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

Abstract

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,more » 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.« less

Authors:
; ; ;  [1];  [2];  [3];  [4];  [2]
  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)
  4. (Austria)
Publication Date:
OSTI Identifier:
20787912
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review. B, Condensed Matter and Materials Physics; Journal Volume: 73; Journal Issue: 5; Other Information: DOI: 10.1103/PhysRevB.73.054502; (c) 2006 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
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

Citation Formats

Haeften, Klaus von, Rudolph, Stephan, Simanovski, Iaroslav, Havenith, Martina, Zillich, Robert E., Fraunhofer ITWM, 67663 Kaiserslautern, Institut fuer Theoretische Physik, Johannes Kepler Universitaet, A-4040 Linz, and Whaley, K. Birgitta. Probing phonon-rotation coupling in helium nanodroplets: Infrared spectroscopy of CO and its isotopomers. United States: N. p., 2006. Web. doi:10.1103/PHYSREVB.73.0.
Haeften, Klaus von, Rudolph, Stephan, Simanovski, Iaroslav, Havenith, Martina, Zillich, Robert E., Fraunhofer ITWM, 67663 Kaiserslautern, Institut fuer Theoretische Physik, Johannes Kepler Universitaet, A-4040 Linz, & Whaley, K. Birgitta. Probing phonon-rotation coupling in helium nanodroplets: Infrared spectroscopy of CO and its isotopomers. United States. doi:10.1103/PHYSREVB.73.0.
Haeften, Klaus von, Rudolph, Stephan, Simanovski, Iaroslav, Havenith, Martina, Zillich, Robert E., Fraunhofer ITWM, 67663 Kaiserslautern, Institut fuer Theoretische Physik, Johannes Kepler Universitaet, A-4040 Linz, and Whaley, K. Birgitta. Wed . "Probing phonon-rotation coupling in helium nanodroplets: Infrared spectroscopy of CO and its isotopomers". United States. doi:10.1103/PHYSREVB.73.0.
@article{osti_20787912,
title = {Probing phonon-rotation coupling in helium nanodroplets: Infrared spectroscopy of CO and its isotopomers},
author = {Haeften, Klaus von and Rudolph, Stephan and Simanovski, Iaroslav and Havenith, Martina and Zillich, Robert E. and Fraunhofer ITWM, 67663 Kaiserslautern and Institut fuer Theoretische Physik, Johannes Kepler Universitaet, A-4040 Linz and Whaley, K. Birgitta},
abstractNote = {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.},
doi = {10.1103/PHYSREVB.73.0},
journal = {Physical Review. B, Condensed Matter and Materials Physics},
number = 5,
volume = 73,
place = {United States},
year = {Wed Feb 01 00:00:00 EST 2006},
month = {Wed Feb 01 00:00:00 EST 2006}
}
  • The catalytic reduction of CO{sub 2} to produce hydrocarbon fuels is a topic that has gained significant attention. Development of efficient catalysts is a key enabler to such approaches, and metal-based catalysts have shown promise towards this goal. The development of a fundamental understanding of the interactions between CO{sub 2} molecules and metal atoms is expected to offer insight into the chemistry that occurs at the active site of such catalysts. In the current study, we utilize helium droplet methods to assemble complexes composed of a CO{sub 2} molecule and a Mg or Al atom. High-resolution infrared (IR) spectroscopy andmore » optically selected mass spectrometry are used to probe the structure and binding of the complexes, and the experimental observations are compared with theoretical results determined from ab initio calculations. In both the Mg–CO{sub 2} and Al–CO{sub 2} systems, two IR bands are obtained: one assigned to a linear isomer and the other assigned to a T-shaped isomer. In the case of the Mg–CO{sub 2} complexes, the vibrational frequencies and rotational constants associated with the two isomers are in good agreement with theoretical values. In the case of the Al–CO{sub 2} complexes, the vibrational frequencies agree with theoretical predictions; however, the bands from both structural isomers exhibit significant homogeneous broadening sufficient to completely obscure the rotational structure of the bands. The broadening is consistent with an upper state lifetime of 2.7 ps for the linear isomer and 1.8 ps for the T-shaped isomer. The short lifetime is tentatively attributed to a prompt photo-induced chemical reaction between the CO{sub 2} molecule and the Al atom comprising the complex.« less
