Dye laser photolysis of n-Pentanal from 280 to 330 nm

Article abstract of DOI:10.1021/jp982487s

The UV photolysis of n-pentanal in the 280-330-nm region has been studied in 5-nm intervals by using dye laser photolysis in combination with cavity ring-down spectroscopy. Absorption cross sections of n-pentanal were measured at each wavelength studied. n-Pentanal exhibited a broad, structureless absorption band similar, in appearance, to that of previously studied short-chain aldehydes. The absorption spectrum peaked at 295 nm with a cross section of (6.56 ± 0.17) × 10^-20 cm² molecule^-1. The formation of the HCO radical, which is a photodissociation product, was monitored in these experiments. The HCO yield was found to be independent of n-pentanal pressure (2-18 Torr) and total pressure (8-480 Torr) except for 325- and 330-nm photolysis where the size of the HCO signal was small and the dissociation was near the threshold. The dependence of the HCO radical yield on the photolysis wavelength was determined. The HCO yields were 0.058 ± 0.006, 0.095 ± 0.009, 0.10 ± 0.02, 0.14 ± 0.01, 0.10 ± 0.02, 0.15 ± 0.02, 0.14 ± 0.02, 0.20 ± 0.06, 0.14 ± 0.02, 0.085 ± 0.034, 0.087 ± 0.015 at 280, 285, 290, 295, 300, 305, 310, 315, 320, 325, and 330 nm, respectively, where uncertainty reflects experimental scatter only. End products from closed-cell photolysis of n-pentanal with and without O₂ were investigated at 290, 310, and 330 nm by using GC/MS and HPLC. Acetaldehyde was found to be a significant product from the photodissociation of n-pentanal/N₂ mixtures. Photolysis rates of n-pentanal to form HCO were calculated for two representative atmospheric conditions (noontime at sea level and 40° N latitude on January 1 and on July 1). The estimated radical formation rate constants from n-pentanal photolysis were about twice as fast as those obtained from acetaldehyde photolysis.

Full text of DOI:10.1021/jp982487s

10274
J. Phys. Chem. A 1998, 102, 10274-10279
Dye Laser Photolysis of n-Pentanal from 280 to 330 nm
Thomas J. Cronin and Lei Zhu*
Wadsworth Center, New York State Department of Health, Department of EnVironmental Health and
Toxicology, State UniVersity of New York, Albany, New York 12201-0509
ReceiVed: June 4, 1998; In Final Form: September 14, 1998
The UV photolysis of n-pentanal in the 280-330-nm region has been studied in 5-nm intervals by using dye
laser photolysis in combination with cavity ring-down spectroscopy. Absorption cross sections of n-pentanal
were measured at each wavelength studied. n-Pentanal exhibited a broad, structureless absorption band similar,
in appearance, to that of previously studied short-chain aldehydes. The absorption spectrum peaked at 295
-
20
2
-1
nm with a cross section of (6.56 ( 0.17) × 10 cm molecule . The formation of the HCO radical, which
is a photodissociation product, was monitored in these experiments. The HCO yield was found to be independent
of n-pentanal pressure (2-18 Torr) and total pressure (8-480 Torr) except for 325- and 330-nm photolysis
where the size of the HCO signal was small and the dissociation was near the threshold. The dependence of
the HCO radical yield on the photolysis wavelength was determined. The HCO yields were 0.058 ( 0.006,
0
0
.095 ( 0.009, 0.10 ( 0.02, 0.14 ( 0.01, 0.10 ( 0.02, 0.15 ( 0.02, 0.14 ( 0.02, 0.20 ( 0.06, 0.14 ( 0.02,
.085 ( 0.034, 0.087 ( 0.015 at 280, 285, 290, 295, 300, 305, 310, 315, 320, 325, and 330 nm, respectively,
where uncertainty reflects experimental scatter only. End products from closed-cell photolysis of n-pentanal
with and without O were investigated at 290, 310, and 330 nm by using GC/MS and HPLC. Acetaldehyde
was found to be a significant product from the photodissociation of n-pentanal/N mixtures. Photolysis rates
2
2
of n-pentanal to form HCO were calculated for two representative atmospheric conditions (noontime at sea
level and 40° N latitude on January 1 and on July 1). The estimated radical formation rate constants from
n-pentanal photolysis were about twice as fast as those obtained from acetaldehyde photolysis.
Introduction
where photochemical thresholds were calculated from the
Aldehydes play an important role in the formation of
photochemical smog, peroxyacetyl nitrate (PAN), and regional
ozone. Photodissociation of aldehydes represents a significant
source of free radicals in the lower atmosphere.1-3 There have
been a relatively large number of studies devoted to the
photodissociation of shorter chain aldehydes, such as CH2O,
CH3CHO, and C2H5CHO.4 However, the photolysis of longer
chain aldehydes is not well-understood. Such an investigation
is important in order to determine the fates of longer chain
aldehydes in the atmosphere and to understand their contribution
to radical formation.
corresponding enthalpy changes. In the absence of experimental
heats of formation for n-C4H9 and C4H9CO, enthalpy changes
for channels (2) and (3) were assumed to be the same as those
for the photolysis of propionaldehyde. This assumption is
reasonable since channel (2) involves the breaking of a
methylene carbonyl bond (-CH2-CHO) and channel (3)
involves the breaking of a carbonyl hydrogen bond (-C(O)-
H) from photolysis of both propionaldehyde and n-pentanal.
