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Dissociation of excited 3-butenoic acid at 193 nm: observation of two channel OH formation dynamics by laser-induced fluorescence
Chemical Physics Letters 367 (2003) 253–258 www.elsevier.com/locate/cplett
Dissociation of excited 3-butenoic acid at 193 nm: observation of two channel OH formation dynamics by laser-induced fluorescence Pradyot K. Chowdhury
*
Radiation Chemistry and Chemical Dynamics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Received 25 September 2002; in final form 9 October 2002
Abstract In contrast to decarboxylation from the ground electronic state, the 1 ðp; p Þ excited 3-butenoic acid dissociates by C–O and C–C bond scissions, producing transient OH and HOCO radicals, respectively. The HOCO further dissociates to OH + CO. Laser-induced fluorescence (LIF) observation of OH showed the primary OH radical formation during 20 ns photolysis pulse, while the secondary OH radicals formed in 2 ls. Doppler spectroscopy showed that 36 kcal mol1 relative translational energy released into C–O bond fission fragments. It appears that the dissociation occurs from the excited state potential energy surface. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction The combination reaction OH þ CO ! HþCO2 is of great interest due to its role in the oxidation of fossil fuels and in atmospheric reactions. The intermediate complex formed in this reaction is HOCO, which may be studied in order to gain a better understanding of the combustion and atmospheric processes. To generate and study the HOCO species, the photolysis of small carboxylic acids such as acetic acid [1] and acrylic acid [2,3] is used. The photodissociation of acetic acid has been studied extensively [4–7], where the OH radical is detected
*
Fax: +91-22-5505151. E-mail address: [email protected] (P.K. Chowdhury).
as a primary product. The OH quantum yields at 222 nm laser photolysis of formic, acetic and propionic acids, directly determined by Singleton et al. [8], are in the range of 0.8–0.15. A crossed lasermolecular beam study on the photodissociation dynamics of acrylic acid has been reported by Butler and coworkers [9,10]. Their photofragment velocity distribution measurements indicate that only C–C and C–O bond fission reactions are major primary pathways, molecular decarboxylation reaction do not occur to a significant extent. Observing both the bond fission processes impart large amount of energy into relative fragment translation, they suggested that these processes occur on electronically excited state potential energy surface, as a simple bond fission in the ground electronic state is generally barrierless.
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 6 2 4 - X
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In 3-butenoic acid, the state excited at 193 nm may arise from a p ! p transition of the C@O group, as the similar assignments are made to acetic acid [4] and acrylic acid [10]. In comparison to the a,b-unsaturated acrylic acid, more facile decarboxylation has been reported in the pyrolysis [11] and infrared multiphoton dissociation [12] of 3-butenoic acid, the smallest b,c-unsaturated carboxylic acid. There is no study reported to our knowledge, on the UV–Vis photochemistry of 3-butenoic acid. We have chosen to study the 1 ðp; p Þ excited state dissociation dynamics of 3-butenoic acid, by using a pulsed ArF photolysis laser and probing the generated OH by laser-induced fluorescence (LIF). The primary photoprocesses proposed in 3-butenoic acid are given as follows: H2 C@CHACH2 COOH þ hm ! H2 C@CHACH2 CO þ OH
ð1Þ
! H2 C@CHACH2 þ COOH ! H3 CACH2 @CH2 þ CO2
ð2Þ ð3Þ
Reactions (1) and (2) involve the cleavage of single bonds C–O and C–C, respectively. While the C–O bond dissociation energy is expected to be similar to acetic acid, the C–C bond dissociation energy may be lower than that of acetic acid due to the allylic radical stabilization energy. Reaction (3) is an exothermic decarboxylation reaction via a sixcentered cyclic transition state, with a low activation energy [11] of 39.3 kcal mol1 . The molecular reaction from the ground electronic state, if formed by nonradiative transition process i.e., internal conversion of the electronic excited 3-butenoic acid, will be predominantly by reaction channel (3). We report here, the real time formation of OH by two channels, with rate constants, k P 108 s1 and k ¼ ð3:6 0:7Þ 106 s1 , respectively, on irradiation of 10 mTorr 3-butenoic acid at 193 nm. The Doppler spectroscopic observation that about 36:0 5:4 kcal/mol of relative translational energy released into the photofragments of reaction (1), suggests that the dissociation occurs from the excited state potential energy surface or by predissociation.
