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2) atoms by small molecules
Chemical Physics Letters 463 (2008) 50–53
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Vacuum UV laser-induced fluorescence study of the collisional removal of Br(2P1/2) atoms by small molecules Kenshi Takahashi a,*, Erika Iwasaki b, Yutaka Matsumi b a b
Kyoto University Pioneering Research Unit, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Solar-Terrestrial Environment Laboratory and Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
a r t i c l e
i n f o
Article history: Received 9 June 2008 In final form 8 August 2008 Available online 13 August 2008
a b s t r a c t This Letter reports on the application of the vacuum ultraviolet laser-induced fluorescence detection of Br(2P1/2) atoms at 157.48 nm to the kinetic study of collisional removal of Br(2P1/2) by small molecules at 295 K. Gas mixtures of a small amount of CH3Br and an excess amount of collision partners are exposed to pulsed laser irradiation at 193 nm. Temporal decay profile of the Br* LIF intensity has been monitored to determine the collisional removal rate coefficients. The collision partners are H2, CO2, CF4, CF2H2, H2O, CH3OH, and SF5CF3, and the results are compared to literature data. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Development of infrared lasers employing the 2P1/2–2P3/2 transition of halogen atoms strongly stimulated the study of the chemical properties of these atoms. Studies of the gas-phase quenching of spin–orbit excited halogen atoms by electronic-to-vibration, rotation, translation energy transfer to molecular partners have thus been studied. Besides to the physical quenching, possible chemical reaction of spin–orbit excited halogen atoms with small molecules is an interesting dynamical question, because formation of ground electronic state products can only occur following non-adiabatic transitions from excited potential energy surfaces correlating with electronically excited products [1]. The spin–orbit states of Br(2P3/2) (=denoted Br) and Br(2P1/2) (=denoted Br*) are separated in energy by 3685 cm 1, with the Br* being higher in energy [2]. Since the pioneering studies using atomic resonance absorption spectroscopy in the vacuum ultraviolet region (RA) by Donovan and Husain [3,4], collisional removal of Br* atoms has extensively been studied. A recent comprehensive review summarizes the experimental and theoretical studies on the chemical properties of electronically excited halogen atoms [5]. As summarized in the review paper [5], the relaxation processes of Br* in collisions with small molecules and rare gas atoms have been studied with a variety of experimental techniques such as RA and resonance fluorescence spectroscopy with an atomic resonance lamp (RF). In our present study, pulsed laser photolysis/vacuum ultraviolet laser-induced fluorescence spectroscopy (PLP/VUV-LIF) technique for direct detection of Br* at 157.48 nm has been developed, and it has been applied to kinetic studies to determine the rate coefficients of collisional removal of Br* atom at 295 ± 2 K. The collision
* Corresponding author. Fax: +81 774 38 4546. E-mail address: [email protected] (K. Takahashi). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.08.027
partners investigated in our present study are H2, CO2, CF4, CF2H2, H2O, CH3OH, and SF5CF3. The tunable VUV laser light around 157.48 nm is generated by two-photon resonance four-wave frequency mixing technique in Xe. The Br* atoms are produced from the photodissociation of CH3Br at 193 nm. The time-resolved measurements of the Br* LIF allow us to determine the collisional removal rate coefficients. The VUV-LIF detection technique was also used in crossed molecular beam experiments to study the dynamics of F + HBr [6] and H + HBr reactions [7,8]. 2. Experimental The PLP/VUV-LIF technique has extensively been applied to the experimental studies of the gas-phase reactions [9–12]. A detail description on the experimental setup for the PLP/VUV-LIF technique was reported in our previous paper [9], therefore only a brief description in relevance to the present study will be given. All the experiments in our present study were performed at 295 ± 2 K. Gas mixtures of a small amount of CH3Br and an excess amount of collision partners (H2, CO2, CF4, CF2H2, H2O, CH3OH, and SF5CF3) diluted in Ar were slowly introduced into a reaction chamber which was evacuated continuously by a rotary pump through a liquid N2 trap. The reactant mixtures in the chamber consisted of 0.7–2.6 mTorr of CH3Br and 8.1 mTorr–5.6 Torr of the collision partner of interest in Ar diluent. The total pressure in the chamber was 2.0 Torr for Br* + H2 system, 8.0 Torr for Br* + CF4 system, and 5.0 Torr for other systems. The gas mixtures were exposed to 193nm laser irradiation (Lambda Physik, COMPex102) to photolyze CH3Br. The nascent spin–orbit branching ratio for Br* formation from the photolysis of CH3Br at 193 nm was reported to be Br*/ Br = 0.20 [13]. Considering the literature value of CH3Br photoabsorption at 193 nm (5.8 10 19 cm2) [14] and the photolysis laser fluence, the initial concentration of Br* atoms in the chamber
K. Takahashi et al. / Chemical Physics Letters 463 (2008) 50–53
