Photoconversion of spiropyran to merocyanine in a monolayer observed using nanosecond pump-probe brewster angle reflectometry

Article abstract of DOI:10.1071/CH12093

A new apparatus for nanosecond-time-resolved Brewster angle reflectometry is described that can be used to measure transient angle-resolved reflectivity changes in thin films and monolayers in a single pulsed laser shot. In order to achieve this, a cylindrical lens is placed in the probe beam path replacing the goniometer that is usually used for angular scanning in other systems. Using two synchronized nanosecond pulsed lasers in pump-probe configuration it is possible to measure the kinetics of photoinduced conformational changes by altering the delay between pump and probe pulses. The system was used to observe nanosecond time-resolved photodynamics in a spiropyran monolayer at the air-water interface. After UV excitation the spiropyran converted to its merocyanine form in two stages. The first stage occurred with a timescale close to the instrument time resolution (tens of nanoseconds) whereas the second stage occurred over a few hundred nanoseconds.

Full text of DOI:10.1071/CH12093

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2012, 65, 283–289
Full Paper
http://dx.doi.org/10.1071/CH12093
Photoconversion of Spiropyran to Merocyanine
in a Monolayer Observed Using Nanosecond
Pump-Probe Brewster Angle Reflectometry
A B C
, ,
D
D
Bernhard Siebenhofer,
Sergey Gorelik, Anton V. Sadovoy,
E
D
A B F G
, , ,
Martin J. Lear, Hong Yan Song, Christoph Nowak,
D G
,
and Jonathan Hobley
A
Austrian Institute of Technology, AIT, Donau-City-Straße 1, 1220 Vienna.
Centre for Biomimetic Sensor Science, 50 Nanyang Drive, Research
Technoplaza, XFrontiers Block, Singapore 63755.
B
C
University of Natural Resources and Life Sciences, Department of Nanobiotechnology,
Muthgasse 11, 1190 Vienna, Austria.
D
Institute of Materials Research and Engineering, IMRE, 3 Research Link,
Singapore 117602.
E
Department of Chemistry, National University of Singapore, 3 Science Drive, Singapore
117543.
F
Centre of Electrochemical Surface Technology, CEST, Viktor-Kaplan-Str. 2,
2700 Wiener Neustadt, Austria.
G
Corresponding authors. Email: hobleyj@imre.a-star.edu.sg; C.Nowak@ait.ac.at
A new apparatus for nanosecond-time-resolved Brewster angle reflectometry is described that can be used to measure
transientangle-resolvedreflectivitychangesinthinfilmsand monolayersina single pulsedlaser shot. Inordertoachieve this,
a cylindrical lens is placed in the probe beam path replacing the goniometer that is usually used for angular scanning in other
systems. Using two synchronized nanosecond pulsed lasers in pump-probeconfiguration it is possible tomeasure the kinetics
of photoinduced conformational changes by altering the delay between pump and probe pulses. The system was used to
observe nanosecond time-resolved photodynamics in a spiropyran monolayer at the air-water interface. After UV excitation
the spiropyran converted to its merocyanine form in two stages. The first stage occurred with a timescale close to the
instrument time resolution (tens of nanoseconds) whereas the second stage occurred over a few hundred nanoseconds.
Manuscript received: 13 February 2012.
Manuscript accepted: 20 February 2012.
Published online: 21 March 2012.
