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Photodissociation dynamics of n-C5H11Br at 234 nm
Optics Communications 265 (2006) 532–536 www.elsevier.com/locate/optcom
Photodissociation dynamics of n-C5H11Br at 234 nm Zhengrong Wei, Ying Tang, Qiusha Zheng, Bing Zhang
*
State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuchang, Xiaohongshanxi 30, Wuhan City 430071, PR China Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China Received 29 November 2005; received in revised form 23 March 2006; accepted 31 March 2006
Abstract The photodissociation of n-C5H11Br at 234 nm has been investigated utilizing the ion velocity imaging technique. The twodimensional images provide detailed information on the translational energy distributions and anisotropy parameters. The obtained translational energy distributions corresponding to the Br and Br* fragments have been fitted by a single Gaussian function, respectively. The anisotropies for the Br and Br* channels are measured to be b = 1.08 and 1.84, respectively. The contributions of each electronic state have been estimated and the probability of nonadiabatic transition between 3Q0 and 1Q1 states is determined to be 0.47. 2006 Elsevier B.V. All rights reserved. Keywords: n-C5H11Br; Photodissociation; Ion imaging; Nonadiabatic transition
1. Introduction Over the past several decades, the ultraviolet (UV) photodissociation of bromine containing compounds have received considerable attention, due to their importance of stratospheric ozone destruction from bromine [1,2]. It is believed that bromine has 40 times more powerful ozone depletion potential than chlorine [1,3]. Our studies help one to create additional information for studying photochemistry and introduce a promising direction for peoples who are working in that field. The A-band of alkyl bromides arises from a r* n transition localized on the C–Br bond, and consists of an overlapping of three excited states, 3Q1, 3Q0 and 1Q1, as denoted by Mulliken [4]. Excitation in the A-band leads to rapid C–Br bond breakage due to the repulsive nature of these excited states. The 3Q0 N transition is polarized parallel to C–Br bond and has dual dissociation paths. The 3 Q0 state is adiabatically correlated with the spin–orbit excited state Br (1P1/2) (denoted Br*) by direct dissociation
*
Corresponding author. Tel.: +86 27 87197441; fax: +86 27 87199291. E-mail address: [email protected] (B. Zhang).
0030-4018/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.03.065
and also contributes to the formation of the ground state Br (1P3/2) (denoted Br) by nonadiabatic transition near the conical intersection of the 3Q0 and 1Q1 surfaces. The 1 Q1 and 3Q1 states are correlated with the spin–orbit ground state Br, and the corresponding transitions to these states are polarized perpendicular to the bond axis. As the simplest organic bromide system, CH3Br has received extensive studies comparing with the others. Underwood et al. [5] investigated the photodissociation dynamics of CH3Br in the red wing of the A-band absorption, evidence of significant methyl fragment rotational excitation was found in the decay channels leading to Br and Br* and vibrational excitation was also found in both channels, studies shown a strong nonadiabatic coupling between spin–orbit states of CH3Br with a distortion from C3v geometry in the interaction region. Kim et al. [6] studied the photodissociation dynamics of CF3Br. They suggested the symmetry reduction from C3v to Cs played an important role. It invoked a nonadiabatic coupling between 3Q0 and 1 Q1 states, distorting the original one-dimensional linear pathway. To get better elucidation about the photodissociation dynamics of organic bromides, it is necessary to study the photodissociation dynamics of molecules with intrinsic Cs symmetry.