2. Results and discussion The experimental measurements are carried out in a pump–probe set-up [7,13], which consists of an ArF photolysis laser and a probe dye laser with frequency doubling and mixing module pumped by a seeded Nd:YAG laser, for the observation of LIF. The fluorescence is collected by a lens and detected by a photomultiplier tube (PMT). The fluorescence signal is gate integrated by a boxcar, averaged for 30 laser shots and fed into an interface for A/D conversion. A personal computer is used to control the scan of the dye laser via RS232 interface and to collect data through GPIB interface using control and data acquisition program. In the present work, the 3-butenoic acid vapor is flowed through the reaction chamber made up of stainless steel, with crossed right angle arms for photolysis and probe lasers, at a flow velocity of approximately 10 cm/ s. The 3-butenoic acid (99% purity, Aldrich) is freeze-pump-thawed for five times before use and its vapor pressure in the reaction chamber is maintained at about 10 mTorr. The OH radicals are produced at 193-nm photolysis of 3-butenoic acid and probed state selectively by exciting A2 Rþ ðv0 ¼ 0Þ X2 P ðv00 ¼ 0Þ transition of OH around 306–309 nm and monitoring the subsequent A ! X fluorescence. Fig. 1
Fig. 1. Portion of the laser-induced fluorescence (LIF) rotational excitation spectrum of OH radical formed in the irradiation of 3-butenoic acid by a ArF excimer laser (20 ns, 150 mJ/ pulse). The vapor pressure of 3-butenoic acid used is 10 mTorr, and the delay between the ArF and the dye laser is 100 ns. The spectral assignments are based on [14].
P.K. Chowdhury / Chemical Physics Letters 367 (2003) 253–258
exhibits a typical LIF spectrum with appropriate assignments [14], where the P, Q, and R branches refer to DN ¼ 1, 0, and 1 rotational transitions, respectively. The relative OH fragment population is determined by normalizing the peak area of the rotational line by pump and probe laser intensities and the respective Einstein absorption coefficient [15]. The spin orbit ratio and the Kdoublet ratio are calculated from the relative population of different states. The Doppler profile associated with the rotational line reveals the translational energy associated with the OH fragments.
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the P, Q and R branches within an experimental error. To determine the population of OH fragment in the v00 ¼ 1 state, the OH (1–1) transition is scanned, but no LIF signal is observed. Based on the Frank–Condon factors for the OH (1,1) and (0,0) transitions and the sensitivity of our experiment, less than 1% of the total OH are estimated to be formed in the v00 ¼ 1 state. The P3=2 =P1=2 ratios multiplied by appropriate statistical weights are plotted against the OH rotational quantum numbers. No preferential population in the P3=2 or P1=2 spin states are observed. A plot of the K-doublet ratio versus the rotational number, N showed the limiting statistical value of unity.
3. Rotational state distribution The nascent rotational state population of the OH radicals, generated in the photodissociation of 3-butenoic acid at 193-nm, is used for a Boltzmann plot providing us with the rotational temperature. The rotational spectrum was measured at 100 ns after the 193 nm photodissociation laser pulse. The Boltzmann plot is depicted in Fig. 2. The rotational temperature obtained by fitting all the data points is 360 40 K. The same rotational temperature is obtained for all the rotational lines of
Fig. 2. Boltzmann plot of the population in various microstates of the nascent OH. The distribution of four different spin and K-doublet states of OH is characterized by a single temperature, T R ¼ 360 40 K.
4. Translational energy in products The component of OH fragment velocity along the probe laser propagation axis z, vz , shifts the central absorption frequency m0 to m by the following equation: m ¼ m0 ð1 vz =cÞ;
ð1Þ
where c is the velocity of light. The linewidth and shape of the Doppler broadened LIF line includes contributions from the fragment molecular velocity, the thermal motion of the parent and the finite probe laser line width. The width of the probe laser spectral profile is obtained from the OH Doppler profile measurement in thermalized condition giving an estimated FWHM to be 0.07 cm1 . The peak profile for P1 (2) is shown in Fig. 3. Measurement of the OH Doppler profile with several other intense rotational lines such as P1 (6), Q1 (2) and Q1 (6) is also made. All the rotational lines are seen to exhibit the same line width within the experimental error. For a completely isotropic OH fragment velocity distribution, the deconvolution of the peak profiles with the instrumental function gives the Doppler width to be 0:56 0:05 cm1 . For the Maxwell–Boltzmann translational energy distribution, this corresponds to an average relative translational energy of 36:0 5:4 kcal mol1 in the center of mass co-ordinate of the photofragments, after due correction for the thermal energy of the parent.
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marginally and then a slower formation is observed with a rise time of 2 ls. The OH radicals are finally decayed by diffusion. The first data point is taken at a probe laser firing 500 ns before the pump laser, and the subsequent data points are taken at 50 ns intervals. Each data is taken after an averaging of 100 laser pulses. Measurement of the OH time evolution with several other intense rotational lines, P1 (6), Q1 (2) and Q1 (6) is also made. All the rotational lines studied show similar timeresolved growth pattern. By fitting the time-dependent data of Fig. 4 to a function (2), the growth rate of the OH formation is computed. Fig. 3. Doppler profile of the P1 (2) rotational line taken at 100 ns after the ArF laser pulse. The solid line drawn through the data points represents a Gaussian fit to the data points. The dotted line represents the instrument function (FWHM ¼ 0.07 cm1 ).
