R(0) b-type at 132313. 2 MHz (OROTRON)
MW-MMW DOUBLE RESONANCE He 8– 12 C 18 O Pumping: R(0) a-type at 31054. 6 MHz (MW synthesizer) Probing: R(0) b-type at 117211. 0 MHz (OROTRON)
FREQUENCY SHIFT OF THE FUNDAMENTAL VIBRATION OF 12 C 18 O completely different from that of “heavy” molecules The dashed line represents the inferred helium nanodroplet value from K. von Haeften, S. Rudolph, I. Simanovski, M. Havenith, R. E. Zillich, K. B. Whaley. Phys. Rev. B 73, 054502 (2006).
ROTATIONAL CONSTANT B VERSUS CLUSTER SIZE N Experimental values (filled blue circles) and those estimated from the IR spectra using the vibrational shift obtained in this work (open blue circle) are shown. The dashed blue line represents the inferred helium nanodroplet limit K. von Haeften, S. Rudolph, I. Simanovski, M. Havenith, R. E. Zillich, K. B. Whaley. Phys. Rev. B 73, 054502 (2006). For comparison, the corresponding plots for OCS, N 2 O, CO 2, HCCCN and the corresponding nanodroplet values are also shown. The B values are plotted on a logarithmic scale. L. A. Surin, A. V. Potapov, B. S. Dumesh, S. Schlemmer, Y. Xu, P. L. Raston, and W. Jäger. Phys. Rev. Lett. 101, 233401 (2008).
ROTATIONAL CONSTANT B VERSUS CLUSTER SIZE N • The B-value turnaround occurs at N = 3, at lower N than in all other probe molecules observed so far “microscopic superfluidity” appears in clusters with as few as four helium atoms • Extremely small moment of inertia of the He. N–CO clusters, which becomes even smaller than the moment of inertia of the He 1 –CO binary complex starting at N = 6 the CO rotation is almost free and carries along the equivalent of less than one helium atom in clusters with N > 5 L. A. Surin, A. V. Potapov, B. S. Dumesh, S. Schlemmer, Y. Xu, P. L. Raston, and W. Jäger. Phys. Rev. Lett. 101, 233401 (2008).
OUTLOOK 3 He N–CO clusters ACKNOWLEDGMENTS F. Lewen W. Jäger, Y. Xu, P. L. Raston A. R. W. Mc. Kellar S. Moroni Deutsche Forschungsgemeinschaft Russian Foundation for Basic Research
J = 1 – 0 TRANSITIONS OF He. N–CO (in MHz) a) the frequency of the upper component of the R(0) transition of He 7–CO, as predicted from the IR data and data from the current study
J = 1 – 0 a-TYPE AND b-TYPE TRANSITIONS OF THE He. N–CO CLUSTERS Calculated frequencies from the CBS+corr potential: T. Škrbić, S. Moroni, and S. Baroni. J. Phys. Chem. A 111, 7640 (2007). Calculated frequencies from the SAPT potential: S. Moroni, private communication. The dashed line indicates the 2 B value inferred from the helium nanodroplet experiments: K. von Haeften, S. Rudolph, I. Simanovski, M. Havenith, R. E. Zillich, K. B. Whaley. Phys. Rev. B 73, 054502 (2006).
J = 1 – 0 b-TYPE TRANSITIONS OF THE He. N– 12 C 16 O, 13 C 16 O, 12 C 18 O, 13 C 18 O CLUSTERS 3 N=6 15 Upper: probability density of finding two He atoms which form a dihedral angle with respect to the molecular axis. Lower: the integrated probability density. P. Cazzato, S. Paolini, S. Moroni, and S. Baroni J. Chem. Phys. 120, 9071 (2004)
FREQUENCY SHIFT OF THE FUNDAMENTAL VIBRATION OF CO completely different from that of “heavy” molecules Theoretical simulations are from T. Škrbić, S. Moroni, and S. Baroni. J. Phys. Chem. A 111, 7640 (2007). The dashed line represents the inferred helium nanodroplet value from K. von Haeften, S. Rudolph, I. Simanovski, M. Havenith, R. E. Zillich, K. B. Whaley. Phys. Rev. B 73, 054502 (2006).
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