Femtosecond transient absorption

1. Femtosecond transient absorption Pump and probe technique

2. Schematic layout of a typical femtosecond transient absorption apparatus

3. Main parts of the instrument • Excitation source • Probe source • Optical delay • Detector

4. Pump Probe t=0 t<0 t>0 Pump and probe concept t

5. Transient signal detection I0 I1

6. Typical temporal characteristics • ~100 fs time resolution • Determined by the laser pulse duration • 1-6 ns time window • Determined by the optical delay length

7. Excitation light sources • Direct Ti-Sapphire laser output • 800 nm • Harmonic generation • SHG 400 nm • THG 267 nm • Optical parametric amplification • 250-12000 nm

8. Probe light sources • White light continuum generation • Advantage • Easy to use • All wavelengths are generated at once • Disadvantage • Somewhat limited spectral range 330-1000 nm

9. Probe light sources • Optical parametric amplification • Advantage • Broad spectral range 250-12000 nm • Disadvantage • Requires more sophisticated equipment • Only narrow spectrum is generated at once, therefore – need to scan

10. The method allows for: • Direct observation of a transient by monitoring a characteristic spectral signature • Observation of the electronic and vibrational energy redistribution • Probing broad spectral and temporal range in automated mode • Observation of stimulated emission

11. The method does not allow for: • Direct observation of emission • Monitoring of structural changes

12. Detectors (CCD)

13. Detectors (CCD) • Advantages • Acquires the whole spectrum at once • Compact • Disadvantages • Reduced speed of data acquisition • More expensive

14. Detectors (diodes)

15. Detectors (diodes) • Advantages • Allows for faster data acquisition • Lower cost • Disadvantages • Limited to a single wavelength • Requires a monochromator • The setup with diodes is generally much more space consuming

16. Typical data example

17. Typical data example