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M. Matsubara et al. / Journal of Photochemistry and Photobiology A: Chemistry 322 (2016) 53–59
the most powerful spectroscopic technique for studying the
molecular structure and reaction mechanism of molecules, ions
and clusters in the gas phase [15,16]. On the other hand, employing
a grating-tuned CO2 laser combined with an FTICR (Fourier
transform ion cyclotron resonance) mass spectrometer, Ehsan et al.
succeeded in obtaining an IRMPD spectrum which is narrower
than that taken with IR-FEL [17].
FEL-TUS (infrared free electron laser at Tokyo University of
Science) possesses a unique temporal pulse structure containing
the train of pico-second pulses. In our previous experiments
[11,12], we showed that this could enhance the efficiency of ladder
climbing of the infrared photons because the interval between
successive pulses is short enough compared with the collisional
interval of molecules in the gas phase, which may minimize the
collisional relaxation of the internal energy and increase the
possibility of IRMPE. In the current paper, we present the results of
IRMPE of 2,3-DHF by FEL-TUS. The experimental results will be
discussed in connection with the calculation.
Fig. 2. Typical gas chromatogram of the sample gas after irradiation of FEL at
ꢁ1580 cmꢀ1 at room temperature. Parent molecule: 2,3-DHF. Initial pressure:
0.2 Torr. Irradiation time: 1 h (18000 shots). A similar spectrum is obtained after
irradiation of FEL at ꢁ1040 cmꢀ1
.
2. Experimental
2,3-DHF (Wako Pure Chemical, >98% purity), CPCA (Tokyo
Chemical Industry Co., >97% purity), trans-CA (Wako Pure
Chemical, >99% purity) were used after degasification without
any purification. Infrared absorption spectra of samples were
measured using a FT-IR infrared spectrometer (JASCO, model 615).
Quantitative analysis of the sample gas after irradiation of FEL was
performed with a GC mass spectrometer (Shimadzu, GCMS-
QP2010). The integrated intensity of the photo isomerization
products, CPCA and CA, on the gas chromatogram is found to be
proportional to the pressure of these compounds, respectively.
Thus by drawing a working curve for each molecule and comparing
the integrated areas on the gas chromatogram, the ratio of the
products is evaluated with an estimated error of 10%.
The structure and the characteristics of FEL-TUS have been
described previously [11]. In brief, the wavelength is variable
within the mid-infrared region of 5–16
TUS provides two kinds of laser pulses, i.e. so-called macropulse
s and a
m
m (625–2000 cmꢀ1). FEL-
and micropulse. The macropulse has a duration of ꢁ2
m
repetition rate of 5 Hz throughout the operation. This macropulse
consists of a train of micropulses with a duration of ꢁ2 ps. The
interval of two consecutive micropulses is 350 ps which corre-
sponds to the RF frequency (2856 MHz) employed for the linear
accelerator.
Accordingly,
one
macropulse
contains
ꢁ6000 micropulses. The irradiation condition is similar to that
of our previous experiment [11,12]. The infrared light beam from
FEL-TUS was reflected on the concaved mirror with a focal length of
500 cm, and the reflected beam was focused into the center of a
reaction cell (diameter; 25 mm, length; 120 mm, windows; BaF2)
by a BaF2 lens with a focal length of typically 30 cm. The energy of
the laser pulse was monitored with an energy meter (Gentec, ED-
500) and the typical pulse energy was 8–10 mJ/macropulse.
However, depending on the daily condition of FEL, the lower
pulse energy (ꢁ5 mJ/macropulse) was provided in a given machine
time. The average fluence is estimated to be ꢁ1 J/cm2. The energy
resolution of FEL-TUS is 1.0–1.5% of the output wavenumbers.
3. Results and discussion
3.1. Irradiation of FEL to 2,3-DHF
Fig. 1 shows IR absorption spectra of 2,3-DHF (upper trace),
CPCA (middle trace), and trans-CA (lower trace). For 2,3-DHF, a
strong band near 1600 cmꢀ1 is assigned to the C
C stretching
¼
mode. In the lower wavenumber region, two prominent bands near
1070 and 1140 cmꢀ1 are assigned to the ring stretching and CCꢀꢀH
bending motions, respectively. On the other hand, only one
prominent band around 1730 cmꢀ1 exists for CPCA and CA
corresponding to the C O stretching mode.
¼
Fig. 2 illustrates a typical gas chromatogram of the sample gas
after the irradiation of FEL (ꢁ1580 cmꢀ1) to 2,3-DHF. A similar gas
chromatogram is obtained at ꢁ1040 cmꢀ1. These wavenumbers
correspond to the maximum efficiency for the isomerization
reactions as discussed later. The peak near retention time of
6.2 min corresponds to the parent molecule, while two signals at
7.5 and 7.6 min to the photo isomerization products, namely, CA
and CPCA, respectively. CA has cis and trans forms; only the latter is
commercially available. The retention time and the mass pattern
for the peak at 7.5 min are identical with that of trans-CA. Because
the retention time of cis-CA is expected to be overlapped with trans
conformer, we cannot estimate the ratio of cis to trans compound
from the gas chromatogram only. In addition to the photo
isomerization products, two photo dissociation products, propyl-
ene (retention time at 3.5 min) and 2-propenal (at 5.2 min) are
identified.
Fig. 3a shows the action spectrum for the isomerization
products, namely, the dependence of the isomerization reaction
Fig. 1. IR absorption spectra of 2,3-DHF (upper trace; 2.3 Torr), CPCA (middle trace;
2.6 Torr), and trans-CA (lower trace; 2.3 Torr).
on the FEL wavenumber near the C C stretching region of 2,3-DHF.
¼