In this paper, we report results obtained from the dye laser
photolysis of n-pentanal in the wavelength region 280-330 nm.
Absorption cross sections of n-pentanal were measured. The
HCO radical yield from the photolysis of n-pentanal was
-12
Aliphatic aldehydes exhibit a weak absorption band in the
wavelength range 240-360 nm as a result of a dipole forbidden
determined by using a sensitive new detection technique, cavity
n f π* transition.1
3,14
Photolysis of n-pentanal can possibly
15,16
ring-down spectroscopy.
The absolute HCO concentrations
occur through the following pathways:
were calibrated relative to those obtained from the photolysis
of formaldehyde (H2CO) at each wavelength. We have also
investigated products from closed-cell photolysis of n-pentanal
with and without oxygen at 290, 310, and 330 nm by using
GC/MS and HPLC.
CH (CH ) CHO + hν f C H + CO
3
2 3
4
10
-1
∆
H° ) -7.8 kJ‚mol (1)
298
f n-C H + HCO
4
9
Experimental Section
-1
∆
H°
298
) 348.3 kJ‚mol (2)
The apparatus, which has been described in detail else-
(
λ e 343 nm)
1
7,18
where,
was modified by the addition of a Lambda Physik
Scanmate II dye laser and a second-harmonic generation unit
which were placed downstream from the excimer laser that
pumped them. The fundamental of an excimer-pumped dye laser
can produce tunable pulsed output from 330 nm to 1.05 µm.
Frequency doubling of the fundamental dye laser output allows
wavelength tunability in the 220-330-nm range. It was the
addition of a dye laser and a frequency doubler that enabled
continuous tuning of the photolysis wavelength. The second-
harmonic crystal used in the present experiments was a KDP
f C H CO + H
4
9
-
1
∆
H°
298
) 365.7 kJ‚mol (3)
(
λ e 327 nm)
f CH CHdCH + CH CHO
3
2
3
-1
∆
H°
298
) 82.2 kJ‚mol (4)
(
λ e 1454 nm)
1
0.1021/jp982487s CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/17/1998

Dye Laser Photolysis of n-Pentanal
J. Phys. Chem. A, Vol. 102, No. 50, 1998 10275
crystal. The following laser dyes were used to cover the spectral
range 280-330 nm: Coumarin 153, Rhodamine 6G, Rhodamine
B, Rhodamine 101, Sulforhodamine 101, and DCM.
The dye laser pumped by a 308-nm XeCl excimer (∼180
mJ/pulse) produced an energy output of ∼20 mJ/pulse (typical
dye efficiency is 8-14% depending upon the laser dyes used).
Frequency doubling of the fundamental output of the dye laser
yielded ∼2 mJ of useable UV light for photolysis. Further losses
along the optical path to the photolysis cell reduced the actual
useable energy to less than 2 mJ/pulse. The photodissociation
dye laser pulse entered the reaction cell at a 15° angle with the
main cell axis through a sidearm and overlapped the probe beam
at the center of the cavity. The probe laser pulse from a nitrogen-
pumped dye laser was directed along the main optical axis of
the cell vacuum-sealed with a pair of high reflectance (∼99.999%
at 614 nm) cavity mirrors. A fraction of the probe laser output
was transmitted into the cavity through the front mirror. The
photon intensity decay inside the cavity was measured from
the weak transmission of light through the rear mirror with a
photomultiplier tube (PMT). The PMT output was amplified,
digitized, and sent to a computer. The decay curve was fit to a
single-exponential decay function from which the total loss per
optical pass was calculated. Sample absorption was determined
by measuring cavity losses in the presence and absence of
resonant absorption. Absorption spectra were recorded by
scanning the wavelength of the probe laser with a digital drive
unit. The photolysis laser pulse energy was measured with a
calibrated Joule meter.
Figure 1. Absorption cross sections in the 280-330-nm region at room
temperature. This work on n-pentanal (b) and previous determinations
of C
solid line), propionaldehyde (dashed line), and n-butyraldehyde (dotted
line).
2
-C
4
aldehyde cross sections by Martinez et al.: acetaldehyde
(
TABLE 1: Absorption Cross Sections and HCO Radical
Yields of n-Pentanal as a Function of Wavelength
Gas pressures at the center of the reaction cell were measured
with a Baratron capacitance manometer. n-Pentanal pressure
used in the experiments ranged from 2 to 18 Torr. All
measurements except spectral scans were carried out at a laser
repetition rate of 0.1 Hz to ensure replenishment of the gas
sample between successive laser shots. Spectrum scans were
performed at a repetition rate of 1 Hz. All experiments were
conducted at room temperature which was 293 ( 2 K.
n-Pentanal is a liquid at room temperature and was obtained
from Aldrich Chemical Co. (∼99% purity) Impurities in
n-pentanal included e0.07% n-butyraldehyde and 1% n-
pentanoic acid. Effects of these impurities on the accuracy of
cross section and HCO yield measurements will be discussed
in the following section. n-Pentanal vapor was used in the
experiments. It was purified by repeated freeze-pump-thaw
cycles and stored under vacuum. Formaldehyde was generated
by the pyrolysis of polymer paraformaldehyde (Aldrich Chemi-
cal Co.; g95% purity) at 110 °C. Nitrogen (g99.999% purity,
UHP grade) and oxygen (g99.994% purity, UHP grade) were
purchased from MG Industries and were used without further
purification.
