Photolysis of glyoxal at 193, 248, 308 and 351 nm

2 August 1996

CHEMICAL PHYSICS LETTERS ELSEVIER

ChemicalPhysicsLetters257 (1996)487-491

Photolysis of glyoxal at 193, 248, 308 and 351 nm Lei Zhu a,b, Daniel Kellis a, Chuan-Fan Ding a a Wadsworth Center, New York State Department o f Health, Albany, IVY 12201-0509, USA b Department o f Ewaironmental Health and Toxicology, State Unioersity o f New York at Albany, Albany, NY 12201-0509, USA

Received29 January 1996;in finalform 16 April 1996

Abstract The UV photochemistry of glyoxal in nitrogen has been investigated by employing excimer laser photolysis at 193, 248, 308 and 351 nm in combination with cavity ring-down spectroscopy. The HCO radical was a photofragmentation product with yields of 0.42 4- 0.21, 0.53 4- 0.24, 0.69 + 0.29, and 1.5 4- 0.6 at 193, 248, 308 and 351 nm. The larger than unity HCO yield at 351 nm suggests the photolysis channel (CHO)2 + hv (351 nm)---)2HCO. The decrease in HCO yields at higher photolysis photon energies is attributed to the opening up of additional glyoxal photolysis pathways.

1. Introduction

(CHO)2 + hl,-'~ 2HCO A°H298 68.5 kcal/mol l; kthreshold < 417 rim) (CHO)2 +/Iv--* H + CO + HCO =

Glyoxal, (CHO) 2, is an important ring-cleavage product in the air photo-oxidation of some aromatic hydrocarbons in the presence of NO x [1]. It has also been identified as a product of OH radical initiated oxidation of acetylene in the atmosphere [2]. The most important removal process for glyoxal in the troposphere is photolysis [3]. Nonetheless, few studies of glyoxal photolysis product channels and quantum yields have been carried out either under atmospheric conditions or under conditions that can be readily extrapolated to those of the atmosphere. Investigation of the pathways and quantum yields of glyoxal photolysis is necessary to assess its atmospheric fates. Photolysis of glyoxal can occur through the following pathways:

A°H298 85.4 kcal/mol; kth~eshold< 334 nm) (CHO)2 -b h p " ) H 2 -I- 2CO =

A°H29s --- -- 2.1 kcal/mol) (CHO)2 + h i , ~ H2CO + CO A°H29s = - 1.7 kcal/mol). Glyoxal photolysis quantum yields, especially radical yields, are of substantial atmospheric interest since free radicals such as HCO (formyl radicals) generated from the photolysis process are readily converted in the atmosphere into HO 2 and OH radiI Enthalpychanges were calculatedusing thermochemicaldata in [4].

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cals, which can speed up photochemical transformations. The yield of radical product following the primary dissociation process can be deduced indirectly by analyzing the stable end products that are formed, but time-resolved measurements immediately after photolysis are necessary to obtain direct information on the primary photodissociation process. We report here yields of HCO radical from photolysis of glyoxal, measured by employing excimer laser photolysis coupled with a sensitive new detection technique, cavity ring-down spectroscopy [5,6]. These photolysis studies were conducted at photolysis wavelengths of 193, 248, 308 and 351 nm. The absolute HCO concentration was calibrated relative to those obtained from 308 nm photolysis of formaldehyde (H2CO) and from the Cl + H2CO ~ HCO + HCI reaction. 2. Experimental The excimer laser photolysis/cavity ring-down spectroscopic probe apparatus has been described in detail elsewhere [7]. Freshly prepared glyoxal/N 2 or formaldehyde/Ne mixture was flowed through the cell. An excimer laser was employed to photodissociate glyoxal or formaldehyde. The output from the photolysis laser (0.008-0.039 J / c m 2) was propagated into the reaction cell at a 15° angle with the main cell axis through a side ann, overlapping the probe beam at the center of the cavity. The probe light pulse ( ~ 0.6 mJ/pulse) was delayed from the photolysis light pulse and was introduced into the system along the main optical axis of the stainless steel cell, which was vacuum-sealed with a pair of high-reflectance ( ~ 99.999% at 614 nm, within the spectral range of the HCO radical) cavity mirrors. A fraction of the probe laser output was injected into the resonant cavity through the front mirror. The photon intensity decay inside the cavity was measured by monitoring the weak transmission of light through the rear mirror with a photomultiplier tube. The output signal of the photomultiplier was amplified, digitized, and sent to a computer. The decay curve was fit to a single-exponential decay function, and the total loss per optical pass was calculated. Sample absorption was determined by measuring cavity losses in the presence and absence of resonant

