Sequential absorption processes in two-photon-excitation transient absorption spectroscopy in a semiconductor polymer
Bibliographic record
Abstract
We investigate two-photon-excited transient absorption (TA) in poly[$9,{9}^{\ensuremath{'}}$-dioctyl-fluorene-co-N-(4-butylphenyl)diphenylamine]. Using excitation pulses at $1.55\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and of around $1\text{\ensuremath{-}}\mathrm{ps}$ duration, we measured the pump intensity dependence of the TA in the visible part of the spectrum and were able to resolve multiple sequential excitation and absorption processes. Polymers in the ground state $({S}_{0})$ were first excited by absorbing two pump photons to the band of two-photon excited states $({S}_{2})$. There is then fast relaxation from ${S}_{2}$ to the lowest singlet excited state $({S}_{1})$. Based on this transient modulation of the population on the excited states, the ${S}_{2}$ and ${S}_{1}$ excitons were further excited by absorbing another pump or tunable probe photon to higher-lying excited states of ${S}_{n}$ and ${S}_{m}$, respectively. Moreover, the probe photon was also absorbed by the ground-state molecules with the aid of a pump photon $({\ensuremath{\omega}}_{\text{pump}}+{\ensuremath{\omega}}_{\text{probe}})$ for the transition from ${S}_{0}$ to ${S}_{2}$. A fraction of higher-energy ${S}_{n}$ and ${S}_{m}$ excitons dissociated into charges that provided an additional absorption channel for the probe pulses. Comparing the simulation and the experimental results, we evaluated quantitatively the contributions from the above absorption channels to the total TA of the probe. The photocurrent measurement in a photodiode structure confirmed that dissociation of excitons from the higher-lying excited states is the dominant mechanism for charge generation in two-photon excitation.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".