Ultraviolet absorption spectrum and cross-sections of vinyl (C2H3) radical in the 225–238 nm region

Chemical Physics 236 Ž1998. 43–51

Ultraviolet absorption spectrum and cross-sections of vinyl ž C 2 H 3 / radical in the 225–238 nm region Askar Fahr ) , Parviz Hassanzadeh, Dean B. Atkinson

1

Physical and Chemical Properties DiÕision, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA Received 22 May 1998

Abstract The room-temperature gas-phase ultraviolet absorption spectrum and cross-sections of vinyl ŽC 2 H 3 . radicals have been determined in the spectral range 225–238 nm, employing cavity ring-down absorption spectroscopy. Vinyl radicals in these experiments were produced from the 193 nm excimer laser photolysis of methyl vinyl ketone ŽCH 3 COC 2 H 3 . and vinyl bromide ŽC 2 H 3 Br.. The spectra obtained from the two systems were nearly identical. The observed spectrum exhibits a relatively broad and featureless absorption with a cross-section of 5.3 = 10y18 cm2 moleculey1 at 230 nm. A combined uncertainty of ; 25% for cross-section values has been assessed. The electronic transitions in the vinyl radical have been calculated by ab initio quantum chemical methods at the CIS, EOM-CCSD, CASSCF and CASPT2 levels of theories which assign the new observed band to the highly allowed and ‘in plane’ p ) Ž2aY . § p Ž1aY . transition. q 1998 Elsevier Science B.V. All rights reserved.

1. Introduction The vinyl radical ŽH 2 C5CH. is among the critical intermediates in hydrocarbon reaction systems pertinent to both the low temperatures of planetary atmospheres and high temperatures of combustion reactions w1–5x. Due to difficulties involved in generating vinyl radicals cleanly and detecting them sensitively, and the high reactivity of radicals, relatively little is known about the spectroscopy and chemical properties of these simple hydrocarbon radicals. Hunziker et al. w6x, using modulation spectroscopy and multipass optics, have observed a relatively weak visible absorption band attributed to an )

Corresponding author. Present address: Chemistry Department, Portland State University, Portland, OR 97207, USA. 1

electronic transition between the ground ŽX 2AX . and first excited state Ž2AY . of vinyl radical. The origin of this absorption is at 499.5 nm with a progression towards the blue and a maximum absorption coefficient of 1.2 = 10y1 9 cm2 moleculey1 at 423.2 nm. Two intense vacuum ultraviolet absorption features at 164.7 and 168.3 nm, with absorption cross-sections of 6.7 = 10y1 7 and 4.5 = 10y1 7 cm2 moleculey1 , respectively, have been observed by Fahr and Laufer w7x following vacuum-UV flash photolysis of several vinyl radical precursors. The features were assigned to vinyl radical on the basis of precursor, kinetic data and the spacing between the bands. These absorptions were used to obtain the first direct measurement of the C 2 H 3 q C 2 H 3 combination and disproportionation reaction rate constants w8x. In the present work we report the ultraviolet absorption spectrum and cross-sections of vinyl radi-

0301-0104r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 Ž 9 8 . 0 0 2 1 3 - 4

44

A. Fahr et al.r Chemical Physics 236 (1998) 43–51

cal in the range 225–238 nm. Excimer laser photolysis and cavity ring-down absorption detection methods have been used. The experimental observations are complemented by ab initio molecular orbital calculations for identifying the electronic states, energies and transitions contributing to the observed spectrum.

