Branching ratios in the dissociative photoionization of iodomethane by photoelectron photoion coincidence.

Iodomethane yields ten fragment ions after valence photoionization, in part by multiple dissociation pathways for each, thanks to a plethora of electronic states available in the parent ion as well as in the fragments. The comprehensive breakdown diagram from 11 eV to the double ionization onset, i.e., 26.7 eV, is recorded at high resolution using double imaging photoelectron photoion coincidence spectroscopy with synchrotron vacuum ultraviolet radiation. Based on fragment ion groupings, the changing branching ratios between these groups and between fragment ions within each group, as well as ancillary thermochemistry, we provide an overview of the dissociation pathways at play. Statistical and impulsive dissociations are identified using kinetic energy release analysis. Finally, a newly observed regime change is discussed in double ionization, whereby coincident H+ + I+ formation dominates over a 4 eV photon energy range, outcompeting the normally prevailing CH3+ + I+ channel.

Multiphoton breakdown of acetylene; formation of organic building block fragments.

Mass resolved REMPI spectra and electron and ion velocity map images were recorded for REMPI of acetylene in the case of two-photon resonant excitations to low lying 3p and 4p Rydberg states. Combined data analysis of ion signal intensities and electron and ion kinetic energy release distribution revealed multiphoton-fragmentation processes in terms of photodissociation and photoionization channels to form the molecular ion, C2H2+ and the fragment ions H+, C+, CH+, CH2+, C2+ and C2H+. The ratio of fragment ion formation over the parent ion formation increases with excitation energy. To a large extent, multiphoton-fragmentation involves the initial breakdown of the molecule into ground and excited state neutral fragments by two-, three- and four-photon dissociation processes prior to multiphoton ionization. The three-photon dissociation processes via superexcited molecular state(s) are found to be the most important and electronically excited fragment species playing a significant role in the overall multiphoton-fragmentation. Furthermore, the data are indicative of the involvement of secondary photodissociation processes and provide information on fragment energetics as well as state interactions. The question, whether acetylene could be an important source of building block fragments for the formation of organic molecules in interstellar space, is addressed.

Coincidence ion pair production (cipp) spectroscopy of diiodine.

Coincidence ion pair production (I+ + I-) (cipp) spectra of I2 were recorded in a double imaging coincidence experiment in the one-photon excitation region of 71 600-74 000 cm-1. The I+ + I- coincidence signal shows vibrational band head structure corresponding to iodine molecule Rydberg states crossing over to ion-pair (I+I-) potential curves above the dissociation limit. The band origin (ν0), vibrational wavenumber (ωe) and anharmonicity constants (ωexe) were determined for the identified Rydberg states. The analysis revealed a number of previously unidentified states and a reassignment of others following a discrepancy in previous assignments. Since the ion pair production threshold is well established, the electric field-dependent spectral intensities were used to derive the cutoff energy in the transitions to the rotational levels of the 7pσ(1/2) (v' = 3) state.

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.

Resolving the F2 bond energy discrepancy using coincidence ion pair production (cipp) spectroscopy.

Coincidence ion pair production (cipp) spectra of F2 were recorded on the DELICIOUS III coincidence spectrometer in the one-photon excitation region of 125 975-126 210 cm-1. The F+ + F- signal shows a rotational band head structure, corresponding to F2 Rydberg states crossing over to the ion pair production surface. Spectral simulation and quantum defect analysis allowed the characterization of five new molecular Rydberg states (F2**): one Π and four Σ states. The lowest-energy Rydberg state spectrum observed (T0 = 125 999 cm-1) lacked some of the predicted rotational structure, which allowed an accurate determination of the ion pair production threshold of 15.62294± 0.00043 eV. Using the well-known atomic fluorine ionization energy and electron affinity, this number leads to a ground state F-F dissociation energy of 1.60129± 0.00044 eV. Photoelectron photoion coincidence (PEPICO) experiments were also carried out on F2 and the dissociative photoionization threshold to F+ + F was determined as 19.0242 ± 0.0006 eV. Using the atomic fluorine ionization energy, this can be converted to an F2 dissociation energy of 1.60132± 0.00062 eV, further confirming the cipp-derived value above. Because the two experiments were independently energy-calibrated, they can be averaged to 1.60130± 0.00036 eV and this value can be used to derive the fluorine atom's 0 K heat of formation as 77.251± 0.017 kJ mol-1. This latter is in excellent agreement with the latest Active Thermochemical Table (ATcT) value but improves its accuracy by almost a factor of three.

Formation of highly excited iodine atoms from multiphoton excitation of CH3I.

Mass resolved REMPI spectra, as well as CH3+and I+ ion and photoelectron images, were recorded for two-photon resonant excitations of CH3I via s, p and d Rydberg states (CH3I**) in the excitation region of 55 700 to 70 000 cm-1. Photoelectron (PE) and ion kinetic energy release spectra (KERs) were derived from the images. The data revealed that after the two-photon resonant excitation, an additional photon is absorbed to form one or more superexcited state(s) (CH3I#), followed by branching into three pathways. The major one is the dissociation of CH3I# to form excited Rydberg states of iodine atoms (I**) along with CH3(X), a phenomenon not commonly observed in methyl halides. The second (minor) pathway involves autoionization of CH3I# towards CH3I+(X), which absorbs another photon to form CH3+ along with I/I* and the third one (minor) is CH3I# dissociation towards the ion pair, CH3+ + I-, prior to I- electron ejection. Furthermore, one-photon non-resonant dissociation of CH3I to form CH3(X) and I/I* prior to three-photon ionization of the fragments is also detected.

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.

Multiphoton Rydberg and valence dynamics of CH3Br probed by mass spectrometry and slice imaging.

