Laser induced fluorescence of tetracene in hydrogen droplets Susumu Kuma, Hiroko Nakahara, and Takamasa Momose The University of British Columbia, Canada

Fluidity of cold H2 clusters H2: candidate for molecular superfluidity Transition temperature n: density m: mass g: degeneracy composite boson (TBEC) Finish within 40 seconds. As Andrey Vilesov told in the opening talk, cold H2 molecules have attracted attention because the superfluid transition temperature is predicted to be 6 K for para and 1.5 K for ortho. < Bulk freezing T I = 0 para TS = 6 K nuclear spin I = 1 ortho TS = 1.5 K 14 K Lower freezing temperature H2 clusters Fluid-like at TS ? Slower solid nucleation rate Small clusters (N ~ several 10) Vilesov, Toennies, Miller, McKellar, Jäger Large clusters (N > 100): our interest To check the fluidity of large H2 clusters at TS

Experimental scheme How to produce large, cold H2 clusters N = 10 ~ 1000: H2 clusters in He droplets T = 0.4 K He gas pulsed nozzle Used the standard technique of He droplet spectroscopy nozzle expansion->skimmed->pickup Advantage-well-defined T. Disadvantage-limited size due to evaporative loss of He. To produce larger clusters nozzle expansion->skimmed Small # of clusters-not direct observation, but probe. Mg-Pc-fluidity was confirmed. Tetracene-to obtain additional information He droplet H2 gas disadvantage:limited size N = 103 ~ 106: H2 “clusters” ~ 4 K H2 gas pulsed nozzle Knuth, Schnemann, and Toennies (1995) H2 cluster skimmer How to monitor the fluidity of the H2 clusters hν LIF of probe tetracene in H2 clusters

Mg-Phthalocyanine Tetracene ~40 cm-1 +H2 ~20 cm-1 Larger shift Larger shift for tetracene-better information from shift ~40 cm-1 n = 0 n = 0 +H2 ~20 cm-1 n = 1 n = 1 –40 –20 cm-1 cm-1 Kuma et al., J. Chem. Phys. 127, 214301(2007) LIndinger et al., J. Chem. Phys. 121, 12282(2004) Larger shift More information

N = 10 ~ 1000 at 0.4 K H2 clusters in He droplets He gas pulsed nozzle He droplet H2 gas

Monitoring the fluidity of clusters Pickup order dependence of tetracene LIF Tetracene -> N H2 He droplet Tetracene H2 (H2)N -> Tetracene or H2 Tetracene solid-like fluid-like LIF spectra of tetracene different same sensitive to the local environment around tetracene

Tetracene is on surface. H2 clusters (N = 10–1000) at 0.4 K nH2 clusters pH2 oH2 : = 1 : 3 Ar clusters He He 000 1ag NHe = 6 × 104 Tetracene -> N H2 Tetracene -> N Ar 250 <N> = 450 (H2)N -> Tetracene ArN -> Tetracene wavenumber (cm-1) wavenumber (cm-1) Same Different Tetracene is inside. Tetracene is on surface.

Pickup order dependence ArN -> Tetracene evaporation energy He 5 cm-1 Shift (cm-1) (H2)N -> Tetracene fusion energy Tetracene -> N H2 Ar ~100 cm-1 Tetracene -> N Ar H2 ~10 cm-1 <N> Tetracene does not melt H2 clusters (N > 50) by the residual energy < 500 cm-1 when it reaches the cluster. Clear difference in Ar clusters (N > 5) Ar clusters = solid-like Tetracene releases most of its energy into helium rapidly and becomes 0.4 K before it reaches the clusters. (130 °C⇒18000 cm-1) Similar shifts in H2 clusters H2 clusters are fluid-like in He droplets at 0.4 K. 3600 He evaporation

Pickup order dependence ArN -> Tetracene Shift (cm-1) (H2)N -> Tetracene Tetracene -> N H2 In addition MgPc:not observed. Tetracene:observed due to larger shift. Tetracene -> N Ar <N> Slightly different shifts in H2 clusters (H2)N -> Tetracene, N = 400 Tetracene -> N H2, N = 150 Tetracene in the (H2)N -> Tetracene pickup does not reside at the center of the nH2 clusters. Possibility: tetracene-induced solidification in fluid-like clusters

H2 clusters in He droplets are fluid-like up to N = 1000 at 0.4 K. Conclusion 1 H2 clusters in He droplets are fluid-like up to N = 1000 at 0.4 K. Short ! In agreement with Mg-Pc doped H2 clusters in He droplets [OHIO 2006 & J. Chem. Phys. 127, 214301(2007)] Impurity-induced solidification can occur in fluid-like clusters.

