_{2}C=C=C:) obtained by negative ion photoelectron spectroscopy

^{1}, Etienne Garand

^{2}, Jongjin Kim

^{2}, Tara I. Yacovitch

^{2}, Christian Hock

^{2}, Amanda S. Case

^{3}, Elisa M. Miller

^{3}, Yu-Ju Lu

^{3}, Kristen M. Vogelhuber

^{3}, Scott W. Wren

^{3}, Takatoshi Ichino

^{1}, John P. Maier

^{4}, Robert J. McMahon

^{5}, David L. Osborn

^{6}, Daniel M. Neumark

^{2}and W. Carl Lineberger

^{3}

### Abstract

A joint experimental-theoretical study has been carried out on electronic states of propadienylidene (H_{2}CCC), using results from negative-ion photoelectron spectroscopy. In addition to the previously characterized electronic state, spectroscopic features are observed that belong to five additional states: the low-lying and states, as well as two excited singlets, and , and a higher-lying triplet, . Term energies (*T* _{0}, in cm^{−1}) for the excited states obtained from the data are: 10 354±11 (); 11 950±30 (); 20 943±11 (); and 13 677±11 (). Strong vibronic coupling affects the and states as well as and and has profound effects on the spectrum. As a result, only a weak, broadened band is observed in the energy region where the origin of the state is expected. The assignments here are supported by high-level coupled-cluster calculations and spectral simulations based on a vibronic coupling Hamiltonian. A result of astrophysical interest is that the present study supports the idea that a broad absorption band found at 5450 Å by cavity ringdown spectroscopy (and coincident with a diffuse interstellar band) is carried by the state of H_{2}CCC.

This work was supported by the U.S. National Science Foundation (NSF) (Grant Nos. PHY1125844 and CHE0809391 to W.C.L.; CHE1012743 to J.F.S.; CHE1011959 to R.J.M.); the U.S. Air Force Office of Scientific Research (Grant No. F49620-03-1-0085 to D.M.N. and Grant No. FA9550-09-1-0046 to W.C.L.); the U.S. Department of Energy Basic Energy Sciences Division (Grant No. DE-FG02-07ER15884 to J.F.S.) and the Robert A. Welch Foundation (Grant No. F-1283 to J.F.S.). J.P.M. and D.L.O. are grateful for JILA Fellowships during 2011. D.L.O. is supported by the Division of Chemical Sciences, Geosciences, and Biosciences, the Office of Basic Energy Sciences, the U.S. Department of Energy. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the National Nuclear Security Administration. We would also like to thank M. C. McCarthy (Harvard) and Tim Schmidt (Sydney) for discussions and their interest in this work.

I. INTRODUCTION

II. EXPERIMENTAL DETAILS

A. Negative ion photoelectron spectroscopy

B. Photoelectron velocity-map imaging

C. Slow electron velocity-map imaging

III. COMPUTATIONAL ASPECTS

IV. RESULTS AND DISCUSSION

A. Overview

B. The electronic state

C. Triplet states

1. and states

2. The state

D. The and singlet states

V. ASTROPHYSICAL RELEVANCE OF EXCITED SINGLET STATES

VI. SUMMARY

### Key Topics

- Absorption spectra
- 19.0
- Absorption spectroscopy
- 9.0
- Photoelectron spectra
- 9.0
- Photoelectron spectroscopy
- 9.0
- Excited states
- 8.0

## Figures

Composite NIPES, VMI, and SEVI spectra over the range of energies studied in this work. The color scheme, which is used throughout this paper, is: NIPES in black, VMI in dark blue, and SEVI in cyan. The state label does not denote the origin transition for this state.

Composite NIPES, VMI, and SEVI spectra over the range of energies studied in this work. The color scheme, which is used throughout this paper, is: NIPES in black, VMI in dark blue, and SEVI in cyan. The state label does not denote the origin transition for this state.

Frontier molecular orbitals of propadienylidene anion. These calculations used the ANO1 basis (see text) and were carried out using restricted open-shell Hartree-Fock theory at the anion geometry. The view is from 45° above the molecular plane, with the methylene group closest and the linear CCC framework extending away from the eye, and is shown schematically in the lower left-hand side of the figure. The electron configuration shown is that for the anion.

Frontier molecular orbitals of propadienylidene anion. These calculations used the ANO1 basis (see text) and were carried out using restricted open-shell Hartree-Fock theory at the anion geometry. The view is from 45° above the molecular plane, with the methylene group closest and the linear CCC framework extending away from the eye, and is shown schematically in the lower left-hand side of the figure. The electron configuration shown is that for the anion.

