dynamics in solid hydrogen below 4 k david t. anderson department of chemistry, university of...
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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
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
0.1
0.2
0.3
0.4
0.5 367
wavenumber (cm-1)
1030 1035 1040 1045
log(
I 0/I
)
0.0
0.1
0.2
0.3
0.4
0.5 352
wavenumber (cm-1)
1030 1035 1040 1045
log(
I 0/I
)
0.0
0.1
0.2
0.3
0.4
0.5 337
wavenumber (cm-1)
1030 1035 1040 1045
log(
I 0/I
)
0.0
0.1
0.2
0.3
0.4
0.5 322
wavenumber (cm-1)
1030 1035 1040 1045
log(
I 0/I
)
0.0
0.1
0.2
0.3
0.4
0.5 307
wavenumber (cm-1)
1030 1035 1040 1045
log(
I 0/I
)
0.0
0.1
0.2
0.3
0.4
0.5 292
wavenumber (cm-1)
1030 1035 1040 1045
log(
I 0/I
)
0.0
0.1
0.2
0.3
0.4
0.5 277
wavenumber (cm-1)
1030 1035 1040 1045
log(
I 0/I
)
0.0
0.1
0.2
0.3
0.4
0.5 262
wavenumber (cm-1)
1030 1035 1040 1045
log(
I 0/I
)
0.0
0.1
0.2
0.3
0.4
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
0.2
0.3
0.4
0.5 97
wavenumber (cm-1)
1030 1035 1040 1045
log(
I 0/I
)
0.0
0.1
0.2
0.3
0.4
0.5 74
wavenumber (cm-1)
1030 1035 1040 1045
log(
I 0/I
)
0.0
0.1
0.2
0.3
0.4
0.5 59
wavenumber (cm-1)
1030 1035 1040 1045
log(
I 0/I
)
0.0
0.1
0.2
0.3
0.4
0.5 0
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