^{1}, Benjamin FitzPatrick

^{1}, Andrew T. Healy

^{1}and David A. Blank

^{1,a)}

### Abstract

The transient absorptionspectrum in the range 500 nm–1000 nm was measured with ultrafast time resolution on a flowing neat, aliphatic, room-temperature ionic liquid following anion photodetachment. In this region the spectrum was shown to be a combination of absorption from the electron and the hole. Spectrally-resolved electron quenching determined a bimodal shape for the hole spectrum in agreement with recent computational predictions on a smaller aliphatic ionic liquid [Margulis *et al.*, J. Am. Chem. Soc.133, 20186 (2011)]10.1021/ja203412v. For time delays beyond 15 ps, spectral evolution qualitatively agrees with recent radiolysis experiments[Wishart *et al.*, Faraday Discuss.154, 353 (2012)10.1039/c1fd00065a]. However, the shape of the spectrum is different, reflecting the contrast in ionization energy between the two methods. Previously unobserved reactivity of the electron was found with a time constant of 300 fs. The results demonstrate solvent control of the rate coefficient for reaction between the electron and proton, with a rapid decline in the rate within the first picosecond.

This work was supported by the (U.S.) Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under SISGR Grant No. DE-FG02-2009ER16118. The authors thank Gary Baker for providing the 1-methyl-1-butyl-pyrrolidinium bis(trifluoromethylsulfonyl)amide samples.

I. INTRODUCTION

II. EXPERIMENTAL

A. Time-resolved pump-probe

B. Ionic liquid samples

III. RESULTS

IV. DISCUSSION

A. The transient absorption spectra

B. Geminate recombination and solvation

C. Electron reactivity

V. CONCLUSION

### Key Topics

- Absorption spectra
- 30.0
- Pump probe experiments
- 12.0
- Diffusion
- 11.0
- Time resolved spectroscopy
- 11.0
- Near infrared imaging
- 10.0

## Figures

The absorption spectrum in a 1 mm path length cell. The inset shows the IL molecular structure.

The absorption spectrum in a 1 mm path length cell. The inset shows the IL molecular structure.

(a) Change in the probe spectrum following excitation at 266 nm. (b) Time dependence for 50 nm wide regions. The circles represent the data and the lines are the fits as described in the text. The associated fitting parameters are listed in Table I.

(a) Change in the probe spectrum following excitation at 266 nm. (b) Time dependence for 50 nm wide regions. The circles represent the data and the lines are the fits as described in the text. The associated fitting parameters are listed in Table I.

The signal at 980 nm and a pump-probe time delay of 500 ps as a function of the 266 nm pump irradiance. The arrow indicates the pump irradiance used for all other presented data. The circles are the data, the line is the fit with a slope of 0.99 ± 0.16, and the error bars span a total of two standard deviations in the signal.

The signal at 980 nm and a pump-probe time delay of 500 ps as a function of the 266 nm pump irradiance. The arrow indicates the pump irradiance used for all other presented data. The circles are the data, the line is the fit with a slope of 0.99 ± 0.16, and the error bars span a total of two standard deviations in the signal.

Change in the signal at 925 nm following excitation at 266 nm and a pump-probe time delay of 1 ps as a function of laboratory time. The first data point is roughly 10 s after the shutter opened to illuminate the sample.

Change in the signal at 925 nm following excitation at 266 nm and a pump-probe time delay of 1 ps as a function of laboratory time. The first data point is roughly 10 s after the shutter opened to illuminate the sample.

Time dependence of changes in the signal at 980 nm following excitation at 266 nm. The data are presented as the fraction of the signal in the absence of HClO_{4}. The circles represent the data and the lines are the fits as described in the text. The associated fitting parameters are listed in Table II.

Time dependence of changes in the signal at 980 nm following excitation at 266 nm. The data are presented as the fraction of the signal in the absence of HClO_{4}. The circles represent the data and the lines are the fits as described in the text. The associated fitting parameters are listed in Table II.

Change in the probe spectrum following excitation at 266 nm in the presence of (a) 179 mM HClO_{4} and (b) 347 mM HClO_{4}. The data are presented as the fraction of the signal in the absence of HClO_{4}.

Change in the probe spectrum following excitation at 266 nm in the presence of (a) 179 mM HClO_{4} and (b) 347 mM HClO_{4}. The data are presented as the fraction of the signal in the absence of HClO_{4}.

Probability distribution of nearest pairwise distances for a random distribution at the concentrations of the HClO_{4} quencher used. The inset shows a comparison of the fraction of the distribution below 0.9 nm (squares), and the fraction of the quenching fit with the 300 fs decay component (circles), see Table II.

Probability distribution of nearest pairwise distances for a random distribution at the concentrations of the HClO_{4} quencher used. The inset shows a comparison of the fraction of the distribution below 0.9 nm (squares), and the fraction of the quenching fit with the 300 fs decay component (circles), see Table II.

Plots of Eq. (5) with τ_{sol} = 300 fs, *k* _{dry} = 4.3 × 10^{12} M^{−1} s^{−1} and the indicated proton concentration. (b) A comparison of the decays in (a) for all values of [H^{+}] after subtraction of the offset at large delay times and scaling to the same initial value. (c) Comparison of the quenching at long time in Eq. (5) and the residual amplitude of the fastest quenching component determined from the fits to the data, (1 – *a* _{1}), see Table II.

Plots of Eq. (5) with τ_{sol} = 300 fs, *k* _{dry} = 4.3 × 10^{12} M^{−1} s^{−1} and the indicated proton concentration. (b) A comparison of the decays in (a) for all values of [H^{+}] after subtraction of the offset at large delay times and scaling to the same initial value. (c) Comparison of the quenching at long time in Eq. (5) and the residual amplitude of the fastest quenching component determined from the fits to the data, (1 – *a* _{1}), see Table II.

## Tables

Fitting parameters for the time dependent changes in optical density shown in Fig. 2(a). The exponential time constants are τ_{ n }, the exponential weighting factors are *a* _{ n }, and *H* is the amplitude of the Heaviside function. Below each value is the 68.2% confidence interval from the nonlinear fitting when all variables are simultaneously optimized.

Fitting parameters for the time dependent changes in optical density shown in Fig. 2(a). The exponential time constants are τ_{ n }, the exponential weighting factors are *a* _{ n }, and *H* is the amplitude of the Heaviside function. Below each value is the 68.2% confidence interval from the nonlinear fitting when all variables are simultaneously optimized.

Fits to the quenching data shown in Fig. 5. The exponential time constants are τ_{ n } and the exponential weighting factors are *a* _{ n }. Below each value is the 68.2% confidence interval from the nonlinear fitting when all variables are simultaneously optimized.

Fits to the quenching data shown in Fig. 5. The exponential time constants are τ_{ n } and the exponential weighting factors are *a* _{ n }. Below each value is the 68.2% confidence interval from the nonlinear fitting when all variables are simultaneously optimized.

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