Energy level diagram for the electronic states of propadienylidene. All energies are based on those determined in this research, except for that of the state, which is taken from Refs. 18 and 72. The (italicized) energy for the state is estimated by adjusting the measured position of its ν_{2} level by the vibrational frequency, as estimated from the calculations. Note the marked difference in singlet–triplet splittings of the excited states, which are represented by the dotted lines. The ground state is obtained by removal of the 2*b* _{1} electron from the anion (see Fig. 2); excited *A* _{2}, *B* _{1}, and *A* _{1} states are obtained by removal of the 2*b* _{2}, 7*a* _{1}, and 1*b* _{1} electrons, respectively.

Energy level diagram for the electronic states of propadienylidene. All energies are based on those determined in this research, except for that of the state, which is taken from Refs. 18 and 72. The (italicized) energy for the state is estimated by adjusting the measured position of its ν_{2} level by the vibrational frequency, as estimated from the calculations. Note the marked difference in singlet–triplet splittings of the excited states, which are represented by the dotted lines. The ground state is obtained by removal of the 2*b* _{1} electron from the anion (see Fig. 2); excited *A* _{2}, *B* _{1}, and *A* _{1} states are obtained by removal of the 2*b* _{2}, 7*a* _{1}, and 1*b* _{1} electrons, respectively.

Photoelectron spectra of H_{2}CCC^{−} from eBE = 14 000 to 17 000 cm^{−1}. The upper black trace is the 364-nm NIPES spectrum; the lower cyan trace is the SEVI data, obtained with a photon energy of 17 790 cm^{−1}. The red sticks along the baseline are from a Franck-Condon simulation, based on the corresponding diagonal block of the effective Hamiltonian used here. The eigenvalues of the model Hamiltonian have been shifted so that the origin feature coincides with that seen experimentally.

Photoelectron spectra of H_{2}CCC^{−} from eBE = 14 000 to 17 000 cm^{−1}. The upper black trace is the 364-nm NIPES spectrum; the lower cyan trace is the SEVI data, obtained with a photon energy of 17 790 cm^{−1}. The red sticks along the baseline are from a Franck-Condon simulation, based on the corresponding diagonal block of the effective Hamiltonian used here. The eigenvalues of the model Hamiltonian have been shifted so that the origin feature coincides with that seen experimentally.

Photoelectron spectra of H_{2}CCC^{−} from eBE = 17 000 to 21 000 cm^{−1}. The upper black trace is the 364-nm NIPES spectrum, and the red sticks from the Franck-Condon calculation are shown. The shift of the model Hamiltonian eigenvalues is as described in Fig. 4. The large number of prominent states in the stick spectrum are various combinations of the Franck-Condon active ν_{2}, ν_{3}, and ν_{4} modes, together with even quantum levels involving ν_{5} and ν_{6}.

Photoelectron spectra of H_{2}CCC^{−} from eBE = 17 000 to 21 000 cm^{−1}. The upper black trace is the 364-nm NIPES spectrum, and the red sticks from the Franck-Condon calculation are shown. The shift of the model Hamiltonian eigenvalues is as described in Fig. 4. The large number of prominent states in the stick spectrum are various combinations of the Franck-Condon active ν_{2}, ν_{3}, and ν_{4} modes, together with even quantum levels involving ν_{5} and ν_{6}.

Photoelectron spectra of H_{2}CCC^{−} from eBE = 24 500 to 26 050 cm^{−1}. The upper black trace is the NIPES spectrum; the lower cyan trace is the SEVI spectrum, obtained with a photon energy of 26 450 cm^{−1}. Green and red sticks shown along the baseline are vibronic levels with *b* _{1} and *a* _{2} vibronic symmetries, respectively, as obtained from the model Hamiltonian assuming equal photodetachment cross sections for the and electronic states. Nominal vibrational quantum numbers are also given, and all states in this range can be assigned as vibrational levels of the state. The simulation has been shifted so that the origin feature coincides with that seen in the SEVI experiment.

Photoelectron spectra of H_{2}CCC^{−} from eBE = 24 500 to 26 050 cm^{−1}. The upper black trace is the NIPES spectrum; the lower cyan trace is the SEVI spectrum, obtained with a photon energy of 26 450 cm^{−1}. Green and red sticks shown along the baseline are vibronic levels with *b* _{1} and *a* _{2} vibronic symmetries, respectively, as obtained from the model Hamiltonian assuming equal photodetachment cross sections for the and electronic states. Nominal vibrational quantum numbers are also given, and all states in this range can be assigned as vibrational levels of the state. The simulation has been shifted so that the origin feature coincides with that seen in the SEVI experiment.

