Photodissociation studies of cyclopentyl bromide at 234 and 266 nm using velocity ion imaging technique

Chemical Physics Letters 511 (2011) 39–44

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Photodissociation studies of cyclopentyl bromide at 234 and 266 nm using velocity ion imaging technique Ahmed Yousif Ghazal, Yuzhu Liu, Yanmei Wang, Changjin Hu, 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 University of the Chinese Academy of Sciences, Beijing 100039, PR China

a r t i c l e

i n f o

Article history: Received 21 July 2010 In final form 8 June 2011 Available online 12 June 2011

a b s t r a c t The photodissociation dynamics of cyclopentyl bromide at 234 and 266 nm were investigated using velocity ion imaging. Translational energy distributions of Br and Br⁄ have been fitted by two Gaussian functions. It is possible that they are originated from different conformational structures. Three Gaussian functions are required to fit the distributions of Br⁄ at 266 nm, which is attributed to the multiphoton dissociative ionization. The rigid radical limits of the impulsive model have been applied to the related energy partitioning. The branching ratios and the relative quantum yields were determined; the results indicated that ground-state bromine was the major dissociation product. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Photodissociation experiments provide very detailed information on molecular interactions and bond-breaking dynamics. Rapid developments in the field of photodynamics have made ion imaging a powerful means of studying molecular photodissociation processes [1,2]. The photodissociation of various organic bromides often involves several dissociation channels and can be complicated [3]. The primary photodissociation of alkyl or alkenyl bromides is more straightforward, with a dominant C–Br fission channel and some contribution from HBr elimination. Once the Br atoms in different spin–orbit states are state-specifically detected, we can accurately determine both the kinetic energy releases and the internal energy of the recoiling hydrocarbon radicals. Many studies have been done on the UV photodissociation of alkyl and alkenyl bromides. Gougousi et al. [4] investigated the photolysis of methyl bromide in the first continuum extensively, resolving the Br(2P3/2)/Br⁄ (2P1/2) spin–orbit branching ratio, the partial absorption cross section, and the vibrational distribution of the nascent CH3 radicals. Park et al. [5] studied the 235 nm photodissociation dynamics of allyl bromide and found a singlepeaked product translational energy distribution P(ET). So far, most studies in the ultraviolet have mainly focused on noncyclic alkyl and alkenyl bromides. Because they have higher symmetry than straight-chain homologous compounds and a propensity to undergo ring-opening and other isomerization, cyclic hydrocarbon radicals have drawn considerable attention in physical chemistry [6,7]. Except for cyclopropyl rings, many theoretical and experimental studies [8–10] have shown that monosubstituted cycloalkanes ⇑ Corresponding author. Fax: +86 27 87198576. E-mail address: [email protected] (B. Zhang). 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.06.014

have more than one conformer in the gas phases. These experimental results show at least two dominant forms as axial and equatorial conformers with respect to relative positions of substituent in a cyclic ring in which one of these two conformers has lower energy due to their difference in the conformational structures. Nevertheless, there have been a few studies on the photolysis of cyclic hydrocarbon halides. Arnold et al. used ion imaging of iodine atoms to perform a photodissociation study of cylcopropyl iodide at 266 and 279.7 nm [11]. In another study, state-selective photofragment translational spectroscopy of iodocyclohexane was performed, and the energy difference between the axial and equatorial conformers was determined by monitoring the I(2PJ) fragments [12]. Cycloalkyl bromides are mono-halogen-substituted cyclic ring systems and have Cs symmetry [13]. The photodissociation dynamics of molecules with Cs symmetry are more complicated than those of a C3v symmetry system. Three states from the r⁄ configuration 3Q0, 3Q1, and 1Q1 in Mulliken’s notation [3] are dipole allowed from a ground state alkyl bromide. The 3Q0 state correlates with the Br⁄ product and the dipole moment is aligned parallel to the C–Br bond while the 3Q1, and 1Q1 states lead to Br formation through a perpendicular transition. Complicated nonadiabatic interactions such as avoided crossing are responsible for the photodissociation of Cs symmetry systems [14]. In this work, the photodissociation dynamics of cyclopentyl bromide were investigated at 234 and 266 nm. The nascent Br and Br⁄ were state-selectively detected via a [2 + 1] resonanceenhanced multiphoton ionization (REMPI) scheme. The relative quantum yields were measured by REMPI with time-of-flight mass spectrometry (TOF-MS). The translational energy distributions and recoil anisotropies were extracted using a photofragment twodimensional ion-imaging technique. The experimental findings