20
2
λ (nm)
σ (10- cm molecule-1)
æ(HCO)
280
285
5.48 ( 0.15
6.23 ( 0.09
6.09 ( 0.15
6.56 ( 0.17
5.80 ( 0.16
5.43 ( 0.09
4.41 ( 0.07
3.74 ( 0.09
2.36 ( 0.10
1.81 ( 0.07
1.03 ( 0.15
0.058 ( 0.006
0.095 ( 0.009
0.10 ( 0.02
0.14 ( 0.01
0.10 ( 0.02
0.15 ( 0.02
0.14 ( 0.02
0.20 ( 0.06
0.14 ( 0.02
0.085 ( 0.034
0.087 ( 0.015
290
295
300
305
310
315
320
325
330
sources of systematic errors are those involving pressure (0.1%)
and path length (0.2%) determinations, and other systematic
errors from the presence of impurity (∼1%) in n-pentanal. Since
n-pentanoic acid only absorbs UV radiation shorter than 260
nm, the presence of 1% n-pentanoic acid in n-pentanal will
systematically lower the values of the absorption cross sections
by 1%. Absorption cross sections of n-butyraldehyde19 are about
1
3
5
% smaller than those of n-pentanal in the wavelength range
studied. As a result, the presence of e0.07% n-butyraldehyde
would decrease n-pentanal cross section values by e0.0035%.
Adding relative (see Table 1) and systematic errors, the overall
uncertainty for n-pentanal cross section measurements is 5%
in the 280-325-nm range, and 16% at 330 nm. Included in
Figure 1 for comparison are previously reported absorption
Results and Discussion
Absorption Cross Sections of n-Pentanal in the 280-330-
nm Region. Depicted in Figure 1 are room-temperature absorp-
tion cross sections of C5H10O as a function of wavelength in
the 280-330-nm range. The absorption cross section at each
wavelength was obtained by measuring the transmitted pho-
tolysis photon intensity as a function of n-pentanal pressure in
the cell and by applying the Beer-Lambert relation to the data
obtained. Error bars quoted (1σ) on Figure 1 are the estimated
precisions of cross section determinations which includes the
standard deviation for each measurement (∼0.5%) plus the
standard deviation about the mean of at least four repeated
experimental runs. Besides random errors, systematic errors also
contribute to the uncertainty in cross section values. The major
1
9
spectra of C2-C4 aldehydes. As can be seen, the spectral shape
of n-pentanal is similar to those reported previously for C2-C4
2
-1
aldehydes. Cross section data (in units of cm molecule , base
e) obtained from this study are also tabulated in Table 1.
HCO Quantum Yield from Photolysis of n-Pentanal in
the 280-330-nm Range. Shown in Figure 2 is a ring-down
absorption spectrum of the product from 290-nm photolysis of
n-pentanal measured at a photolysis/probe laser delay of 15 µs
in the wavelength region 613-616 nm. The resemblance of the
photoproduct absorption spectrum to the reported absorption
20-23
spectrum
of HCO in the same spectral region indicates that

10276 J. Phys. Chem. A, Vol. 102, No. 50, 1998
Cronin and Zhu
formation channel around 300 nm.13 Assuming that this channel
is also the only important radical formation channel around 300
nm for n-pentanal, then the time dependence of HCO decay
following photolysis can be modeled by the following kinetic
scheme consisting of three coupled second-order equations:
HCO + HCO f H CO + CO
(5)
(6)
2
HCO + C H f products
4
9
C H + C H f C H
18
(7a)
4
9
4
9
8
f C H + C H
8
4
10
4
(7b)
To extract rate constants from HCO concentration decays, we
compared time-resolved HCO decay profiles from the photolysis
of 4-, 8-, and 16-Torr n-pentanal with those calculated using
the ACUCHEM simulation program. Input parameters for the
ACUCHEM program included initial estimates for kHCO+HCO
and kHCO+C H (eqs 5 and 6, respectively) as well as input values
4
9
for kC H +C H (eq 7) and HCO initial concentration ([HCO]0).
4
9
4 9
The simulated decay profile was compared with the experi-
mental HCO decay profile, and then kHCO+HCO and kHCO+C4H9
were adjusted until an optimum fit of experimental data was
Figure 2. The lower trace is a low-resolution cavity ring-down
absorption spectrum of the product following the photolysis of 4-Torr
n-pentanal at 290 nm. The upper trace is a time-resolved intracavity
-12
3
-1 -1
accomplished. A value of 2.0 × 10
cm molecule
s
for
-12
kC H +C H was used in the fitting (rate constants of 1.9 × 10
1
0
4
9
4 9
laser absorption spectrum of the (00 0) f (09 0) vibronic transition of
3
-1 -1
-12
3
-1 -1
cm molecule
s
and 2.0 × 10
cm molecule
s
were
HCO following the 266-nm photolysis of 0.1-Torr CH
Torr Ar (adapted from ref 22).
