1 62nd ohio state university international symposium on molecular spectroscopy, talk tj06, 19 june...

162nd Ohio State University International Symposium on Molecular Spectroscopy, Talk TJ06, 19 June 2007, Columbus, OH.

Approved for Public Release; Distribution Unlimited. AAC/PA 06-07-07-440.

Mario E. Fajardo, C. Michael LindsayAFRL/MNME, Energetic Materials Branch, Ordnance Division,

U.S. Air Force Research Laboratory, 2306 Perimeter Road, Eglin AFB, FL 32542-5910. [email protected]

&

David T. AndersonDepartment of Chemistry, University of Wyoming, Laramie, WY 82071-3838.

* Gas phase (HCl)2 – “geared” internal rotation/tunneling.

* “Broken-symmetry allowed” transitions in H35Cl-H37Cl.

* High-res IR spectra of (HCl)2 isotopomers in solid pH2.

* “Tunneling” splittings in H35Cl-H37Cl/pH2 (± 0.002 cm-1).

* CAVEAT! spectroscopic assignments incomplete, so conclusions in re tunneling still "tentative."

HCl Dimer in Solid Parahydrogen Revisited



Gas Phase (HCl)2

donor (2)

acceptor (1)

(HCl)2:

nearly prolate symmetric top( -1.0)

donor (2) mode||, a-type, Ka=0,

acceptor (1), b-type, Ka=±1

tunneling interconversion of HCl subunits; "geared" bending;R‡

HCl-HCl < 0.004Å



Broken-Symmetry Transitions

M.D. Schuder, C.M. Lovejoy, R. Lascola, and D.J. Nesbitt, J. Chem. Phys. 99, 4346 (1993).

+

+

2+ 2

15.5 cm-1

3.2 cm-1

H35Cl-H35Cl & H37Cl-H37Clrigorous inversion symmetry

+ - selection rules

+

+

2+ 2

H35Cl-H37Cl & H37Cl-H35Clno inversion symmetry

new IR transitions; 12x weaker



Low-lying (HCl)2 States


Ka0 1

gas-

phas

e ro

vibr

atio

nal o

rigin

s (c

m-1

)

0

10

202850

2860

2870

2880

2890

2900

--35-35--

--35-35--

--35-35--

--35-35--

--35-35--

--35-35--

--35-35--

--35-35--

--35-35--

--35-35--

--35-35--

--35-37--

--35-37--

--35-37--

--35-37--

--35-37--

--35-37--

--35-37--

--35-37--

--35-37--

--35-37--

--35-37--

--37-37--

--37-37--

--37-37--

--37-37--

--37-37--

--37-37--

--37-37--

--37-37--

--37-37--

--37-37--

=0 =1

=0 =1



HCl & (HCl)2 in solid pH2

3.8 Å

gas phase HCl dimer dimer is nearly perfect fit!

pH2 nearest neighbor spacing

BpH2(HCl) 0.93 Bgas(HCl) monomer is nearly free rotor*

3.8 Å

Can A-axis rotation & internal dynamics survive immersion in solid pH2 environment?

* D.T. Anderson, R.J. Hinde, S. Tam and M.E. Fajardo, J. Chem. Phys. 116, 594 (2002).



Rapid vapor deposition of solid pH2

M.E. Fajardo and S. Tam, J. Chem. Phys. 108, 4237 (1998).S. Tam and M.E. Fajardo, Rev. Sci. Instrum. 70, 1926 (1999).S. Tam and M.E. Fajardo, J. Low Temp. Phys. 122, 345 (2001).

Deposition conditions: Tconv = 15 K

100 ppm oH2

Tconv = 19 K

1000 ppm oH2

npH2/t = 200 mmol/hr

x/t = 3.1 mm/hr Tsub = 2.4 K 2.9 K

TpH2 4.6 K

Result: mixed fcc/hcp microstructure in as-deposited samples;can anneal to pure hcp.



Low Temperature Stick Spectrum


wavenumber (cm-1)

2820 2830 2840 2850 2860 2870 2880 2890 2900 2910 2920

sim

ulat

ed a

bsor

banc

e (a

rb.)

