High-resolution Threshold Photoelectron Spectrum Research Articles

Photoionization of Nitrile-substituted Naphthalene and Benzene: Cation Spectroscopy, Photostability, and Implications for Photoelectric Gas Heating

We investigate the vacuum ultraviolet (VUV) photodynamics of gas phase 1- and 2-cyanonaphthalene and cyanobenzene, recently detected in the Taurus molecular cloud, by combining synchrotron radiation and a double imaging electron/ion coincidence setup. The high-resolution threshold photoelectron spectra (TPES) of all three molecules are obtained experimentally from which the adiabatic ionization energies are reported with very high accuracy, particularly for 2-cyanonaphthalene, for which no data exist at this level of precision. Theoretical calculations are performed to compare with the TPES for the ground electronic state of the cations. Furthermore, the different features observed in the extended TPES have been assigned to the different molecular orbitals with the help of the outer valence Green's function calculations. The present experiments also shed light on the kinetic energy distribution of the photoelectrons as a function of the incident photon energy, to describe their contribution to the photoelectric heating effect in the interstellar medium. In this context, we show how kinetic energy distributions can be obtained from our data for any given photon energy, such as the omnipresent Lyα line, or any given interstellar radiation field (ISRF). In addition, from the total ion yields, we estimate the photorates for a few ISRFs. Finally, we discuss the photodissociation of the two cyanonaphthalenes, quoting the activation energies of the dissociation channels with the help of Rice–Ramsperger–Kassel–Marcus modeling. It is observed that CN substitution does not cause any appreciable change to the VUV dissociative photoionization relaxation channel.

High resolution threshold photoelectron spectrum and autoionization processes of S2 up to 15.0 eV

VUV photoionization dynamics of the S2 molecule were re-investigated from threshold up to 15.0 eV, using synchrotron radiation coupled with double imaging photoelectron/photoion coincidence featuring high resolution capabilities. We measured the first threshold photoelectron spectrum of S2 achieving higher resolution than previous literature to derive accurate spectroscopic constants for a few electronic states of the cation including the X2ΠΩ,g ground state and the a4Πu, b4Σg–, and B2Σg– states. We also recorded the total ion yield for S2 up to a photon energy of 15.0 eV which, combined with the threshold photoelectron spectrum, led to the assignment of various autoionizing Rydberg series.

Threshold photoelectron spectroscopy of the HO2 radical.

We report a synchrotron radiation vacuum ultraviolet photoionization study of the hydroperoxyl radical (HO2), a key reaction intermediate in combustion and atmospheric chemistry as well as astrochemistry, using double imaging photoelectron photoion coincidence spectroscopy. The HO2 radical is formed in a microwave discharge flow tube reactor through a set of reactions initiated by F atoms in a CH4/O2/He gas mixture. The high-resolution threshold photoelectron spectrum of HO2 in the 11 eV-12 eV energy range is acquired without interferences from other species and assigned with the aid of theoretically calculated adiabatic ionization energies (AIEs) and Franck-Condon factors. The three vibrational modes of the radical cation HO2 +, the H-O stretch, the H-O-O bend, and the O-O stretch, have been identified, and their individual frequencies are measured. In addition, the AIEs of the X3A″ ground state and the a1A' first excited electronic state of HO2 + are experimentally determined at 11.359 ± 0.003 eV and 11.639 ± 0.005 eV, respectively, in agreement with high-level theoretically computed results. Furthermore, the former AIE value provides validation of thermochemical networks used to extract the enthalpy of formation of the HO2 radical.

The Threshold Photoelectron Spectrum of Fulvenone: A Reactive Ketene Derivative in Lignin Valorization.