2. Procedures 2.1. Experimental method Excimer laser photolysis and cavity ring-down UV absorption spectroscopic techniques were used for the production and detection of vinyl radicals. Only a brief summary of our methods are presented here. A more detailed description of the experimental procedure has previously been reported w9x. The cavity ring-down technique offers direct absorption measurements with a high detection sensitivity Ž; 10y6 fractional absorption. w9,10x. The method is based on the detection of the enhanced rate of decay of light intensity within a stable optical cavity when an absorber is present. A Spectrasil 2 flow tube sealed at both ends with highly reflective mirrors forms the optical cavity and the laser photolysis reactor. A frequency-doubled dye laser pumped by a XeCl excimer laser ŽLambda-Physik EMG-201-MSC q FL2002 with Coumarin 450 dye q Inrad Autotracker II with BBO-0 crystal. produces the probe beam. The probe beam is injected into the cell along the axis of the cavity through one of the two mirrors and circulates between the mirrors as loss mechanisms cause each excited cavity mode to undergo first-order decay. The presence of absorbing species shortens the photon decay with respect to the empty cavity case. A greatly attenuated laser beam exits through the second cavity mirror. A photomultiplier

2 Certain commercial instruments, materials and computational programs are identified in this paper to adequately specify the experimental or computational procedures. In no case does such identification imply recommendation or endorsement by NIST, nor does it imply that the instruments, materials and computational programs identified are necessarily the best available for the purpose.

tube monitors this event via the light transmitted through the exit mirror upon each reflection. Our method uses a second excimer laser ŽLambda-Physik EMG-203-MSC, ArF-193 nm, nominal 20 ns pulsewidth, 50 mJ pulsey1 maximum energy. to produce the photolysis beam which enters the reactor through the Spectrasil walls normal to the propagation direction of the probe beam. A pulse delay generator, programmed by a data acquisition computer, controls the timing relationship between the photolysis and probe lasers. The output of the photomultiplier is read by a digital oscilloscope which records the time evolution of the ring-down signal induced by each probe laser pulse. Following each probe laser pulse, the digital data are transferred as an integer array from the oscilloscope to the computer. The extraction of the decay constant is accomplished using a LabView virtual instrument which subtracts the pre-ring-down base line, truncates the decay at the first non-positive value, converts the array into double precision floating-point numbers, and fits the ring-down decay to a linear function of the form ln Ž I0rIt . s b t ,

Ž 1.

where b is the intensity loss rate for the laser pulse in the cavity, I0 is the initial light intensity, and It is the light intensity at time t. For each laser pulse a bI is calculated and the average b is calculated from the linear average over N pulses, that is ² b : s Ý bIrN, for the current photolysis-to-probe laser delay time. Extraction of the fractional intensity loss per pass through the cavity is obtained by multiplying b by the time the light takes to traverse the cavity. The decay rate obtained for an evacuated cavity, bempty , is the summation of all loss mechanisms extant in the cavity and is dominated for these cavities by the finite mirror reflectivity. In photolysis experiments the base loss rate against which absorption is measured, b base , also includes any absorption loss contributed by the pre-photolysis gas mixture Ž b base s bempty q bgas .. This is an important consideration in the current studies, where the detection was seriously affected and eventually limited at shorter wavelengths by the increasingly strong absorption of the photolytic precursors, bgas . These effects also contributed a greater than normal uncer-

A. Fahr et al.r Chemical Physics 236 (1998) 43–51

tainty to the cavity ring-down absorbances determined in these studies. The photolytic introduction of the absorbing transient species into the cavity accelerates the loss rate to babs . Average absorbance, ² A:, is obtained from the difference between the measured rates, ² A: s Ž Lrc . Ž ² babs : y ² b base : . ,

Ž 2.

where L is the cavity length and c is the speed of light. The base decay rate is measured before each measurement of a time-delayed Žkinetic. decay rate by moving the probe laser to a time before the photolysis laser has fired, giving any radicals which are produced the entire settling time to exit the reactor. Assuming Beer–Lambert behavior, ² A: s nl abs s ,

Ž 3.

