- Email: [email protected]
Studies on photodissociation of alkyl bromides at 234 and 267 nm
Chemical Physics Letters 392 (2004) 493–497 www.elsevier.com/locate/cplett
Studies on photodissociation of alkyl bromides at 234 and 267 nm Ying Tang, Lei Ji, Bifeng Tang, Rongshu Zhu, Song Zhang, Bing Zhang
*
State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, PR China Graduate School of Chinese Academy of Sciences, Beijing, PR China Received 11 March 2004; in final form 27 May 2004 Available online 19 June 2004
Abstract We studied the photodissociation dynamics of alkyl bromides (CHBr3 , CH2 Br2 , C2 H5 Br, C2 H4 Br2 ) near 234 and 267 nm using resonance-enhanced multiphoton ionization (REMPI) with time-of-flight mass spectrometer. After photodissociation of alkyl bromides, bromine fragments of Br (2 P01=2 ) (denoted Br*) and Br (2 P03=2 ) (denoted Br) released. Branching ratios of NðBr Þ=N ðBrÞ were determined, and a possible photodissociation mechanism of alkyl bromides was proposed. Additionally, the REMPI spectrum of bromine atoms has been recorded between 231 and 268 nm, 42 transition lines were found and assigned, and most of them coincided with the previous results except 12 atomic transitions that were new. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction The chemical characters of bromine have recently received considerable attention because of their importance to stratosphere ozone destruction [1,2]. Gordon and coworkers [3] studied the 4p–5p transitions of bromine by resonance-enhanced multiphoton ionization (REMPI) between 250 and 285 nm, and observed 21 atomic transitions; Chen et al. [4] observed other four transitions in the same range. Additionally, Park et al. [5] observed 13 REMPI lines of bromine in the region of 232–236 nm when studying the photodissociation of CCl3 Br. In this Letter, we report two-photon resonant three-photon ionization spectrum of bromine generated by photodissociation of CH2 Br2 in the region of 231– 268 nm, in which 42 atomic transitions were observed, and most of them coincide with the results in [3–5] except 12 atomic transitions that are new. Under the UV laser radiation, bromine atoms from photodissociation of alkyl bromides are Br (2 P03=2 ) state or Br (2 P01=2 ) state. Many works have been completed on branching ratio N ðBr Þ=N ðBrÞ: For CBrCl3 [6] and Br2 [7] at 234 and 265 nm, CHBr3 at 267 nm, [8] CH2 Br2 *
Corresponding author. Fax: +86-27-87199291. E-mail address: [email protected] (B. Zhang).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.06.003
cations at 355 nm, [9] CH3 Br in the region of 218–245 nm [10], and for C3 H5 Br under 234 nm laser radiation [11]. The branching ratios of halogen atoms from other alkyl halides such as alkyl chlorides, alkyl iodides [12–14] etc. have also been studied in many other works. In this Letter, we report the branching ratios of alkyl bromides CHBr3 , CH2 Br2 , C2 H5 Br, and C2 H4 Br2 at 234 and 267 nm. In addition, we recorded the power dependencies of Br and Br signals which were generated from the photodissociation of the alkyl bromide.
2. Experimental The apparatus consisted of two stainless-steel chambers that had been pumped with two mechanical pumps and two diffusion pumps. The ionization chamber housed a piezoelectric pulsed valve with a 0.2 mm nozzle and three electrode plates: a repeller, an accelerator, and a ground plate. The background pressure was about 5.0 104 Pa during the experiment. The samples of alkyl bromides CHBr3 , CH2 Br2 , C2 H5 Br, C2 H4 Br2 (99.5% purity) in the experiment were used without further purification. The liquid samples seeded in helium were introduced into the ionization chamber as a supersonic jet through a pulsed nozzle.
Y. Tang et al. / Chemical Physics Letters 392 (2004) 493–497
3. Results and discussion 3.1. Time-of-flight mass spectra and power dependencies of signals
0.5
Log Signal (arb. unit)
UV light was obtained from a dye laser [ScanMate 2E OG, Lambda Physik] pumped by a frequency-tripled light of an Nd: YAG laser [YG981E10, Quantel] operating at 10 Hz. The output of the dye laser was doubled by BBO crystal and the energy of the doubled light was about 0.8 mJ/pulse. The laser dye coumarin 307(LC 5000) [Lambda Physik] was used to cover the wavelength region of 485–546 nm with the peak at 508 nm, and coumarin 102(LC 4800) [Lambda Physik] to cover 462–497 nm with the peak at 480 nm. After focused by a lens with a focal length of 18 cm, the laser beam entered into the ionization chamber in a direction that was perpendicular to the molecular beam. All wavelengths noted in this Letter were optogalvanic (OG) calibrated by the atomic resonance lines of Ne. Positive high voltages were added to the repeller and the accelerator to collect the total ion signals. After flying through a 50 cm field-free flight tube, the ions were detected by dual microchannel plate (MCP). The signals were then sent to a boxcar integrator and a digital oscilloscope [100 MHz] that were interfaced with a computer.
