MODULE 26
Instrumentation for Time-resolved Absorption Spectrometry
(Excerpted from Henbest and Rodgers’ chapter in “Handbook of Electron Transfer”, V. Balzani,
ed., Wiley-VCI, 2001.)
In this section two operational instruments are described which are based on the principles
discussed in Module 25. One is useful for monitoring processes with reaction times in the 10-8 s
to >100 s range-a so-called suprananosecond kinetic spectrometer. The other-an ultrafast kinetic
spectrometer-is employed for reactions occurring in the range <10-12 s to >10-9 s. Both these
spectrometers are in use in the Rodgers’ laboratory for investigating photo-induced processes.
A suprananosecond kinetic spectrometer
A schematic of the system is shown in Figure 25.1.
X
S
L
A
L
C
M
PM
L
O
PD
Q
PC
Figure 25.1.
Schematic layout of a typical suprananosecond kinetic spectrometer:
Q: Q-switched laser with harmonic generators, L: lens, X: cw monitoring lamp, S: shutter, A: aperture (see Figure
3 for detail), C: sample cuvette, M: monochromator, PM, PD: photodetectors, O: digital oscilloscope, PC: personal
computer.
1
The excitation source is a Q-switched Nd:YAG laser (Continuum Surelite I) which is capable of
a 10 Hz repetition rate but is typically used in the “replicate-one-shot” mode.
The nominal 6 ns pulse can contain up to 450 mJ at 1064 nm, which decreases after harmonic
conversions to 532 nm, 355 nm, or 266 nm. In addition to these harmonic lines the 355 nm line
can be used to pump an OPO (Opotek Magicprism), which provides tunable radiation in the
range 420 nm to ca 900 nm. The selected photolysis beam is incident on one face of a 10 mm x
10 mm quartz cuvette containing the sample.
In experiments in fluid solutions where absorbance at the excitation wavelength exc) can be
controlled, it is advantageous to arrange that the deposition of photons, and thereby the
production of excited states is as homogeneous as possible in the interrogated volume. The
Beer-Lambert law ensures that the concentration of excited states will decline exponentially with
distance from the laser input face. However, as stated below, the interrogation volume is
restricted to the first 1 mm from the laser-input face and thus absorbance values at exc of 1 per
cm or even greater can provide close to homogeneous energy deposition across 1 mm. The
monitoring light source is either a 150 W xenon arc lamp61-64 or a 100 W quartz-halogen lamp.
The path of the monitoring light is at right angles to the excitation beam and it interrogates the
first 1 mm of the irradiated volume behind the laser input face (see Figure 25.2). This is
arranged by the appropriate placement of lens and an aperture.
TOP VIEW
A
EXCITATION
BEAM
MONITORING
BEAM
APERTURE
1 mm
SAMPLE
1 cm x 1 cm
Figure 25.2
Detail of the geometrical relationships of the laser beam, the
cuvette and the monitoring beam.
B
SIDE VIEW
MONITORED
REGION
EXCITATION
BEAM
IRRADIATED
VOLUME
2
Xenon arc lamps operate at high pressure (20 atm) with a short arc configuration and require a
highly stable DC power supply for low baseline ripple. The lamp output is a smooth continuum
with weak superposition of lines in the visible and strong lines in the near IR. Xenon arc lamps
are prone to suffer from baseline ripple with a frequency less than ca 1 kHz. This can be caused
by power supply fluctuations or by wandering of the arc. These ripples can cause severe
distortions in the time profile of low intensity transient absorptions on the millisecond or longer
timescales. The current supply to the xenon lamp can be pulsed to high intensities65-68 to increase
the radiance. A 250 W xenon lamp with a 450 amp pulse has been reported to have a 700 times
increase in radiance in the UV, 300 times increase in the visible and 50 times in the IR. 65
Tungsten and quartz-halogen lamps69-73 are less subject to ripple and provide much more stable
outputs albeit with much reduced intensities. Tungsten emits a continuum from the near UV into
the IR.74
The quartz-halogen lamps have quartz filaments and are run at higher temperatures
than tungsten lamps, viz. 3000 to 3200K which gives somewhat higher UV outputs. A quartzhalogen lamp can be used as a standard for calibrating the spectral response of spectrometers.75
The high wattage lamps are used with shutters to protect the sample from undue photolysis, and
the detector from unnecessary fatigue.