where n is the concentration of the absorber, s is the absorption cross-section at lCD RS , and l abs is the absorption pathlength, which in this case is defined by the width of the photolysis laser. The measured photolysis laser fluence before the cell, when the laser was at the optimum operating energy, was ; 10 mJrcm2 . The cell was operated in continuous flow mode with gases regulated by calibrated mass flow controllers and the total pressure measured with a capacitance manometer. Reagent concentrations were calculated from the total pressure and the calibrated flows. Vinyl radicals in these experiments were produced through the 193 nm excimer laser photolysis of methyl vinyl ketone ŽCH 3 COC 2 H 3 . and vinyl bromide ŽC 2 H 3 Br.. 2.2. Ab initio computational methods The equilibrium geometry of the ground state of C 2 H 3 was calculated at the CCSD level, a couple cluster with all single and double substitutions, using 6-311 q GŽ3df, 2p. basis sets. All electrons were active in the dynamic electron correlation treatments. The 6-311q GŽ3df, 2p. basis sets were adopted from Gaussian 94 programs w11x. Vertical excitation energies were computed by complete active space self-consistent field ŽCASSCF. alone and CASSCF including dynamic electron correlation effect at the second-order Møller–Plesset perturbation theory ŽCASPT2.. In the CASPT2Ž3, 3. treatment, all electrons except the 1s 2 of carbon

45

atoms were included in the electron correlation treatments. Atomic natural orbitals ŽANO. of Pierloot et al. w12x contracted as C: 3s 2p 1d and H: 2s 1p as supplied with the MOLCAS programs were used w13,14x. Vertical excitation energies were also calculated with configuration interaction with single substitution ŽCIS. and equation-of-motion coupled cluster with all single and double substitutions ŽEOMCCSD. using 6-311q GŽ3df, 2p. basis sets. All electrons were active in the electron correlation treatments. The multi configuration calculations ŽCASSCF and CASPT2. were performed using MOLCAS programs running on an IBM RS-6000r590 computer. The CCSD, CIS, and EOM-CCSD calculations were carried out using ACESII programs w14x on a CRAY C-90 computer.

3. Results and discussion 3.1. UV absorption spectrum of C2 H3 Absorption–time traces were determined at various wavelengths in the range of 225–238 nm, following the 193 nm excimer laser photolysis of flowing CH 3 COC 2 H 3rN2 or C 2 H 3 BrrN2 mixtures. The production of vinyl radicals using these precursors and 193 nm photolysis has been confirmed in previous studies w15–18x through kinetic determinations and end product analysis by gas chromatography and mass spectrometric detection ŽGCrMS.. The GCrMS results showed products of vinyl radical reactions, including the self-reaction product 1,3butadiene, as well as products of the other expected reaction pathways. The 193 nm excimer laser photolysis of CH 3 COC 2 H 3 simultaneously yields identical concentrations of C 2 H 3 and CH 3 radicals w15x. An example of the time-dependent absorption signal, observed following photolysis of a CH 3 COC 2 H 3rN2 mixture, is shown in Fig. 1. The absorbance at any wavelength is taken as the maximum in the real-time absorption curve, which always promptly follows the photolysis event. The variation of the maximum absorbance with wavelength is presented in Fig. 2. The spectrum and decay kinetics obtained from the two precursors, methyl vinyl ketone and vinyl bromide, were nearly identical, which is a strong evi-

46

A. Fahr et al.r Chemical Physics 236 (1998) 43–51

Fig. 1. An example of a time-dependent absorption signal observed following 193 nm photolysis of a mixture with CH 3 COC 2 H 3 concentration of 3.9 = 10 15 molecule cmy3 in 6.6 kPa N2 .

dence for production and detection of vinyl radicals in these systems. The photolysis of methyl vinyl ketone resulted in a higher yield of radicals while the steady-state absorption of the precursor at the monitoring wavelengths was much lower. This combination resulted in a better signal to noise ratio for the CRD absorbance measurements and methyl vinyl ketone was subsequently used for most of the spectral characterization and cross-section measurements. The observed spectrum exhibits a broad and featureless absorption band between 225 and 238 nm which becomes more intense towards the blue. Experimen-

tal limitations resulting from an inaccessible wavelength region between the ranges of our highly reflective cavity mirrors, prevented us from extending the spectroscopic determinations to shorter wavelengths. The major reactions following the 193 nm photolysis of methyl vinyl ketone are as follows: hn

