Abstract.
A Fourier – Transform Infrared (FTIR) spectrometer (Bruker Optics IFS-55) has been upgraded to
operate in step-scan time-resolved mode with 20 ns time resolution. This allows one to follow mid-IR
spectra changes for a photoreaction on the nanosecond- to millisecond time scale and to visualize
short-lived reaction intermediates1-3. The setup has been tested in the laser–photolysis ( = 355 nm) of
2,2-dimethyl-propiophenone in hexane solution. A time–dependent IR peak at 1828 cm-1 corresponding
to the C=O stretching vibration of the benzoyl radical was detected. A time-resolved (TR) IR spectrum
of the benzoyl radical reacting with CCl3Br was recorded. This revealed an absorption grows in at 1780
cm-1. This has been assigned to benzoyl bromide formation.
Method.
The step-scan technique (Fig.1) is a way to introduce time resolution to the conventional FTIR
spectroscopy4-6. In the step-scan mode the moving mirror of the spectrometer is stepped to a fixed
position and held there. While the mirror is stationery the photoexcitation event is repeatedly triggered
and the IR light intensity A(xi,t) from the detector is digitized and recorded as a function of time. Then
the mirror is stepped to the next position and the procedure repeated. This generates an array of
intensities A(xi,t) when the complete set of mirror positions has been covered. The IR intensities are
read out from the array as a function of the mirror position at a fixed time to form an interferogram to be
converted to a spectral time slice corresponding to the fixed delay after the trigger. In this way the
temporal resolution of the order of a few nanoseconds can be achieved which is determined by the
response time of the IR detector coupled with the analog bandwidth of the electronics.
Fig. 1. STEP - SCAN TIME – RESOLVED FTIR TECHNIQUE
time
L1
D
L2
X1 X2 X3…XN
S
Set of interferograms
X1
t1 A11
t2 A21
… …
tM AM1
X2
A12
A22
…
AM2
X3
A13
A23
…
AM3
X4
A14
A24
…
AM4
X5
A15
A25
…
AM5
…
…
…
…
…
XN
A1N
A2N
…
AMN
Set of
spectra
FFT
Experimental setup (Fig.2).
A Spectra Physics GCR series pulsed solid-state YAG:Nd3+ laser (third harmonics at 355 nm,
10 Hz repetition rate) was the excitation source. The FTIR spectrometer, fast electronics for
signal processing, a Tektronix TDS380 digital oscilloscope and Spectrum PAD82 computer
ADC board were employed for data visualization and acquisition. Synchronization of the laser
pulse and interferometer data acquisition was controlled by a Stanford Research System
(Model DG535) timing generator. Two channels were used to fire the laser flash lamps and the
Q-switch and another single channel was used to trigger the PAD82 transient recorder.
The photochemical system.
The photoexcitation, benzoyl radical formation and chemistry scheme are presented in Fig.3.
3-D spectra.
The TR FTIR spectrum of a 0.04 M solution of 1 in hexane with 20 ns resolution is presented in
Fig.4. A single peak corresponding to the C=O absorption of benzoyl radical at 1828 cm -1
grows in within one hundred nanoseconds and decays within several microseconds, which is
entirely consistent with the previous findings7. The spectrum of the same sample with the
0.003 M of CCl3Br added is shown in Fig.5. The carbonyl band of benzoyl radical decays faster
indicating the shortening of the radical lifetime. In addition, the second band at 1780 cm -1
grows in as the transition at 1828 cm-1 decays. This peak corresponds to the C=O fundamental
vibrational frequency of benzoyl bromide.
Kinetic data.
Decay profiles at 1828 cm-1 for TR FTIR spectra of 2 recorded at two different initial
concentrations of 1 (0.01 and 0.05 M) are presented in Fig.6a. The plot of kobs vs.
concentration of 1 is presented in Fig.6b. From this plot, the bimolecular rate constant (kq1) of
7.2x106 M-1s-1 was obtained. Clearly the quenching of benzoyl radical by starting phenone
takes place, hence, the addition of CCl3Br to solution introduces the second quenching path for
the benzoyl radical and resulting kinetic traces have to be described by biexponential function.
The kobs resulted from the faster component of the biexponential function was plotted vs.
concentration of CCl3Br over the range of 0.0013-0.01 M (Fig.6c). The initial concentration of 1
was 0.04 M. A quenching constant of 4.1x108 M-1s-1 was obtained, close to the 2.2x108 M-1s-1
literature value7.
References.
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J. Am. Chem. Soc., 1997, 119, 7013-18.
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5. M. S. Hutson, M. S. Braiman. Applied Spectrosc. 1998, 52, 974-984.
6. W. Uhman, A. Becker, C. Taran, F. Siebert. Applied Spectrosc., 1991, 45, 390-7.
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