Unveiling reaction mechanisms by isomer-selective detection of reactive intermediates requires advanced spectroscopic knowledge. We study the photoionization of fulvenone (c-C5 H4 =C=O), a reactive ketene species relevant in catalytic pyrolysis of lignin, which was generated by pyrolysis of 2-methoxy acetophenone. The high-resolution threshold photoelectron spectrum (TPES) with vacuum ultraviolet synchrotron radiation revealed well-resolved vibrational transitions, assigned to ring deformation modes of the cyclopentadiene moiety. The adiabatic ionization energy was determined to be 8.25±0.01 eV and is assigned to the 2 A2 ← 1 A1 transition. A broad and featureless band arising at 9 eV is associated with the 2 B1 ← 1 A1 excitation. A conical intersection is responsible for the ultrafast relaxation of the fulvenone cation from the into the state resulting in a featureless and lifetime broadened band. These insights will increase the detection capabilities for fulvenone and thereby help to elucidate reaction mechanisms in lignin catalytic pyrolysis.

Threshold photoelectron spectroscopy of the methoxy radical.

We present here a synchrotron radiation vacuum ultraviolet photoionization study of the simplest alkoxy radical, CH3O, a key reaction intermediate in atmospheric and combustion chemistry. A microwave discharge fast flow tube connected to a molecular beam sampling system is employed as a chemical reactor to initiate reactions and generate radicals. The CH3O+ cation from direct ionization of the CH3O radical is detected successfully in the photoionization mass spectrum close to its ionization threshold. In addition, after identifying and removing the contribution of the 13C-isotopic formaldehyde H2 13CO with the same isobaric mass m/z = 31, the high-resolution threshold photoelectron spectrum of CH3O is obtained and assigned with the aid of calculated Franck-Condon factors. The adiabatic ionization energy of CH3O is determined at 10.701 eV with an accuracy of 0.005 eV.

Dissociative photoionization of quinoline and isoquinoline.

Two nitrogen-containing polycyclic aromatic hydrocarbon isomers of C9H7N composition, quinoline, and isoquinoline have been studied by imaging photoelectron photoion coincidence spectroscopy at the VUV beamline of the Swiss Light Source. High resolution threshold photoelectron spectra have been recorded and are interpreted applying a Franck-Condon model. Dissociative ionization mass spectra as a function of the parent ion internal energy are analyzed with the use of breakdown diagrams. HCN loss and H loss are the dominant dissociation paths for both C9H7N(•+) isomers at photon energies below 15.5 eV. Computed C9H7N(•+) potential energy surfaces suggest that the lowest energy path leading to HCN-loss yields the benzocyclobutadiene cation. A statistical model is used to fit the breakdown diagram and-to account for the kinetic shift-the time-of-flight mass spectra that reveal the dissociation rates. We have derived appearance energies of 11.9 ± 0.1 (HCN loss) and 12.0 ± 0.1 (H loss), as well as 11.6 ± 0.2 (HCN loss) and 12.1 ± 0.2 (H loss) eV, for the dissociative ionization of quinoline and isoquinoline, respectively. The results are compared to a recent study on the dissociative ionization of naphthalene. Implications for the formation and destruction of nitrogenated PAHs in the interstellar medium and in Titan's atmosphere are highlighted.

Satellite structures of Kr and Xe studied by high-resolution threshold photoelectron spectroscopy

A high-resolution threshold photoelectron spectrum of Xe was measured in the photon energy region of the 5s25p4nl states. The peak width of the measured Xe 5s−1 main line was close to 5 meV. A previously measured spectrum of Kr was also analysed. Rich satellite peaks were clearly observed, including many peaks not observed in previous optical, photoelectron and threshold photoelectron studies. The satellite peak energies and their assignments for Kr and Xe are listed. The observed Rydberg series converging to the Xe2+ ground state was assigned to the (3P2)ng series by detailed analysis. The energy level of the Xe2+ 3P2 state as the converging limit of the observed Rydberg series was obtained as 38.355 eV.

