This disclosure describes a reaction vessel assembly that includes the following: a reaction vessel including a housing component; a reaction chamber defined by the housing component; and a light absorbing layer conforming to a portion of an interior-facing surface of the housing component that defines the reaction chamber, the light absorbing layer comprising a multiple discrete regions; and an energy source configured to direct light through at least a portion of the housing component at one or more of the discrete regions of the light absorbing layer.