(a) Br Slope =2.98
0.0
-0.5
2.2
0.5
2.4
2.5
2.6
2.4
2.5
(b) Br* Slope =3.14
0.0
-0.5
-1.0 2.1
The TOF mass spectra of CHBr3 were recorded at the resonance wavelengths separately for Br (2 P01=2 ) and Br (2 P03=2 ), and the TOF spectra at the off-resonance wavelengths were also recorded. These spectra were all achieved under the same conditions except the wavelengths, hence the intensities of the photofragments signals except Brþ were found comparable. In the offresonance TOF spectrum, the double peaks of Brþ arose from the two isotopes of Br, Br79 and Br81 , which appeared to have roughly equal natural abundances. However, in the on-resonance TOF mass spectra, the intensity of Brþ signal was much stronger, but we could not distinguish the double peaks. It was probably due to space charge effects, in which too many ions were formed in the small volume of the reaction region. For other alkyl bromides, there were many similarities with CHBr3 in the TOF spectra. The intensities of fragments ions from the photodissociation of parent ions were found proportional to the intensity of the parent ions [15]. In our experiment, the parent ion signal was negligible compared to Brþ signal off-resonance, and we considered that Brþ was not from the dissociation of the parent ions, but from the dissociations of the neutral parent molecules and then photoionized. In order to make clear of the photolysis mechanism of the alkyl bromides, we studied the power dependencies of signals intensities for transitions corresponding to Br and Br ,
2.3
Log Fluence (arb. unit.)
Log Signal (arb. unit)
494
2.2
2.3
Log Fluence (arb. unit.) Fig. 1. Dependence of Br (2 P03=2 ) and Br (2 P01=2 ) signal as a function of laser energy. The dots are the data of experiments and the solid lines are linear fits. The wavelengths are 233.618 and 233.951 nm for the data shown in the (a) and (b) panels, respectively.
which are shown in Fig. 1. The signals were linear in a log–log plot over the range of powers used in the present study. The lines exhibit a slope of about 3.0 which is consistent with one-photon dissociation followed by 2 + 1 REMPI of the bromine atoms, assuming that the ionization step is saturated. 3.2. REMPI spectra of bromine atoms In alkyl bromides photolysis, two possible dissociation channels exist in the UV wavelength region. Take CH2 Br2 for an example, CH2 Br2 þ hm ! CH2 Br þ Brð2 P01=2 Þ
ð1Þ
P03=2 Þ
ð2Þ
! CH2 Br þ Brð
2
The ionization potential of bromine atoms is 11.8 eV [16], so at least three UV photons are necessary to reach the ionization threshold. Absorbing photons continuously, two-photon resonant three-photon ionization took place. Scanning the laser wavelength, we recorded
Y. Tang et al. / Chemical Physics Letters 392 (2004) 493–497
495
ments, what we were interested in was the branching ratio of Br and Br*: N ðBr Þ=N ðBrÞ, where N(Br ) and N(Br) are the numbers of bromine atoms Br (2 P01=2 ) and Br (2 P03=2 ) resulting from the photodissociation of alkyl bromides. The ratio of ion intensities between Br* and Br measured in the TOF mass spectra is proportional to the branching ratio between Br* and Br atoms by [11] N ðBr Þ SðBr Þ ¼k ; N ðBrÞ SðBrÞ
230
240
250
260
270
Wavelength / nm Fig. 2. REMPI spectrum of Br atoms generated from photodissociation of CH2 Br2 at wavelength between 231 and 268 nm.
the REMPI spectrum of bromine. The REMPI spectrum of Br atoms between 231 and 268 nm is shown in Fig. 2. Based on Moore’s work [17], we assigned all the transitions observed in our experiment (See Table 1). In the range of 250–268 nm, 18 two-photon transition lines were observed in our work, 12 of which arose from the ground state of bromine atom Br (4p2 P03=2 ), and the other six from the first excited state of bromine atom Br (4p2 P01=2 ). All the 18 transition lines observed were excited from Br (4p2 P03=2 ) or Br (4p2 P01=2 ) to the upper states 5p. The spectrum of 250–268 nm that we recorded is quite consistent with the previous result [4]. In the range of 231–250 nm, 24 two-photon transition lines were observed, 19 of which were excited from Br (4p2 P01=2 ), and the other five from Br (4p2 P03=2 ). Because of the stronger energy of photons, the transition lines were separately excited to upper states such as 5p, 6p, 7p etc. Comparing with the previous results in the region of 232–236 nm [5], we observed 15 transitions lines, of which 12 were quite consistent with [5], whereas one missed and three were newly found. In summary, 42 transition lines were found and assigned between 231 and 268 nm, and most of them coincided with the previous results except 12 that were new. From Table 1, it could be noted that the transition lines at 233.618 and 233.951 nm were corresponding to the transitions with the lower states Br (2 P03=2 ) and Br (2 P01=2 ). Additionally, the transitions lines at 266.551 and 266.613 nm were corresponding to the transitions with the lower states Br (2 P03=2 ) and Br (2 P01=2 ). We chose them to study the branching ratio of Br (2 P03=2 ) and Br (2 P01=2 ). 3.3. Branching ratio and dissociation dynamics As Eqs. (1) and (2) show, both Br (2 P01=2 ) and Br are the fragments of photolysis. In our experi-
(2 P03=2 )
