^{1,a)}, Benjamin J. Knurr

^{1}and J. Mathias Weber

^{1,b)}

### Abstract

We present low-energy velocity map photoelectron imaging results for bare and Ar-solvated 1-nitropropane and 1-nitrobutane anions. We report the adiabatic electron affinity of 1-nitropropane as (223 ± 6) meV and that of 1-nitrobutane as (240 ± 6 meV). The vertical detachment energies of these two species are found to be (0.92 ± 0.05) and (0.88 ± 0.05) eV, respectively. The photoelectron spectra are discussed in the framework of Franck-Condon simulations based on density functional theory. We observe unusual resonances in the photoelectron spectra of both ions under study, whose kinetic energy is independent of the photonenergy of the detaching radiation. We discuss possible origins of these resonances as rescattering phenomena, consistent with the experimental photoelectron angular distributions.

We gratefully acknowledge funding from the National Science Foundation (NSF) through the NSF Physics Frontier Center at JILA (PHY-0551010). We thank Dr. Thomas Sommerfeld who in a discussion brought up the idea that only one vibrational mode may be active in the decay of a vibrational Feshbach resonance. We also thank Dr. Chris Greene, Dr. Anna Krylov, and Dr. Ksenia Bravaya for helpful discussions.

I. INTRODUCTION

II. EXPERIMENTAL

III. COMPUTATIONAL

IV. RESULTS AND DISCUSSION

A. Experimental photoelectron spectra

1. 1-nitropropane anion

2. 1-nitrobutane anion

B. Franck-Condon simulations

1. 1-nitropropane anion

2. 1-nitrobutane anion

C. Angular distributions

D. Remarks on the anomalous feature in photodetachment from 1-NP and 1-NB anions

V. SUMMARY

### Key Topics

- Photoelectron spectra
- 30.0
- Photons
- 28.0
- Anisotropy
- 11.0
- Vibration resonance
- 8.0
- Angular distribution
- 6.0

## Figures

Photoelectron spectrum of 1-NP anion obtained at a photon energy of 2.331 eV.

Photoelectron spectrum of 1-NP anion obtained at a photon energy of 2.331 eV.

Photoelectron spectra of 1-NP anion taken at photon energies of 393 meV (3170 cm^{−1}), 368 meV (2968 cm^{−1}), 329 meV (2650 cm^{−1}), and 273 meV (2200 cm^{−1}), from top to bottom. The data on the left are plotted as functions of binding energy while the data on the right are shown as functions of kinetic energy. The labels A and B denote two features that are consistently observed at the same binding energy. The label F denotes a prominent feature found at a fixed kinetic energy in the spectra taken at photon energies of 329 meV (2650 cm^{−1}) and 368 meV (2968 cm^{−1}). The dotted lines in the spectra on the left show the positions of features A and B; the dotted line in the spectrum on the right shows the position of feature F (see text).

Photoelectron spectra of 1-NP anion taken at photon energies of 393 meV (3170 cm^{−1}), 368 meV (2968 cm^{−1}), 329 meV (2650 cm^{−1}), and 273 meV (2200 cm^{−1}), from top to bottom. The data on the left are plotted as functions of binding energy while the data on the right are shown as functions of kinetic energy. The labels A and B denote two features that are consistently observed at the same binding energy. The label F denotes a prominent feature found at a fixed kinetic energy in the spectra taken at photon energies of 329 meV (2650 cm^{−1}) and 368 meV (2968 cm^{−1}). The dotted lines in the spectra on the left show the positions of features A and B; the dotted line in the spectrum on the right shows the position of feature F (see text).

Photoelectron spectra of CH_{3}(CH_{2})_{2}NO_{2} ^{−} (upper curve) and CH_{3}(CH_{2})_{2}NO_{2} ^{−}·Ar (lower curve) taken at a photon energy of 393 meV. The arrows represent the shift of the spectrum by (56 ± 7) meV upon argon solvation.

Photoelectron spectra of CH_{3}(CH_{2})_{2}NO_{2} ^{−} (upper curve) and CH_{3}(CH_{2})_{2}NO_{2} ^{−}·Ar (lower curve) taken at a photon energy of 393 meV. The arrows represent the shift of the spectrum by (56 ± 7) meV upon argon solvation.

Photoelectron spectra of 1-NB anion, obtained at a photon energy of 2.331 eV.

Photoelectron spectra of 1-NB anion, obtained at a photon energy of 2.331 eV.