CH 3 COC 2 H 3 ™ CH 3 q C 2 H 3 q CO M

C 2 H 3 q C 2 H 3 ™ C 4 H 6 Ž 1,3 y butadiene. M

C2 H3 qC2 H3 ™ C2 H2 qC2 H4 M

CH 3 q CH 3 ™ C 2 H 6 M

C 2 H 3 q CH 3 ™ C 3 H 6 Ž propylene. M

C 2 H 3 q CH 3 ™ C 2 H 2 q CH 4

Fig. 2. The ultraviolet absorption spectrum of vinyl ŽC 2 H 3 . radical determined following 193 nm photolysis of CH 3 COC 2 H 3 rN2 mixtures with CH 3 COC 2 H 3 concentrations in the range Ž2.0–3.=10 15 molecule cmy3 and total pressure of 2–12 kPa.

Ž R1. Ž R2. Ž R3. Ž R4. Ž R5a. Ž R5b.

The rate constants for the above radical reactions have previously been determined and reaction products have been identified using GCrMS product analysis w15,16x. Inspection of the time-dependent absorption traces as shown in Fig. 1, reveal three distinctive components. First, the rapid rise is coincident with the photolysis laser pulse, due to the direct photolytic production of vinyl radical from the photodissocia-

A. Fahr et al.r Chemical Physics 236 (1998) 43–51

tion of the precursor. The other transient photolysis product, methyl radical, has a very sharp and intense absorption at 216.4 nm which has previously been characterized and reported in numerous papers w19x. No other absorption features in the spectral range of the present study Ž l ) 224 nm. have been observed and assigned to CH 3 . This has previously been tested in our laboratory through spectroscopic survey of methyl radical absorption following photolysis of several known precursors such as acetone and azomethane. The second is the decay component of the absorption signal which can be attributed to the vinyl radical loss through reactions ŽR2., ŽR3. and ŽR5.. The third is an absorption component, appearing at longer delay times as non-zero baseline Žabsorption ‘tail’., that can be attributed to the absorptionŽs. of stable productŽs. at the monitoring wavelength. The observation times, for most experiments, were chosen to be long enough such that no time variation of the absorption ‘tail’ was observed. Among all of the above reaction products, only 1,3-butadiene has a significant absorption in the spectral range of these studies w20x. All of the other smaller hydrocarbon products have appreciable absorptions only at wavelengths shorter than ; 200 nm w20,21x. Therefore, the absorption at long observation times Žabsorption ‘tail’. can be primarily attributed to 1,3-butadiene. 3.2. Absorption cross-sections of C2 H3 To determine the absolute absorption cross-sections, the initial concentration of vinyl radical Ž n V . must be evaluated. This has been achieved here indirectly by using the multicomponent time-depen-

47

dent absorption traces. At any particular monitoring wavelength, as discussed above, the maximum absorbance at short delay time, t 1 , is primarily due to the nascent vinyl radical A t1 s A V s n V l abs s V

Ž 4.

while the absorbance at long delay time, t 2 Žabsorption ‘tail’., is mainly due to 1,3-butadiene A t 2 s A bd s n bd l abs s bd .

Ž 5.

Therefore, the ratio of the maximum absorbance to the absorption ‘tail’ derived from the absorption–time traces can be related to the concentrations and absorption cross-sections of vinyl radical and butadiene as follows A t1rA t 2 s A V rA bd s n V s V rn bd s bd .

Ž 6.

