Two-dimensional resonance enhanced multiphoton ionization of H(i)Cl; i = 35, 37: state interactions, photofragmentations and energetics of high energy Rydberg states

High energy state interactions, energetics and multiphoto-fragmentation processes of HI

Mass resolved multiphoton ionization data for two-photon resonant excitations (REMPI) in the 69 000-79 000 cm^-1 region were recorded for HI. REMPI spectra of fragment and molecular ions were derived from the data and analysed to obtain information relevant to the state interactions, energetics and photofragmentation processes of intermediate Rydberg and ion-pair states (HI**). Spectral perturbations observed as line shifts and intensity anomalies acted complementarily to demonstrate the effects of the state interactions. The interaction strength and character mixing of Rydberg states and Rydberg and ion-pair states of different interaction types and the states energetics were quantified by deperturbation analysis for the high energy region of 75 000-79 000 cm^-1, which is dense in states. Energetics of new, not previously observed, Rydberg states, detected in the lower energy excitation region of 69 000-75 000 cm^-1 was characterized by simulation calculations. Ion intensity borrowing effects, found in the spectra of interacting states, are evidence of alterations in two-photon transition probabilities due to state mixing. Based on variations in relative spectral line intensities the major photofragmentation processes involved are proposed. These involve one-photon excitation of the intermediate states (HI**) to form repulsive superexcited states (HI^#) followed by autoionization, dissociation, photodissociation and photoionization processes to form ions. The importance of state interactions in multiphoton-fragmentation processes is evident from the work.

High energy Rydberg and ion-pair states, state mixing and excitation dynamics of HI

Mass resolved multiphoton ionization data for two-photon resonant excitations (REMPI) in the region of 74 000-75 000 cm-1 were recorded for HI. Spectra structures of fragment and molecular ions derived from the data were analyzed to derive information relevant to the energetics and state mixing of ion-pair and Rydberg resonance states as well as for the excitation dynamics. Four new ion-pair vibrational states (V1Σ+(v' = m + i); i = 16-19) and two Rydberg states (j3Σ-(0+; v' = 1) and N1Π1(v' = 2)) were identified and characterized. Spectral perturbations allowed characterization and quantization of both homogeneous and heterogeneous state interactions and mixing of the Rydberg states and ion-pair states. Intensity alterations, with respect to energy level excitations and ion masses, are found to be clear indications of state mixing as well as branching into different fragmentation (both photodissociation and photoionization) channels.

Two-color studies of CH3Br excitation dynamics with MPI and slice imaging

Two-color pump-probe experiments were performed to explore the multiphoton dynamics of CH3Br at high excitation energies of 8-10 eV, involving two-photon resonant excitations to a number of np and nd Rydberg states (pump) followed by REMPI detection (probe) of the Br, Br* and CH3(X) photoproducts. Slice images of Br+ and CH3+ ions were recorded in pump-only, probe-only and pump and probe experiments. Kinetic-energy release spectra (KERs), as well as spatial anisotropy parameters, were extracted from the images to identify the processes and the dynamics involved. Predissociation channels, following the two-photon resonant excitations and non-resonant photodissociation forming CH3(X) and Br/Br*, were identified and characterized. Furthermore, the probe excitations for CH3(X) involved near-resonant excitations to lower energy 5s Rydberg states of CH3Br. In three-photon excitation processes, a striking contrast is seen between excitations via the p/d and the s Rydberg states. Involvement of high-energy interactions between Rydberg and ion-pair states is identified.

Excitation dynamics involving homogeneous multistate interactions: one and two color VMI and REMPI of HBr

Velocity map imaging (VMI) data and mass resolved REMPI spectra are complementarily utilized to elucidate the involvement of homogeneous multistate interactions in excited state dynamics of HBr. The H¹Σ⁺(v' = 0) and E¹Σ⁺(v' = 1) Rydberg states and the V¹Σ⁺(v'= m + 7) and V¹Σ⁺(v'= m + 8) ion-pair states are explored as a function of rotational quantum number in the two-photon excitation region of 79 100-80 700 cm^-1. H⁺ and Br⁺ images were recorded by one- as well as two-color excitation schemes. Kinetic energy release (KER) spectra and angular distributions were extracted from the data. Strong-to-medium interactions between the E(1) and V(m + 8)/V(m + 7) states on one hand and the H(0) and V(m + 7)/V(m + 8) states on the other hand were quantified from peak shifts and intensity analysis of REMPI spectra. The effects of those interactions on subsequent photoionization and photolytic pathways of HBr were evaluated in one-color VMI experiments of the H⁺ and two-color VMI experiments of the Br⁺ photoproducts.

