INFRARED-ACTIVE VIBRON BANDS ASSOCIATED WITH RARE GAS SUBSTITUTIONAL IMPURITIES IN SOLID HYDROGEN

PAUL L. RASTON and DAVID T. ANDERSON, Department of Chemistry, University of Wyoming, Laramie, WY 82071

Ne H2 Ar Kr Xe

* Important for understanding basic van der Waals interactions.

* Model systems with regard to determination of accurate potential energy surfaces from experimental data.

* Attractive to study because of the relative simplicity and fundamental nature of these systems.

H2-Rg: R. J. Le Roy and J. M. Hutson, J. Chem. Phys. 86, 837 (1987).

H2-H2: P. Diep, and J. K. Johnson, J. Chem. Phys. 112, 4465 (2000).

Calculated Isotropic Potentials

R (Å)3.0 3.5 4.0 4.5 5.0 5.5

V (

cm-1

)

-60

-40

-20

0

Ne-H2

H2-H2

Ar-H2

Kr-H2

Xe-H2

H2-Rare gas (Rg) Systems

E. J. Allin, W. F. J. Hare, and R. E. MacDonald, Phys. Rev. 98, 554 (1955).

* Isolated gas phase nH2 infrared

inactive.

* High pressure gas phase intermolecular interactions allow for infrared activity.

* Low temperature condensed phase longer duration of molecular interactions give rise to sharper spectra.

* The induced Q1(0) (vibrational) and

S1(0) (vibrational - rotational)

transitions are reported for Rg doped solid pH2 in this study.

Gas Phase (high pressure) vs. Condensed Phase Spectra of nH2

Q1(0) S1(0)

room temperature dopant Rg

vacuum shroud

radiation shield

optical substrateT = 2.4 K

IR beam

atmosphere

vacuum

Fe(OH)3

nH2

cold tip of cryostat T = 14.5 K

Rapid Vapour Deposition

pH2

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

Pure pH2 - net = 0 Doped pH2 - net ≠ 0

* In pure pH2 Q1(0) transition will not be observed

* The presence of an impurity Rg atom will induce a net dipole moment observed transition.

pH2Rg v=1 pH2

Impurity Induced Transitions

Wavenumber (cm-1)

4145 4150 4155 4160

Abs

orba

nce

0.00

0.04

0.08

0.12

0.16

[Ne]=1000 ppm

[Ar]=1300 ppm

[Xe]=260 ppm

[Kr]=970 ppm

[oH2]=100 ppm

v=0, j=0

v=1, j=0

Kr Doped pH2

Wavenumber (cm-1)

4000 4200 4400 4600 4800 5000

Inte

nsity

0.00

0.05

0.10

0.15

0.20

0.25

0.30

* Q1(0) Transition (4153 cm-1) not allowed in pure pH2.

Dopant Induced pH2 Q1(0) Vibron Band

* Larger rare gas atoms induce a greater transition moment in the surrounding pH2 molecules There is an exponential increase in the intensity of the

induced Q1(0) feature in going from Ne to Ar to Kr to Xe.

Trends in Q1(0) Rg Induced Region

Q1(0) Rg Induced Intensity vs. Re (for Rg-H2)

Re (Å)

3.2 3.4 3.6 3.8 4.0

(

cm)

2x10-21

4x10-21

6x10-21

8x10-21

10x10-21

12x10-21

Ne

Ar

Kr

Xe

)cm()cm mol(

~)~(

)cm(3

(0)Q1

dC

dA

Rg

Mean Q1(0) Red Shift vs. Well Depth (for Rg-H2)

Potential well depth (cm-1)

20 30 40 50 60 70

Mea

n r

ed s

hif

t (c

m-1

)

0

1

2

3

4

Ne

Ar

Kr

Xe

* Data also fits the prediction based on the relative potential well depths, i.e. the stronger the Rg-H2 interaction, the further red-shifted is the H2’s vibrational frequency.

R. J. Hinde, J. Chem. Phys. 119, 6 (2003).

ΔE=0.25cm-1

ΔE=2.0cm-1

Theoretical models of impurity induced infrared-active vibron bands

Calculated impurity-induced Q1(0)

spectrum as a function of the detuning parameter ΔE

(ΔE: magnitude the dopant shifts the Q1(0) vibrational frequency from its value in pH2 )

* ΔE=2.0cm-1 induced vibron band comparable to Xe induced Q1(0) band

* ΔE=1.5cm-1 induced vibron band comparable to Kr induced Q1(0) band

* ΔE=0.25cm-1 induced vibron band comparable to Ar induced Q1(0) band

Delocalized vibron

Localized vs. Delocalized Vibron

Localized vibron

Q1(0) Rg Induced Transitions in solid pH2

Wavenumber (cm-1)

4148 4150 4152 4154 4156

Abs

orba

nce

0.00

0.04

0.08

0.12

0.16

[Ne]=1000 ppm

[Ar]=1300 ppm

[Xe]=260 ppm

[Kr]=970 ppm

[oH2]=100 ppm

Wavenumber (cm-1)

4480 4482 4484 4486 4488 4490

Abs

orba

nce

0.0

0.2

0.4

0.6

0.8

[Ne]=1000 ppm

[Ar]=1300 ppm

[Xe]=260 ppm

[Kr]=970 ppm

[oH2]=100 ppm

v=0, j=0

v=1, j=2

* S1(0) transition (4486 cm-1) allowed even in pure pH2.