The absorption cross-sections for 1,3-butadiene have been previously determined and reported w22x. The concentration of the initially produced vinyl radical Ž n V . can be related to the concentration of the 1,3-butadiene product Ž n bd . through the analysis of the reaction sequence Ž2. to Ž6.. An exact analytical solution relating the initial radical concentrations to the final products is not feasible. However, multicomponent reaction kinetic problems are often modeled employing numerical integration routines. We have used the Acuchem w22x program for modeling of the reactions following the 193 nm photolysis of methyl vinyl ketone. The Acuchem kinetic modeling program requires an input file containing the kinetic mechanism with initialized rate constants and initial species concentrations. An output file is generated which lists the concentrations of all of the species on a pre-selected time grid, and can be displayed as a

Table 1 Reactions, rate constants and initial vinyl radical concentrations for modeling of the methyl vinyl ketone system and the yield of 1,3-butadiene Reaction

k Ž10y10 cm3 moleculey1 sy1 .

n V s wC 2 H 3 x0

n bd s wC 4 H 6 x

ŽR2. C 2 H 3 q C 2 H 3 ™ M C 4 H 6 ŽR3. C 2 H 3 q C 2 H 3 ™ M C 2 H 2 q C 2 H 4 ŽR4. CH 3 q CH 3 ™ M C 2 H 6 ŽR5. C 2 H 3 q CH 3 ™ M products

0.94 0.30 0.45 1.53

1 = 10 13 2 = 10 13 4 = 10 13 10 = 10 13 n V rn bd s 5.06 " 0.01

0.197 = 10 13 0.395 = 10 13 0.791 = 10 13 1.98 = 10 13

Rate constants from Refs. w15,23x. Concentrations in molecule cmy3 .

A. Fahr et al.r Chemical Physics 236 (1998) 43–51

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user directed graph. The rate constants for reactions ŽR2. to ŽR5. have previously been determined in our laboratory and were used as fixed parameters w15,23x. Several values for the initial concentrations of vinyl and methyl radicals, in a range expected under our experimental conditions, were chosen. The parameters used in the modeling and the derived concentrations of the product 1,3-butadiene are listed in Table 1. The results of the modeling, presented in Table 1, illustrate that in this reaction system, irrespective of the initial vinyl radical concentration, the ratio wC 2 H 3 x0rwC 4 H 6 xf s n V rn bd s 5. Therefore, Eq. Ž6. can be reduced to: A V rA bd s 5s V rs bd .

Ž 7.

The value of the ratio A V rA bd can be determined from the real-time absorption traces. Then by using the known absorption cross-section of butadiene at the monitoring wavelength, the cross-section value for vinyl radical at that particular wavelength can be evaluated. Table 2 lists the results of such analysis for the vinyl radical absorption cross-section determination. The derived absorption cross-sections at various wavelengths are plotted in Fig. 3.

Table 2 The vinyl radical absorbance Ž A V ., and absorption cross-sectionsa of 1,3-butadiene Ž s bd . and C 2 H 3 Ž s V . at room temperature, in the wavelength range 225–238 nm Wavelength Žnm.

AV Ž=10y6 .

s bd

sV

225 226 227 228 228.5 230 231 232 233 233 234 234 235 236 237 238

3093 2686 2525 2176 2059 1696 1600 1283 1060 1094 1030 995 990 889 788 686

44.9 33.7 25.1 18.9 16.4 11.0 8.76 6.93 5.67 5.67 4.59 4.59 3.76 3.12 2.71 2.14

9.6 8.4 7.9 6.8 6.4 5.3 5.0 4.1 3.3 3.4 3.2 3.1 3.1 2.8 2.4 2.1

a

Cross-sections in cm2 moleculey1 =10 18 .

Fig. 3. The ultraviolet absorption spectrum and cross-sections of C 2 H 3.

