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B. Xiang, L. Zhu / Chemical Physics Letters 450 (2007) 31–38
where photochemical thresholds were calculated from the
corresponding enthalpy changes [9]. Pathway (R1) is a
ketene formation channel. Pathways (R2) and (R3) are
radical formation channels. Pathways (R4) and (R5) are
molecular elimination channels.
In this Letter, we report results obtained from absorp-
tion cross section measurements of E,E-2,4-hexadienedial
at 248 nm and in the 290–430 nm region. The HCO radical
was observed after 248 nm photolysis of E,E-2,4-hexa-
dienedial, and its quantum yield has been determined.
Ketene formation was observed at 248 nm photolysis
wavelength. The end-product yields from 248 and 308 nm
photolysis of E,E-2,4-hexadienedial have been estimated
using FTIR.
sample. For cross section measurements, the pressure of
E,E-2,4-hexadienedial vapor inside the cell was varied
between 8 · 10ꢁ4 Torr and 4 · 10ꢁ3 Torr. The cell was
evacuated before another pressure of E,E-2,4-hexadiene-
dial was introduced into the cell to minimize the effect of
outgassing. The cavity loss of the evacuated cell was about
the same when going from one E,E-2,4-hexadienedial pres-
sure to another suggesting that E,E-2,4-hexadienedial did
not stick to the cavity mirrors. The cross section measure-
ments were made under static condition, and the E,E-2,4-
hexadienedial sample was stable (we did not observe
deposit on the mirrors or in the absorption cell). An H
NMR analysis described later in this section indicated that
the E,E-2,4-hexadienedial sample had a minimum purity of
P99.9%. It took up to 2 min. to fill 3 · 10ꢁ3 Torr of E,E-
2,4-hexadienedial in the cell. Once the cell was filled with
the sample, it took about 8 s to perform a ring-down mea-
surement with a 1 Hz laser repetition rate and eight count
average. Considering the cell degassing rate and the sample
filling time, as well as the accuracy of pressure read-out, the
maximum uncertainty in E,E-2,4-hexadienedial concentra-
tion measurement is estimated about 17%.
Photolysis of E,E-2,4-hexadienedial was carried out in a
stainless steel cell. Detailed descriptions of our experimen-
tal setup can be found elsewhere [12–14]. The output from
an excimer laser was propagated into the reaction cell at a
15ꢁ angle with the main cell axis, through a side arm. The
probe laser beam, used to monitor the HCO radical gener-
ated from the photolysis process, was directed along the
main optical axis of the cell; the cell had been vacuum-
sealed with a pair of highly reflective cavity mirrors. The
probe laser beam overlapped with the photolysis beam at
the center of the cavity. The photolysis/probe laser overlap
region can be conceptualized as a rectangular solid with its
center overlapping that of the cell, with its width and height
defined by those of the photolysis beam, and with the
length of the rectangular solid defined by (beam
width) · (tan15ꢁ)ꢁ1, where 15ꢁ is the crossing angle
between the pump and probe laser beams. The length of
the photolysis/probe laser overlap region is defined by
(beam width) · (sin15ꢁ)ꢁ1. For a 12 mm wide photolysis
beam, the length of the photolysis/probe laser overlap
region is about 4.6 cm. The HCO absorption resulting from
the photolysis of E,E-2,4-hexadienedial was obtained by
measuring the cavity losses with and without photolysis
in the 613–617 nm region (HCO X2A00 (0,0,0) ! A2A0
(0,9,0) transition). A pulse/delay generator was used to
vary the delay time between the firings of the photolysis
and the probe lasers. The incident photolysis beam energy
into the cell (E0) was measured by a calibrated Joulemeter
placed in front of the cell. The incident beam energy inside
the cell was corrected for photolysis beam transmission loss
at the front cell window, and for reflection of the photolysis
beam from the rear cell window. E,E-2,4-Hexadienedial
pressures of 6 · 10ꢁ4–3 · 10ꢁ3 Torr were used in the
HCO quantum yield measurements. The end-products
from 248 and 308 nm photolysis of E,E-2,4-hexadienedial
2. Experimental technique
The absorption cross sections of E,E-2,4-hexadienedial
in the 290–430 nm region were determined by cavity ring-
down spectroscopy [10,11]. The cylindrical cell used for
cross section measurements was 46 cm long; the two ends
of the cell were sealed by a pair of high-reflectance cavity
mirrors. Four pairs of high-reflectance cavity mirrors with
centering wavelengths at 285 nm (covering 290–300 nm
region), 320 nm (covering 300–340 nm region), 377 nm
(covering 340–410 nm region), and 455 nm (covering 410–
430 nm region) were used in the cross section measure-
ments. The fundamental or the second harmonic output
from a XeCl excimer-pumped dye laser (but at a much
reduced fluence) was transmitted into the ring-down cavity
through the front cavity mirror. The laser dyes used to
cover the 290–430 nm region were rhodamin 6G, rhodamin
B, rhodamin 101, DCM, PTP, BBQ, DPS, and stilbene 3.
The photon intensity decay inside the cavity was monitored
with a photomultiplier tube (PMT) placed behind the rear
cavity mirror. The PMT output was amplified, digitized,
and sent to a computer. The decay curve was fitted to a sin-
gle-exponential decay function, from which the ring-down
time constant (s) and the total loss (C) per optical pass were
calculated. When cavity mirrors were properly aligned, fit-
ting of ring-down curve to single-exponential decay had a
maximum uncertainty of 5%. By measuring the cavity
losses with and without E,E-2,4-hexadienedial vapor in
the cavity, we obtained optical loss due to E,E-2,4-hexadi-
enedial absorption. Ring-down decay times for an empty
cavity were about 0.79 ls at 290 nm, 1.54 ls at 320 nm,
0.42 ls at 360 nm, and 1.17 ls at 430 nm. With E,E-2,4-
hexadienedial vapor in the cavity, ring-down decay times
reduced to as low as 0.61 ls at 290 nm, 1.30 ls at
320 nm, 0.36 ls at 360 nm, and 1.06 ls at 430 nm. The
gas pressure inside the cell was monitored by an MKS Bar-
atron capacitance manometer (1 Torr full scale, pressure
measurement accuracy is about 10ꢁ4 Torr), which can mea-
sure pressures down to 10ꢁ4 Torr. Before each experiment,
the cell was pumped with a combination of rotary and dif-
fusion pumps to 10ꢁ5 Torr. The cell had a degassing rate of
1 · 10ꢁ4 Torr/min in the absence of E,E-2,4-hexadienedial