was estimated to be about(2.2–8.1) 1011 atoms cm 3. Ar gas used as a buffer is an inefficient quencher for Br* with a reported rate coefficient<2 10 16 [5]. Physical quenching of Br* by Ar can safely be ignored under our experimental conditions. Ar gases thermalize the translational energy distribution of Br* produced photolytically within 1 ls delay time between the photolysis and probe laser pulses. All the experiments were carried out under pseudo-first-order conditions with [collision partner] [Br*]0, where [Br*]0 is the initial Br(2P1/2) atom concentration. Br* atoms were directly detected by the VUV-LIF spectroscopy technique at 157.48 nm (4p5 2P1/2–4p4 5s 2P3/2 transition). The tunable VUV radiation was generated by two-photon resonant fourwave difference frequency mixing (2x1–x2) in a cell containing 15 Torr of Xe, using two dye lasers pumped by a single XeCl excimer laser (Lambda Physik, COMPex 201, FL3002 and Scanmate 2E). The fundamental output of the x1 laser at 499.26 nm was frequency-doubled in a BBO crystal to generate 249.63 nm light which is two-photon resonant with the 6p[1/2]0 level of Xe. The wavelength of the x2 laser was 601.6 nm. Typical pulse energies of the x1 and x2 lasers were 0.3 mJ and 4 mJ, respectively. The x1 and x2 laser beams were overlapped and focused into the Xe cell by a fused silica lens (f = 200 mm). The Kr gas cell and the reaction chamber were separated by a thin MgF2 window. The LIF of Br* atoms was detected by a solar-blind photomultiplier tube (EMR, 541J-08-17) mounted at right angles to the propagation direction of the VUV probe beam and the 193 nm photolysis beam. The 193 nm laser and the VUV laser beams were orthogonal to each other. The linewidth of the VUV laser was about 0.7 cm 1, which was estimated from the spectral shape of the resonance line for thermal Br* atoms. Bromine has two stable isotopes 79Br and 81Br with their natural abundances of 50.52% and 49.48%, respectively, and each isotope has a nuclear spin dipole, I, I = 3/2. Hyperfine structure, F = I + J, is thus generated. The energy splitting among the hyperfine structure [15,16] is much smaller than the laser linewidth in our present study. The photolysis and probe lasers were operated at repetition rate of 10 Hz. The time delay between the dissociation and probe laser pulses was controlled by a digital delay generator (Stanford Research, DG535). The jitter of the delay time was less than 20 ns. There was no discernible Br* atom LIF signal when the 193 nm laser light was turned off, indicating that photolysis of CH3Br at 157.48 nm is not a complication in the present work. Each of reagents H2O, CH3OH, CF2H2, and SF5CF3 was diluted in Ar and stored in 10 L glass bulbs which were blackened to avoid any photochemistry. The reagents were introduced into the reaction cell through mass flow controllers (Horiba STEC, SEC400MARK3). Each of reagents H2, CO2, and CF4 was supplied directly from the cylinders into the reaction cell through mass flow controllers. The total pressure in the reaction cell was monitored by a capacitance manometer (MKS Baratron, Model 622A, 10 Torr full scale). The gases used in the experiments had the following stated purities: H2 >99.99% (Iwatani Industrial Gases); CO2 >99.99% (Showa Tansan); CF4 >99.99% (Japan Fine Products); CF2H2 >98% (PCR); CH3OH >99.9% (Wako Pure Chemical); SF5CF3 >99% (Oakwoods Products); CH3Br >98% (Tokyo Chemical Industry), and Ar >99.999% (Japan Fine Products). H2O and CH3OH samples were degassed and subjected to freeze–pump–thaw cycles prior to their storage in glass bulbs. CF2H2, SF5CF3 and CH3Br gases were also subjected to freeze–pump–thaw cycles. H2, CO2, CF4, and Ar were used in the experiments without further purification.
3. Results and discussion Fig. 1 shows a typical trace of the temporal profile of the LIF signal of Br*, in which a reactant mixture of 1.9 mTorr of CH3Br,
51
67 mTorr of CH3OH in 4.5 Torr of Ar diluent was exposed to 193nm laser irradiation. An initial jump at delay time t = 0 indicating photolytic formation of Br* atoms from CH3Br is followed by a slow decay due to collisional removal by CH3OH molecules. Fig. 2 shows pseudo-first-order plots for collisional removal of Br* in collisions with H2, CO2, CF4, CF2H2, H2O, CH3OH, and SF5CF3. The straight lines in Fig. 2 are the results of linear least squares fit analysis of the experimental data. The slopes give second-order rate coefficients as summarized in Table 1. Quoted uncertainties include two standard deviations from the least-squares fit analysis and our estimate of potential systematic errors associated with measurement of the reactant concentrations. Table 1 lists the present results in comparison with previous studies. There have been some experimental techniques to measure the rate coefficients for collisional removal of Br*. In our present paper, the laser-induced fluorescence spectroscopy has been demonstrated as a new technique for direct detection of Br* atoms, and it has been applied to kinetic studies of collisional removal of Br* atoms at room temperature. For CF2H2, CH3OH, and SF5CF3, no literature data were available and our present study provided the first determinations of the removal rate coefficients. For Br* + H2 system, the literature data are quite scattered and our present result is within the range of those data. For Br* + CO2 system, our present data is in good agreement with the values of Hariri and Wittig [17] and Sedlacek et al. [18]. For Br* + CF4 system, our present result is in agreement with the value of Donovan and Husain [3]. For Br* + H2O system, our present result is consistent with the value of Hariri and Witting [19] within each uncertainties. Fig. 3 shows a semi-logarithmic plot of the collisional removal rate coefficients as a function of energy deficit, |DE|, where the energy deficit is defined as energy difference between the spin–orbit energy of Br* (3685 cm 1) and fundamental vibrational frequency which is the nearest to 3685 cm 1 among the vibrations of a colliding molecule. Fundamental vibrational frequencies of the collision partners investigated here were referred to Stewart and Nielsen [20] for CF2H2, Rinsland et al. [21] for SF5CF3 and NIST chemistry webbook for others [22]. There is a moderate correlation between the removal rate coefficients and DE. This suggests that electronicto-vibrational (E–V) energy transfer plays a significant role in the collisional removal of Br*.
