The effect of the matrix on film properties in matrix-assisted pulsed laser evaporation

Thin films of polyethylene glycol of average molecular weight 1400 amu have been deposited by matrix-assisted pulsed laser evaporation (MAPLE). The deposition was carried out in vacuum ${\text{(∼10}}^{\text{−6}} \mathrm{Torr})$ with an ArF $\text{(λ=193\hspace{0.05em}}\mathrm{nm})$ laser at a fluence of 220–230 mJ/cm². Films were deposited on NaCl plates and glass microscope slides. Both deionized water $({\mathrm{H}}_{\mathrm{2}}\mathrm{O})$ and chloroform $({\mathrm{CHCl}}_{\mathrm{3}})$ were used as matrices. The physiochemical properties of the films are compared via Fourier transform infrared spectroscopy, and electrospray ionization mass spectrometry. The results show that the matrix used during MAPLE can greatly affect the chemical structure and molecular weight distribution of the deposited film. The infrared absorption spectrum shows evidence for C–Cl bond formation when ${\mathrm{CHCl}}_{\mathrm{3}}$ is used as a matrix, while there is little evidence in the IR data for photochemical modification when ${\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ is used as a matrix. Time-of-flight analysis was performed using a quadrupole mass spectrometer to monitor evaporation of a frozen ${\mathrm{CHCl}}_{\mathrm{3}}$ target during laser exposure. Using this approach, we determined that the TOF spectra for $\text{m/z=35}$ (Cl) and $\text{m/z=85}$ $({\mathrm{CHCl}}_{\mathrm{2}})$ differed significantly in both width and peak arrival time, indicating that neutral chlorine atoms were produced at the target surface. We attribute the reduction in molecular weight and structural modification of the film deposited using ${\mathrm{CHCl}}_{\mathrm{3}}$ to the presence of these highly reactive species.