Vernier VSP-FP User manual

© Ultrafast Systems LLC/Vernier Software & Technology
Vernier Flash
Photolysis
Spectrometer
(order code: VSP-FP)
Educational Flash Photolysis Spectrometer
Reference Manual
(version: 12/14/2015)

© Ultrafast Systems LLC/Vernier Software & Technology
Table of Contents
1General Information ............................................................................................................................... 4
1.1Introduction...................................................................................................................................... 4
1.2Specifications .................................................................................................................................. 4
1.2.1Spectrometer Specifications .................................................................................................... 4
1.2.2Equipment Specifications......................................................................................................... 6
1.2.3Detector Specifications ............................................................................................................ 7
2Safety Information.................................................................................................................................. 8
3Layout and Basic Principles of Operation......................................................................................... 10
4Hardware Description .......................................................................................................................... 12
4.1Turning the Spectrometer On and Off........................................................................................... 12
4.2Optical Filters................................................................................................................................. 12
4.3Software Description ..................................................................................................................... 13
4.3.1Graph Controls....................................................................................................................... 14
5The Basic Principles of Chemical Kinetics ....................................................................................... 15
5.1Stoichiometry................................................................................................................................. 15
5.2Reaction Rate................................................................................................................................ 15
5.3The Rate Expression and the Rate Constant................................................................................ 16
5.4Order of Reaction .......................................................................................................................... 17
5.5The Experimental Approach.......................................................................................................... 18
5.6The Relationships Between Rate Constant, Half Life, and Life Time ........................................... 18
5.7Pseudo Order ................................................................................................................................ 20
6Data Treatment ..................................................................................................................................... 21
6.1Data Acquisition............................................................................................................................. 21
6.2Kinetic Analysis ............................................................................................................................. 23
6.3The Basics of Photochemistry....................................................................................................... 25
7Flash Photolysis Experiments ............................................................................................................ 26
7.1Base Catalysis of the cis-trans Isomerization of Congo Red ........................................................ 26
7.2Isomerization of Mercury Dithizonate ............................................................................................ 31
7.3Determination of the Activation Energy of the Thermal Back Reaction of One Spiropyran in
Toluene.......................................................................................................................................... 34
8Appendix ............................................................................................................................................... 38
8.1Spectra of Flash Lamp and White Light LED ................................................................................ 38
8.2Spectra of Optical Filters ............................................................................................................... 40

© Ultrafast Systems LLC/Vernier Software & Technology
List of Figures
Figure 1: Examples of Vernier Flash Photolysis Spectrometer data ........................................................... 5
Figure 2: Detector responsivity vs. wavelength ........................................................................................... 7
Figure 3: Schematic of the Vernier Flash Photolysis Spectrometer .......................................................... 10
Figure 4: Screenshot of Vernier Flash Photolysis Spectrometer Software................................................ 13
Figure 5: Concentration time profiles of reactant and product................................................................... 18
Figure 6: Example of a voltage-time waveform after importing into Logger Pro........................................ 21
Figure 7: Time and voltage (raw data) and absorbance (calculated) ........................................................ 22
Figure 8: Example of a temporal profile of the scattered excitation light. .................................................. 23
Figure 9: Absorption (left) and emission (right) kinetic profilesincluding the scattered excitation light
picked up by the detector............................................................................................................ 24
Figure 10: Decay portion of the A-time profile ......................................................................................... 24
Figure 11: Congo red, MW=696.67............................................................................................................ 26
Figure 12: Ground state absorption spectrum of Congo red (trans conf.) in 20% water in ethanol .......... 27
Figure 13: Kinetic profile of Congo red in 20% water in ethanol................................................................ 29
Figure 14: Absorption kinetic profile of Congo red in 20% water in ethanol with 2 mM OH-ions.............. 30
Figure 15: Photoinduced isomerization of mercury dithizonate ................................................................. 31
Figure 16: Ground state absorption spectrum of mercury dithizonate (trans conf.) in ethanol.................. 32
Figure 17: Trifluoroacetic acid (TFA); MW = 114.03.................................................................................. 33
Figure 18: Kinetic profile of mercury dithizonate in ethanol ....................................................................... 33
Figure 19: Structure and photochromic reaction of 6-NO2-BIPS ............................................................... 34
Figure 20: Ground state absorption spectrum of one spiropyran in toluene.............................................. 36
Figure 21: Transient absorption at 600 nm of one spiropyran in toluene .................................................. 36
Figure 22: Example decay and exponential fit for 55°C measurement ..................................................... 37
Figure 23: Actual data example of ln() as a function of 1000/T................................................................ 37
Figure 24: Emission spectrum of xenon flash lamp ................................................................................... 38
Figure 25: Emission spectrum of white light LED ...................................................................................... 39
Figure 26: Transmission spectrum of 600 nm bandpass filter ................................................................... 40

