Molecular beam cooled HCl was state selected by two-photon excitation of the V1(0+)[v=9,1113,15], E1(0+)[v=0], and g3(0+)[v=0] states through either the Q(0) or Q(1) lines of the respective 1,3(0+)X1(0+) transition. Similarly, HBr was excited to the V1(0+) [v=m+3, m+5m+8], E1(0+)[v=0], and H1(0+)[v=0] states through the Q(0) or Q(1) lines. Following absorption of a third photon, protons were formed by three different mechanisms and detected using velocity map imaging. (1) H*(n=2) was formed in coincidence with Pi2 halogen atoms and subsequently ionized. For HCl, photodissociation into H*(n=2)+Cl(P122) was dominant over the formation of Cl(P322) and was attributed to parallel excitation of the repulsive [(2)Π24lλ] superexcited (Ω=0) states. For HBr, the Br(P322)Br(P122) ratio decreases with increasing excitation energy. This indicates that both the [(3)Π1225lλ] and the [B25lλ] superexcited (Ω=0) states contribute to the formation of H*(n=2). (2) For selected intermediate states HCl was found to dissociate into the H++Cl ion pair with over 20% relative yield. A mechanism is proposed by which a bound [A2nlσ]1(0+) superexcited state acts as a gateway state to dissociation into the ion pair. (3) For all intermediate states, protons were formed by dissociation of HX+[v+] following a parallel, ΔΩ=0, excitation. The quantum yield for the dissociation process was obtained using previously reported photoionization efficiency data and was found to peak at v+=67 for HCl and v+=12 for HBr. This is consistent with excitation of the repulsive A122 and (2)Π2 states of HCl+, and the (3)Π2 state of HBr+. Rotational alignment of the Ω=0+ intermediate states is evident from the angular distribution of the excited H*(n=2) photofragments. This effect has been observed previously and was used here to verify the reliability of the measured spatial anisotropy parameters.

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