An Ar+ laser, operating at excitation wavelengths of 488.0 and 514.5 nm and a power of 5 mW, and a Kr+ laser, operating at an excitation wavelength of 647.1 nm and a power of 5 mW, were the light sources. A 10% neutral density filter, which decreased the laser power at the surface to 0.5 mW, was introduced where necessary to reduce the risk of damage from laser light. Integration times of 1-10 s for 10-100 accumulations were used to obtain spectra of sufficient quality for the current study.