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

2. 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

3. 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

4. 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, (2007)
LIndinger et al.,
J. Chem. Phys. 121, 12282(2004)
Larger shift
More information

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

6. 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

7. 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.

8. 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

9. 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

10. 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, (2007)]
Impurity-induced solidification can occur in fluid-like clusters.

11. 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

12. 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 ~
T ~ 3-4 K
Slow: N ~
Similar spectra for para

13. Large H2 “clusters” (N = 103~106)
LIF spectra
Pnozzle = 5 bar
Tnozzle = 45 K
<N> ~
Similar spectra in the fast group
Tetracene is inside.
24 K
with 360 H2 evaporation
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:
<N> = 450
(H2)N -> Tetracene
Slow:
in He
Temperature
3~4 K
wavenumber (cm-1)
wavenumber (cm-1)

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

15. 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.

16. 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)

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

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

20. 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)