Photoelectron spectra of H_{2}CCC^{−} from eBE = 25 800 to 29 000 cm^{−1}. The cyan trace is the SEVI data obtained at photon energies of 27 550 cm^{−1} and 31 740 cm^{−1}, with the boundary between the two clearly demarcated. The lower blue trace is the VMI spectrum obtained with a photon energy of 32 260 cm^{−1} (4.0 eV). The stick spectrum, obtained from the model Hamiltonian, is shown along the baseline, with red sticks indicating levels with *a* _{2} vibronic symmetry and green indicating *b* _{1} vibronic symmetry. The dominant lower energy peak in the spectrum is the ν_{4} band of the state which is also seen in the preceding figure; the position of the apparent origin is indicated with an arrow. The shift applied to the calculated spectrum is the same as that used in Figure 4. The red trace is obtained from the model Hamiltonian spectrum, with each peak in the stick spectrum convoluted with a Gaussian with FWHM 40 cm^{−1}.

Photoelectron spectra of H_{2}CCC^{−} from eBE = 25 800 to 29 000 cm^{−1}. The cyan trace is the SEVI data obtained at photon energies of 27 550 cm^{−1} and 31 740 cm^{−1}, with the boundary between the two clearly demarcated. The lower blue trace is the VMI spectrum obtained with a photon energy of 32 260 cm^{−1} (4.0 eV). The stick spectrum, obtained from the model Hamiltonian, is shown along the baseline, with red sticks indicating levels with *a* _{2} vibronic symmetry and green indicating *b* _{1} vibronic symmetry. The dominant lower energy peak in the spectrum is the ν_{4} band of the state which is also seen in the preceding figure; the position of the apparent origin is indicated with an arrow. The shift applied to the calculated spectrum is the same as that used in Figure 4. The red trace is obtained from the model Hamiltonian spectrum, with each peak in the stick spectrum convoluted with a Gaussian with FWHM 40 cm^{−1}.

Photoelectron VMI spectrum of H_{2}CCC^{−} from eBE =34 500 to 38 000 cm^{−1} obtained with a photon energy of 37 940 cm^{−1}. Along the baseline is a stick spectrum obtained from the model Hamiltonian in the Franck-Condon approximation. Note the resonance between ν_{2} and 2ν_{4} that results in the polyad structure of the spectrum.

Photoelectron VMI spectrum of H_{2}CCC^{−} from eBE =34 500 to 38 000 cm^{−1} obtained with a photon energy of 37 940 cm^{−1}. Along the baseline is a stick spectrum obtained from the model Hamiltonian in the Franck-Condon approximation. Note the resonance between ν_{2} and 2ν_{4} that results in the polyad structure of the spectrum.

Simulated photoelectron spectrum of H_{2}CCC^{−} to the and states of H_{2}CCC, as calculated from the purely *ab initio* Hamiltonian (the relative energies of the two neutral states are unadjusted). Red sticks correspond to vibronic levels of *a* _{2} symmetry, while the green sticks indicate *b* _{1} vibronic levels. Due to the strong vibronic coupling that mixes these two states, only the origin has a “clean” assignment; general assignments of the various features are given in the figure. The energy range shown extends just beyond what roughly corresponds to the 3ν_{2} vibrational level in the state, which exhibits very little activity. At higher energies, no significant features are present. The simulated spectrum has been shifted so that the origin is coincident with the *T* _{0} feature measured by SEVI.

Simulated photoelectron spectrum of H_{2}CCC^{−} to the and states of H_{2}CCC, as calculated from the purely *ab initio* Hamiltonian (the relative energies of the two neutral states are unadjusted). Red sticks correspond to vibronic levels of *a* _{2} symmetry, while the green sticks indicate *b* _{1} vibronic levels. Due to the strong vibronic coupling that mixes these two states, only the origin has a “clean” assignment; general assignments of the various features are given in the figure. The energy range shown extends just beyond what roughly corresponds to the 3ν_{2} vibrational level in the state, which exhibits very little activity. At higher energies, no significant features are present. The simulated spectrum has been shifted so that the origin is coincident with the *T* _{0} feature measured by SEVI.

Experimental VMI photoelectron spectrum of H_{2}CCC^{−} in the region of eBE = 27 500 to 33 200 cm^{−1}, obtained at a photon energy of 33 590 cm^{−1}. The stick spectrum (same color scheme as in Fig. 6) from the simulation is also shown, where the vertical gap between the and states has been shifted by −800 cm^{−1} (0.1 eV) due to the higher-order correlation effects that principally affect the state (see text). Apart from moving the vibronic features associated with the state to lower energy, note that shifting the gap has a profound effect on the intensity profile (see Fig. 9 for the unadjusted spectrum) in the vicinity of origin.

