Picosecond real time study of the bimolecular reaction O([sup 3]P)+C[sub 2]H[sub 4] and the...

JOURNAL OF CHEMICAL PHYSICS VOLUME 109, NUMBER 4 22 JULY 1998

Picosecond real time study of the bimolecular reaction O „

3P…1C2H4and the unimolecular photodissociation of CH 3CHO and H2CO

Osama K. Abou-Zieda) and J. Douglas McDonaldb)

Department of Chemistry, University of Illinois, Urbana–Champaign, Urbana, Illinois 61801

~Received 15 December 1997; accepted 14 April 1998!

The bimolecular reaction of O(3P) with ethylene and the unimolecular photodissociation ofacetaldehyde and formaldehyde have been studied using a picosecond pump/probe technique. Thebimolecular reaction was initiated in a van der Waals dimer precursor, C2H4•NO2, and the evolutionof the vinoxy radical product monitored by laser-induced fluorescence. The NO2 constituent of thecomplex was photodissociated at 266 nm. The triplet oxygen atom then attacks a carbon atom ofC2H4 to form a triplet diradical (CH2CH2O) which subsequently dissociates to vinoxy (CH2CHO)and H. The rise time of vinoxy radical production was measured to be 217 (175225) ps. RRKMtheory was applied and a late high exit barrier was invoked in order to fit the measured rise time.The structure and binding energy of the van der Waals complex have been modeled usingLennard-Jones type potentials and the results were compared with other systems. The unimolecularside of the potential energy surfaces of this reaction has been investigated by photodissociatingacetaldehyde at the same pump energy of 266 nm. The resulting photoproducts, acetyl radical(CH3CO) and formyl radical~HCO!, have been monitored by resonance enhanced multiphotonionization ~REMPI! combined with a time-of-flight mass spectrometer. The similarity in themeasured evolution times of both radicals indicates the same photodissociation pathway of theparent molecule. The photodissociation rate of acetaldehyde is estimated from RRKM theory to bevery fast (331012 s21). The T1←S1 intersystem crossing~ISC! rate is found to be the ratedetermining step to photodissociation and increases with energy. The REMPI mechanism for theproduction of CH3CO1 is proposed to be the same as that of HCO1(211). The HCO product fromthe photodissociation of formaldehyde at 266 nm reveals a fasterT1←S1 ISC rate than inacetaldehyde. ©1998 American Institute of Physics.@S0021-9606~98!00528-5#

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I. INTRODUCTION

Several radicals are found to play important rolescombustion processes1,2 and atmospheric reactions.3 Amongthese radicals, which are the focus of this paper, are for~HCO!, acetyl (CH3CO), and vinoxy (CH2CHO). For spec-troscopic studies, these and other radicals are usuallytained by unimolecular photodissociation of small orgacompounds such as formaldehyde, acetaldehyde, and ethmethyl vinyl ether. Important examples in combustion pcesses and in atmospheric chemistry are the reactions ofgen atoms with a number of unsaturated hydrocarboncies.

Reactions of both singlet and triplet oxygen atomscommon. The reaction of triplet oxygen with ethylene hbeen studied by molecular beam scattering4 and by laserfluorescence,5–8 as well as by infrared chemiluminescence9

Consider the Born–Oppenheimer potential enesurfaces4,7,10,11 shown in Fig. 1~adapted from Refs. 7 an10!. Basically, there are three channels observed for theaction of O(3P) with ethylene,

a!Present address: Department of Chemistry, California Institute of Techogy, Pasadena, California 91125.

b!Author to whom correspondence should be addressed.

1290021-9606/98/109(4)/1293/9/$15.00

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O~3P!1C2H4→CH2CHO1H, DH5217 kcal/mol ~1!

→CH31HCO, DH5228 kcal/mol ~2!

→CH2CO1H2, DH5282 kcal/mol. ~3!