5. Time-resolved OH formation The production of OH (m00 ¼ 0) as a function of the delay time between the two pump–probe lasers is obtained by monitoring the intensity of the P1 (2) LIF signal, which is shown in Fig. 4. The vapor pressure of 3-butenoic acid is kept at 10 mTorr. Some of the OH radicals are formed instantaneously during the pump laser pulse, which decays
IðtÞ ¼ A½1 expðktÞ kd t;
ð2Þ
where k is the growth rate constant and kd is the OH diffusion rate. The OH formation rate constant is computed to be ð3:6 0:7Þ 106 s1 . While the experiment was repeated at a lower pressure of 5 mTorr and a higher pressure of 20 mTorr, respectively, the OH formation rate constant remained unchanged. This rules out the influence of the OH formation rate by collisional up pumping of HOCO population from lower internal energies up to states above the dissociation barrier. In another study, the 193-nm ArF excimer laser energy was varied from 2 to 20 mJ/pulse and the LIF intensity of the P1 (2) rotational line of OH was monitored. The log–log plot of the LIF signal versus photolysis laser energy yields a slope of 0:9 0:1. This indicates that the OH radicals are produced in a single-photon process.
6. Dissociation dynamics The primary dissociation pathways in 3-butenoic acid are as follows: H2 C@CH–CH2 COOH ! H2 C@CH–CH2 CO þ OH DH o ¼ 110:3 kcal mol1
ð1Þ
! H2 C@CH–CH2 þ COOH Fig. 4. LIF intensity of the OH generated in the photodissociation of CH2 @CH–CH2 COOH at 193 nm as a function of the pump–probe delay. The vapor pressure of 3-butenoic acid is kept at 10 mTorr.
DH o ¼ 71:2 kcal mol1
ð2Þ
! H3 C–CH2 @CH2 þ CO2 DH o ¼ 5:1 kcal mol1
ð3Þ
P.K. Chowdhury / Chemical Physics Letters 367 (2003) 253–258
The C–O dissociation energy is taken from acetic acid [4], the C–C dissociation energy is calculated from the heat of formation of HOCO [16], propene and 3-butenoic acid [17] and the exothermicity of the decarboxylation reaction (3) is taken from Smith and Blau [11]. Reaction (3) occurs via a six-centered cyclic transition state, with as low a barrier of activation energy [11] as 39.3 kcal/mol. The fate of the 3-butenoic acid from the ground electronic state, if formed by the nonradiative process (internal conversion) of the electronic excited state, will be predominantly by decarboxylation reaction. Further, the possibility of OH formation by slow dissociation from the secondary dissociation of CH2 COOH is not feasible due to energetic constraints. On irradiation of 3-butenoic acid at 193 nm, there are two channels observed for the OH formation, a prompt channel with a rate constant, k P 108 s1 and a slow channel with a rate constant, k ¼ 3:6 106 s1 . The prompt OH formation is assigned to the direct C–O bond fission, where the Doppler profile measurement gives a relative translation energy of the photofragments as 36 kcal mol1 . The excess energy available in this photodissociation reaction (1) is about 38 kcal mol1 . Imparting such a large amount of translation energy into the products suggests that the dissociation occurs on the excited state potential energy surface with a large barrier or by predissociation. Similar observation of large translational energy imparted with the C–C bond fission products (50 kcal mol1 as translation energy out of 54 kcal mol1 available energy) generated from the excited state dissociation has been reported by Butler and coworkers [10]. The slow formation of OH with a rate constant, k ¼ 3:6 106 s1 , has been assigned to the secondary HOCO dissociation to OH + CO. The HOCO may dissociate by two competing channels such as: HOCO ! OH þ CO DH o ¼ 34:6 kcal mol1 o
1
! H þ CO2 DH ¼ 10:2 kcal mol
ð2aÞ ð2bÞ
There is a significant energy barrier in the (2b) reaction. The theoretical calculations on the above reactions [18,19] have predicted that both the
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barrier heights are nearly equal, and thereby proceed in competition. The reaction of H atom with CO2 has been studied experimentally by Radhakrishnan et al. [20], using the van der Waals complex CO2 –HBr. They found that the hot H atoms produced at 193 nm photolysis of HBr constituent approaches CO2 mainly along the direction of the O–C–O axis. The generated OH radical was detected by LIF. Observing the vibrational, rotational and spin–orbit population distribution under molecular beam and bulk conditions, the lifetime of the HOCO intermediate was inferred to be short. In our photolysis study of 3-butenoic acid, the HOCO appears to be formed just above the barrier height [21] of 39.7 kcal mol1 , from the available energy of 77 kcal mol1 . Since statistical energy distribution may provide only half of the required energy, it appears that the reaction (2) also occurs on the excited state potential energy surface with a preferential energy partitioning in favor of HOCO internal energy. Since the excited state of the 3-butenoic acid is generated via a p ! p transition of the C@O group, it is likely that a fraction of the energy remains trapped in the CO group, which is unavailable for product energy distribution during the primary dissociation.
