The Journal of Physical Chemistry, Vol. 83, No. 5, 1979 569
Laser-Induced Photodecomposition of H&O
Laser-Induced Photodecomposition of Formaldehyde (A‘ A,) from Its Single Vibronic “1 II Levels. Determination of the Quantum Yield of H Atom by HNO” (A A ) Chemiluminescence Kenneth Y. Tang,+ Paul W. Fairchild, and Edward K. C. Lee* Department of Chemistry, University of California, Irvine, California 927 17 (Ueceived July 24, 1978) Publication costs assisted by the Office of Naval Research
Photodecomposition quantum yields for radical formation process from 11 single vibronic level in the A’Az state of H2C0 have been measured by monitoring the red HNO* chemiluminescence from the H + NO + M recombination. The quantum yield of H atom increases smoothly from -0.4 to 0.8 over -3000 cm-l of excess vibrational energy. Experimental details of the laser induced HNO* chemiluminescence excitation spectroscopy are given.
Introduction The photodecomposition of formaldehyde occurs through two distinct product channels: a radical process (1)and a molecular elimination process (2). Because of HzCO + h~ H + HCO (1) +
4
H2
+ CO
(2)
the fact that the products of the radical process can also react further to give Hz and CO, it has not been simple to determine unambiguously the quantum yield of process 1 (al)or that of process 2 (a2).In spite of this wellrecognized ambiguity, for the lack of a better analytical method, much of the recent photolysis studies in the 270-360-nm region have measured either the quantum yield of molecular hydrogen (cPHP) and/or the quantum yield of CO (rgc0),1-5the product ratios of Hz, HD, and Dz from HDC0F7 or the mixtures of H2C0 and DzC0.738The kinetic treatment of these product yield data obtained in the absence and in the presence of a radical scavenger species (NO, olefin, and 0,) has provided the much sought quantum yields for the two primary processes with varying degrees of reliability, depending upon the validity of the mechanistic assumptions made and the experimental conditions employed. In our view, the chief objections to the above measurements have been the lack of data at low pressures where “isolated” molecule conditions can be maintained for the precursors and the indirectness of the method. For this reason, we have recently explored the feasibility of two other analytical methods by which the quantum yield of the radical process can be measured. Our first methodg involves the product measurement of the absolute H-atom yield in the low pressure limit of added olefinic scavenger, e.g., cis-2-butene, by measuring the yield of propylene produced from the well-studied unimolecular decompositionlo of the “hot” secondary butyl radical formed initially by the H-atom addition to the butene. Our second methodll involves the spectroscopic measurement of the relative yield of H atom in the presence of NO, by the 760-nm chemiluminescence emission from the electronically excited HNO*(AIAi‘) formed through the well-studied H f NO recombination reaction.l2-l4 Since the HNO* chemiluminescence method is quite sensitive and the H-atom yields can be measured conveniently from the rotationally selected15as well as the vibrationally selected states,ll we wish to report on the improvements made on the chemiluminescence excitation Maxwell Laboratories, Inc., San Diego, Calif. 92123. 0022-3654/79/2083-0569$0 1.OO/O
spectroscopy using a pulsed tunable dye laser and a gated boxcar averager.
