CLUSTERS IN THE DUNCAN LAB
Metal-Carbide and Oxide Cages and Nanocrystals
Metal carbon clusters are produced using the same techniques originally used to produce C60 ("Buckminsterfullerene"). These species are now recognized to form metal-carbon cages (so-called "met-cars" clusters - M8C12) or metal carbon "nanocrystals" (M13C14), depending on the metals employed and the growth conditions. We produce these clusters and study their chemistry via gas phase adsorption reactions and we study their decomposition via mass-selected photodissociation. Our laboratory was the first to document the competition between met-cars cage and nanocrystal production, the first to show that certain nanocrystals could dissociatively reconstruct to form met-cars cages, and the first to show that laser excitation of large nanocrystals leads to photo-induced crystal cleavage to produce smaller nanocrystals.
Exciting work has been done in collaboration with the research group of Prof. Gerard Meijer at the University of Nijmegen, The Netherlands. In their lab, we have used the Free Electron Laser for Infrared eXperiments ("FELIX") together with a metal cluster experiment. (visit the FELIX lab) Using FELIX, we have obtained the first-ever infrared spectra of gas phase metal clusters for the met-cars and nanocrystal species. The experiments use resonance-enhanced multiphoton ionization with infrared radiation (IR-REMPI) in the 400-1700 cm-1 region from the pulsed free electron laser at the F.O.M. Institute for Plasma Physics (Utrecht). Infrared spectra are obtained for Ti8C12, Ti14C13 and for many larger metal carbide nanocrystals. The nanocrystals have a strong resonance at 500 cm-1(20 microns), and these spectra are essentially unchanged with the size of the cluster. In a new collaboration with astronomers, these TiC nanocrystal spectra have been identified as having the identical signature of the so-called "21 micron line" (which actually occurs at 20.1 microns) seen in the IR spectra of carbon rich stars as they burn out in the last stage of their growth. This identifies metal carbide nanocrystals in space for the first time and provides significant new insight into interstellar dust formation.
Closely related to the work on metal carbide clusters are studies of other metal compound systems (metal oxides, nitrides, etc.) which also form strongly bound clusters with novel geometries, and which also have potential astrophysical significance.
This research is sponsored by the Air Force Office of Scientific Research and by the U.S. Department of Energy.
M. A. Duncan, "Laser vaporization cluster sources," Rev. Sci. Instrum. 83, 041101/1-19 (2012)(invited review).
C. J. Dibble, S. T. Akin, S. Ard, C. P. Fowler, M. A. Duncan, "Photodissociation of cobalt and nickel oxide cluster cations," J. Phys. Chem. A 116, 5398 (2012).
A. M. Ketch, C. L. Anfuso, K. S. Molek and M. A. Duncan, "Photodissociation of small indium oxide cluster cations," Int. J. Mass Spectrom. 304, 29 (2011).
O. Kostko, S. R. Leone, M. A. Duncan, M. Ahmed, "Determination of ionization energies of small silicon clusters: Vacuum ultraviolet (VUV) photoionization experiments and ab initio calculations," J. Phys. Chem. A 114, 3176 (2010).
B. W. Ticknor, B. Bandyopadhyay and M. A. Duncan, “Photodissociation of noble metal-doped carbon clusters,” J. Phys. Chem. A 112, 12355 (2008).
K. S. Molek, C. Anfuso-Cleary and M. A. Duncan, “Photodissociation of iron oxide cluster cations,” J. Phys. Chem. A 112, 9238 (2008).
Z. D. Reed and M. A. Duncan, "Photodissociation of yttrium and lanthanum oxide cluster cations," J. Phys. Chem. A 112, 5354 (2008).
L. Belau, S. E. Wheeler, B. W. Ticknor, M. Ahmed, S. R. Leone, W. D. Allen, H. F. Schaefer, and M. A. Duncan, "Ionization thresholds of small carbon clusters: Tunable VUV experiments and theory," J. Am. Chem. Soc. 129, 10229 (2007).
K. S. Molek, Z. D. Reed, A. M. Ricks and M. A. Duncan, “Photodissociation of chromium oxide cluster cations,” J. Phys. Chem. A, 111, 8080 (2007).
J. B. Jaeger, T. D Jaeger, M. A. Duncan, "Photodissociation of metal-silicon clusters: Encapsulated versus surface-bound metal," J. Phys. Chem. A 110, 9310 (2006).
K. S. Molek, T.D. Jaeger and M. A. Duncan, "Photodissociation of vanadium, niobium and tantalum oxide cluster cations," J. Chem. Phys. 123, 144313 (2005).