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provide an insight into photodissociation dynamics of brominesubstituted cyclic ring systems with Cs symmetry. 2. Experimental The experimental setup for the two-dimensional photofragment velocity ion imaging has been described in detail elsewhere [15]. To summarize briefly, our system consisted of a homebuilt time-of-flight (TOF) mass spectrometer and a two-dimensional position sensitive detector. A mixture of cyclopentyl bromide, typically 1.3%, (P99% purity) seeded in helium gas at a backing pressure of 1 atm was expanded supersonically into the vacuum chamber via a pulsed valve. The molecular beam was skimmed before entering the reaction chamber. The 355 nm light from the third harmonic of a Nd: YAG laser (YG981E, Quantel) was used to pump a dye laser (ScanMate 2E OG, Lambda Physik). The visible output of the dye laser was frequency doubled using a BBO crystal, then aligned using a half-wave retardation plate to obtain the desired polarization. Finally, we used a 250 mm focal length lens to focus the linearly polarized laser on the molecular beam interaction region. The [2 + 1] REMPI technique was employed to selectively ionize Br (6p4P3/2 state at 233.62 nm, 5p4P3/2 state at 266.57 nm), and Br⁄ (6p2S1/2 state at 233.96 nm, 5p4S1/2 state at 266.63 nm). Fragments formed in the photodissociation were extracted and accelerated by an electrostatic lens and projected onto a two-dimensional detector, which consisted of a microchannel plate (MCP) coupled with a P47 fast phosphor screen and a charge-coupled device (CCD) camera. In order to minimize cluster formation, photolysis was performed on the rising edge of the molecular beam pulse.

Figure 1. Raw ion imaging of Br and Br⁄ fragments from the photolysis of cyclopentyl bromide at 234 nm (a, b) and 266 nm (c, d). In all images the linear ⁄ polarization vector of photolysis laser is vertical: (a) Br at 234 nm; (b) Br at ⁄ 234 nm; (c) Br at 266 nm; (d) Br at 266 nm.

< ET > ; Eav l

3. Results

fT ¼

Our experiment investigated the following reaction at 234 and 266 nm:

Eav l ¼ hv  D0  Eel þ Eint ;

hm

c-C5 H9 Br ! c-C5 H9 þ Brð2 P1=2 Þ; Brð2 P3=2 Þ D0 ¼ 285:3 kJ=mol: ð1Þ ⁄

The raw images of Br and Br obtained from the photolysis of cyclopentyl bromide at 234 and 266 nm are presented in Figure 1. The background was removed by subtracting a reference image taken under the same conditions with the laser off resonance. The raw images are two-dimensional projections of the original three-dimensional speed and angular distributions, which have cylindrical symmetry around the polarization axis of the photolysis laser [16]. By performing an inverse Abel transformation, we can reconstruct the three-dimensional velocity distribution from the two-dimensional projection. The polarization vector of the photolysis laser is parallel to the vertical direction of the image plane. By integrating the reconstructed 3D velocity distributions over all angles for each speed, we obtained the speed distributions P(m) of the two atomic fragments as shown in Table 1. P(m) can be converted to the center-of-mass translational energy distribution P(E) using the relationship,