7. Conclusions The photodissociation of 3-butenoic acid, H2 C@CH–CH2 COOH, at 193 nm occurs by two channels, generating H2 C@CH–CH2 CO þ OH and H2 C@CH–CH2 þ COOH as primary products. The HOCO undergoes secondary dissociation to OH + CO. The OH radicals are probed by LIF spectroscopy. The real time observation of the OH formation within the pump laser pulse duration, gives a dissociation rate of H2 C@CH–CH2 COOH to be k P 108 s1 . The slower formation of OH with a rate constant of 3:6 106 s1 has been assigned to the secondary dissociation of HOCO. No vibrational excitation with OH could be observed. Neither any preferential OH formation in either spin doublets or K-doublets could be observed. Boltzmann rotational temperature of the primary OH is found to be 360 40 K. Doppler line-width
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measurement, with the primary OH formed from 3-butenoic acid, shows that the translational energy released with the photofragments in the center of mass co-ordinate is 36:0 5:4 kcal mol1 . The observation of the primary OH radicals with a small amount of rotational energy, no vibrational energy and a large translational energy suggests that 3-butenoic acid dissociates from the excited state potential energy surface or by predissociation. Acknowledgements It is a pleasure to acknowledge the valuable discussions with Dr. Jai P. Mittal and his kind support. The help of Dr. Hari P. Upadhyaya with the initial experiments is highly acknowledged. The author wishes to thank Dr. T. Mukherjee and Dr. A. V. Sapre for their keen interest in this work. References [1] T.J. Sears, W.M. Fawzy, P.M. Johnson, J. Chem. Phys. 97 (1992) 3996. [2] T.J. Petty, J.A. Harrison, C.B. Moore, J. Phys. Chem. 97 (1993) 11194. [3] A. Miyoshi, A. Matsui, N. Washida, J. Chem. Phys. 100 (1994) 3532.
[4] J.C. Owrutsky, A.P. Baronavski, J. Chem. Phys. 111 (1999) 7329. [5] S.S. Hunnicutt, L. D Waits, J.A. Guest, J. Phys. Chem. 95 (1991) 562. [6] S.S. Hunnicutt, L.D. Waits, J.A. Guest, J. Phys. Chem. 93 (1989) 5188. [7] P.D. Naik, H.P. Upadhyaya, A. Kumar, A.V. Sapre, J.P. Mittal, Chem. Phys. Lett. 340 (2001) 116. [8] D.L. Singleton, G. Paraskevopoulos, R.S. Irwin, J. Phys. Chem. 94 (1990) 695. [9] M.F. Arendt, P.W. Browning, L.J. Butler, J. Chem. Phys. 103 (1995) 5877. [10] D.C. Kitchen, N.R. Forde, L.J. Butler, J. Phys. Chem. A 101 (1997) 6603. [11] G.G. Smith, S.E. Blau, J. Phys. Chem. 68 (1964) 1231. [12] J.L. Buechele, E. Weitz, F.D. Lewis, Chem. Phys. Lett. 77 (1981) 280. [13] P.K. Chowdhury, H.P. Upadhyaya, P.D. Naik, Chem. Phys. Lett. 344 (2001) 292. [14] G.H. Dieke, H.M. Crosswhite, J. Quant. Spectrosc. Radiat. Transfer 2 (1961) 97. [15] I.L. Chidsey, D.R. Crosley, J. Quant. Spectrosc. Radiat. Transfer 23 (1980) 187. [16] B. Ruscic, M. Schwartz, J. Berkowitz, J. Chem. Phys. 91 (1989) 6780. [17] S.W. Benson, H.E. OÕNeal, Kinetic Data on Gas Phase Unimolecular Reactions, NSRDS – NBS 21 (1970). [18] M. Aoyagi, S. Kato, J. Chem. Phys. 88 (1988) 6409. [19] M. Mozurkewich, S.W. Benson, J. Phys. Chem. 88 (1984) 6429. [20] G. Radhakrishnan, S. Buelow, C. Wittig, J. Chem. Phys. 84 (1986) 727. [21] A. Miyoshi, H. Matsui, J. Chem. Phys. 100 (1994) 3532.
