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SPECTROSCOPY OF LARGE, COLD MOLECULAR CLUSTERS Andrey F. Vilesov University of Southern California Los Angeles 1. Formation of large NH 3 clusters in He droplets 2. Superfluid hydrogen ?
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Structure of large NH 3 clusters in He droplets (HCN) n T = 0.38 K Trapping in local minima (NH 3 ) n Nauta, Miller, Science, 283 (1999) 1985
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Superfluidity and aggregation kinetics DLA: δ = d, N = 4 10 4 BA: δ = 128 d, N = 2 10 5 Diffusion limited (DLA) vs. Ballistic (BA) aggregation in large He droplets Alves, Vilesov, Ferreira, to be published (2008)
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Formation of large clusters (n≈10 4 ) requires huge He droplets N He = 1.6 10 7 ; Evaporation energy - 10 4 eV Mozhayskiy, Slipchenko, Adamchuk, Vilesov, J.C.P. 127 (2007) 094701
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Mean size of He droplets Toennies, Vilesov, Angew.Chem., 43 (2004) 2622 Droplet size distribution: N
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Laser spectroscopy
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3 µm bands of NH 3 vs. cluster size N H H H 1 (a 1 ) = 3336 cm -1 H N H H 3 (e) = 3444 cm -1 N H H H 2 4 (a 1 ) = 3217 cm -1 2 4 (e) = 3239 cm -1 V = 36.5 cm -1 Guelashvili, et. al., JMS, 133 (1989) 345
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Small (NH 3 ) n clusters at different average number of captured molecules Slipchenko, Sartakov, Vilesov, Xantheas, JPCA 111 (2007) 7460 =0.14 =2 =0.7 2ν 4 (a 1, e ) ν 1 (A 1 ) ν 3 (e)
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He droplet: A nanocalorimeter Depletion signal: I n h 10 A 10 [M n ] E laser h
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Total intensity of the (NH 3 ) n 3 m bands per H-bond IR Intensity, km/mol Slipchenko,Kuyanov, Sartakov, Vilesov, JCP 124 (2006) 241101
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Size dependence of the spectra of large NH 3 clusters = 80 2 4 1 3 = 12 = 900 = 5 = 1 = 0.2 Slipchenko, Sartakov, Vilesov, JCP 128 (2008) 134509
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Frequency and relative intensity of the 3 µm bands in ammonia vs. cluster size Slipchenko, Sartakov, Vilesov, JCP 128 (2008) 134509
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Spectra in different phases of NH 3 In cooling cell Jetzki, Bonnamy, Signorell, JCP 120 (2004) 11775 T=10 K film deposition Holt, Sadoskas, Pursell, JCP 120 (2004) 7153 Sill, Fink, Ferraro, JOSA 70, (1980) 724 Crystal structure
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Packing of ammonia molecules in clusters Slipchenko, Sartakov, Vilesov, JCP 128 (2008) 134509
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Large clusters (NH 3, CH 4, HCl) formed in He droplets are compact HCN in bulk
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2. Superfluid hydrogen ? only feasible superfluid substance besides 4 He and 3 He small BEC fraction may constitute a new class of molecular superfluids anisotropic interaction Rotational angular momentum Nuclear spin, mass H 2 (I = 0, 1), D 2 (I=0,1,2), HD(I=1/2,3/2)
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Helium vs. Hydrogen H 2 molecule para-hydrogen ortho-hydrogen J = 0, 2, 4, … levels allowed J = 1, 3, 5, … levels allowed I = 0 I = 1 4 HeH2H2 BosonI = 0I = 0, 1 MassM = 4 auM = 2 au Interactionε m = 11 Kε m = 34 K StateSuperfluid at T <2.2 K Liquid at T = 0 K Superfluid at T < 1.1 K Solid at T = 13.8 K 2 He 1 H
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He vs. H 2 2 He 1 H SF Metastable SF phase
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Calculated superfluid fraction vs. size of pH 2 cluster at T = 1 K Mezzacapo, Boninsegni, PRL 97 (2006) 045302 Sindzingre, Ceperley, Klein, PRL, 67 (1991) 1871 - liquid state is thermodynamically stable is small clusters
