Dynamics in Solid Hydrogen below 4 KDynamics in Solid Hydrogen below 4 K

David T. AndersonDepartment of Chemistry, University of Wyoming

Laramie, WY 82071-3838

danderso@uwyo.edu

60th International Symposium on Molecular Spectroscopy

OutlineOutline

• Growing H2 clusters in the condensed phase

• Impurity induced H2 nuclear spin conversion

• Chemical reaction dynamics at low temperature

Preferential formation of ortho-H2-HF in slit jet spectra

j=0

H2 H F H FH

H

j=1

D0 = 29.71 cm-1 D0 = 48.28 cm-1

Expt: C. M. Lovejoy, D. D. Nelson, Jr., and D. J. Nesbitt, JCP 87, 5621 (1987).Theory: D. C. Clary and P. J. Knowles, JCP 93, 6334 (1990).

para-H2-HF ortho-H2-HF

A. R. W. McKellar is studying X(H2)n clusters in the gas-phase

13.8 K

32.98 K

Grow H2 crystals via rapid vapor deposition

S. Tam and M. E. Fajardo, Rev. Sci. Instrumen. 70, 1926 (1999).

15 K2 K

Growing solid parahydrogen crystals

S. Tam and M. E. Fajardo, Rev. Sci. Instrumen. 70, 1926 (1999).

room temperature dopant (e.g., CH3F)

vacuum shroud

radiation shield

optical substrateT = 2.4 K

IR beam

atmosphere

vacuum

cryostatcold tip

catalytic converterFe(OH)3

Experimental set-up

IR

ortho/paraconverter

p-H2 gas

turbopump

dopant

Specifications

sample-in-a-vacuum liquid helium cryostat (1.7 – 5.0 K)

variable temperature ortho/para converter(10 - 80 K)

180 l s-1 turbo pump (<10-4 torr during deposition)

IR diagnostics – Bruker IFS120 (0.008 cm-1)

Studying CH3F(ortho-H2)n clusters in solid parahydrogen

para-H2

London dispersionweaker

ortho-H2

dipole-quadrupolestronger

• binding energies that differ by 5 cm-1 (60 J mol-1) are significant at T ≤ 4 K

CH3F(ortho-H2)n clusters in solid hydrogen

wavenumber (cm-1)

1032 1034 1036 1038 1040

abso

rban

ce

0.0

0.5

1.0

1.5

2.0 01

23

456

7

89

1011

12

5x

0.5x

100

2200

7200

17000

[ortho] /ppm

K. Yoshioka and D.T. Anderson, JCP 119, 4731(2003).

[CH3F] = 0.5 - 5 ppm n =

ortho-H2 clusters around CH3F

H3CF

para-H2

ortho-H2

n = 1

n = 2 n = 3

n = 0

wavenumber (cm-1)

1030 1032 1034 1036 1038 1040

abso

rban

ce

0.0

0.2

0.4

0.6

0.8

1.0012312 456

(a)

(b)

as-deposited

1st annealing

1st annealing increases cluster size

2.5 K

4.4 K

7 ppm CH3F5.7% ortho-H2

wavenumber (cm-1)

1032 1034 1036 1038 1040

abso

rban

ce

0.0

0.5

1.0

1.512312 456

(a)

(b)

(c)

(d)

(e)

Takes time to approach low temperature cluster equilibrium

4.4 K → 1.8 K

0 hr

1 hr

2 hr

3 hr

difference

• at 1.8 K spectrum is still changing after 3 hours!

ortho-H2 can move by a process called “rotational diffusion”

afterbefore

• this process can be facile even at T = 2 K!!

wavenumber (cm-1)

1030 1032 1034 1036 1038 1040

abso

rban

ce

0.0

0.5

1.0

1.5

2.0

(a)

(d)

(b)

(c)

4.3 K

1.8 K

?