Z. Wei et al. / Optics Communications 265 (2006) 532–536
Compared with the wide studies of CX3Br (X = H, F, D, Cl) [5–8], CH2ClBr [9], CF2ClBr [10], CF2Br2 [11], CHBr3 [12], the photodissociation dynamics of alkyl bromides with a long C–C chain has not been studied in such detail. Literatures about these compounds are very rare. The behavior of alkyl bromides with a long C–C chain, which belong to Cs symmetry, is more similar to that of the common polyatomic molecules. In the present study, we have investigated the photodissociation of n-C5H11Br at A-band utilizing the ion velocity imaging technique. The anisotropies for the Br and Br* channels are measured; the translational energy distributions are also obtained. The results provide an insight into the photodissociation dynamics of molecule with Cs symmetry and can be helpful for the studies of other organic bromide systems. 2. Experiment The experiments are performed using an ion velocity imaging system; which has been described elsewhere [13] is similar to that Eppink and Parker had reported [14,15] and will be discussed briefly here. It consists of two stainless steel chambers that all pumped with separate turbo molecular pumps. The first chamber houses a pulsed valve with a 0.6 mm nozzle. Under experimental condition, the typical pressure in this chamber is 1.6 · 104 Pa. The samples (99.9%) are purchased commercially without additional purification. The sample mixture is prepared with n-C5H11Br seeded in 1.5 atm helium. The second chamber houses a standard TOF-MS and detector. The ion optics of the TOF-MS consists of three plates: a repeller, an accelerator, and a ground plate, which is the entrance to the flight tube. High voltages of the appropriate ratio for ion imaging are applied to the repeller and accelerator to focus the ions into the flight tube that followed the molecular beam flight direction. The typical voltages are 3200 and 2325 V for obtaining good focus. The pressure in the flight tube is 4.2 · 105 Pa. The detector assembly consists of a dual microchannel plate and a fast P47 phosphor screen. A CCD camera and a photomultiplier tube are mounted behind the screen to collect images and TOF mass spectra, respectively. The 355 nm output of a Nd:YAG laser (YG981E10, Quantel) is used to pump a dye laser (ScanMate 2E OG, Lambda Physik), the wavelength of the visible dye laser output lays in the region of 462–497 nm. A BBO crystal is used for frequency doubling to generate linearly polarized 234 nm laser light, which is subsequently aligned using a Soleil–Babinet compensator. A lens with a focal length of 200 mm is inserted in front of the window to focus the laser light into the second chamber. Inside the chamber the molecular beam intersects the laser light. A single UV laser pulse is used for excited n-C5H11Br and ionized Br and Br*. The [2+1] REMPI technique is employed to selectively ionize Br (233.62 nm, via 6p4 P03=2 state), Br* (233.96 nm, via 6p4 P01=2 state). The wavelengths are calibrated with an optogalvanic (OG) signal, which is produced by a neon
533
hollow cathode lamp, and represents the absorption spectrum of neon. Over 30000 shots are integrated to construct an image. The laser is scanned properly in order to detect all velocity component of bromine fragment. To minimize background noise, images obtained at off-resonance wavelengths under the same condition are subtracted from those at resonance wavelengths. 3. Results and discussion The raw images of Br* and Br are displayed in Fig. 1(a) and (c), each raw imaging is the two-dimensional (2D) projection of a three-dimensional (3D) velocity distribution. The polarization vectors of laser beams are vertically aligned in both cases. The cylindrical symmetry of a velocity distribution allows one to reconstruct a three-dimensional image. A three-dimensional velocity distribution is reconstructed from a raw image by performing an inverse Abel transformation. Their relative inverse Abel images are 1(b) and (d). The anisotropy parameter b, is extracted by fitting the ion signal intensity I(h), with a standard form [16,17]: IðhÞ / 1 þ bP 2 ðcos hÞ:
ð1Þ
The parameter h represents the angle between the laser polarization axis and the recoil velocity of fragment, P2(cosh) is the second Legendre polynomial. As shown in Fig. 2(a) and (b), the anisotropy parameters determined for the overall ranges of velocity distributions are b = 1.84 for Br* and 1.08 for Br. Implying that an electronic transition with dipole moment parallel to the recoil axis (C–Br) provides the main contribution to the generation
Fig. 1. Experimental raw images: (a) Br*, (c) Br and the reconstructed 3D recoiled distribution (b), (d) respectively. Arrow represents the polarization vector of photolysis laser.
534
Z. Wei et al. / Optics Communications 265 (2006) 532–536 4.5
(a)
1.0
4.0
(a)
*
Br at 234 nm
*
Br at 234 nm
3.5
0.8
Intensity, arb.unit
3.0
Intensity, arb.unit
2.5 2.0 1.5 1.0 0.5 0.0
0.6
0.4
0.2
-0.5 -1.0
0
20
40
60
80
100
120
140
160
0.0
180
Angle, degree 0
500
1000
4.0
1500
2000
2500
3000
3500
2500
3000
3500
-1
Velocity, ms
(b) 3.5
Br at 234 nm
3.0
1.0
(b)
Br at 234 nm
Intensity, arb.unit
2.5 2.0
0.8
Intensity, arb.unit
1.5 1.0 0.5 0.0 -0.5 -1.0
0.6
0.4
0.2 0
20
40
60
80
100
120
140
160
180
Angle, degree 0.0
Fig. 2. Angular distributions: (a) Br* and for (b) Br fragment in photodissociation of C5H11Br at 234 nm.