3.3. EÕaluation of uncertainty in cross-section determinations In general, a measurement procedure has imperfections that gives rise to the measurement error. Errors may be random or systematic. A random error arises from unpredictable variations in measurements. A systematic error is introduced by an imperfect knowledge of the values for known parameters, faulty calibration or uncertainty of technique. These systematic errors are often called biases in measurement w24x. As unknown parameters are extracted through modeling of a multicomponent reaction system, it is important to quantify a realistic uncertainty for the derived parameterŽs.. Here we asses the contribution of uncertainties of each known parameter, used in determining the final butadiene concentration wC 4 H 6 x bd s n bd . Table 3 is constructed to illustrate the biases on the derived n bd due to the uncertainty in the known parameters used in the modeling. A condition when the initial vinyl radical concentration is wC 2 H 3 x 0 s n V s 4 = 10 13 molecule cmy3 has been chosen for this analysis. We usually do not know the exact values for the known kinetic parameters, but only an average value with plus and minus uncertainty. In developing rows 3 through 6 of Table 3, the rate constant values used in the modeling are fixed either at best known Žcentral. values or within their uncertainties. Values of the known parameters were chosen to the fullest extent. If a cell is blank the central value for that parameter is employed. The combined uncertainty on the measured parameter may be approximated through the vector addition of the systematic errors

A. Fahr et al.r Chemical Physics 236 (1998) 43–51

49

Table 3 The biases on n V rn bd due to uncertainty of the parameters used in the modeling Known parameters used at their central values Žblank. or values within expected errors as given Parameter Central value

k2

k3

k4

y11

y11

9.4 = 10

y11

3.0 = 10

Derived parameters

k5

n bd y11

4.5 = 10

15.3 = 10

q20% y20% q20% y20% q12% y12% q40% y40%

n V rn bd

Bias

5.06

0.0

8.64 7.08

4.63 5.65

0.43 0.59

7.68 8.20

5.21 4.88

0.15 0.18

8.04 7.81

4.97 5.12

0.08 0.06

6.74 9.75

5.93 4.10

0.87 0.96

7.9 = 10

12

An initial vinyl radical concentration n V s 4 = 10 13 molecule cmy3 has been chosen.

Žbiases. engendered by using a parameter at a noncentral value and the random error. For evaluation of the combined error, we assume the contribution of the biases for each known parameter as the average of the upper and lower biases determined at the uncertainty limits of each known parameter. Thus the total combined uncertainty due to the systematic and random errors would be; 2

2

s 2 Ž n V rn bd . s Ž 0.51 . q Ž 0.16 . q Ž 0.07 . 2

2

2

q Ž 0.91 . q Ž 0.01 . ,

s Ž n V rn bd . s 1.06 . From this analysis a more realistic value n V rn bd s 5 " 1 is derived. The repetitive measurement of the absorbances A V and A bd resulted in ; 4% measurement error. Therefore, a combined uncertainty of ; 25% is expected for the vinyl absorption crosssection values Ž s V . determined here.

orbitals Ž1aX . 2 Ž2aX . 2 Ž3aX . 2 Ž4aX . 2 Ž5aX . 2 Ž6aX . 2 Ž1aY . 2 Ž7aX .1 and the lower virtual orbital Ž2aY . Ž8aX . Ž9aX .. In the CASSCFŽ3, 3., the active electrons, as illustrated in Fig. 5, are the three valence electrons Ž1aY . 2 Ž7aX .1 ; and the active space consisted of these valence orbitals and lowest virtual orbital Ž2aY .. This scheme allows the lowest accessible electronic excitations involving the Ž px . 2 electrons with 1aY symmetry and Ž s y .1 electron with 7aX symmetry ŽTable 3.. The transition energies calculated at CIS level of theory are qualitatively as expected from simple MO theory ŽFig. 5., that is, the energy increases in the order of s Ž7aX . § p Ž1aY . - p ) Ž2aY . § s Ž7aX . p ) Ž2aY . § p Ž1aY .. However, the calculated energies for the latter two transitions are almost identical at the EOM-CCSD and CASSCF levels of theories. The CASPT2 excitation energies are not only in agreement with the simple MO theory but are also lower in magnitudes than the corresponding CIS

3.4. Ab initio predictions for electronic transitions The optimized planar Ž Cs . geometry and the active electrons and orbitals used in our calculations are shown in Figs. 4 and 5. The optimized geometry of the ground electronic state of the vinyl radical is in good agreement with previous calculations w25x and it was used for predicting the vertical excitation energies. The Hartree–Fock electronic configuration of the ground state of vinyl radical consists of the occupied

Fig. 4. Optimized planar Ž Cs . geometry at CCSDr6-311q GŽ3df, 2p. level, bond lengths are in angstroms ˚ ¨ and bond angles in degrees.

