You might have difficulty if you just use the light level you see on the diode with the A/D. A flash is in the order of 1mS long so you have to make sure that the voltage out of the sensor is read at least that often.
I would be inclined to use the very fast duration of the flash to detect it. That means putting the output of the sensor through an amplifier and then a high pass filter ( a small capacitor should do) followed by a comparator and then to trigger an interrupt.