Dissociation of excited 3-butenoic acid at 193 nm: observation of two channel OH formation dynamics by laser-induced fluorescence Pradyot K. Chowdhury
*
Radiation Chemistry and Chemical Dynamics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Received 25 September 2002; in final form 9 October 2002
Abstract In contrast to decarboxylation from the ground electronic state, the 1 ðp; p Þ excited 3-butenoic acid dissociates by C–O and C–C bond scissions, producing transient OH and HOCO radicals, respectively. The HOCO further dissociates to OH + CO. Laser-induced fluorescence (LIF) observation of OH showed the primary OH radical formation during 20 ns photolysis pulse, while the secondary OH radicals formed in 2 ls. Doppler spectroscopy showed that 36 kcal mol1 relative translational energy released into C–O bond fission fragments. It appears that the dissociation occurs from the excited state potential energy surface. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction The combination reaction OH þ CO ! HþCO2 is of great interest due to its role in the oxidation of fossil fuels and in atmospheric reactions. The intermediate complex formed in this reaction is HOCO, which may be studied in order to gain a better understanding of the combustion and atmospheric processes. To generate and study the HOCO species, the photolysis of small carboxylic acids such as acetic acid [1] and acrylic acid [2,3] is used. The photodissociation of acetic acid has been studied extensively [4–7], where the OH radical is detected
*
Fax: +91-22-5505151. E-mail address: [email protected] (P.K. Chowdhury).
as a primary product. The OH quantum yields at 222 nm laser photolysis of formic, acetic and propionic acids, directly determined by Singleton et al. [8], are in the range of 0.8–0.15. A crossed lasermolecular beam study on the photodissociation dynamics of acrylic acid has been reported by Butler and coworkers [9,10]. Their photofragment velocity distribution measurements indicate that only C–C and C–O bond fission reactions are major primary pathways, molecular decarboxylation reaction do not occur to a significant extent. Observing both the bond fission processes impart large amount of energy into relative fragment translation, they suggested that these processes occur on electronically excited state potential energy surface, as a simple bond fission in the ground electronic state is generally barrierless.
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 6 2 4 - X
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In 3-butenoic acid, the state excited at 193 nm may arise from a p ! p transition of the C@O group, as the similar assignments are made to acetic acid [4] and acrylic acid [10]. In comparison to the a,b-unsaturated acrylic acid, more facile decarboxylation has been reported in the pyrolysis [11] and infrared multiphoton dissociation [12] of 3-butenoic acid, the smallest b,c-unsaturated carboxylic acid. There is no study reported to our knowledge, on the UV–Vis photochemistry of 3-butenoic acid. We have chosen to study the 1 ðp; p Þ excited state dissociation dynamics of 3-butenoic acid, by using a pulsed ArF photolysis laser and probing the generated OH by laser-induced fluorescence (LIF). The primary photoprocesses proposed in 3-butenoic acid are given as follows: H2 C@CHACH2 COOH þ hm ! H2 C@CHACH2 CO þ OH
ð1Þ
! H2 C@CHACH2 þ COOH ! H3 CACH2 @CH2 þ CO2
ð2Þ ð3Þ
Reactions (1) and (2) involve the cleavage of single bonds C–O and C–C, respectively. While the C–O bond dissociation energy is expected to be similar to acetic acid, the C–C bond dissociation energy may be lower than that of acetic acid due to the allylic radical stabilization energy. Reaction (3) is an exothermic decarboxylation reaction via a sixcentered cyclic transition state, with a low activation energy [11] of 39.3 kcal mol1 . The molecular reaction from the ground electronic state, if formed by nonradiative transition process i.e., internal conversion of the electronic excited 3-butenoic acid, will be predominantly by reaction channel (3). We report here, the real time formation of OH by two channels, with rate constants, k P 108 s1 and k ¼ ð3:6 0:7Þ 106 s1 , respectively, on irradiation of 10 mTorr 3-butenoic acid at 193 nm. The Doppler spectroscopic observation that about 36:0 5:4 kcal/mol of relative translational energy released into the photofragments of reaction (1), suggests that the dissociation occurs from the excited state potential energy surface or by predissociation.