where SðBr Þ and SðBrÞ refer to the intensities of Brþ that generated separately from the ionization of the Br and Br atoms, and they are achieved by integrating the ion signal intensities over the proper range containing the Doppler width and the probe laser bandwidth. The factor k is determined by the REMPI probability ratios, relative detection efficiencies, and other instrumentation factors. It is obtained by performing a calibration experiment of Br2 photolysis under the same conditions. Because the branching ratio N ðBr Þ=N ðBrÞ is unity, k becomes (SðBr Þ=SðBrÞ)1 [7], which was determined to be 0.349 at 234 nm and 0.578 at 267 nm in our experiments. All branching ratios of the alkyl bromides CHBr3 , CH2 Br2 , C2 H5 Br, C2 H4 Br2 under the 234 and 267 nm laser radiation are shown in Table 2, and we noted that all branching ratios at 267 nm are obviously larger than the results at 234 nm. As Fig. 3 shows, with high symmetry of C3v , the alkyl bromides are excited from the ground state to three dissociative excited states that are assigned as 1 Q1 (E), 3 Q0 (A1 ) and 3 Q1 (E), with curve crossing between 1 Q1 (E) and 3 Q0 (A1 ) [6]. The 1 Q1 (E) and 3 Q1 (E) states correlate with the Br production, while the 3 Q0 (A1 ) state is corresponding to the Br production. As the symmetry debases, such as Cs symmetry, the degeneration disappears, and energy levels split. The previous three-channel dissociation turns into a fivechannel dissociation, which, however, will not influence the curve crossing between the 1 Q1 (E) and 3 Q0 (A1 ) states. Taking C2 H5 Br for example, the energy levels split because of Cs symmetry [18]. It leads to the five-channel photodissociation, but does not influence the curve crossing. On one hand, the energy of one photon at 234 nm is larger than that at 267 nm, hence alkyl bromides are mostly excited to the 1 Q1 (E) state under the 234 nm laser radiation, and then dissociate directly to Br. The sources of Br are as follows: (1) alkyl bromides are excited to the 1 Q1 (E) state, partially coupled to the 3 Q0 (A1 ) state and dissociate; (2) excited directly to the 3 Q0 (A1 ) and dissociate. The sources of Br are: (1) directly excited to the 1 Q1 (E) state and dissociate; (2) excited to the 3 Q0 (A1 ) state and partially coupled to the 1 Q1 (E) state and then dissociate; (3) only a few are
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Y. Tang et al. / Chemical Physics Letters 392 (2004) 493–497
Table 1 Br atoms two-photon transitions at wavelength of 231–268 nm
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 30 31 32 33 34 35 36 37 38 39 40 41 42
Wavelength/nm
mexp ð2mÞ/cm1
DS
DL
DJ
Assignment
231.76 231.87 232.19 232.24 232.49 232.60 232.83 232.93 232.98 233.15 233.62 233.95 234.43 234.68 235.17 235.81 235.94 238.57 239.80 242.97 243.32 243.38 244.12 244.29 250.33 250.88 252.55 253.53 254.14 254.71 256.10 260.54 262.47 263.05 263.73 264.14 264.77 264.86 265.96 266.55 266.61 267.21
86295.78 86254.66 86137.65 86117.55 86023.70 85984.61 85898.10 85863.23 85842.68 85782.07 85610.01 85488.00 85314.61 85221.53 85044.70 84813.69 84768.22 83833.80 83403.21 82315.58 82196.49 82175.98 81926.00 81870.66 79893.58 79719.07 79191.61 78886.44 78695.54 78520.98 78093.27 76763.94 76200.35 76031.17 75836.00 75718.86 75537.26 75511.31 75200.41 75032.55 75015.10 74846.66
1 0 0 1 0 1 1 1 1 1 1 0 1 0 0 0 0 0 0 1 1 1 1 1 0 0 0 0 1 0 1 1 0 0 1 1 1 0 0 1 1 0
0 0 1 0 0 1 0 1 0 1 0 )1 )1 1 2 0 0 0 0 1 1 0 0 0 )1 1 0 0 )1 1 1 1 )1 1 0 1 1 0 0 0 )1 0
0 1 2 1 0 0 2 1 )1 2 0 0 1 2 2 0 1 0 1 1 2 0 1 2 )1 0 0 )1 1 1 )1 0 0 1 )1 1 2 1 0 0 2 2
2
excited to the 3 Q1 (E) state and dissociate. On the other hand, alkyl bromides are mostly excited to 3 Q0 (A1 ) state and then dissociate to Br directly. At that time, the dominating origins of Br are: (1) excited to 3 Q0 (A1 ) state directly and dissociate; (2) Table 2 Branching ratios of Br atoms dissociated from alkyl bromides at 234 and 267 nm
CH2 Br2 CHBr3 C2 H5 Br C2 H4 Br2
234 (nm)
267 (nm)
0.312 0.207 0.205 0.160
0.399 0.435 0.459 0.213
P01=2 –7p4 P01=2 P01=2 –5p002 P03=2 2 0 P1=2 –5p0 2 D05=2 2 0 P1=2 –7p4 P03=2 2 0 P1=2 –5p002 P01=2 2 0 P3=2 –6p4 D03=2 2 0 P1=2 –7p4 P05=2 2 0 P3=2 –6p4 D05=2 2 0 P3=2 –6p4 P01=2 2 0 P3=2 –6p4 D07=2 2 0 P3=2 –6p4 P03=2 2 0 P1=2 –6p2 S01=2 2 0 P1=2 –6p4 S03=2 2 0 P1=2 –6p2 D03=2 2 0 P1=2 –5p02 F05=2 2 0 P1=2 –5p0 2 P01=2 2 0 P1=2 –5p0 2 P03=2 2 0 P1=2 –6p2 P01=2 2 0 P1=2 –6p2 P03=2 2 0 P1=2 –6p4 D03=2 2 0 P1=2 –6p4 D05=2 2 0 P1=2 –6p4 P01=2 2 0 P1=2 –6p4 P03=2 2 0 P1=2 –6p4 P05=2 2 0 P3=2 –5p2 S01=2 2 0 P3=2 –5p2 D03=2 2 0 P3=2 –5p2 P03=2 2 0 P3=2 –5p2 P01=2 2 0 P3=2 –5p4 S03=2 2 0 P3=2 –5p2 D05=2 2 0 P3=2 –5p4 D01=2 2 0 P3=2 –5p4 D03=2 2 0 P1=2 –5p2 S01=2 2 0 P1=2 –5p2 D03=2 2 0 P3=2 –5p4 P01=2 2 0 P3=2 –5p4 D05=2 2 0 P3=2 –5p4 D07=2 2 0 P1=2 –5p2 P03=2 2 0 P1=2 –5p2 P01=2 2 0 P3=2 –5p4 P03=2 2 0 P1=2 –5p4 S03=2 2 0 P1=2 –5p2 D05=2 2
partially coupled to the 3 Q0 (A1 ) state after excited to the 1 Q1 (E) state and dissociate. The leading origins of Br are as follows: (1) partially coupled to the 1 Q1 (E) state after excited to the 3 Q0 (A1 ) state and dissociate; (2) partially excited to the 1 Q1 (E) state and 3 Q1 (E) state directly, then dissociate. In general, under the UV laser radiation of 234 nm, the quantity of alkyl bromides generating Br is more than that at 267 nm, while the quantity of bromides producing Br is less. For this reason, the branching ratio at 267 nm is greater than that at 234 nm. For other alkyl bromides in the experiment, there are many similarities with C2 H5 Br in the photodissociation mechanism.