proven to be successful at making observations.[8–11] PM-FTIR
can also be used on other substrates. However, fast nanosecond
kinetics of chemical processes has not been reported with either
SPR or PM-FTIR. Cavity ring-down spectroscopy (CRDS)
can be used to make thin film measurements by making the
probe-beam travel through the sample with multiple passes,
thereby increasing the effective pathlength.[12–17] However time
resolution on CRDS will be limited by the multi-pass require-
ment. Also, small absorbance or scattering losses from the
substrate are problematic witha multi-pass configuration. CRDS
has therefore not yet been used for fast kinetic studies. Brewster
angle microscopy (BAM) was invented as a technique to study
adsorbed molecules on solids and floating monolayers at the air/
liquid interface.[4–7] It is based on the fact that p-polarized light
is fully refracted and therefore not reflected when it is incident at
an interface at Brewster’s angle. However, the presence of a
monolayer on the reflecting surface alters the refractive index at
the interface and some reflection occurs with an intensity that
depends on the refractive index of the monolayer. This means
Introduction
Self assembled layers that form at interfaces between dissimilar
materials are interesting regions where unique chemical reac-
tions and biological processes can occur.[1–3] This is because
within such layers the environment is quite different compared
with both of the bulk media that create the interface. Molecules
that exist within such regions are often highly ordered, yet fluid,
states in which molecules move predominantly in two dimen-
sions and where specific molecular interactions strongly hold
the layer together.[4–7] Additionally these interfaces can exhibit
potential gradients due to forced conformations of molecules at
the interface.[3] However the study of reactions in such regions is
somewhat limited by the fact that these ordered layers are too
thin to see optically in transmission as they are sub-light
wavelength in thickness and give a tiny phase shift or absor-
bance to light passing through them. To overcome this, several
techniques have been successfully applied. On metallic surfaces
surface plasmon resonance (SPR) and polarization modulation
Fourier transform infrared spectroscopy (PM-FTIR) have
Journal compilation Ó CSIRO 2012
www.publish.csiro.au/journals/ajc

284
B. Siebenhofer et al.
Pump beam
Probe beam
CCD array
Signal
O
NO2
Delay
N
generator
Reference
O2N
Sample area
UV, Vis, heat
R
NO2
NO2
UV, Vis, heat
Pump
laser
Polarizer
Analyzer
Nꢀ
ꢁO
Vertical
slit
R: C10H20OOH
R
Cylindrical
lens
Scheme 1.
Computer
Monolayer
that if we image a monolayer at Brewster’s angle we can obtain a
high contrast image of the lateral morphology of the layer with
an inherently dark background. It has been shown that, using
pulsed pump-probe laser configurations, it is possible to measure
time-resolved events using BAM.[18–21]
Water
RecentlythepresentauthorsreportedtheuseofBrewsterangle
reflectometry (BAR) to measure very small absorbance changes
down to ,10ꢀ6 in a single pulse transient absorption measure-
ment in spiropyran monolayers on quartz and water. The system
developed to achieve this utilized a precision motorized goniom-
eter to scan small angle steps in the vicinity of Brewster’s
angle.[22,23] That work showed that transient absorption measure-
ments in spiropyran monolayers were possible with a continuous
wave (CW) probe laser and a nanosecond pulsed laser as an
excitation source. In principle that apparatus could therefore
achieve nanosecond time-resolution, however this was not dem-
onstratedinthatwork. Onedrawbackofthetechniquewasthatthe
angle resolution was only achievable with a precise goniometer
and by scanning the angle whilst measuring many kinetic traces.
This method is effective for single shot kinetic traces, but a more
cost-effective approach may be attractive. Also it may sometimes
be desirable to determine the angle dependence in a single shot.
This work describes the development of an alternative
strategy for BAR, using a fixed angle for the incident laser
whilst still achieving angle resolution, by using a cylindrical lens
to achieve a range of angles at the sample on the interface. This
eliminates the need for an expensive precision motorized
goniometer. The new apparatus was used to make observations
on photoactive spiropyran monolayers at an air-water interface.
Amphiphilic spiropyrans (Scheme 1) can often form mono-
layers at an air-water interface.[19–22,24–26] When colourless
spiropyran is irradiated by UV light it converts to a ring-open
planar merocyanine with increased dipole moment and which
also absorbs visible light.[27–32] In the dark or photochemically,
merocyanine converts back to spiropyran. Although many
works describe the photoactivity of spiropyran monolayers,
very few actually attempt to mechanistically understand the
photoconversion of the spiropyran to its merocyanine.[24–26]
This lack of mechanistic study is no doubt because transient
measurements have been hampered by the short optical path-
length of the monolayer. The current work aims to give
information on the kinetic processes in a spiropyran monolayer
on water following pulsed UV excitation and to determine the
rate of formation of the merocyanine form of the molecule.
Fig. 1. (Top) Experimental setup for pump-probe Brewster angle reflec-
tometry. The top insert shows the method of self referencing measurements.
(Bottom) Photograph of the apparatus showing its actual layout.
