1. Ultrafast Excited State Raman Spectroscopy
Griffin Canning
Spring 2016

2. What is it? Provides vibronic information on a chemical system
Can eliminate ground state signal
Allows researchers to probe internal conversions and intersystem crossings
Based on transient absorption experiments
Liebel, et. al., PRL, 112, , 2014

3. Introduction and motivation
TIPS-Pentacene undergoes singlet fission
Models predict both a direct mechanism via ISC and a mediated mechanism via charge transfer states
Ultrafast Raman can track vibrational coherence during the process
Fig 1, TIPS-pentacene and its absorption spectrum in thin film
Fig. 2, electronic state ordering
Musser, et. al., Nat. Phys., 11, , 2015

4. Singlet Fission
Smith et al., Chem. Rev. 2010

5. Experimental Setup Amplifier Probe pulse Sapphire SHG Δx Detector
Sum freq. harm
NOPA G
Pulse compression
Sample
Pump pulse
Probe pulse
Dump pulse
Detector
Δt = 300 fs Δx

6. Two-pulse experiment Step 1: pump off/on
Transmittance of white-light probe pulse measured as a function of λ
Pump on
Pump pulse populates excited electronic states
After ∆t, white-light probe interacts with ground and excited states.
Transmittance of probe is measured as a function of λ, t
Musser, et. al., Nat. Phys., 11, , 2015

7. Two-pulse experiment Step 2: differential transmittance
Pump: 10 fs (broadband)
Probe : 300 fs (white-light broadband)
∆t steps of fs, integration time 100 fs
∆T = T2 – T1
∆T/T is the relative differential transmittance
Fig. 3, showing contour maps

8. Two-pulse experiment Step 3: electronic slow kinetics fitting
In figure 4a, zoomed-in excerpt of figure 3a
In figure 4b, integrated kinetics for T1  T2 ( nm)
Fig. 4

9. Two-pulse experiment Step 4: residuals
In figure 5a, integrated kinetics for T1 T3 ( nm), plus a subtraction of a fitting function
In figure 5b, the fitting and subtraction is done over all the wavelengths
Fig. 5

10. Two-pulse experiment Step 5: Fast Fourier Transform
Fig. 6a
Fig. 6b

11. Three-pulse experiment Steps 1, 2: dump off/on
Dump on/Dump off experiment:
Pump: 10 fs
Dump: 650 fs, tuned to 860 nm (T1  T2 )
Probe: 300 fs
Pump-Dump delay: 300 fs fixed
Fig. 6
Fig. 7

12. Three-pulse experiment Steps 2,3: residuals
In figure 8a, Integrated kinetics for T1 T2 ( nm) for Dump off minus Dump on
In figure 8b, applying same process as in figure 8a for all the wavelengths and subtracting 2 pulse from 3 pulse
Fig. 8

13. Three-pulse experiment: Step 5: Fast Fourier Transform
|FFT|2
Fig. 9b
Fig. 9a

14. Two-pulse and three-pulse experiment comparison
FFT on GSB region, fig.8.a
Reproduces spontaneous Raman
FFT on T1 T3 region, fig.8.a
Contribution from excited state
FFT on T1 T3 region, fig.8.c
GS modes are gone, new modes appear
Fig. 10a
Fig. 10b

15. Advantages, disadvantages, limitations
Advantages Provides high-quality, background-free Raman spectra of electronic excited states and their dynamics Allows for the experimental isolation of the vibrational spectra of the excited state of interest Extremely high signal-to-noise ratio compared with traditional Raman
Disadvantages The instrumentation needed is expensive and the experiments demand high accuracy The limitation comes from the precision on the time domain: - the width of the pulses - the limited period of time of the measurement

16. THANK YOU!

17. Complementary information

18. Raman spectroscopy
The molecules need to exhibit polarizability for the incident light to undergo Raman scattering
Raman signal has a very small cross section (~10-30 cm2/sr):
The excitation light can overwhelm the Raman signal
Any luminiscence can overwhelm the Raman signal (CS ~10-20 cm2/sr)

19. Some concepts of photochemistry
IC  internal conversion ISC  intersystem crossing

20. Conclusions by Musser et al.
Singlet exciton delocalized before singlet fission
“Direct model” and “Mediated mode” possible, “Coherent model” ruled out
Conical intersection between delocalized singlet and TT assisted by vibronic coupling
Similarities between TIPS-pentacene (singlet fission) and DPO (internal conversion)
Fig. 10

21. History
Although ultrafast spectroscopy is not a new technique, the increase in the precision and accuracy of the instrumentation is allowing the advent of novel applications like the Ultrafast Vibronic Spectroscopy
Introduced in 2013 by Matz Liebel and Philipp Kukura(1)
So far, it has been applied to study internal conversion in β-carotene(3), singlet fission in TIPS-pentacene(4), internal conversion in DPO(4)