Effect of a triplet to singlet state interaction on photofragmentation dynamics: highly excited states of HBr probed by VMI and REMPI as a case study

Analysis of mass resolved spectra as well as velocity map images derived from resonance enhanced multiphoton ionization (REMPI) of HBr via resonance excitations to mixed Rydberg (6pπ ³Σ^-(v' = 0)) and valence (ion-pair) (V ¹Σ⁺(v' = m + 17)) states allows characterization of the effect of a triplet-to-singlet state interaction on further photoexcitation and photoionization processes. The analysis makes use of rotational spectra line shifts, line intensity alterations, kinetic energy release spectra as well as angular distributions. Energy-level-dependent state mixing of the resonance excited states is quantified and photoexcitation processes, leading to H⁺ formation, are characterized in terms of the states and fragmentation processes involved, depending on the state mixing.

Long term puzzles of the CH and CD energetics and related phenomena revisited; solutions sought through REMPI-photofragmentations of bromomethanes

Ever since the pioneering work by Herzberg and Johns in 1969 (The Astrophysical Journal, 1969, 158, 399) the spectral assignment and the energetics of the fundamental molecular fragment CH, in the region of 63 000-65 000 cm(-1) (7.81-8.06 eV), have remained a puzzle to a large extent. The dissociation of bromoform and deuterated bromoform following two-photon resonance excitations to molecular Rydberg states forms the fragment species CH* and CD* in the excited state A(2)Δ(v' =0) as well as carbon and bromine atoms in the ground and first excited states, C/C* and Br/Br*. Further (1r + 1i)REMPI of CH* and CD* resonance excites the fragments to the energy region of concern, whereas the atom fragments were identified by further (2r + 1i)REMPI. Analysis based on spectral simulations, isotope shifts and comparison with other data allowed spectral identifications, assignments and partial characterization of four highly excited bound states for each of the molecular fragments (CH**/CD**); including the (3)(2)Π valence state and the (4)(2)Π Rydberg state, for the first time. Perturbations, shown as line-shifts, line-intensity and/or line-width alterations, due to the level-to-level state interactions between the bound states and predissociations by a repulsive state are recognized. Recording of C(+) signals in REMPI of several bromomethanes for a one-photon energy of about 40 333 cm(-1) allows the clarification of a mystery concerning a broad C(+) band frequently observed. This work, presented, demonstrates the usefulness of molecular REMPI for fragment analysis.

Revealing photofragmentation dynamics through interactions between Rydberg states: REMPI of HI as a case study

High energy regions of molecular electronic states are largely characterized by the nature and involvement of Rydberg states. Whereas there are a number of observed dynamical processes that are due to interactions between Rydberg and valence states, reports on the corresponding effect of Rydberg-Rydberg state interaction in the literature are scarce. Here we report a detailed characterization of the effects of interactions between two Rydberg states on photofragmentation processes, for a hydrogen halide molecule. Perturbation effects, showing as rotational line shifts, intensity alterations and line-broadenings in REMPI spectra of HI, for two-photon resonance excitations to the j(3)Σ(-)(0(+); v' = 0) and k(3)Π1(v' = 2) Rydberg states, are analyzed. The data reveal pathways of further photofragmentation processes involving photodissociation, autoionization and photoionization affected by the Rydberg-Rydberg state interactions as well as the involvement of other states, close in energy. Detailed mechanisms of the involved processes are proposed.

State interactions and illumination of hidden states through perturbations and observations of new states: High energy resonance enhanced multiphoton ionization of HI

Hydrogen iodide, a Hund's case (c) molecule, serves as a benchmark compound for studying rich molecular state interactions between Rydberg and valence states as well as between Rydberg states at high energies (72,300-74,600 cm(-1)) by mass resolved resonance enhanced multiphoton ionization (REMPI). Perturbations in the spectra appearing as deformations in line-positions, line-intensities, and linewidths are found to be either due to near-degenerate or non-degenerate interactions, both homogeneous and heterogeneous in nature. Perturbation analyses allow indirect observation as well as characterization of "hidden states" to some extent. Furthermore, new observable spectral features are assigned and characterized.