0
of Br and Br*. The more distinct structure in the raw image of Br* in comparison to that of Br indicates a higher anisotropy in the angular distribution. The speed distribution P(m) can be extracted by integrating the reconstructed three dimensional speed distributions over all angles at each speed. The center-of-mass translational energy distribution, P(E), is obtained by converting the speed distribution using the following equations: P ðEÞ ¼ P ðmÞ
dv ; dE
ð2Þ
1 mBr 2 ET ¼ ðmBr þ mC5 H11 Þ m; 2 mC5 H11
ð3Þ
where ET, mX, and m denote the total translational energy, the mass of X (X = Br, C5H11), and the velocity of the bromine fragments, respectively. The translational energy distributions of Br and Br* are shown in Fig. 3. The available energies (Eavl stands for available energy of Br and Eavl for Br*) can be determined from the energy conservation relations. ðf Þ
Eavl ¼ hm D0 þ Epint ¼ Et þ Eint ;
ð4Þ
Eavl
ð5Þ
¼ Eavl þ Eso ¼
Et
þ
Eint ;
500
1000
1500
2000
Velocity, ms-1 Fig. 3. The speed distributions: (a) Br* and (b) Br channels. The solid line indicates the curve fitted with Gaussian functions.
fT ¼
ET ; Eavl
ð6Þ ðf Þ
where hm is the photon energy 512 kJ mol1, D0 the dissociation energy of the C–Br bond, which can be calculated 287.5 kJ mol1 by using the values of Table 1. The internal energy of parent molecule Epint is estimated to be zero since the rotational and vibrational excitations are negligible in a supersonic molecular beam. Eso = 44 kJ mol1 represents the spin–orbit excitation energy of the Br atom. Using these values, the fT values for Br* and Br are determined to be 0.34 and 0.28, respectively (as in Table 2). In 234 nm photodissociation of n-C5H11Br, the speed distributions of Br and Br* are fitted well by a single Gaussian function, implying that Br and Br* are generated as a result of a direct dissociation via repulsive potential energy surfaces after absorbing one UV photon, the direct dissociation is a ultrafast process, molecule dissociates in several hundred femtosecond after absorbing a photon [20,21]
Z. Wei et al. / Optics Communications 265 (2006) 532–536
much faster than the time scale of molecular rotation. The effect of rotational motion on angular distribution for direct dissociation can be ignored. Therefore, under our experimental condition the effect on angular distribution of fragments can be exclude. The state-dependent total translational energy distributions show average energies of 64 and 61 kcal mol1 for the Br and Br* channels (as in Table 2). In order to estimate the partitioning of the available energy, we employed the soft fragment impulsive model [22,23], which has been shown to provide a reasonable description of prompt dissociation on repulsive potential energy surfaces, according to this model, the ratio of the available energy appearing as product translation is given by lC–Br fTsoft ¼ : ð7Þ lC5 H11 Br Value of fTsoft = 0.28 is determined, where lC–Br is the reduced mass of carbon and bromine atoms, and lC5 H11 Br is the reduced mass of C5H11 radial and bromine fragments. The good agreement between fT and f Tsoft suggested the high vibrational excitation in the photodissociation of n-C5H11Br is obtained. Kang et al. [24] compared the energy partitioning of alkyl iodides in photodissociation at various excitation wavelength, studies shown: the energy partitioning of excitation energy was closely related with alkyl radial size; the energy partitioning for bigger alkyl radials can be explained by the soft model. Our results is agreement with this tendency, which has been interpreted in terms of the accommodation of the excess energy by a larger number of small vibration modes in the alkyl radial. Alkyl bromides display a first absorption band (A-band) centered about 220 nm. The A-band consists of three tran-
