Quantum yield for H atom formation in the methane dissociation after photoexcitation at the Lyman-α (121.6 nm) wavelength

rL~'7, r'T ~

21 February 1997

CHEMICAL PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 266 (1997) 259-266

Quantum yield for H atom formation in the methane dissociation after photoexcitation at the Lyman-oL ( 121.6 nm) wavelength R.A. Brownsword, M. Hillenkamp, T. Laurent, R.K. Vatsa l H.-R. Volpp *, J. Wolfrum Physikalisch-Chemisches lnstitut der Universiti~t Heidelberg, Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany

Received 16 October 1996; in fmal form 10 December 1996

Abstract The gas-phase photodissociation dynamics of methane after excitation at the Lyman-a wavelength (121.6 nm) was investigated under collision-free conditions at room temperature. Narrow-band tunable Lyman-a laser radiation was generated by the resonant third-order sum-difference frequency conversion of pulsed-dye laser radiation and used both to photodissociate the parent molecules and to detect the photolytically produced hydrogen atoms via ( 2 p 2 p ~ ls2S) laser-induced fluorescence. H atom Doppler profiles were recorded and the absolute quantum yield for H atom formation, ~ . = ( 0 . 4 7 _ 0.11), was determined by means of a photolytic calibration method where the Lyman-a photolysis of H 2 0 was used as a reference source of well-defined H atom concentrations, This value will be used in combination with previous results from H 2 and CH yield measurements to estimate the relative importance of the different product pathways in the CH 4 photodissociation.

I. Introduction

Methane is the most abundant hydrocarbon in the Earth's troposphere [1]. It plays an important role as a greenhouse gas [2,3] and it modulates via the reaction OH + CH 4 the tropospheric concentration of the OH free radical [4,5]. In the upper atmosphere, photodissociation by solar radiation represents a significant loss mechanism for C H 4 [6]. Due to the absence of lone-pair electrons, the optical absorption spectrum begins only far in the vacuum-ultraviolet

* Corresponding author. On sabbatical leave from Chemistry Division, Bhabha Atomic Research Centre, Bombay 400 085, India.

(VUV) region ( < 145 nm), and exhibits continuous absorptions with a broad maximum at about 94 nm [7]. The ionization potential of methane has been measured to be (12.99 + 0.01) eV corresponding to a wavelength of 94.6 nm [8] and it has been suggested that below 77 nm all the absorption is due to photoionization, while at wavelengths above 94.6 nm all of the absorption is due to dissociative processes [9]. As early as 1935, Mulliken attributed, in a molecular orbital description, the 'long wavelength absorption' of methane to a (:~ tA I --->IT2) transition and suggested that it can be described as a (lt 2 ~ 3s) symmetry-allowed Rydberg transition [10]. Later on, however, the bands in the 143 nm wavelength region downwards were assigned to a (lt 2 ~ 3airy*) valence shell transition [11]. A detailed discussion of

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R.A. BrowrL~'wordet al. / Chemical Physics Letters 266 (1997) 259-266

experimental and theoretical results supporting the earlier Rydberg assignment can be found in the book by Robin [7]. Extensive calculations described in Ref. [12] placed the (lt 2 ~ 3s) Rydberg excitation at 119.2 nm and (lt 2 ~ 3a~cr * ) valence shell type excitation at 48.4 nm. In a number of theoretical studies it was confirmed that in the Franck-Condon region the low-lying excited states of alkanes can be well described as Rydberg states [13]. Photochemical studies by Mahan and Mandal suggested that CH 2 and H 2 a r e the major dissociation products after photoexcitation at 123.6 nm [14]. For the same excitation wavelength, an absolute quantum yield of ~cH = 0.06 for CH formation was determined by Rebbert and Ausloos [15], and Laufer and McNesby [16] reported a lower limit for the total quantum yield of 1.01 ~ @tot. The fact that in the latter experiments the total quantum yield must exceed unity was attributed to possible secondary fragmentation processes of the primary products such as C H 2 ~ H + CH, CH 3 ~ H + CH 2. The results of Refs. [14-16] were obtained via stable end-product analysis under collisional photolysis conditions, thus no information about the dynamics of the primary photolysis process could be derived. In the following, spin-allowed dissociation pathways which are energetically possible after excitation at the Lyman-cx wavelength (hCOL~,=984.0 kJ/mol) are listed (atomic and molecular hydrogen is in the l s2S and X ~ g+ electronic ground states, respectively):