Experimental Section Any determination of the photolytic quantum yield inust involve either separate measurements of both the amount of the actinic light absorbed and the amount of the resulting photochemical events or a direct measurement of the ratio of the two. Although in principle the direct iratio measurement is most universally desirable, it is selldom realizable in practice. Hence, the separate measurements are often made with varying degrees of instrumental efficiency and complexity. The HNO* chemiluminescence excitation method for the H-atom quantum yield is not an absolute method but a relative method, since it permits only a relative quantum yield measurement. However, when the calibration of the instruments has been made against the absolute method at a given excitation wavelength and other pertinent experimental conditions, it can provide an absolute value of the quantum yieldgJ1as shown by the previous work done in our laboratory. In the previous work,ll the tunable monochromatic light (fvvhm = 0.5 nm) for photoexcitation was provided by the conventional combination of a Xe compact arc lamp and a 1-m grating monochromator, and the chemiluminescence signal from the photomultiplier was recorded with a photon counting setup. In the present experimental setulp, a tunable, flash-lamp-pumped dye laser and a gated boxcar averager were used to increase the overall sensitivity and the spectral resolution in excitation. The arrangements of the apparatus are shown in Figure 1. We used arrangement (a) for the chemiluminescence measurement and arrangement (b) for the absorption measurement. The output pulse length of the frequency-doubled dye laser (Chromatix CMX-4) was slightly less than 1ps, and the laser was operated at the repetition rate of 30 Hz. The second harmonic frequency output was continuously scanned at a spectral bandwidth of 6 c d , while its fundamental frequency output was eliminated by two Corning CS 7-54 filters. Three different dyes (rhodamine 6G, rhodamine B, and Kiton Red s)were used to cover the frequency doubled wavelength range of 290-330 nm. The absorption spectrum of HgCO was taken with arrangement (b), using the identical spectral bandwidth as in the chemiluminescence excitation spectrum of HzCO taken with arrangement (a), since this requirement is critical for a precise quantum yield measurement when sharp absorption features are present.16 Two RCA 11’28 0 1979 American Chemical Society
570
The Journal of Physical Chemistry, Vol. 83,
1-1
No. 5, 1979
h M 1
Dye Loser
Dl PMT 1
I I
Trigger
1
-
Gated B
Boxcar
-
A / B Outpui L o g 1 4 / 6 1 Output
K. Y. Tang, P. W. Fairchild, and E. K. C. Lee
signal averaging made possible in the present experiments. The useful output of the laser was in the range of 0.03-0.3 mJ/pulse, and we were concerned about the photolytic degradation of the sample during long scan periods lasting up to several hours. To check the degradation, the output signals of the repeated scans were compared against each other, but no significant difference was observed. All of the experiments were done at 23 “C. Formaldehyde was prepared by heating paraformaldehyde (Aldrich Chemicals) using the method of Spence and Wild” as described e1~ewhere.l~ The monomer was collected and stored under vacuum at 77 K until its use. Nitric oxide (Matheson, “99%” stated purity) was used after trap-to-trap purification.
Dye Loser
i i i - n-r l v r.I
/
BS3
I
Cell
8 5 4 PD6
PD2
Figure 1. A schematic diagram for the chemiluminescence measurement (a) and for the absorption measurement (b): F I (Corning CS 7-54) F2 (760-nm interference filter); MI-3 (Ai mirror); BS 1-4 (quartz plate beam splitter); QD 1-5 (frosted quartz diffuser plate); PD (RCA 935); PMT 1 (EM1 9659); PMT 2-4 (RCA 1P28).
photomultipliers (PMT) were used to record the intensities of the laser beam incident to and transmitted through the sample. Quartz diffuser plates were placed in front of both photomultipliers in order to illuminate the photocathodes evenly. A cylindrical Suprasil cell having an optical path length of 45.0 cm and a diameter of 3.75 cm was filled with 3.0 torr of H2C0 for all absorption measurements. The output of the phototube (RCA 935 A) monitoring the laser pulse triggered the boxcar averager (Princeton Applied Research Model 162) equipped with two gated integrator modules (PAR Model 164). The output of the “signal” PMT was fed to the A integrator and the output of the “reference” P M T was fed to the B integrator. The log (A/B) output from the boxcar was recorded as a function of the laser frequency on a strip chart recorder, since it is linear in absorbance. The results obtained with different laser dyes were consistent where