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

Abstract

Molecular beam cooled HCl was state selected by two-photon excitation of the V (1) summation operator(0(+)) [v=9,11-13,15], E (1) summation operator(0(+)) [v=0], and g (3) summation operator(-)(0(+)) [v=0] states through either the Q(0) or Q(1) lines of the respective (1,3) summation operator(0(+))<--<--X (1) summation operator(0(+)) transition. Similarly, HBr was excited to the V (1) summation operator(0(+)) [v=m+3, m+5-m+8], E (1) summation operator(0(+)) [v=0], and H (1) summation operator(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 (2)P(i) halogen atoms and subsequently ionized. For HCl, photodissociation into H(*)(n=2)+Cl((2)P(12)) was dominant over the formation of Cl((2)P(32)) and was attributed to parallel excitation of the repulsive [(2) (2)Pi4llambda] superexcited (Omega=0) states. For HBr, the Br((2)P(32))Br((2)P(12)) ratio decreases with increasing excitation energy. This indicates that both the [(3) (2)Pi(12)5llambda] and the [B (2) summation operator5llambda] 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) summation operatornlsigma] (1) summation operator(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, DeltaOmega=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) summation operator(12) 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.

Two-dimensional (2+n) resonance enhanced multiphoton ionization of HCl: State interactions and photorupture channels via low-energy triplet Rydberg states

“…More detailed investigations of excitations via various Rydberg states and the V 1 ⌺ + ion-pair state using photofragment imaging and/or mass-resolved REMPI techniques have revealed several ionization channels depending on the nature of the resonance excited state. [18][19][20][21][22] Results are largely based on analysis of excitations via the E 1 ⌺ + Rydberg state and the V 1 ⌺ + ion-pair state, which couple strongly to produce the mixed ͑adiabatic͒ B 1 ⌺ + state with two minima. Also, analysis of excitations via the F 1 ⌬ 2 ͑vЈ =1͒ Rydberg state and the V 1 ⌺ + ͑vЈ =14͒ state has shown the characteristic effects of near-resonance interactions on photoionization channels.…”

“…HCl + could be formed partly by direct ionization and partly by autoionization. 19 ͑2͒ Several ionization channels, via the noncoupled ion-pair state, have been proposed, [18][19][20][21] involving ͑iii͒ onephoton autoionization via a repulsive superexcited state that correlates with H + Cl ‫ء‬ to form HCl + largely in high vibrational ͑v + ͒ levels, 19 followed by ͑iv͒ a second onephoton excitation to a repulsive ion state ͑͑2͒ 2 ⌸͒ and dissociation ͓analogous to ͑ii͔͒, ͑v͒ one-photon excitation to repulsive triplet superexcited states, 20,21 forming H and Cl ‫ء‬ ͑Cl ‫ء‬ =Cl ‫ء‬ ͑4s ,4p ,3d͒͒, followed by onephoton ionization of Cl ‫ء‬ to form Cl + , ͑vi͒ one-photon excitation to a repulsive superexcited state ͑HCl ‫ء‬ , 1 ⌺ + ͒, forming H ‫ء‬ ͑n =2͒ and Cl, followed by one-photon ionization of H ‫ء‬ ͑n =2͒ to form H + , and ͑vii͒ one-photon excitation to a bound superexcited state, which acts as a gateway state to dissociation into the ion-pair H + +Cl − . 18 More channels have been proposed 18,20 via the "noncoupled" ion-pair state but these are believed to be of minor importance.…”

Three-dimensional velocity map imaging: Setup and resolution improvement compared to three-dimensional ion imaging

“…The detailed dynamics of this process have been investigated in detail by our group and by Loock and co-workers. 23,44,45 Only peak II in Fig. 9͑b͒ is important for the determination of the resolution, since this peak emerges from the dissociation into H + and Cl − which do not have any internal excitation ͑peak I and the propagation of peaks denoted as III are due to the dissociation of super excited HCl states yielding H͑n =2͒ atoms and the photodissociation of vibrationally excited HCl + , respectively, 23,44,45 ͒.…”

“…One cannot expect the overall 3D resolution to be as good as the space resolution achieved in refined versions of the 2D velocity map imaging or slice imaging technique, e.g., reported by Loock and co-workers. 44,45 However, the superb 2D resolution of those experiments goes along with the complete loss of information about the third dimension, which has consequently been termed a "pancaking" or "ion sphere crushing" technique. Similarly, slicing techniques which can provide a very large spatial resolution and can be utilized without extraordinary efforts have the principal deficiency that for increased time resolution the slice approaches a mere sheet.…”

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.…”