G. von Helden, A. G. G. M. Tielens, D. van Heijnsbergen, M. A. Duncan, S. Hony, L. B. F. M. Waters and G. Meijer, "Titanium carbide nanocrystals in circumstellar environments," Science 288, 313 (2000).
Synthesis of Macroscopic Amounts of Metal Clusters and Metal Nanoparticles
A new laser vaporization flow reactor (LVFR) has been constructed consisting of a laser ablation cluster source combined with a fast flowtube reactor for the production and isolation of ligand-coated metal clusters. The source includes high repetition rate laser vaporization with a 100 Hz KrF (248 nm) excimer laser, while cluster growth and passivation with ligands takes place in a flowtube with ligand addition via a nebulizer spray. Samples are isolated in a low temperature trap and solutions containing the clusters are analyzed with laser desorption time-of-flight mass spectrometry. Initial experiments with this apparatus have trapped Tix(ethylenediamine)y complexes which apparently have linear metal units with octahedral ligand coordination. Other experiments have produced and isolated clusters of the form TixOy(THF)z that apparently have linear metal oxide cores and larger (TiO2)x(THF)y nanoparticle species, where x=10-14 and y=5-8. The isolation of these new cluster species suggest that the LVFR instrument has considerable potential for the production of new nanocluster materials.
This research is sponsored by the Air Force Office of Scientific Research.
S. Ard, C. Dibble, S. T. Akin, M. A. Duncan, "Ligand-coated vanadium oxide nanoclusters: Capturing gas phase magic numbers in solution," J. Phys. Chem. C 115, 6438 (2011).
T. M. Ayers, J. L. Fye, Q. Li, M. A. Duncan, “Synthesis and isolation of titanium metal cluster complexes and ligand-coated nanoparticles with a laser ablation flowtube reactor,” J. Clus. Sci. 14, 97 (2003).
Laser Desorption Mass Spectrometry
We have built a special version of a time-of-flight mass spectrometer for analysis of involatile materials. Recent studies show that fast pulsed laser excitation can lead to volatilization and ionization of molecules with virtually no intrinsic vapor pressure. We have used this technique to analyze novel polymer films of C60 and metal colloidal "quantum dot" materials. Additional studies in our lab use Matrix Assisted Laser Desorption Ionization ("MALDI") to produce mass spectra of proteins, enzymes and polymers. We are collaborating to analyze particle sizes and extent of polymerization in materials produced in other laboratories, and we are investigating the mechanism of laser desorption/ionization in its various forms.
This research is sponsored by the University of Georgia Research Foundation and the Air Force Office of Scientific Research.
This is the laser desorption mass spectrometer system. A "MiniLite" YAG laser is used to desorb samples.
T. C. Cheng, S. T. Akin, C. J. Dibble, S. Ard and M. A. Duncan, "Tunable infrared laser desorption ionization of fullerene films," Int. J. Mass. Spectrom. 354–355, 159 (2013).
A. M. Rao, P. C. Eklund, U. D. Venkateswaran, J. Tucker, M. A. Duncan, G. Bendele, P. W. Stephens, J. L. Hodeau, L. Marques, M. Nunez-Regueiro, I. O. Bashkin, E. G. Ponyatovsky and A. P. Morovsky, "Properties of C60 polymerized under high pressure and temperature," Appl. Phys. A 64, 231 (1997).
A. M. Rao, M. Menon, K. A. Wang, P. C. Eklund, K. R. Subbaswamy, D. S. Cornett, M. A. Duncan, I. J. Amster, "Photoinduced polymerization of solid C70 films," Chem. Phys. Lett. 224, 106 (1994).
D. S. Cornett, M. A. Duncan, I. J. Amster, "Liquid mixtures for matrix assisted laser desorption mass spectrometry of proteins," Anal. Chem. 65, 2608 (1993).
D. S. Cornett, I.J. Amster, M. A. Duncan, A. M Rao, P. C. Eklund, "laser desorption mass spectrometry of photopolymerized C60 films," J. Phys. Chem. 97, 5036 (1993).
A. M. Rao, P. Zhou, K. A. Wang, G. T. Hager, J. M. Holden, Y. Wang, W. T. Lee, X. X. Bi, P. C. Eklund, D. S. Cornett, M. A. Duncan, I. J. Amster, "Photo-induced polymerization of solid C60 films," Science 259, 955 (1993).