Channels~2! and ~3! which produce formyl radical andketene occur on the ground singlet surface produced(triplet→singlet) intersystem crossing~ISC! at theCH2CH2O diradical and could be quite slower.4 The firstchannel produces a vinoxy radical which can be reachetwo ways: one directly through3TS0, the triplet state ofCH2CH2O, and3TS1; the other by ISC to singlet CH2CH2O,across1TS1 to acetaldehyde, across1TS2 to vinyl alcohol,and then uphill to vinoxy1H. The latter is exceedingly im-probable at energies just above threshold~210 kcal/mol inFig. 1! because several products can be reached directly fS0 acetaldehyde by lower energy processes. The formethe only reaction channel that comes directly from tripCH2CH2O, and it competes only with ISC. So most vinoxwill come from the direct channel. The part of the potentsurface to the right of the diradical in Fig. 1 can be easstudied by unimolecular photodissociation of acetaldehyd

The above O1C2H4 reaction has already been studiedlaser-induced fluorescence~LIF! starting from a van derWaals complex of ethylene with NO2. It is found from LIFthat the vinoxy product from the van der Waals complwith a 355 nm pump is cold,7 as is that found in a crossebeam LIF study at the same energy.5 At 266 nm pump in the

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1294 J. Chem. Phys., Vol. 109, No. 4, 22 July 1998 O. K. Abou-Zied and J. D. McDonald

beam, Loisonet al.7 find vinoxy to still be colder than thestatistical limit. At a translational energy of 8 kcal/mSchmoltneret al.4 found a broad distribution of productranslational energies, with a very large amount of prodnear the high velocity energetic limit. In bulk, Loisonet al.8

indicate hot vinoxy by LIF, but their experiment is sstrongly collisional that it really has no bearing on the sincollision experiment. However, Loisonet al. find no evi-dence of a HCO radical from the complex at a time debetween pump and probe of 20 ns. At longer times theya HCO radical, but indicate that this is under collision coditions. Schmoltneret al. reported the ratio of the relativcross sections for the production of HCO and CH2CHO to be2.560.9 in their crossed molecular beam experiment unthe assumption that there is no contribution from other retion channels.

In this paper, we further investigate the above compsystem under cold isolated conditions in a pump–probeperiment. We report our picosecond observation of the tevolution of the vinoxy radical produced from the van dWaals complex of ethylene with NO2. NO2 is used as aprecursor of atomic oxygen since photodissociation of N2

has been well studied12,13 and it is known that it dissociateat 400.l.244 nm to produce a high yield of O(3P). Thegeometry of the complex is calculated using a Lennard-Jo6–12 atom–atom potential which shows that the O atattacks the ethylene molecule at one end. We also extendinvestigation of this chemical system to the unimolecuphotodissociation of acetaldehyde. This photodissociacan be studied by probing both HCO and CH3CO radicals asthe products. We compare the time evolution of HCO pduced from acetaldehyde to that from the photodissociaof formaldehyde at the same photolysis energy. The RiRamsperger–Kassel–Marcus~RRKM! theory is applied herein order to estimate the reaction barriers to dissociation.

II. EXPERIMENT

The laser system used in this study provides two pisecond pulses which are continuously tunable over a br

FIG. 1. Energy levels of the reaction of O(3P)1C2H4 and of the acetalde-hyde photodissociation. Energies are in correct order but not precplaced. Solid lines representing reaction coordinates represent tripdashed ones represent singlets.~Taken from Refs. 7 and 10.!


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spectrum. The details of the experimental apparatustechniques employed have been described in deelsewhere.14,15Briefly, a cw mode-locked dye laser generatnear picosecond laser pulses~0.2 nJ 563–700 pulses 0.6 plong! by pumping it with a pulse-compressed~6 ps longcompressed part! Nd:YAG laser. Residual 1.064mm pulsesfrom the cw Nd:YAG laser drive a multistage amplifiewhich produces about 10 mJ each at 532 and 355 nmrepetition rate of 12 Hz.

A three-stage amplifier chain generates 50–150mJ of563 nm light which is used for supercontinuum generatifor producing other wavelengths, 363–900 nm. A tunabbandwidth-selectable, nontime dispersive monochromaselects the probe wavelength from the continuum. Thisamplified to about 80mJ in a three-stage dye amplifier chai

In one experiment, Exalite 398 or DPS dyes were usto produce 395–425 nm light, which was used directly. Toutput of this channel was 60–90mJ. In another experimentlight at 640–710 nm amplified in LDS 698 or DCM was thedoubled to produce 5–8mJ pulses at 320–360 nm. In aexperiments, the pump light was provided by 15mJ of qua-drupled YAG laser output at 266 nm. This light was senconstant distance to the interaction region which was tused to dissociate NO2, acetaldehyde, and formaldehydThe probe light was variably delayed prior to entering tinteraction region. A typical cross correlation of the pumand probe is presented in Fig. 2. The zero time for theperiment was established by overlapping the two beamsternally using a pinhole and then carrying out a pump/protype experiment.