535
sitions from the ground state to three dissociative excited states, designate as 3Q1, 3Q0, and 1Q1, which originate from 1 (n, r*) and 3(n, r*) states of the C–Br bond. The 1Q1 and 3 Q1 states correlated to Br production channel. The 3Q0 state is adiabatically correlated with the Br* production channel by direct dissociation and also contributes to the ground state Br by the curve crossings between the 3Q0 and 1Q1 potential energy surfaces. However, for the molecule shows intrinsic Cs symmetry these excited states will split. The 3Q0 state transforms to A 0 and the double degenerate 1Q1 and Q1 states split into A 0 and A00 components. The relative fraction of individual pathways can be determined using the following relations: lim bBr ¼ aBr1 blim par þ aBr2 bper ;
lim bBr ¼ aBr1 blim par þ aBr2 bper ;
aBr1 þ aBr2 ¼ a þ a ¼ 1; /Br aBr1 /Br aBr2 /Br aBr1 /Br aBr2 1 3 Q0 Þ f ð1 Q1 Þ þ f ð3 Q1 Þ f ð Q1 ; ¼ 1 f ð3 Q 0 Þ f ð3 Q 0 Q1 Þ Br1
ð8Þ ð9Þ
Br2
ð10Þ
where bBr, bBr are the observed anisotropy parameters of lim lim Br and Br*, blim par and bper are two limiting values, bpar ¼ 2, lim bper ¼ 1. And aBr1, aBr2, aBr 1 ,a Br 2 represent the productions of Br and Br* come from either a parallel or perpendicular transitions. UBr and UBr are relative quantum yields, which have been studied by Zhu et al. [25] UBr was 0.447 and UBr was 0.553, respectively. And f(1Q1), f(3Q0), f(3Q1) are contributions of the individual pathway. Fractions of the individual pathway corresponding to Br* and Br channels are shown in Table 3. Using the values of Table 3, the curve-crossing probabilities can be extracted
Table 1 Standard enthalpies of dissociation
1
Species
DH0f ðkJ mol1 Þ
n-C5H11Br C5H11 Br
129.8 [18] 45.81 [19] 111.86 [18]
Q1 0.802
3
Q0
3
Q1 0.420
C5H11 + Br*
0.382
234nm
C5H11 + Br
Table 2 Energy partitioning in the photodissociation of n-C5H11Br in 234 nm Bromine
Eavl
ET
Eint
fT
Br* Br
181 225
61 64
120 161
0.34 0.28
Ground state
1
Fig. 4. Correlation diagram for n-C5H11Br photodissociation.
Energies are in kJ mol .
Table 3 Fractions of the individual pathway corresponding to Br* and Br channels at 234 nm
n-C5H11Br
aBr
aBr2
aBr1
aBr2
f(3Q0)
f(1Q1) + f(3Q1)
f(3Q0
0.69
0.31
0.94
0.06
0.420
0.171
0.027
1
Q1)
f(1Q1 0.382
3
Q0)
536
Z. Wei et al. / Optics Communications 265 (2006) 532–536
3 f ð1 Q1 Q0 Þ ¼ 0:47; 1 3Q Þ þ f ð Q1 0 1 f ð3 Q 0 Q1 Þ P down ¼ 1 3 1Q Þ f ð Q1 Þ þ f ð Q0 1 3 f ð3 Q0 Q1 Þ P 1 ¼ 0:13; 1Q Þ f ð Q1 Þ þ f ð3 Q1 Þ þ f ð3 Q0 1
P up ¼
f ð3 Q0 Þ
ð11Þ ð12Þ ð13Þ
where Pup and Pdown denote the up-crossing probability from 3Q0 to 1Q1 state and the down-crossing probability from 1Q1 to 3Q0 state, respectively. Values of Pup = 0.47 and Pdown = 0.13 are determined. The photodissociation of n-C5H11Br still being dominates by 3Q0 state in 234 nm, which carrying 80% of transition strength (as in Fig. 4). This is similar with other alkyl bromides. There is a strong crossing between 1Q1 and 3Q0 states, the existing of high crossing probability may be arising from the molecular symmetry reduction motion. 4. Summary In this work, the photodissociation dynamics of nC5H11Br has been investigated utilizing the ion velocity imaging technique. Base on the recoil anisotropies and relative quantum yields, the relative contributions of parallel and perpendicular transitions to the generation of bromine fragments have been obtained for the Br and Br* channels. This work reveals a strong crossing probability existing between 1Q1 and 3Q0 states, which may be arise from the molecular symmetry reduction motion. Acknowledgement The authors gratefully acknowledge the support from the innovation Foundation of the Chinese Academy of Sciences.
