Transcript P 0

Transient Absorption (Courtesy of Kenneth Hanson, Florida
University): The technique applied to molecular dynamics
Source
Source
hn
Detector
hn
Sample
1
Excited State Decay
Steady-state Emission
Time-resolved Emission
Absorption
Spectroscopy
NMR
Mass-spec
x-ray…
Non-radiative Decay
Events in Time
Isomerization
Photochemistry
Intersystem Crossing
Excitation
Fluorescence
Phosphorescence
Internal Conversion
1 fs
1 ps
1 ns
1 ms
1 ms
1s
femto
pico
nano
micro
milli
seconds
Events in Time
Transient Absorption Spectroscopy
Source
Source
hn
S2
S1
Detector
hn
T2
T1
E
Sample
Transient Absorption
1) High intensity pulse of light.
2) Monitor absorption spectrum over time.
S0
Excitation
Internal Conversion
Fluorescence
Non-radiative decay
Intersystem Crossing
Phosphorescence
Transient Absorption Spectroscopy
Spectroscopy Timeline
Visual Spectroscopy
“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.”
10 ms or 0.01 s
100 ms or 0.1 s
Time
Perceived as green and then red.
Time
Perceived as yellow.
We are missing out!
70 Hz
14 ms per cycle
Transient Absorption Spectroscopy
Source
Source
hn
Detector
hn
Sample
Transient Absorption (Pump-Probe Experiment)
1) High intensity pulse of light.
2) Monitor absorption spectrum over time.
Transient Absorption Spectroscopy
Electron Transfer Dynamics
hn
A
C
A
C*
A-
C+
Transient Absorption Spectroscopy
1.2
1.2
1.0
1.0
1.0
0.8
0.6
0.4
0.2
Absorbance (a.u.)
1.2
Absorbance (a.u.)
Absorbance (a.u.)
Electron Transfer Dynamics
0.8
0.6
0.4
0.2
400
500
600
700
Wavelength (nm)
A
C
800
0.6
0.4
0.2
0.0
0.0
0.0
0.8
400
500
600
700
Wavelength (nm)
A
C*
800
400
500
600
700
Wavelength (nm)
A-
C+
800
Transient Absorption Spectroscopy
Transient Absorption Spectroscopy
C
C*
Excited State Absorption Spectra
1) Excitation (hnpump)
2) Absorption Spectra (hnprobe)
Basics of TA Measurement
Source (2)
Source
hn
Events:
Detector
hn
(1) (3)
1) Absorption Spectra
2) Excitation Flash
Sample
(1) (3)
3) Absorption spectra
Excited State
Ground State pump
probe
probe
probe
Time
Difference Spectra
4 excited states/100 molecules
S1
hn
E
S0
1.2
1.0
Absorbance (a.u.)
Absorbance (a.u.)
1.2
0.8
0.6
0.4
0.2
0.0
400
500
600
700
800
Wavelength (nm)
A for xS0 molecules
1.0
0.8
0.6
0.4
0.2
0.0
400
500
600
700
800
Wavelength (nm)
A for (x - y)S0 + yS1 molecules
Difference Spectra
A(t) - A(0) = DA
A(0) = absorption without laser pulse
A(t) = absorption at time t after laser pulse
A(t)
DA at time t
A(0)
0.01
0.00
0.8
0.6
0.4
0.2
Absorbance (a.u.)
Absorbance (a.u.)
-
1.0
1.0
=
0.8
0.6
0.4
Delta A
1.2
1.2
-0.01
-0.02
-0.03
0.2
0.0
0.0
400
500
600
700
Wavelength (nm)
800
400
500
600
700
Wavelength (nm)
800
-0.04
400
450
500
550
600
650
Wavelength (nm)
A for
(x - y)S0 + yS1
A for
xS0
- yS0 + yS1
700
750
Difference Spectra
∝ S1 generated
∝ S0 lost
We don’t get to measure absorbance!
Difference Spectra
We measure transmittance!
Sample
P0
P
(power in)
(power out)
Absorbance:
A = -log T = log P0/P
A(t) - A(0) = DA
P0(t)
A(t) = log
P(t)
Probe source
is the Same
Then:
P0(t) = P0(0)
DA = log
P(0)
P(t)
P0(0)
A(0) = log
P(0)
P(0) = power out before pump
P(t) = power out after pump
TA Measurement
Source (2)
Source
hn
Events:
Detector
hn
(1) (3)
1) Measure P(0)
2) Pump
Sample
DA = log P(0)
P(t)
(1) (3)
3) Measure P(t)
P(0) = power out before pump
P(t) = power out after pump
TA Measurement
Single l detection
Full spectra detection
Pump
Pump
Detector
Probe
hn
Probe
hn
Sample
Delta OD
0.00
10 ns
750 ns
1490 ns
2230 ns
2970 ns
3710 ns
4450 ns
5190 ns
5930 ns
-0.01
-0.02
-0.03
400
450
500
hn
Sample
0.01
-0.04
hn
550
600
650
Wavelength (nm)
700
750
Detector
Single Wavelength to Full Spectrum
Single Wavelength
Full Spectrum Data
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"
Femtosecond TA (10-15 s)
(1)
Pump
(2)
Probe
(3)
Delay Stage
(4)
Detector
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
Femtosecond TA (10-15 s)
DA = log P(0)/P(t)
Pump
Intensity
Transient
Concentration
DA
Graph of t vs DA
time
time
td1
P(t)
Intensity
blank
P(0) pump probe
Transmitted
Light at time 1
P(t1)
time
time
probe
Intensity
Intensity
pump
td2
time
Transmitted
Light at time 1
P(t2)
time
Femtosecond TA (10-15 s)
P(t) < P(0)
DA = log P(0)/P(t)
blank
P(0) pump probe
Intensity
Intensity
blank
P(0) pump probe
td1
Decrease Transmitted
light P(t)
time
Increased
Transmitted
light P(t)
time
DA
Graph of t vs DA
td1
time
P(t)
P(t)
time
DA
P(t) > P(0)
Graph of t vs DA
time
time
New species after laser pulse.
Loss of species after laser pulse.
Single Wavelength to Full Spectrum
Single Wavelength
Full Spectrum Data
Femtosecond TA (10-15 s)
A striking example
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 DT/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
DT/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.
Suggested mechanism
To complete the description of photoinduced
dynamics in rhodopsin, we report the portion of the
DT/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.