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.

Multiphoton Ionization and Fragmentation of Hydrogen Chloride: A Diatomic Still Good for a Surprise

“…The observed angular anisotropies, however, suggest the participation of another fine structure component than the one with Ω = 0 which is accessed from B 1 Σ + . The equivalent background H + ion signal was not observed for excitation of the F 1 Δ state, in agreement with the notion of a spin-forbidden transition to the gateway state which is neither facilitated by the peculiar shape of the double-well B state nor by an allowed transition within the triplet manifold [22]. In addition, the strong H + ion signal enhancement observed for the near resonance f 3 Δ 2 Q(5) transition is associated with a slightly smaller kinetic energy release and a lowered anisotropy of the angular distribution compared to the adjacent rotational levels (cf.…”

“…A small, but nonnegligible fragmentation yielding H + ions was observed for all rotational levels belonging to the f 3 Δ state [18,22]. It is believed to be due to the spin-allowed transition from the resonantly excited 3 Δ intermediate state to the 3 Π gateway system leading to autoionization and dissociation as described in the previous section.…”

“…Both groups investigated the ionization/fragmentation pathways through various vibrational bands of the B 1 Σ + (0 + ) state. Very recently, Kauczok et al [22] have reported similar studies on the singlet and triplet Δ states that previously were identified as being in resonance with selected rovibrational states of the B 1 Σ + (0 + ) state [16][17][18].…”

“…The colour coding matches that of Figure 3. Recently, Kvaran and coworkers have detailedly studied Rydberg states of HCl with either Λ > 0 or S > 0 (i.e., belonging to the triplet manifold) or both, some of which exhibit near resonances with rovibrational levels of the B 1 Σ + state [16][17][18] Kauczok et al have recently studied the kinetic energy and the angular distributions of these fragments in order to identify those channels which become available to the HCl molecule in the near resonance situation [22]. They have found that those near-resonance rotational levels in the f 3 Δ and F 1 Δ states behave identically to the B 1 Σ + state levels.…”

Two-dimensional resonance enhanced multiphoton ionization of HiCl; i = 35, 37: State interactions, photofragmentations and energetics of high energy Rydberg states

“…Perturbations due to Rydberg-Rydberg mixings have also been predicted and identified. [23][24][25][26][27][28][29][30] The number of REMPI studies performed by our group for resonance excitations to the F 1 2 Rydberg state 16,27 and several triplet Rydberg states 16,27 as well as the V 1 + ion-pair states have revealed near-resonance interactions between the Rydberg and the ion-pair states. Such quantitative data on molecule-photon interactions are of interest in understanding stratospheric photochemistry as well as being relevant to the photochemistry of planetary atmospheres and the interstellar medium.…”