The lamp housing is equipped with a rear reflector and a fused quartz, four element, spherically
corrected f/0.7 lens assembly (Oriel) to collect the light and transfer as much as possible through
the 1 mm aperture in front of the cuvette. The exiting beam is re-collimated and focused onto the
entrance slit of a 0.22 m monochromator (Spex 1681), which disperses the white light into its
component wavelengths and isolates a narrow range of wavelengths at the exit slit. The grating
may be either ruled76-81 or holographic.80,82-85 Ruled gratings can contain periodic ruling errors
that result in ghost images in the dispersion plane and therefore holographic grating are
preferred. Holographic gratings are manufactured from photo-resist materials that are exposed to
interference fringes produced from diffracted monochromatic light. They are free of ghost
images. There is a multitude of different types of monochromators with different
configurations.76-78,86,87 One important consideration is the f-number, defined as
f = fc/dc
(13)
where fc is the focal length and dc the diameter of the collimating mirror. The lower the fnumber, the more light is transmitted by the device.
3
In designing an optical system it is
important to arrange for the beam that is incident on the entrance slit of the monochromator to
have a convergence that matches the f-number of the monochromator. If the entrance optics has
a lower f-number than the monochromator then the grating will be overfilled and stray light may
be increased. If the f-number of the entrance optics is larger than that of the monochromator then
the area of the grating that is illuminated is decreased and the resolution of the instrument
suffers.77,80
The grating in most frequent use is a holographic type and covers the range from
230-800 nm with a blaze at 400 nm.
The photo-detector that monitors the intensity and time profile of the monochromatic light
issuing from the exit slit of the monochromator is a side-on photomultiplier tube (Hamamatsu
928) which has an excellent response in the UV, visible and near IR to about 900 nm.
Photomultiplier tubes (PMTs) are light detection devices capable of measuring weak fluxes of
light as low as 10-15 W. They depend on the photoelectric effect for their function, in which
electrons are emitted from the illuminated part of the active surface.
It consists of a
photosensitive cathode, an electron multiplier section (dynodes), and an electron collector
(anode) all sealed in a vacuum tube. When photons of the appropriate frequency are incident on
the photocathode (held at a high negative potential) electrons are emitted and are directed onto
the first of the dynode series where secondary emission of electrons occurs. Each dynode is held
at a lower negative voltage than its predecessor so that the arriving electrons possess about 100200 V of energy and thus each arriving electron generates several secondary electrons, thereby
producing a cascade that contributes to the multiplication process. In a typical 11-dynode stage
PMT each stage provides an amplification of ca 2.5 and an overall gain of 106 (2.5 11) or so.
The electrons leaving the final dynode are collected by the anode, and this charge, when run to
ground through a load resistance produces the output signal voltage. The PMT employed in the
current instrument uses just 5 stages of gain, with the 6th employed as collector electrode. In
this way gain is sacrificed for improved time response.88
[SEE MODULE 25 FOR DISCUSSION OF THE NANOSECOND BARRIER]
Photodiode/fast amplifier combinations can replace PMTs and this is particularly advantageous
in the near IR where PMT photocathodes tend to have poor quantum efficiencies.89
It is
important that the photodetector must respond linearly to the light intensity. This is checked by
4
inserting neutral density filters between the lamp and the monochromator entrance slit and
confirming that the detector output corresponds to that expected from the transmittance of the
filter. Excellent descriptions of the comparisons between detectors and the spectral response
characteristics of different photocathodes have been published.90-91 The current waveform on the
anode of the photo-detector is fed to the input connector of a transient digitizer/digital CRO and
the voltage generated is converted into digital format and read into a PC. The resulting stored
data can be processed for spectral/dynamic content by one of several available commercially
available data manipulation software packages.
The suprananosecond instrument described, in common with almost every other such instrument,
is a single beam spectrometer inasmuch as it records the absorbance difference between the prepulse and post-pulse values i.e. the combined spectra of the ground state and product state are
involved. Only when the sample has zero absorbance prior to the excitation event does the
experiment provide absolute absorption spectra. An example of this is presented in Figure 25.3,
which shows the transient difference spectrum of the triplet state of a metallophthalocyanine
compound, measured point by point from absorbance-time profiles such as the one shown in the
inset.