2. Results and discussion The experimental measurements are carried out in a pump–probe set-up [7,13], which consists of an ArF photolysis laser and a probe dye laser with frequency doubling and mixing module pumped by a seeded Nd:YAG laser, for the observation of LIF. The fluorescence is collected by a lens and detected by a photomultiplier tube (PMT). The fluorescence signal is gate integrated by a boxcar, averaged for 30 laser shots and fed into an interface for A/D conversion. A personal computer is used to control the scan of the dye laser via RS232 interface and to collect data through GPIB interface using control and data acquisition program. In the present work, the 3-butenoic acid vapor is flowed through the reaction chamber made up of stainless steel, with crossed right angle arms for photolysis and probe lasers, at a flow velocity of approximately 10 cm/ s. The 3-butenoic acid (99% purity, Aldrich) is freeze-pump-thawed for five times before use and its vapor pressure in the reaction chamber is maintained at about 10 mTorr. The OH radicals are produced at 193-nm photolysis of 3-butenoic acid and probed state selectively by exciting A2 Rþ ðv0 ¼ 0Þ X2 P ðv00 ¼ 0Þ transition of OH around 306–309 nm and monitoring the subsequent A ! X fluorescence. Fig. 1
Fig. 1. Portion of the laser-induced fluorescence (LIF) rotational excitation spectrum of OH radical formed in the irradiation of 3-butenoic acid by a ArF excimer laser (20 ns, 150 mJ/ pulse). The vapor pressure of 3-butenoic acid used is 10 mTorr, and the delay between the ArF and the dye laser is 100 ns. The spectral assignments are based on [14].
P.K. Chowdhury / Chemical Physics Letters 367 (2003) 253–258
exhibits a typical LIF spectrum with appropriate assignments [14], where the P, Q, and R branches refer to DN ¼ 1, 0, and 1 rotational transitions, respectively. The relative OH fragment population is determined by normalizing the peak area of the rotational line by pump and probe laser intensities and the respective Einstein absorption coefficient [15]. The spin orbit ratio and the Kdoublet ratio are calculated from the relative population of different states. The Doppler profile associated with the rotational line reveals the translational energy associated with the OH fragments.
255
the P, Q and R branches within an experimental error. To determine the population of OH fragment in the v00 ¼ 1 state, the OH (1–1) transition is scanned, but no LIF signal is observed. Based on the Frank–Condon factors for the OH (1,1) and (0,0) transitions and the sensitivity of our experiment, less than 1% of the total OH are estimated to be formed in the v00 ¼ 1 state. The P3=2 =P1=2 ratios multiplied by appropriate statistical weights are plotted against the OH rotational quantum numbers. No preferential population in the P3=2 or P1=2 spin states are observed. A plot of the K-doublet ratio versus the rotational number, N showed the limiting statistical value of unity.
3. Rotational state distribution The nascent rotational state population of the OH radicals, generated in the photodissociation of 3-butenoic acid at 193-nm, is used for a Boltzmann plot providing us with the rotational temperature. The rotational spectrum was measured at 100 ns after the 193 nm photodissociation laser pulse. The Boltzmann plot is depicted in Fig. 2. The rotational temperature obtained by fitting all the data points is 360 40 K. The same rotational temperature is obtained for all the rotational lines of
Fig. 2. Boltzmann plot of the population in various microstates of the nascent OH. The distribution of four different spin and K-doublet states of OH is characterized by a single temperature, T R ¼ 360 40 K.
4. Translational energy in products The component of OH fragment velocity along the probe laser propagation axis z, vz , shifts the central absorption frequency m0 to m by the following equation: m ¼ m0 ð1 vz =cÞ;
ð1Þ
where c is the velocity of light. The linewidth and shape of the Doppler broadened LIF line includes contributions from the fragment molecular velocity, the thermal motion of the parent and the finite probe laser line width. The width of the probe laser spectral profile is obtained from the OH Doppler profile measurement in thermalized condition giving an estimated FWHM to be 0.07 cm1 . The peak profile for P1 (2) is shown in Fig. 3. Measurement of the OH Doppler profile with several other intense rotational lines such as P1 (6), Q1 (2) and Q1 (6) is also made. All the rotational lines are seen to exhibit the same line width within the experimental error. For a completely isotropic OH fragment velocity distribution, the deconvolution of the peak profiles with the instrumental function gives the Doppler width to be 0:56 0:05 cm1 . For the Maxwell–Boltzmann translational energy distribution, this corresponds to an average relative translational energy of 36:0 5:4 kcal mol1 in the center of mass co-ordinate of the photofragments, after due correction for the thermal energy of the parent.
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marginally and then a slower formation is observed with a rise time of 2 ls. The OH radicals are finally decayed by diffusion. The first data point is taken at a probe laser firing 500 ns before the pump laser, and the subsequent data points are taken at 50 ns intervals. Each data is taken after an averaging of 100 laser pulses. Measurement of the OH time evolution with several other intense rotational lines, P1 (6), Q1 (2) and Q1 (6) is also made. All the rotational lines studied show similar timeresolved growth pattern. By fitting the time-dependent data of Fig. 4 to a function (2), the growth rate of the OH formation is computed. Fig. 3. Doppler profile of the P1 (2) rotational line taken at 100 ns after the ArF laser pulse. The solid line drawn through the data points represents a Gaussian fit to the data points. The dotted line represents the instrument function (FWHM ¼ 0.07 cm1 ).