The molecular beam apparatus was changed in ordesuit two different kinds of experimental detection, resonanenhanced multiphoton ionization~REMPI! and LIF. ForREMPI, experiments were conducted in a doubly skimmdouble-differentially pumped three-chamber vacuum systThe initial chemical expansion occurs there through a scially designed corrosion-resistant pulsed solenoid vawith a 0.76 mm orifice.16 REMPI products are produced i

lyts,FIG. 2. A characteristic cross correlation of the output from the pump beat lpump5532 nm and the probe beam from one of the continuum amplichains atlprobe5692 nm.


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1295J. Chem. Phys., Vol. 109, No. 4, 22 July 1998 O. K. Abou-Zied and J. D. McDonald

the source of a Wiley–McLaren time-of-flight mass spetrometer~TOFMS! with a flight path of 1 m. The ultimatemass resolution of this spectrometer is 1:110 amu. Thelaser beams enter at the base of the TOFMS, mutuallythogonal to the detection axis and the molecular beamare tightly focused and crossed exactly in the region ofteraction. Ions are detected with a cascade pair of leaglass microchannel plate electron multipliers. Transient dfrom the TOFMS are handled with a 2048 channel transdigitizer ~Biomation model 8100!. The transient recordecollects a complete mass spectrum from the TOFMS weach laser or electron pulse, allowing many mass peaks tmonitored simultaneously. Data acquisition and experimtal control are handled by a personal computer. Electrimpact methods are also employed for monitoring beam cditions.

For LIF experiments, only a single vacuum chamberused. A pulsed, unskimmed General Valve pulsed va~Part 9-181-900! is used as the beam source. Typical opering conditions were obtained with a nozzle diameter ofmm and exciting 5–10 mm downstream from the noz(x/D510– 25) in order to optimize the signal. The pulsvalve is operated at 12 Hz~for both REMPI and LIF!, basedon the capacity of the vacuum pumps. The LIF signalfocused and detected by an RCA 7265 photomultiplier tuThe two laser beams enter the main chamber and exthrough four pairs of homemade black rings which actbaffles placed inside the chamber in order to reduceamount of scattered laser light. The two laser beams arefocused in order to illuminate a large area of the sam~about 2–3 mm in diameter!.

Ultrahigh purity NO2 ~Air Products and Chemicals, Inc!and C2H4 ~99.999%, MG Industries! were used as receiveand premixed before expansion in a separate light-shiecontainer to obtain a mixture of about 1%–2% NO2 and2%–5% C2H4 in 20–30 psig He~ultrahigh purity, Mathe-son!. No reaction was observed between NO2 and C2H4 inthe dark for long time mixing. Cutoff filters were usedorder to avoid scattered laser light and to block any detecof NO2 emission produced by excitation of N2O4.

17 Thisemission was strongly detected when only NO2 was ex-panded without ethylene, as observed by Loisonet al.6 Fluo-rescence from vinoxy was detected in the wavelength ra~360–440! nm which hosts most of its emission transitiofollowing excitation to its 00

0 band origin.18 Our short timedelay between the laser pulses made dissociation of NO2 at355 nm impossible due to interference by the excitation lascattering.

Acetaldehyde~99%, Aldrich Chemical Company, Inc.!was used after distillation and was cooled down to240 °Cin a mixture of dry ice in 1:1o-~97%! andm-~99%! xylenes~Aldrich Chemical Company, Inc.! and then allowed to flowslowly by bubbling about 5 psig of He through it prior texpansion. This procedure greatly minimized the dimer fmation of acetaldehyde to essentially zero as can be seeour mass spectra.

Formaldehyde monomers were obtained by mixabout 3 g of paraformaldehyde~95%, Aldrich ChemicalCompany, Inc.! and 3 g ofanhydrous MgSO4 ~Fisher Scien-


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tific! in a plastic tube connected to the valve and containwadded cotton wool prior to the nozzle in order to filter oparticulates. The tube and the nozzle were heated to a70 °C causing the paraformaldehyde to liberate formalhyde monomers and water. The latter was trapped byMgSO4. Formaldehyde vapor was then expanded intonozzle in 5 psig He.

III. RESULTS

The time evolution of fluorescence from the vinoxy radcal produced from a C2H4•NO2 van der Waals complex isshown in Fig. 3. In this experiment, pump light at 266 nmused to dissociate NO2 into NO and O(3P). O(3P) subse-quently interacts with C2H4 in the restricted geometry complex to yield the vinoxy radical@reaction~1!#. The radical isdetected by tuning the probe pulse to its 00

0 @B(2A9)←X(2A9)# transition at 347.2 nm19 and observing its LIFevolution as a function of the time delay between the puand the probe pulses.