0.1
0.0
-0.1
0.07
-0.2
0.05
A 520 nm
A
0.06
-0.3
5 s
59 s
180 s
0.04
 = 129  s
0.03
0.02
0.01
0.00
0
100
300
200
300
400
5 00
time / s
-0.4
400
500
600
700
wavelength / nm
5
Figure 25.3: Main Figure: transient absorbance spectra recorded at three times (5, 59 and 180 s) post 355 nm
excitation of a solution of bis(trihexylsiloxy) phthalocyaninato- Sn(IV) in toluene solution. Inset: the time profile of
the absorption decay at 520 nm.
The spectrum in Figure 25.3 has positive and negative values of difference absorbance. When
the absorbance is negative, as shown at 630 nm, the ground state removed by the pulse has a
higher  value than the excited state generated concomitantly. When positive (530 nm), the
excited state has a higher  than that of the ground state. When both states absorb equally, there
is a zero value in the absorbance difference spectrum.
In some laboratories92,93 the spectrometer outlined above has been modified to include an optical
multi-channel analyzer (OMA) for obtaining spectral information in a “single shot". In this
arrangement the monitoring beam is steered into a spectrograph (“polychromator”) after the
sample, which disperses the white light over a diode array/CCD detector situated at its focal
plane. Time resolution requires some kind of “gating” which can be provided by (i) an electromechanical shutter in the monitoring beam, (ii) by using a pulsed lamp as the monitoring source,
or (iii) by applying a gating pulse to an intensification stage on the detector. The detector thus
provides a dispersed absorption spectrum of the species produced in the sample by
photoexcitation, each pixel corresponding to a particular wavelength interval. The recorded
spectrum is an average over the temporal width of the monitoring beam pulse.
Kinetic
information can be obtained by repeating the spectrum collection at a series of times between the
excitation and the interrogating pulses arriving. While the OMA technique will generate a
dispersed absorption spectrum from a single excitation event, in practice multiple repetitions are
employed to improve the signal-to-noise response.
The preceding narrative has focused on the uv-visible spectral region for interrogating the
transient states generated. However, in recent years there has been an increasing activity in midinfrared detection of time-resolved spectra. The major advantage that IR spectrometry brings is
that it offers the possibility of structural as well as kinetic information, and it provides distinct
absorption bands for some species that have only indistinct uv-visible absorptions. The major
drawback to IR observations is that extinction coefficients are generally much smaller than in the
visible region, which puts additional demands on instrumental sensitivity. Nevertheless several
6
interesting reports of suprananosecond studies have appeared.94-97 and one recently constructed
in the Rodgers’ laboratory will be described elsewhere.
An Ultrafast Kinetic Spectrometer
A schematic layout of the system extant in the Rodgers’ laboratory is shown in Figure 25.4.
D
C
800 +/- 40 nm
1 kHz; 1W;
100 fs
100 fs
E
A
B
F
G
H
Figure 25.4
Schematic layout of an ultrafast pump-probe spectrometer. Top left panel; A: cw diode-pumped, frequency-doubled,
Nd:YAG laser, B: mode locked Ti-S oscillator, C: regenerative Ti-S amplifier, D: Q-switched, frequency-doubled
Nd:YLF amplifier pump laser, E: second/third harmonic generators or OPA. Lower right panel; F: continuum
generation, G: sample, H: CCD spectrograph. The double arrow indicates the optical delay stage, and the dashed
line indicates the pump beam trajectory.
The laser assembly is Titanium-Sapphire based with a Ti-S oscillator (SpectraPhysics, Tsunami)
that is pumped by a frequency-doubled, diode pumped 5 W cw Nd:YAG laser (SpectraPhysics,
Millennia). The oscillator output (ca 10nJ, 70fs, 80 MHz) at 800±30 nm is steered into a Ti-S
7
regenerative amplifier (Positive Light, Spitfire) which is pumped with a frequency-doubled (527
nm) Q-switched Nd:YLF laser (Positive Light, Merlin) operating at 1 kHz. Before entering the
amplifier cavity the 70 fs pulse is stretched to lower its peak power, then steered into the cavity
where its amplitude is increased by ca 105. The amplified pulse is subsequently recompressed to
ca 100 fs. The output of the amplifier is typically 1mJ per pulse at a repetition rate of 1kHz. It
can be wavelength-tuned over a range 780 nm to 820 nm.
To obtain useful excitation
wavelengths (400 nm, 266 nm), the output of the amplifier is coupled into a harmonic generator
(CSX, SuperTripler). To achieve tunable excitation wavelengths in the visible and near IR, the
800 nm beam is employed as pump for an optical parametric amplifier (OPA-800,
SpectraPhysics).
