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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.

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Presentation on theme: "SPECTROSCOPY OF LARGE, COLD MOLECULAR CLUSTERS Andrey F. Vilesov University of Southern California Los Angeles 1. Formation of large NH 3 clusters in He."— Presentation transcript:

1 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 ?

2 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

3 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)

4 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

5 Mean size of He droplets Toennies, Vilesov, Angew.Chem., 43 (2004) 2622 Droplet size distribution:  N 

6 Laser spectroscopy

7 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

8 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)

9 He droplet: A nanocalorimeter Depletion signal:  I n  h 10  A 10  [M n ]  E laser h

10 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

11 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

12 Frequency and relative intensity of the 3 µm bands in ammonia vs. cluster size Slipchenko, Sartakov, Vilesov, JCP 128 (2008) 134509

13 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

14 Packing of ammonia molecules in clusters Slipchenko, Sartakov, Vilesov, JCP 128 (2008) 134509

15 Large clusters (NH 3, CH 4, HCl) formed in He droplets are compact HCN in bulk

16 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)

17 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

18 He vs. H 2 2 He 1 H SF Metastable SF phase

19 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

20 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

21 ► Neat large (n > 1000) pH 2 clusters ► Raman spectroscopy

22 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)

23 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

24 Vibrational Q 1 (0) line in p-H 2 clusters vs. X% of pH 2 in expanding He gas Kuyanov, Vilesov, unpublished results

25 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

26 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)

27 S 0 (0) line in clusters formed from liquid pH 2 Kuyanov, Vilesov, unpublished results Free pH 2

28 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)

29 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

30 Cluster growth in gas expansion L = 5 mm d 0 = 1 mm T 0 = 20 K

31 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

32 ► Warm clusters are solid ► Cold clusters are liquid

33 Homogeneous nucleation: freezing via formation and growth of a nucleation center r

34 Nucleation center formation and rate of freezing Nucleation Rate ≈ exp(-ΔG*(T)/kT)

35 Rate of classical and quantum homogeneous nucleation in liquid pH 2 vs. T Maris, Seidel, Williams, PRB 36 (1987 ) 6799 classical quantum

36 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

37 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

38 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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