NanoFilm EP3TM BAM with nanosecond laser excitation,
except that it has an additional photomultiplier tube (PMT)
detector, connected to a Tektronics Oscilloscope (TDS 2014B
1GS sꢀ1, 100 MHz) with 50 O termination for determining
nanosecond kinetics of reflectivity change. This apparatus was
used both to visualize the monolayer (with its CCD camera) and
to determine kinetic information (with its PMT) after UV ex-
citation of the monolayer. The new BAR apparatus developed in
this work was custom built (Fig. 1): A pulsed probe laser (8 ns,
532 nm, 10 Hz Nd : YAG Litron Laser Nano-L-280–10, 140 mJ)
was mounted on a sloped platform approximately set to the
calculated Brewster-angle of the air-water interface. The probe
light was p-polarized using a Glan-Taylor polarizer. A combi-
nation of a vertical slit and a cylindrical lens were used to focus a
vertical slit cross-section of the initially round laser beam onto a
point on the sample surface so that the incident beam has varying
angles of incidence at the sample as determined by the focal
length of the cylindrical lens. The reflected beam therefore
diverges over a range of angles. As the approximate centre of the
laser beam is incident at the Brewster angle, reflectivity will be
minimized there. According to Fresnel’s equations for
p-polarized light this leads to a parabolic reflectivity increase
around Brewster’s angle.[23]
Experimental
Brewster Angle Microscope
Two types of BAR apparatus are used in this work. The first has
been described previously[22,23] and is essentially based on a
The reflected light was collected using a charge-coupled
device (CCD) camera (DU420-BV, Andor Technology).

Photoconversion of Spiropyran to Merocyanine in a Monolayer
285
1.6 ꢂ 10ꢁ5
The probe laser was appropriately attenuated using neutral
density filters so it did not saturate the camera. This camera
features a two dimensional array of pixels that allows the
collection of the fully expanded probe beam on a shot to shot
basis. For dark current reduction the camera is thermoelectri-
cally cooled to ꢀ808C. Additionally the CCD chip was analyzed
in two halves. One-half measured the intensity profile of probe
light that had reflected from a UV irradiated region on the
sample and the other half monitored light from a non-irradiated
part of the sample. The irradiated and non-irradiated zones were
controlled by inserting a razor blade to partially mask the
sample on the probed area. In this way every pulsed laser shot
had its own internal reference (no-pump) beam so that reflec-
tance changes and angular shifts could be accurately determined
without shot to shot noise due to beam profile changes or noise
from vibrations in the laboratory. In this case the main noise is
expected to be shot-to-shot noise in the pump-laser beam
intensity (usually ,10 %).
2.5 ꢂ 10ꢁ6
2.0 ꢂ 10ꢁ6
1.5 ꢂ 10ꢁ6
1.0 ꢂ 10ꢁ6
5.0 ꢂ 10ꢁ7
1.4 ꢂ 10ꢁ5
1.2 ꢂ 10ꢁ5
1.0 ꢂ 10ꢁ5
8.0 ꢂ 10ꢁ6
1.46 1.48 1.50 1.52 1.54 1.56 1.58
n
6.0 ꢂ 10ꢁ6
4.0 ꢂ 10ꢁ6
2.0 ꢂ 10ꢁ6
0
n ꢃ 1.463
n ꢃ 1.503
n ꢃ 1.543
n ꢃ 1.583
52.7 52.8 52.9 53.0 53.1 53.2 53.3 53.4 53.5 53.6
Angle [°]
Fig. 2. Simulation of Brewster angle minimum upon variation of the real
part of the refractive index.
The 8 ns pulse from a Litron NANO-S-60–30 Nd : YAG laser
was used to excite the sample. Either 355 nm 3rd or 266 nm 4th
harmonics can be used depending on the respective samples
absorption spectrum. The laser was equipped with a variable
attenuator that could be used to vary the output energy from
0–10 mJ for 355 nm and 0–6 mJ for 266 nm. A Plano-convex
lens was used to reduce the pump laser spot size to a diameter of
,2 mm on the sample surface. Maximum fluences of
,120 mJ cmꢀ2 were estimated for this configuration.
Two delay generators (Stanford Research Systems DG535
and Berkley Nucleonics Corporation BNC-465) were used to
synchronize the flashlamps and Q-switches of each laser, both
lasers relative to each other and to the shutter of the CCD
camera. Variation of the delay time between the pump and probe
lasers allowed the measurement of kinetics with nanosecond
time resolution. The time-resolution is limited only by the
temporal convolution of pulse lengths of the pump and
probe lasers.