3
CHO and 10-
2
4,25
reported
for C2H5 + C2H5 and n-C3H7 + n-C3H7 reactions,
respectively). An average of four experiments gave kHCO+HCO
and kHCO+C4H9 of (7.3 ( 1.7) × 10
-
11
-11
and (6.5 ( 1.5) × 10
3
-1 -1
cm molecule s , respectively. Errors are 1σ and represent
experimental scatter only. Systematic errors such as uncertainties
in the values of [HCO]0, kC H +C H , and time resolution of the
4
9
4 9
cavity ring-down technique (ring-down time of ∼12 µs at the
wavelength range studied) also contributed to overall uncertainty
in the fitted values of kHCO+HCO and kHCO+C H . We estimate
4
9
systematic error because of the time resolution of the cavity
ring-down technique to be a maximum of 5%. If we assume
errors in the estimation of [HCO]0 and kC H +C H to be 15%
4
9
4 9
and 10%, respectively, then the overall error, including the
experimental scatter in the fitted value of kHCO+HCO and
kHCO+C H , is ∼53%. The extracted kHCO+HCO and kHCO+C H rate
4
9
4 9
constants are in good agreement with the recommended rate
2
6
-11
3
constant for the HCO + HCO reaction (k ) 5.0 × 10 cm
-
1
-1
molecule
s
at 300 K; ∆log k ) (0.3) and the previously
2
4
reported rate constants for the HCO + C2H5 ((7.2 ( 1.6) ×
-
-
11
11
3
-1 -1
1
10
0
cm molecule s ) and HCO + n-C3H7 ((6.5 ( 1.3) ×
Figure 3. Typical time profile of the HCO radical. Photolysis at 290
nm of 4-Torr n-pentanal. Also shown is a fit to the kinetic model
3
-1 -1
cm molecule s ) reactions.
involving HCO + HCO, HCO + n-C
4
H
9
, and n-C
4
H
9
+ n-C
4
H
9
Although HCO radicals were generated the whole length of
reactions.
the level arm through which the photolysis laser pulse traveled,
the cavity ring-down technique only monitored HCO radical
concentration in the region where the photolysis and probe laser
beam overlapped. This region could be envisioned as a
rectangular solid with width and height defined by those of the
HCO is a photolysis product. The sharp absorption bands
correspond to a vibronic transition from the vibrationless level
2
1
of the ground state A′ (00 0) to the vibrationally and electroni-
2
0
cally excited A′′ (09 0) state. The cavity ring-down spectrom-
eter was tuned to a HCO absorption resonance at 613.8 nm (R
bandhead), and the HCO concentration as a function of time
was monitored. The time dependence of the HCO concentration
was obtained by varying the delay time between the firing of
the photolysis and the probe lasers and measuring the corre-
sponding changes in absorption losses. Presented in Figure 3 is
a typical time profile of the HCO radical along with a fit to the
data. As seen from Figure 3, HCO appeared immediately after
the 290-nm photolysis pulse and its concentration decayed as a
function of time. Experimental studies of smaller aldehydes
-
1
photolysis beam, and length defined by (beam width) × tan
(15°), where 15° is the angle between the photolysis beam and
probe beam. The yield of HCO was derived from the ratio of
the HCO concentration produced in this overlapping region to
the absorbed photon density in the same region. Absorption of
the photolysis beam by n-pentanal in the photolysis/probe laser
overlapping region was calculated from the difference in the
transmitted photolysis photon intensity at the beginning and at
the end of the overlapping region. The incident light intensity
was determined with a Joule meter, which was calibrated by
1
3
(RCHO) have shown that R + HCO is the only important radical
acetone photolysis actinometry, by measuring the photolysis

Dye Laser Photolysis of n-Pentanal
J. Phys. Chem. A, Vol. 102, No. 50, 1998 10277
photon energy into the cell. To correct for photon transmission
loss at the front window of the cell, photolysis photon intensity
before and after an empty cell was determined. Transmission
loss was ∼8%/window. Once the absorption cross section of
n-pentanal at a given photolysis wavelength and the light
intensity incident onto n-pentanal were known, the absorbed
photon density in the overlapping region of the photolysis and
the probe lasers could be calculated for a given initial n-pentanal
pressure. The HCO concentration produced at a given photolysis
wavelength was obtained from lossmeter measurements of the
HCO absorption intensity at 613.8 nm at a photolysis-to-probe
laser delay of 15 µs. To convert HCO absorption losses into
absolute concentrations, the absorption cross section of HCO
at the probe laser wavelength needed to be known. The
absorption cross section of HCO was determined in the present
study relative to the photolysis reaction H2CO + hν f HCO
26
+
H, for which the HCO quantum yield is known. Since H2CO
is easily polymerizable, it was produced immediately prior to
each calibration run. The H2CO sample purity was checked
using FTIR; H2O and CO2 impurities were not observed. The
purity of H2CO was estimated by comparing the absorption cross
section of H2CO determined at each photolysis wavelength with
literature values. The H2CO absorption cross section was
determined by measuring transmitted photolysis photon intensity
as a function of H2CO pressure in the cell and by applying the
Beer-Lambert relation to the data obtained. Except for 290 and
Figure 4. HCO quantum yield as a function of the n-pentanal
photolysis wavelength.