© Ultrafast Systems LLC/Vernier Software & Technology
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1 General Information
1.1 Introduction
The Vernier Flash Photolysis Spectrometer is designed to be a simple, user-friendly device for
demonstrating to chemistry students the fundamental principles of chemical kinetics. Every process that
we label as “chemical” occurs as the result of chemical bonds being broken or formed, i.e., nuclei change
their spatial positions with respect to each other. Chemical kinetics is one of the major divisions of
physical chemistry and is basically the quantitative study of the rates and mechanisms of chemical
reactions. Chemical change can be induced in a variety of ways—the one employed in the Vernier Flash
Photolysis Spectrometer is by absorption of light. Thus the instrument serves also as a way of introducing
the student to the concepts and practice of photochemistry.
1.2 Specifications
1.2.1 Spectrometer Specifications
Spectral Coverage
450–750 nm
Spectral Resolution
Determined by an interference filter used. Typically ~10 nm
Temporal Resolution
~100 µs
Time Window
15 ms

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Software
The Vernier Flash Photolysis Spectrometer comes with data-acquisition software necessary to
collect the signal waveform from the photodetector. Subsequent data processing, such as conversion of
the acquired voltage waveform into temporal profile of Aand fitting the Aprofile to an appropriate
function in order to extract the rate constants, can be done by exporting the data into Vernier Logger Pro
software or Microsoft®Excel®.
Data Format
The Vernier Flash Photolysis Spectrometer produces Avs. time data in a form of an ASCII CSV
file that can be easily processed with Logger Pro.
Figure 1: Examples of Vernier Flash Photolysis Spectrometer data processed in Logger Pro

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1.2.2 Equipment Specifications
Weight: 10 lbs
Footprint: 8" x 8" (20.3 cm x 20.3 cm)
UL94 rating: V–0
Meets UL796 requirements
Voltage range: 100–120 V & 210–240 V
Frequency range: 50 Hz & 60 Hz
Power rating: 50 W maximum
DC battery: A23 12 V
USB cable: 28 AWG shielded
AC power cable: 18 AWG 300 V

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1.2.3 Detector Specifications
All measurements were performed with a 50 load unless stated otherwise.
Detector Silicon PIN
Active area 3.6 x 3.6 mm (13 mm2)
Wavelength range 350 to 1100 nm
Peak wavelength p 970 nm
Rise time tr 14 ns
Bias voltage VR 10 V
Dark current (with 1 Mload) ID 0.35 nA
Output voltage VOUT 0 to 10 V
Figure 2: Detector responsivity vs. wavelength

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2 Safety Information
The Vernier Flash Photolysis Spectrometer is designed and manufactured for kinetic analysis of a
chemical specimen by means of transient absorption spectrometry. It is not sold, nor intended for, nor
should ever be used for any other purpose. The product should be used solely in accordance with the
instructions provided.
Use a compliant surge protector when exposing equipment to voltage. Failure to do so may result
in damaging the unit.
Do not use this equipment in or near water.
Use extreme caution in handling the cuvette when it is filled with liquid. Never fill or refill the
cuvette when it is in the sample holder. Failure to use a cuvette with a dry outer surface or a
cuvette that is leaking liquid can result in the flash housing to malfunction or short.
This system is designed to work optimally at room temperature or an approximation of room
temperature.
Do not disassemble the spectrometer case. Doing so may result in damaging the excitation light
source and electric shock.
If the spectrometer does not appear to be functioning properly, do not attempt to repair it yourself.
Please contact Vernier Technical Support for assistance.
Every effort has been made to ensure that the information in this manual is accurate. All
information in this document is subject to change without notice. Ultrafast Systems makes no
representation or warranty, either expressed or implied, with respect to this document. In no event will
Ultrafast Systems be liable for any direct, indirect, special, incidental, or consequential damages resulting
from any defects in this documentation.
If this equipment is used in a manner not specified in this manual, the protection provided by this
equipment may be impaired.
The following are definitions of the Warnings, Cautions, and Notes that are used throughout this
manual to call your attention to important information regarding your safety, the safety and preservation of
your equipment, or an important tip.