PðEÞ ¼ PðmÞ ET ¼

dm ; dE

1 mBr 2 ðmBr þ mC5 H9 Þ m : 2 mC5 H9 Br

ð2Þ

ð3Þ

The total translational energy distributions for Br and Br⁄ are shown in Figure 2. The fractional translational energy fT gives the ratio of the average transitional energy to the available energy Eavl,

ð4Þ ð5Þ

where hm is the photon energy (512 kJ/mol at 234 nm and 449 kJ/mol at 266 nm), D0 is the dissociation energy of the c-C5H9–Br molecule, and D0 = 285.3 kJ/mol [17]. For the case of a supersonic molecular beam, the internal energy of the parent molecule Eint is taken to be zero. The electronic energy level Eel of the atomic bromine is assumed to be 0 kJ/mol for Br and 44 kJ/mol for Br⁄. The and /Eavl values are listed in Table 1. The Br⁄ image at 266 nm is different from the photofragment images of Br and Br⁄ at 234 nm. The angular distributions of the fragments P(h) in Figure 3 can be obtained by integrating a reconstructed three-dimensional velocity distribution over an appropriate range of the speed at each angle. The parameter (h) is the angle between the recoil velocity of the photofragment and the polarization axis of the photolysis laser. The angular distribution can be characterized by the anisotropy parameter b by,

IðhÞ ¼ ð4pÞ1 ½1 þ bP2 ðcos hÞ;

ð6Þ

where P2 (cos h) is the second-order Legendre polynomial [18]. The values of the anisotropy parameter b for Br and Br⁄ photolysis at 234 and 266 nm photolysis are listed in Table 1. The recoil anisotropy parameters we obtained are between the two limit values of +2 and 1. The relative contributions of the parallel and perpendicular transitions to the observed recoil anisotropies can be resolved by using the following relationships:

vII þ v? ¼ 1; and b ¼ vII bII þ v? b?

ð7Þ

vII and v? are the relative fractions of the Br and Br⁄ fragments produced via the parallel and perpendicular transitions, respectively. bII and b? are limit values of the anisotropy parameters for the parallel

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A.Y. Ghazal et al. / Chemical Physics Letters 511 (2011) 39–44 Table 1 Energy partitioning, anisotropy parameter and velocities of the bromine fragments in the photodissociation of cyclopentyl bromide, energies are in kJ/mol. Channels

hm

Wavelength (nm)

Speed (m/s)

Eavl

< ET >low Gs

/Eavl

b

vII

v?

0.25 0.45 0.30 0.50 0.22 0.46 0.23 0.50 0.87

0.49 ± 0.06 1.01 ± 0.05 0.88 ± 0.05 1.98 ± 0.05 0.28 ± 0.05 0.76 ± 0.05 0.65 ± 0.07 1.01 ± 0.07 -

0.50 0.67 0.63 0.99 0.43 0.58 0.55 0.67 -

0.50 0.33 0.37 0.01 0.57 0.42 0.45 0.33 -

< ET >high Gs C5H9 + Br

233.62

512.05

C5H9 + Br

233.95

511.33

C5H9 + Br

266.48

448.91

C5H9 + Br⁄

266.62

448.67

⁄

226.75

163.61 119.37

0.6

0.4

0.2

(b) Br* at 234nm (β) of low-E t=0.88 (β) of high-Et=1.98

1.0

0.8

intensity/arb.unit

0.8

57.33 103.69 54.21 91.59 36.90 76.61 27.88 59.72 104.63

182.03

(a) Br at 234nm (β) of low-E t=0.49 (β) of high-E t=1.01

1.0

intensity/arb.unit

8 15 1 096 7 93 1 030 654 942 5 68 8 32 1 101

0.0

0.6

0.4

0.2

0.0

-20 0

20 40 60 80 100 120 140 160 180 200 220 240 260 280 300

-20

Total Translational Energy (kJ/mol)

