Measurement of sulfur L2,3 and carbon K edge XANES in a polythiophene film using a high harmonic supercontinuum - PubMed (original) (raw)

. 2016 Oct 17;3(6):062603.

doi: 10.1063/1.4964821. eCollection 2016 Nov.

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Measurement of sulfur L2,3 and carbon K edge XANES in a polythiophene film using a high harmonic supercontinuum

A S Johnson et al. Struct Dyn. 2016.

Abstract

We use a high harmonic generated supercontinuum in the soft X-ray region to measure X-ray absorption near edge structure (XANES) spectra in polythiophene (poly(3-hexylthiophene)) films at multiple absorption edges. A few-cycle carrier-envelope phase-stable laser pulse centered at 1800 nm was used to generate a stable soft X-ray supercontinuum, with amplitude gating limiting the generated pulse duration to a single optical half-cycle. We report a quantitative transmission measurement of the sulfur _L_2,3 edge over the range 160-200 eV and the carbon K edge from 280 to 330 eV. These spectra show all the features previously reported in the XANES spectra of polythiophene, but for the first time they are measured with a source that has an approximately 1 fs pulse duration. This study opens the door to measurements that can fully time-resolve the photoexcited electronic dynamics in these systems.

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Figures

FIG. 1.

FIG. 1.

Full system for generating SXR harmonics from a table-top laser source and performing static XANES measurements. The system can be generally described in three parts. (a) Mid-IR pulse generation: the output of our CPA system is passed to a commercial OPA and downconverted. (b) Mid-IR pulse compression: the OPA output is sent through a HCF, spectrally broadened, and compressed in FS wedges. (c) High harmonic SXR supercontinuum generation: the few-cycle mid-IR pulse is focused into a tube filled with Ne and the emitted harmonics are detected in a spectrometer. See text for details.

FIG. 2.

FIG. 2.

Temporal profile of our few-cycle 1800 nm pulse generated from HCF pulse compression. The intensity (blue crosses) and phase (green-dashed) are both indicated and were determined with SEA-F-SPIDER. The main peak, containing 52% of the energy, has a duration of 7.2 fs FWHM, with the smaller post-pulse having no effect upon the HHG process.

FIG. 3.

FIG. 3.

CEP stability of our few-cycle 1800 nm source over 70 min, determined from a 2f-3f interferometry measurement. The CEP stability remains below 250 mrad indefinitely.

FIG. 4.

FIG. 4.

Harmonic spectrum generated in a Ne gas cell by our few-cycle laser source after an Al filter (transmission indicated by the dashed green line). The spectrum does not display any harmonic structure, indicating an isolated harmonic pulse.

FIG. 5.

FIG. 5.

Harmonic spectrum as a function of the CEP of the driving field. Clear half-cycle cut-offs are observable at the highest energies. The red lines are classic trajectory fits to the half-cycle cut-offs which we use to inform an SFA simulation. Dotted red lines indicate trajectories born more than one half cycle before the peak of the field which have weaker amplitude.

FIG. 6.

FIG. 6.

Visible absorption spectrum of our 200 nm free-standing polythiophene films. Inset: Photo of the corresponding film showing the high uniformity.

FIG. 7.

FIG. 7.

Polythiophene sulfur _L_2,3 edge XANES spectrum obtained with our high harmonic supercontinuum. The first three transitions are resolved and are enumerated in Table I.

FIG. 8.

FIG. 8.

Polythiophene carbon K edge XANES spectrum obtained with our high harmonic supercontinuum. All transitions are resolved and are enumerated in Table II.

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