are plotted in Figure 4 and listed in Table 1. Errors quoted reflect
experimental scatter (1σ) which are estimated on the basis of
at least two experimental runs consisting of about 8 aldehyde
pressures per run. Systematic errors in the yield measurements
include uncertainties in the determination of the following
parameters: H2CO concentration and absorption cross section
3
10 nm, our H2CO cross section data agreed within (15% of
(
3
∼24%), n-pentanal absorption cross section (5% in the 280-
4
those obtained by Moortgat et al. at λ e 300 nm and by Cantrell
et al.27 in the 300-330-nm region. At 290 nm, an average of
seven cross section measurements yielded an H2CO cross section
25-nm region; 16% at 330 nm), pulse energy (5%), and dye
laser width. To estimate the effect of uncertainty in the
determination of dye laser width on the absolute value of HCO
yield, we calculated the HCO yield using a photolysis beam
width of 0.15 and 0.20 cm. We found that HCO yield only
differed by a maximum of 5% when the beam width varied
from 0.15 to 0.20 cm. The presence of 1% n-pentanoic acid in
n-pentanal will systematically lower the value of HCO yield
by 1%. Photolysis of n-butyraldehyde also leads to the formation
-
21
2
of (6.98 ( 1.04) × 10
cm , which is 86% smaller than that
-20
2
4
of 1.30 × 10 cm reported by Moorgat et al. H2CO exhibits
a structured absorption spectrum in the 280-330-nm region.
The cross section reported by Moorgat et al. was averaged over
a 0.5-nm interval. Our photolysis laser has a wavelength
-
1
resolution of 0.15 cm . If there were a dip in the H2CO
absorption spectrum, it would be reflected in the cross section
28,29
of HCO. An HCO yield of 0.34 was reported at 313 nm.
27
value obtained. Cantrell et al. reported absorption cross
Assuming the HCO channel has a quantum yield of 0.34 in the
-
20
2
-21
2
sections of 2.22 × 10
cm and 9.31 × 10
cm at 308.75
2
80-330-nm region, the presence of 0.07% n-butyraldehyde
and 311.25 nm, respectively. Their extrapolated cross section
will lead to a maximum estimated uncertainty of 0.4% in HCO
yield. The overall uncertainties of HCO yield measurements
including both systematic error and experimental scatter are
-
20
2
at 310 nm is 1.58 × 10
cm , which is 70% higher than our
-
21
2
value of (9.25 ( 1.70) × 10
cm . Since cross sections
changed dramatically in this wavelength region, linear extrapo-
lation of their data may not be valid. We considered using the
Cl + H2CO f HCO + HCl reaction to calibrate absolute HCO
concentration. However, we did not use this calibration scheme
since H2CO photolysis would also have led to the formation of
HCO.
∼50% at 280, 285, and 295 nm; ∼55% at 305, 310, and 320
nm; ∼60% at 290 and 300 nm; 68% at 330 nm; 70% at 315
nm; 80% at 325 nm. As seen from Figure 4, the HCO yields
increased to a maximum in the vicinity of 315 nm and then
decreased at both longer and shorter wavelength ends. The
decrease in HCO yields at higher photolysis photon energies
can possibly be attributed to the opening up of additional
n-pentanal photodissociation pathways (Product studies de-
scribed later might also be suggestive of the C4H10 + CO
channel at higher photolysis photon energies). The reduced HCO
yields at the longer wavelength tail may be the result of
photodissociation at a near-threshold wavelength.
The dependence of the HCO radical yield on n-pentanal
pressure was examined. The HCO radical yield was found to
be independent of n-pentanal pressure when photolysis was
conducted in the wavelength range 280-325 nm. At 330-nm
photolysis, the HCO radical yield decreased by ∼25% when
the n-pentanal pressure was increased from 2 to 18 Torr. Since
the size of the HCO signal was small at 330 nm, it is unclear
whether this decrease in the HCO yield was just the experi-
mental error limit or it is due to dissociation at a near-threshold
wavelength. The photolysis laser pulse energy was repeatedly
monitored during the course of the HCO yield measurements
so that any drop in photolysis photon energy with time was
corrected. Also, the HCO yield at a given n-pentanal pressure
was repeatedly measured during the course of the experiments
in order to ensure the consistency of the results. HCO radical
yields as a function of the photolysis wavelength thus obtained
The dependence of HCO radical yields on total pressure was
examined by maintaining the constant n-pentanal pressure and
varying the nitrogen carrier gas pressure. HCO radical yields
were found to be independent of the total pressure when the
total pressure was varied between 8 and 480 Torr with photolysis
wavelengths shorter than 325 nm.