  • High-resolution infrared laser spectroscopy is used to study the CH{sub 3}...HF and CD{sub 3}...HF radical complexes, corresponding to the exit-channel complex in the F+CH{sub 4}{yields}HF+CH{sub 3} reaction. The complexes are formed in helium nanodroplets by sequential pickup of a methyl radical and a HF molecule. The rotationally resolved spectra presented here correspond to the fundamental v=1(leftarrow)0 H-F vibrational band, the analysis of which reveals a complex with C{sub 3v} symmetry. The vibrational band origin for the CH{sub 3}...HF complex (3797.00 cm{sup -1}) is significantly redshifted from that of the HF monomer (3959.19 cm{sup -1}), consistent with the hydrogen-bonded structure predictedmore » by theory [E. Ya. Misochko et al., J. Am. Chem. Soc. 117, 11997 (1995)] and suggested by previous matrix isolation experiments [M. E. Jacox, Chem. Phys. 42, 133 (1979)]. The permanent electric dipole moment of this complex is experimentally determined by Stark spectroscopy to be 2.4{+-}0.3 D. The wide amplitude zero-point bending motion of this complex is revealed by the vibrational dependence of the A rotational constant. A sixfold reduction in the line broadening associated with the H-F vibrational mode is observed in going from CH{sub 3}...HF to CD{sub 3}...HF. The results suggest that fast relaxation in the former case results from near-resonant intermolecular vibration-vibration (V-V) energy transfer. Ab initio calculations are also reported (at the MP2 level) for the various stationary points on the F+CH{sub 4} surface, including geometry optimizations and vibrational frequency calculations for CH{sub 3}...HF.« less
  • Here, the entrance channel complex in the exothermic OH + CH 4 → H 2O + CH 3 reaction has been isolated in helium nanodroplets following the sequential pick-up of the hydroxyl radical and methane. The a-type OH stretching band was probed with infrared depletion spectroscopy, revealing a spectrum qualitatively similar to that previously reported in the gas phase, but with additional substructure that is due to the different internal rotation states of methane (j CH4 = 0, 1, or 2) in the complex. We fit the spectra by assuming the rotational constants of the complex are the same formore » all internal rotation states; however, subband origins are found to decrease with increasing j CH4. Measurements of deuterated complexes have also been made (OD–CH 4, OH–CD 4, and OD–CD 4), the relative linewidths of which provide information about the flow of vibrational energy in the complexes; vibrational lifetime broadening is prominent for OH–CH 4 and OD–CD 4, for which the excited OX stretching state has a nearby CY 4 stretching fundamental (X, Y = H or D).« less
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  • The ejection dynamics of Rydberg atoms and molecular fragments from electronically excited helium nanodroplets are studied with time-resolved extreme ultraviolet ion imaging spectroscopy. At excitation energies of 23.6 {+-} 0.2 eV, Rydberg atoms in n= 3 and n= 4 states are ejected on different time scales and with significantly different kinetic energy distributions. Specifically, n= 3 Rydberg atoms are ejected with kinetic energies as high as 0.85 eV, but their appearance is delayed by approximately 200 fs. In contrast, n= 4 Rydberg atoms appear within the time resolution of the experiment with considerably lower kinetic energies. Major features in themore » Rydberg atom kinetic energy distributions for both principal quantum numbers can be described within a simple elastic scattering model of localized perturbed atomic Rydberg atoms that are expelled from the droplet due to their repulsive interaction with the surrounding helium bath. Time-dependent kinetic energy distributions of He{sub 2}{sup +} and He{sub 3}{sup +} ions are presented that support the formation of molecular ions in an indirect droplet ionization process and the ejection of neutral Rydberg dimers on a similar time scale as the n= 3 Rydberg atoms.« less