5.0 ꢂ 10ꢁ6
k
k
k
k
ꢃ
ꢃ
ꢃ
ꢃ
0
53.25
53.20
53.15
0.08
0.16
0.24
4.0 ꢂ 10ꢁ6
53.10
53.05
3.0 ꢂ 10ꢁ6
2.0 ꢂ 10ꢁ6
1.0 ꢂ 10ꢁ6
0
0
0.05 0.10 0.15 0.20 0.25
Kappa
53.0
53.1
53.2
53.3
53.4
Angle [°]
Fig. 3. Simulation of Brewster angle minimum upon variation of the
imaginary part of the refractive index.
For measurements on spiropyran monolayers a high
intensity white light source (Thorlabs OSL1-IC Fibre Illumi-
nator, 150 W) was used to fully photobleach the monolayer
back to the spiropyran form before the experiments on
photocoloration kinetics were started. This was done because
the equilibrated form of this particular spiropyran is predom-
inantly the merocyanine form if the sample is left in the
dark.[27–31]
Results and Discussion
Theoretical Considerations
The reflectivity and transient absorption changes of the interface
can be simulated using Fresnel equations.[21–23] For this work
WINSPALL (Ju¨rgen Worm, MPIP Mainz) is used. This soft-
ware can be used to compute the reflectivity of multilayer sys-
tems utilizing Fresnel equations and matrix formalism. Figs 2
and 3 show the behaviour of Brewster’s angle upon variation of
real and imaginary part of the refractive index of a floating
monolayer on water. This shows that a one shot Brewster angle
measurement can be used to simultaneously access transient
absorption via the imaginary part of refractive index Kappa (k)
and reflectivity changes caused by changes in n, the real part of
refractive index. Additionally it can be seen that the angular shift
with absorption is linear over the range of values simulated,
which are also the range of angular shifts found experimentally.
In addition, the change in the reflectivity with the real part
of refractive index can be approximated to linear (with
slight deviation) over the range of intensity changes found in
this work.
Sample Preparation
A small Langmuir-Blodgett trough (7.5 cm by 0–12 cm surface
area) was cleaned using ethanol, acetone, chloroform, and
deionized water. The bulk liquid sub-phase was deionized water
(ELAGA Laboratory Water, Purelab option-Q). The floating
layer was deposited using a Hamilton micro-syringe. The pho-
tochromic dye solution was added drop by drop with the needle
parallel and as close as possible to the surface without actually
touching it. Before use the syringe was cleaned with ethanol,
acetone, chloroform, and deionized water. The monolayer
was compressed to give
0.34 nm2 moleculeꢀ1
a molecular monolayer with
.
Amphiphilic spiropyran, 11-(30,30-dimethyl-6,8-dinitrospiro
[chromene-2,20-indoline]-10-yl) undecanoic acid, was synthe-
sized from 2,3,4-trimethylindolenium 11-bromoundecanoic
acid and 3,5-dinitrosalicylaldehyde via a two-step substitution
and condensation reaction.[22,23]
Spiropyran Monolayer Characterization and Kinetics
Fig. 4 shows a BAM image of a monolayer made by 30 mL of a
1 mM spiropyran in chloroform spreading solution on an area of

286
B. Siebenhofer et al.
20000
17500
15000
12500
10000
7500
5000
Signal
Reference
52.4
52.6 52.8 53.0 53.2
53.4
53.6 53.8
54.0
Fig. 4. BAM image of a spiropyran monolayer recorded with modified
Nanofilm EP3TM. The right part of the image shows the reflectivity change
after one shot irradiation at 355 nm.
Angle [°]
Fig. 6. Reflectivity changes in the irradiated part of the sample (signal)
compared with the reflectivity of the non-irradiated part (reference)
measured directly from the two halves of the Andor CCD camera (4 ms
pump-probe delay). Parabolic fits are shown for each curve.
3.0
Merocyanine
Spiropyran
2.5
Fig. 6 shows typical single shot signal for reflectivity
changes for a 355 nm excited monolayer compared with its
reference reflectivity angular dependence. The minima of the
datasets can be adequately fitted by a parabolic function and this
fitting protocol is used to determine both the intensity and
angular shifts for the kinetic studies.
One shot UV
0.15
2.0
1.5
0.10
Figs 7 and 8 show the kinetics of angular and intensity shift
for the spiropyran monolayer. The rise in reflectivity corre-
sponds to a rising value of the real part of the refractive index.