Y. Tang et al. / Chemical Physics Letters 392 (2004) 493–497
C 3v symmetry
under the 267 nm laser radiation. However, whether at 234 nm or at 267 nm, the in equation N(Br )/N(Br) <1 always existed. It is proved that the bromine atoms from the photodissociation of alkyl bromides mostly existed in the ground state Br (2 P03=2 ).
C s symmetry
1 Q 1 (E)
3 Q 0 (A1)
497
Crossing
Acknowledgements
3 Q 1 (E) Br*
All the authors gratefully acknowledge support from National Natural Science Foundation of China (299730397).
Br
References
Ground State Fig. 3. Dissociation channels of alkyl bromides at the reaction of UV laser.
4. Conclusion Under the UV laser radiation of 234 and 267 nm, alkyl bromides of CHBr3 , CH2 Br2 , C2 H5 Br, C2 H4 Br2 dissociated and released bromine atoms. (2 + 1) REMPI of bromine atoms took place after absorbing photons continuously. Scanning the UV laser wavelength, we recorded the REMPI spectrum of bromine atoms in the region of 231–268 nm, in which 42 transition lines were observed and assigned, and most of them coincided with the previous results except 12 that were new. Bromine atoms produced from photolysis of alkyl bromides under UV laser radiation are Br (2 P03=2 ) state or Br (2 P01=2 ) state. Studying the branching ratio of them, we found that all branching ratios at 267 nm were obviously larger than those at 234 nm, because there exist three dissociation channels from higher to lower: 1 Q1 (E), 3 Q0 (A1 ), 3 Q1 (E), with a curve crossing between 1 Q1 (E) and 3 Q0 (A1 ). Under the 234 nm laser radiation, much more alkyl bromides molecules were excited to the 1 Q1 (E) state and dissociated to get Br (2 P03=2 ), and less
[1] S.C. Wofsy, M.B. McElroy, Y.L. Yung, Geophys. Res. Lett. 2 (1975) 215. [2] Y.L. Yung, J.P. Pinto, R.J. Watson, S.P. Sander, J. Atmos. Sci. 37 (1980) 339. [3] S. Arepalli, N. Presser, D. Robie, R.J. Gordon, Chem. Phys. Lett. 117 (1985) 64. [4] Y. Chen, L.S. Pei, J. Jin, Q. Ran, C.X. Chen, S.Q. Yu, X.X. Ma, Acta Phys. Sin. (Overseas Edition) 7 (1999) 490. [5] M.S. Park, Y.J. Jung, S.H. Lee, D.C. Kim, K.H. Jung, Chem. Phys. Lett. 322 (2000) 429. [6] Y.J. Jung, M.S. Park, Y.S. Kim, K.H. Jung, J. Chem. Phys. 111 (1999) 4005. [7] Y.J. Jee, Y.J. Jung, K.H. Jung, J. Chem. Phys. 115 (2001) 9739. [8] D.D. Xu, J.S. Francisco, J.H. Huang, W.M. Jackson, J. Chem. Phys. 117 (2002) 2578. [9] J.H. Huang, D.D. Xu, W.H. Fink, W.M. Jackson, J. Chem. Phys. 115 (2001) 6012. [10] T. Gougousi, P.C. Samartzis, T.N. Kitsopoulos, J. Chem. Phys. 108 (1998) 5742. [11] M.S. Park, K.W. Lee, K.H. Jung, J. Chem. Phys. 114 (2001) 10368. [12] W.K. Kang, K.W. Jung, D.C. Kim, K.H. Jung, J. Chem. Phys. 104 (1996) 5815. [13] H.F. Xu, Y. Guo, S.L. Liu, X.X. Ma, J. Chem. Phys. 117 (2002) 5722. [14] W.K. Kang, K.W. Jung, K.H. Jung, J. Phys. Chem. 98 (1994) 1525. [15] B.D. Koptlitz, J.K. McVey, J. Chem. Phys. 11 (1984) 81. [16] W.D. Zhou, L.S. Sheng, G.H. Wu, H. Gao, F. Qi, Y.W. Zhang, Acta Phys.-Chim. Sin. 15 (1999) 948. [17] Charlotte E. Moore. Atomic Energy Levels, vol. I, NSRDS-NBS 35, 1971. [18] W.S. McGivern, R.J. Li, P. Zou, S.W. North, J. Chem. Phys. 111 (1999) 5771.