Fig. 1. A trace of the temporal decay of Br* LIF signal. A mixture of 1.9 mTorr CH3Br and 67 mTorr of CH3OH in 4.5 Torr of Ar diluent was exposed to 193-nm laser irradiation. The Br* atoms were directly detected by laser-induced fluorescence spectroscopy at 157.48 nm associated with the Br(4p5 2P1/2–4p4 5s 2P3/2) electronic transition. The initial jump in the profile reflects the photolytic formation of Br* atoms from CH3Br at 193 nm. The solid curve is a least-squares first order decay fit to the data. The insert shows a semi-log plot of the time profile.
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K. Takahashi et al. / Chemical Physics Letters 463 (2008) 50–53
Fig. 2. Pseudo-first-order plots for collisional removal of Br*. The lines through the data are linear least squares fits. (a): CH3OH (open circle), CO2 (open diamond), and H2O (open triangle), (b): H2 (filled diamond), (c): CF4 (open square), (d): CF2H2 (filled circle), and SF5CF3 (filled triangle).
Table 1 Rate coefficients for collisional removal of Br* at room temperature Collision partner
Rate coefficient
H2
4.7 18 ± 2 2.7 ± 0.3 6.3 ± 1.0 2.35 ± 0.22 9.7 ± 1.6 15 ± 1 15 ± 2 15 ± 6 13.2 ± 0.3 17.1 ± 2.5 0.21 0.62 ± 0.006 0.26 ± 0.03 32 62 ± 12 51 ± 3 76.7 ± 3.8 2.46 ± 0.65 64.0 ± 6.6 1.41 ± 0.46
CO2
CF4
H2O
CF2H2 CH3OH SF5CF3
a
Detection method b
References
RA RF SE SE SE PLP/VUV-LIF IR emission of CO2 IR absorption of CO2 IR emission of CO2 SE PLP/VUV-LIF RA SE PLP/VUV-LIF RA IR emission of H2O FCL-AS PLP/VUV-LIF PLP/VUV-LIF PLP/VUV-LIF PLP/VUV-LIF
Donovan and Husain [4] Wiesenfeld and Wolk [25] Grimley and Houston [26] Nesbitt and Leone [24] Johnson et al. [27] This work Hariri and Wittig [17] Sedlacek et al. [18] Reisler and Wittig [28] Johnson et al. [27] This work Donovan and Husain [4] Johnson et al. [27] This work Donovan and Husain [4] Hariri and Wittig [19] Taatjes et al. [29] This work This work This work This work
a
Rate coefficient for collisional removal of Br*. In units of cm3 molecule 1 s 1. b Experimental method for detection of Br* atoms; PLP/LIF: pulsed laser photolysis/laser-induced fluorescence, RA: resonance absorption detection of Br* using an atomic resonance lamp, RF: resonance fluorescence detection of Br* using an atomic resonance lamp, FCL-AS: F-center-color laser detection for Br*–Br absorption, SE: spontaneous infrared emission associated with Br* ? Br transition. 10
12
Among the collision partners investigated in the present study, H2O and CH3OH molecules deactivate the Br* atoms most efficiently. Fundamental vibrational frequency of the O–H bond in CH3OH molecule in gas phase is 3682 cm 1 [22]. Fundamental vibrational frequencies of v1 and v3 modes in H2O molecule are
Fig. 3. Semi-log plot for the collisional removal rate coefficients of Br* as a function of energy deficit, |DE| (see text).