One might wonder if the HCO radical yield is dependent on
n-pentanal pressure when n-pentanal pressure is on the order
of hundreds of mTorr and if quenching of excited n-pentanal
below its dissociation threshold has already occurred at the

1
0278 J. Phys. Chem. A, Vol. 102, No. 50, 1998
Cronin and Zhu
TABLE 2: Products from Closed-Cell Photolysis of
n-pentanal pressures we worked with. To answer this question,
we used a 308-nm excimer laser to photolyze n-pentanal. The
much higher photon fluence from the excimer laser (∼0.032
n-Pentanal at Several Wavelengths
products observed and yieldsa
photolysis
2
J/cm ) and the larger overlapping distance between the pho-
sample
wavelength
GC/MS
HPLC
tolysis and the probe lasers (4.6 cm) allowed us to determine
the HCO yield as a function of n-pentanal pressure over 2 orders
of magnitude of n-pentanal pressure. n-Pentanal pressures of
n-pentanal/N
2
290 nm
1-butene
CH
H CO (2.5%)
2
3
CHO (84%)
1
-propene
butane
CH (CH ) CHO (12%)
3
2
2
0
.1, 0.2, 0.5, 1, 1.5, 2, 3, 4, 6, 8, and 10 Torr were used. The
n-octane
photolysis/probe laser delay was set at 5 µs. The HCO yield
was found to be independent of n-pentanal pressure in the 0.1-
3
10 nm
CH
3
CHO (91%)
H
2
CO (2.4%)
4
-Torr range. The HCO yield was about 15% lower from the
photolysis of 10-Torr n-pentanal than that from 0.1 Torr. If HCO
HCO, HCO + C4H9, and C4H9 + C4H9 reactions at 5 µs
after photolysis of 10-Torr n-pentanal were corrected for
CH (CH CHO (6.8%)
3
2 2
)
330 nm
290 nm
1-butene
1-propene
3
CH CHO (68%)
H CO (1.1%)
2
+
CH
3
(CH
2
)
2
CHO (28%)
1
3
-3
(
[HCO]0 ) [C4H9]0 ) 8.3 × 10 cm ), this difference amounts
n-pentanal/O
2
1-butene
CH
3
CHO
1
-propene
2
H CO
to only 8% over a 100-fold change in n-pentanal pressure. Also,
the size of the HCO yield at 308 nm (∼0.13) compared with
those at 305 and 310 nm does not support the occurrence of
significant quenching.
butane
CH
CH
CH
3
3
3
CH
COCH
(CH
2
CHO
3
2
)
2
CHO
3
10 nm
CH
3
CHO
Analysis of End Products from the Photolysis of n-
Pentanal. End products from the photolysis of n-pentanal/N2
and n-pentanal/O2 mixtures in a closed cell with ∼15000 laser
shots were analyzed by GC/MS and HPLC. HPLC was used to
probe aldehyde products. GC/MS was employed to monitor
nonpolar products. Limited use of analytical apparatus (GC/
MS and HPLC) outside of our laboratory represented our best
current effort to make quasi-steady-state end-product determina-
tions. One major technical problem was that n-pentanal is sticky,
making it difficult to transfer n-pentanal quantitatively from one
vessel to another. We found that when we added the same
n-pentanal pressure into a 25-mm × 36-cm cylindrical Pyrex
cell, and then transferred the unphotolyzed sample to an HPLC
cartridge or to a GC/MS sampling canister, the GC/MS and
HPLC analysis did not show the same absolute response.
Product studies therefore were qualitative. n-Pentanal, oxygen,
and nitrogen pressures of 10, 160, and 160 Torr, respectively,
were used in the experiments. Acetaldehyde was observed by
HPLC after photolysis of an n-pentanal/N2 mixture. This
suggests the photofragmentation channel:
H
2
CO
CH CH
CH COCH
3
2
CHO
3
3
CH
3
(CH
2
)
2
CHO
330 nm
1-butene
1-propene
CH CHO
H CO
2
CH
CH
CH
3
3
3
3
CH
COCH
(CH
2
CHO
3
2
)
2
CHO
a
Yields were estimated assuming that total carbonyls were conserved
before and after photolysis of an n-pentanal/N mixture. n-Pentanal
2
photolyzed was generally less than 10%.
oxidation products such as n-butyraldehyde, propionaldehyde,
acetone, and formaldehyde were also detected by HPLC. More
n-pentanal was consumed when an n-pentanal/O2 mixture was
photolyzed compared to that of an n-pentanal/N2 mixture. Also,
the relative product distributions were changed. On one hand,
relative product ratios from photolysis of an n-pentanal/O2
mixture at 290, 310, and 330 nm are acetaldehyde/propional-
dehyde ∼ 1, 0.15, and 0.054, respectively; acetaldehyde/acetone
∼
1, 0.0043, and 0.005, respectively; acetaldehyde/H2CO ∼ 1,
CH (CH ) CHO + hν f CH CHdCH + CH CHO (4)
3
2 3
3
2
3
0
1
.030, and 0.22, respectively; acetaldehyde/n-butyraldehyde ∼
, 0.32, and 0.065, respectively. On the other hand, acetalde-
Estimates of the quantum yields of aldehydes formed from the
photolysis of an n-pentanal/N2 mixture can be derived in the
following manner. Total carbonyls are expected to be conserved
before and after the photolysis of an n-pentanal/N2 mixture.