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Aggregation of pH 2 molecules around OCS in mixed 4 He/ 3 He droplets Grebenev, Sartakov, Toennies, Vilesov, Science 289 (2000) 1532 - strong interaction with OCS - anisotropic H 2 exchange - small number of particles - overwhelming effect of surface - Low Temperature Physics meets Molecular Spectroscopy
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► Neat large (n > 1000) pH 2 clusters ► Raman spectroscopy
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Pulsed CARS spectroscopy of pH 2 clusters ν=1 ν=0 PMT T = 10-30 K X% pH 2 (100-X)% He H2H2 Pulsed cryogenic nozzle: Slipcheko, Kuma, Momose, Vilesov, RSI, 73 (2002) 3600 CARS experiment: Beck, Nibler, CPL, 248 (1988) 271 Q 1 (0) line (v' = 1, J' = 0) ← (v" = 0, J" = 0)
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CARS spectrum of Q 1 (0) line of para-H 2 clusters of N H2 ≈ 6 10 4 (expansion of X = 0.5% pH 2 in He at 17 K) Cluster surface Cluster core Free p-H 2 Molecules p-H 2 in bulk Kuyanov, Vilesov, unpublished results Liquid, 18 K T ≈ 1.2 K
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Vibrational Q 1 (0) line in p-H 2 clusters vs. X% of pH 2 in expanding He gas Kuyanov, Vilesov, unpublished results
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Liquid and solid pH 2 : Density and Q 1 (0) frequency vs. T Bhatnagar, Allin, Welsh, Can. J. Phys. 40 (1962) 9 Kerr, Momose, Welily, Gabrys, Oka, P.R.L. 72 (1994) 3957 Uetake, Sihombing, Hakuta, Opt. Lett. 27 (2002) 421 Model: Δν = -c·ρ 2
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J=2 J=0 Rotational CARS in p-H 2 clusters S 0 (0) line S 0 (0) line (v' = 0, J' = 2) ← (v" = 0, J" = 0)
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S 0 (0) line in clusters formed from liquid pH 2 Kuyanov, Vilesov, unpublished results Free pH 2
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Rotational Raman Spectra in condensed H 2 Bhatnagar, Allin, Welsh, Can. J. Phys. 40 (1962) 9 hcp 350 Electric Q-Q interaction S 0 (0) (J' = 2 J'' = 0)
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S 0 (0) line in clusters formed from liquid pH 2 Free pH 2 hcp fcc ► Liquid pH 2 freezes rapidly into an irregular solid T = 4-14 K
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Cluster growth in gas expansion L = 5 mm d 0 = 1 mm T 0 = 20 K
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S 0 (0) line in clusters formed from gaseous pH 2 seeded in He T ≈ 1.2 K T ≈ 10 K Kuyanov, Vilesov, unpublished results T ≈ 2 K 10% pH 2 in He 1% pH 2 in He 0.5% pH 2 in He in liquid pH 2
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► Warm clusters are solid ► Cold clusters are liquid
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Homogeneous nucleation: freezing via formation and growth of a nucleation center r
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Nucleation center formation and rate of freezing Nucleation Rate ≈ exp(-ΔG*(T)/kT)
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Rate of classical and quantum homogeneous nucleation in liquid pH 2 vs. T Maris, Seidel, Williams, PRB 36 (1987 ) 6799 classical quantum
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Region of stability of liquid hydrogen at low T ► observation of large liquid pH 2 clusters at T ≈1 K ► coming into the T range of predicted SF transition ► at low T pH 2 cluster grow via liquid state
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Towards superfluidity in H 2 ► Spectroscopic manifestations of SF ► Formation of liquid pH 2 clusters in He droplets at 0.38 K ► Continuous variation of temperature L-He L-pH 2 Kuma, Goto, Slipchenko, Vilesov, Khramov, Momose, JCP 127 (2007) 214301
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Acknowledgements USC Myong Yong Choi Hiromichi Hoshina Kirill Kuyanov Vadim Mozhaiskiy Dmitri Skvortsov Mikhail Slipchenko Russell Sliter Luis Gomes Kyoto - Vancouver Takamasa Momose Susumu Kuma Moscow Boris Sartakov OSU - Corvalis Joseph Nibler Support: USC, PRF, NSF WH13 WH06
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