Proton formation dynamics in the REMPI[2+n] process via the F Δ12 and f Δ32 Rydberg states of HCl investigated by three-dimensional velocity mapping

Abstract

HCl in the bulk gas phase at a pressure of 10(-5) mbar has been excited via selected Q-lines of the two-photon transition band systems F (1)Delta(2)<--X (1)Sigma(+)(1,0) [Q(8)], V (1)Sigma(+)<--X (1)Sigma(+)(14,0) [Q(8), Q(7)] and f (3)Delta(2)<--X (1)Sigma(+)(0,0) [Q(2-6)]. Concerning the V<--X excitation, subsequent photon absorption is known to yield HCl(+), H(n=2)+Cl, H(+)+Cl(-) and H+Cl(4s,4p,3d). Vibrationally excited HCl(+) (v(+) > or = 5) can be photodissociated to H(+)+Cl, and excited atoms can be easily photoionized by absorption of a fourth photon, respectively. Using three-dimensional velocity map imaging, the spatial proton velocity distributions resulting from these processes for these particular transitions were studied for the first time. Kvaran et al. [J. Chem. Phys. 131, 044324 (2009); J. Chem. Phys. 129, 164313 (2008)] recently reported a substantial increase in the formation of chlorine and hydrogen ions in single rovibrational transitions of the F (1)Delta(2) and f (3)Delta(2) band systems using mass resolved resonance enhanced multiphoton ionization spectroscopy and explained this by the vicinity of single rovibrational levels of the V (1)Sigma(+) state for which photorupture is the main feature. Thus, the known dissociation dynamics of the V (1)Sigma(+) state should also leave their fingerprint in the spatial proton velocity distribution emerging from the photodissociation of those states. Accordingly, we found a strong increase in the H(+) ion signal for the Q(5) line of the f (3)Delta(2)<--X (1)Sigma(+)(0,0) transition, the extra signal resulting from dissociation into H(n=2)+Cl((2)P(1/2)) and the ion pair. No increase for the HCl(+)(v(+) > or = 5) photodissociation channel or dissociation into H(n=2)+Cl((2)P(3/2)) has been observed. Furthermore, H(+) distributions from the Q transitions of the f (3)Delta(2)<--X (1)Sigma(+)(0,0) band system were found to show the two features previously ascribed to the "gateway" state [(4)Pi...4s](3)Pi(0), i.e., autoionization into HCl(+)(5 < or = v(+) < or = 8) and nonadiabatic dissociation into H(n=2)+Cl((2)P(3/2)). The F (1)Delta(2)<--X (1)Sigma(+)(1,0) band system only showed significant proton formation for the Q(8) line. The speed distribution is the same as for the Q(8,7) lines of the V (1)Sigma(+)<--X (1)Sigma(+)(14,0) transition while the excitation history is conserved in the angular distribution confirming the resonance interpretation.