(c) Br at 266nm (β) of low-E t=0.28 (β) of high-Et=0.76

1.0

0.6

0.4

0.2

0.0

20 40 60 80 100 120 140 160 180 200 220 240 260 280 300

(d) Br* at 266nm (β) of low-E t=0.65 (β) of high-E t=1.01

1.0

0.8

intensity/arb.unit

intensity/arb.unit

0.8

0

Total Translational Energy (kJ/mol)

0.6

0.4

0.2

0.0 -20 0

20 40 60 80 100 120 140 160 180 200 220 240 260 280 300

Total Translational Energy (kJ/mol)

-20 0

20 40 60 80 100 120 140 160 180 200 220 240 260 280 300

Total Translational Energy (kJ/mol)

Figure 2. The total kinetic-energy distributions of Br and Br⁄ fragments from the photolysis of cyclopentyl bromide at 234 nm and 266 nm. The solid line indicates the curve fitted with a Gaussian function.

and perpendicular transitions, respectively. The corresponding results are listed in Table 1. The branching ratios at 234 and 266 nm were obtained in a one-color experiment by measuring the total ion signal for Br and Br⁄ transitions, while correcting for the relative absorption cross sections between the wavelengths. According to Eq. (8), the ratios N(Br⁄)/N(Br) are proportional to the measured ion signal ratio in the TOF mass spectra,

photolysis were found to be 0.80 and 0.71, respectively. The branching ratios N(Br⁄)/N(Br) were found using Eq. (8) and are listed in Table 2. The relative quantum yields of U(Br⁄) and U(Br) were determined by Eq. (9). The results are also listed in Table 2.

NðBr Þ SðBr Þ ¼k ; NðBrÞ SðBrÞ

ð8Þ

4.1. Ground Br (2P3/2) atoms from c-C5H9Br photolysis

UðBrÞ ¼ 1  UðBr Þ;

ð9Þ

where S(Br) and S(Br⁄) are the signal intensity of bromine atoms, N(Br) and N(Br⁄) are the numbers of atoms produced in the photodissociation, and the factor k is the proportionality constant. The k value is determined by the relative detection efficiency of Br and Br⁄ and the instrument factor. We find it by performing Br2 photodissociation under the same experimental conditions, since the value of N(Br⁄)/N(Br) was well known for Br2 photodissociation at 234 and at 266 nm [19]. The k values for photolysis at 234 and 266 nm

4. Discussion

The formation dynamics of the Br atoms for the dissociation at 234 and 266 nm are similar. As shown in Figure 2, the total translational energy distributions at these two wavelengths can both be fit by two Gaussian curves. The fractions of energy from the high-Et component and from the low-Et component are also very similar at two wavelengths, as shown in Table 1. The translational energy of each channel decreases with decreasing excitation laser photon energy, but the energy distributions among different product channels are the same. This indicates that the Br products at 234 and 266 nm have the same formation dynamics. For this prompt dissociation, two models have been proposed for two limiting cases

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A.Y. Ghazal et al. / Chemical Physics Letters 511 (2011) 39–44 2.0

(a) (Br of h-Et at 234 nm) β=1.01

1.5

Intensity/arb.unit

Intensity/arb.unit

2.0

1.0

0.5

0.0

(b) (Br*of h-Et at 234 nm) β=1.98

1.5

1.0

0.5

0.0

-20

0

20

40

60

80

100

120

140

160

180

-20

200

0

20

40

2.0

80

100

140

120

160

180

200

2.0

(c) (Br of h-Et at 266 nm) β=0.76

1.5

Intensity/arb.unit

Intensity/arb.unit

60

Angle (degree)

Angle (degree)

1.0

0.5

0.0

(d) (Br*of h-Et at 266 nm) β=1.01

1.5

1.0

0.5

0.0

-20

0

20

40

60

80

100

120

140

160

180

200

-20

0

20

40

60

80

100

140

120

160

180

200

Angle (degree)

Angle (degree)

Figure 3. Angular distributions of Br and Br⁄ fragments from the photolysis of cyclopentyl bromide at 234 nm and 266 nm.

