Laser photofragmentation-fluorescence spectrometric determination of B-Diketonate complexes

A major shortcoming of fluorescence spectrometry is its limited range of applicability. A way to broaden this range can be found in laser photofragmentation-fluorescence spectrometry. An experimental system has been developed wherein a pulsed laser beam photofragments molecules of a gaseous sample introduced into a vacuum chamber. Fluorescence of fragments so produced is observed. Studies of fragment emission resulting from the ArF (193 nm) laser photolysis of nitromethane are reported here. Strong emission occurs at 431 nm, due to the CH(A²Δ→X²Π) transition. A limit of detection of 190 pg is reported. The linear dynamic range extends over 3.5 decades in nitromethane pressure. The fragment fluorescence measurements can be made with excellent precision (3.7% relative standard deviation).

Studies of emission by photoproducts of the excimer laser photolysis of the fluorinated β-diketonate complexes of Al, Cr, Cu, Fe, and Ga at 193 nm are are reported here. The principal fragment emission occurs at 431 nm, due to the CH fragment, and at wavelengths correspond ing to atomic emission by the "bare" metal atom. Quantitative measurements were made with bis(1,1,1,5,5,5-hexafluoro-2,4-pentanedione)diaquo copper (II), by monitoring the intense atomic copper emission at 324.75 nm. A limit of detection for this chelate of 3.8 ng is reported. The linear dynamic range extends over almost three decades in sample pressure. The fragment fluorescence measurements can be made with good precision (7% relative standard deviation).

Enhancement of the copper emission signal was observed by exciting 2 0 ground-state copper atoms using a probe laser tuned to the Cu(²P⁰_3/2 → ²S_1/2 transition at 324.75 nm. A maximum factor of enhancement of ~250 is reported. Extrapolating this factor of enhancement to pressures below that for the limit of detection reported above, an "enhanced" limit of detection of 15 pg is calculated. The probe-laser enhanced copper emission measurements can be made with excellent precision (relative standard deviation of less than 2%). Attempts at enhancing the Al emission signal, resulting from photolysis of tris(1,1,1,5,5,5-hexafluoro-2,4-pentanedione) aluminum (III), using a probe laser tuned to the Al(²S_1/2 → ²P⁰_1/2) transition at 394.40 nm, are also discussed. It is reported that no measureable enhancement of this fluorescence signal was observed.