Valence-Shell Photoionization of C4H5: The 2-Butyn-1-yl Radical.

We present new high-resolution data on the photoionization of the 2-butyn-1-yl radical (CH3C≡C-•CH2) formed by H atom abstraction from 2-butyne by F atoms. The spectra were recorded from 7.7 to 11 eV by using double-imaging, photoelectron-photoion coincidence spectroscopy, which allows the unambiguous correlation of photoelectron data and the mass of the species. The photoionization spectrum shows significant resonant autoionizing structure converging to excited states of the C4H5+ cation, similar to what is observed in the closely related propargyl radical (HC≡C-•CH2). The threshold photoelectron spectrum, obtained with a resolution of 17 meV, is also reported. This spectrum is consistent with previous measurements of the first photoionization band but has been extended to higher energy to allow the observation of bands corresponding to excited electronic states of the ion. A refined value of the adiabatic ionization energy is extracted: IE(C4H5) = 7.93 ± 0.01 eV. A determination of the absolute photoionization cross section of the 2-butyn-1-yl radical at 9.7 eV is also reported: σion(C4H5) = 6.1 ± 1.8 Mb.

Structure and dynamics of the radical cation of ethane arising from the Jahn-Teller and pseudo-Jahn-Teller effects.

The pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of C2H6 has been recorded in the region of the adiabatic ionization threshold. The partially rotationally resolved spectrum indicates the existence of several vibronic states of C2H6+ with less than 600 cm-1 of internal excitation. The analysis of the rotational structures assisted by ab initio calculations enabled the determination of the adiabatic ionization energy of C2H6 and the investigation of the structure and dynamics of C2H6+ at low energies. The ground state of C2H6+ is found to be a 2Ag state of diborane-like structure with strongly mixed (a1g)-1 and (eg)-1 configurations. The vibrational structure reveals the importance of large-amplitude nuclear motions involving the diborane distortion modes, the C-C stretching motion, and the internal rotation at elongated C-C distances. The spectrum is analyzed in the light of the information obtained in earlier studies of C2H6+ by ab initio quantum chemistry, EPR spectroscopy and photoelectron spectroscopy.

Photoelectron angular distributions from rotationally resolved autoionizing states of N2.

The single-photon, photoelectron-photoion coincidence spectrum of N2 has been recorded at high (∼1.5 cm-1) resolution in the region between the N2+ X Σg2+, v+ = 0 and 1 ionization thresholds by using a double-imaging spectrometer and intense vacuum-ultraviolet light from the Synchrotron SOLEIL. This approach provides the relative photoionization cross section, the photoelectron energy distribution, and the photoelectron angular distribution as a function of photon energy. The region of interest contains autoionizing valence states, vibrationally autoionizing Rydberg states converging to vibrationally excited levels of the N2+ X Σg2+ ground state, and electronically autoionizing states converging to the N2+A2Π and B 2Σu+ states. The wavelength resolution is sufficient to resolve rotational structure in the autoionizing states, but the electron energy resolution is insufficient to resolve rotational structure in the photoion spectrum. A simplified approach based on multichannel quantum defect theory is used to predict the photoelectron angular distribution parameters, ß, and the results are in reasonably good agreement with experiment.

Spin-orbit interaction and Renner-Teller effect in HCCCCH+ studied by high-resolution photoelectron spectroscopy.

The photoelectron spectra of the X+ 2Πg ← X 1Σ photoionizing transition of diacetylene (HCCCCH) and d2-diacetylene (DCCCCD) have been recorded at high resolution using the technique of pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy. The partially resolved rotational structure of the origin band of the spectra of HCCCCH and DCCCCD has enabled the determination of the adiabatic ionization energies of HCCCCH [Ead = 82072.2(5) cm-1] and DCCCCD [Ead = 82090.0(10) cm-1] and of the spin-orbit coupling constant [A = -31.1(4) cm-1] of the ground vibronic state of HCCCCH+, which is smaller than the value of -33.3 cm-1 commonly used since the work of Callomon (J. H. Callomon, Can. J. Phys., 1956, 34, 1046). Several excited vibrational levels of HCCCCH+ and DCCCCD+, including some affected by the Renner-Teller effect and Fermi interactions, have been observed and the fundamental wavenumber of the mode ν9 has been determined in both HCCCCH+ (200.0(10) cm-1) and DCCCCD+ (192.6(20) cm-1). Possible assignments for several of these levels are discussed and deficiencies in the current understanding of the energy-level structure of the radical cation of diacetylene are pointed at.

Infrared spectroscopy of molecular ions in selected rotational and spin-orbit states.

First results are presented obtained with an experimental setup developed to record IR spectra of rotationally state-selected ions. The method we use is a state-selective version of a method developed by Schlemmer et al. [Int. J. Mass Spectrom. 185, 589 (1999); J. Chem. Phys. 117, 2068 (2002)] to record IR spectra of ions. Ions are produced in specific rotational levels using mass-analyzed-threshold-ionization spectroscopy. The state-selected ions generated by pulsed-field ionization of Rydberg states of high principal quantum number (n ≈ 200) are extracted toward an octupole ion guide containing a neutral target gas. Prior to entering the octupole, the ions are excited by an IR laser. The target gas is chosen so that only excited ions react to form product ions. These product ions are detected mass selectively as a function of the IR laser wavenumber. To illustrate this method, we present IR spectra of C2H2 (+) in selected rotational levels of the (2)Πu,3/2 and (2)Πu,1/2 spin-orbit components of the vibronic ground state.