Table 2 The relative quantum yields in the photodissociation of cyclopentyl bromide at 234 and 266 nm. Wavelength (nm)

k

S(Br⁄)/S(Br)

N[Br⁄]/N[Br]

U (Br)

U (Br⁄)

234 266

0.80 0.71

0.91 ± 0.035 0.79 ± 0.056

0.72 ± 0.028 0.56 ± 0.052

0.578 ± 0.009 0.641 ± 0.020

0.421 ± 0.009 0.359 ± 0.020

[20]: the rigid model that ignores the flow of energy into the vibrational degrees of freedom of the fragments and the soft radical limit which is discussed in more detail in Section 4.3.1. The angular distributions of the high-Et components of Br fragments at 234 and 266 nm are shown in Figure 3a and c, and the obtained corresponding anisotropy parameter b values are listed in Table 1. As shown in Table 1, the values we found were between the parallel transition limit value of b = +2 and the perpendicular transition limit of b = 1. The contribution of parallel transition is 67% for Br product at 234 nm. But for the Br at 266 nm, 58% of the product is from parallel transition and 42% is from perpendicular. 4.2. Excited Br⁄ (2P1/2) atoms from c-C5H9Br photolysis At 234 nm, Br⁄ and Br atom formation dynamics are similar. The translational distributions can be fitted using two Gaussian curves as show in Figure 2b. In addition, the /Eavl value of this component for high-Et is 0.50, which is also very close to the Br high-Et component. Our results for Br⁄ formation at 266 nm were different. The total translational energy distribution is fit by three Gaussian functions to get three components, the low-Et, the high-Et (the middle one), and the highest-Et as shown in Fig. 2d. The high-Et and the low-Et components correspond to the high-Et and low-Et components of Br⁄ at 234 nm, respectively, because they have similar /Eavl values and are in accordance with the trend that the translational energy decreases with decreasing excitation photon energy. The highest-Et component of Br⁄ at 266 nm cannot be explained by the impulsive model, and may be from:

(i) Photodissociation of the parent ion c-C5H9Br+,

c-C5 H9 Br þ 3hm ! c-C5 H9 Brþ þ e ;

ð10Þ

c-C5 H9 Brþ þ hm ! c-C5 Hþ9 þ Br=Br :

ð11Þ

(ii) Multiphoton dissociative ionization of c-C5H9Br,

c-C5 H9 Br þ 2hm ! c-C5 H9 Br þ hm ! c-C5 Hþ9 þ Br=Br þ e : c-C5 Hþ 9

ð12Þ 

The appearance energy for c-C5 H9 Br ! þ Br þ e was reported to be 10.9 eV [21], and the ionization potential energy of c-C5H9Br was reported to be 9.94 ± 0.02 eV [22]. After a very thorough search for the existence of the parent ion in the experiment, we did not find any trace of c-C5H9Br+ signal in the TOF mass spectrum (TOF-MS). Given the instability of the c-C5H9Br+ parent ion during multiphoton ionization, it seems unlikely that mechanism (i) can account for the formation of the Br⁄ product. Therefore, we attribute the highest-Et component of Br⁄ at 266 nm to mechanism (ii). Two photons at 266.62 nm have an energy of 9.30 eV, which is in the energy region of low-n Rydberg states, 0.639 eV lower than the ionization potential energy of c-C5H9Br. The low-n Rydberg states are accessed with two-photons excitation. If the photons were on resonance, that would help to absorb the third photon. In addition, the images of Br⁄ of 266 nm have been collected at three different laser intensities. It is found that the ratio of the intensity of the highest-Et component was increased when the laser intensity became higher and higher. This helps to conclude the multiphoton dissociative ionization mechanism. Multiphoton dissociative ionization has been observed for many systems, for example ICl [23], C6H5I [24] and t-C4H9Br [25].

