Photodissociation Dynamics of 2,5-Dihydroxyacetophenone

Photodissociation of 2,5-dihydroxyacetophenone (DHAP), an important matrix compound in matrix-assisted laser desorption/ionization (MALDI), was studied in a molecular beam at 193 nm using multimass ion imaging techniques. Two major dissociation channels were observed, including (1) C6H3(OH)(2)COCH3 -> OC6H3(OH)COCH3 + H and (2) C6H3(OH)(2)COCH3 -> C6H3(OH)(2) + COCH3. The minor channels include C6H(OH)(2)COCH3 -> C6H3(OH)(2)CO + CH3 and/or C6H3(OH)(2)COCH3 - C6H3(OH)(2) + CO + CH3. The photofragment translational energy distribution suggests that reaction I occurs at an excited state in which the potential along the O-H bond distance is repulsive. Comparison to the branching ratios from RRKM calculations suggests that reaction 2 does not occur at either the ground state or the triplet state or that if it does occur at one of these states it must not follow the RRKM-model. A comparison to the photodissociation dynamics of acetophenone and phenol and its derivatives was made.