Assuming all the carbonyl groups from the photolysis of an
n-pentanal/N2 mixture existed as aldehydes and were detected
by HPLC, then the fraction of individual aldehyde on the
chromatogram can be estimated by summing up the peak areas
of all the aldehydes and dividing the individual aldehyde peak
area by the sum. The relative amount of individual aldehyde
formed was equal to the difference in the ratio of the aldehyde
peak area to the total peak area before and after photolysis. The
relative amount of n-pentanal photolyzed was equal to the
difference in the ratio of n-pentanal over the total peak area
before and after photolysis. The estimated individual aldehyde
yield was obtained by dividing the amount of aldehyde formed
by the amount of n-pentanal photolyzed. Since carbonyl groups
from the photolysis of an n-pentanal/N2 mixture may also exist
as CO, and it was not detected by HPLC, the analysis mentioned
above only provided a crude estimate of aldehyde yields. The
acetaldehyde quantum yields thus obtained are ∼84%, 91%,
and 68% at 290, 310, and 330 nm, which are also listed in Table
hyde/n-butyraldehyde ratios of 1, 1.6, and 0.34 at 290, 310, and
3
30 nm were obtained from n-pentanal/N2 photolysis.
GC/MS detection of photolysis products suggested the
existence of several product channels leading to the formation
of alkenes and alkanes. For instance, photolysis of an n-pentanal/
N2 mixture at 330 nm led to the formation of 1-propene and
1
-butene. 1-Propene can be generated from the acetaldehyde
channel (see eq 4). 1-Butene may be produced from the
following channel:
CH (CH ) CHO + hν f CH CH CHdCH + H CO (8)
3
2 3
3
2
2
2
At 290 nm, the photolysis of an n-pentanal/N2 mixture led to
the formation of additional products such as butane and n-octane.
Butane is possibly a product of the following photodissociation
channel:
CH (CH ) CHO + hν f C H + CO
(1)
3
2 3
4
10
The observation of butane at the shorter photodissociation
wavelength is consistent with results from the wavelength-
dependent HCO yield measurements which could be suggestive
of additional photolysis pathways at higher photon energies.
2
. When an n-pentanal/O2 mixture was photolyzed, secondary

Dye Laser Photolysis of n-Pentanal
J. Phys. Chem. A, Vol. 102, No. 50, 1998 10279
Because of the overlapping GC/MS peaks of n-butane and other
products, we could not extract an n-butane yield from the 290-
nm photolysis of an n-pentanal/N2 mixture. n-Octane can be
formed from the recombination of C4H9:
(see Table 2)). Thus, the radical formation capability of longer
chain aldehydes such as n-pentanal is significant.
Acknowledgment. We thank Mr. Andrew Levine for his
assistance in the experiments and for calculating the photolysis
rates. We are grateful to Dr. Xianliang Zhou, Ms. Guo Hong,
Dr. Amarjit Narang, and Ms. Michele Losavio for performing
HPLC analysis of photolysis products, and to Dr. Ken Aldous
and Mr. Michael Force for GC/MS analysis of photodissociation
products. Helpful discussions and suggestions by Professor
Robert Keesee, Dr. Xianliang Zhou, Dr. Stephen Riley, and Dr.
Geoffrey Tyndall are acknowledged. This work was supported
by the National Science Foundation under Grant No. ATM-
C H + C H f C H
18
(9)
4
9
4
9
8
In the presence of oxygen, n-octane disappeared possibly due
to the formation of butyl peroxy radicals (C4H9O2):
C H + O f C H O
2
(10)
4
9
2
4
9
9
610285.
A summary of the product study using GC/MS and HPLC is
included in Table 2.
References and Notes
A previous study24 on the photolysis of n-butyraldehyde at
(
1) Graedel, T. E.; Farrow, L. A.; Weber, T. A. Atmos. EnViron. 1976,
3
08 nm indicated the yield of ethylene (a coproduct of the
10, 1095.
(
(
2) Grosjean, D. EnViron. Sci. Technol. 1982, 16, 254.
3) Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Atmospheric Chemistry; John
acetaldehyde channel) decreased by 45% when 500-Torr air was
added to 20-Torr n-butyraldehyde. It would be very interesting
to examine the dependence of the acetaldehyde yield as a
function of the oxygen pressure and total pressure from
n-pentanal photolysis.
Atmospheric Photodissociation Rate Constant To Form
the HCO Radical. The photodissociation rate constant (krad)
for n-pentanal to form the HCO radical (or HO2 in the presence
of air) was calculated from the actinic solar flux (J(λ)) reported
by Demerjian et al.,30 the absorption cross section (σ(λ)) of
n-pentanal and the HCO radical yield (Φ(rad, λ)) from n-
pentanal photolysis, both from the present study, using the
relationship
Wiley: New York, 1986.
(4) Moortgat, G. K.; Seiler, W.; Warneck, P. J. Chem. Phys. 1983, 78,
185.
