The Journal of
Physical Chemistry
0 Copyright, 1987, by the American Chemical Society
VOLUME 91, NUMBER 13 JUNE 18, 1987
LETTERS Direct Determination of the Quantum Yield of OH in the Laser Photolysis of Formic Acid at 222 nm+ Gurvinder S. Jolly,* Donald L. Singleton,* and George Paraskevopoulos* Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada K l A OR9 (Received: February 25, 1987)
The primary quantum yield for formation of OH radicals by photolysis of the monomer and the dimer of formic acid vapor at 222 nm is determined for the first time by a laser photolysis-resonance absorption technique. The measurement is relative to the quantum yield of OH in the photolysis of nitric acid, taken as unity. For the monomer of formic acid, the quantum yield of OH is found to be unity, and for the dimer, essentially zero. The photochemicalimplicationsof the results are discussed.
Introduction The photolytic channels of formic acid have not been quantitatively determined despite its position as the simplest member of the carboxylic acids. Differences in the behavior of the monomer and dimer of formic acid would provide insight into the influence of the hydrogen bonds in the dimer on the photolytic process, which should be amenable to theoretical calculations. The UV absorption spectrum of formic acid monomer consists A* transition beginning at about 260 nm and of a weak n merging at shorter wavelengths with a strong A A* transition which peaks at about 180 nm.1-3 The stable products observed on photolysis in this spectral region are CO, H 2 0 , C 0 2 , and H2, and the decomposition has been discussed in terms of processes 1-5, based on analysis of stable products.4-8 Although early work +
+
+ h~ HCOOH + h~ HCOOH + hu HCOOH
+
-
H2
+ C02
+ CO O H + HCO
+
HlO
(1)
(2) (3)
+ hu HCOOH + hu HCOOH
"RCC Research Associate. Present address: Northern Telecom Electronics, Ottawa, Ontario, Canada KIY 4H7.
0022-3654/87/2091-3463$01.50/0
+ COOH HCOO + H H
(4)
(5)
using parahydrogens and the antimony mirror technique6 indicated that H atoms were not formed at wavelengths greater than 190 nm, subsequent work by Gorden and Ausloos' showed that yields of H2 were depressed by addition of C2H4,which scavenges H. They concluded that reaction 3 is the most important of the processes forming free radicals, but a quantitative assessment of its importance was not possible. Photolysis of the dimer was not discussed. Also, emission from excited OH(A2Z+) formed by photolysis of formic acid in the vacuum-UV (>130 nm) has been reported.9-" Despite these studies, a quantitative understanding (1) (2) 1966. (3) (4) (5)
(6) (7)
'NRCC No. 27599
-
+
(8)
(9)
Ng, T. L.; Bell, S. J . Mol. Spectrosc. 1974, 50, 166. Calvert, J. G.; Pitts, J. N., Jr. Photochemistry; Wiley: New York, Barnes, E. E.; Simpson, W. T. J . Chem. Phys. 1963, 39, 670. Ramsperger, H. C.; Porter, C. W. J . Am. Chem. SOC.1926, 48, 1267. Gorin, E.; Taylor, H. S . J . A m . Chem. SOC.1934, 56, 2042. Burton, M. J . Am. Chem. SOC.1936, 58, 1655. Gorden, R.; Ausloos, P. J . Phys. Chem. 1961, 65, 1033. Yankwich, P. E.; Steigelmann, E. F. J . Phys. Chem. 1963, 67, 757. Dyne, P. J.; Style, D. W. G.Discuss. Faraday SOC.1947, 2, 159.
Published 1987 by the American Chemical Society
3464 The Journal of Physical Chemistry, Vol. 91, No. 13, I987 of the photolytic pathways of the monomer and dimer of formic acid has not been achieved. We recently recognized that the quantum yield of OH is significant in a study of the O H + formic acid reaction in which we used the laser photolysis of formic acid as a source of OH.I2 Also, we have recently used a direct spectroscopic method to determine the primary quantum yield of O H in the laser photolysis of nitric acid,I3 which gave results in agreement with the measurements of Johnston et al.I4 of the quantum yield of the coproduct, NO2. In the present work, the method is extended to the determination of the quantum yield of O H from the monomer and dimer of formic acid, using the photolysis of nitric acid and its known quantum yield of O H to calibrate the system, thus avoiding the requirement to know the photolysis volume, the oscillator strength of OH, and the absolute intensity of the photolysis radiation.