A Near-Threshold Shape Resonance in the Valence-Shell Photoabsorption of Linear Alkynes.

The room-temperature photoabsorption spectra of a number of linear alkynes with internal triple bonds (e.g., 2-butyne, 2-pentyne, and 2- and 3-hexyne) show similar resonances just above the lowest ionization threshold of the neutral molecules. These features result in a substantial enhancement of the photoabsorption cross sections relative to the cross sections of alkynes with terminal triple bonds (e.g., propyne, 1-butyne, 1-pentyne, ...). Based on earlier work on 2-butyne [ Xu et al., J. Chem. Phys. 2012, 136, 154303 ], these features are assigned to excitation from the neutral highest occupied molecular orbital (HOMO) to a shape resonance with g (l = 4) character and approximate π symmetry. This generic behavior results from the similarity of the HOMOs in all internal alkynes, as well as the similarity of the corresponding gπ virtual orbital in the continuum. Theoretical calculations of the absorption spectrum above the ionization threshold for the 2- and 3-alkynes show the presence of a shape resonance when the coupling between the two degenerate or nearly degenerate π channels is included, with a dominant contribution from l = 4. These calculations thus confirm the qualitative arguments for the importance of the l = 4 continuum near threshold for internal alkynes, which should also apply to other linear internal alkynes and alkynyl radicals. The 1-alkynes do not have such high partial waves present in the shape resonance. The lower l partial waves in these systems are consistent with the broader features observed in the corresponding spectra.

High-resolution vacuum-ultraviolet photoabsorption spectra of 1-butyne and 2-butyne.

The absolute photoabsorption cross sections of 1- and 2-butyne have been recorded at high resolution by using the vacuum-ultraviolet Fourier-Transform spectrometer at the SOLEIL Synchrotron. Both spectra show more resolved structure than previously observed, especially in the case of 2-butyne. In this work, we assess the potential importance of Rydberg states with higher values of orbital angular momentum, l, than are typically observed in photoabsorption experiments from ground state molecules. We show how the character of the highest occupied molecular orbitals in 1- and 2-butyne suggests the potential importance of transitions to such high-l (l = 3 and 4) Rydberg states. Furthermore, we use theoretical calculations of the partial wave composition of the absorption cross section just above the ionization threshold and the principle of continuity of oscillator strength through an ionization threshold to support this conclusion. The new absolute photoabsorption cross sections are discussed in light of these arguments, and the results are consistent with the expectations. This type of argument should be valuable for assessing the potential importance of different Rydberg series when sufficiently accurate direct quantum chemical calculations are difficult, for example, in the n ≥ 5 manifolds of excited states of larger molecules.

High-resolution photoabsorption spectrum of jet-cooled propyne.

The absolute photoabsorption cross section of propyne was recorded between 62,000 and 88,000 cm(-1) by using the vacuum-ultraviolet, Fourier-transform spectrometer at the Synchrotron Soleil. This cross section spans the region including the lowest Rydberg bands and extends above the Franck-Condon envelope for ionization to the ground electronic state of the propyne cation, X̃(+). Room-temperature spectra were recorded in a flowing cell at 0.9 cm(-1) resolution, and jet-cooled spectra were recorded at 1.8 cm(-1) resolution and a rotational temperature of ~100 K. The reduced widths of the rotational band envelopes in the latter spectra reveal new structure and simplify a number of assignments. Although nf Rydberg series have not been assigned previously in the photoabsorption spectrum of propyne, arguments are presented for their potential importance, and the assignment of one nf series is proposed. As expected from previous photoelectron spectra, Rydberg series are also observed above the adiabatic ionization threshold that converge to the v3(+) = 1 and 2 levels of the C≡C stretching vibration.

On the adiabatic ionization energy of the propargyl radical.

The photoionization and pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the propargyl radical have been recorded in the vicinity of the origin of the X(+) (1)A1←X(2)B1 photoionizing transition. An internally cold sample of propargyl with a rotational temperature of ~45 K was produced in a supersonic expansion of 1,3-butadiene in helium. Propargyl was generated by excimer laser (ArF, 193 nm) photolysis of 1,3-butadiene in a quartz capillary mounted at the exit of a pulsed valve. The rotational structure of the origin band of the photoelectron spectrum was partially resolved and an improved value of the adiabatic ionization energy of propargyl (E(I)/hc = 70174.5(20) cm(-1)) was determined.

Near-threshold shape resonance in the photoionization of 2-butyne.

Photoelectron velocity map imaging is combined with one- and two-photon ionization to study the near threshold photoionization of the 2-butyne molecule. In this region, the photoabsorption and photoionization cross sections display a very intense broad feature that is assigned to an l = 4, π(g) shape resonance. The effect of this shape resonance on the vibrational branching ratios and photoelectron angular distributions is explored. Theoretical calculations of the photoionization cross section and photoelectron angular distributions are in good agreement with the experiments. The results for 2-butyne are compared with those of acetylene, propyne, and 1-butyne, none of which show such significant enhancements near threshold, and the differences are rationalized in terms of the symmetries and orbital angular momenta of the highest occupied orbitals and the corresponding shape resonances. Expectations for larger alkynes and alkynyl radicals are also discussed. A preliminary measurement of the ionization energy of the 2-butyne dimer is also presented.