3657 and 3755 cm 1, respectively [22]. Those vibrational energies are very close to the spin–orbit energy of Br* atom. Hariri and Wittig [19] observed IR emissions from H2O (v1 = 1 and v3 = 1) produced by the collisions of Br* and H2O, in which the energy transfer to the H2O (0 0 1) state is endothermic by 71 cm 1 while that to the H2O (1 0 0) is exothermic by 33 cm 1. Spectral overlap between the emissions from the (0 0 1) ? (0 0 0) and (1 0 0) ? (0 0 0) bands and strong collisional coupling of the states prevented identification of the detailed energy transfer processes. For Br* + CO2 system, simultaneous excitation of several vibrational levels in CO2 was reported, in which the Br + CO2 (1 0 1) product channel can contribute significantly with a negligibly small endothermic energy (30 cm 1) [17]. Only the fundamental vibrational frequency of 2385 cm 1(v3) in CO2 is considered to plot the rate coefficient value on Fig. 3. Taking the Br + CO2 (1 0 1) product channel into account, the data point for CO2 will be shifted to almost
K. Takahashi et al. / Chemical Physics Letters 463 (2008) 50–53
near resonant region, thereby the correlation between all the data and DE can be more consistent. For SF5CF3 and CH2F2, our data are the first measurements and no other report is available. Although the DE value in Br* + SF5CF3 system is similar to that in Br*+CF4, the removal rate coefficient for Br*+SF5CF3 system is apparently larger than that for Br* + CF4 system, as shown in Fig. 3. Similarly, although the DE value in Br* + CH2F2 system is similar to that in Br* + H2, the removal rate coefficient for Br* + CH2F2 system is apparently smaller than that for Br* + CH2F2 system. Experimental and theoretical investigations of electronic-to-rotation and translation (E–R, T) as well as E–V energy transfer processes will be beneficial to account for the trend in the observed collisional removal rate coefficients. An intriguing issue is that there has been considerable experimental and theoretical interest in electronically non-adiabatic chemical reaction channel in the collisional removal of spin–orbit excited halogen atoms. For Br* + H2 system, Takayanagi and Kurosaki [23] reported three-dimensional quantum scattering calculations study for the Br* + H2(v = 0,1) ? H + HBr channel. They concluded that the contribution of the electronically non-adiabatic channel is small, which is in contrast to an experimental view that a substantial fraction of the Br* + H2(v = 1) collisions can result in the H + HBr products [24]. For Br* + CH3OH system, the Br* + CH3OH ? CH2OH + HBr channel is exothermic by 454 cm 1 (1.31 kcal mol 1), while the Br + CH3OH ? CH2OH + HBr channel is endothermic by 3228 cm 1 (9.23 kcal mol 1). The gas-phase enthalpy data were taken from the NASA/JPL databook [16]. Neither experimental nor theoretical work has been reported on the nonadiabatic chemical reaction channel of Br* + CH3OH ? CH2OH + HBr. Taketani et al. [11] reported the experimental study on the branching ratio between the non-adiabatic chemical reaction and physical quenching channels for Cl* + CH3OH system, using the PLP/VUV-LIF technique with Cl*/Cl detection. 4. Conclusion In this Letter, the PLP/VUV-LIF technique has been applied to kinetic measurements of the collisional removal of Br* by small molecules for the first time. The results have been discussed in terms of a correlation between the rate coefficients and the energy deficit. It has been suggested that further experimental and theoretical investigations on E–R, T energy transfer processes and non-adiabatic chemical reaction channels as well as E–V energy transfer processes are required to come to complete understanding of the chemical properties of halogen atoms. The experimental technique reported here will be a powerful tool to study the non-adiabatic
53
chemical reaction channels in the collisional decay of Br*, as it had successfully revealed the branching ratios between the nonadiabatic chemical reaction and physical quenching channels in Cl* + small hydrocarbons. Acknowledgement This study was partly supported by Program for Improvement of Research Environment for Young Researchers from Special Coordination Funds for Promoting Science and Technology (SCF) commissioned by the MEXT of Japan (K.T.). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
