# Simulated and Experimental Time-Resolved Photoelectron Spectra of the Intersystem Crossing Dynamics in 2-Thiouracil

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## Abstract

**:**

## 1. Introduction

## 2. Experimental Details

## 3. Computational Details

#### 3.1. Excited-State Dynamics Simulations

#### 3.2. TRPES Simulations

## 4. Results and Discussion

#### 4.1. Experimental Results

#### 4.2. Comparison of One- and Two-Photon TRPES Data

#### 4.3. Simulated TRPE Spectra

#### 4.3.1. Overall Spectrum

#### 4.3.2. State-Wise Decomposition

#### 4.3.3. Time-Averaged Spectra

#### 4.3.4. Energy-Integrated Yields

#### 4.4. Discussion

## 5. Conclusions

## Supplementary Materials

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**2D time-resolved photo-electron spectrum of 2-thiouracil recorded with 293 nm excitation and 194 nm one-photon ionization (

**top row, first column**). Individual contributions to the 2D spectrum from global analysis techniques are plotted in the bottom row. Summation of these contributions yields the total fit (

**top row, second column**) and when subtracted from the signal results in the residuals (

**top row, third column**). All 2D spectra are plotted as color maps with the pump-probe delay, t (ps), along the y-axis and the electron binding energy, ${E}_{b}$ (eV), along the x-axis; signal intensities are represented according to the color bar on the bottom. The time traces (

**top row, fourth column**) correspond to the signal, fit, and individual contributions integrated over all electron binding energies. Background subtraction errors in the probe-pump region amount to 3% of the maximum signal.

**Figure 2.**Time traces showing the total photoelectron yield as a function of pump-probe delay, t (ps). The TRPE spectra recorded at 293 nm + 194 nm (red) and 292 nm + 2 × 330 nm (light blue) were integrated over all electron binding energies ${E}_{b}\le 10.6$ eV. The TRPE spectrum at 292 nm + 2 × 330 nm was also integrated for ${E}_{b}\le 11.7$ eV (plotted in dark blue), i.e., over the entire range observable with the available (1 + 2’) total photon energy. All time traces are scaled so that their photoelectron yields match at long pump-probe delays in the 3–4 ps range. The grey box indicates the region where one- and two-photon experiments differ significantly.

**Figure 3.**Electronic populations from the Sharc simulations (

**a**) and simulated TRPE spectra without (

**b**) and with (

**c**) temporal broadening (full width at half maximum of 190 fs) added. ${E}_{b}$ is the binding energy, whose upper limit (10.6 eV) is equal to the sum of pump and probe energies (4.2 eV and 6.4 eV). The rectangular line shape function and intensities proportional to Dyson norms were used.

**Figure 4.**Simulated TRPE spectra of 2TU for the individual neutral states with temporal convolution (FWHM 190 fs). Each column of panels shows the spectral contribution of one initial (neutral) state (${S}_{2}$, ${S}_{1}$, ${T}_{1-3}$), with the total sum given in the first column. The top row was computed with Dyson norms and rectangle line shape functions, the bottom row by setting all relevant Dyson norms to unity. As above, 4.2 eV pump energy and 6.4 eV probe energy were assumed in the computation of the binding energy. The color palette is the same as in Figure 3.

**Figure 5.**In (

**a**,

**b**), we show the simulated photoelectron spectra of each electronic state, computed by integrating over the whole simulation time, for Dyson norms and unity norms, respectively. For better comparison, each spectrum is normalized by the time-averaged population of each state and shifted by +0.65 eV to higher binding energies (the unshifted spectra are shown as thin, dashed lines). In (

**c**), we show the experimental decay-associated spectra (DAS) for the four fitted time constants ${\tau}_{1}$ to ${\tau}_{4}$. The coloring of the four DAS was done in line with the assignment of ${\tau}_{1}$ to ${S}_{2}\to {S}_{1}$, ${\tau}_{2}$ to ${S}_{1}\to T$, and ${\tau}_{3}$ to $T\to {S}_{0}$. In panel (

**d**), we also show the predicted ionization spectra of the vibrationally cold ${S}_{1}$ and ${T}_{1}$ minima from Ref. [71].

**Figure 6.**Integration of the simulated TRPES in Figure 3b for binding energies below 9.95 eV (i.e., 10.6 eV minus the estimated shift of 0.65 eV). Black dots show the integrated, temporally not broadened data, based on either Dyson norms or unity norms. The red curves are mono-exponential fits of the data, where the data in the first 20 fs (containing the large intensity spike at the Franck–Condon point) were excluded from the fit.

**Figure 7.**Integrated TRPES yields decomposed by state (${S}_{2}$, ${S}_{1}$, ${T}_{1-3}$). The labels within the plots give the obtained time constant from fits with a three-component sequential kinetic model. Note that, in the fit, we assumed identical intensities for ${S}_{1}$ and T, based on the experimental intensities and the fact that the total yields are almost constant.

**Figure 8.**Dyson orbitals computed at the ${S}_{1}$ and ${T}_{1}$ minima of 2TU for ionization from ${S}_{1}$ or ${T}_{1}$ to ${D}_{0}$ or ${D}_{1}$. Note that, in order to show the renormalized Dyson orbitals, for each plot, we chose the isosurface value as $0.07\sqrt{\left|D\right|}$, where $\left|D\right|$ is the relevant Dyson norm.

**Table 1.**Time constants ${}^{a}$ for the excited-state dynamics of 2TU obtained with time-resolved photoelectron spectroscopy (TRPES) and nonadiabatic SHARC (surface hopping including arbitrary couplings) dynamics simulations.

Method | Pump (nm) | Probe (nm) | ${\mathit{\tau}}_{1}$ (fs) | ${\mathit{\tau}}_{2}$ (fs) | ${\mathit{\tau}}_{3}$ (ps) | ${\mathit{\tau}}_{4}$ (fs) | Remark |
---|---|---|---|---|---|---|---|

—experimental— | |||||||

Two-photon TRPES | 293 | 2 × 330 | <50 | 775 | 203 | [27,28] | |

Two-photon TRPES | 260 | 2 × 330 | 67 | 285 | 85.6 | [28] | |

One-photon TRPES | 293 | 194 | 83 | 750 | 203 | −80 | [present work] |

One-photon TRPES | 260 | 194 | <50 | 246 | 85.6 | −80 | [present work] |

—simulated— | |||||||

SHARC populations | 295–317 | none | ∼60 | ∼400 | [35] | ||

Simulated TRPES | 295–317 | 194 | ∼45 | ∼500 | [present work] |

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**MDPI and ACS Style**

Mai, S.; Mohamadzade, A.; Marquetand, P.; González, L.; Ullrich, S.
Simulated and Experimental Time-Resolved Photoelectron Spectra of the Intersystem Crossing Dynamics in 2-Thiouracil. *Molecules* **2018**, *23*, 2836.
https://doi.org/10.3390/molecules23112836

**AMA Style**

Mai S, Mohamadzade A, Marquetand P, González L, Ullrich S.
Simulated and Experimental Time-Resolved Photoelectron Spectra of the Intersystem Crossing Dynamics in 2-Thiouracil. *Molecules*. 2018; 23(11):2836.
https://doi.org/10.3390/molecules23112836

**Chicago/Turabian Style**

Mai, Sebastian, Abed Mohamadzade, Philipp Marquetand, Leticia González, and Susanne Ullrich.
2018. "Simulated and Experimental Time-Resolved Photoelectron Spectra of the Intersystem Crossing Dynamics in 2-Thiouracil" *Molecules* 23, no. 11: 2836.
https://doi.org/10.3390/molecules23112836