cn4(x 'A, ) + h OJL,~ -, H + cH3(:

:x:), ek o

545 2kJ/mol

(la) H 2 + CH2(5.~A,), Ekin ~< 479.1 k J / m o l (lb) H + H + CH2(~.IAj), Eki. ~< 43.1 k J / m o l (lc) n + H + C H 2 ( X 3B,), Eki .-%<80.7kJ/mol

(ld) H + H 2 + CH(X 2I]), Eki~ ~< 94.9 k J / m o l . (le)

The amount of energy available to the products for the different channels was calculated using the enthalpies of formation, A H~°(298 K), compiled in Ref. [ 17]. Emission from the CHzCb IB I ~ l A l) transition was observed after dissociative excitation of CH 4 at 108.6, 117.6, 121.6 [18], 104.8 and 123.6 nm [19]. Lee and Chiang [20] measured the photoabsorption and fluorescence cross section of CH4 in the range 106-142 nm using synchrotron radiation and concluded that CH 2 is produced mainly in the 5~A~ state. Using the photofragment ion imaging technique [21], Zare, Chandler and co-workers investigated molecular hydrogen formation after two-photon excitation at 210 nm of CH 4 [22]. In these experiments, H 2 was state-selectively detected in several rovibrational levels and evidence for the existence of two distinct H 2 formation channels was found, both leading to an isotropic fragment distribution. It was observed that H 2 elimination forms highly rotationally excited H 2 , with significant population even in the H2(v = 1, J = 13) state. Furthermore, it was found that the H2(v = 1) rotational distribution could be well described by a Boltzmann-like distribution with a rotational temperature of 4700 K. Recent studies by Mordaunt et al. [23] using H atom photofragment time-of-flight (TOF) spectroscopy and by Zare, Chandler and co-workers [24] using the H atom photo-fragment ion imaging technique revealed that following Lyman-e~ excitation simple C - H bond fission via reaction (la) is an important primary dissociation process, as predicted by the calculations of Karplus and Bersohn [25]. In Ref. [24], two distinct H atom formation pathways were observed. The one leading to 'fast' H atoms was attributed to dissociation channel (la) while the product channels ( l c - e ) were suggested to be responsible for the formation of a 'slow' H atom. In Ref. [23], based on a detailed analysis of the measured H atom TOF spectrum, it was suggested that the observed slow H atom component can originate either from direct simultaneous three-body fragmentation or from subsequent unimolecular decay of internally excited CH3(X 2X'2) formed via channel (la). RRKM models for the unimolecular decay of CH3(X 2P¢'2) were employed in order to determine for different scenarios the relative contributions of

R.A. Brownsword et al./ Chemical Physics Letters 266 (1997) 259-266

product channel (la) versus that of product channels

(lc and d) [23]. In order to put such relative quantities described above on an absolute basis, as well as to allow a comparison with the results of CH [15,26] and H 2 [16] quantum yield measurements, the knowledge of accurate values for the absolute H atom quantum yield q~u in the Lyman-ot photolysis of C H 4 is essential. However, there is considerable uncertainty in the literature regarding this quantity. A lower limit of 0.42 for q~n was determined in Ref. [16] after photoexcitation at 123.6 rim. For the Lyman-a photolysis a q~n value of about 0.4 was reported [27], whereas for the same photolysis wavelength in Ref. [28] a value of q~H = 1.16 was estimated. So far, only one measurement at the Lyman-a wavelength was carried out under collision-free conditions in a molecular beam apparatus using the H atom TOF method which gave a value of I~) H = (1 " 0 +°'6~ [23]. - 0,4-' In the work to be presented in this Letter, we