Studies on photodissociation of alkyl bromides at 234 and 267 nm Ying Tang, Lei Ji, Bifeng Tang, Rongshu Zhu, Song Zhang, Bing Zhang
*
State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, PR China Graduate School of Chinese Academy of Sciences, Beijing, PR China Received 11 March 2004; in final form 27 May 2004 Available online 19 June 2004
Abstract We studied the photodissociation dynamics of alkyl bromides (CHBr3 , CH2 Br2 , C2 H5 Br, C2 H4 Br2 ) near 234 and 267 nm using resonance-enhanced multiphoton ionization (REMPI) with time-of-flight mass spectrometer. After photodissociation of alkyl bromides, bromine fragments of Br (2 P01=2 ) (denoted Br*) and Br (2 P03=2 ) (denoted Br) released. Branching ratios of NðBr Þ=N ðBrÞ were determined, and a possible photodissociation mechanism of alkyl bromides was proposed. Additionally, the REMPI spectrum of bromine atoms has been recorded between 231 and 268 nm, 42 transition lines were found and assigned, and most of them coincided with the previous results except 12 atomic transitions that were new. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction The chemical characters of bromine have recently received considerable attention because of their importance to stratosphere ozone destruction [1,2]. Gordon and coworkers [3] studied the 4p–5p transitions of bromine by resonance-enhanced multiphoton ionization (REMPI) between 250 and 285 nm, and observed 21 atomic transitions; Chen et al. [4] observed other four transitions in the same range. Additionally, Park et al. [5] observed 13 REMPI lines of bromine in the region of 232–236 nm when studying the photodissociation of CCl3 Br. In this Letter, we report two-photon resonant three-photon ionization spectrum of bromine generated by photodissociation of CH2 Br2 in the region of 231– 268 nm, in which 42 atomic transitions were observed, and most of them coincide with the results in [3–5] except 12 atomic transitions that are new. Under the UV laser radiation, bromine atoms from photodissociation of alkyl bromides are Br (2 P03=2 ) state or Br (2 P01=2 ) state. Many works have been completed on branching ratio N ðBr Þ=N ðBrÞ: For CBrCl3 [6] and Br2 [7] at 234 and 265 nm, CHBr3 at 267 nm, [8] CH2 Br2 *
Corresponding author. Fax: +86-27-87199291. E-mail address: [email protected] (B. Zhang).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.06.003
cations at 355 nm, [9] CH3 Br in the region of 218–245 nm [10], and for C3 H5 Br under 234 nm laser radiation [11]. The branching ratios of halogen atoms from other alkyl halides such as alkyl chlorides, alkyl iodides [12–14] etc. have also been studied in many other works. In this Letter, we report the branching ratios of alkyl bromides CHBr3 , CH2 Br2 , C2 H5 Br, and C2 H4 Br2 at 234 and 267 nm. In addition, we recorded the power dependencies of Br and Br signals which were generated from the photodissociation of the alkyl bromide.
2. Experimental The apparatus consisted of two stainless-steel chambers that had been pumped with two mechanical pumps and two diffusion pumps. The ionization chamber housed a piezoelectric pulsed valve with a 0.2 mm nozzle and three electrode plates: a repeller, an accelerator, and a ground plate. The background pressure was about 5.0 104 Pa during the experiment. The samples of alkyl bromides CHBr3 , CH2 Br2 , C2 H5 Br, C2 H4 Br2 (99.5% purity) in the experiment were used without further purification. The liquid samples seeded in helium were introduced into the ionization chamber as a supersonic jet through a pulsed nozzle.
Y. Tang et al. / Chemical Physics Letters 392 (2004) 493–497
3. Results and discussion 3.1. Time-of-flight mass spectra and power dependencies of signals
0.5
Log Signal (arb. unit)
UV light was obtained from a dye laser [ScanMate 2E OG, Lambda Physik] pumped by a frequency-tripled light of an Nd: YAG laser [YG981E10, Quantel] operating at 10 Hz. The output of the dye laser was doubled by BBO crystal and the energy of the doubled light was about 0.8 mJ/pulse. The laser dye coumarin 307(LC 5000) [Lambda Physik] was used to cover the wavelength region of 485–546 nm with the peak at 508 nm, and coumarin 102(LC 4800) [Lambda Physik] to cover 462–497 nm with the peak at 480 nm. After focused by a lens with a focal length of 18 cm, the laser beam entered into the ionization chamber in a direction that was perpendicular to the molecular beam. All wavelengths noted in this Letter were optogalvanic (OG) calibrated by the atomic resonance lines of Ne. Positive high voltages were added to the repeller and the accelerator to collect the total ion signals. After flying through a 50 cm field-free flight tube, the ions were detected by dual microchannel plate (MCP). The signals were then sent to a boxcar integrator and a digital oscilloscope [100 MHz] that were interfaced with a computer.
