[1] Univ Puerto Rico, Coll Nat Sci, Dept Phys, Rio Piedras Campus, San Juan, PR 00925 USA
[2] Univ Puerto Rico, Mol Sci Res Ctr, San Juan, PR 00927 USA
[3] Univ Puerto Rico, Coll Arts & Sci, Dept Phys, Mayaguez Campus, Mayaguez, PR 00682 USA
[4] Univ Puerto Rico, Coll Nat Sci, Dept Chem, Rio Piedras Campus, San Juan, PR 00925 USA