Selected ion flow tube cation–molecule reaction studies and threshold photoelectron photoion coincidence spectroscopy of cyclic-C5F8

The product ion branching ratios and rate coefficients have been measured using a selected ion flow tube (SIFT) at 298 K for the bimolecular reactions of cyclic-C5F8 with several atomic and molecular cations. The majority of reactions occur at the collisional rate calculated by the modified average dipole orientation theory, with the exception of H2O+ for which the reaction efficiency is only 55%. Apart from H2O+ and N+, the similarity of the product ion branching ratios determined from threshold photoelectron photoion coincidence (TPEPICO) and ion-molecule data suggests that long-range electron transfer is the dominant mechanism for reactions involving ions with recombination energies between 12 and 17 eV. For N+, the product ion branching ratios are very different to those produced by photoionisation; this result may be explained if some of the N-atom products are formed electronically excited. The onset of an ionisation signal of c-C5F8 measured by TPEPICO spectroscopy occurs at 12.25 +/- 0.05 eV. This is much higher than the value of the first adiabatic ionisation energy determined from electron ionisation (11.24 +/- 0.10 eV), He (I) photoionisation (11.30 +/- 0.05 eV), and an independent high resolution threshold photoelectron spectrum (11.237 +/- 0.002 eV). The ground electronic state of c-C5F8+ has very weak intensity under threshold electron conditions. The TPEPICO spectrum of c-C5F8 recorded from 12-23 eV shows detection of the parent ion and the daughter ions C4F6+ and C5F7+, with their appearance energies increasing in this order. Ion yield curves and branching ratios have been determined. Using Gaussian 03, the enthalpy of formation of c-C5F8 at 298 K has been determined to be -1495 kJ mol(-1).

High-resolution threshold photoelectron study of the Kr 4s satellite states

High-resolution threshold photoelectron spectra of Kr were measured in the photon energy region between the Kr + 4s −1 main line and the Kr 2+ 1D 2 state. The measured peak width was close to 7 meV. Rich satellite structures, 178 peaks including 48 newly observed peaks, were found in these spectra. Assignments of these levels were performed by comparison with previous optical, photoelectron, and threshold photoelectron measurements. The Rydberg series converging to the Kr 2+ 3P 2 states, which had not been observed, as well as three Rydberg series converging to the Kr 2+ 1D 2 states, some levels of which had been observed, were observed. The energy levels of the Kr 2+ 3P 2 and 1D 2 states as converging limits of the observed Rydberg series were obtained as 38.355 and 40.180 eV, respectively.

High-resolution threshold photoelectron spectra of molecular oxygen in the 18–24 eV region

A high-resolution threshold photoelectron spectrum of molecular oxygen has been studied between 18 and 24 eV using the penetrating field technique and a synchrotron radiation source. Nine ionic states are observed, and three of them for the first time. Higher vibrational levels of the b 4Σg− state and the 2Φu state are also studied. Furthermore, the doublet structure of the 2Φu state is resolved for the first time. Two new states with quartet multiplicity are assigned to the 4Πg and the 4Πu states which dissociate to the limit, [O(3P)+O+(2D)], at 22.059 eV. Also we tentatively assign another newly observed band to the 2Πu state. The long vibrational progression of the 3 2Πu state is clearly observed in the region 22.3–23.8 eV with doublet splitting which increases with increasing of the vibrational quantum number.

High resolution threshold photoelectron spectrum of oxygen in the 12–19 eV region

The threshold photoelectron spectrum of oxygen has been measured with high resolution in the photon energy range of 12–19 eV using a penetrating field technique and a synchrotron radiation source. The long vibrational progression of the X2Πg state of O2+ was observed to v′=45. An extrapolation in a Birge–Sponer plot (ΔG versus v′+1/2) gives a maximum vibrational level vmax′=56. The A2Πu state was also measured in detail. Most vibrational levels of the A2Πu state of O2+ (from v′=0 to 24) were observed for the first time. A Birge–Sponer plot of the A2Πu state gives a maximum vibrational level vmax′=31. The dissociation limit of the A2Πu state is slightly higher (76 meV) than the well-known dissociation limit O(3P2)+O+(3S0) 18.733 eV. Therefore the A2Πu state may be considered to have a hump. We observed new states which were assigned to Σg+4 and Σu+2 according to a previous theoretical investigation [Beebe et al., J. Chem. Phys. 64, 2080 (1976)]. Potential curves of the X2Πg and A2Πu states were calculated by the Rydberg–Klein–Rees method using both the rotational constants obtained previously by others [K. P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure, Constants of Diatomic Molecules Volume IV (Van Nostrand, New York, 1974)] and the vibrational energy values observed in the present experiment. A Le Roy–Bernstein plot as well as a Birge–Sponer plot were used to determine the dissociation energy of the X2Πg state. However, the plot gave an unreasonably large dissociation limit.

