Transcript RI-02_OSU-2006.ppt

An Empirical Relationship Between the
Centrifugal Distortion and Rotational Constants
of Rotors Solvated in Superfluid Helium
Droplets
Mike Lindsay* and Roger Miller
University of North Carolina at Chapel Hill
OSU International Symposium on Molecular Spectroscopy, TI02, 6/22/2006
* Current address: AFRL/MNME, Energetic Materials Branch, Ordnance Division, U.S. Air Force Research Lab,
2306 Perimeter Rd., Eglin AFB, FL 32542-5910
Dynamics of rotors in liquid helium
• Helium provides a weakly interacting, homogeneous
environment for impurities
• Rotationally resolved spectra
Adiabatic following
– Observed, even for fairly large molecules,
– Relatively slow rotational relaxation rates for
Erot < Ephonon (~5 cm-1)
• Rotational structure
– Exhibits the same symmetry as that in the
gas phase system.
– Provides structural information about the
rotor
• Moment of inertia is modified
by the helium:
– B reduced by factor of 2-6 (for large rotors)
– Some of the helium density rotates with rotor
Dynamics of rotors in liquid helium
Simulations
Experiments
*
New Systems
?
5 kV/cm
Field Free
3284.7 3285.0 3285.3
“Rules of thumb” on the solvent interactions
are becoming increasingly necessary to extract
information about novel systems.
*POITSE-DMC Calculations of F. Paesani and B Whaley, private communication.
Centrifugal distortion in helium: Early experiments
SF6
SF6
B
D
•
He
Gas
1019 MHz 2730 MHz
1.12 MHz 1.66×10-4 MHz
OCS
OCS
÷ 2.7
× 6700
B
D
He
Gas
2212 MHz 6063 MHz
11.4 MHz 1.31× 10-3 MHz
÷ 2.7
× 8700
Large effective centrifugal distortion constants observed in the early experiments
SF6 - M. Hartmann, R. E. Miller, J. P. Toennies, and A. Vilesov, Phys. Rev. Lett. 75 1566 (1995).
OCS - Grebenev, Hartmann, Havenith, Sartakov, Toennies, and Vilesov, J. Chem. Phys. 112, 4485 (2000).
Centrifugal distortion in He: Anisotropy dependence on J
•
Dozens of systems have been studied:
×102-106 times larger than in the gas
phase.
•
Anomalously large values (~.02 cm-1)
for HCCH and C2H4
•
Deff in all systems is positive,
not negative!
•
Classically, w = 2BJ
*
– Coupling decreases with increased B
– Coupling increases with increased J
•
Lehmann’s toy model
He He He
He
He
– Increased anisotropy with higher J
reduces the superfluid fraction
He
I1
He
He
He He
* M. Hartmann, R. E. Miller, J. P. Toennies, and A. Vilesov, Phys. Rev. Lett. 75 1566 (1995).
K. K. Lehmann, J. Chem. Phys, 114 4643 (2001)
Centrifugal distortion in He: Another contribution
Bulk Helium density of states
J=2
• In addition to the
anisotropy, Zillich &
Whaley* showed that
when B is sufficiently
large, higher levels
interact with the phononroton modes.
• Effect should be present
in spectra of small
molecules with
B > 1 cm-1
J=1
J=0
B =1 cm-1
Free rotor
B =1 cm-1
Rotor (in He)
* R. E. Zillich and K. B. Whaley, Phys. Rev. B 69 1014517 (2004)
Compilation of He data
• Compilation of reported and unreported spectroscopic constants in helium
were performed as part of a recent review*
• Data includes ~50 systems (molecules, weakly bound clusters, stronglybound clusters, open shell systems, many body clusters, etc…)
Linear Plot
Logarithmic Plot
Power law
dependence!
* Miller group, Int. Rev. Phys. Chem. 25, 15-75 (2006)
Fit and comparison to gas phase centrifugal distortion
•
Good power law
correlation, spanning
over four orders
of magnitude
•
All He data present
included in fit except
CH4, *
•
Nearly quadratic
dependence on Beff
*
Fit and comparison to gas phase centrifugal distortion
•
Good power law
correlation, spanning
over four orders
of magnitude
•
All He data present
included in fit except
CH4, *
•
Nearly quadratic
dependence on Beff
•
No correlation to
exists in gas phase
*
Gas phase data taken from Herzberg, Townes & Schalow, NIST spectral database, and helium droplet papers
Simulations of Deff
• Monte Carlo
simulations
predict correct
magnitude of Deff
CBF-DMC calc. of
Zillich and Whaley
POITSE-DMC calc. of
Paesani and Whaley
• Two very
different
approaches for
light and heavy
rotors
References for Calculations:
HCN, DCN Phys. Rev. B, 69 1014517 (2004)
HCCH
Phys. Rev. Lett., 93 250401 (2004)
CO
Phys. Rev. B, 73 1 (2006)
N2O
OCS
CO2
J. Chem. Phys. 121 5293 (2004)
J. Chem. Phys. 121 4180 (2004)
Phys. Rev. Lett. 94 1 (2005)
Simulations of Deff
• Monte Carlo
simulations
predict correct
magnitude of Deff
HCCH Potential,
vary rotor energy
level spacings
• Two very
different
approaches for
light and heavy
rotors
Varying only the energy level spacings (i.e. phono-roton coupling) does not
capture the trend…
Discussion
Deff=0.029(5)×Beff1.78(5)
HCN in p-H2
CO in p-H2
• What does it all mean?!?!
• Is this a general phenomenon? CO and HCN in solid p-H2…
Summary
• Effective centrifugal distortion constants
appear to vary approximately quadratically
with effective rotational constant.
• Qualitative picture is unclear, but Monte
Carlo simulations contain the underlying
mechanism
• Similar effect may also be occurring in
rotors in solid-parahydrogen… (see next
talk!)
Deff=0.029(5)×Beff1.78(5)
Acknowledgements: N$F
Robert Zillich and Francesco Paesani (Whaley Group)
Miller Group
The students who slaved over the spectra of all these systems…