R.J. Donovan, D. Husain, Chem. Rev. 70 (1970) 489. J.D. Campbell, J.V.V. Kasper, Chem. Phys. Lett. 10 (1971) 436. R.J. Donovan, D. Husain, Trans. Faraday Soc. 62 (1966) 2643. R.J. Donovan, D. Husain, Trans. Faraday Soc. 62 (1966) 2987. A.I. Chichinin, J. Phys. Chem. Ref. Data 35 (2006) 869. J.W. Hepburn, K. Liu, R.G. Macdonald, F.J. Northrup, J.C. Polanyi, J. Chem. Phys. 75 (1981) 3353. J.W. Hepburn, D. Klimel, K. Liu, R.G. Mcdonald, F.J. Northrup, J.C. Polanyi, J. Chem. Phys. 74 (1981) 6226. J.W. Hepburn, D. Klimek, K. Liu, J.C. Polanyi, S.C. Wallace, J. Chem. Phys. 69 (1978) 4311. K. Hitsuda, K. Takahashi, Y. Matsumi, T.J. Wallington, J. Phys. Chem. A 105 (2001) 5131. F. Taketani, A. Yamasaki, K. Takahashi, Y. Matsumi, Chem. Phys. Lett. 406 (2005) 259. F. Taketani, K. Takahashi, Y. Matsumi, T.J. Wallington, J. Phys. Chem. A 109 (2005) 3935. K. Takahashi, E. Iwasaki, T. Nakayama, Y. Matsumi, T.J. Wallington, Int. J. Chem. Kinet. 39 (2007) 328. G.N.A. Van Veen, T. Baller, A.E. De Vries, Chem. Phys. 92 (1985) 59. S.P. Sander et al., Chemical Kinetics and Photochemical Data for use in Atmospheric Studies, Evaluation No. 15, JPL Publication 06-2, 2006. J.G. King, V. Jaccarino, Phys. Rev. 94 (1954) 1610. H.H. Brown, J.G. King, Phys. Rev. 142 (1966) 53. A. Hariri, C. Wittig, J. Chem. Phys. 67 (1977) 4454. A.J. Sedlacek, R.E. Weston Jr., G.W. Flynn, R.E. Weston, J. Chem. Phys. 93 (1990) 2812. A. Hariri, C. Wittig, J. Chem. Phys. 68 (1978) 2109. H.B. Stewart, H.H. Nielsen, Phys. Rev. 75 (1949) 640. C.P. Rinsland, S.W. Sharpe, R.L. Sams, J. Quant. Spectrosc. Radiat. Transfer 82 (2003) 483. National Institute of Standard and Technology (NIST) Standard Reference Database Number 69, June 2005,
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Vacuum UV laser-induced fluorescence study of the collisional removal of Br(2P1/2) atoms by small molecules Kenshi Takahashi a,*, Erika Iwasaki b, Yutaka Matsumi b a b
Kyoto University Pioneering Research Unit, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Solar-Terrestrial Environment Laboratory and Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
a r t i c l e
i n f o
Article history: Received 9 June 2008 In final form 8 August 2008 Available online 13 August 2008
a b s t r a c t This Letter reports on the application of the vacuum ultraviolet laser-induced fluorescence detection of Br(2P1/2) atoms at 157.48 nm to the kinetic study of collisional removal of Br(2P1/2) by small molecules at 295 K. Gas mixtures of a small amount of CH3Br and an excess amount of collision partners are exposed to pulsed laser irradiation at 193 nm. Temporal decay profile of the Br* LIF intensity has been monitored to determine the collisional removal rate coefficients. The collision partners are H2, CO2, CF4, CF2H2, H2O, CH3OH, and SF5CF3, and the results are compared to literature data. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Development of infrared lasers employing the 2P1/2–2P3/2 transition of halogen atoms strongly stimulated the study of the chemical properties of these atoms. Studies of the gas-phase quenching of spin–orbit excited halogen atoms by electronic-to-vibration, rotation, translation energy transfer to molecular partners have thus been studied. Besides to the physical quenching, possible chemical reaction of spin–orbit excited halogen atoms with small molecules is an interesting dynamical question, because formation of ground electronic state products can only occur following non-adiabatic transitions from excited potential energy surfaces correlating with electronically excited products [1]. The spin–orbit states of Br(2P3/2) (=denoted Br) and Br(2P1/2) (=denoted Br*) are separated in energy by 3685 cm 1, with the Br* being higher in energy [2]. Since the pioneering studies using atomic resonance absorption spectroscopy in the vacuum ultraviolet region (RA) by Donovan and Husain [3,4], collisional removal of Br* atoms has extensively been studied. A recent comprehensive review summarizes the experimental and theoretical studies on the chemical properties of electronically excited halogen atoms [5]. As summarized in the review paper [5], the relaxation processes of Br* in collisions with small molecules and rare gas atoms have been studied with a variety of experimental techniques such as RA and resonance fluorescence spectroscopy with an atomic resonance lamp (RF). In our present study, pulsed laser photolysis/vacuum ultraviolet laser-induced fluorescence spectroscopy (PLP/VUV-LIF) technique for direct detection of Br* at 157.48 nm has been developed, and it has been applied to kinetic studies to determine the rate coefficients of collisional removal of Br* atom at 295 ± 2 K. The collision
* Corresponding author. Fax: +81 774 38 4546. E-mail address: [email protected] (K. Takahashi). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.08.027