The multiphoton dynamics of CH3Br were probed by Mass Resolved MultiPhoton Ionization (MR-MPI), Slice Imaging and Photoelectron Imaging in the two-photon excitation region of 66 000 to 80 000 cm-1. Slice images of the CH3+ and Br+ photoproducts of ten two-photon resonant transitions to np and nd Rydberg states of the parent molecule were recorded. CH3+ ions dominate the mass spectra. Kinetic energy release spectra (KERs) were derived from slice and photoelectron images and anisotropy parameters were extracted from the angular distributions of the ions to identify the processes and the dynamics involved. At all wavelengths we observe three-photon excitations, via the two-photon resonant transitions to molecular Rydberg states, forming metastable, superexcited (CH3Br#) states which dissociate to form CH3 Rydberg states (CH3**) along with Br/Br*. A correlation between the parent Rydberg states excited and CH3** formed is evident. For the three highest excitation energies used, the CH3Br# metastable states also generate high kinetic energy fragments of CH3(X) and Br/Br*. In addition for two out of these three wavelengths we also measure one-photon photolysis of CH3Br in the A band forming CH3(X) in various vibrational modes and bromine atoms in the ground (Br) and spin-orbit excited (Br*) states.

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.

Dissociative Photoionization of 1-Halogenated Silacyclohexanes: Silicon Traps the Halogen.

The threshold photoelectron spectra and threshold photoionization mass spectra of 1-halogenated-1-silacyclohexanes, for the halogens X = F, Cl, Br, and I, have been obtained using synchrotron vacuum ultraviolet radiation and photoelectron photoion coincidence spectroscopy. As confirmed by a similar ionization onset and density functional theory molecular orbitals, the ionization to the ground state is dominated by electron removal from the silacyclohexane ring for X = F, Cl, and Br, and from the halogen lone pair for X = I. The breakdown diagrams show that the dissociative photoionization mechanism is also different for X = I. Whereas the parent ions decay by ethylene loss for X = F to Br in the low-energy regime, the iodine atom is lost for X = I. The first step is followed by a sequential ethylene loss at higher internal energies in each of the compounds. It is argued that the tendency of silicon to lower bond angles stabilizes the complex cation in which C2H4 is η2-coordinated to it, and which precedes ethylene loss. Together with the relatively strong silicon-halogen bonds and the increased inductive effect of the silacyclohexane ring in stabilizing the cation, this explains the main differences observed in the fragmentation of the halogenated silacyclohexane and halogenated cyclohexane ions. The breakdown diagrams have been modeled taking into account slow dissociations at threshold and the resulting kinetic shift. The 0 K appearance energies have been obtained to within 0.08 eV for the ethylene loss for X = F to Br (10.56, 10.51, and 10.51 eV, respectively), the iodine atom loss for X = I (10.11 eV), the sequential ethylene loss for X = F to I (12.29, 12.01, 11.94, and 11.86 eV, respectively), and the minor channels of H loss for X = F (10.56 eV) and propylene loss in X = Cl (also at 10.56 eV). The appearance energies for the major channels likely correspond to the dissociative photoionization reaction energy.

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.

Resonance-enhanced multiphoton ionization of CH2Br2: Rydberg states, photofragmentation, and CH spectra.

Mass-resolved (2 + n) resonance-enhanced multiphoton ionization (REMPI) spectra of CH2Br2 in the two-photon resonance excitation region from 71 200 to 82 300 cm(-1) were recorded and analyzed. Spectral structures allowed characterization of new molecular Rydberg states. C*((1)D2) was found to be an important intermediate in the photodissociation processes. A broad spectral feature peaking at about 80 663 cm(-1) in the C(+) spectrum and frequently seen in related studies is reinterpreted and associated with switching between three- and two-photon ionization of C*((1)D2). Analysis of band structures due to transitions from the A(2)Δ state of CH* that were seen in the CH(+) and C(+) REMPI spectra allowed characterization of three electronic states of CH, assigned as E(2)Π, D(2)Π, and F(2)Σ(+), which clarifies a long-term puzzle concerning the energetics of the CH radical. Predissociation of the E, D, and F states to form C*((1)D2) occurs. Bromine atomic lines were observed and are believed to be associated with bromine atom formation via predissociation of CH2Br2 Rydberg states.

Conformational Properties of 1-Halogenated-1-Silacyclohexanes, C5H10SiHX (X = Cl, Br, I): Gas Electron Diffraction, Low-Temperature NMR, Temperature-Dependent Raman Spectroscopy, and Quantum-Chemical Calculations.

The molecular structures of axial and equatorial conformers of cyclo-C5H10SiHX (X = Cl, Br, I) as well as the thermodynamic equilibrium between these species was investigated by means of gas electron diffraction, dynamic nuclear magnetic resonance, temperature-dependent Raman spectroscopy, and quantum-chemical calculations applying CCSD(T), MP2, and DFT methods. According to the experimental and calculated results, all three compounds exist as a mixture of two chair conformers of the six-membered ring. The two chair forms of Cs symmetry differ in the axial or equatorial position of the X atom. In all cases, the axial conformer is preferred over the equatorial one. When the experimental uncertainties are taken into account, all of the experimental and theoretical results for the conformational energy (Eaxial - Eequatorial) fit into a remarkably narrow range of -0.50 ± 0.15 kcal mol-1. It was found by NBO analysis that the axial conformers are unfavorable in terms of steric energy and conjugation effects and that they are stabilized mainly by electrostatic interactions. The conformational energies for C6H11X and cyclo-C5H10SiHX (X = F, Cl, Br, I, At) were compared using CCSD(T) calculations. In both series, fluorine is predicted to have a lower conformational preference (cyclohexane equatorial, silacyclohexane axial) than Cl, Br, and I. It is predicted that astatine would behave very similarly to Cl, Br, and I within each series.

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.