focused on the collision-free measurement of q5 H using an alternative method. The results to be presented were obtained in a flow system using the laser photolysis/vacuum-ultraviolet laser-induced fluorescence (LP/VUV-LIF) 'pump-and-probe' technique. In addition, we report the results from direct measurements of the optical absorption cross section of

261

methane at the H atom Lyman-a wavelength, which is needed for an accurate quantum yield determination.

2. Experimental The VUV photodissociation studies were carried out in a flow system (Fig. 1) similar to the one used in previous photodissociation [29] and bimolecular reaction dynamics studies [30]. A brief summary of the experimental method will be given in the following. C H 4 ( > / 9 9 . 8 % , Messer Griesheim) was pumped through the reactor at room temperature without further purification. The CH a flow was controlled by a Tylan flowmeter. For the calibration measurement room temperature H 2 0 (deionized and doubly distilled) was passed through the reactor. The H 2 0 flow was regulated by a glass valve. Typical pressures during the photodissociation experiments were 20-60 reTort, as measured by an MKS Baratron. Narrow-band VUV laser light - tunable around the H atom Lyman-c~ transition at 121.567 nm - was generated by resonant third-order sum-difference frequency conversion of pulsed-dye laser radiation in a phase-matched K r - A r mixture [31] and used to photodissociate the C H 4 molecule as well as to

sum-difference frequency conversion in Krypton

4p 5 5p (1/2, 0) J=0

II Xvuv ~8~ 308~

XeCI Excimer Laser

BBOII

ground state (4p 6 1S0)

DyeLaserA Filter DyeLaserB B Laser

II ~ ~ r / / / / / / / , / ¥ / / / / _ / / ~

(KdAr)-Mixture

Baratran~

H20

CH4 Fig, 1, Experimental apparatus used for the H atom quantum yield measurements. The four-wave mixing scheme for the generation of tunable VUV laser radiation in Kr is shown as an inset (details are given in the text).

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R.A. Brownsword et al. / Chemical Physics Letters 266 (1997) 259-266

detect the nascent photolytically produced H atoms via (2p2p *-- l s2S) laser-induced fluorescence (LIF) within the same laser pulse. The duration of the laser pulse was about 15-20 ns, which ensures the collision-free detection of the photolytically produced H atoms under the low-pressure conditions of the experiment. In the Kr mixing scheme (shown as an inset in Fig. 1) via which the VUV radiation (tovuv = 2 t o R -- tOT) was generated, the laser radiation of tOR (/~R = 212.55 nm) is resonant with the Kr 4p-5p (1/2, 0) two-photon transition and was held fixed during the experiments, while tOT was tuned from 844 to 846 nm to generate VUV laser radiation covering the H atom Lyman-et transition. The laser radiation was obtained from two dye lasers (Lambda Physik FL 2002), simultaneously pumped by a XeC! excimer laser (Lambda Physik EMG 201 MSC). In the first dye laser (denoted as A in Fig. 1), Coumarin 120 dye was used to generate the 425.10 nm radiation which was subsequently frequency-doubled in a BBO II crystal to get A R = 212.55 nm. AT = 844846 nm was obtained by operating the second dye laser (denoted as B in Fig. l) with Styryl 9 dye. The generated Lyman-ct light was separated from the fundamental laser light by a lens monochromator (denoted as LM in Fig. l) followed by a light baffle system. A bandwidth of A PEa = 0.4 cm-i was determined for the Lyman-(x laser radiation in separate experiments by measuring H atom profiles under thermalized conditions (Tt. . . . ~ 3 0 0 K ) . The H atom LIF signal was measured through a bandpass filter (ARC, Model 122-VN-ID, )tcenter = 122 nm, FWHM = 20 nm) by a solar blind photomultiplier (Hamamatsu Model RI290R) positioned at right angles to the VUV laser beam (PM 1 in Fig. 1). During the experiments the change in the VUV laser beam intensity was monitored with an additional solar blind photomultiplier of the same kind (PM 2 in Fig. 1). In order to obtain a satisfactory S / N ratio, each point of the recorded H atom Doppler profiles was averaged over 30 laser shots. The measurements were carried out at a laser repetition rate of 6 Hz. The H atom LIF signal and VUV beam intensities were recorded with a two-channel boxcar integrator system (SRS 250) and transferred to a microcomputer where the H atom LIF signal was normalized point-by-point to the square of the VUV laser intensity.

