A Transient Absorption Spectrometer Using an Ultra Bright
Laser-driven Light Source
Man-Nung Su and Jim Jr-Min Lin
Oscillation of the EQ-99 light source
We measured the time-resolved output of the EQ-99 light source with a photodiode and the
result is shown in Figure S1. Significant oscillation in the light intensity was observed. This
oscillation should originate from the laser-driven plasma. There is a physical phenomenon
called ‘plasma oscillation’, in which the charged particles all oscillate together at a
characteristic frequency that depends on the nature of the particular plasma. It seems that the
fluctuation of our spectral baseline was caused by this plasma oscillation. Because the
oscillation period is about 4.8 s, this effect would not be observed if the integration time is
much longer. (The manufacture claims the stability of such light sources is 0.55%
(peak-to-peak) at 8 ms integration time over 8 sec.1)
FIG. S1. Oscilloscope trace showing the intensity oscillation of the EQ-99 light source
recorded by a silicon photodiode. The peak-to-peak value of the oscillation is 260 mV, which
may contribute an error up to 0.07 in absorbance unit. The oscillation period is about 4.8 s.
Spectra and brightness of the EQ-99 light source
The spectrum of the EQ-99 light source recorded by the iCCD spectrometer is shown in
Figure S2. The decline in intensity near 400 nm is mostly due to the decrease in the quantum
yield of the intensifier photocathode. By moving PM2, the width of the focusing spot was
measured to be about 200 m, which corresponds to ~100 m width at the source because the
magnifying factor of our focusing system (PM1 and PM2) is 101.6/50.8 = 2. See Appendix
for the brightness measurement.
In Figure S2, the signal is 5.84x105 counts (0.3 nm)1(20 s)1 at 550 nm. Considering the
quantum efficiency of the iCCD photocathode is about 20% and one photoelectron
contributed to 1.06 counts at our intensifier setting, the signal counts can be converted to
4.6x1011 photons nm1 sec1 or 1.8x104 mW nm1. The solid angle at the slit is about 0.025
sr (18 mm beam diameter, 101.6 mm focal length) and the slit width is 10 m. If we further
assume a height of 0.4 mm for the focus spot at the slit, we can estimate the brightness of the
light, which is 1.8 mWmm2nm1sr1, about 9% of the claimed source brightness (20
mWmm2nm1sr1). We think this is a quite reasonable value, considering the grating
efficiency is 70% at 550 nm and the light has passed through one neutral density filter of
0.5OD (T=32%) and 10 optics (PM1, HR3, HR2, 2 windows of the sample cell, HR1, F, PM2
and two mirrors inside the spectrometer). It is noteworthy that for a smaller gate width, the
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Relative Intensity
Counts/20 s
slit can be a bit wider (e.g., 30 m) to pass more light without sacrificing the spectral
resolution (the spectrograph dispersion in the current system is 0.65 nm/30 m). In addition,
we have tested the iris effect of the intensifier. We found the central pixels have about 41%
relative efficiency at a small gate width of 5 ns, it reaches 63% at 10 ns gate width, 79% at 20
ns, and 95% at 40 ns.
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Mirror translation (m)
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Wavelength (nm)
FIG. S2. Reference spectrum of the laser driven plasma light source (EQ-99) recorded by the
iCCD spectrometer. The decline in intensity near 400 nm is mostly due to the decrease in the
photocathode quantum yield of the iCCD. Inset: Horizontal intensity distribution on the
entrance slit of the spectrometer recorded by translating the parabolic mirror (PM2) before
the entrance slit. The symbols represent experimental data, and the line is a Gaussian fit. (Slit
width: 10 m; iCCD gate width: 200 ns; total 100 accumulations)
Analysis on the effect of synchronization
In Figure S3, we took 24 and 19 absorption spectra (with no sample) with and without
synchronization control, respectively. Each absorption spectrum was deduced from two
consecutive spectra of which each was accumulated for 100 pulses (the total exposure time is
then 200 ns x 100 = 20 s) and the time interval between taking these two spectra was about
10 minutes. The former one was chosen as the reference spectrum. The intensity growth of
the EQ-99 light source in a longer time scale explains the overall negative offset in the
absorbance. It is obvious that the free-running experiment has a much bigger range of the
baselines (that is, its baseline fluctuates much more).
(a) Synchronized
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(b) Free running
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Wavelength (nm)
FIG. S3. Statistical analysis of blank absorption spectra (vacuum in the sample tube) with and
without synchronization to the oscillation of the EQ-99 light source. The symbol and error
bar indicate the average value and ±2 range (= standard deviation) of the absorbance at a
bandwidth of 30 nm. 24 and 19 absorption spectra were used in the statistical analyses for the
synchronized and free-running conditions, respectively.
Comparison with D2 lamp as light source
To illustrate the effect of brightness of the light source, Figure 7 shows the reference and
blank absorption spectra obtained by using the laser-driven plasma light source (EQ-99) and
by using a common D2 lamp (Ocean Optics, D-2000) with other experimental conditions kept
similar. Under the same accumulation time (200 ns x 100 = 20 s), the signal of the EQ-99
light source is about 100 times stronger than that of the D2 lamp. To make the noise level in
the absorption spectrum using the D2 lamp comparable to that using the EQ-99 light source,
the accumulation time has to be about 100 times longer (200 ns x 10000 = 2 ms). For a low
repetition rate experiment (e.g., 1 Hz), the time to average the data for 10000 pulses is very
long (~3 hours). In other words, to use a bright light source results in a good signal-to-noise
ratio in a much shorter accumulation time; this allows us to investigate more deeply and
widely the transient absorption spectra by performing more measurements at different
experimental conditions.
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Signal (count)
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D2 lamp 10000 accumulations
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EQ-99 100 accumulations
D2 lamp 100 accumulations
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Wavelength (nm)
FIG. S4. Comparison of performance for the EQ-99 light source and a D2 lamp. Another
iCCD detector (Andor iStar DH320T-18F-E3 with WE-AGT photocathode) was used to
better cover the spectral range of the D2 lamp. The upper graph is the reference spectra and
the lower graph is the blank absorption spectra (air in the absorption tube). The blank
absorption spectra using the D2 lamp (10000 accumulations) and the EQ-99 light source (100
accumulations) are shifted upward by 0.2 and 0.4, respectively, for visualization. Slit width:
10 m; iCCD gate width: 200 ns.
Experimental details of the transient absorption spectra of excited NO3
The N2O5 sample was synthesized by mixing NO2 and O3.2 The product gas was then trapped
in dry ice/acetone cooling bath and white N2O5 solid was formed. During the experiment, the
N2O5 solid was stored at about 57 oC, and the buffer gas, such as nitrogen or SF6, was
flowed through the storage cell and then into the sample cell.
1
Peter J. Dwyer, Energetiq Technology, Inc., private communication.
2
G. Schott and N. Davidson, J. Am. Chem. Soc. 80(8), 1841 (1958).