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.

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.

Dissociative Photoionization of Methyl Vinyl Ketone-Thermochemical Anchors and a Drifting Methyl Group.

The dissociative photoionization of methyl vinyl ketone (MVK), an important intermediate in the atmospheric oxidation of isoprene, has been studied by photoelectron photoion coincidence spectroscopy. In the photon energy range of 9.5-13.8 eV, four main fragment ions were detected at m/z 55, 43, 42, and 27 aside from the parent ion at m/z 70. The m/z 55 fragment ion (C2H3CO+) is formed from ionized MVK by direct methyl loss, while breaking the C-C bond on the other side of the carbonyl group results in the acetyl cation (CH3CO+, m/z 43) and the vinyl radical. The m/z 42 fragment ion is formed via a CO-loss from the molecular ion after a methyl shift. The lightest fragment ion, the vinyl cation (C2H3+ at m/z 27), is produced in two different reactions: acetyl radical loss from the molecular ion and CO-loss from C2H3CO+. Their contributions to the m/z 27 signal are quantified based on the acetyl and vinyl fragment thermochemical anchors and quantum chemical calculations. Based on the experimentally derived appearance energy of the m/z 43 fragment ion, a new, experimentally derived heat of formation is proposed herein for gaseous methyl vinyl ketone (ΔfH0K = -94.3 ± 4.8 kJ mol-1; ΔfH298K = -110.5 ± 4.8 kJ mol-1), together with cationic heats of formation and bond dissociation energies.

Dissociative photoionization of 1,3-dioxolane: We need six channels to fit the elephant.

The dissociative photoionization of 1,3-dioxolane was studied by photoelectron photoion coincidence (PEPICO) spectroscopy in the photon energy range of 9.5-13.5 eV. Our statistical thermodynamics model shows that a total of six dissociation channels are involved in the formation of three fragment ions, namely, C3 H5 O2 + (m/z 73), C2 H5 O+ (m/z 45), and C2 H4 O+ (m/z 44), with two channels contributing to the formation of each. By comparing the results of ab initio quantum chemical calculations to the experimentally derived appearance energies of the fragment ions, the most likely mechanisms for these unimolecular dissociation reactions are proposed, including a description of the relevant parts of the potential energy surface.

The Vagabond Fluorine Atom: Dissociative Photoionization of trans-1,3,3,3-Tetrafluoropropene.

The dissociative photoionization of trans-1,3,3,3-tetrafluoropropene (HFO-1234ze) was investigated by imaging photoelectron photoion coincidence (PEPICO) spectroscopy. From the threshold photoelectron spectrum (TPES), an adiabatic ionization energy of 10.91 ± 0.05 eV is determined and reported for the first time. Over a 4 eV wide range, internal-energy selected trans-1,3,3,3-tetrafluoropropene cations decay by three parallel dissociative photoionization channels, which were modeled using statistical theory. The 0 K appearance energies of CF2CHCF2 (H-loss, m/z 113), CFHCHCF2 (F-loss, m/z 95), and CH2═CF2 (CF2-loss, m/z 64) fragment ions were determined to be 12.247 ± 0.030, 12.66 ± 0.10, and 12.80 ± 0.05 eV, respectively. From the last, the heat of formation of neutral trans-1,3,3,3-tetrafluoropropene was determined to be -779.9 ± 9.7 kJ/mol. While the lowest-energy fluorine loss occurs directly, the first H-loss and CF2-loss channels involve both a fluorine- and a hydrogen-migration prior to dissociation. At higher internal energies, several other rearrangement pathways open up, which involve fluorine and hydrogen transfer and, through fluorine loss, lead to the formation of several additional isomeric allylic fragment ions.