Byron, while I don't have any experience with DLC, the excitation energy can have a significant impact on your results.  To get a better idea of how, you need to consider the possible absorption processes in your sample. 

Generally, exciting a sample with light above the bandgap energy produces largely consistent results. The reason for this is that intra-band scattering occurs very quickly compared to most other relevant energy transfer processes, so small differences in energy/wavelength doesn't make much difference once you're above the bandgap.

If you're exciting a sample with light below the bandgap energy, you will see dramatic differences.  Here, you may be directly exciting defect states in your material without causing band-band recombination as you would in the previous example.  Intrinsic defects and intentional impurities can be studied using this approach.

The excitation-dependence of PL spectra is often studied using a method called Photoluminescence Excitation (PLE) spectroscopy and has been published on often.  Photoconductivity is also a related, but different technique where the photo generation of charge carriers is characterized for different excitation wavelengths.  Both techniques serve to probe the optical absorption mechanisms in materials.

In addition to wavelength, also note that continuous-wave (CW) excitation can produce different experimental results from pulsed excitation.  Furthermore, the excitation power density and pulse duration/repetition rate can also be experimental variables.

From my own experience, the  peak wavelength of PL is depend on the concentration of PL material. Through the absorption spectrum and excitation spectrum, u could get the best excitation wavelength. The shift you got maybe be involved by the concentration  and other proterties of PL material.