100

"~.

80

c

60

a) CH 4

n = (2.0 ± 0.2)

b) H 2 0

n = (2.0 + 0.2)

e-

a._ _.1

40

E "r

20

80

6o t-

it_

40

_J

E

o

"1-

20

L.I/ 0

20

40

60

80

1O0

Lyman-cc Intensity [a.u.]

Fig. 2. Dependence of the observed H atom LIF signal on the Lyman-e( laser intensity: (a) for CH 4 and (b) for H 2 0 . Solid lines are the results of a fit to the experimental data to determine the power-dependence n of the H atom L1F signal. The obtained values for n are given in the figure.

Because sequential multiphoton absorption (n > 2) might distort the results, the (n = I + l)-photon nature (one-photon dissociation of the parent molecule followed by one-photon H atom LIF detection) of the process was carefully checked in separate experiments where the VUV laser intensity was varied. In Fig. 2, plots of the measured H atom LIF signal versus the Lyman-a VUV laser intensity are shown for C H 4 and H 2 0 photolyses, respectively. The solid lines represent the results of a fit assuming a (IL~)" dependence of the H atom LIF signal on the Lyman-et intensity, IL~. The obtained numerical values for n are shown in Fig. 2. From the observed quadratic dependence we conclude that the subsequent photodissociation of primary photolysis products is negligible and need not be considered in the analysis of the present results.

R.A. Browra'word et al. / Chemical Physics Letters 266 (1997) 259-266

263

1.6

Secondary photodissociation of CH 2 has been suggested to be partially responsible for the appearance of much larger concentrations of CH in the high-intensity VUV-flash photolysis than in low-intensity photolysis studies at 123.6 nm [26].

a) OH 4 Lyman-e~ absorption

.

1.4

/

1.2 1 o

0.8 0.6

3. Results and discussion

0.4 0.2

3.1. H atom quantum yield in the Lyman-a photolysis of CH 4 Absolute quantum yields ~H for photolytic H atom formation were obtained by calibrating the H atom signal SH(CH 4) measured in the CH 4 photodissociation against the H atom signal SH(H20) from well-defined H atom number densities generated by photolyzing H20. VUV photodissociation of H 2 0 has been studied in great detail [32,33] and an H atom quantum yield of ~bH(H20) = 1.02 was measured after H 2 0 excitation at the Lyman-ot wavelength [28]. In the present study, the absolute H atom quantum yield for methane was determined using the following equation:

0

/{S H(H 2O) O'c." Pc..}.