A.Y. Ghazal et al. / Chemical Physics Letters 511 (2011) 39–44

The angular distributions of high-Et (not the highest one) components of Br⁄ fragments are shown in Figure 3b and d at 234 and 266 nm, respectively. At 234 nm, the anisotropy parameter b = 1.98 of high-Et component is very close to the parallel transition limit value b = +2. At 266 nm, the b value of its high-Et component is 1.01. The contributions of the parallel and perpendicular transition are listed in Table 1. It is obvious that the Br⁄ product is nearly from pure parallel at 234 nm. The contribution of parallel transition is 99%, but for the Br⁄ at 266 nm, 67% of the product is from parallel transition and 33% is from perpendicular. And the non-adiabatic transition from the 3Q0 to 1Q1 states results in the generation of the parallel component observed for the Br⁄ channel. 4.3. Comparison of ground Br (2P3/2) and excited Br⁄ (2P1/2) atoms 4.3.1. Estimate the Impulsive model of the c-C5H9Br conformers Based on studies using temperature-dependent infrared and Raman spectroscopy, microwave spectroscopy and ab initio calculations of c-C5H5Br, Harris et al. [13] reported the presence of a mixture of the axial and the equatorial forms with low pseudorotational barrier. In their recent microwave and ab initio investigations, Durig and et al. [26] observed and assigned microwave transitions to both the axial and the equatorial forms. Cyclopentyl bromide molecule was characterized by two potential energy minimums of unequal depth, which correspond to equatorial and axial envelope conformations by Diky and et al. [27]. The optimized geometries and conformational stabilities have also been obtained from ab initio MP2/6-311+G (d,p) calculations and from density functional theory calculations by the B3YLP method with several different basis sets. The enthalpy difference (2.79 ± 0.28 kJ/mol) between the two stable forms of the bromide has been obtained by variable temperature studies of the infrared spectra of rare gas solutions with the axial conformer which is the more stable form [28]. In the photodissociation of cyclopentyl iodide [29] and cyclohexyl iodide [12], the rate of interconversion between the two conformers is slower than the rate of bond breaking in the excited state. It is reasonable to assume that the conversion time for the cyclopentyl bromide is on a similar timescale. Accordingly, the observed intensities of the two peaks in Figure 2 could reflect the populations of the two conformers, the axial-c-C5H9–Br and the equatorial-c-C5H9–Br. The energy partition can be estimated by introducing either of two radical limits of the impulsive model. Using geometrical parameters optimized by the ab initio calculation of Durig et al. [26], the /Eavl values of the rigid model [20] can be calculated with the following equations, 2

Er ¼ sin x½ð1  la =lf Þ1  cos2 x1 Eav l ;

ð13Þ

ET ¼ Eav l  Er ;

ð14Þ

where la is the reduced mass of the a-carbon and bromine atom at each end of a breaking bond and lf is the reduced mass of the recoiling fragments. v is the bond angle of (centre of mass of C5H9)-Ca–Br, which can be estimated using the geometrical parameters optimized by the ab initio calculation in [26]. The /Eavl value calculated using the soft model is 0.28. The soft radical limit of the impulsive model is not a suitable interpretation of the dissociation dynamics. Furthermore, the C5 ring of the c-C5H9 fragment should be fairly stiff, not soft [30][27]. In the rigid impulsive model, the /Eavl value depends on the impact parameter. From conservation of angular momentum [20], one can find that the rotational energy of the c-C5H9 product is proportional to the square of the impact parameter of the two-body process of c-C5H9 + Br. This rotational energy would be quite significant if the impact parameter is not small. For the axial-c-C5H9–Br conformer the rotational energy of c-C5H9 was estimated by using Eq. (13) with Er = 0.69 Eavl, and

43

Figure 4. The 3-dimensional ground-state geometries for the two conformational structures of cyclopentyl bromide.