A. Fahr et al.r Chemical Physics 236 (1998) 43–51

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225–238 nm is thus assigned to the more intense p ) Ž2aY . § p Ž1aY . transition. The predicted weaker p ) Ž2aY . § s Ž7aX . transition is either at lower energy compared to the p ) Ž2aY . § p Ž1aY . transition or hidden under this band. The blue shift of the calculated absorptions compared to the experimental values has also been observed in several calculations of the ultraviolet absorption bands of propargyl w26x and peroxy radicals w18,27x. These quantitative discrepancies are expected to be minimized by using larger basis sets, larger active space, and the inclusion of a higher level of dynamic electron correlations.

4. Conclusions

Fig. 5. Active electrons and orbitals used in CASŽ3, 3. multi-configuration calculations and allowed transitions.

results. The CIS absorptions are also blue shifted compared to those calculated with the EOM-CCSD method. A previously observed spectrum of vinyl radical in the visible region shows an absorption band between 368.5 and 499.5 nm with a maximum at 403 nm w6x. The lowest transition, s Ž7aX . § p Ž1aY ., calculated at the CASPT2Ž3, 3. level ŽTable 4. is 363 nm which is blue shifted compared to the experimentally observed value. The next transitions are predicted in the near ultraviolet around 260 and 225 nm for p ) Ž2aY . § s Ž7aX . and p ) Ž2aY . § p Ž1aY ., respectively. The new observed band in the range of

The ultraviolet absorption spectrum of vinyl ŽC 2 H 3 . radicals, in the spectral range 225–238 nm, has been identified and characterized using excimer laser photolysis and cavity ring-down absorption detection. Vinyl radicals were produced from the 193 nm photolysis of CH 3 COC 2 H 3 and C 2 H 3 Br. The spectrum exhibits a relatively broad absorption with an increased intensity towards the shorter wavelengths. By employing a comparative method, the cross-section values for vinyl radical were derived from the composite time-dependent absorption traces and the known absorption cross-sections of 1,3butadiene, the vinyl combination product. A detailed error analysis for determination of the cross-sections has been presented. The electronic transitions contributing to the vinyl radical spectrum were identified by ab initio molecular orbital calculations. These calculations suggest that the observed absorption band belongs to the ‘in plane’ p ) Ž2aY . § p Ž1aY . transition.

Table 4 Vertical electronic absorptions Žnm. in vinyl radical calculated at CIS, EOM-CCSD, CASSCF, and CASPT2 levels of theories Electronic transition

Transition symmetry Žintensity.

Y Y p ) Ž2a . § p Ž1a . X Y s Ž7a . § p Ž1a . Y X p ) Ž2a . § s Ž7a .

A Žstrong. Y A Žweak. Y A Žweak.

X

CISr6-311q GŽ3df, 2p.

EOM-CCSDr 6-311 q GŽ3df, 2p.

CASSCFŽ3, 3.r ANO

CASPT2Ž3, 3.r ANO

Experimentally observed

159 246 200

258 351 251

220 380 220

225 363 260

225–238 Žthis work. 368.5–499.5 6

A. Fahr et al.r Chemical Physics 236 (1998) 43–51

Acknowledgements The authors wish to acknowledge support for this work by NASA’s Planetary Atmospheres Program. We thank Dr. Jeffrey W. Hudgens and Dr. Russell D. Johnson for helpful comments.

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w25x w26x w27x

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