5. Time-resolved OH formation The production of OH (m00 ¼ 0) as a function of the delay time between the two pump–probe lasers is obtained by monitoring the intensity of the P1 (2) LIF signal, which is shown in Fig. 4. The vapor pressure of 3-butenoic acid is kept at 10 mTorr. Some of the OH radicals are formed instantaneously during the pump laser pulse, which decays
IðtÞ ¼ A½1 expðktÞ kd t;
ð2Þ
where k is the growth rate constant and kd is the OH diffusion rate. The OH formation rate constant is computed to be ð3:6 0:7Þ 106 s1 . While the experiment was repeated at a lower pressure of 5 mTorr and a higher pressure of 20 mTorr, respectively, the OH formation rate constant remained unchanged. This rules out the influence of the OH formation rate by collisional up pumping of HOCO population from lower internal energies up to states above the dissociation barrier. In another study, the 193-nm ArF excimer laser energy was varied from 2 to 20 mJ/pulse and the LIF intensity of the P1 (2) rotational line of OH was monitored. The log–log plot of the LIF signal versus photolysis laser energy yields a slope of 0:9 0:1. This indicates that the OH radicals are produced in a single-photon process.
6. Dissociation dynamics The primary dissociation pathways in 3-butenoic acid are as follows: H2 C@CH–CH2 COOH ! H2 C@CH–CH2 CO þ OH DH o ¼ 110:3 kcal mol1
ð1Þ
! H2 C@CH–CH2 þ COOH Fig. 4. LIF intensity of the OH generated in the photodissociation of CH2 @CH–CH2 COOH at 193 nm as a function of the pump–probe delay. The vapor pressure of 3-butenoic acid is kept at 10 mTorr.
DH o ¼ 71:2 kcal mol1
ð2Þ
! H3 C–CH2 @CH2 þ CO2 DH o ¼ 5:1 kcal mol1
ð3Þ
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The C–O dissociation energy is taken from acetic acid [4], the C–C dissociation energy is calculated from the heat of formation of HOCO [16], propene and 3-butenoic acid [17] and the exothermicity of the decarboxylation reaction (3) is taken from Smith and Blau [11]. Reaction (3) occurs via a six-centered cyclic transition state, with as low a barrier of activation energy [11] as 39.3 kcal/mol. The fate of the 3-butenoic acid from the ground electronic state, if formed by the nonradiative process (internal conversion) of the electronic excited state, will be predominantly by decarboxylation reaction. Further, the possibility of OH formation by slow dissociation from the secondary dissociation of CH2 COOH is not feasible due to energetic constraints. On irradiation of 3-butenoic acid at 193 nm, there are two channels observed for the OH formation, a prompt channel with a rate constant, k P 108 s1 and a slow channel with a rate constant, k ¼ 3:6 106 s1 . The prompt OH formation is assigned to the direct C–O bond fission, where the Doppler profile measurement gives a relative translation energy of the photofragments as 36 kcal mol1 . The excess energy available in this photodissociation reaction (1) is about 38 kcal mol1 . Imparting such a large amount of translation energy into the products suggests that the dissociation occurs on the excited state potential energy surface with a large barrier or by predissociation. Similar observation of large translational energy imparted with the C–C bond fission products (50 kcal mol1 as translation energy out of 54 kcal mol1 available energy) generated from the excited state dissociation has been reported by Butler and coworkers [10]. The slow formation of OH with a rate constant, k ¼ 3:6 106 s1 , has been assigned to the secondary HOCO dissociation to OH + CO. The HOCO may dissociate by two competing channels such as: HOCO ! OH þ CO DH o ¼ 34:6 kcal mol1 o
1
! H þ CO2 DH ¼ 10:2 kcal mol
ð2aÞ ð2bÞ
There is a significant energy barrier in the (2b) reaction. The theoretical calculations on the above reactions [18,19] have predicted that both the
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barrier heights are nearly equal, and thereby proceed in competition. The reaction of H atom with CO2 has been studied experimentally by Radhakrishnan et al. [20], using the van der Waals complex CO2 –HBr. They found that the hot H atoms produced at 193 nm photolysis of HBr constituent approaches CO2 mainly along the direction of the O–C–O axis. The generated OH radical was detected by LIF. Observing the vibrational, rotational and spin–orbit population distribution under molecular beam and bulk conditions, the lifetime of the HOCO intermediate was inferred to be short. In our photolysis study of 3-butenoic acid, the HOCO appears to be formed just above the barrier height [21] of 39.7 kcal mol1 , from the available energy of 77 kcal mol1 . Since statistical energy distribution may provide only half of the required energy, it appears that the reaction (2) also occurs on the excited state potential energy surface with a preferential energy partitioning in favor of HOCO internal energy. Since the excited state of the 3-butenoic acid is generated via a p ! p transition of the C@O group, it is likely that a fraction of the energy remains trapped in the CO group, which is unavailable for product energy distribution during the primary dissociation.