1
(
(
5) Carmely, Y.; Horowitz, A. Int. J. Chem. Kinet. 1984, 16, 1585.
6) Moore, C. B.; Weisshaar, J. C. Annu. ReV. Phys. Chem. 1983, 34,
5
25.
(7) Ho, P.; Bamford, D. J.; Buss, R. J.; Lee, Y. T.; Moore, C. B. J.
Chem. Phys. 1982, 76, 3630.
(
8) Horowitz, A.; Calvert, J. G. J. Phys. Chem. 1982, 86, 3105.
(9) Meyrahn, H.; Moortgat, G. K.; Warneck, P. Presented at the 15th
Informal Conference on Photochemistry, Stanford, CA, July 1982.
(
(
10) Shepson, P. B.; Heicklen, J. J. Photochem. 1982, 19, 215.
11) Heicklen, J.; Desai, J.; Bahta, A.; Harper, C.; Simonaitis, R. J.
Photochem. 1986, 34, 117.
(12) Terentis, A. C.; Knepp, P. T.; Kable, S. H. J. Phys. Chem. 1995,
9
9, 12704.
13) Calvert, J. G.; Pitts, J. N., Jr. Photochemistry; John Wiley: New
York, 1966.
(
krad
)
∫
σ(λ)Φ(rad,λ)J(λ) dλ
(11)
(
(
(
14) Lee, E. K. C.; Lewis, R. S. AdV. Photochem. 1980, 12, 1.
15) O’Keefe, A.; Deacon, D. A. G. ReV. Sci. Instrum. 1988, 59, 2544.
16) O’Keefe, A.; Scherer, J. J.; Cooksy, A. L.; Sheeks, R.; Heath, J.;
The photolysis rate constants of n-pentanal to form HCO for
noontime on January 1 and July 1 under cloudless conditions
at sea level and a latitude of 40° N were calculated to be 5.0 ×
Saykally, R. J. Chem. Phys. Lett. 1990, 172, 214.
17) Zhu, L.; Johnston, G. J. Phys. Chem. 1995, 99, 15114.
(
(18) Zhu, L.; Kellis, D.; Ding, C.-F. Chem. Phys. Lett. 1996, 257, 487.
(19) Martinez, R. D.; Buitrago, A. A.; Howell, N. W.; Hearn, C. H.;
Joens, J. A. Atmos. EnViron. 1992, 26A, 785.
-
6
-5 -1
1
0
and 1.6 × 10 s , respectively. If both primary HCO
yield from the photolysis of n-pentanal and secondary HCO
yield from the photolysis of the acetaldehyde product were
included by using literature acetaldehyde absorption cross
section and quantum yield data,26 calculated radical formation
rate constants for noontime on January 1 and on July 1 are 6.9
(
34.
20) Herzberg, G.; Ramsay, D. A. Proc. R. Soc. (London) 1955, A233,
(21) Johns, J. W. C.; Priddle, S. H.; Ramsay, D. A. Discuss. Faraday
Soc. 1963, 35, 90.
22) Stoeckel, F.; Schuh, M. D.; Goldstein, N.; Atkinson, G. H. Chem.
(
Phys. 1985, 95, 135.
(23) Reilly, J. P.; Clark, J. H.; Moore, C. B.; Pimentel, G. C. J. Chem.
Phys. 1978, 69, 4381.
-
6
-5
-1
×
10
and 2.5 × 10
s , respectively. Acetaldehyde
photodissociation rate constants that lead to radical formation
(24) Baggott, J. E.; Frey, H. M.; Lightfoot, P. D.; Walsh, R. J. Phys.
-
6
-1
-5 -1
are 2.6 × 10
s
for January 1 and 1.1 × 10
s
for July
Chem. 1987, 91, 3386.
(25) Adach, H.; Basco, N. Int. J. Chem. Kinet. 1981, 13, 367.
1
. The calculated radical formation rate constants from n-
(
26) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Kerr,
J. A.; Troe, J. J. Phys. Chem. Ref. Data, 1992, 21, 1125.
27) Cantrell, C. A.; Davidson, J. A.; McDaniel, A. H.; Shetter, R. E.;
pentanal photolysis are 1.9-2.7 times and 1.5-2.3 times as fast
as those obtained from acetaldehyde photolysis for January 1
and July 1 conditions, respectively (the lower limit in the range
did not include the HCO radical formed from the photolysis of
the acetaldehyde product; the upper limit assumed acetaldehyde
yields from the photolysis of n-pentanal at atmospheric pressure
were 0.84, 0.91, and 0.68 at 290, 310, and 330 nm, respectively
(
Calvert, J. G. J. Phys. Chem. 1990, 94, 3902.
(28) F o¨ rgeteg, S.; B e´ rces, T.; D o´ b e´ , S. Int. J. Chem. Kinet. 1979, 11,
19.
2
(
(
29) Blacet, F. E.; Calvert, J. G. J. Am. Chem. Soc. 1951, 73, 667.
30) Demerjian, K. L.; Schere, K. L.; Peterson, J. T. AdV. EnViron. Sci.
Technol. 1980, 10, 369.