Experimental VMI photoelectron spectrum of H_{2}CCC^{−} in the region of eBE = 27 500 to 33 200 cm^{−1}, obtained at a photon energy of 33 590 cm^{−1}. The stick spectrum (same color scheme as in Fig. 6) from the simulation is also shown, where the vertical gap between the and states has been shifted by −800 cm^{−1} (0.1 eV) due to the higher-order correlation effects that principally affect the state (see text). Apart from moving the vibronic features associated with the state to lower energy, note that shifting the gap has a profound effect on the intensity profile (see Fig. 9 for the unadjusted spectrum) in the vicinity of origin.

## Tables

Cartesian coordinates (in bohr) of the state of the propadienylidene anion, along with Cartesian representations (unit length) of the totally symmetric reduced normal coordinates. The approximate descriptions (and positive phase conventions) for the normal coordinates, which can be inferred from the data below, are: *q* _{1} C_{1}H symmetric stretch (compression); *q* _{2} C_{2}C_{3} stretch (compression); *q* _{3} HC_{1}H scissor (closing); *q* _{4} C_{1}C_{2} stretch (expansion). The atomic numbering follows Fig. 2.

Cartesian coordinates (in bohr) of the state of the propadienylidene anion, along with Cartesian representations (unit length) of the totally symmetric reduced normal coordinates. The approximate descriptions (and positive phase conventions) for the normal coordinates, which can be inferred from the data below, are: *q* _{1} C_{1}H symmetric stretch (compression); *q* _{2} C_{2}C_{3} stretch (compression); *q* _{3} HC_{1}H scissor (closing); *q* _{4} C_{1}C_{2} stretch (expansion). The atomic numbering follows Fig. 2.

Linear and quadratic parameters (in cm^{−1}) for quasidiabatic Hamiltonian, expressed in the reduced normal coordinate representation of the state of the propadienylidene anion. See Eqs. (4) and (5) for definitions of all parameters.

Linear and quadratic parameters (in cm^{−1}) for quasidiabatic Hamiltonian, expressed in the reduced normal coordinate representation of the state of the propadienylidene anion. See Eqs. (4) and (5) for definitions of all parameters.

Cubic and quartic parameters (in cm^{−1}) for the quasidiabatic Hamiltonian, expressed in the reduced normal coordinate representation of the state of the propadienylidene anion. See Eqs. (4) and (5) for definitions of the various parameters.

Cubic and quartic parameters (in cm^{−1}) for the quasidiabatic Hamiltonian, expressed in the reduced normal coordinate representation of the state of the propadienylidene anion. See Eqs. (4) and (5) for definitions of the various parameters.

Parameters associated with adiabatic potential energy surfaces of H_{2}CCC. All values for the singlet states are obtained with the CCSDT treatment of electron correlation, with EOMEE-CCSDT used for the excited states. For triplet states and the anion, the CCSD(T) treatment of correlation is used. The ANO1 basis set and the frozen-core approximation were adopted in all calculations. Bond lengths in Ångstroms, angles in degrees; harmonic frequencies and energies are in cm^{−1}. Normal coordinates are in dimensionless units.

Parameters associated with adiabatic potential energy surfaces of H_{2}CCC. All values for the singlet states are obtained with the CCSDT treatment of electron correlation, with EOMEE-CCSDT used for the excited states. For triplet states and the anion, the CCSD(T) treatment of correlation is used. The ANO1 basis set and the frozen-core approximation were adopted in all calculations. Bond lengths in Ångstroms, angles in degrees; harmonic frequencies and energies are in cm^{−1}. Normal coordinates are in dimensionless units.

Assigned vibrational levels in the ground electronic state of propadienylidene. All energies are in units of cm^{−1}.

Assigned vibrational levels in the ground electronic state of propadienylidene. All energies are in units of cm^{−1}.

Observed features and assignments for the ground vibronic state and selected excited state levels of H_{2}CCC in the photoelectron spectrum of H_{2}CCC^{−}. All energies in cm^{−1}, with reported uncertainties of ∼ 1σ. While the various experimental results are consistent within the indicated uncertainty, the recommended values are shown in boldface. Energies in the upper row are the electron binding energies (relative to the ground state of the anion); italicized values in the row below are relative to the ground vibronic level of the neutral.

Observed features and assignments for the ground vibronic state and selected excited state levels of H_{2}CCC in the photoelectron spectrum of H_{2}CCC^{−}. All energies in cm^{−1}, with reported uncertainties of ∼ 1σ. While the various experimental results are consistent within the indicated uncertainty, the recommended values are shown in boldface. Energies in the upper row are the electron binding energies (relative to the ground state of the anion); italicized values in the row below are relative to the ground vibronic level of the neutral.

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