<n>=9<n>≥12

Reversibly grow larger clusters at low temperature

4.3 K

1.8 K

Reversibly “boil-off” outer ortho-H2 molecules

T = 1.8 K T = 4.4 K

4

1

RV quaddip

Low temperature irreversible process –CH3F induced H2 nuclear spin conversion

1H ½ 2.7928519F ½ 2.62887

atom spin nuclear magnetic moment

wavenumber (cm-1)

1030 1035 1040 1045

log(

I 0/I

)

0.0

0.1

0.2

0.3

0.4

0.5 382

wavenumber (cm-1)

1030 1035 1040 1045

log(

I 0/I

)

0.0

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1030 1035 1040 1045

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I 0/I

)

0.0

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1030 1035 1040 1045

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I 0/I

)

0.0

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1030 1035 1040 1045

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I 0/I

)

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1030 1035 1040 1045

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I 0/I

)

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1030 1035 1040 1045

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I 0/I

)

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1030 1035 1040 1045

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I 0/I

)

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1030 1035 1040 1045

log(

I 0/I

)

0.0

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0.5 262

wavenumber (cm-1)

1030 1035 1040 1045

log(

I 0/I

)

0.0

0.1

0.2

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0.5 247

wavenumber (cm-1)

1030 1035 1040 1045

log(

I 0/I

)

0.0

0.1

0.2

0.3

0.4

0.5 232

wavenumber (cm-1)

1030 1035 1040 1045

log(

I 0/I

)

0.0

0.1

0.2

0.3

0.4

0.5 217

wavenumber (cm-1)

1030 1035 1040 1045

log(

I 0/I

)

0.0

0.1

0.2

0.3

0.4

0.5 202

wavenumber (cm-1)

1030 1035 1040 1045

log(

I 0/I

)

0.0

0.1

0.2

0.3

0.4

0.5 187

wavenumber (cm-1)

1030 1035 1040 1045

log(

I 0/I

)

0.0

0.1

0.2

0.3

0.4

0.5 172

wavenumber (cm-1)

1030 1035 1040 1045

log(

I 0/I

)

0.0

0.1

0.2

0.3

0.4

0.5 157

wavenumber (cm-1)

1030 1035 1040 1045

log(

I 0/I

)

0.0

0.1

0.2

0.3

0.4

0.5 142

wavenumber (cm-1)

1030 1035 1040 1045

log(

I 0/I

)

0.0

0.1

0.2

0.3

0.4

0.5 127

wavenumber (cm-1)

1030 1035 1040 1045

log(

I 0/I

)

0.0

0.1

0.2

0.3

0.4

0.5 112

wavenumber (cm-1)

1030 1035 1040 1045

log(

I 0/I

)

0.0

0.1

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0.4

0.5 97

wavenumber (cm-1)

1030 1035 1040 1045

log(

I 0/I

)

0.0

0.1

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0.5 74

wavenumber (cm-1)

1030 1035 1040 1045

log(

I 0/I

)

0.0

0.1

0.2

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0.4

0.5 59

wavenumber (cm-1)

1030 1035 1040 1045

log(

I 0/I

)

0.0

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CH3F catalyzed ortho-H2 → para-H2 conversion

minutes

SummarySummary

• ortho-H2 molecules preferentially cluster around polar impurities even in solid molecular hydrogen

• solid molecular hydrogen is dynamic at liquid helium temperatures

• CH3X catalyzes H2 nuclear spin conversion while OCS or N2O do not

The usual suspects (2004)

KazukiYoshioka Subrahmanyam

Garimella

PaulRaston

AdamOman

BritneyLorenz

Becker

FundingThe American Chemical SocietyResearch CorporationDepartment of Defense (AFOSR)The National Science Foundation

CollaboratorsMario E. Fajardo, Eglin AFB, FloridaR. J. Hinde, University of Tennessee

wavenumber (cm-1)

2221 2222 2223 2224

abso

rban

ce

0.00

0.01

0.02

0.03

0.04

2221 2222 2223 2224

abso

rban

ce

0.0

0.1

0.2

0.3

0.4

annealed, 1.83 K

annealed, 1.67 K

N2O(ortho-H2)n

N2O(ortho-D2)n

Assigning X(H2)n cluster spectra in SMH