SPECTROSCOPY IN THE DUNCAN LAB
Metal Ion Complexes
Metal cation complexes are models for metal-ligand bonding, metal ion solvation and atmospheric meteor ablation chemistry. We study these systems by mass selecting them and performing photodissociation spectroscopy in a specially designed reflectron time-of-flight mass spectrometer (design developed in our group). Over the last several years, these complexes have been studied with electronic spectroscopy in wavelength regions near atomic transitions on metal cations or with infrared spectroscopy probing the ligand vibrations.
If the electrostatic bonding is weak, the complexes will have electronic spectra in the wavelength region near that of the free metal ion transitions. Systems studied included Mg+-L and Ca+-L complexes, where L = Ne, Ar, Kr, Xe, N2, CO2, and H2O. The highlight of this work is the first determination for the structure (bond distances and H-O-H bond angle) and spectroscopy of metal-water complexes (Mg+-H2O and Ca+-H2O). These studies determined the bond energies and the vibrational frequencies for the slightly distorted water molecules. Other systems studied include metal-benzene, -acetylene, and -ethylene complexes where p bonding interactions are investigated.
New experiments employ infrared optical parametric oscillator (OPO/OPA) systems to investigate the infrared spectroscopy of these metal ion complexes through IR photodissociation and IR-optical double resonance experiments. Infrared experiments are harder technically than UV-Visible experiments because of the available laser sources and because transitions are weaker in the IR than they are in the UV-Visible. Additionally, the lower energy in IR photons make it more difficult to break the bonds in the cluster, and dissociation is required in our experiment in order to measure the spectrum. Better interactions with theory are possible however, when measurements can be made in the electronic ground state. These experiments excite ligand or solvent vibrational modes, which then lead to dissociation of the cluster. Metal ion complexes with strong IR active ligands (CO2, H2O, etc.) are investigated to measure the perturbation caused by binding to metal and especially the onset of solvation and multilayer clustering. These new experiments have been successful so far for M+(CO2)n complexes (M=Al, Mg, Si, Fe, Ni, V) with and without added argon in the cluster, for M+(acetylene)n complexes (M=Ni, Co, Fe, Ag), which are especially interesting because of the possibility of cyclization chemistry that may occur to form cyclobutadiene, benzene or cyclo-octatetraene, metal-benzene complexes, metal-carbonyl complexes and many metal ion-water complexes (M=Al, Mg, V, Fe, Co, Mn, Cr, Sc, Cu, Ag, Au, Nb). New methods make it possible to obtain doubly charged cation-water complexes, and their infrared spectroscopy can now be compared to corresponding singly charged complexes. Argon or neon atoms are often added to the cluster to promote efficient fragmentation without introducing significant perturbations. The loss of the rare gas "tag" provides the mass change needed to detect light absorption with high sensitivity.
Figure 1. The infrared spectrum of various M+(C2H2) complexes showing the two C-H stretching modes. The positions and intensities of these bands confirm the predictions of theory, which indicate the formation of p-complexes for all the complexes except V+(C2H2), where a three-membered ring "metalla-cycle" is formed.
Figure 2. The infrared spectrum of the Li+(H2O) complex in the OH stretching region compared to the predictions of theory. The symmetric stretch band occurs as a single feature at 3629 cm-1, while the asymmetric stretch has three main peaks associated with its rotational structure.
Figure 3. The infrared spectrum of Ta(CO)7+, which is an unusual seven-coordinate carbonyl made stable by its 18-electron configuration.
This research is sponsored by the National Science Foundation (main group metal and protonated systems) and by the U.S. Department of Energy (transition metal systems).
Infrared Spectroscopy of Metal Ion Complexes:
M. Castro, R. Flores Mena, and M. A. Duncan, "Theoretical Study of Nascent Solvation in Ni+(benzene)m, m=3 and 4, Clusters," J. Phys. Chem. A, submitted.
A. D. Brathwaite and M. A. Duncan, "Infrared Spectroscopy of Vanadium Oxide Carbonyl Cations," J. Phys. Chem. A, in press.
B. Bandyopadhyay, K. N. Reishus and M. A. Duncan, " Infrared spectroscopy of solvation in small Zn+(H2O)n Complexes," J. Phys. Chem. A, in press.
A. D. Brathwaite and M. A. Duncan, " Infrared photodissociation spectroscopy of saturated group IV (Ti, Zr, Hf) metal carbonyl cations," J. Phys. Chem. A, in press. (Curt Wittig Festschrift).