References [1] S.C. Wofsy, M.B. McEloroy, 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] R.P. Wayne, The Chemistry of Atmospheres, second ed., Oxford University Press, New York, 1991, and references therein. [4] R.S. Mulliken, J. Chem. Phys. 3 (1935) 513. [5] J.G. Underwood, I. Powis, Phys. Chem. Chem. Phys. 1 (2000) 747. [6] T.K. Kim, M.S. Park, K.W. Lee, K.H. Jung, J. Chem. Phys. 115 (2001) 10745. [7] W.P. Hess, D.W. Chandler, J.W. Thoman, Chem. Phys. 163 (1992) 277. [8] Y.J. Jung, M.S. Park, Y.S. Kima, K.H. Jung, H.R. Volpp, J. Chem. Phys. 111 (1999) 4005. [9] S.H. Lee, Y.J. Jung, K.H. Jung, Chem. Phys. 260 (2000) 143. [10] S.H. Lee, K.H. Jung, Chem. Phys. Lett. 350 (2001) 306. [11] M.S. Park, T.K. Kim, S.H. Lee, K.H. Jung, H.R. Volpp, J. Wolfrum, J. Phys. Chem. A 105 (2001) 5606. [12] D.D. Xu, J.S. Francisco, J.H. Huang, M. Jackson, J. Chem. Phys. 117 (2002) 2578. [13] S. Zhang, Y.M. Wang, B.F. Tang, Q.S. Zheng, B. Zhang, Chem. Phys. Lett. 413 (2005) 129. [14] A.T.J.B. Eppink, D.H. Parker, Rev. Sci. Instrum. 68 (1997) 3477. [15] D.H. Parker, A.T.J.B. Eppink, J. Chem. Phys. 107 (1997) 2357. [16] R.N. Zare, D.R. Hersdhbach, proc. IEEE. 51 (1963) 173. [17] R.N. Zare, Mol. Photochem. 4 (1972) 1. [18] Available from:
Photodissociation dynamics of n-C5H11Br at 234 nm Zhengrong Wei, Ying Tang, Qiusha Zheng, Bing Zhang
*
State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuchang, Xiaohongshanxi 30, Wuhan City 430071, PR China Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China Received 29 November 2005; received in revised form 23 March 2006; accepted 31 March 2006
Abstract The photodissociation of n-C5H11Br at 234 nm has been investigated utilizing the ion velocity imaging technique. The twodimensional images provide detailed information on the translational energy distributions and anisotropy parameters. The obtained translational energy distributions corresponding to the Br and Br* fragments have been fitted by a single Gaussian function, respectively. The anisotropies for the Br and Br* channels are measured to be b = 1.08 and 1.84, respectively. The contributions of each electronic state have been estimated and the probability of nonadiabatic transition between 3Q0 and 1Q1 states is determined to be 0.47. 2006 Elsevier B.V. All rights reserved. Keywords: n-C5H11Br; Photodissociation; Ion imaging; Nonadiabatic transition
1. Introduction Over the past several decades, the ultraviolet (UV) photodissociation of bromine containing compounds have received considerable attention, due to their importance of stratospheric ozone destruction from bromine [1,2]. It is believed that bromine has 40 times more powerful ozone depletion potential than chlorine [1,3]. Our studies help one to create additional information for studying photochemistry and introduce a promising direction for peoples who are working in that field. The A-band of alkyl bromides arises from a r* n transition localized on the C–Br bond, and consists of an overlapping of three excited states, 3Q1, 3Q0 and 1Q1, as denoted by Mulliken [4]. Excitation in the A-band leads to rapid C–Br bond breakage due to the repulsive nature of these excited states. The 3Q0 N transition is polarized parallel to C–Br bond and has dual dissociation paths. The 3 Q0 state is adiabatically correlated with the spin–orbit excited state Br (1P1/2) (denoted Br*) by direct dissociation
*
Corresponding author. Tel.: +86 27 87197441; fax: +86 27 87199291. E-mail address: [email protected] (B. Zhang).
0030-4018/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.03.065
and also contributes to the formation of the ground state Br (1P3/2) (denoted Br) by nonadiabatic transition near the conical intersection of the 3Q0 and 1Q1 surfaces. The 1 Q1 and 3Q1 states are correlated with the spin–orbit ground state Br, and the corresponding transitions to these states are polarized perpendicular to the bond axis. As the simplest organic bromide system, CH3Br has received extensive studies comparing with the others. Underwood et al. [5] investigated the photodissociation dynamics of CH3Br in the red wing of the A-band absorption, evidence of significant methyl fragment rotational excitation was found in the decay channels leading to Br and Br* and vibrational excitation was also found in both channels, studies shown a strong nonadiabatic coupling between spin–orbit states of CH3Br with a distortion from C3v geometry in the interaction region. Kim et al. [6] studied the photodissociation dynamics of CF3Br. They suggested the symmetry reduction from C3v to Cs played an important role. It invoked a nonadiabatic coupling between 3Q0 and 1 Q1 states, distorting the original one-dimensional linear pathway. To get better elucidation about the photodissociation dynamics of organic bromides, it is necessary to study the photodissociation dynamics of molecules with intrinsic Cs symmetry.
