Becker & Hickl GmbH
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10829 Berlin
Tel.
030 / 787 56 32
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030 / 787 57 34
email: info@becker-hickl.de
http://www.becker-hickl.de
PCSABS.DOC
Transient Absorption Measurements with the PCS-150 / PCI-200
Boxcar Modules
General Arrangement
To measure the absorption of molecules in excited states the sample is pumped with a strong
laser pulse, and, after a variable delay, the absorption of a second ‘probe’ pulse is measured.
The lifetime of the excited state is derived from dependence of the measured absorption on
the delay between the pump and the probe pulse.
In the figure below a simple arrangement for transient absorption measurements is shown.
Prism
Optical
Delay
Probe
Beam
D1
P1
Dye Laser
Pulsed Laser
M2
P2
Pump
M1
Sample
Beam
P3
Filter
PDM-400
D2
PDI-400
D3
Trigger
Channel A
Channel B
PCI-200
or
PCS-150
PDI-400
Transient Absorption Measurement
The output of a high power pulsed laser (i.e. N2 laser, excimer laser or frequency multiplied
diode laser pumped YAG) is divided into two parts. One part is used to pump the sample, the
other part pumps a dye laser which generates a light pulse of the appropriate wavelength to
probe the absorption of the excited molecules in the sample. The detector D1 is a fast PDM400 photodiode module which generates a trigger pulse for the PCI-200 Boxcar Module. The
1
absorption in the sample is measured by the detectors D2 and D3. D1 and D2 are PDI-400
integrating photodiode modules and deliver energy proportional output pulses of some 100ns
duration. The amplitudes of these pulses are recorded by the two signal channels of the
PCI-200 Boxcar module. The PCI-200 is run in the ‘Fixed Delay’ mode. Thus, it records a
curve consisting of subsequent averages over a selectable number of D2 and D3 intensity
values. If the optical delay is continuously changed during the measurement and the quotient
A/B is displayed the result shows the decay of the absorption of the excited state species in
the sample.
Instead of the PCI-200 also the PCS-150 module can be used. However, due to its small gate
width the PCS-150 has a higher noise so that the accuracy is lower than with the PCI-150.
In the simple setup shown above there is no fixed relationship between the optical delay and
the x axis of the recorded absorption decay function. In the figures below some solutions to
accurately control the probe pulse delay are shown.
The first solution is to drive the delay unit by a step motor which is controlled by an STP-240
step motor controller. The STP-240 is controlled by the PCI-200 software so that a defined
delay step is made after the specified number of shots has been averaged.
Step Motor
Prism
Optical
Delay
Probe
Beam
D1
P1
Dye Laser
Pulsed Laser
M2
Filter
D2
P2
Pump
M1
PDM-400
PDI-400
Sample
Beam
D3
P3
Trigger
Channel A
Channel B
PCI-200
or
PCS-150
STP-240
PDI-400
Transient Absorption Measurement
Delay control via step motor and STP-240 step motor controller
Another solution is to use an external step motor controller which is triggered by the laser
shots. The controller is programmed either to make a small step after each laser shot or to
make a bigger step after the same number of shots that are averaged for one curve point in the
PCI-200.
Step Motor
Step Motor
Controller
Prism
Optical
Delay
Probe
Beam
D1
P1
Dye Laser
Pulsed Laser
M2
Filter
P2
Pump
M1
Sample
Beam
P3
PDM-400
D2
PDI-400
D3
Trigger
Channel A
Channel B
PCI-200
or
PCS-150
PDI-400
Transient Absorption Measurement
Delay control via external step motor driver triggered by the laser
2
Another problem in the shown setup is that pump and the probe beam must be kept in perfect
alignment when the prism of the delay unit is moved. Furthermore, the probe beam diameter
must be kept constant. Although there are self-aligning optical systems which compensate for
these errors, this is not a simple task. The next figure shows a way to avoid all these
problems.
Delay
DEL-150
Diode
Laser
Probe
Beam
D1
Filter
PDM-400
D2
P2
Pulsed Laser
Pump
Sample
PDI-400
Beam
D3
P3
Trigger
Channel A
Channel B
PCI-200
or
PCS-150
PDI-400
Transient Absorption Measurement
Probe beam generated by diode laser triggered via electronic delay unit
For the probe laser a diode laser is used. Because a diode laser can be triggered electronically
with a high accuracy the optical delay unit can be replaced by an electronic one which is
triggered from the pump laser via the photodiode module D1. For the delay unit, the DEL-150
ps delay generator is available. The DEL-150 is a PC module and can be placed in the same
computer as the PCI-200.
Of course, the diode laser suffers from its restriction to some discrete wavelenths. However, a
lot of different wavelegths from 635nm to 850nm are available in the red part of the spectrum,
and green and blue lasers have become available recently. The pulse width of fast diode lasers
is some 10ps, so that the method is worth to be considered.
Nonlinear Optical Absorption Measurements
Another approach to the lifetimes of excited states is the measurement of the intensitydependence of the light absorption in organic dyes. A suitable arrangement is shown in the
figure below.
A high power pulsed laser (i.e. nitrogen laser or pulsed dye laser) generates short pulses
(< 1ns) with high energy (1mJ). The intensity is controlled by a suitable optical attenuator.
The beam is split into two parts by the glass plate P2. The main part of the light is focused
into the sample cell C1. The other part is fed through the reference cell C2. Both light signals
are fed through a filter to the Detectors D1 and D2. D1 and D2 are PDI-400 integrating
photodiode modules and deliver energy proportional output pulses of some 100ns duration.
These pulses are recorded by the two signal channels of the PCI-200. The trigger pulse for the
PCI-200 is generated by the photodiode PD3. Due to the long duration of the signal pulses,
delay lines in the signal path are not required. The gate width and the delay of the PCI-200
are set to sample a signal portion near the peak of the input pulses.
The main problem in non-linear optical absorption measurements is, that an absorption
accuracy of better than one percent over several orders of magnitude of the intensity is
required. To reach the required absorption accuracy, the shown setup uses a second signal
path trough a reference cell and the detector module D3. By using a common replaceable
3
filter for both channels the signal intensity can be held inside the useful input voltage range of
the PCI-200 without degrading the accuracy of the measured absorption values.
Filter
P1
Pulsed Laser
optical
Attenuator
D3
PDM-400
P2
C1
D1 PDI-400
C2
D2 PDI-400
M
Trigger
Channel B
PCI-200
Channel A
Measurement of non-linear absorption
The measurement delivers pairs of signal values from which the intensity and the ratio of
small signal and large signal absorption can be derived. By referring the A value (large signal
absorption) to the B value (intensity and small signal absorption) the influence of the laser
instability and the error of the optical attenuator do not appear in the measured absorption
values. The apparatus is able to measure absorption variations as small as 1 %.
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