A. M. Ricks, A. D. Brathwaite, M. A. Duncan, "Coordination and spin states of V+(CO)n clusters revealed by IR spectroscopy," J. Phys. Chem. A 117, 1001–1010 (2013).
B. Bandyopadhyay and M. A. Duncan, "Infrared spectroscopy of V2+(H2O) complexes," Chem. Phys. Lett. 530, 10-15 (2012).
R. Garza-Galindo, M. Castro and M. A. Duncan, "Theoretical study of nascent hydration in the Fe+(H2O)n system," J. Phys. Chem. A 116, 1906-1913 (2012).
A. D. Brathwaite and M. A. Duncan, "Infrared spectroscopy of Si(CO)n+ complexes: Evidence for asymmetric coordination," J. Phys. Chem. A 116, 1375-1382 (2012).
A.M. Ricks, Z.E. Reed and M. A. Duncan, "IR spectroscopy of gas phase metal carbonyl cations," J. Mol. Spec. 266, 63-74 (2011) (invited feature article; cover art).
A. D. Brathwaite, Z. D. Reed and M. A. Duncan, "Infrared spectroscopy of copper carbonyl cations," J. Phys. Chem. A 115, 10461-10469 (2011).
A. M. Ricks, L. Gagliardi and M. A. Duncan, "Oxides and superoxides of uranium detected by IR spectroscopy in the gas phase," J. Phys. Chem. Lett. 2, 1662 (2011).
A. D. Brathwaite, Z. D. Reed and M. A. Duncan, "Infrared spectroscopy of copper carbonyl cations," J. Phys. Chem. A 115, 10461 (2011).
B. Bandyopadhyay, P. D. Carnegie, and M. A. Duncan, "Infrared spectroscopy of Mn+(H2O)n and Mn2+(H2O) complexes via argon complex predissociation," J. Phys. Chem. A 115, 7602 (2011).
M. A. Duncan, "IR spectroscopy of gas phase metal carbonyl cations," J. Mol. Spec. 266, 63 (2011) (invited feature article; cover art).
P. D. Carnegie, B. Bandyopadhyay and M. A. Duncan, "Infrared spectroscopy of Sc+(H2O) and Sc2+(H2O) via argon complex predissociation: The charge dependence of cation hydration," J. Chem. Phys. 134, 014302 (2011).
A. M. Ricks, L. Gagliardi and M. A. Duncan, "Infrared spectroscopy of extreme coordination: The carbonyls of U+ and UO2+," J. Am. Chem. Soc. 132, 15905 (2010).
Z. D. Reed and M. A. Duncan, "Infrared Spectroscopy and Structures of Manganese Carbonyl Cations, Mn(CO)n+ (n=1-9)," J. Am. Soc. Mass Spectrom. 21, 739 (2010).
A. M. Ricks, Z. D. Reed and M. A. Duncan, "Seven-coordinate homoleptic metal carbonyls in the gas phase," J. Am. Chem. Soc. 131, 9176 (2009).
A. M. Ricks and M. A. Duncan, "IR spectroscopy of Co+(CO)n complexes in the gas phase," J. Phys. Chem. A 113, 4701 (2009).
P. D. Carnegie, A. B. McCoy and M. A. Duncan, "Infrared spectroscopy and theory of Cu+(H2O)Ar2 and Cu+(D2O)Ar2: Fundamentals and combination bands," J. Phys. Chem. A 113, 4849 (2009).
P. D. Carnegie, B. Bandyopadhyay and M.A. Duncan, “Infrared spectroscopy of Cr+(H2O) and Cr2+(H2O): The role of charge in cation hydration,” J. Phys. Chem. A 112, 6237 (2008).
M.A. Duncan, “Structures, energetics and spectroscopy of gas phase transition metal ion-benzene complexes,” Int. J. Mass Spectrom. 272, 99 (2008) (review).
J. Velasquez, III, B. Njegic, M. S. Gordon and M. A. Duncan, "IR Photodissociation Spectroscopy and Theory of Au+(CO)n Complexes: Nonclassical Carbonyls in the Gas Phase," J. Phys. Chem. A 112, 1907 (2008).
J. D. Mosley, T. C. Cheng, S. Hasbrouck, A. M. Ricks and M. A. Duncan, "Electronic Spectroscopy of Co+-Ne via Mass-selected Photodissociation," J. Chem. Phys. 135, 104309 (2011).
M. A. Duncan, "Frontiers in the Spectroscopy of Mass-Selected Molecular Ions," Intl. J. Mass Spectrom. 200, 545 (2000) (review).
