1. Transient Absorption (Courtesy of Kenneth Hanson, Florida University): The technique applied to molecular dynamics
Source hn
Sample
Detector

2. Excited State Decay Steady-state Emission Time-resolved Emission
Absorption Spectroscopy
NMR
Mass-spec
x-ray…
Non-radiative Decay

3. Events in Time Photochemistry Isomerization Intersystem Crossing
Excitation
Fluorescence
Phosphorescence
Internal Conversion
1 s
1 ms
1 ns
1 ps
1 fs
femto
pico
nano
micro
milli
seconds

4. Events in Time

5. Transient Absorption Spectroscopy
Source hn
Sample
Detector S0 S1 S2 E T1 T2
Transient Absorption
1) High intensity pulse of light.
2) Monitor absorption spectrum over time.
Excitation
Internal Conversion
Fluorescence
Non-radiative decay
Intersystem Crossing
Phosphorescence

6. Transient Absorption Spectroscopy

7. Spectroscopy Timeline

8. Visual Spectroscopy Perceived as green and then red.
“The human eye and its brain interface, the human visual system, can process 10 to 12 separate images per second (10 Hz), perceiving them individually.”
100 ms or 0.1 s
10 ms or 0.01 s
Time
Time
Perceived as green and then red.
Perceived as yellow.

9. We are missing out!
70 Hz
14 ms per cycle

10. Transient Absorption Spectroscopy
Source hn
Sample
Detector
Transient Absorption (Pump-Probe Experiment)
1) High intensity pulse of light.
2) Monitor absorption spectrum over time.

11. Transient Absorption Spectroscopy Electron Transfer Dynamicshn A C A C* A- C+

12. Transient Absorption Spectroscopy Electron Transfer Dynamics

13. Transient Absorption Spectroscopy

14. Transient Absorption Spectroscopy
Excited State Absorption Spectra
1) Excitation (hnpump)
2) Absorption Spectra (hnprobe)

15. Basics of TA Measurement
Source hn
Sample
Detector
(2)
Events:
1) Absorption Spectra
2) Excitation Flash
3) Absorption spectra
(1)
(3)
(1)
(3)
Excited State
Ground State
pump
probe
probe
probe
Time

16. 4 excited states/100 molecules
Difference Spectra
4 excited states/100 molecules S0 S1 E hn
A for xS0 molecules
A for (x - y)S0 + yS1 molecules

17. - = Difference Spectra A(t) - A(0) = DA A(t) A(0) DA at time t A for
A(0) = absorption without laser pulse
A(t) = absorption at time t after laser pulse
A(t)
A(0)
DA at time t - =
A for
(x - y)S0 + yS1
A for
xS0
- yS0 + yS1

18. We don’t get to measure absorbance!
Difference Spectra
∝ S1 generated
∝ S0 lost
We don’t get to measure absorbance!

19. We measure transmittance!
Difference Spectra
We measure transmittance! P0
Sample
(power in) P
(power out)
Absorbance:
A = -log T = log P0/P
A(t) - A(0) = DA
P0(t)
P0(0)
A(t) = log
A(0) = log
P(t)
P(0)
Probe source
is the Same
P0(t) = P0(0)
P(0) = power out before pump
P(t) = power out after pump
P(0)
Then:
DA = log
P(t)

20. TA Measurement P(0) DA = log P(t) (2) Events: 1) Measure P(0) 2) Pump
Source hn
Sample
Detector
(2)
Events:
1) Measure P(0)
2) Pump
3) Measure P(t)
(1)
(3)
(1)
(3)
P(0)
P(0) = power out before pump
P(t) = power out after pump
DA = log
P(t)

21. Full spectra detection
TA Measurement
Probe hn
Sample
Detector
Full spectra detection
Pump
Single l detection
Probe hn
Sample
Detector
Pump

22. Single Wavelength to Full Spectrum
Full Spectrum Data

23. Femtosecond TA (10-15 s) First developed in the 1980s (A. H. Zewail)
1999 Nobel Prize in Chemistry “for his studies of the transition states of chemical reactions using femtosecond spectroscopy"

24. Femtosecond TA (10-15 s) (1) (2) (3) (4) 1) Femtosecond laser pulse
Pump
(2)
Probe
Delay Stage
(3)
(4)
1) Femtosecond laser pulse
2) Beam splitter (into Pump and Probe)
3) Probe Travels through Delay Stage
4) Pump hits sample (exciation)
5) Probe hits sample
6) Transmitted Probe hits detector
Detector

25. Femtosecond TA (10-15 s) DA = log P(0)/P(t) Transient Concentration
Pump
Graph of t vs DA
Transient Concentration
Intensity DA
time
time
blank P(0)
pump
probe
Transmitted Light at time 1
P(t1)
td1
Intensity
P(t)
time
time
pump
probe
Transmitted Light at time 1
P(t2)
td2
Intensity
Intensity
time
time

26. Femtosecond TA (10-15 s) DA = log P(0)/P(t) P(t) < P(0)
blank P(0)
blank P(0)
pump
probe
pump
probe
td1
td1
Intensity
Intensity
time
time
Increased Transmitted
light P(t)
Decrease Transmitted light P(t)
P(t)
P(t)
time
time
Graph of t vs DA
Graph of t vs DA DA DA
time
time
New species after laser pulse.
Loss of species after laser pulse.

27. Single Wavelength to Full Spectrum
Full Spectrum Data

28. Femtosecond TA (10-15 s)

29. A striking example

30. We initiated the photoisomerization reaction in the retinal chromophore of purified rhodopsin by 10-fs 500-nm pump pulses resonant with the ground-state absorption. The photoinduced dynamics were then probed by delayed ultra-broadband few-optical-cycle probe pulses, either in the visible wavelength region (500–720 nm) or in the near-infrared (NIR, 820–1,020 nm), generated by synchronized optical parametric amplifiers. The temporal resolution was <20 fs over the entire monitored spectral range.
Immediately following excitation from the ground state (S0) to the first excited singlet state (S1), we observed a positive T/T signal (blue in the figure) with maximum intensity at, 650 nm, which is assigned to stimulated emission from the excited S1 state due to the negligible ground-state absorption in this wavelength range. The stimulated emission signal rapidly shifts to the red while losing intensity and disappearing to wavelengths longer than 1,000 nm within ,75 fs. At this time, the T/T signal changes sign and transforms into a weak photoinduced absorption signal (red in the figure), which initially appears at 1,000 nm and then gradually shifts to the blue and increases in intensity. For delays longer than 200 fs, the photoinduced absorption signal stabilizes as a long-lived band peaking at 560 nm, indicating the formation of the all-trans photoproduct.

31. Suggested mechanism

32. To complete the description of photoinduced dynamics in rhodopsin, we report the portion of the T/T map probing the response of the system in the visible region, from 495 nm to 610 nm (Fig. 3a). In
agreement with previous studies we observe the delayed formation of the photoinduced absorption band of the photorhodopsin photoproduct, which peaks at 560 nm and is complete within 200–250 fs.
The signal does not display exponential build-up dynamics, but appears rather abruptly, starting at 150 fs (see time trace at 550 nm in Fig. 3b), which is the time needed for the wave packet to cross the
conical intersection and enter the probed wavelength window on the photoproduct side. The blue region of the spectrum is dominated by the photobleaching signal from the ground state of the parent rhodopsin molecule, peaking at, 510 nm. These two spectral signatures partially overlap, so that the photobleaching band shrinks in time
as the photoinduced absorption signal forms and blue-shifts.