Attempts were made to detect the REMPI of the vinoradical by probing at the frequency of its 00

0 band through112 REMPI, or probing its 111 REMPI in the blue tail ofits absorption spectrum. All these attempts failed andattribute this to the very small signal-to-noise ratio~S/N!.The mass spectra resulting from these attempts did not sany evidence of van der Waals formation or vinoxy. Wshould mention here that the area illuminated by the tcrossed laser beams in the REMPI experiment is msmaller than that in the LIF experiment as discussed in SII. This will greatly lower the possibility of targeting some othe complexes formed in the beam. We carried out an etron impact ionization experiment in order to detect homuch complex is formed. The mass spectrum of this expment is shown in Fig. 4, which shows the amount of tcomplex between ethylene and NO2 to be no more thanabout;1% of the NO2 mass peak. It is difficult to measurfrom such ionization spectra the exact distribution of clustin a beam,20 but it is not difficult to ensure that the smallecluster, i.e., the dimer, is dominant. We did that by looking

FIG. 3. Photoinitiated reaction starting from C2H4•NO2 precursor. The tran-sient signal is the fluorescence of CH2CHO probed at 347.2 nm. The ristime is estimated as 217638 ps according to a single-exponential fit.


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higher cluster masses in our C2H41NO2 beam and havefound that, at the concentrations employed in our expment, the higher cluster ion concentration drops off byorder of magnitude for every added molecule.

The transient shown in Fig. 3 is the sum of several scin order to enhance S/N. This time evolution is fitted tosingle exponential rise time of 217638 ps~statistical uncer-tainty only!, using a nonlinear least-squares fitting routinWe did not fit the whole range starting from the zerotiming because of the large slow rise resulting from the ovlapping of the two laser beams in time within the regi0–250 ps~see Fig. 2!. It is our opinion that such fitting is noa valid procedure since even slight pump laser saturationchange the shape of the signal in this region. Changingstart point from 250 ps changes the fit rise time more th638 ps. A good guess at possible systematic errors is250175 ps.

Acetaldehyde and formaldehyde were both dissociaby the 266 nm pump light. The probe wavelength was tunto the REMPI of HCO at about 400 nm by two-photon ecitation to mostly the 3p 2P state, followed by ionization byabsorption of one more photon (211).21 We measured theREMPI-TOFM spectra shown in Fig. 5. It is clear that boacetaldehyde and formaldehyde dissociate cleanly at 266As shown in Fig. 5, we also detected other fragments sucCO1 ~from formaldehyde! and CH3CO1 ~from acetalde-hyde! at the same probe wavelength. Several very wpeaks were observed in our mass spectra similar to somthese observed by Fisanicket al.22 and Shinet al.,23 whichare assigned to several fragments from the parent acethyde. By shining only the pump light we still see some framents which indicates multiphoton absorption processethe pump wavelength. Multiphoton ionization~MPI! was de-tected by exciting acetaldehyde to its 3s←n Rydberg state inthe wavelength range 363–354 nm.22,23 We could minimizethe signals due to the pump only by lowering the pump lapower, but could not avoid them completely. This probledoes not affect time-dependence scans since the signals

FIG. 4. An electron impact spectrum used to characterize the molecbeam of C2H4 and NO2 in the interaction region. Most unmarked smapeaks are residual background.


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to the pump light are considered as a constant backgrothroughout the time scan. The observed intensity ofCH3CO1 peak at m/z543 is twice that of HCO1 (m/z529) in the mass spectrum of acetaldehyde. This obsetion is in contrast to the previous work22,23 in which the masspeak of HCO1 was found to be the most intense peak.

We measured evolution times for both HCO and CH3COat the same probe wavelength of 400 nm as products fthe photodissociation of acetaldehyde. The time evolutionHCO and CH3CO are shown in Fig. 6. Both scans exhibsingle-exponential rise times of 622634 and 667643 ps, re-spectively~with a systematic error of1100–50 ps!. Chang-ing the probe wavelength to a region away from t

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FIG. 5. Mass-selected ion yield spectra of the photodissociation of formdehyde~upper! and acetaldehyde~lower!. The wavelengths used arelpump

5266 nm andlprobe5400 nm.

FIG. 6. Transient signals for HCO1 and CH3CO1 produced from the pho-todissociation of acetaldehyde atlpump5266 nm and probed atlprobe

5400 nm. The REMPI intensity is recorded as a function of the pumprobe delay. The rise times are estimated as 662634 ps for HCO1 and667643 ps for CH3CO1 according to single-exponential fits.