Kr Doped pH2

Wavenumber (cm-1)

4000 4200 4400 4600 4800 5000

Inte

nsity

0.00

0.05

0.10

0.15

0.20

0.25

0.30

S1(0) Rg Induced Transitions in Solid pH2

* Complicated spectrum – why a minimum of 6 satellite peaks…

Wavenumber (cm-1)

4481.5 4482.0 4482.5 4483.0 4483.5 4484.0

Abs

orba

nce

0.00

0.05

0.10

0.15

0.20

0.25

0.30

S1(0) Xe Induced Transitions in Solid pH2

-8

-6

-4

-2

0

090

180270

360

3060

90120

150

180

V (

cm-1

)

Theta

Phi

v=1, j=2 pH2

Computed Anisotropic Potential for H2 in Lattice

Xe

H2-Xe: R. J. Le Roy and J. M. Hutson, J. Chem. Phys. 86, 837 (1987).

H2-H2: P. Diep, and J. K. Johnson, J. Chem. Phys. 112, 4465 (2000).

* Inherent axial symmetry to each substitutional crystal site in hcp lattice lifting of the J=2 upper state giving three rotational states with mJ = ±2, ±1, and 0.

* Two different substitutional sites exist for a Xenon impurity in a pH2 hcp lattice, in-plane (IP) and out-of-plane (OP) Potential explanation for the 3 x 2 = 6 excited state levels…

356.02 cm-1

354.54 cm-1

355.28 cm-1

357.56 cm-1

Calculated magnitude of splitting of J=2 level by Xe in hcp lattice

3cm-1 (total observed splitting is ~1.5cm-1)

mJ=±1

mJ=±2

mJ=0

IP OP

* Calculations predict the difference between IP and OP substitution sites is not significant enough to cause further lifting to the degree which is observed experimentally.

* Likely scenario is that Xe distorts the local lattice resulting in 2 more distinctly different pH2 environments than considered in the calculations.

In-Plane vs. Out-of-Plane Substitution

356.02 cm-1

354.84 cm-1

355.43 cm-1

357.24 cm-1

2.4cm-1

mJ=±1

mJ=±2

mJ=0

Calculated Potential Surface and J=2 Splitting for Kr Dopant

-6

-5

-4

-3

-2

-1

0

090

180270

360

030

6090

120

150

180

V (

cm-1

)

Theta

Phi

Kr

H2-Kr: H. Wei, and R. J. Le Roy, J. Chem. Phys. 122, 84321 (2005).

H2-H2: P. Diep, and J. K. Johnson, J. Chem. Phys. 112, 4465 (2000).

S1(0) Kr Induced Transitions in Solid pH2

Wavenumber (cm-1)

4483.5 4484.0 4484.5 4485.0A

bsor

banc

e

0.00

0.05

0.10

0.15

* Total calculated splitting induced by Kr is less than that for Xe (3cm-1).

* This is to be expected as Kr-H2 interaction is weaker.

Conclusions

* The line shapes of the Q1(0) features induced by Rg atoms in solid pH2 provide information on the extent of localization of the vibron.

* The intensity of the Rg atom induced Q1(0) feature provides information on the induction mechanism and on the Rg-pH2 intermolecular potential.

* Satellite lines in the S1(0) region induced by the presence of Rg atoms provide information on the anisotropy of the Rg-pH2 potential, and on the anisotropy of the crystal structure when the pH2 molecule is in a J=2 rotational state.

Future Work

* Calculations that take into account distortions in the hcp lattice induced by Xenon…

Acknowledgements

* M. E. Fajardo

* R. J. Hinde

* R. J. Le Roy

* B. D. Lorenz

* G. V. Subrahmanyam

* Funding Sources:*Petroleum Research Fund*The Research Corporation*National Science Foundation

End

Q1(0) Xe Induced Transition in solid oD2 (~4% pD2)

Wavenumber (cm-1)2979 2982 2985 2988 2991 2994

Abs

orba

nce

0.00

0.01

0.02

0.03

D2 x 2.0

[Xe]=110 ppm

S1(0) Xe Induced Transition in solid oD2 (~4% pD2)

Wavenumber (cm-1)3146 3148 3150 3152 3154

Abs

orba

nce

0.00

0.04

0.08

0.12

0.16

D2 x 2.0

[Xe]=110 ppm

Isotope Atomic mass (amu) Natural abundance (%)

124Xe 123.9058942 0.09

126Xe 125.904281 0.09

128Xe 127.9035312 1.92

129Xe 128.9047801 26.44

130Xe 129.9035094 4.08

131Xe 130.905072 21.18

132Xe 131.904144 26.89

134Xe 133.905395 10.44

136Xe 135.907214 8.87

Complex Vo (cm-1) Ro (*10-10 m) Re (*10-10 m)

       

D2-D2 2993.96 3.642 3.478

       

H2-H2 4162.06 3.789 3.478

       

H2-He      

D2-He      

       

H2-Ne 4161.15 3.99  3.3

D2-Ne 2993.52 3.66  

       

H2-Ar 4160.08 3.94  3.59

D2-Ar      

       

H2-Kr 4159.54 4.07  3.72

D2-Kr      

       

H2-Xe 4158.66 4.25  3.94

D2-Xe      

       

* All values are gas phase experimental or theoretical.