The stripping of photoresists from a silicon wafer using an rf oxygen plasma has been monitored using the optical emission from electronically excited OH and CO species in the ultraviolet region of the spectrum. The band systems at 283.0 nm (CO*, OH*), 297.7 nm (CO*), and 308.9 nm (OH*) are intense and spectrally isolated from other systems and arise from plasma-induced oxidation of the polymeric photoresist material. The endpoint of plasma stripping and the amount of stripped material is easily determined quantitatively. In addition, variations in stripping rate of photoresist as a function of wafer position in the reaction chamber can be detected.
Energy-transfer processes in the algal light-harvesting proteins, the phycocyanins, have been studied by means of picosecond absorption spectroscopy. After excitation at 530 nm, the absorption at several wavelengths in the range 480--669 nm decayed with a short time constant (picosecond) and a long time constant (greater than 1 ns). For C-phycocyanin, energy transfer from the beta to the alpha subunits is interpreted as being a likely candidate for the short time constant; the long time constant probably is the excitation lifetime of the chromophore on the alpha subunits. The time constants for energy transfer in monomers, trimers, and hexamers of C-phycocyanin extracted from a blue-green alga, Phormidium luridum, were measured as approximately 85, approximately 56, and approximately 32 ps, respectively. The corresponding time constant in the cryptomonad phycocyanin 645 from Chroomonas species was found to be less than 5 ps.
Picosecond studies of carboxymyoglobin (MbCO) and oxymyoglobin (MbO2) reveal that excitation at 530 nm induces photodissociation at less than 8 ps. The kinetic and structural changes were monitored by following absorbance changes at selected wave-lengths in the Soret (B) band and in the Q band. Within the 10 ps-0.45 ns period of time over which our experiments were conducted, the absorbance changes in the Soret and Q bands for MbCO and MbO2 correspond to the conventional long-term, steady-state deoxymyoglobin difference spectra (Mb-MbCO and Mb-MbO2), as determined by comparison of isosbestic, maximum, and minimum points. In addition, MbCO exhibits a decay to a steady state in the Soret band (monitored at 440 nm). The onset of the decay immediately follows photodissociation and has a rate of (8 +/- 3) X 10(9) s-1 (tau = 125 +/- 50 ps). During the 10 ps-0.45 ns observation window, relaxation is not seen for MbO2 in the Soret band, nor is relaxation observed in the Q band for either MbCO or MbO2. We conclude from these results that the steady state that we observed for MbCO and MbO2 is most likely the stable form of deoxymyoglobin, and the relaxational differences between MbCO and MbO2 observed in the Soret band indicate that the electronic destabilization after ligand detachment is very different for these molecules. We believe that these relaxational differences may be related to differences in tertiary structural changes, or due to the fact that the MbCO (S = 0) molecule passes through an intermediate spin Mb (S = 1) state before relaxing the the Mb (S = 2) state.
Publication costs assisted by Bell LaboratoriesDual-beam picosecond absorption spectroscopy was used to measure the time constant of the trans to cis isomerization of l-phenylazo-2-hydroxynaphthalene as a function of viscosity. We find that the isomerization time constant at 21.5 °C is 14 ± 3 ps in methylcyclohexane (viscosity, = 0.729 cP), 28 ± 5 ps in 5:2 methylcyclohexane-cyclohexanol mixed solvent ( = 2.06 cP), 107 ± 29 ps in 5:9 methylcyclohexane-cyclohexanol mixed solvent ( = 9.63 cP).
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