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WARNING
Situation has the potential to cause bodily harm or
death.
NOTE
Additional information the user or operator should consider.
CAUTION
Situation has the potential to cause damage to
property or equipment.

© Ultrafast Systems LLC /Vernier Software & Technology
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3 Layout and Basic Principles of
Operation
Figure 3: Schematic of the Vernier Flash Photolysis Spectrometer
Detector
photodiode
F
LED
Digitizer
Sample
L1 L2
Xe flash lamp

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A white light LED is utilized for probing the spectral changes taking place after the sample has
been exposed to an excitation flash. The white light is sent through a sample cuvette via lens L1 and
subsequently delivered to the photodetector diode by passing through an optical interference filter and
objective lens L2. The optical interference filter is used to select the desired wavelength out of the broad
emission spectrum of the LED. While the continuous wave (CW) output from the LED is being monitored
by the photodiode, the photoexcitation flash is sent through the sample. The photo-induced transient
species cause the absorption of the sample to deviate from what it was before the excitation flash. This
results in changes in the intensity of the LED light passing through the sample. These changes are
detected by the monitoring photodiode. The output voltage waveform from the photodiode is collected
and digitized by an internal DAQ device and transferred to the computer for further manipulation.

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4 Hardware Description
4.1 Turning the Spectrometer On and Off
Connect the power cord to the AC input located at the rear of the Vernier Flash Photolysis
Spectrometer.
Connect the Vernier Flash Photolysis Spectrometer to the computer via a USB cable.
Wait until Windows®recognizes the new USB device.
Start the Vernier Flash Photolysis Spectrometer Software.
NOTE
The 12 V battery that powers the photodiode can be accessed by removing the battery compartment
lid on the side of the housing.
4.2 Optical Filters
A 600 nm interference filter is included with the Vernier Flash Photolysis Spectrometer. Insert the
filter into the filter housing that is also included. These filters select the wavelength monitored by the
photodiode. Additional filters can be purchased from any filter company. The size that fits the housing is
12.5 mm (OD), 9.0 mm (ID).

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4.3 Software Description
The user interface of the Vernier Flash Photolysis Spectrometer Software is rather intuitive
(Figure 4). The I0value (PD level, V) at the top-right of the screen reflects the amount of probe light
reaching the detector photodiode. Please keep in mind that at 2500 mV the detector will saturate, and
your signal will be distorted. A typical I0should not be larger than 2 V for absorption measurements. If the
I0values are lower than 500 mV or higher than 2000 mV, please contact Vernier Technical Support. For
emission measurements, the LED light turns off and the I0value should drop close to zero (<10 mV).
During data acquisition, the Vernier Flash Photolysis Spectrometer will wait for approximately
2 seconds between flashes to allow enough time for the flash capacitors to recharge. By default, the
software will adjust the axis scales automatically: the y-axis range will adjust to display the signal only,
cutting off the large scattered flash pulse around time = 0. The default x-axis setting is “Auto Scale.” To
change this, right-click anywhere on the plot and choose the desired option. Alternatively, you can use the
graph controls on the top of the screen (Figure 4).
The data are saved in CSV (comma delimited values) ASCII format. The first column is time in
milliseconds and the second column is light intensity in millivolts.
Figure 4: Screenshot of Vernier Flash Photolysis Spectrometer Software

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4.3.1 Graph Controls
Graph controls on the top of the screen allow you to zoom in the graph after measurements have
been completed. By default, the cursor mode on the graph is y-axis zoom, with the following graph
controls:
/ Axis Auto Scale mode OFF and ON.
Autoscale axis once
Pan graph
Select between graph zoom modes:
Select X and Y region to zoom in.
Select X region to zoom in, keep Y limits unchanged.
Default, select Y region to zoom in, keep X limits unchanged.
Autoscale both x- and y-axes once to fit graph.
Zoom in; Zoom out.