35-3535-3737-3735-37 B.S.monomer

monomerR(0)

BS

BS



wavenumber (cm-1)

2725 2750 2775 2800 2825 2850 2875 2900 2925

log

10(I

0/I)

(ab

s.)

0.0

0.5

1.0

1.5

monomerR(0)

dimer

dimer

trimer(HCl)3

tetramer(HCl)4

88 ppm HCl/pH2 (annealed)

HCl/pH2

0=2876 cm-1

number of HCl molecules

0 1 2 3 4 5

vibr

atio

nal s

hift

(cm

-1)

-150

-100

-50

0

50

gas phase*pH2

* M. Farnik, S. Davis, and D.J. Nesbitt, Faraday Disc. 118, 63 (2001).

*



(HCl)2/pH2

wavenumber (cm-1)

2800 2810 2820 2830 2840 2850 2860 2870 2880 2890 2900

log

10(I

0/I)

(ab

s.)

0.0

0.5

1.0

1.5

monomerR(0)

BS



2+ region (HCl)2/pH2

wavenumber (cm-1)

2832 2833 2834 2835 2836 2837

log

10(I

0/I)

(ab

s.)

0

1

23535

3535

3537/

3537

3537

3537 37

37

3737

a

b

c

brokensymmetry

2+ ?!

naturalabundance

37Cl enriched

35Cl enriched



Eglin AFB Experimental Setup

s

p

HCP c-axis ??

p-H2

gold substrate

s

p

HCP c-axis ??

p-H2

gold substrate



Polarization Dependence

wavenumber (cm-1)

2832 2833 2834 2835 2836 2837

log

10(I

0/I)

(ab

s.)

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

3537

3537

3537

3537

3535

3535

3737

3737

(a)

(b)

* oH2 - HCl complex in-plane (HCl)2

|| out-of-plane (HCl)2



H35Cl-H37Cl Tunneling Splittings (?)

2+ K=00 gas phase solid pH2 gas - pH2 ip - oop

H35Cl-H35Cl 2857.235 2835.701 21.534 0.630

2836.331 || 20.904

H35Cl-H37Cl 2856.449 2835.153 21.296 0.594

2835.747 || 20.702

H37Cl-H37Cl 2855.106 2833.557 21.549 0.638

2834.195 || 20.911

H35Cl-H37Cl 2852.717 2832.902 19.815 0.511broken symmetry 2833.413 || 19.304

v2=1 H35Cl-H37Cl: Egas = 3.7320(4) cm-1, Eip = 2.250(1) cm-1, Eoop = 2.335(1) cm-1



1+ region (HCl)2/pH2

wavenumber (cm-1)

2860 2865 2870 2875 2880

log

10(I

0/I)

(ab

s.)

0.00

0.05

0.10

0.15

0.20

0.25

3537

3535

3737

(a)

(c)

(b)

oH2 -

H35

Cl

oH2 -

H35

Cl

oH2 -

H37

Cl

oH2 -

H37

Cl

naturalabundance

37Cl enriched

35Cl enriched

brokensymmetry

1+ ?!



Unassigned Hot Bands

wavenumber (cm-1)

2800 2810 2820 2830 2840 2850 2860 2870 2880 2890 2900

log

10(I

0/I)

(ab

s.)

0.0

0.1

0.2

0.3

T = 4.8 K

T = 2.4 K



Summary

* Observed several rovibrational transitions of (HCl)2 isotopologes in solid pH2; assignments incomplete.

* New polarization measurements show that ubiquitous 0.5 cm-1 splitting in 2

+ region arises from in-plane (ip) and out-of-plane (oop) HCl dimers in hcp solid.

* Assignment of 2+ region consistent with observation of

broken symmetry transition of mixed H35Cl-H37Cl dimer.

* Assignment implies tunneling splittings for v2=1 of H35Cl-H37Cl: Eip = 2.250(1) cm-1, Eoop = 2.335(1) cm-1.

* Assignment of weak hot-bands underway.

1 62nd ohio state university international symposium on molecular spectroscopy, talk tj06, 19 june...

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