[ovin8k]

"Photochemical reactions are initiated by the absorption of a photon, typically in the wavelength range 290–700 nm (at the surface of the Earth). The energy of an absorbed photon is transferred to electrons in the molecule and briefly changes their configuration (i.e., promotes the molecule from a ground state to an excited state). The excited state represents what is essentially a new molecule. Often excited state molecules are not kinetically stable in the presence of O2 or H2O and can spontaneously decompose (oxidize or hydrolyze). Sometimes molecules decompose to produce high energy, unstable fragments that can react with other molecules around them. The two processes are collectively referred to as direct photolysis or indirect photolysis, and both mechanisms contribute to the removal of pollutants."

I'm curious, what causes and why do o2 and h2 to make excited molecules not stable?

(Sorry if this is the wrong forum).

[Corribus]

Water isn't a big (chemical) issue but oxygen certainly is. A primary reason is that photoexcited organic molecules often end up in a longish-lived triplet excited state, which can efficiently photosensitize production of singlet oxygen. Singlet oxygen is basically a molecular wrecking ball that tears apart organic matter... often including the original molecule that generated it. Actually, despite being a strong oxidant, oxygen - being a ground state triplet - is kinetically stable. Strictly speaking, it can't react with organic molecules, which most of the time are ground state singlets. Such reaction events are formally spin forbidden. When either organic molecules are photoexcited to their triplet states, that kinetic bottleneck disappears. Oxidation events between organic molecules and substrates are formally always due to thermal or photophysical activation of an organic substrate to its excited triplet or oxygen to its excited singlet. That those energy gaps are so large (and the probability of thermal activation is correspondingly low) is the only reason stable organic molecules can exist in an oxygenated atmosphere at room temperature. 

Unless studying generation of singlet oxygen is the primary goal, experimentalists usually take great pains to deaerate samples prior to doing sophisticated spectroscopy experiments.

Water does have strong intermolecular interactions and can greatly affect rates of photophysical events. So it is frequently preferable to do spectroscopic measurements in dry, nonpolar (and deoxygenated) solvents ... but it all depends on what you're trying to do. Sometimes your substrate is intended to be dissolved in water, so in that case, water it is!