N = 103 ~ 106 at 4 K H2 “clusters” H2 gas pulsed nozzle H2 cluster Larger clusters at 4 K, produced by nozzle expansion of hydrogen gas. H2 gas pulsed nozzle H2 cluster skimmer

Expansion of cold H2 gas TOF measurement Two kinds of clusters Pnozzle = 5 bar Pnozzle = 20 bar Tnozzle = 45 K Tnozzle = 41 K Q-mass pulsed nozzle H9+ 24 K 36 K nH2 gas 23 K 30 K ×10 Short ! Two components ~1 m 21 K 17 K 20 bar Time-of-flight (ms) Time-of-flight (ms) 5 bar critical point at 13 bar Knuth, Schnemann, and Toennies (1995) Two kinds of clusters Fast: N ~ 103-104 T ~ 3-4 K Slow: N ~ 105-106 Similar spectra for para

Large H2 “clusters” (N = 103~106) LIF spectra Pnozzle = 5 bar Tnozzle = 45 K <N> ~ 103-104 Similar spectra in the fast group Tetracene is inside. 24 K with 360 H2 evaporation 103-104 Melting by tetracene should be considered, but we expect H2 clusters are fluid-like. 23 K H2 evaporative energy ~50 cm-1 21 K wavenumber (cm-1) Tetrene -> N H2 in He (H2)N -> Tetracene in He Time-of-flight (ms) H2 expansion Size Fast: 103-104 <N> = 450 (H2)N -> Tetracene Slow: 105-106 in He Temperature 3~4 K wavenumber (cm-1) wavenumber (cm-1)

Large H2 “clusters” (N = 103~106) LIF spectra Pnozzle = 5 bar Tnozzle = 45 K <N> ~ 103-104 24 K 103-104 23 K 21 K 105-106 wavenumber (cm-1) Time-of-flight (ms) Strong scattering due to large size prevents the LIF observation. Size Fast: 103-104 Slow: 105-106 Solid-like No pickup ? Temperature 3~4 K

H2 clusters in the fast group H2 clusters in the slow group Conclusion 2 Large H2 clusters (N = 103~106) at 4 K produced by nozzle expansion H2 clusters in the fast group . Similar spectra as mean Two compoments. In the fast, expected to be fluid-like. In the slow, cannot conclude whether the slow H2 clusters are fluid-like or solid-like. H2 clusters are fluid-like? H2 clusters in the slow group LIF was not observed.

Ortho/para dependence H2 clusters (N = 10–1000) in He droplets at 0.4 K pH2 -> Tetracene Tetracene -> pH2 Shift (cm-1) nH2 -> Tetracene Tetracene -> nH2 <N> Tetracene -> H2 H2 ->Tetracene Similar width & shape –500 cm-1 –475 cm-1 normal No clear difference other than shift –470 cm-1 –445 cm-1 para wavenumber (cm-1) wavenumber (cm-1)

Summary Large H2 clusters picked up in He droplets are fluid-like. N = 10-1000 at T = 0.4 K Large H2 clusters produced by nozzle expansion of H2 gas are fluid-like? In the fast group, N = 103-104 at T = 4 K Solidification of H2 clusters induced by tetracene

Acknowledgments Dr. Andrey F. Vilesov Dr. Mikhail N. Slipchenko Members of Momose lab @ UBC especially, Akira Takahashi & Majd Mustafa $$$ from JSPS (Japan) JST (Japan) NSERC (Canada)

Sharp Solid-like ? N > 150 Fluid-like ? N < 150 pH2 ->Tetracene in He droplets Sharp Solid-like ? (induced by tetracene?) N > 150 Fluid-like ? (too small to freeze?) N < 150 wavenumber (cm-1)