7. Conclusions The photodissociation of 3-butenoic acid, H2 C@CH–CH2 COOH, at 193 nm occurs by two channels, generating H2 C@CH–CH2 CO þ OH and H2 C@CH–CH2 þ COOH as primary products. The HOCO undergoes secondary dissociation to OH + CO. The OH radicals are probed by LIF spectroscopy. The real time observation of the OH formation within the pump laser pulse duration, gives a dissociation rate of H2 C@CH–CH2 COOH to be k P 108 s1 . The slower formation of OH with a rate constant of 3:6 106 s1 has been assigned to the secondary dissociation of HOCO. No vibrational excitation with OH could be observed. Neither any preferential OH formation in either spin doublets or K-doublets could be observed. Boltzmann rotational temperature of the primary OH is found to be 360 40 K. Doppler line-width
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measurement, with the primary OH formed from 3-butenoic acid, shows that the translational energy released with the photofragments in the center of mass co-ordinate is 36:0 5:4 kcal mol1 . The observation of the primary OH radicals with a small amount of rotational energy, no vibrational energy and a large translational energy suggests that 3-butenoic acid dissociates from the excited state potential energy surface or by predissociation. Acknowledgements It is a pleasure to acknowledge the valuable discussions with Dr. Jai P. Mittal and his kind support. The help of Dr. Hari P. Upadhyaya with the initial experiments is highly acknowledged. The author wishes to thank Dr. T. Mukherjee and Dr. A. V. Sapre for their keen interest in this work. References [1] T.J. Sears, W.M. Fawzy, P.M. Johnson, J. Chem. Phys. 97 (1992) 3996. [2] T.J. Petty, J.A. Harrison, C.B. Moore, J. Phys. Chem. 97 (1993) 11194. [3] A. Miyoshi, A. Matsui, N. Washida, J. Chem. Phys. 100 (1994) 3532.
[4] J.C. Owrutsky, A.P. Baronavski, J. Chem. Phys. 111 (1999) 7329. [5] S.S. Hunnicutt, L. D Waits, J.A. Guest, J. Phys. Chem. 95 (1991) 562. [6] S.S. Hunnicutt, L.D. Waits, J.A. Guest, J. Phys. Chem. 93 (1989) 5188. [7] P.D. Naik, H.P. Upadhyaya, A. Kumar, A.V. Sapre, J.P. Mittal, Chem. Phys. Lett. 340 (2001) 116. [8] D.L. Singleton, G. Paraskevopoulos, R.S. Irwin, J. Phys. Chem. 94 (1990) 695. [9] M.F. Arendt, P.W. Browning, L.J. Butler, J. Chem. Phys. 103 (1995) 5877. [10] D.C. Kitchen, N.R. Forde, L.J. Butler, J. Phys. Chem. A 101 (1997) 6603. [11] G.G. Smith, S.E. Blau, J. Phys. Chem. 68 (1964) 1231. [12] J.L. Buechele, E. Weitz, F.D. Lewis, Chem. Phys. Lett. 77 (1981) 280. [13] P.K. Chowdhury, H.P. Upadhyaya, P.D. Naik, Chem. Phys. Lett. 344 (2001) 292. [14] G.H. Dieke, H.M. Crosswhite, J. Quant. Spectrosc. Radiat. Transfer 2 (1961) 97. [15] I.L. Chidsey, D.R. Crosley, J. Quant. Spectrosc. Radiat. Transfer 23 (1980) 187. [16] B. Ruscic, M. Schwartz, J. Berkowitz, J. Chem. Phys. 91 (1989) 6780. [17] S.W. Benson, H.E. OÕNeal, Kinetic Data on Gas Phase Unimolecular Reactions, NSRDS – NBS 21 (1970). [18] M. Aoyagi, S. Kato, J. Chem. Phys. 88 (1988) 6409. [19] M. Mozurkewich, S.W. Benson, J. Phys. Chem. 88 (1984) 6429. [20] G. Radhakrishnan, S. Buelow, C. Wittig, J. Chem. Phys. 84 (1986) 727. [21] A. Miyoshi, H. Matsui, J. Chem. Phys. 100 (1994) 3532.