(a) Br Slope =2.98
0.0
-0.5
2.2
0.5
2.4
2.5
2.6
2.4
2.5
(b) Br* Slope =3.14
0.0
-0.5
-1.0 2.1
The TOF mass spectra of CHBr3 were recorded at the resonance wavelengths separately for Br (2 P01=2 ) and Br (2 P03=2 ), and the TOF spectra at the off-resonance wavelengths were also recorded. These spectra were all achieved under the same conditions except the wavelengths, hence the intensities of the photofragments signals except Brþ were found comparable. In the offresonance TOF spectrum, the double peaks of Brþ arose from the two isotopes of Br, Br79 and Br81 , which appeared to have roughly equal natural abundances. However, in the on-resonance TOF mass spectra, the intensity of Brþ signal was much stronger, but we could not distinguish the double peaks. It was probably due to space charge effects, in which too many ions were formed in the small volume of the reaction region. For other alkyl bromides, there were many similarities with CHBr3 in the TOF spectra. The intensities of fragments ions from the photodissociation of parent ions were found proportional to the intensity of the parent ions [15]. In our experiment, the parent ion signal was negligible compared to Brþ signal off-resonance, and we considered that Brþ was not from the dissociation of the parent ions, but from the dissociations of the neutral parent molecules and then photoionized. In order to make clear of the photolysis mechanism of the alkyl bromides, we studied the power dependencies of signals intensities for transitions corresponding to Br and Br ,
2.3
Log Fluence (arb. unit.)
Log Signal (arb. unit)
494
2.2
2.3
Log Fluence (arb. unit.) Fig. 1. Dependence of Br (2 P03=2 ) and Br (2 P01=2 ) signal as a function of laser energy. The dots are the data of experiments and the solid lines are linear fits. The wavelengths are 233.618 and 233.951 nm for the data shown in the (a) and (b) panels, respectively.
which are shown in Fig. 1. The signals were linear in a log–log plot over the range of powers used in the present study. The lines exhibit a slope of about 3.0 which is consistent with one-photon dissociation followed by 2 + 1 REMPI of the bromine atoms, assuming that the ionization step is saturated. 3.2. REMPI spectra of bromine atoms In alkyl bromides photolysis, two possible dissociation channels exist in the UV wavelength region. Take CH2 Br2 for an example, CH2 Br2 þ hm ! CH2 Br þ Brð2 P01=2 Þ
ð1Þ
P03=2 Þ
ð2Þ
! CH2 Br þ Brð
2
The ionization potential of bromine atoms is 11.8 eV [16], so at least three UV photons are necessary to reach the ionization threshold. Absorbing photons continuously, two-photon resonant three-photon ionization took place. Scanning the laser wavelength, we recorded
Y. Tang et al. / Chemical Physics Letters 392 (2004) 493–497
495
ments, what we were interested in was the branching ratio of Br and Br*: N ðBr Þ=N ðBrÞ, where N(Br ) and N(Br) are the numbers of bromine atoms Br (2 P01=2 ) and Br (2 P03=2 ) resulting from the photodissociation of alkyl bromides. The ratio of ion intensities between Br* and Br measured in the TOF mass spectra is proportional to the branching ratio between Br* and Br atoms by [11] N ðBr Þ SðBr Þ ¼k ; N ðBrÞ SðBrÞ
230
240
250
260
270
Wavelength / nm Fig. 2. REMPI spectrum of Br atoms generated from photodissociation of CH2 Br2 at wavelength between 231 and 268 nm.
the REMPI spectrum of bromine. The REMPI spectrum of Br atoms between 231 and 268 nm is shown in Fig. 2. Based on Moore’s work [17], we assigned all the transitions observed in our experiment (See Table 1). In the range of 250–268 nm, 18 two-photon transition lines were observed in our work, 12 of which arose from the ground state of bromine atom Br (4p2 P03=2 ), and the other six from the first excited state of bromine atom Br (4p2 P01=2 ). All the 18 transition lines observed were excited from Br (4p2 P03=2 ) or Br (4p2 P01=2 ) to the upper states 5p. The spectrum of 250–268 nm that we recorded is quite consistent with the previous result [4]. In the range of 231–250 nm, 24 two-photon transition lines were observed, 19 of which were excited from Br (4p2 P01=2 ), and the other five from Br (4p2 P03=2 ). Because of the stronger energy of photons, the transition lines were separately excited to upper states such as 5p, 6p, 7p etc. Comparing with the previous results in the region of 232–236 nm [5], we observed 15 transitions lines, of which 12 were quite consistent with [5], whereas one missed and three were newly found. In summary, 42 transition lines were found and assigned between 231 and 268 nm, and most of them coincided with the previous results except 12 that were new. From Table 1, it could be noted that the transition lines at 233.618 and 233.951 nm were corresponding to the transitions with the lower states Br (2 P03=2 ) and Br (2 P01=2 ). Additionally, the transitions lines at 266.551 and 266.613 nm were corresponding to the transitions with the lower states Br (2 P03=2 ) and Br (2 P01=2 ). We chose them to study the branching ratio of Br (2 P03=2 ) and Br (2 P01=2 ). 3.3. Branching ratio and dissociation dynamics As Eqs. (1) and (2) show, both Br (2 P01=2 ) and Br are the fragments of photolysis. In our experi-
(2 P03=2 )