Z. Wei et al. / Optics Communications 265 (2006) 532–536
Compared with the wide studies of CX3Br (X = H, F, D, Cl) [5–8], CH2ClBr [9], CF2ClBr [10], CF2Br2 [11], CHBr3 [12], the photodissociation dynamics of alkyl bromides with a long C–C chain has not been studied in such detail. Literatures about these compounds are very rare. The behavior of alkyl bromides with a long C–C chain, which belong to Cs symmetry, is more similar to that of the common polyatomic molecules. In the present study, we have investigated the photodissociation of n-C5H11Br at A-band utilizing the ion velocity imaging technique. The anisotropies for the Br and Br* channels are measured; the translational energy distributions are also obtained. The results provide an insight into the photodissociation dynamics of molecule with Cs symmetry and can be helpful for the studies of other organic bromide systems. 2. Experiment The experiments are performed using an ion velocity imaging system; which has been described elsewhere [13] is similar to that Eppink and Parker had reported [14,15] and will be discussed briefly here. It consists of two stainless steel chambers that all pumped with separate turbo molecular pumps. The first chamber houses a pulsed valve with a 0.6 mm nozzle. Under experimental condition, the typical pressure in this chamber is 1.6 · 104 Pa. The samples (99.9%) are purchased commercially without additional purification. The sample mixture is prepared with n-C5H11Br seeded in 1.5 atm helium. The second chamber houses a standard TOF-MS and detector. The ion optics of the TOF-MS consists of three plates: a repeller, an accelerator, and a ground plate, which is the entrance to the flight tube. High voltages of the appropriate ratio for ion imaging are applied to the repeller and accelerator to focus the ions into the flight tube that followed the molecular beam flight direction. The typical voltages are 3200 and 2325 V for obtaining good focus. The pressure in the flight tube is 4.2 · 105 Pa. The detector assembly consists of a dual microchannel plate and a fast P47 phosphor screen. A CCD camera and a photomultiplier tube are mounted behind the screen to collect images and TOF mass spectra, respectively. The 355 nm output of a Nd:YAG laser (YG981E10, Quantel) is used to pump a dye laser (ScanMate 2E OG, Lambda Physik), the wavelength of the visible dye laser output lays in the region of 462–497 nm. A BBO crystal is used for frequency doubling to generate linearly polarized 234 nm laser light, which is subsequently aligned using a Soleil–Babinet compensator. A lens with a focal length of 200 mm is inserted in front of the window to focus the laser light into the second chamber. Inside the chamber the molecular beam intersects the laser light. A single UV laser pulse is used for excited n-C5H11Br and ionized Br and Br*. The [2+1] REMPI technique is employed to selectively ionize Br (233.62 nm, via 6p4 P03=2 state), Br* (233.96 nm, via 6p4 P01=2 state). The wavelengths are calibrated with an optogalvanic (OG) signal, which is produced by a neon
533
hollow cathode lamp, and represents the absorption spectrum of neon. Over 30000 shots are integrated to construct an image. The laser is scanned properly in order to detect all velocity component of bromine fragment. To minimize background noise, images obtained at off-resonance wavelengths under the same condition are subtracted from those at resonance wavelengths. 3. Results and discussion The raw images of Br* and Br are displayed in Fig. 1(a) and (c), each raw imaging is the two-dimensional (2D) projection of a three-dimensional (3D) velocity distribution. The polarization vectors of laser beams are vertically aligned in both cases. The cylindrical symmetry of a velocity distribution allows one to reconstruct a three-dimensional image. A three-dimensional velocity distribution is reconstructed from a raw image by performing an inverse Abel transformation. Their relative inverse Abel images are 1(b) and (d). The anisotropy parameter b, is extracted by fitting the ion signal intensity I(h), with a standard form [16,17]: IðhÞ / 1 þ bP 2 ðcos hÞ:
ð1Þ
The parameter h represents the angle between the laser polarization axis and the recoil velocity of fragment, P2(cosh) is the second Legendre polynomial. As shown in Fig. 2(a) and (b), the anisotropy parameters determined for the overall ranges of velocity distributions are b = 1.84 for Br* and 1.08 for Br. Implying that an electronic transition with dipole moment parallel to the recoil axis (C–Br) provides the main contribution to the generation
Fig. 1. Experimental raw images: (a) Br*, (c) Br and the reconstructed 3D recoiled distribution (b), (d) respectively. Arrow represents the polarization vector of photolysis laser.