partners investigated in our present study are H2, CO2, CF4, CF2H2, H2O, CH3OH, and SF5CF3. The tunable VUV laser light around 157.48 nm is generated by two-photon resonance four-wave frequency mixing technique in Xe. The Br* atoms are produced from the photodissociation of CH3Br at 193 nm. The time-resolved measurements of the Br* LIF allow us to determine the collisional removal rate coefficients. The VUV-LIF detection technique was also used in crossed molecular beam experiments to study the dynamics of F + HBr [6] and H + HBr reactions [7,8]. 2. Experimental The PLP/VUV-LIF technique has extensively been applied to the experimental studies of the gas-phase reactions [9–12]. A detail description on the experimental setup for the PLP/VUV-LIF technique was reported in our previous paper [9], therefore only a brief description in relevance to the present study will be given. All the experiments in our present study were performed at 295 ± 2 K. Gas mixtures of a small amount of CH3Br and an excess amount of collision partners (H2, CO2, CF4, CF2H2, H2O, CH3OH, and SF5CF3) diluted in Ar were slowly introduced into a reaction chamber which was evacuated continuously by a rotary pump through a liquid N2 trap. The reactant mixtures in the chamber consisted of 0.7–2.6 mTorr of CH3Br and 8.1 mTorr–5.6 Torr of the collision partner of interest in Ar diluent. The total pressure in the chamber was 2.0 Torr for Br* + H2 system, 8.0 Torr for Br* + CF4 system, and 5.0 Torr for other systems. The gas mixtures were exposed to 193nm laser irradiation (Lambda Physik, COMPex102) to photolyze CH3Br. The nascent spin–orbit branching ratio for Br* formation from the photolysis of CH3Br at 193 nm was reported to be Br*/ Br = 0.20 [13]. Considering the literature value of CH3Br photoabsorption at 193 nm (5.8 10 19 cm2) [14] and the photolysis laser fluence, the initial concentration of Br* atoms in the chamber
K. Takahashi et al. / Chemical Physics Letters 463 (2008) 50–53
was estimated to be about(2.2–8.1) 1011 atoms cm 3. Ar gas used as a buffer is an inefficient quencher for Br* with a reported rate coefficient<2 10 16 [5]. Physical quenching of Br* by Ar can safely be ignored under our experimental conditions. Ar gases thermalize the translational energy distribution of Br* produced photolytically within 1 ls delay time between the photolysis and probe laser pulses. All the experiments were carried out under pseudo-first-order conditions with [collision partner] [Br*]0, where [Br*]0 is the initial Br(2P1/2) atom concentration. Br* atoms were directly detected by the VUV-LIF spectroscopy technique at 157.48 nm (4p5 2P1/2–4p4 5s 2P3/2 transition). The tunable VUV radiation was generated by two-photon resonant fourwave difference frequency mixing (2x1–x2) in a cell containing 15 Torr of Xe, using two dye lasers pumped by a single XeCl excimer laser (Lambda Physik, COMPex 201, FL3002 and Scanmate 2E). The fundamental output of the x1 laser at 499.26 nm was frequency-doubled in a BBO crystal to generate 249.63 nm light which is two-photon resonant with the 6p[1/2]0 level of Xe. The wavelength of the x2 laser was 601.6 nm. Typical pulse energies of the x1 and x2 lasers were 0.3 mJ and 4 mJ, respectively. The x1 and x2 laser beams were overlapped and focused into the Xe cell by a fused silica lens (f = 200 mm). The Kr gas cell and the reaction chamber were separated by a thin MgF2 window. The LIF of Br* atoms was detected by a solar-blind photomultiplier tube (EMR, 541J-08-17) mounted at right angles to the propagation direction of the VUV probe beam and the 193 nm photolysis beam. The 193 nm laser and the VUV laser beams were orthogonal to each other. The linewidth of the VUV laser was about 0.7 cm 1, which was estimated from the spectral shape of the resonance line for thermal Br* atoms. Bromine has two stable isotopes 79Br and 81Br with their natural abundances of 50.52% and 49.48%, respectively, and each isotope has a nuclear spin dipole, I, I = 3/2. Hyperfine structure, F = I + J, is thus generated. The energy splitting among the hyperfine structure [15,16] is much smaller than the laser linewidth in our present study. The photolysis and probe lasers were operated at repetition rate of 10 Hz. The time delay between the dissociation and probe laser pulses was controlled by a digital delay generator (Stanford Research, DG535). The jitter of the delay time was less than 20 ns. There was no discernible Br* atom LIF signal when the 193 nm laser light was turned off, indicating that photolysis of CH3Br at 157.48 nm is not a complication in the present work. Each of reagents H2O, CH3OH, CF2H2, and SF5CF3 was diluted in Ar and stored in 10 L glass bulbs which were blackened to avoid any photochemistry. The reagents were introduced into the reaction cell through mass flow controllers (Horiba STEC, SEC400MARK3). Each of reagents H2, CO2, and CF4 was supplied directly from the cylinders into the reaction cell through mass flow controllers. The total pressure in the reaction cell was monitored by a capacitance manometer (MKS Baratron, Model 622A, 10 Torr full scale). The gases used in the experiments had the following stated purities: H2 >99.99% (Iwatani Industrial Gases); CO2 >99.99% (Showa Tansan); CF4 >99.99% (Japan Fine Products); CF2H2 >98% (PCR); CH3OH >99.9% (Wako Pure Chemical); SF5CF3 >99% (Oakwoods Products); CH3Br >98% (Tokyo Chemical Industry), and Ar >99.999% (Japan Fine Products). H2O and CH3OH samples were degassed and subjected to freeze–pump–thaw cycles prior to their storage in glass bulbs. CF2H2, SF5CF3 and CH3Br gases were also subjected to freeze–pump–thaw cycles. H2, CO2, CF4, and Ar were used in the experiments without further purification.