Er = 0.49 Eavl, for the equatorial-c-C5H9–Br. The product rotational energy in the equatorial conformer case would be smaller, given the ground state geometry as shown in Figure 4 b. Consequently, the /Eavl value of the axial-c-C5H9–Br is 0.31, while the /Eavl value of the equatorial-c-C5H9–Br is 0.51. Hence, our /Eavl values for the high-Et components of Br and Br⁄ at 234 and 266 nm are very close to the /Eavl rigid value of the equatorial-c-C5H9–Br that we found from equation (14) to be 0.51. The /Eavl values of low-Et components at 234 and 266 nm are very close to the /Eavl rigid value of the axial-c-C5H9–Br. In addition, the ratio of the intensities of the high and low-Et components does not depend on the laser power, which suggests that neither component results from nonlinear processes. Furthermore, no low-Et components were observed in the velocity spectra of 1-bromopentane [31], which is a noncyclic molecule with much smaller interconversion barriers than those of cyclic compounds. Finally, we did not see any trace of any cluster peak in our TOF mass spectra. Other sources of atomic Br fragments such as the secondary photodissociation of HBr are assumed to be negligible. From above discussion and according to the /Eavl rigid values, we suggest that the low-Et components in Figure 2 may correspond to the axial conformer and the dominant high-Et components may correspond to the equatorial conformer. That is, the high-Et component is due to the bromine atoms produced from the photodissociation of eq-C5H9Br and the low-Et component is due to the bromine atoms from ax-C5H9Br. Comparatively, Freitas et al. have performed a photodissociation study of cylcohexyl iodide by monitoring the I(2PJ) fragments, translational energy and angular distributions. It is found that the cyclohexyl radical produced from the axial iodocyclohexane has less internal energy than the cyclohexyl radical produced from the equatorial conformer [12]. In our experiment, /Eavl values for Br and Br⁄ of the high-Et components at the two excitation wavelengths were found to be close to each other. At given wavelength, /Eavl for Br was less than /Eavl for Br⁄, which indicated that the C5H9 fragment from the Br product channel was more vibrationally excited. This was also supported by the comparatively broader raw images and broader P(m) distributions of Br fragments. 4.3.2. Angular distribution and the Branching ratio We found that the anisotropy parameters of Br and Br⁄ fragments decreased with increasing wavelength. Furthermore, the percent contributions of the parallel and perpendicular transitions also changed with the wavelengths. As shown in Table 1, the contribution of the parallel transition decreased from 67% to 58% for Br and from 99% to 67% for Br⁄. In Table 2 we see that for each compound, the U(Br⁄) values are smaller than the U(Br) values, which indicates that ground state bromine was a major product of the photodissociation of cyclopentyl bromide at 234 and 266 nm. Secondly, the U(Br⁄) from the photodissociation of c-C5H9Br molecules increased with increasing photon energy, which showed the same trend as CH3Br photodissociation [32]. The anisotropy parameters b, listed in Table 1, are lower for the low-Et components than they are for high-Et components and the anisotropy parameters b for the low-Et are not as obvious as b of the high-Et components.

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A.Y. Ghazal et al. / Chemical Physics Letters 511 (2011) 39–44

5. Conclusion The photodissociation of cyclopentyl bromide at 234 and 266 nm was studied using velocity ion imaging. Two dissociation channels for C–Br bond fission were observed, and they showed competition between a dominant high kinetic energy product and a less prevalent low kinetic energy product. According to the /Eavl values estimated by the rigid impulsive model, the high-Et components are probably due to the bromine atoms produced from the photodissociation of eq-C5H9Br and the low-Et components are probably due to the bromine atoms from ax-C5H9Br. We attribute the highest-Et component of Br⁄ at 266 nm to a multiphoton dissociative ionization mechanism. U(Br⁄) was smaller than U(Br), which indicated that ground state bromine was a major product. Acknowledgment All the authors gratefully acknowledge support from the National Natural Science Foundation of China under Grant Nos. 20703060 and 20973194. References [1] [2] [3] [4] [5]

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