I

I

I

i

i

I

i

1

2

3

4

5

6

7

n x ~' [1016 c m -2] 2.5 b) H20 Lyman-~x absorption 2

1.5

v

1

0.5

0

~ . = {S,(CH4) ~ . ( H 2 0 ) o-.2oPH2o}

•

0

f

I

I

I

I

I

I

2

4

6

8

10

12

14

nx E [1016 cm-2] (2)

where o-H2o and ~rCH4 are the optical absorption cross sections of H 2 0 and CH 4 at the Lyman-c~ wavelength. Optical absorption cross sections for H20 and methane at the Lyman-a wavelength were measured in the course of the present experiments (Fig. 3 shows the corresponding Lambert-Beer plots for room temperature samples). The following values were obtained: O's2o = (1.6 + 0.1) × 10-17 cm 2, O'CH4 = (2.0 + 0 . 1 ) × 10 -17 cm 2, and were found to be in good agreement with earlier measurements of Ref. [34] for water and Refl [20] for CH 4 at room temperature; S H are the integrated areas under the measured H atom Doppler profiles and PH20 and PCH, are the pressures of H 2 0 and CH4, respectively. The pressures of H 2 0 used in the calibration runs were similar to those used in the CH 4 measurements, so that absorption of the VUV laser beam within the reaction cell was comparable in each case. In Fig. 4, typical H atom Doppler profiles from the H20 and CH 4 Lyman-e~ photodissociation are

Fig. 3. Lambert-Beer plots of the absorption of C H 4 and H20 at the Lyman-et wavelength. The absorption path length l was 20 cm (n stands for the numberdensity of absorbing molecules).The optical absorption cross sections determined are indicated in the figure.

shown. In independent calibration runs, integrated areas under the fluorescence curves were determined for the H 2 0 and CH 4 photodissociation under identical experimental conditions which, using Eq. (2), gave the following average value for the H atom quantum yield: ~H = (0.47 + 0.11). The experimental error was determined from the I tr statistical uncertainty of the experimental data (10%), the uncertainties of the measured optical absorption cross sections (given above) and the uncertainty of the measured H atom quantum yield of water (estimated to be 20%) using simple error propagation. The present result is in agreement with the lower limit of 0.42 ~< ~ . reported by Laufer and McNesby [16] and the value of ~ . = 0.4 measured by Slanger

264

R.4. Brownsword et a l . / Chemical Physics Letters 266 (1997) 259-266

-:"

5

-~

4

3.2. Estimate of the relative importance of the CH+ dissociation product pathways

a) CH+

t-

"~

3

It.

•~ E

2

0

-io

I

'-7,

5

I

I

I

I

b) H20

4 3 ii --

_1

E

2

1-

2

1 0

-15

i

i

i

i

i

-10

-5

0

5

10

15

Doppler shift [cm -1] Fig. 4. Comparison of the total H atom signal (defined as the integrated area of the line profile) produced in the Lyman-~t photolysis of 57 mTorr CH 4 with the signal observed for 60 mTorr H20. Details of the calibration method are explained in the text. The centres of the LIF spectra correspond to the H atom Lyman-a transition ( 121.567 nm).

[27] but considerably smaller than the values of 1.16 and q0H =(1.01_01 +0 6+) reported in Refs. [28,23], respectively. However, the lower limit of the H atom quantum yield obtained in the latter experiments comes close to the upper limit of the present result. The results of Ref. [23] were obtained by a calibration method using Lyman-a photolysis of HCN as an H atom reference source. In the evaluation of their measurements Mordaunt et al. used values of (1.9_+02)× 10 - 1 7 and (3.1 _+0.6)× 10 - 1 7 c m 2 for the Lyman-ct optical absorption cross sections of methane and HCN, respectively, which were estimated from the room temperature absorption spectra depicted in Refs. [20,35]. However, the actual H atom quantum yield calibration experiments of Ref. [23] were carried out in a 'cold' molecular beam where the unknown temperature dependence of the optical absorption cross sections of methane and HCN leads to the large uncertainty of the measured quantum yield.