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(211) resonant transition of the fragments causes a stetime equal to 0 between the two lasers whose rise timcontained in the first 250 ps, which are disregarded infits. This step is due to a (11111) REMPI resonant transition in acetaldehyde.

The formation of the acetyl radical~which is isomeric tovinoxy radical! from acetaldehyde was verified by tuning thprobe to the 00

0 transition of vinoxy radical at 347.2 nm anmeasuring its LIF~similar to ethylene and NO2 experiment!.We did not observe any vinoxy fluorescence at this walength, which indicates that the vinoxy radical is not onethe products in the acetaldehyde photodissociation.

Finally, the evolution of HCO produced from the dissciation of formaldehyde is shown in Fig. 7. A singleexponential rise time of 390664 ps was obtained for HCOagain with a larger possible systematic error1150– 100 ps.

IV. DISCUSSION

A. Reaction of O „

3P… with C 2H4

The interaction between C2H4 and NO2 is initiated fromthe precursor dimer, C2H4•NO2. This van der Waals complex is nonreactive and contains the reactants in a restrigeometry, which determines the initial conditions of theaction upon photodissociation. For example, by orientingtwo molecules in the complex relative to each other,dimer structure determines the initial separation of NO2 fromC2H4 and the resultant recoil direction of the particles upphotodissociation. The equilibrium geometry of the compis not known experimentally, through high resolution spetroscopy, although it is reasonably well known for homogous systems such as C2H4•SO2 and C2H4•O3.

24,25

We have calculated the geometry of C2H4•NO2 by usingLennard-Jones~LJ! 6-12 atom–atom potential functions.26 Inthese calculations, the geometries of both C2H4 and NO2 aregiven by their ground state equilibrium molecular structu

FIG. 7. Transient signal for HCO1 produced from the photodissociation oformaldehyde atlpump5266 nm and probed atlprobe5400 nm. The REMPIintensity is recorded as a function of the pump–probe delay. The rise timestimated as 390664 ps according to a single-exponential fit.


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determined by spectroscopic methods.27 These geometriesare considered to be retained in the complex. The bindenergy is calculated as a function of the position of NO2 withrespect to C2H4. The binding energy is calculated at eageometry by a pairwise summation of attractive and repsive atom–atom potentials. Accordingly, the ground stbinding energy,E(R), is given by26

E~R!5(i

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whereAi j andBi j are empirically determined attractive anrepulsive terms, respectively, for the interactions betweenoms i ~in C2H4) and j ~in NO2) and their values are calculated from the parameters given by Nemethyet al.26 Ri j isthe distance between atomsi and j . Electrostatic effects arelarge for the similar molecule C2H4•SO2. Using only elec-trostatic attractive terms, Andrewset al.24 calculated a varia-tion from minimum to maximum of about 800 cm21 in theirfigure corresponding to our Fig. 8. However, for C2H4•NO2,these effects are only 18% (160 cm21) as great due to thesmaller dipole moment of NO2 and the most stable geometris predicted to be exactly the same as for the pure LJ calations. Hence all simple calculations and experimentssimilar systems predict the same geometryC2H4•NO2.

24,25

The results of these calculations show that the mstable conformation has a plane of symmetry which bisethe O–N–Oangle and the CvC bond of ethylene. This geometry places the N atom closest to the CvC bond, whereaseach oxygen atom is closest to one carbon atom of ethylFigure 8 displays this geometry and shows how the potenenergy curve changes with the distance between the N aand the center of the CvC bond in a Morse-type potentiaThe equilibrium geometry places the NO2 molecule with atilt angle of 38° with respect to thexz plane~the coordinatesystem is shown in Fig. 8!. The ethylene molecule is alstilted with respect to the same plane by 27°. The tilt of bo

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FIG. 8. One-dimensional potential energy surface for the C2H4•NO2 com-plex calculated using the sum of Lennard-Jones atom–atom potentialsfunction of they distance between both adducts. The coordinate systemshown and has its origin at the nitrogen atom. The equilibrium geometrthe complex is also shown in two views.


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molecules affects only the positions of the oxygen atomsNO2 and the hydrogen atoms of C2H4. The resulting overallsymmetry of the complex isCs . The binding energy of themost stable conformation is calculated to be2431 cm21

~perhaps2500 cm21 including the dipole–quadrupole term!and the equilibrium distance between the N atom andcenter of the CvC bond is 3.36 Å. The distance between tO atom and the C atom is calculated to be 2.73 Å.