Vibrational structure of the N+2 ground state observed by threshold photoelectron spectroscopy

The long vibrational progression of the ground state of N+2 was observed in a high resolution threshold photoelectron spectrum obtained using the penetrating field technique and synchrotron radiation. Vibrational states were observed up to v′=67 for the first time and the final vibrational level was deduced to be v′=77 from extrapolation. The complete molecular constants of this state were obtained and the entire potential curve was then drawn.

Threshold photoelectron spectrum of O 2 in 31–34 eV region

High resolution threshold photoelectron spectra of O 2 were obtained in the region of 31–34 eV. Three vibrational progressions were observed. These structures are assigned to two ionic Rydberg states converging to the O 2 2+ ground state and to a valence state.

Threshold photoelectron spectroscopy of HF

The high-resolution threshold photoelectron spectrum of the HF molecule has been measured using pulsed field ionization after single-photon excitation by coherent vacuum ultra-violet radiation. The adiabatic ionization potential for HF was found to be 129422.4±1 cm −1. In contrast to HCl, only small changes in total angular momentum of the ion core are observed. The experimental results show a preference for the F 1 spin—orbit component for both vibrational states of the ion, whereas the vibrational branching ratio was found to be similar to the relative intensities obtained from the Franck-Condon principle.

High-resolution threshold photoelectron spectrum of molecular oxygen

The threshold TPE photoelectron spectrum of molecular oxygen has been reinvestigated using a partially cooled effusive molecular jet and the monochromatised VUV synchrotron radiation from super-ACO in Orsay. Three vibrational progressions are identified. One of them corresponds to and confirms that observed recently by Baltzer et al. (Phys. Rev. A 45 (1992) 4374), the two other ones are observed for the first time in TPE spectroscopy. Possible assignments for these progressions are discussed. The vibrational progression in the X 2Π g state has been seen up to v +=26 and the two spin-orbit components 2Π 1 2 and 2Π 3 2 ) are for the first time fully resolved in TPES. High-lying vibrational levels with v + >20 of the X state are seen to overlap with the a state levels. The relative contribution of both states is obtained through a deconvolution procedure. The vibrational progression in the b 4Σ − g is extended to v +=18. The threshold photoelectron spectrum around 20 eV shows a particularly high density of lines. Possible assignments of these lines to new progressions are discussed with the help of a series of time-of-flight photoelectron spectra (TOF-PES) measured at a series of excitation energies between 18 and 20 eV.

High resolution threshold photoelectron spectra of neon and argon in the valence satellite region

Using a newly designed zerovolt-electron spectrometer, high resolution threshold photoelectron spectra of neon above the 2s and of argon above the 3s photoelectron thresholds were taken following excitation with synchrotron radiation. From these spectra we were able to derive quantitatively the multiplet splitting of two neon satellites and almost all argon photoelectron satellites up to 38eV. Strong deviations from statistical behavior of most multiplet components were found. This unusual behavior is explained by autoionization of double-excited neutral states into the continuum of the satellite states just above threshold.

High-resolution threshold photoionization of N2O

Pulsed field ionization (PFI) has been used in conjunction with a coherent vuv source to obtain high-resolution threshold photoelectron spectra for the (000), (010), (020), and (100) vibrational states of the N2O+ cation. Simulations for the rotational profiles of each vibronic level were obtained by fitting the Buckingham–Orr–Sichel equations [A. D. Buckingham, B. J. Orr, and J. M. Sichel, Philos. Trans. R. Soc. London, Ser. A 268, 147 (1970)] using accurate spectroscopic constants for the ground states of the neutral and the ion. The relative branch intensities are interpreted in terms of the partial waves of the outgoing photoelectron to which the ionic core is coupled and in terms of the angular momentum transferred to the core. The PFI technique also allows us to report an improved value for the ionization potential of N2O of 103 963±5 cm−1.