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5 The Basic Principles of
Chemical Kinetics
Most chemical reactions that occur, either in nature or in the laboratory, are complex. That is,
they occur via a series of simple, or elementary, processes, which can proceed at different rates. An
elementary process is a single-step reaction in which there are no chemically-identifiable intermediates.
For a given complex reaction, its rate will be determined by the rate of the slowest of the participating
elementary reactions. The major advantage of using photons to initiate reactions is that in this way it is
relatively straightforward to initiate elementary processes and it is these that are best used to understand
the fundamental principles of chemical kinetics.
Now some necessary definitions:
5.1 Stoichiometry
Every chemical change can be represented by a stoichiometric, or balanced, equation in symbolic
form. For example, the acid-base reaction between sulfuric acid and a metal hydroxide is represented by
24 24 2
22
H
SO NaOH Na SO H O


This equation tells us nothing about the mechanism of the process; it simply expresses the
overall chemical equivalency.
5.2 Reaction Rate
The rate of a chemical reaction is expressed as the variation with time of the concentration of
either reactants or products. In solution phase reactions, as studied here, the units of reaction rate are
concentration units per second, i.e., mole per liter per second, Ms-1.
In the (elementary) reaction
A
BC Equation 1
The rate (change in concentration as a function of time) is expressed in the form of a derivative
Rate = [] [] []dA dB dC
dt dt dt
 Ms-1 Equation 2

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For the rate to be a positive quantity the derivatives of [A] and [B] with time are negative because
the concentrations of these reactants decrease as time proceeds.
In the case of the symbolic elementary reaction
2
A
CEquation 3
which could represent a dimerization process, the rate is given by
Rate = 1[] []
2
dA dC
dt dt

Equation 4
since A is removed twice as quickly as the product is formed.
5.3 The Rate Expression and the Rate Constant
Empirical observations of the rates of chemical processes led to the Law of Mass Action which
states that
The rate of a chemical reaction is directly proportional to the active masses of the reacting
species.
For reactions taking place in dilute homogeneous solution, the term “active masses” can be
replaced by molar concentrations (mole L-1, or M).
For the elementary reaction
A
B products Equation 5
the mathematical formulation of the Mass Action Law is
[] [][] dA
A
B
dt
 Ms-1 Equation 6
and replacing the proportionality sign with a constant of proportionality, k, we find
[] [][] dA kA B
dt
 Ms-1 Equation 7
Equation 7 is known as the rate expression, or rate equation, and the proportionality constant is
referred to as the rate constant. This equation tells us that since the concentrations of the reactants
decrease during the course of the reaction, then the rate (but not the rate constant) decreases with time

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elapsed. Thus the rate constant provides the experimenter with a useful measure of the velocity of a
reaction, and it is this quantity that we usually seek in a study of chemical kinetics.
5.4 Order of Reaction
In Equation 7, the concentrations of A and B both appear to the first power, and we say that this
reaction is first order in [A] and first order in [B], and that it is second order overall. However the rate
expression for the elementary process 3 will be
2
[] [][] [] dA kA A kA
dt
  Ms-1 Equation 8
For this reaction it is seen that the concentration of A appears as the 2nd power, and we say that
the reaction is second order in [A] and second order overall.
Also, the symbolic elementary process
A
products
would be expected to have a rate expression such as
[] [] dA kA
dt

Ms-1 Equation 9
and the reaction would be said to be first order in [A] (and first order overall). Orders higher than 2 and
also fractional and zero orders are found in special cases. These are not considered here because of lack
of relevance to the Vernier Flash Photolysis Spectrometer concept.
SPECIAL NOTE: It is important to realize that assignment of order by inspection is only valid for
elementary reactions; we cannot do this for stoichiometric equations.