J. E. Reddic and M.A. Duncan, "Photodissociation Spectroscopy of Ca+-Ne," J. Chem. Phys. 112, 4974 (2000).
J. E. Reddic and M.A. Duncan, "Photodissociation Spectroscopy of Mg+-Ne," J. Chem. Phys. 110, 9948 (1999).
M. A. Duncan, "Spectroscopy of Metal Ion Complexes: Gas Phase Models for Solvation, " Ann. Rev. Phys. Chem. 48, 69 (1997) (review).
Protonated Water Clusters, H+(H2O)n
Our group has recently obtained the first infrared spectroscopy for protonated water clusters, H+(H2O)n, in the size range of n=1-30. Smaller clusters (n=1-8) had been studied before, but these are the first experiments to make measurements on the larger clusters. In the late 1970's, John Fenn and coworkers noticed a strange occurrence in the mass spectrum of such clusters. The n=21 mass peak was much larger than others, indicating that it had special stability. Fenn suggested that a near-spherical cage structure could explain this. A hydrogen bonding network can be constructed with 20 water molecules forming a "dodecahedron cage," leaving one left-over molecule. Fenn suggested that this molecule would go inside the cage, explaining why n=21 was apparently more stable than n=20. Because hydrogen bonding is so important throughout chemistry and biology, many labs have tried to do experiments and theory on these clusters. Until the recent work in Duncan's lab, however, there was no spectroscopy that might reveal the exact structures of these clusters.
Our work uses a variation of the laser plasma source that we use for metal complexes to produce the protonated water clusters in a supersonic molecular beam. They are then mass-selected in the same kind of time-of-flight mass spectrometer, and studied with infrared photodissociation spectroscopy. The spectra obtained this way do indeed find that the n=21 cluster is special. The spectra contain strong vibrational bands in the free-OH region near 3700 cm-1. Clusters smaller than n=21 have a multiplet here, which gradually evolves into a single strong peak at n=21. This proves that this cluster has a high symmetry structure like the proposed dodecahedron. Larger clusters have a more complex multiplet of peaks in this region.
Figure 4. The infrared spectrum of H+(H2O)5. The broad peaks in the hydrogen bonding region and the sharp ones in the O-H stretching region agree well with the predictions for a single isomeric structure, as shown.
T. C. Cheng, B. Bandyopadhyay, M. A. Duncan, "Protonation in water-benzene nanoclusters: Hydronium, zundel and eigen at a hydrophobic interface," J. Am. Chem. Soc. 134, 13046 (2012).
G. E. Douberly, R. S. Walters, J. Cai, K. D. Jordan and M. A. Duncan, "Infrared spectroscopy of small protonated water clusters H+(H2O)n (n=2-5): Isomers, argon tagging and deuteration," J. Phys. Chem. A 114, 4570 (2010).
G. E. Douberly, A. M. Ricks and M. A. Duncan, "Infrared spectroscopy of perdeuterated protonated water clusters in the vicinity of the clathrate cage structure," J. Phys. Chem. A 113, 8449 (2009) (communication).
J. Headrick, E.G. Diken, R.S. Walters, N.I. Hammer, R.A. Christie, J. Cui, E.M. Myshakin, M.A. Duncan, M.A. Johnson and K.D. Jordan, "Spectral signatures of hydrated proton vibrations in water clusters," Science 308, 1765 (2005).
J.-W. Shin, N.I. Hammer, E.G. Diken, M.A. Johnson, R.S. Walters, T.D. Jaeger, M.A. Duncan, R.A. Christie and K.D. Jordan, “Infrared signature of structural motifs associated with the H+(H2O)n, n=6-27, clusters,” Science 304, 1137 (2004).
Other Protonated Molecular Clusters
We have synthesized a variety of other molecular clusters containing protons or shared protons. These species are studied with infrared photodissociation spectroscopy. The free proton stretch occurs at high frequency (near 3500-3800 cm-1, while shared proton motions occur at very low frequency (1000-1200 cm-1). These data and their theoretical modeling are relevant for proton transfer reactions that occur throughout Chemistry and Biology.
This research is sponsored by the National Science Foundation.
J. W. Young, T. C. Cheng, B. Bandyopadhyay and M. A. Duncan, "Infrared photodissociation spectroscopy of H7+, H9+ and their deuterated analogs," J. Phys. Chem. A, in press. (Joel Bowman Festscrift).