3. Results and discussion Fig. 1 shows a typical trace of the temporal profile of the LIF signal of Br*, in which a reactant mixture of 1.9 mTorr of CH3Br,
51
67 mTorr of CH3OH in 4.5 Torr of Ar diluent was exposed to 193nm laser irradiation. An initial jump at delay time t = 0 indicating photolytic formation of Br* atoms from CH3Br is followed by a slow decay due to collisional removal by CH3OH molecules. Fig. 2 shows pseudo-first-order plots for collisional removal of Br* in collisions with H2, CO2, CF4, CF2H2, H2O, CH3OH, and SF5CF3. The straight lines in Fig. 2 are the results of linear least squares fit analysis of the experimental data. The slopes give second-order rate coefficients as summarized in Table 1. Quoted uncertainties include two standard deviations from the least-squares fit analysis and our estimate of potential systematic errors associated with measurement of the reactant concentrations. Table 1 lists the present results in comparison with previous studies. There have been some experimental techniques to measure the rate coefficients for collisional removal of Br*. In our present paper, the laser-induced fluorescence spectroscopy has been demonstrated as a new technique for direct detection of Br* atoms, and it has been applied to kinetic studies of collisional removal of Br* atoms at room temperature. For CF2H2, CH3OH, and SF5CF3, no literature data were available and our present study provided the first determinations of the removal rate coefficients. For Br* + H2 system, the literature data are quite scattered and our present result is within the range of those data. For Br* + CO2 system, our present data is in good agreement with the values of Hariri and Wittig [17] and Sedlacek et al. [18]. For Br* + CF4 system, our present result is in agreement with the value of Donovan and Husain [3]. For Br* + H2O system, our present result is consistent with the value of Hariri and Witting [19] within each uncertainties. Fig. 3 shows a semi-logarithmic plot of the collisional removal rate coefficients as a function of energy deficit, |DE|, where the energy deficit is defined as energy difference between the spin–orbit energy of Br* (3685 cm 1) and fundamental vibrational frequency which is the nearest to 3685 cm 1 among the vibrations of a colliding molecule. Fundamental vibrational frequencies of the collision partners investigated here were referred to Stewart and Nielsen [20] for CF2H2, Rinsland et al. [21] for SF5CF3 and NIST chemistry webbook for others [22]. There is a moderate correlation between the removal rate coefficients and DE. This suggests that electronicto-vibrational (E–V) energy transfer plays a significant role in the collisional removal of Br*.
Fig. 1. A trace of the temporal decay of Br* LIF signal. A mixture of 1.9 mTorr CH3Br and 67 mTorr of CH3OH in 4.5 Torr of Ar diluent was exposed to 193-nm laser irradiation. The Br* atoms were directly detected by laser-induced fluorescence spectroscopy at 157.48 nm associated with the Br(4p5 2P1/2–4p4 5s 2P3/2) electronic transition. The initial jump in the profile reflects the photolytic formation of Br* atoms from CH3Br at 193 nm. The solid curve is a least-squares first order decay fit to the data. The insert shows a semi-log plot of the time profile.
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K. Takahashi et al. / Chemical Physics Letters 463 (2008) 50–53
Fig. 2. Pseudo-first-order plots for collisional removal of Br*. The lines through the data are linear least squares fits. (a): CH3OH (open circle), CO2 (open diamond), and H2O (open triangle), (b): H2 (filled diamond), (c): CF4 (open square), (d): CF2H2 (filled circle), and SF5CF3 (filled triangle).
Table 1 Rate coefficients for collisional removal of Br* at room temperature Collision partner
Rate coefficient
H2
4.7 18 ± 2 2.7 ± 0.3 6.3 ± 1.0 2.35 ± 0.22 9.7 ± 1.6 15 ± 1 15 ± 2 15 ± 6 13.2 ± 0.3 17.1 ± 2.5 0.21 0.62 ± 0.006 0.26 ± 0.03 32 62 ± 12 51 ± 3 76.7 ± 3.8 2.46 ± 0.65 64.0 ± 6.6 1.41 ± 0.46
CO2
CF4
H2O
CF2H2 CH3OH SF5CF3
a
Detection method b
References
RA RF SE SE SE PLP/VUV-LIF IR emission of CO2 IR absorption of CO2 IR emission of CO2 SE PLP/VUV-LIF RA SE PLP/VUV-LIF RA IR emission of H2O FCL-AS PLP/VUV-LIF PLP/VUV-LIF PLP/VUV-LIF PLP/VUV-LIF
Donovan and Husain [4] Wiesenfeld and Wolk [25] Grimley and Houston [26] Nesbitt and Leone [24] Johnson et al. [27] This work Hariri and Wittig [17] Sedlacek et al. [18] Reisler and Wittig [28] Johnson et al. [27] This work Donovan and Husain [4] Johnson et al. [27] This work Donovan and Husain [4] Hariri and Wittig [19] Taatjes et al. [29] This work This work This work This work
a
Rate coefficient for collisional removal of Br*. In units of cm3 molecule 1 s 1. b Experimental method for detection of Br* atoms; PLP/LIF: pulsed laser photolysis/laser-induced fluorescence, RA: resonance absorption detection of Br* using an atomic resonance lamp, RF: resonance fluorescence detection of Br* using an atomic resonance lamp, FCL-AS: F-center-color laser detection for Br*–Br absorption, SE: spontaneous infrared emission associated with Br* ? Br transition. 10
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Among the collision partners investigated in the present study, H2O and CH3OH molecules deactivate the Br* atoms most efficiently. Fundamental vibrational frequency of the O–H bond in CH3OH molecule in gas phase is 3682 cm 1 [22]. Fundamental vibrational frequencies of v1 and v3 modes in H2O molecule are
Fig. 3. Semi-log plot for the collisional removal rate coefficients of Br* as a function of energy deficit, |DE| (see text).
