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Multiphoton Ionization Spectroscopy of AlAr_N Clusters

Spotts, J. M. and Wong, C.-K. and Johnson, M. S. and Okumura, M. and Boatz, J. A. and Hinde, R. J. and Sheehy, J. A. and Langhoff, P. W. (2003) Multiphoton Ionization Spectroscopy of AlAr_N Clusters. Journal of Physical Chemistry A, 107 (36). pp. 6948-6965. ISSN 1089-5639. doi:10.1021/jp030399o.

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Experimental and theoretical studies are reported of the multiphoton ionization spectroscopy of selected AlAr_N clusters (N = 2−54). Resonantly enhanced 1_(uv) + 1_(vis) and 2_(vis) + 1_(vis) ionization spectra are recorded of neutral clusters employing a laser-ablation/pulsed supersonic expansion source and time-of-flight mass spectrometric cluster-ion detection. The spectra are dominated by broad red- and blue-shifted asymmetric bands in the neighborhood of the 308 and 303 nm atomic Al 3p → 3d and 4p lines. The detailed structures of these bands and the observed degree of their spectral shifts with increasing cluster size are attributed on the basis of concomitant ab initio theoretical calculations to interplay among a number of factors, including (i) the comparable strengths of spin−orbit-split anisotropic (^2P_(1/2))Al−(^1S_0)Ar interactions and Ar−Ar mutual attractions, responsible for predicted external-site Al atom locations on distorted icosahedral Ar_N structures, (ii) avoided crossings in the nearly degenerate AlAr_N potential energy surfaces accessed by one- and two-photon atomic Al 3p → 3d and 4p excitations, giving rise to the red- and blue-shifted spectral profiles, and (iii) significant dynamical rearrangement and parent cluster-ion fragmentation following ionization, resulting in Al+Ar_M signals that generally reflect the absorption cross sections of an ensemble of larger prior clusters (AlAr_N, N > M). Additionally, nonuniformity in the cluster-size distribution of the incident molecular beam is inferred from the calculated and measured spectra and must be incorporated in the development for a completely satisfactory accounting between theory and experiment. Comparisons with the results of earlier experimental studies of the ionization potentials of AlAr_N clusters also underscore the importance of dynamical parent-ion rearrangement and fragmentation, consequent of the increased Ar solvation of the Al+ radical in the equilibrium Al+Ar_M cluster-ion structures. The reported multiphoton ionization cluster-ion spectra are evidently highly sensitive to the details of the atomic Ar arrangements around the Al chromophore and accordingly provide a spectroscopic probe of the nature and evolution of the Al trapping sites and cluster geometries with increasing cluster size when the complex electronic and vibrational phenomena underlying the measurements are appropriately interpreted.

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Okumura, M.0000-0001-6874-1137
Additional Information:© 2003 American Chemical Society. Received: March 31, 2003; In Final Form: June 5, 2003. Publication Date (Web): August 15, 2003. This work was supported in part by grants from the US Air Force Office of Scientific Research (Grants F49620-92-J-0537 and F49620-93-I-0326) under the auspices of the High Energy Density Matter Program administered by Dr. Michael Berman, by a National Science Foundation Fellowship for J.M.S., and by National Research Council Senior and Summer Fellowships and other support from AFOSR and the Air Force Research Laboratory for P.W.L. and R.J.H. We thank Drs. Mario E. Fajardo and Jeffrey D. Mills for helpful collaboration and ongoing comments during the course of the investigation and Professor S. T. Manson for providing Hartree−Slater estimates of the atomic Al 3d and 4p photoionization cross sections.
Funding AgencyGrant Number
Air Force Office of Scientific Research (AFOSR)F49620-92-J-0537
Air Force Office of Scientific Research (AFOSR)F49620-93-I-0326
National Research CouncilUNSPECIFIED
Air Force Research Laboratory (AFRL)UNSPECIFIED
Issue or Number:36
Record Number:CaltechAUTHORS:20170426-102550498
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Official Citation:Multiphoton Ionization Spectroscopy of AlArN Clusters J. M. Spotts, C.-K. Wong, M. S. Johnson, M. Okumura, J. A. Boatz, R. J. Hinde, J. A. Sheehy, and P. W. Langhoff The Journal of Physical Chemistry A 2003 107 (36), 6948-6965 DOI: 10.1021/jp030399o
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:76945
Deposited By: Tony Diaz
Deposited On:26 Apr 2017 17:55
Last Modified:15 Nov 2021 17:04

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