COMMUN I CAT IONS
Production of O,( ' A ~ ) by Energy Transfer from Excited Benzaldehyde R. H. Kummler and M. H. Bortner General Electric Co., Space Sciences Laboratory, Philadelphia, Pa. 19101
Emission of the Oz(alA,) 4 Oz(X'Z,-) 0, 0 band at 1.27 microns has been observed when benzaldehyde-oxygen mixtures are irradiated with ultraviolet light, providing the first direct confirmation of the Kautsky mechanism pertinent to urban environments. The apparatus and preliminary results are described.
R
ecent interest in singlet molecular oxygen in the environmental sciences has been evoked by the increasing body of evidence supporting its high reactivity (Arnold, Kubo, et af., 1968; Broadbent, Gleason, et af., 1968; DASA Handbook, 1967; Falick, Mahan, et a f . , 1965; McNeal and Cook, 1967) and by the discovery of new, potentially large sources of OZ*in the urban atmosphere (Kummler, Bortner, et a f . , 1969; Pitts, 1967; Khan, Pitts, et al., 1967). The possible role of singlet oxygen in the as yet incompletely understood NO to NOz conversion and in the material balance of hydrocarbon oxidation has been summarized recently by Pitts, et a f .(1969). The focal point of that and previous studies was the classic Kautsky (1939) mechanism for the production of singlet oxygen from excited triplet donor molecules. The over-all mechanism for the generation of Oz('A,)-the singlet oxygen molecule of primary concern in air pollution photochemistry (Kummler, Bortner, et a f . , 1969)-may be summarized as follows D(So) D(sl)
+ hv
+
intersystem crossing
D(Si)
(1)
qTd
where D represents the donor organic molecule which undergoes the primary absorption in the near ultraviolet, and So, SI,and TI represent its ground state, lowest excited singlet and triplet states, respectively, as in the Pitts, Khan, et a f . (1969) notation. This mechanism avoids the inherent difficulties of other previously discussed (Kummler, Bortner, et a[., 1969) production sources for OZ(lAg): the low transition probabilities for direct absorption of sunlight by Oz(XT,),and the critical dependence of the ozone photolysis upon the toe of the solar flux at 3000-2100 A. The extinction coefficients for the organics under consideration are, in general, large, and the absorption bands extend well into the available solar wavelength regime (Calvert and Pitts, 1966). Theoretically, the efficiency of Process 3 has been predicted to be high for many cases (Kawaoka, Khan, et a f . , 1967; Foote, 1968). In condensed phases, there is a large body of indirect evidence to support this position. When this work was undertaken, how944 Environmental Science & Technology
ever, no direct evidence had as yet been reported, nor had any experimental evidence been provided for the over-all process in the gas phase. This communication provides evidence on both counts, using benzaldehyde as the donor molecule because of its probable presence in contaminated atmospheres as a result of the oxidation of abundant aromatic hydrocarbons contained in automobile exhaust (Begeman, 1964), its large extinction coefficient (Calvert and Pitts, 1966) in the near ultraviolet, and the known rapidity and completeness of conversion from the direcly excited singlet state to the triplet state (Calvert and Pitts, 1966). Since this work, Snelling (1968) has reported a similar observation using benzene. Experimental The apparatus employed is illustrated schematically in Figure 1. It consisted of two Princeton Applied Research (PAR)BZ 1, two blade choppers connected back to back with a cylindrical commercial grade quartz cell 12 inches long and 2 inches in diameter placed between the respective blades of the chopper and the signal chopper illustrated in Figure 1. One revolution of the blade corresponds to two cycles. The respective blades were placed 90" out of phase, so that the illumination by the 500-watt Hg-Xe arc lamp was provided during the first quarter of a cycle and darkness prevailed during the remainder. The finite size of the chopped aperture permits some light to enter during the second and fourth quarters. In the third quarter, fluoresence from the cell at 1.27 microns was sought, using