absorption. Typical concentrations of HCO formed from photolysis ranged from 4 × 10 m2 to 7 × 1013 molecules cm-3. To examine the time dependence of HCO concentration in the cell, the delay between the photolysis and the probe laser pulses was varied by using a pulse/delay generator, usually between 4 p.s and 5 ms. Absorption spectra were acquired by scanning the wavelength of the probe laser through the spectral region containing an electronic origin by using a digital drive unit. The photolysis laser pulse energy was measured with a calibrated Joulemeter. Gas pressures were measured at the center of the reaction cell with a Baratron capacitance manometer. All measurements except spectral scans were carried out at a laser repetition rate of 0.2 Hz to ensure replenishment of the gas sample between successive laser shots. Spectrum scans were performed at a laser repetition rate of 1 Hz. All experiments were performed at room temperature, T = 298 K. Glyoxal was produced by heating glyoxal trimeric dihydrate in the presence of P205 [8]. Formaldehyde was generated by pyrolysis of polymer paraformaldehyde at 110°C [9]. Glyoxal and formaldehyde sampies were purified by repeated freeze-pump-thaw cycles and were stored at liquid-nitrogen temperature before use. Glyoxal and formaldehyde were prepared as mixtures with nitrogen and the purity of the samples was examined by using FTIR. Glyoxal trimeric dihydrate ( > 99.2%) and paraformaldehyde ( > 95%) were obtained from Aldrich Chemical Co. Nitrogen ( > 99.999% purity, UHP Grade) was purchased from MG Industries and was used without further purification. 3. Results and discussion Shown in Fig. 1 is the lossmeter absorption spectrum of the product from 351 nm photolysis of glyoxal measured at a photolysis/probe laser delay of 15 I~s in the wavelength region 613-618 nm. The resemblance of the photoproduct lossmeter spectrum to the reported absorption spectrum [10,11] of HCO in the same spectral region indicates that HCO is a photolysis product. The sharp absorption bands around 614 nm correspond to a vibronic transition from the vibrationless level of the ground state 2K (00t0) to the vibrationally and electronically excited 2A" (0900) state. The absorption continuum beneath

L. Zhu et aL / Chemical Physics Letters 257 (1996) 487--491

i:

~

-o

~ 613

614

61S

616

617

•

0

Fig. 1. Lossmeter absorption spectrum of the product from 351 nm photolysis of 10% glyoxal in N 2. Ptotal = 20 Ton';, delay = 20 p.s.

the sharp bands is a A band possibly due to an 2~ (0010) to 2A" (0920) vibronic transition. The rotational temperature of the HCO product was determined by measuring the integrated band strength of several individual rotational lines and fitting the HCO populations (assumed to be proportional to integrated band strengths) to a Boltzmann distribution. A rotational temperature of 282 + 20 K was obtained at 5 tLs after 351 nm photolysis of glyoxal at a total pressure of 1 Torr, as shown in Fig. 2. Since rotational relaxation occurs on a single collision time scale ( ~ 0.1 tLs for 1 Ton" of gas), it was not surprising that rotational equilibrium had been established at the pressure ( > 1 Torr) and detection time (5 ~s) used. HCO rotational temperatures of approximately 300 K were also obtained at photolysis wavelengths of 193 nm, 248 nm, and 308 rim.

