Probing phonon-rotation coupling in helium nanodroplets: Infrared spectroscopy of CO and its isotopomers

Journal Article · Wed Feb 01 00:00:00 EST 2006 · Physical Review. B, Condensed Matter and Materials Physics

DOI:https://doi.org/10.1103/PHYSREVB.73.0· OSTI ID:20787912

Haeften, Klaus von; Rudolph, Stephan; Simanovski, Iaroslav; Havenith, Martina ^[1]; Zillich, Robert E ^[2]; Fraunhofer ITWM, 67663 Kaiserslautern ^[3]; Whaley, K Birgitta ^[2]

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1. Lehrstuhl fuer Physikalische Chemie II, Ruhr-Universitaet Bochum, D-44780 Bochum (Germany)
2. Department of Chemistry and Pitzer Center for Theoretical Chemistry, University of California, Berkeley, California 94720 (United States)
3. Germany

We have recorded the R(0){nu}{sub CO}=1(leftarrow)0 IR spectrum of CO and its isotopomers in superfluid helium nanodroplets. For droplets with average size N > or approx. 2000 helium atoms, the transition exhibits a Lorentzian shaped linewidth of 0.034 cm{sup -1}, indicating a homogeneous broadening mechanism. The rotational constants could be deduced and were found to be reduced to about 60% of the corresponding gas-phase values (63% for the reference {sup 12}C {sup 16}O species). Accompanying calculations of the pure rotational spectra were carried out using the method of correlated basis functions in combination with diffusion Monte Carlo (CBF/DMC). These calculations show that both the reduction of the rotational B constant and the line broadening can be attributed to phonon-rotation coupling. The reduction in B is confirmed by path integral correlation function calculations for a cluster of 64 {sup 4}He atoms, which also reveal a non-negligible effect of finite size on the collective modes. The phonon-rotation coupling strength is seen to depend strongly on the strength and anisotropy of the molecule-helium interaction potential. Comparison with other light rotors shows that this coupling is particularly high for CO. The CBF/DMC analysis shows that the J=1 rotational state couples effectively to phonon states, which are only present in large helium droplets or bulk. In particular, they are not present in small clusters with n{<=}20, thereby accounting for the much narrower linewidths and larger B constant measured for these sizes.

• OSTI ID: 20787912
• Journal Information: Physical Review. B, Condensed Matter and Materials Physics, Vol. 73, Issue 5; Other Information: DOI: 10.1103/PhysRevB.73.054502; (c) 2006 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA); ISSN 1098-0121
• Country of Publication: United States
• Language: English

Related Subjects

75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
ABSORPTION SPECTROSCOPY
ANISOTROPY
ATOMS
CARBON 12
CARBON MONOXIDE
COMPARATIVE EVALUATIONS
CORRELATION FUNCTIONS
DIFFUSION
DROPLETS
ELECTRON-PHONON COUPLING
HELIUM
HELIUM 4
INFRARED SPECTRA
ISOTOPE EFFECTS
LINE BROADENING
LINE WIDTHS
MOLECULES
MONTE CARLO METHOD
OXYGEN 16
PATH INTEGRALS
PHONONS
POTENTIALS
ROTATION
ROTATIONAL STATES
SUPERFLUIDITY
VIBRATIONAL STATES