Ab initio calculations on a similar system (C2H4•O3),show that in order for the complex to reach the transitstate the distance between the oxygen and the carbon ashould be;2.0– 2.3 Å,25 or 1.88 Å.10 We have used theabove potentials with an estimated electrostatic term to strotation of the NO2 molecule with respect to the C2H4 moi-ety. The barrier to the rotation of the oxygen atom directoward ethylene~rotation around thex axis! to form a C2vsymmetry is only about 100 cm21. The barrier to placing oneoxygen atom directly above the CvC bond, with the nitro-gen atom also directly above it~rotation around thez axis!, ismuch larger, greater than 200 cm21.

The calculated equilibrium geometry for the complmay not lead directly to the transition state if the oxygatom leaves NO after dissociation with a zero impact paraeter, however three factors should be considered: first, tare large amplitude zero-point quantum fluctuations ingeometry of the complex. Second, the average exit chaimpact parameter for oxygen leaving NO is about 1 Å at ourpump energy.28 Third, excitation at 266 nm leaves NO2

highly vibrationally excited before dissociation. Based onresults on similar systems,10,25 the oxygen atom has to traveonly about 1 Å toward the carbon atom of ethylene in ordto reach the transition state. These three factors are expeto place the oxygen atom in the exact direction which weventually lead to the reaction for a substantial fractionNO2 dissociations. Although NO may need to travel awfrom the interaction region in order to be completely inactto the diradical intermediate, the left over NO fragmentthe vicinity of the diradical is not expected to affectthrough what is known as the ‘‘three body effect’’ o‘‘trapped atom effect.’’ In comparison with other systemsstudy was conducted on the HI•CO2 system,29,30in which thevery heavy I atom is ‘‘left over’’ in the vicinity of the actuareaction, and sees no overt effects of its presence onreaction. However, dissociation of ozone is apparentlyverely perturbed by complexation even with the normarather inert water molecule,31 so that the leaving oxygen mabe in a different electronic state. We do not expect this prlem to afflict our system, since neither NO nor O has a dferent electronic state accessible at our pump energy, othan low energy spin–orbit states.

After dissociation of NO2, the formation of the vinoxyradical from the reaction of O with C2H4 takes place byelectrophilic addition of the oxygen atom in its3P state toone of the carbon atoms of ethylene. Cvetanovic32,33 andSchmoltneret al.4 have suggested that, in the absence of sbilizing collisions, this first step leads to the formation of ttriplet diradical CH2CH2O (3D in Fig. 1!, followed by frag-mentation to hydrogen atom and vinoxy radical. Busset al.34

and Schmoltneret al.4 confirm, through their molecula


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beam experiment, that the formation of vinoxy is a maproduct in this bimolecular reaction from the measuremeof product angular distributions~obtained by extrapolationfrom lower energies! but not~at their energy! the most com-mon product. In our beam experiment, dissociation of NO2 at266 nm leaves an average of 18 kcal of energy availabletranslation of O relative to NO, in a very broad distribution35

Relative to the ethylene, this becomes 7.5 kcal/mol. Thisslightly higher than what Schmoltneret al. used.4 Schmolt-ner et al. also indicate that the vinoxy angular distributiocould suggest a lifetime of the triplet diradical of about orotational period at about 6 kcal incoming translational eergy. However, their data appear to us to indicate thatlifetime at the averageJ value would be at least 4 times throtational period~3 ps! or 12 ps.36 Our measured rise time ovinoxy ~of 217 ps at an even higher energy! does not agreewith this argument and we believe that, given the difficultiof their experiment, their distribution is within their systematic errors of being actually flat.

The formation of vinoxy radical takes place via the3TS1

transition state5 ~see Fig. 1!. We calculated the rate of dissociation of the diradical to vinoxy and H by RRKM theoryThe vibrational frequencies for the diradical intermediaused in our calculations were taken from theab initio calcu-lated values by Dupuiset al.37 ~reduced 10%!. Using theenergies calculated by Fuenoet al.10 for the diradical inter-mediate (3D), and for the exit channel (3TS1), the calculatedrate is 31.7 ps, which is about sevenfold faster than our msured value. Given the nature of their calculations ofenergy barriers~based on inadequate basis sets or inadeqconfigurations! their calculated values must be considerapproximate65 kcal/mol or even worse. In order to reproduce our measured rise time of vinoxy, the3TS1 transitionstate must be placed as a late high barrier at about 2.5 kmol above the 0 energy in Fig. 1, which is 4 kcal/mol highthan the calculated energy10 or the energy of the tripletdiradical reduced to235.5 kcal/mol ~14.9 kcal/mol lowerthan the calculated energy!, or some combination of thestwo changes. This new barrier may very well explain thekcal energy content in vinoxy measured by Loisonet al.,6

despite the 3 kcal available as translational energy inincoming O(3P) from their dissociation at 355 nm. Thnumber we measure is only a lower limit to the intrinsdissociation lifetime of the diradical, because 1/tobs51/tdiss