From the H atom speed distribution determined in Ref. [24] the relative branching ratio of H atoms formed with o H >~ 13 700 m / s (Eki . 1> 94.9 k J / m o l ) can be estimated to be about 80%. As has been suggested in Ref. [23], 'stable' CH 3 radicals can be formed only in combination with these H atoms. H atoms with a lower kinetic energy are produced by either a simultaneous or a sequential three-body fragmentation mechanism [23]. Using the value qbH = (0.47 + 0.11) the relative branching ratio for CH 3 radical formation can be converted to an absolute quantum yield of ~CH3 = 0.38. Although the actual three-body dissociation mechanism leading to the H atoms with Ek~n ~ 94.9 k J / m o l cannot be assigned unambiguously, the absolute CH quantum yield of qbcn = 0.08 (determined in Ref. [28] from data measured at 123.6 and 104.8 nm [15]) can be used to determine the sum of the absolute H atom yields of channels (lc) and (ld) to be ~H(lc/d)= ~rt -- ~CH~ - - ~CH = 0.01. In combination with the absolute H 2 quantum yield qbH: = 0.59 [16], measured after CH 4 photoexcitation at 123.6 n m - assuming that the H 2 quantum yield does not change much between 123.6 and 121.6 nm - the relative importance of the product pathways ( l a - e ) and the resulting total destruction yield of C H 4 c a n be assessed. In Table l, the relative branching ratios for the C H 4 photodissociation product channels are listed (column A) and compared with an estimate (column B) as currently used in photochemical models of atmospheres [4]. The data of column B is based on a previous H atom quantum yield measurement [28]. Table 1 Relative contributions ~ (in %) of the CH 4 dissociation product channels (I a-e) after photoexcitation in the Lyman-ct wavelength region

Ftla) FOb) ~( Ic,d) ~i~)

A

B

39 52 1

=

8

0 47 45 8

A: This work in combination with the CH and H 2 quantum yields of Refs. [15,16]. B: Values reported in Ref. [28] (based on q~H = 1.16 and the CH and H 2 quantum yields of Refs. [15,16]).

R.A. Brownsword et al. / Chemical Physics Letters 266 (1997) 259-266

The present results (column A) correspond to a total CH 4 destruction yield of 0.97 which can be regarded as a reasonable mass balance taking into consideration both the uncertainty of the H, CH and H 2 yield measurements as well as the fact that the latter two ones have so far not been directly measured at the Lyman-a wavelength.

265

authors gratefully acknowledge financial support of the European Union under Contract No. ISC* CT940096 of the International Scientific Cooperation programme between the University of Heidelberg and the Ben-Gurion University of the Negev (Beer-Sheva, Israel) as well as support by the Deutsche Forschungsgemeinschaft. Thanks are due to P. Farmanara for assistance in the experiments and D.W. Chandler for helpful communications.

4. Conclusion The absolute H atom quantum yield q~H = (0.47 + 0.11) measured in the present study in combination with the available literature values for the CH [15] and H 2 [16] quantum yields suggests that the two-body dissociation channels (1 a and b) dominate the primary methane photochemistry in the Lyman-a wavelength region and that three-body paths (lc and d) play a less important role than previously thought [4,28]. In agreement with the conclusions of Mordaunt et al. [23], the present results indicate a considerable primary yield for methyl radical formation in the C H 4 Lyman-a photolysis. The direct formation pathway (la) for methyl radicals has so far not been considered to be an important step in chemical mechanisms of the atmospheres of Jupiter, Saturn and its moon Titan, for example [4], where it could have an impact on the predicted relative abundances of higher hydrocarbons, such as that of ethane versus acetylene. In order to validate the suggested branching ratios for the product pathways as well as to get dynamics information about the underlying dissociation mechanisms further quantum-state-resolved photodissociation studies at the Lyman-c~ wavelength are needed. Measurements of the kinetic energy and nascent internal state distributions of H e, CH and CH 2 fragments could help to distinguish between different fragmentation mechanisms (simultaneous versus sequential) even though they might lead to chemically identical products.

Acknowledgements RKV wishes to acknowledge the DLR Bonn for a fellowship and its extension under the Indo-Gerrnan bilateral agreement (Project No. CHEM-19). The

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