534
Z. Wei et al. / Optics Communications 265 (2006) 532–536 4.5
(a)
1.0
4.0
(a)
*
Br at 234 nm
*
Br at 234 nm
3.5
0.8
Intensity, arb.unit
3.0
Intensity, arb.unit
2.5 2.0 1.5 1.0 0.5 0.0
0.6
0.4
0.2
-0.5 -1.0
0
20
40
60
80
100
120
140
160
0.0
180
Angle, degree 0
500
1000
4.0
1500
2000
2500
3000
3500
2500
3000
3500
-1
Velocity, ms
(b) 3.5
Br at 234 nm
3.0
1.0
(b)
Br at 234 nm
Intensity, arb.unit
2.5 2.0
0.8
Intensity, arb.unit
1.5 1.0 0.5 0.0 -0.5 -1.0
0.6
0.4
0.2 0
20
40
60
80
100
120
140
160
180
Angle, degree 0.0
Fig. 2. Angular distributions: (a) Br* and for (b) Br fragment in photodissociation of C5H11Br at 234 nm.
0
of Br and Br*. The more distinct structure in the raw image of Br* in comparison to that of Br indicates a higher anisotropy in the angular distribution. The speed distribution P(m) can be extracted by integrating the reconstructed three dimensional speed distributions over all angles at each speed. The center-of-mass translational energy distribution, P(E), is obtained by converting the speed distribution using the following equations: P ðEÞ ¼ P ðmÞ
dv ; dE
ð2Þ
1 mBr 2 ET ¼ ðmBr þ mC5 H11 Þ m; 2 mC5 H11
ð3Þ
where ET, mX, and m denote the total translational energy, the mass of X (X = Br, C5H11), and the velocity of the bromine fragments, respectively. The translational energy distributions of Br and Br* are shown in Fig. 3. The available energies (Eavl stands for available energy of Br and Eavl for Br*) can be determined from the energy conservation relations. ðf Þ
Eavl ¼ hm D0 þ Epint ¼ Et þ Eint ;
ð4Þ
Eavl
ð5Þ
¼ Eavl þ Eso ¼
Et
þ
Eint ;
500
1000
1500
2000
Velocity, ms-1 Fig. 3. The speed distributions: (a) Br* and (b) Br channels. The solid line indicates the curve fitted with Gaussian functions.
fT ¼
ET ; Eavl
ð6Þ ðf Þ
where hm is the photon energy 512 kJ mol1, D0 the dissociation energy of the C–Br bond, which can be calculated 287.5 kJ mol1 by using the values of Table 1. The internal energy of parent molecule Epint is estimated to be zero since the rotational and vibrational excitations are negligible in a supersonic molecular beam. Eso = 44 kJ mol1 represents the spin–orbit excitation energy of the Br atom. Using these values, the fT values for Br* and Br are determined to be 0.34 and 0.28, respectively (as in Table 2). In 234 nm photodissociation of n-C5H11Br, the speed distributions of Br and Br* are fitted well by a single Gaussian function, implying that Br and Br* are generated as a result of a direct dissociation via repulsive potential energy surfaces after absorbing one UV photon, the direct dissociation is a ultrafast process, molecule dissociates in several hundred femtosecond after absorbing a photon [20,21]
Z. Wei et al. / Optics Communications 265 (2006) 532–536
much faster than the time scale of molecular rotation. The effect of rotational motion on angular distribution for direct dissociation can be ignored. Therefore, under our experimental condition the effect on angular distribution of fragments can be exclude. The state-dependent total translational energy distributions show average energies of 64 and 61 kcal mol1 for the Br and Br* channels (as in Table 2). In order to estimate the partitioning of the available energy, we employed the soft fragment impulsive model [22,23], which has been shown to provide a reasonable description of prompt dissociation on repulsive potential energy surfaces, according to this model, the ratio of the available energy appearing as product translation is given by lC–Br fTsoft ¼ : ð7Þ lC5 H11 Br Value of fTsoft = 0.28 is determined, where lC–Br is the reduced mass of carbon and bromine atoms, and lC5 H11 Br is the reduced mass of C5H11 radial and bromine fragments. The good agreement between fT and f Tsoft suggested the high vibrational excitation in the photodissociation of n-C5H11Br is obtained. Kang et al. [24] compared the energy partitioning of alkyl iodides in photodissociation at various excitation wavelength, studies shown: the energy partitioning of excitation energy was closely related with alkyl radial size; the energy partitioning for bigger alkyl radials can be explained by the soft model. Our results is agreement with this tendency, which has been interpreted in terms of the accommodation of the excess energy by a larger number of small vibration modes in the alkyl radial. Alkyl bromides display a first absorption band (A-band) centered about 220 nm. The A-band consists of three tran-