2

-T,I

0

I

I

I

I

I

50

100

150

200

250

0000

,5- 20

61|

Wavelength(rim)

0

489

300

J(J+l)

Fig. 2. Boltzmann plot for HCO product following 351 nm photolysis of glyoxal. Poyox.~= 1 Tort; delay = 5 tts; Tm= 282 +20K.

0

•

I

I

I

I

f

400

800

1200

1800

2000

2400

t(~s)

Fig. 3. Typical time profile of the HCO radical. Photolysis at 351 nm of 10% glyoxal in N2; Ptmal = 2 0 To~, Aprobe: 615.75 nm.

The cavity ring-down spectrometer was tuned to HCO absorption resonances at 616.37 nm (Pll), 615.75 nm (P8), 614.37 nm ( P / Q bandheads), and 613.69 nm (R bandhead), and the HCO concentration as a function of time was monitored. The time dependence of the HCO concentration was acquired by varying the delay time between the firing of the photolysis and the probe lasers and measuring the corresponding absorption losses at the center and at the edge of individual absorption lines. The on-line loss was subtracted from the off-line loss and HCO concentration as a function of time was obtained. Exhibited in Fig. 3 is a typical time profile of the HCO radical measured by its absorption at 615.75 rim. As can be seen from Fig. 3, HCO appeared immediately after the 351 nm photolysis pulse and its disappearance rate is second-order in HCO concentration, possibly owing to the decay channel HCO + HCO --, H2CO + CO (Since HCO decay rate was found to be independent of total pressure, diffusion did not play any significant role in the decay of HCO signal). Fig. 4 shows a plot of 1/[HCO] vs t. A kHco+~c o of 6.3 × 10 -II cm 3 molecule -I s - l was extracted from the slope of the regression line (cr615.Ts m = 4 . 9 × 10-'9 cm2/molecule determined in the present study was used to convert HCO absorption losses at 615.75 nm into absolute HCO concentrations). The rate constant for the HCO + HCO reaction thus determined agrees with the recommended value [12] of 5 × 10 -11 cm 3 molecule- l s - i. The dependence of HCO signal intensity on photolysis laser power and total pressure was also examined. Altering the excimer laser power by a factor of

490

L. Zhu et a l . / Chemical Physics Letters 257 (1996) 487-491 2.0

1.5

1.0

O

0.5

0,0

|

I

I

i

200

400

600

800

1000

t(~s) Fig. 4. Second-order fit of HCO decay. Conditions as in Fig. 3.

ten changed the magnitude of H C O signal proportionally, suggesting that photolysis is a single-photon process. The H C O signal intensity was found to be independent of total pressure when the total pressure was varied between 20 Tort and 360 Ton'. The yield of H C O was derived from the ratio of the H C O concentration produced in the photolysis/probe laser overlapping region to the absorbed photon density in the same region. The number of absorbed photons in the overlapping region of the photolysis and the probe lasers 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 intensities were determined in these experiments with a Joulemeter which was calibrated by chemical actinometers. 248 nm and 308 nm photon energies were measured by analyzing CO product formed from gas phase photolysis of acetone at these wavelengths. 351 nm light intensity was monitored using the potassium ferrioxaiate actinometer. We also tried to employ 193 nm photolysis of gaseous hydrogen bromide as actinorneter, but chain reactions involving Br 2 product made HBr actinometry unreliable. The absorption cross section of glyoxai at a given photolysis wavelength was calculated using the Beer-Lambert relation on data obtained by measuring the intensity of the transmitted photolysis beam as a function of glyoxal pressure in the cell. The absorption cross sections of glyoxal thus determined were 4.8 x 10 -19 cm2/molecule, 1.3 x 10 -2° cm2/mole cule, 2.9 X 10 -2° cm2/molecule, and 5.1 x 10 -21 cm2/molecule at 193, 248, 308 and 351 nm, respec-