Experimental Section The essential features of the apparatus are similar to those described previouslyi3 for the direct determination of the O H quantum yield from the photolysis of H N 0 3 . Formic acid was photolyzed at 222 nm with a KrCl excimer laser (Lumonics TE-860-3) directed down the axis of a 90-cm-long cylindrical Suprasil reaction cell. Intensities of the laser radiation (Io,I) were measured with a disk calorimeter (Scientech Model 36-0001) which was calibrated actinometrically from the amount of H2 formed during photolysis of HBr at 222 nm, as described before.I3 The O H produced was monitored by time-resolved resonance absorption at 308.3 nm using the Ql(4) line of the (0,O) band of the A2Z+ X211transition generated by a microwave-powered resonance lamp. The analyzing beam was directed coaxially with the photolysis beam along the length of the photolysis cell. The signal corresponding to the transmitted 308.3-nm light was accumulated in a signal averager for 1000-3000 laser pulses to improve the signal-to-noise ratio. Pressure was measured with capacitance manometers. Formic acid (98%), obtained from Aldrich, was degassed by repeated freeze-pumpthaw cycles. Gas chromatographic analysis confirmed the presence of 2% H 2 0 . Because the vapor is enriched in formic acid with respect to the liquid phase (formic acid + water),15 it was not deemed necessary to correct for the small amount of water vapor in the gas phase. Nitric acid was prepared and handled as described previo~sly.'~
-
Results The equilibrium constant between the monomer and the dimer, (HCO0H)Z 2HCOOH (6) K = [HCOOH]2/[(HCOOH)2],was taken as 6.55 X 10l6 molecules cm-3 at 296 K, the average temperature in the present work, based on Halford's analysisi6of data over the temperature range 283-353 K.I7 Extrapolation of recent higher temperature data'*J9 to 296 K gives values within +14% and -6% of the adopted value, which would lead to a maximum difference of 8% in the partial pressures of monomer and dimer over the pressure range 1-20 Torr. In order to determine the quantum yield of O H from photolysis of the monomer and dimer, the amount of 222-nm radiation absorbed by each must be known. The optical absorption coefficients of the monomer and the dimer, uM,and uD, respectively, were obtained by measuring the fractional transmission of the KrCl -+
(IO) Terenin, A.; Neujmin, H. J . Chem. Phys. 1935, 3, 436. (11) Vinogradov, I. P.; Vilesov, F. I . Khim. Vys. Energ. 1977, 11, 25; Chem. Abstr. 1977, 86, 163518q. (12) Jolly, G. S.; McKenney, D. J.; Singleton, D. L.; Paraskevopoulos, G.; Bossard, A. R. J . Phys. Chem. 1986, 90, 6557. ( 1 3) Jolly, G. S.; Singleton, D. L.; McKenney, D. J.; Paraskevopoulos, G. J . Chem. Phys. 1986, 84, 6662. (14) Johnston, H . S.;Chang, S . G . ; Whitten, G. J . Phys. Chem. 1974, 78, I. ( I 5) Timmermans, J. The Physico-chemical Constants of Binary Systems in Concentrated Solutions; Interscience: New York, 1960; Vol. 4, p 347. (16) Halford, J. 0. J . Chem. Phys. 1942, 10, 5 8 2 . (17) Coolidge, A. S. J . Am. Chem. Soc. 1928, 50, 2166. (18) Barton, J. R.; Hsu, C. C. J . Chem. Eng. Data 1969, 14, 184. (19) Biittner, R.;Maurer. G. Ber. Bunsen-Ges. Phys. Chem. 1983.87, 877.
Letters c
I
I
I
I
I
I
I
m0-
-.
(D
0 0-
CI
t 0-
- 1
0
I
I
1
2
I
3
I
4
I
5
I
I
6
7
Formic Acid x lo-'' (molecule cm?) Figure 1. Fractional transmission, I,/l,,, of 222-nm radiation (KrCI excimer laser) for various total pressures of formic acid. The line is calculated via eq I, with uM = 1.31 X and uD = 1.99 X IOTL9cm2
molecule-'. excimer line at 222 nm, I/Zo, over a range of formic acid pressures in a 10.2-cm-long Suprasil cell. Equation I was fitted to the data
= exp(-(uM[Mvi] + uDIDI)I) (1) by nonlinear least squares, where I is the optical path length and [MI and [D] are the concentrations of monomer and dimer. The data in Figure 1 give the results uM = (1.31 f 0.10) X and uD = (1.99 f 0.06) X cm2 molecule-', where the indicated uncertainties are one standard deviation. The overall quantum yield of formation of O H from both the monomer and dimer of formic acid is defined as (number of OH radicals formed)/(number of photons absorbed). This is expressed in terms of experimental quantities by eq 11. The number of +(OH) = c In (io/i)f=o/{bZ,,(l - I / I o ) )
(11)
photons absorbed is obtained as described previously from the expression bZo(l - Z/Io), where Z/Io is the ratio of transmitted intensities at 222 nm with formic acid in the photolysis cell and with the photolysis cell empty, b is a constant for converting from power to the number of photons, and c is a calibration factor defined below. A small correction is applied for additional absorption of radiation due to reflection of the beam (7%) by the inner surface of the windows. Io is determined by measuring the transmitted power with the cell evacuated and correcting for the measured transmission efficiency of the exit window of the cell and the reflection efficiency of the movable mirror. Although absolute values of the number of photons absorbed are calculated, only relative values are needed (and used) for the quantum yield determinations. The correction factors for the calorimeter, cell window transmission, and mirror reflectance, as well as the unit conversion factor b, cancel as a result of determining the calibration factor c by photolysis of HNO,, as described below. The number of O H radicals formed is proportional to the absorbance at 308.3 nm extrapolated to t = 0 (the instant that the laser was triggered), In (io/i)f=o.The absorbance is timedependent due to the reaction of O H with formic acid. Because [OH]
