Proton formation in 2+1 resonance enhanced multiphoton excitation of HCl and HBr via (Ω=0) Rydberg and ion-pair states

Molecular beam cooled HCl was state selected by two-photon excitation of the V (1)Sigma(0(+)) [v=9,11-13,15], E (1)Sigma(0(+)) [v=0], and g (3)Sigma(-)(0(+)) [v=0] states through either the Q(0) or Q(1) lines of the respective (1,3)Sigma(0(+))<-<- X (1)Sigma(0(+)) transition. Similarly, HBr was excited to the V (1)Sigma(0(+)) [v=m+3, m+5-m+8], E (1)Sigma(0(+)) [v=0], and H (1)Sigma(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 P-2(i) halogen atoms and subsequently ionized. For HCl, photodissociation into H-*(n=2)+Cl(P-2(1/2)) was dominant over the formation of Cl(P-2(3/2)) and was attributed to parallel excitation of the repulsive [(2) (2)Pi center dot center dot 4l lambda] superexcited (Omega=0) states. For HBr, the Br(P-2(3/2))/Br(P-2(1/2)) ratio decreases with increasing excitation energy. This indicates that both the [(3) (2)Pi(1/2)center dot center dot 5l lambda] and the [B (2)Sigma center dot center dot 5l lambda] superexcited (Omega=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 [A (2)Sigma center dot center dot nl sigma] (1)Sigma(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, Delta Omega=0, excitation. The quantum yield for the dissociation process was obtained using previously reported photoionization efficiency data and was found to peak at v(+)=6-7 for HCl and v(+)=12 for HBr. This is consistent with excitation of the repulsive A(2)Sigma(1/2) and (2) (2)Pi states of HCl+, and the (3) (2)Pi state of HBr+. Rotational alignment of the Omega=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. (C) 2007 American Institute of Physics.