tively. Absorption cross sections of 7.3× I0 -21 cm2/molecule, 2.8 × 10 -20 cm2/molecule, and 0 cm2/molecule have been reported by Plum et al. at 248, 308 and 351 nm, respectively [3]. Glyoxal absorption cross section at 308 n m obtained in the present study is in good agreement with that reported by Plum et al. On the other hand, our cross sections at 248 n m and 351 n m arc significantly larger than those of Plum et al. W e have checked our glyoxal purity and have repeatedly measured its absorption cross sections at these photolysis wavelengths, and wc have confidence in these results. Once the absorption cross section of glyoxal at a given photolysis wavelength and the incident light intensity was known, the absorbed photon density in the overlapping region of the photolysis and the probe lasers could be calculated for a given initialglyoxai pressure. The H C O concentration produced at a given photolysis wavelength was obtained from lossmeter measurements of the H C O absorption intensities at 616.37 nm (P11), 615.75 ran (P8), 614.37 nm ( P / Q bandheads), and 613.69 nm (R bandhead) at photolysis to probe laser delay of 15 I~s. To convert HCO absorption losses into absolute concentrations, the absorption cross sections of HCO at these probe laser wavelengths needed to be known. The absorption cross sections of HCO were determined in the present study relative to the photolysis reaction H2CO + hv(308 nm) ~ HCO + H, for which the HCO quantum yield [13] is well-known (q~Hco = 0.78), and were (3.9 + 0.7) X 10 -19 cm2/molecule, (4.9 + 1.0) x 10-19 crn2/molecule, (1.2 -1- 0.3) X 10-18 cm2/molecule, and (1.6 -6 0.3) X 10-is cmZ/molecule at 616.37 nm, 615.75 rim, 614.37 nm, and 613.69 nm, respectively. The absorption cross sections of HCO were also calibrated by measuring HCO from the CI + HzCO ~ HCO + HC1 reaction, using 248 nm photolysis of CCI 4 as a CI atom precursor. The HCO absorption cross sections thus determined were 3.9 × 10-19, 5.0 X 10-19, 1.3 X 10 -Is, and 1 . 6 X 1 0 -Is cm2/molecule at 616.37 nm, 615.75 nm, 614.37 nm, and 613.69 nm, in good agreement with those obtained from the 308 nm photolysis of H2CO. The number of HCO radicals produced per photolysis photon absorbed by glyoxal was independent of glyoxal pressure and total pressure, and was 0.42 .6 0.21, 0.53 .6 0.24, 0.69 .6 0.29, and 1.5 .6 0.6 at 193 nm, 248 rim, 308 nm, and 351