11/t ISC, and Schmoltneret al. find that an appreciableamount of HCO product comes from the ISC channel. Mlecular beam measurements such as theirs are not, howvery accurate measurements of relative product yield.

B. Photodissociation of acetaldehyde

We now turn to the right-hand side of the potential eergy surfaces shown in Fig. 1. According to the workseveral groups~Refs. 38–42, and references therein!, acetaldehyde is statistical limit for internal conversion~IC! fromS1 to S0 but small molecule for ISC fromS1 to T1 at itssinglet origin (;336 nm). From 336 to 317 nm it is still thestatistical limit for IC but an intermediate case for ISC


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Above;317 nm it is the statistical limit for both. The latteidentifies the onset of photodissociation which results in oor more of the following products:

CH3CHO→CH31HCO ~5!

→CH3CO1H ~6!

→CH41CO. ~7!

The triplet state density apparently rises dramatically jabove the reaction threshold and the production of HCOCH3CO radicals@channels~5! and ~6!, respectively# willcome exclusively from the triplet pathway.38–41,43However,at high energy (ldiss<280 nm) Ohta and Baba suggest thHCO may come from higher vibrational levels ofS1 . On theother hand, channel~7! is only correlated to theS0 of acetal-dehyde via the IC fromS1 , and the barrier for the productioof molecular CO and CH4 was calculated to be about 8kcal/mol.44 This barrier is at the same energy as the vibtionless level ofS1 acetaldehyde.

The energy available in the CH31HCO or H1CH3COfragments after dissociation at our 266 nm pump is aboukcal/mol. This energy should appear as vibrational and rotional quanta in the photofragments. Atkinson aco-workers45–47 have studied the kinetics following thpopulation of different vibrational and rotational levelsnascent HCO, but their experiment is entirely different froours. ~Their study was conducted in the gas phase usingintracavity laser spectroscopy technique with time resolutin the microsecond scale.! Due to our intrinsically low spec-tral resolution, we have not attempted to measure the prostate distribution.

The transients shown in Fig. 6 for formyl and aceradicals suggest the same mechanism for the productioboth radicals based on their appearance time (t5622 ps forHCO andt5667 ps for CH3CO). We tried to observe similar transients at different pump wavelengths~in the range280–315 nm!, keeping the probe wavelength unchanged,could not observe any time evolution from either radicwithin our time delay limit between the pump and the propulses of;2 ns. This observation can be explained by tresults of Ohta and Baba.37 They measured the lifetime oacetaldehyde fluorescence in bulb at different energiesfound it to decrease from 5.2 ns at excitation energy nearS1 origin to 1.0 ns at excitation down to 280 nm, the,0.6 ns atldiss5275– 270 nm, within their time resolutionIn our beam experiment, we did not observe any evolutfrom either radical in this wavelength range which indicathat acetaldehyde dissociates in a time longer than 2 ns~ourmaximum scan range! in this region of the spectrum undecollisionless conditions. The evolution of the radicals at 2nm pump energy implies that acetaldehydestarts to dissoci-ate in a time shorter than;100 ps ~our pump durationpulse!.

Two rates should be considered which control the ptodissociation process, the rate of theT1←S1 ISC and thephotodissociation rate from the triplet state to radicals. Trate of the triplet photodissociation following ISC was esmated roughly to be;108 and ;109 s21 for 315 and 310nm excitation, respectively.39 If this rate is the rate determin


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ing step, we could have detected the photoproducts atldiss

<315 nm. We have calculated this rate at our pump eneof 266 nm by RRKM theory~for which, unlike the vinoxycase, the energetics are well known! which yields 3.031012 s21. This indicates that the rate of photodissociatiof acetaldehyde does not contribute to our measuredtimes. The radical’s appearance rates are then due mainthe ISC process which increases monotonicly with enerAn exit channel from higherS1 levels cannot be ruled out asuch high energy as suggested by Ohta and Baba39 andshould also be considered. However, the single-exponerise times observed here indicate that the radical productsdue only to one channel.