the output wavelength overlaps. The chemiluminescence excitation spectrum was taken with a typical sample mixture of 1.0 torr of H2C0 and 10 torr of NO contained in a cross-shaped, fused silica cell made with 3.75-c-m diameter stock. The chemiluminescence of HNO*(AIA”) at -760 nm was monitored at a right angle to the laser beam by a red-sensitive, dry-ice cooled photomultiplier tube (EM1 9659 B, extended S-20 response, quoted quantum efficiency of 5% at 760 nm). A 715-nm sharp cutoff filter and a 760-nm interference filter (15 nm bandwidth at 10% transmission with the peak transmission of 75704) were used to isolate the (000-000) emission of HNO*(AIA”). A 2-ps delay (with a 1.5-ps aperture) was used between the trigger and the signal pulses to separate the slow decaying HNO* chemiluminescence from the scattered laser pulse as well a_s the fast decaying fluorescence emission from H2CO*(A1A2) which extends significantly into the red region with decay times in the 10-100-ns range.17 The ratio of the above sample (A) to the reference laser signal (B) monitored by an RCA P M T was processed by the boxcar averager and the (A/B) output was recorded as a function of the excitation frequency. An identical spectral resolution (6 cm-l) was used in both absorption and excitation experiments. With the instrumental parameters used here, no fluorescence signal from H2CO(A1A2)interferred with the recording of the HNO* chemiluminescence signal. The signal-to-noise ratio and the reproducibility were greatly improved over the previous experiments,” because of the better spectral and temporal discriminations and the better
Results and Discussion A. Chemiluminescence Kinetics. The recent experimental results based upon the final product analysis of H2 and CO indicate that the sum of the quantum yields for processes 1and 2 is close to unity below 335 nm,314and the fluorescence quantum yield is less than 1% below the excitation wavelength of 335 nm.17 Since the onset of process 1 is around 330 nm,3-6J1,20 it is possible to observe quantitatively the chemiluminescence below 330 nm. However, a more practical limit was set near 325 nm because N0(Z2n1,, 2) can el_ectronicallyquench and also vibrationally relax” H2CO(A1A2)with a moderate efficiency at 10 torr pressure and therefore the integrity of the single vibronic level (SVL) excitation cannot be maintained at the excitation wavelength longer than 325 nm. The kinetics of the photoexcited chemiluminescence system can be described adequately by the reaction scheme shown in eq 3-11 based on earlier ~ t u d i e s . ~ J ~Previous -~~$~ H2CO + hu [HzCO*] H + HCO (3)
+
-
+
H
+ NO
+ NO
H
M
HNO*((t?lA”)
(44
HNO(PA’)
(4b)
+
HNO*
+M
-HNO+M HNO*(A’A”) HNO*
HNO(2’A’)
-+
+Q
(5b)
+ hu
+Q
+ HCO HCO + NO CO + HNO HNO + 2 N 0 N2 + H N 0 3 2HNO H20 + N2O H
+ HzCO
HNO
--*
+
Hz
-
+
+
(54 (6) (7)
(8) (9) (10) (11)
studies _have shown that the electronically excited HNO*(A’A”) is formed by a three-body process (5a), but we haved added a two-body mechanism (4a) for the sake of completeness. Although the lifetime of HNO* has not been measured directly, it has been estimated to be in the s in the chemiluminescent since order of (010)-(000) and (001)-(000) emission bands near 700 nm have been 0b~erved.l~ Since the formation of HNO* by the recombination of HCO and NO is quite endothermic, it is sufficient to consider the lower energy product channel (reaction 9) only. We have left out the H HCO reaction in the above reaction scheme, since this will be even slower than the H + HzCO reaction (reaction 8) under our experimental condition. We estimate that each laser pulse of 0.3-mJ energy generates initially HCO with the maximum concentration of 1 x 10l2molecules/cm3 for 1torr of HzCO
+
-
The Journal of Physical Chemistry, Vol. 83, No.
Laser-Induced Photodecomposition of H2C0
sample assuming Q1 = 0.5 a t 305 nm. The reaction rate of 1 3 X lo2 s-I is calculated for the H HCO reaction using a collision-rate controlled rate constant24of 1 3 X lo-") cm3 molecule-' s-l. The rate of H + H2C0 reaction a t 1.0 torr of H2C0 is calculated as 1.6 X lo3 s-l using a cm3 molecule-' s-l. rate constant25 of k8 N 5 x 10-l~ Therefore, under the experimental conditions employed here the H HCO reaction is certainly less important than the H H2C0 reaction (reaction 8) which in turn is significantly less important than the H NO + M reaction (reaction 5) to be shown below. The recent m e a ~ u r e m e n t of l ~ the ~ three-body recombination rate constant (k5b) of the H NO + M reaction for M = for M = NO and 4.63 X gives 5.26 X Hz as compared to an earlier value of 6.3 X cm6 molecuk2 s-l for M = Hz.26 Since it is known that in the torr pressure range, k4b