an intrinsic germanium photodiode (NEP of 5 X watts at 1.27 microns) and cooled preamplifier system (Pullan, 1965) with an OCLI 1285 filter. The entire system was optically isolated to prevent stray light from reaching the detector. The chopping system was sufficiently light-tight that no signal due to the arc lamp could be observed, either visually or with the detector, looking along the axis directly at the lamp with the flask evacuated. A reference signal was taken from the exciting light chopper as input to a PAR HR 8 phase-sensitive lock-in amplifier, along with the signal from the germanium detector. The entire chopper assembly was driven externally by a pulley with a variable speed motor to permit data analysis by frequency variation. Motor stability in this system provided early difficulty due to drift of the reference signal. The most stable system was attained with an RMS Co. 25 in. oz. motor and controller, which was variable over a narrow frequency range about 20 Hz. The quartz cell was initially evacuated to 10-6 Torr. Eastman Kodak Organic Chemical Co. (chlorine-free grade) benzaldehyde was introduced into the vacuum system without further processing, reduced to dry ice-acetone temperature and pumped on to eliminate entrapped air. The pumps were closed, and the system was permitted to pressure-equilibrate at room temperature (-27" C.). Then Matheson ultrahighpurity oxygen was introduced. The pressure was monitored
Hq - Xe LAMP
TO P U M P AND GAS I NTRGDUCTION TO PAR PHASE SENSITIVE LOCK I N AMPLIFIER PHOTO DIODE
/
CHOPPING MOTOR AND PULLEY SYSTEiM
FLUORESCEK IT r l ^ . . . .^ , , > I b N H L LHOPPER
ll
I
/
Figure 1. Schematic of apparatus
on a Wallace and Tiernan gage. The evacuated flask (a), benzaldehyde alone (b), and benzaldehyde-oxygen mixtures (c) each were irradiated in like manner over the range of variables, as was pure oxygen (d) after an evacuation step. The light intensity at the detector end of the flask was estimated, using a Corning 7-59 filter and an Epply 4812 thermopile, The integrated value of the energy was 0.1 watt per cm.2, which, from the nominal filter and source spectral characteristics, yields approximately 1016+1 photons per cm.2 sec. in the ultraviolet (2000 to 4000 A.). Preliminary Results and Discussion In cases (a), (b), and (d), no signal was observed by the germanium detector, as indeed was expected from the spectrum of benzaldehyde and previous experiments with pure oxygen; direct absorption at 1.27 microns is insufficient to produce detectable signals in this geometrical configuration. In case (c), the benzaldehyde-oxygen mixtures, small (signal to noise ~ 2 / 1 but ) reproducible signals were observed at frequencies of at least 17 to 25 Hz,under conditions of approximately 0.1 Torr of benzaldehyde and 20 to 30 Torr of 0 2 . These signals can be qualitatively interpreted in terms of the Kautsky (1939) mechanism (Reactions 1 to 3) which may be rewritten in more complete form as: D
+ hv -, ' D
(1)
ID -, aD
(2)
+ 302
'D
+ ' 0 2
+D
aD+M-,D+M 'D
+ M-,
D
+M
+ hv ID -,D + hv * + hv lo2+ a02 -, 302 + a02 + D a02+ D aD -,D
' 0 2
' 0 2
302
-+
(3) (3 '1 (4) (5)
(6) (7) (8) (9)
where D 3 benzaldehyde in the ground state D(So), ' D D(SJ, ' D D(TI), ' 0 2 E 04.IA g), M is either 0 2 or D , and ki is the rate constant for the zth reaction. Processes 4 and 6 are slow (Calvert and Pitts, 1966) with respect to Process 2,
and Process 7 is slow with respect to Process 8 (Kummler and Bortner, 1967) under the experimental conditions. If we assume that the effective rate constant for Reaction 1 is modw ko sin wt. This form is lated at frequency w , then k l = ko not strictly correct, but the actual waveform can be thought of as a superposition of higher order modes in a Fourier series, so that it is conceptually equivalent. The constant, ko, is a function of the lamp intensity and the benzaldehyde absorption coefficient. The modulated signal, Z, after a short induction period is given by:
+
where a1 =
a2 =
tan-I(w/kz)
tar1[w/(k3'
a3 = tan-'[w/(ksOz
+ ka)l + ksD)l
- -+
Since the signal occurs at a phase angle of 90" f 30", this implies that a1 a2 a3 90" f 30".Since w