The rate of the appearance of HCO from the photodsociation of formaldehyde at the sameldiss of 266 nm isfaster than that from acetaldehyde. We have measuredrate to be 390 ps~Fig. 7!. The onset of photodissociation oHCHO to H1HCO has been measured to be around 3nm.48,49 The energy surfaces representing the photodissotion processes of formaldehyde are shown in Fig. 9~adoptedfrom the work of Moore49!. The quantum yield of the HCOradical increases asl decreases. According to Fig. 9, HCcan be produced without a barrier fromS0 formaldehyde. Asthe dissociation energy increases, the rate ofT1←S1 ISCincreases and a barrier to dissociation occurs fromT1 at

FIG. 9. Energy levels for formaldehyde photodissociation. The solid lrepresenting reaction coordinates represents the triplet, dashed onessent singlets.~Adapted from Ref. 49.!


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about 311 nm for HCO production. This situation resembthe production of HCO from the triplet acetaldehyde. Trate of ISC in formaldehyde was measured to be3107 s21 near the barrier.49 This rate, as in the case of acetaldehyde, gets faster as the energy increases over therier. The photodissociation rate must be greater than th31012 s21 of acetaldehyde photodissociation. The observsingle-exponential rise time of HCO also indicates thatdetect only one exit channel at such high energy whichmost likely the triplet photodissociation.

We now return to the observation of acetyl radicalREMPI. Little is known about the spectroscopyCH3CO.50–52The lack of structure in its absorption spectrusuggests that it strongly predissociates in the UV visispectral region into CH31CO.50 All indications are that itselectronic structure is exactly what would be expectedanalogy to HCO.

The measured rate of appearance of CH3CO in Fig. 6 issimilar to that measured for HCO at the same probe eneof 400 nm. This means that a similar mechanism is involvin the production of both radicals from the photodissociatof acetaldehyde and a similar mechanism for the producof both ions by REMPI.

The question now is, which REMPI mechanism tacetyl radical follows to ionization. We propose the REM(211) mechanism based on the following observation. Wperformed a power-dependent experiment by measuringion intensity as a function of the probe power. This expment did not lead to any conclusive result, but in all casesion intensity ratio CH3CO1/HCO1 remains the same~;2!.We know from the work of Tjossemet al.21 that the REMPImechanism for the production of HCO1 is (211). Accord-ingly, the same mechanism can be proposed for CH3CO1.Also, the ionization potential of CH3CO is 6.84 eV.53 Theprobe energy at 400 nm is well above this threshold ifconsider a~211! mechanism.

V. CONCLUSION

The reaction between O(3P) and C2H4 has been studiedin a restricted geometry under cold-isolated conditions usa pump–probe technique with LIF detection. The decomsition lifetime of the intermediate triplet diradical was mesured to be 217 ps. This intermediate (CH2CH2O) is formedimmediately after the oxygen atom attacks one carbon aof the ethylene molecule. This step is followed by fragmetation to hydrogen and vinoxy through a late high barriThe geometry of the precursor complex C2H4•NO2 has beencalculated using the sum of 6–12 atom–atom potentials.calculated geometry fits the model described for similar vder Waals molecules.

The unimolecular side of the potential energy surfacesthe above reaction has been investigated by studyingphotodissociation of acetaldehyde. Fragments from this ptodissociation were detected by REMPI-TOFMS. The rtime evolution of HCO and CH3CO photofragments weremeasured and found to be close within the experimentarors. The results imply a similar mechanism of dissociatiThe ISC rate fromS1 to T1 is found to be the rate determining step. At about 7.5 kcal/mol of translational energy, t


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evolution of HCO and CH3CO could be observed with ristimes of 622 and 667 ps, respectively. A (211) REMPImechanism is suggested for the production of CH3CO1 bycomparing the results of both CH3CO1 and HCO1.

The evolution of HCO from the photodissociationformaldehyde is measured to be shorter than the evolutioHCO from acetaldehyde by about 200 ps indicating a fasT1←S1 ISC rate in the former.

ACKNOWLEDGMENT

This material is based upon work supported by the Ntional Science Foundation under Award No. CHE-95-288

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Picosecond real time study of the bimolecular reaction O([sup 3]P)+C[sub 2]H[sub 4] and the...

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Transcript of Picosecond real time study of the bimolecular reaction O([sup 3]P)+C[sub 2]H[sub 4] and the...