The Brewster-angle shift to greater angles means that the
transient absorption of merocyanine is bigger than that of
spiropyran, as expected. These results are in good agreement
with previous works by Gorelik et al.[22]
1.0
0.5
0
525
550
575
400
500
600
700
800
900
1000
The kinetics determined from the single shot angle depen-
dent measurements with different time delays are consistent
with a two-step mechanism for the ring opening of the spir-
opyran. According to this data, the first stage has a lifetime of
,30 ns and is therefore highly convoluted into the time response
for the system which is determined by the temporal overlap of
the pump and probe pulses. The second stage in the reaction is
much slower and has a lifetime of 230 ns (angular shift) and
570 ns (intensity shift). We can therefore estimate the lifetime
to be on the order of a few 100 ns, as this difference is not
significant compared with the experimental noise.
Fig. 9 shows the angular shift dependency on the pump laser
power. The dependence is approximately linear and the maxi-
mum conversion achieved at the highest fluence was 10 %
spiropyran to merocyanine. This is consistent with the low
conversions achievable in chloroform solution. In fact this
particular spiropyran appears to have a very low quantum yield
for coloration compared with its high quantum yield for
bleaching.[22,27–32]
It is worth commenting on the fluences that the monolayer is
exposed to without apparent damage, or saturation effects. First,
exact laser fluences are rather hard to determine as the exact spot
size is rather uncertain, so the values given should be considered
somewhat relative. Second, saturation is not apparent and the
photochemistry appears to be single photon in nature.
We attribute this to the inherently low quantum yield for
photo-merocyanine formation for this compound. We should
also consider temperature effects and possible destruction of the
monolayer. In fact the monolayer will absorb ,1/1000th of the
Wavelength [nm]
Fig. 5. Spiropyran and merocyanine absorption spectrum in chloroform.
Insert shows the increase in absorbance after a single, 355 nm pulse of laser
excitation at the same fluences used in the kinetic studies. In this case the
measurement is in a 0.5 cm quartz cell.
67.5 cm2 (,0.34 nm2 moleculeꢀ1). From this picture it is evi-
dent that the layer is homogeneous enough for obtaining reliable
self referenced measurements. In this image it can be seen that
the reflectivity of the monolayer increases in the region irradi-
ated by the UV laser. The reflectivity change was reversible and
the irradiation did not lead to destruction of the monolayer or the
molecules within the monolayer. All further studies are done on
monolayers with exactly this surface coverage.
Fig. 5 shows the absorption spectra for the spiropyran and
merocyanine form of the molecules used in the monolayers in
chloroform. The peak maximum is at ,560 nm, however
because water is more polar than chloroform we may expect a
significant blue shift of tens of nm in the monolayer.[31] Also in
Fig. 5 we can see the absorption change in a 0.5 cm pathlength
small volume cell upon a single pulse of 355 nm 10 mJ excita-
tion of spiropyran. The entire cell volume was irradiated and the
laser pulse was nearly 90 % absorbed. We can see that despite
the high absorbance of the 355 nm pulse by the spiropyran
the absorbance increase is only ,0.01. This means that the
conversion to the merocyanine in the monolayer is expected to
be very small, since the monolayer absorbance would only be
,10ꢀ3 at 355 nm.

Photoconversion of Spiropyran to Merocyanine in a Monolayer
287
2.5 ꢂ 10ꢁ3
0.25
0.20
2.0 ꢂ 10ꢁ3
1.5 ꢂ 10ꢁ3
1.0 ꢂ 10ꢁ3
0.15
2.0 ꢂ 10ꢁ3
0.20
0.10
0.15
1.5 ꢂ 10ꢁ3
1.0 ꢂ 10ꢁ3
5.0 ꢂ 10ꢁ4
0
0.10
5.0 ꢂ 10ꢁ4
0.05
0.05
0
0
0
ꢁ
5.0 ꢂ 10ꢁ4
ꢁ0.05
100
200
300
400
500
ꢁ
100
0
ꢁ
5.0 ꢂ 10ꢁ4
ꢁ
0.05
0
2000
4000
6000
8000
10000
Pump-probe delay time [ns]
Fig. 7. Kinetics of the angular shift in a pumped spiropyran monolayer and its exponential approximation.
Axis legends of the insert are identical to the main graph.
