A low power He-Ne laser beam is shown to induce layer instabilities in a smectic A liquid crystal cell possessing initial homeotropic orientation. The effect occurs when the smectic A phase is doped with a photosensitive ate-compound. The most plausible mechanism for the instability is photo-transformation of the guest molecules (e.g. trans-cis isomerization), and a subsequent decrease in the smectic layer spacing.