3657 and 3755 cm 1, respectively [22]. Those vibrational energies are very close to the spin–orbit energy of Br* atom. Hariri and Wittig [19] observed IR emissions from H2O (v1 = 1 and v3 = 1) produced by the collisions of Br* and H2O, in which the energy transfer to the H2O (0 0 1) state is endothermic by 71 cm 1 while that to the H2O (1 0 0) is exothermic by 33 cm 1. Spectral overlap between the emissions from the (0 0 1) ? (0 0 0) and (1 0 0) ? (0 0 0) bands and strong collisional coupling of the states prevented identification of the detailed energy transfer processes. For Br* + CO2 system, simultaneous excitation of several vibrational levels in CO2 was reported, in which the Br + CO2 (1 0 1) product channel can contribute significantly with a negligibly small endothermic energy (30 cm 1) [17]. Only the fundamental vibrational frequency of 2385 cm 1(v3) in CO2 is considered to plot the rate coefficient value on Fig. 3. Taking the Br + CO2 (1 0 1) product channel into account, the data point for CO2 will be shifted to almost
K. Takahashi et al. / Chemical Physics Letters 463 (2008) 50–53
near resonant region, thereby the correlation between all the data and DE can be more consistent. For SF5CF3 and CH2F2, our data are the first measurements and no other report is available. Although the DE value in Br* + SF5CF3 system is similar to that in Br*+CF4, the removal rate coefficient for Br*+SF5CF3 system is apparently larger than that for Br* + CF4 system, as shown in Fig. 3. Similarly, although the DE value in Br* + CH2F2 system is similar to that in Br* + H2, the removal rate coefficient for Br* + CH2F2 system is apparently smaller than that for Br* + CH2F2 system. Experimental and theoretical investigations of electronic-to-rotation and translation (E–R, T) as well as E–V energy transfer processes will be beneficial to account for the trend in the observed collisional removal rate coefficients. An intriguing issue is that there has been considerable experimental and theoretical interest in electronically non-adiabatic chemical reaction channel in the collisional removal of spin–orbit excited halogen atoms. For Br* + H2 system, Takayanagi and Kurosaki [23] reported three-dimensional quantum scattering calculations study for the Br* + H2(v = 0,1) ? H + HBr channel. They concluded that the contribution of the electronically non-adiabatic channel is small, which is in contrast to an experimental view that a substantial fraction of the Br* + H2(v = 1) collisions can result in the H + HBr products [24]. For Br* + CH3OH system, the Br* + CH3OH ? CH2OH + HBr channel is exothermic by 454 cm 1 (1.31 kcal mol 1), while the Br + CH3OH ? CH2OH + HBr channel is endothermic by 3228 cm 1 (9.23 kcal mol 1). The gas-phase enthalpy data were taken from the NASA/JPL databook [16]. Neither experimental nor theoretical work has been reported on the nonadiabatic chemical reaction channel of Br* + CH3OH ? CH2OH + HBr. Taketani et al. [11] reported the experimental study on the branching ratio between the non-adiabatic chemical reaction and physical quenching channels for Cl* + CH3OH system, using the PLP/VUV-LIF technique with Cl*/Cl detection. 4. Conclusion In this Letter, the PLP/VUV-LIF technique has been applied to kinetic measurements of the collisional removal of Br* by small molecules for the first time. The results have been discussed in terms of a correlation between the rate coefficients and the energy deficit. It has been suggested that further experimental and theoretical investigations on E–R, T energy transfer processes and non-adiabatic chemical reaction channels as well as E–V energy transfer processes are required to come to complete understanding of the chemical properties of halogen atoms. The experimental technique reported here will be a powerful tool to study the non-adiabatic
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chemical reaction channels in the collisional decay of Br*, as it had successfully revealed the branching ratios between the nonadiabatic chemical reaction and physical quenching channels in Cl* + small hydrocarbons. Acknowledgement This study was partly supported by Program for Improvement of Research Environment for Young Researchers from Special Coordination Funds for Promoting Science and Technology (SCF) commissioned by the MEXT of Japan (K.T.). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
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