where SðBr Þ and SðBrÞ refer to the intensities of Brþ that generated separately from the ionization of the Br and Br atoms, and they are achieved by integrating the ion signal intensities over the proper range containing the Doppler width and the probe laser bandwidth. The factor k is determined by the REMPI probability ratios, relative detection efficiencies, and other instrumentation factors. It is obtained by performing a calibration experiment of Br2 photolysis under the same conditions. Because the branching ratio N ðBr Þ=N ðBrÞ is unity, k becomes (SðBr Þ=SðBrÞ)1 [7], which was determined to be 0.349 at 234 nm and 0.578 at 267 nm in our experiments. All branching ratios of the alkyl bromides CHBr3 , CH2 Br2 , C2 H5 Br, C2 H4 Br2 under the 234 and 267 nm laser radiation are shown in Table 2, and we noted that all branching ratios at 267 nm are obviously larger than the results at 234 nm. As Fig. 3 shows, with high symmetry of C3v , the alkyl bromides are excited from the ground state to three dissociative excited states that are assigned as 1 Q1 (E), 3 Q0 (A1 ) and 3 Q1 (E), with curve crossing between 1 Q1 (E) and 3 Q0 (A1 ) [6]. The 1 Q1 (E) and 3 Q1 (E) states correlate with the Br production, while the 3 Q0 (A1 ) state is corresponding to the Br production. As the symmetry debases, such as Cs symmetry, the degeneration disappears, and energy levels split. The previous three-channel dissociation turns into a fivechannel dissociation, which, however, will not influence the curve crossing between the 1 Q1 (E) and 3 Q0 (A1 ) states. Taking C2 H5 Br for example, the energy levels split because of Cs symmetry [18]. It leads to the five-channel photodissociation, but does not influence the curve crossing. On one hand, the energy of one photon at 234 nm is larger than that at 267 nm, hence alkyl bromides are mostly excited to the 1 Q1 (E) state under the 234 nm laser radiation, and then dissociate directly to Br. The sources of Br are as follows: (1) alkyl bromides are excited to the 1 Q1 (E) state, partially coupled to the 3 Q0 (A1 ) state and dissociate; (2) excited directly to the 3 Q0 (A1 ) and dissociate. The sources of Br are: (1) directly excited to the 1 Q1 (E) state and dissociate; (2) excited to the 3 Q0 (A1 ) state and partially coupled to the 1 Q1 (E) state and then dissociate; (3) only a few are
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Table 1 Br atoms two-photon transitions at wavelength of 231–268 nm
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 30 31 32 33 34 35 36 37 38 39 40 41 42
Wavelength/nm
mexp ð2mÞ/cm1
DS
DL
DJ
Assignment
231.76 231.87 232.19 232.24 232.49 232.60 232.83 232.93 232.98 233.15 233.62 233.95 234.43 234.68 235.17 235.81 235.94 238.57 239.80 242.97 243.32 243.38 244.12 244.29 250.33 250.88 252.55 253.53 254.14 254.71 256.10 260.54 262.47 263.05 263.73 264.14 264.77 264.86 265.96 266.55 266.61 267.21
86295.78 86254.66 86137.65 86117.55 86023.70 85984.61 85898.10 85863.23 85842.68 85782.07 85610.01 85488.00 85314.61 85221.53 85044.70 84813.69 84768.22 83833.80 83403.21 82315.58 82196.49 82175.98 81926.00 81870.66 79893.58 79719.07 79191.61 78886.44 78695.54 78520.98 78093.27 76763.94 76200.35 76031.17 75836.00 75718.86 75537.26 75511.31 75200.41 75032.55 75015.10 74846.66
1 0 0 1 0 1 1 1 1 1 1 0 1 0 0 0 0 0 0 1 1 1 1 1 0 0 0 0 1 0 1 1 0 0 1 1 1 0 0 1 1 0
0 0 1 0 0 1 0 1 0 1 0 )1 )1 1 2 0 0 0 0 1 1 0 0 0 )1 1 0 0 )1 1 1 1 )1 1 0 1 1 0 0 0 )1 0
0 1 2 1 0 0 2 1 )1 2 0 0 1 2 2 0 1 0 1 1 2 0 1 2 )1 0 0 )1 1 1 )1 0 0 1 )1 1 2 1 0 0 2 2
2
excited to the 3 Q1 (E) state and dissociate. On the other hand, alkyl bromides are mostly excited to 3 Q0 (A1 ) state and then dissociate to Br directly. At that time, the dominating origins of Br are: (1) excited to 3 Q0 (A1 ) state directly and dissociate; (2) Table 2 Branching ratios of Br atoms dissociated from alkyl bromides at 234 and 267 nm
CH2 Br2 CHBr3 C2 H5 Br C2 H4 Br2
234 (nm)
267 (nm)
0.312 0.207 0.205 0.160
0.399 0.435 0.459 0.213
P01=2 –7p4 P01=2 P01=2 –5p002 P03=2 2 0 P1=2 –5p0 2 D05=2 2 0 P1=2 –7p4 P03=2 2 0 P1=2 –5p002 P01=2 2 0 P3=2 –6p4 D03=2 2 0 P1=2 –7p4 P05=2 2 0 P3=2 –6p4 D05=2 2 0 P3=2 –6p4 P01=2 2 0 P3=2 –6p4 D07=2 2 0 P3=2 –6p4 P03=2 2 0 P1=2 –6p2 S01=2 2 0 P1=2 –6p4 S03=2 2 0 P1=2 –6p2 D03=2 2 0 P1=2 –5p02 F05=2 2 0 P1=2 –5p0 2 P01=2 2 0 P1=2 –5p0 2 P03=2 2 0 P1=2 –6p2 P01=2 2 0 P1=2 –6p2 P03=2 2 0 P1=2 –6p4 D03=2 2 0 P1=2 –6p4 D05=2 2 0 P1=2 –6p4 P01=2 2 0 P1=2 –6p4 P03=2 2 0 P1=2 –6p4 P05=2 2 0 P3=2 –5p2 S01=2 2 0 P3=2 –5p2 D03=2 2 0 P3=2 –5p2 P03=2 2 0 P3=2 –5p2 P01=2 2 0 P3=2 –5p4 S03=2 2 0 P3=2 –5p2 D05=2 2 0 P3=2 –5p4 D01=2 2 0 P3=2 –5p4 D03=2 2 0 P1=2 –5p2 S01=2 2 0 P1=2 –5p2 D03=2 2 0 P3=2 –5p4 P01=2 2 0 P3=2 –5p4 D05=2 2 0 P3=2 –5p4 D07=2 2 0 P1=2 –5p2 P03=2 2 0 P1=2 –5p2 P01=2 2 0 P3=2 –5p4 P03=2 2 0 P1=2 –5p4 S03=2 2 0 P1=2 –5p2 D05=2 2
partially coupled to the 3 Q0 (A1 ) state after excited to the 1 Q1 (E) state and dissociate. The leading origins of Br are as follows: (1) partially coupled to the 1 Q1 (E) state after excited to the 3 Q0 (A1 ) state and dissociate; (2) partially excited to the 1 Q1 (E) state and 3 Q1 (E) state directly, then dissociate. In general, under the UV laser radiation of 234 nm, the quantity of alkyl bromides generating Br is more than that at 267 nm, while the quantity of bromides producing Br is less. For this reason, the branching ratio at 267 nm is greater than that at 234 nm. For other alkyl bromides in the experiment, there are many similarities with C2 H5 Br in the photodissociation mechanism.