535
sitions from the ground state to three dissociative excited states, designate as 3Q1, 3Q0, and 1Q1, which originate from 1 (n, r*) and 3(n, r*) states of the C–Br bond. The 1Q1 and 3 Q1 states correlated to Br production channel. The 3Q0 state is adiabatically correlated with the Br* production channel by direct dissociation and also contributes to the ground state Br by the curve crossings between the 3Q0 and 1Q1 potential energy surfaces. However, for the molecule shows intrinsic Cs symmetry these excited states will split. The 3Q0 state transforms to A 0 and the double degenerate 1Q1 and Q1 states split into A 0 and A00 components. The relative fraction of individual pathways can be determined using the following relations: lim bBr ¼ aBr1 blim par þ aBr2 bper ;
lim bBr ¼ aBr1 blim par þ aBr2 bper ;
aBr1 þ aBr2 ¼ a þ a ¼ 1; /Br aBr1 /Br aBr2 /Br aBr1 /Br aBr2 1 3 Q0 Þ f ð1 Q1 Þ þ f ð3 Q1 Þ f ð Q1 ; ¼ 1 f ð3 Q 0 Þ f ð3 Q 0 Q1 Þ Br1
ð8Þ ð9Þ
Br2
ð10Þ
where bBr, bBr are the observed anisotropy parameters of lim lim Br and Br*, blim par and bper are two limiting values, bpar ¼ 2, lim bper ¼ 1. And aBr1, aBr2, aBr 1 ,a Br 2 represent the productions of Br and Br* come from either a parallel or perpendicular transitions. UBr and UBr are relative quantum yields, which have been studied by Zhu et al. [25] UBr was 0.447 and UBr was 0.553, respectively. And f(1Q1), f(3Q0), f(3Q1) are contributions of the individual pathway. Fractions of the individual pathway corresponding to Br* and Br channels are shown in Table 3. Using the values of Table 3, the curve-crossing probabilities can be extracted
Table 1 Standard enthalpies of dissociation
1
Species
DH0f ðkJ mol1 Þ
n-C5H11Br C5H11 Br
129.8 [18] 45.81 [19] 111.86 [18]
Q1 0.802
3
Q0
3
Q1 0.420
C5H11 + Br*
0.382
234nm
C5H11 + Br
Table 2 Energy partitioning in the photodissociation of n-C5H11Br in 234 nm Bromine
Eavl
ET
Eint
fT
Br* Br
181 225
61 64
120 161
0.34 0.28
Ground state
1
Fig. 4. Correlation diagram for n-C5H11Br photodissociation.
Energies are in kJ mol .
Table 3 Fractions of the individual pathway corresponding to Br* and Br channels at 234 nm
n-C5H11Br
aBr
aBr2
aBr1
aBr2
f(3Q0)
f(1Q1) + f(3Q1)
f(3Q0
0.69
0.31
0.94
0.06
0.420
0.171
0.027
1
Q1)
f(1Q1 0.382
3
Q0)
536
Z. Wei et al. / Optics Communications 265 (2006) 532–536
3 f ð1 Q1 Q0 Þ ¼ 0:47; 1 3Q Þ þ f ð Q1 0 1 f ð3 Q 0 Q1 Þ P down ¼ 1 3 1Q Þ f ð Q1 Þ þ f ð Q0 1 3 f ð3 Q0 Q1 Þ P 1 ¼ 0:13; 1Q Þ f ð Q1 Þ þ f ð3 Q1 Þ þ f ð3 Q0 1
P up ¼
f ð3 Q0 Þ
ð11Þ ð12Þ ð13Þ
where Pup and Pdown denote the up-crossing probability from 3Q0 to 1Q1 state and the down-crossing probability from 1Q1 to 3Q0 state, respectively. Values of Pup = 0.47 and Pdown = 0.13 are determined. The photodissociation of n-C5H11Br still being dominates by 3Q0 state in 234 nm, which carrying 80% of transition strength (as in Fig. 4). This is similar with other alkyl bromides. There is a strong crossing between 1Q1 and 3Q0 states, the existing of high crossing probability may be arising from the molecular symmetry reduction motion. 4. Summary In this work, the photodissociation dynamics of nC5H11Br has been investigated utilizing the ion velocity imaging technique. Base on the recoil anisotropies and relative quantum yields, the relative contributions of parallel and perpendicular transitions to the generation of bromine fragments have been obtained for the Br and Br* channels. This work reveals a strong crossing probability existing between 1Q1 and 3Q0 states, which may be arise from the molecular symmetry reduction motion. Acknowledgement The authors gratefully acknowledge the support from the innovation Foundation of the Chinese Academy of Sciences.
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