L. Zhu et al. / Chemical Physics Letters 257 (1996) 487-491

nm, respectively. The lack of giyoxal pressure dependence of HCO yields (Pgtyoxal varied from 0.5 Torr to 6 Tort for 248 and 351 nm experiments, Pgtyo~a]= 0.25-2.5 Torr at 308 nm, Pslyox,d= 0.1--1 Torr at 193 nm) seems to indicate that HCO radicals are vibrationally deactivated to the ground state (000) at the detection time used, especially at longer photolysis wavelengths. This is consistent with a study by Langford and Moore [8] who reported a relaxation time from higher vibrational level of HCO to (010) of ~ 0.3 p.s and a relaxation time from HCO (010) to (000) of 1.2 p~s following 308 nm photolysis of 4.02 Ton" glyoxal. The yields obtained have been corrected for transmission losses of laser fluence at the front window of the photolysis cell (7-8% correction at 248, 308 and 351 nm; ~ 12% at 193 nm) and the enhancement in the fluence in the reactor due to light that is reflected from the rear window (3-10%). The indicated error limits include these corrections along with uncertainties in fluence due to divergence of the excimer beam (15-20%) and its pulse-to-pulse intensity fluctuations ( ~ 10%). Our HCO yield from 308 nm photolysis is consistent with that of 0.8 + 0.4 reported by Langford and Moore [ 14]. The larger than unity HCO yield at 351 nm suggests the existence of the photolysis channel (CHO) 2 + h v (351 n m ) ~ 2HCO, with more than one HCO radical formed per photolyzed (CHO) 2 molecule. The decrease in HCO yields with increasing excitation photon energies can be attributed to the opening up of additional glyoxal photolysis pathways such as ( C H O ) 2 + h v - - * H - t - C O + H C O at higher photolysis photon energies. The non-negligible photodissociation quantum yield of giyoxal obtained from the present study at 351 nm agrees with that suggested by Plum et al. [3], who measured the photolysis rate ratio of glyoxal relative to NO 2 (kglyoxal/kNo2) in a smog chamber. Their observed photolysis rate ratio (0.008) is significantly smaller than the maximum value (0.29) calculated by using cpAffi 1 for giyoxal at all photolysis wavelengths (A = 290-470 nm), but is much larger than the photolysis rate ratio (0.00038) obtained by using cpA= 1 at k ~: 340 nm and q)x -- 0 at k > 340 nm. The non-negligible photolysis yield of glyoxal at

491

351 nm is also supported by thermochemical calculations. The photolysis channel (CHO)2 + h i, ~ 2HCO is predicted to be energetically possible at A ~ 417 nm by using A Hf° values of - 5 0 . 7 kcal/mol and 8.9 kcal/mol for giyoxal and HCO [4].

Acknowledgements The authors are grateful to Dr. Stephen J. Riley, Professor John F. Hershberger, Dr. Kopin Liu, Professor Robert G. Kcesee, and Dr. Brian Bush for their stimulating discussions and helpful suggestions. We also thank Dr. Liaquat Husain for his help. Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research through grant-in-aid PRF#27927-G4.

References [1] E.C. Tuazon, H. MacLeod, R. Atkinson and W.P.L. Carter, Environ. Sci. Technoi. 20 (1986) 383. [2] V. Schmidt, G.Y. Zhu, K.H. Becker and E.H. Fink, Ber. Bunsanges. Phys. Chem. 89 (1985) 321. [3] C.N. Plum, E. Sanhueza, R. Atkinson, W.P.L. Carter and J.N. Pitts, Jr., Environ. Sci. Technol. 17 (1983) 479. [4] R. Atkinson, D.L. Baulch, R.A. Cox, R.F. I-lampson, Jr., J.A. Kerr and J. Troe, J. Phys. Chem. Ref. Data 21 (1992) 1125. [5] A. O'Keefe and D.A.G. Deacon, Rev. Sci. Instmm. 59

(]988) 2544. [6] A. O'Keefe, J.J. Scherer, A.L. Cooks)', R. Sheeks, J. Heath and R.J. Saykally, Chem. Phys. Letters 172 (1990) 214. [7] L. Zhu and G. Johnston, J. phys. Chem. 99 (1995) 15114. [8] A.O. Langford and C.B. Moore, J. Chem. phys. 80 (1984) 4204. [9] G. Moortgat and P. Wameck, J. Chem. phys. 70 (1979) 3639. [10] G. Herzberg and D.A. Ramsay, Proc. Roy. Soc. (London) A233 (1955) 34. [11] J.W.C. Jolms, S.H. Priddle and D.A. Ramsay, Discuss. Faraday Soc. 35 (1963) 90. [12] D.L. Banich, C.J. Cobos, R.A. Cox, C. Esser, P. Frank, Th. Just, J.A. Kerr, MJ. Pilling, J. Troe, R.W. Walker and J. Warnatz, J. Phys. Chem. Ref. Data 21 (1992) 411. [13] G.K. Moortgat, W. Seiler and P. Warneck, J. Chem. phys. 78 (1983) 1185. [14] A.O. Langford and C.B. Moore, J. Chem. Phys. 80 (1984) 4211.