Y. Tang et al. / Chemical Physics Letters 392 (2004) 493–497
C 3v symmetry
under the 267 nm laser radiation. However, whether at 234 nm or at 267 nm, the in equation N(Br )/N(Br) <1 always existed. It is proved that the bromine atoms from the photodissociation of alkyl bromides mostly existed in the ground state Br (2 P03=2 ).
C s symmetry
1 Q 1 (E)
3 Q 0 (A1)
497
Crossing
Acknowledgements
3 Q 1 (E) Br*
All the authors gratefully acknowledge support from National Natural Science Foundation of China (299730397).
Br
References
Ground State Fig. 3. Dissociation channels of alkyl bromides at the reaction of UV laser.
4. Conclusion Under the UV laser radiation of 234 and 267 nm, alkyl bromides of CHBr3 , CH2 Br2 , C2 H5 Br, C2 H4 Br2 dissociated and released bromine atoms. (2 + 1) REMPI of bromine atoms took place after absorbing photons continuously. Scanning the UV laser wavelength, we recorded the REMPI spectrum of bromine atoms in the region of 231–268 nm, in which 42 transition lines were observed and assigned, and most of them coincided with the previous results except 12 that were new. Bromine atoms produced from photolysis of alkyl bromides under UV laser radiation are Br (2 P03=2 ) state or Br (2 P01=2 ) state. Studying the branching ratio of them, we found that all branching ratios at 267 nm were obviously larger than those at 234 nm, because there exist three dissociation channels from higher to lower: 1 Q1 (E), 3 Q0 (A1 ), 3 Q1 (E), with a curve crossing between 1 Q1 (E) and 3 Q0 (A1 ). Under the 234 nm laser radiation, much more alkyl bromides molecules were excited to the 1 Q1 (E) state and dissociated to get Br (2 P03=2 ), and less
[1] S.C. Wofsy, M.B. McElroy, Y.L. Yung, Geophys. Res. Lett. 2 (1975) 215. [2] Y.L. Yung, J.P. Pinto, R.J. Watson, S.P. Sander, J. Atmos. Sci. 37 (1980) 339. [3] S. Arepalli, N. Presser, D. Robie, R.J. Gordon, Chem. Phys. Lett. 117 (1985) 64. [4] Y. Chen, L.S. Pei, J. Jin, Q. Ran, C.X. Chen, S.Q. Yu, X.X. Ma, Acta Phys. Sin. (Overseas Edition) 7 (1999) 490. [5] M.S. Park, Y.J. Jung, S.H. Lee, D.C. Kim, K.H. Jung, Chem. Phys. Lett. 322 (2000) 429. [6] Y.J. Jung, M.S. Park, Y.S. Kim, K.H. Jung, J. Chem. Phys. 111 (1999) 4005. [7] Y.J. Jee, Y.J. Jung, K.H. Jung, J. Chem. Phys. 115 (2001) 9739. [8] D.D. Xu, J.S. Francisco, J.H. Huang, W.M. Jackson, J. Chem. Phys. 117 (2002) 2578. [9] J.H. Huang, D.D. Xu, W.H. Fink, W.M. Jackson, J. Chem. Phys. 115 (2001) 6012. [10] T. Gougousi, P.C. Samartzis, T.N. Kitsopoulos, J. Chem. Phys. 108 (1998) 5742. [11] M.S. Park, K.W. Lee, K.H. Jung, J. Chem. Phys. 114 (2001) 10368. [12] W.K. Kang, K.W. Jung, D.C. Kim, K.H. Jung, J. Chem. Phys. 104 (1996) 5815. [13] H.F. Xu, Y. Guo, S.L. Liu, X.X. Ma, J. Chem. Phys. 117 (2002) 5722. [14] W.K. Kang, K.W. Jung, K.H. Jung, J. Phys. Chem. 98 (1994) 1525. [15] B.D. Koptlitz, J.K. McVey, J. Chem. Phys. 11 (1984) 81. [16] W.D. Zhou, L.S. Sheng, G.H. Wu, H. Gao, F. Qi, Y.W. Zhang, Acta Phys.-Chim. Sin. 15 (1999) 948. [17] Charlotte E. Moore. Atomic Energy Levels, vol. I, NSRDS-NBS 35, 1971. [18] W.S. McGivern, R.J. Li, P. Zou, S.W. North, J. Chem. Phys. 111 (1999) 5771.