Rydberg and valence state excitation dynamics: a velocity map imaging study involving the E-V state interaction in HBr

Photoexcitation dynamics of the E((1)Σ(+)) (v' = 0) Rydberg state and the V((1)Σ(+)) (v') ion-pair vibrational states of HBr are investigated by velocity map imaging (VMI). H(+) photoions, produced through a number of vibrational and rotational levels of the two states were imaged and kinetic energy release (KER) and angular distributions were extracted from the data. In agreement with previous work, we found the photodissociation channels forming H*(n = 2) + Br((2)P3/2)/Br*((2)P1/2) to be dominant. Autoionization pathways leading to H(+) + Br((2)P3/2)/Br*((2)P1/2) via either HBr(+)((2)Π3/2) or HBr(+)*((2)Π1/2) formation were also present. The analysis of KER and angular distributions and comparison with rotationally and mass resolved resonance enhanced multiphoton ionization (REMPI) spectra revealed the excitation transition mechanisms and characteristics of states involved as well as the involvement of the E-V state interactions and their v' and J' dependence.

Photofragmentation, state interaction, and energetics of Rydberg and ion-pair states: resonance enhanced multiphoton ionization of HI

Mass resolved resonance enhanced multiphoton ionization data for hydrogen iodide (HI), for two-photon resonance excitation to Rydberg and ion-pair states in the 69,600-72,400 cm(-1) region were recorded and analyzed. Spectral perturbations due to homogeneous and heterogeneous interactions between Rydberg and ion-pair states, showing as deformations in line-positions, line-intensities, and line-widths, were focused on. Parameters relevant to photodissociation processes, state interaction strengths and spectroscopic parameters for deperturbed states were derived. Overall interaction and dynamical schemes to describe the observations are proposed.

Photofragmentations, state interactions, and energetics of Rydberg and ion-pair states: resonance enhanced multiphoton ionization via E and V (B) states of HCl and HBr

(2 + n) resonance enhanced multiphoton ionization mass spectra for resonance excitations to diabatic E(1)Σ(+) (v') Rydberg and V (1)Σ(+) (v') ion-pair states (adiabatic B(1)Σ(+)(v') states) of H(i)Cl (i = 35,37) and H(i)Br (i = 79,81) were recorded as a function of excitation wavenumber (two-dimensional REMPI). Simulation analyses of ion signal intensities, deperturbation analysis of line shifts and interpretations of line-widths are used to derive qualitative and quantitative information concerning the energetics of the states, off-resonance interactions between the E states and V states, closest in energy as well as on predissociation channels. Spectroscopic parameters for the E(1)Σ(+) (v')(v' = 1) for H(35)Cl and v' = 0 for H(79)Br states, interaction strengths for E - V state interactions and parameters relevant to dissociation of the E states are derived. An overall interaction and dynamical scheme, to describe the observations for HBr, is proposed.

Photofragmentations, state interactions, and energetics of Rydberg and ion-pair states: two-dimensional resonance enhanced multiphoton ionization of HBr via singlet-, triplet-, Ω = 0 and 2 states

Mass spectra were recorded for one-colour resonance enhanced multiphoton ionization (REMPI) of H(i)Br (i = 79, 81) for the two-photon resonance excitation region 79,040-80,300 cm(-1) to obtain two-dimensional REMPI data. The data were analysed in terms of rotational line positions, intensities, and line-widths. Quantitative analysis of the data relevant to near-resonance interactions between the F(1)Δ(2)(v' = 1) and V(1)Σ(+)(v' = m + 7) states gives interaction strengths, fractional state mixing, and parameters relevant to dissociation of the F state. Qualitative analysis further reveals the nature of state interactions between ion-pair states and the E(1)Σ(+) (v' = 1) and H(1)Σ(+)(v' = 0) Rydberg states in terms of relative strengths and J' dependences. Large variety in line-widths, depending on electronic states and J' quantum numbers, is indicative of number of different predissociation channels. The relationship between line-widths, line-shifts, and signal intensities reveals dissociation mechanisms involving ion-pair to Rydberg state interactions prior to direct or indirect predissociations of Rydberg states. Quantum interference effects are found to be important. Moreover, observed bromine atom (2 + 1) REMPI signals support the importance of Rydberg state predissociation channels. A band system, not previously observed in REMPI, was observed and assigned to the k(3)Π(0)(v' = 0) ←← X transition with band origin 80,038 cm(-1) and rotational parameter B(v('))=7.238 cm(-1).