A fraction of the amplifier output light (800 nm) is split from the main beam for continuum
generation prior to harmonic generation. This is accomplished by focusing the red light (a few
tens of J) into a 1-mm thick sapphire plate where, at sufficiently high peak powers, non-linear
optical processes such as stimulated Raman and multi-wave mixing occur within the beam waist.
This process (self-phase modulation) results in the production of a coherent white light beam
along the pump beam direction and having similar time duration. Before continuum generation
the red light beam traverses an optical delay line that provides an experimental time window of
1.6 ns with a step resolution of 6.6 fs. The energy of the probe pulses is ca 5J /cm2 at the
sample.
The pump beam is typically up to 50 J per pulse, although with OPA output this is rarely
possible. The spot size at the sample is normally about 2mm diameter, but again this varies with
conditions. A mechanical chopper operating at 30 Hz is positioned in the pump beam upstream
of the sample. This provides a series of pump on-pump off exposures (see below). The angle
between pump and probe beam is kept as small as possible and is typically 5-7o. The liquid
sample is held in a reservoir and pumped through a flow cell (path length of 2mm, or less). After
traversing the sample the probe beam is focused into a 400 m fiber optic cable and input into a
CCD spectrograph (Ocean Optics, SD1000) capable of 0.5 nm per pixel resolution over the range
400 nm to 800 nm. Spectral information is passed from the spectrograph to a PC where it is
sorted according to whether it is a pump on or pump off sequence. The chopper sync-out signal
8
is used by the software as an indicator of the state of the chopper. The pump off signals provide
the I0 data, and the pump on provides the corresponding I values. Typically 4000 excitation
pulses are averaged to obtain the transient spectrum at a particular delay time. A PC using
LabView (National Instruments) software routines acquires data from the CDD spectrograph and
operates the delay line. Repetition of the sequence at a series of delay line settings allows the
generation of a dynamic surface, and absorbance data at a particular wavelength as a function of
delay time provides time profiles.
The instrument response function (10% to 90% of rise) of the ultrafast spectrometer is ca 250fs.
Figure 25.5 shows an example of a dynamic surface and Figure 25.6 depicts a reaction time
profile obtained on the instrument described above. The system described is a typical example
of similar spectrometers to be found in laboratories around the world. There are variants, largely
concerning the pump laser. For example the pump-probe technique has been used to great effect
with dye laser sources from which sub-picosecond pulses are available.98-99 Similarly modelocked Nd:YAG and Nd:glass lasers have been employed. As described in an earlier section
pulse durations for these laser types are supra-picosecond but they have proven very useful
where reaction times are 50 ps and longer.100,101 As with the ultrafast systems continua can be
generated using, for example, residual near IR light after the harmonic generation process and
the detection system has been based on diode array spectrography.102-105, In addition there have
been reports of streak cameras being employed as time-resolved detectors in experiments where
continuum generation was avoided.106-108
9
0.08
0.06
A
C
absorption
0.04
0.02
0.00
-0.02
B
e/
tim
-0.04
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
ps
600 625 650
575
550
525
450 475 500
th / nm
waveleng
Figure 25.5
A dynamic surface generated by absorption of 400 nm light pulses in an aqueous solution of tetra-(kissulfonatophenyl) porphinato Ni(II). The positive feature indicated by "A" is an immediately formed absorption with
a peak at 490 nm. This decays with fast and slow components (see Figure 25.6). The negative feature indicated by
"B" is loss of absorption at the wavelength of the ground state due to removal of some fraction of its population
upon photo-excitation. This recovers only slightly over the 5 ps time window. The positive feature labeled "C"
which decays only slightly over the 5 ps time window is possibly another part of the longer-lived contributor at 490
nm. Careful scrutiny of the surface shows that the fast component at "A" has a wavelength maximum at 490 nm,
whereas the slower contributor has a maximum near 460 nm. Surfaces such as this contain a wealth of information
on a single page.
10
0
.
0
8
0
.
0
6
0
.
0
4
absorance
1
k
=
1
.
1
6
p
s
f
a
s
t
1
k
=
0
.
0
8
p
s
s
l
o
w
0
.
0
2
0
.
0
0
0
1
2
3
4
5
6
7
8
9 1
0
t
i
m
e
/
p
s
Figure 25.6
A two-dimensional slice from the surface shown in Figure 25.5 at 490 nm, allowing rate parameter evaluation. The
fast component has a lifetime of some 860 fs; the slower component has a lifetime of 12.5 ps.
11
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14