Photoelectron spectra of 1-NB anion at photon energies of 444 meV (3580 cm^{−1}), 424 meV (3420 cm^{−1}), 404 meV (3260 cm^{−1}), 384 meV (3100 cm^{−1}), and 329 meV (2650 cm^{−1}), from top to bottom. The data on the left are presented as a function of binding energy while the data on the right are shown as a function of kinetic energy. The labels A^{′} and B^{′} denote two features that are consistently observed at the same binding energy. The label F^{′} denotes a prominent feature found at a fixed kinetic energy in the spectra. The dotted lines in the spectra on the left show the positions of features A^{′} and B^{′}; the dotted line in the spectrum on the right shows the position of feature F^{′} (see text).

Photoelectron spectra of 1-NB anion at photon energies of 444 meV (3580 cm^{−1}), 424 meV (3420 cm^{−1}), 404 meV (3260 cm^{−1}), 384 meV (3100 cm^{−1}), and 329 meV (2650 cm^{−1}), from top to bottom. The data on the left are presented as a function of binding energy while the data on the right are shown as a function of kinetic energy. The labels A^{′} and B^{′} denote two features that are consistently observed at the same binding energy. The label F^{′} denotes a prominent feature found at a fixed kinetic energy in the spectra. The dotted lines in the spectra on the left show the positions of features A^{′} and B^{′}; the dotted line in the spectrum on the right shows the position of feature F^{′} (see text).

The photoelectron spectrum of bare 1-NB anion (top curve) and argon-solvated 1-NB anion (bottom curve), taken at a photon energy of 384 meV. The arrow represents the shift of the 0 ← 0 band upon Ar solvation. Features are labeled as in Figure 5.

The photoelectron spectrum of bare 1-NB anion (top curve) and argon-solvated 1-NB anion (bottom curve), taken at a photon energy of 384 meV. The arrow represents the shift of the 0 ← 0 band upon Ar solvation. Features are labeled as in Figure 5.

The Franck-Condon simulations of conformers I and II of 1-NP anion, using an anion temperature of 250 K. An inset in each graph illustrates the 1-NP anion conformer along with its relative energy. The open circles, solid lines, and vertical bars represent the experimental spectrum, simulated curve, and the individual calculated transitions, respectively.

The Franck-Condon simulations of conformers I and II of 1-NP anion, using an anion temperature of 250 K. An inset in each graph illustrates the 1-NP anion conformer along with its relative energy. The open circles, solid lines, and vertical bars represent the experimental spectrum, simulated curve, and the individual calculated transitions, respectively.

The Franck-Condon simulations of conformers i-iii of 1-NB anion, using an anion temperature of 250 K. An inset in each graph illustrates the 1-NB anion conformer along with its relative energy. The open circles, solid lines, and vertical bars represent the experimental spectrum, simulated curve, and the individual calculated transitions, respectively.

The Franck-Condon simulations of conformers i-iii of 1-NB anion, using an anion temperature of 250 K. An inset in each graph illustrates the 1-NB anion conformer along with its relative energy. The open circles, solid lines, and vertical bars represent the experimental spectrum, simulated curve, and the individual calculated transitions, respectively.

Average anisotropy parameter as a function of kinetic energy for the four mid-IR PE images of 1-NP anion (top) and 1-NB anion (bottom). The minimum in *β* (dashed line) is located at a kinetic energy of (166 ± 5) meV in the case of 1-NP and at (167 ± 5) meV in the case of 1-NB.

Average anisotropy parameter as a function of kinetic energy for the four mid-IR PE images of 1-NP anion (top) and 1-NB anion (bottom). The minimum in *β* (dashed line) is located at a kinetic energy of (166 ± 5) meV in the case of 1-NP and at (167 ± 5) meV in the case of 1-NB.

Model to explain the anomalous feature observed in the PE spectra of 1-NP and 1-NB; X^{−} = anion ground state; X^{0} = neutral ground state; hν(IR) = infrared photon energy; ℏω(VFR) = energy of vibration giving rise to the VFR; *E* _{ B }(DBS) = binding energy of the DBS; *E* _{ K } = ℏω(VFR)– *E* _{ B }(DBS) = kinetic energy of electrons in anomalous feature.

Model to explain the anomalous feature observed in the PE spectra of 1-NP and 1-NB; X^{−} = anion ground state; X^{0} = neutral ground state; hν(IR) = infrared photon energy; ℏω(VFR) = energy of vibration giving rise to the VFR; *E* _{ B }(DBS) = binding energy of the DBS; *E* _{ K } = ℏω(VFR)– *E* _{ B }(DBS) = kinetic energy of electrons in anomalous feature.

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