1.2
1.0
0.8
0.6
0.4
2.5 ꢂ 10ꢁ3
2.0 ꢂ 10ꢁ3
1.5 ꢂ 10ꢁ3
2.5 ꢂ 10ꢁ3
2.0 ꢂ 10ꢁ3
1.5 ꢂ 10ꢁ3
1.0 ꢂ 10ꢁ3
1.2
1.0
1.0 ꢂ 10ꢁ3
5.0 ꢂ 10ꢁ4
0
0.8
0.6
0.2
0
0.4
5.0 ꢂ 10ꢁ4
0.2
0
0
ꢁ0.2
ꢁ0.4
ꢁ5.0 ꢂ 10ꢁ4
ꢁ0.2
ꢁ5.0 ꢂ 10ꢁ4
ꢁ100
0
100
200
300
400
500
0
2000
4000
6000
8000
10000
Pump-probe delay time [ns]
Absorbance
Reflectivity shift
Fitted absorbance
Fitted reflectivity shift
Fig. 8. Kinetics of the intensity shift in a pumped spiropyran monolayer and its exponential
approximation with angular shifts for comparison. Axis legends of the insert are identical to the main
graph.
incident laser beam (i.e. a maximum of ,0.3 mJ cmꢀ2). This
energy would be expected to dissipate to the sub-phase, which
has a high heat capacity, on the picosecond timescale,[20] hence
the monolayer is unlikely to get hot enough to damage. This is
verified in BAM images which show reversible reflectivity
changes without monolayer damage, as exemplified in Fig. 4.
For comparison, the current methodology was compared
with the EP3 system at a single angle of reflection using the
photomultiplier detector. This data is shown in Fig. 10 with the
fits from the reflectivity and angular shifts determined with
the newer apparatus. It is quite apparent that both systems give
very consistent kinetic traces, giving us confidence in the
kinetics that can be derived.
monolayer occurs in two stages. The first occurs in several tens
of nanoseconds and the second occurs in several hundred
nanoseconds. Two scenarios could account for these observa-
tions, and these will be explored next.
The first possibility is that the mechanism involves a fast
singlet manifold and a slower triplet manifold. Non-nitrated
spiropyrans are known to convert to their merocyanines very
rapidly in sub-nanosecond timescales[28] and the fast component
in the current case could then be singlet state conversion to
merocyanine on similar timescales. Should a triplet state of
spiropyran form it would be quenched by oxygen in the
subphase in ,200 ns, since the solubility of oxygen in water
is ,10ꢀ3 M and with an expected quenching rate constant of
Therefore with some confidence we can state that the
transformation from spiropyran to merocyanine in the
,5 ꢁ 109 dm3 molꢀ1 sꢀ1 [33]
.
This timescale is consistent with
the current experimental observation and a triplet manifold may

288
B. Siebenhofer et al.
0.20
0.15
0.10
0.05
0
2.0 ꢂ 10ꢁ3
1.5 ꢂ 10ꢁ3
1.0 ꢂ 10ꢁ3
5.0 ꢂ 10ꢁ4
0
molecules in solution. This would also be expected to slow down
the process of isomerization from cis- to trans-forms.
Conclusions
An apparatus was constructed that allows easy one-shot angle-
resolved reflectivity measurements, probing Brewster’s angle
and its vicinity simultaneously. Qualitative measurements on
organic photochromic dyes have shown that the setup is feasible
for conversion kinetics measurements on monolayers on water.
Spiropyran films studied in this way exhibited rather slow
photoconversion to the final merocyanine over a few hundred
nanoseconds, probably because of steric hindrance to bond
rotations in the packed monolayer.
0
20
40
60
80
100
120
2
Pump laser fluence [mJ cmꢁ
]
Fig. 9. Angular shift at 1 ms pump probe delay time depending on pump
laser fluence.
Acknowledgements
This work was funded by the International Graduate School Bio-Nano-Tech,
a joint venture of University of Natural Resources and Life Sciences Vienna
(BOKU), the Austrian Institute of Technology (AIT) and the Nanyang
Technological University (NTU). Equipment and materials were supported
by A-STAR JCO and A-STAR-JST funding.
Previous data with its best fit
Previous data with fit to
new intensity shift data
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0
500
1000
1500
2000
Pump-probe delay time [ns]
Fig. 10. Comparison of the fits obtained to data from the new methodology,
with the data and fit for the previously reported method with angle scanning
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