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5.5 The Experimental Approach
The data that are obtained in a kinetics experiment could look something like those presented in
Figure 5, which shows the change in concentration of reactant A (open circles) and product B (filled
circles) as a function of time; M is the generic species (A or B).
Figure 5: Concentration time profiles of reactant and product
On the plot you can see a pair of horizontal lines, one blue (at 0.5) and one red (at 0.37). The
blue one is drawn at the point of 50% reaction (half of the initial concentration of A has been converted
into B). By inspection this occurs after an elapsed time of about 28 milliseconds. This represents the half
life, or half time, of the reaction. The red line is drawn when about 37% of the initial concentration of A
has been converted to B; this represents the reaction life time.
5.6 The Relationships Between Rate Constant, Half Life,
and Life Time
First, we shall consider a first order reaction symbolized by
Aproducts
The rate of this reaction is as written in Equation 9 as
[] [] dA kA
dt

Ms
-1

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Setting [A]0as the initial concentration of A, the integrated form of this differential rate equation is
given as
0
[] []exp( )
A
Akt
Equation 10
which shows us that the concentration of A decreases as an exponential function of time. The shape of
the decrease of [A] curve in the above Figure 5 is exponential in time. Since the argument of the
exponential term must be dimensionless, it is clear that the units of k, the first order rate constant, are
time-1, and conventionally, time is measured in seconds, or sub-multiples of seconds such as ms, µs, or
ns.
At the half-life point,
50% 0
[] 0.5[]
A
Aand 1/ 2 exp( ) 1/ 2kt

,
whence 1/2 ln 2 / 0.693/tkk and a measurement of t1/2 leads to the rate constant.
The lifetime of a reaction, usually written as
 ,is defined as the inverse of the rate constant, 1/k


.
Thus Equation 10 can be written as
0
[] []exp( /)
A
At


Equation 11
and at t =  ,
00
[ ] [ ] exp( 1) 0.368[ ]
A
AA


or, at the life-time point, the concentration of A has fallen to 36.8% (about 37%) of its initial value.
Note that in processes that are first order overall, the half life and the life time are independent of
time and initial concentration.
So far we have considered only the reaction that can be written
A
products
This implies that we are considering the spontaneous conversion of some compound into another,
chemically distinct, species. Such a process could be the spontaneous decay of an electronically excited
state of a molecule that has been generated by the absorption of photons (see Section 6.3). In this case
the product(s) could be the original ground state of the molecule, or an isomer thereof (cis-trans
photoisomerization). Such reactions are termed unimolecular because there is only one molecule
involved. Such reactions will show first order kinetics overall.
Now let us consider the elementary process
A
B products


Clearly this is now bimolecular because it requires a molecule of A and a molecule of B to react
together. At first sight, we would expect this process to show second order kinetics overall, but, as we
shall see, the order actually observed in an experiment will be determined by the reaction conditions.

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Under conditions where the initial concentrations of A and B are equal, the rate equation is as in
Equation 7, viz
[] [][] dA kA B
dt

Integrating this and setting [A] = [B] leads to
11
0
[] []
A
Akt


Equation 12
This is also the result for the case of the reaction
2
A
products
At the half-time point [A] = 0.5[A]0then
1/ 2
00 0
1/ 2 0
21 1
[] [] []
or 1/ [ ]
kt
AA A
tkA


Thus we see that the half time depends on the initial concentration, unlike the first order case.
5.7 Pseudo Order
In the bimolecular elementary process
A
B products


If the initial concentration of one of the reacting species, say B, is very much higher than that of
the other, A, then changes in concentration of B through reaction become negligible compared to those of
A, which go from 100% to zero during the course of the reaction. Then the rate equation
[] [][] dA kA B
dt

becomes
0
[] [][] '[] dA kB A k A
dt
 
where k’ = k[B]0
Upon integration this becomes
0
[] []exp( ')
A
Akt


This bimolecular process is seen to be first order in A and first order overall. This is referred to as
“pseudo-first order” by some practitioners, but the important thing to realize is that the process behaves in
a first order manner in the experiment. This behavior can be detected by evaluating the first order rate
constant at different initial concentrations of B, when the behavior will be found to depend linearly on [B].