T. C. Cheng, L. Jiang, K. R. Asmis, Y. Wang, J. M. Bowman, A. M. Ricks, M. A. Duncan, "Mid- and Far-IR spectra of H5+ and D5+ compared to the predictions of anharmonic theory," J. Phys. Chem. Lett. 3, 3160–3166 (2012).
J. D. Moseley, A. M. Ricks, J. I. Wu, P. v. R. Schleyer, M. A. Duncan, "IR spectroscopy of α- and β-protonated pyrrole via argon complex photodissociation," J. Phys. Chem. A 116, 9689–9695 (2012).
J. D. Moseley, T. C. Cheng, A. B. McCoy, M. A. Duncan, "Infrared spectroscopy of the mass 31 cation: Protonated formaldehyde vs methoxy," J. Phys. Chem. A 116, 9287 (2012).
T. C. Cheng, B. Bandyopadhyay, S. Wheeler, M. A. Duncan, "Vibrational spectroscopy and theory of the protonated benzene dimer and trimer," J. Phys. Chem. A 116, 2065 (2012).
B. Bandyopadhyay, T. C. Cheng and M. A. Duncan, "Proton sharing in hydronium-nitrogen clusters probed with infrared spectroscopy," Int. J. Mass Spectrom. 297, 124 (2010).
T. C. Cheng, B. Bandyopadhyay, Y. Wang, B. J. Braams, J. M. Bowman, Michael A. Duncan, "The shared proton mode lights up the infrared spectrum of fluxional cations H5+ and D5+," J. Phys. Chem. Lett. 1, 758 (2010).
A. M. Ricks, G. E. Douberly and M. A. Duncan, "Infrared spectroscopy of proton-bridged nitrogen dimers: Complex vibrational dynamics of the shared proton," J. Chem. Phys. 131, 104312 (2009).
G.E. Douberly, A.M. Ricks, B.W. Ticknor and M.A. Duncan, “Infrared spectroscopy of protonated acetone and its dimer,” Phys. Chem. Chem. Phys. 10, 77 (2008).
G.E. Douberly, A.D. Ricks, B.W. Ticknor and M.A. Duncan, “The structure of protonated carbon dioxide clusters: Infrared photodissociation spectroscopy and ab initio calculations,” J. Phys. Chem. A 112, 950 (2008).
G.E. Douberly, A.M. Ricks, B.W. Ticknor, W.C. McKee, P.v.R. Schleyer and M.A. Duncan, “Infrared photodissociation spectroscopy of protonated acetylene and its clusters,” J. Phys. Chem. A 112, 1897 (2008).
A number of small carbocations (formerly known as carbonium ions) have been produced with pulsed discharge cluster sources and studied with infrared photodissociation spectroscopy. Unsaturated ions of this sort are expected to have several isomeric structures and some have close energies. Some of these species are also believed to be present in interstellar gas clouds and may contribute to unassigned infrared or optical emission from these environments.
Figure 5. The infrared spectrum of C3H3+ ions compared to the predictions of theory for the cyclopropenyl cation (red) and propargyl cation (blue). As shown, both are present in the experiment.
M. A. Duncan, "Infrared laser spectroscopy of mass-selected carbocations," J. Phys. Chem. A 116, 11477–11491 (2012) (invited Feature Article).
A. M. Ricks, G. E. Douberly, P. v. R. Schleyer and M. A. Duncan, "The infrared spectrum of gas phase C3H3+: The cyclopropenyl and propargyl cations," J. Chem. Phys. 132, 051101 (2010) (communication).
A. M. Ricks, G. E. Douberly, P. v. R. Schleyer and M. A. Duncan, "Infrared spectroscopy of protonated ethylene: The nature of proton binding in the non-classical structure," Chem. Phys. Lett. 480, 17 (2009).
A. M. Ricks, G. E. Douberly and M. A. Duncan, "The infrared spectroscopy of protonated naphthalene and its relevance for the unidentified infrared bands," Astrophys. J. 702, 301 (2009).
G.E. Douberly, A.M. Ricks, B.W. Ticknor, P.v.R. Schleyer and M.A. Duncan, “Infrared spectroscopy of gas phase benzenium ions: Protonated benzene and protonated toluene from 750 to 3400 cm-1,” J. Phys. Chem. A 112, 4869 (2008) (letter).
G.E. Douberly, A.M. Ricks, P.v.R. Schleyer and M.A. Duncan, “Infrared spectroscopy of gas phase C3H5+: The allyl and 2-propenyl cations,” J. Chem. Phys. 128, 021102 (2008) (communication).