^{1}, U. Jacovella

^{1}, B. Ruscic

^{1}, S. T. Pratt

^{1,a)}and R. R. Lucchese

^{2}

### Abstract

Photoelectron velocity map imaging is combined with one- and two-photonionization 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.

We would like to thank David Osborn of the Combustion Research Facility of Sandia National Laboratory for pointing out the large threshold photoionization cross section of 2-butyne. This work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under Contract Nos. DE-AC02-06CH11357 and DE-FG02-01ER15178. R.R.L. acknowledges the support of the Robert A. Welch Foundation under Grant No. A-1020. This work was also supported by the Texas A&M University Supercomputing Facility.

I. INTRODUCTION

II. METHODS

A. Experiment

B. Theory

III. RESULTS

A. Single-photon ionization

B. Two-photonionization

C. The 2-butyne dimer

IV. DISCUSSION

V. CONCLUSIONS

### Key Topics

- Photoionization
- 34.0
- Ionization
- 29.0
- Multiphoton processes
- 16.0
- Photoelectron spectra
- 16.0
- Atomic and molecular beams
- 13.0

## Figures

The experimental photoabsorption spectrum and photoionization spectrum of 2-butyne from Refs. 25 and 6, respectively. Also shown is the theoretical photoionization spectrum from the present work.

The experimental photoabsorption spectrum and photoionization spectrum of 2-butyne from Refs. 25 and 6, respectively. Also shown is the theoretical photoionization spectrum from the present work.

(a) The reconstructed photoelectron image following single-photon ionization of 2-butyne at 10.320 eV. (b) The angle-integrated photoelectron kinetic energy spectrum extracted from the reconstructed image.

(a) The reconstructed photoelectron image following single-photon ionization of 2-butyne at 10.320 eV. (b) The angle-integrated photoelectron kinetic energy spectrum extracted from the reconstructed image.

The angle-integrated photoelectron kinetic energy spectra extracted from the photoelectron images recorded following single-photon ionization at several different photon energies. (a) 9.739 eV, (b) 9.835 eV, (c) 9.933 eV, and (d) 10.347 eV. For these spectra, the electron energy and ionization potential have been subtracted from the photon energy, and the x axis gives the internal energy of the photoion.

The angle-integrated photoelectron kinetic energy spectra extracted from the photoelectron images recorded following single-photon ionization at several different photon energies. (a) 9.739 eV, (b) 9.835 eV, (c) 9.933 eV, and (d) 10.347 eV. For these spectra, the electron energy and ionization potential have been subtracted from the photon energy, and the x axis gives the internal energy of the photoion.

The experimental and theoretical β_{2} values for the near-threshold, single-photon ionization of 2-butyne. The experimental β_{2} values are plotted separately for ionic final states with v_{2} ^{+} = 0 and 1. The error bars for the v_{2} ^{+} = 0 and 1 values are smaller than the symbol diameters; however, these error bars only represent the counting statistics and do not include systematic effects. The theoretical curve has been displaced to lower energy by 0.411 eV, which corresponds to the shift between the experimental and theoretical resonance maxima in the cross section data of Figure 1.

The experimental and theoretical β_{2} values for the near-threshold, single-photon ionization of 2-butyne. The experimental β_{2} values are plotted separately for ionic final states with v_{2} ^{+} = 0 and 1. The error bars for the v_{2} ^{+} = 0 and 1 values are smaller than the symbol diameters; however, these error bars only represent the counting statistics and do not include systematic effects. The theoretical curve has been displaced to lower energy by 0.411 eV, which corresponds to the shift between the experimental and theoretical resonance maxima in the cross section data of Figure 1.

(a) The reconstructed photoelectron image following two-photon ionization with a photon energy of 4.967 eV (the two-photon energy is 9.933 eV). (b) The corresponding angle-integrated photoelectron kinetic energy distribution.

(a) The reconstructed photoelectron image following two-photon ionization with a photon energy of 4.967 eV (the two-photon energy is 9.933 eV). (b) The corresponding angle-integrated photoelectron kinetic energy distribution.

(a) The reconstructed photoelectron image following two-photon ionization with a photon energy of 4.870 eV (the two-photon energy is 9.739 eV). (b) The corresponding angle-integrated photoelectron kinetic energy distribution.

(a) The reconstructed photoelectron image following two-photon ionization with a photon energy of 4.870 eV (the two-photon energy is 9.739 eV). (b) The corresponding angle-integrated photoelectron kinetic energy distribution.

Representations of the highest occupied and lowest unoccupied orbitals (HOMO and LUMO, respectively) for acetylene, propyne, 1-butyne, and 2-butyne. For 1-butyne, the higher energy orbital plotted is actually the LUMO+1.

Representations of the highest occupied and lowest unoccupied orbitals (HOMO and LUMO, respectively) for acetylene, propyne, 1-butyne, and 2-butyne. For 1-butyne, the higher energy orbital plotted is actually the LUMO+1.

Plots of the resonant wave function for a photoelectron of e symmetry for ionization from the HOMO of 2-butyne that occurs in the model calculations at a photon energy of 10.2 eV and with a width of 1.9 eV.

Plots of the resonant wave function for a photoelectron of e symmetry for ionization from the HOMO of 2-butyne that occurs in the model calculations at a photon energy of 10.2 eV and with a width of 1.9 eV.

## Tables

Vibrational Frequencies for the 2-Butyne Ground State Neutral and Ion.

Vibrational Frequencies for the 2-Butyne Ground State Neutral and Ion.

Observed vibrational frequencies for the 2-butyne Ion.

Observed vibrational frequencies for the 2-butyne Ion.

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