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DC slice ion imaging of the ultraviolet photodissociation of BrCN
Chemical Physics Letters 426 (2006) 242–247 www.elsevier.com/locate/cplett
DC slice ion imaging of the ultraviolet photodissociation of BrCN Cunshun Huang a, Wen Li b
a,b
, Ruchira Silva a, Arthur G. Suits
a,b,*
a Department of Chemistry, Wayne State University, Detroit, MI 48202, USA Department of Chemistry, Stony Brook University, Stony Brook, NY 11794, USA
Received 21 April 2006; in final form 12 May 2006 Available online 10 June 2006
Abstract The photodissociation of jet-cooled BrCN molecules has been investigated by DC slice ion imaging of the Br atom product at wavelengths of 193 and 234 nm. The images reveal that both the ground state (2P3/2) and spin–orbit excited (2P1/2) bromine atoms are produced at both photolysis energies studied. The translational energy measurements show that at 193 nm the CN fragments are vibrationally cold, while the vibrational excitation is much higher in dissociation at 234 nm. Furthermore, the CN vibrational excitation in the spin–orbit excited Br product channel is higher at both wavelengths studied. 2006 Elsevier B.V. All rights reserved.
1. Introduction The cyanogen halides are an interesting class of molecules because they behave both chemically and physically much like diatomic halogen molecules [1]. This is due to the strength of the C–N bond (7.76 eV), as well as the very large electron affinity of the CN radical. The cyanogen halides have been a favorite system for those interested in photodissociation dynamics dating to the earliest days of photofragment translational spectroscopy [2]. The CN radical can be detected by laser-induced-fluorescence (LIF) [3–13], while halogen atoms can be detected by resonance-enhanced ionization (REMPI) time-of-flight spectroscopy [2,14–19]. Both methods make the products excellent probes for studying the dynamics of these dissociation processes. The first absorption band in BrCN is a weak continuum beginning at about 250 nm and extending below 185 nm, with a peak around 210 nm. Although the excited states have not been clearly identified in the case of BrCN, at least two states are involved based on the production of ground state and spin–orbit excited Br as discussed below. For *
Corresponding author. Address: Department of Chemistry, Wayne State University, Detroit, MI 48202, USA. E-mail address: [email protected] (A.G. Suits). 0009-2614/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.05.124
ICN, a particularly well-studied and closely related system, the A-band continuum is shifted about 40 nm to the red. Using LIF, Baronavski and McDonald [3] detected CN fragments created in the dissociation of ICN at 266 nm. Sabety-Dzvonik and Cody [20,21] measured the nascent CN distributions from flashlamp photolysis of ICN and ClCN. An extensive literature on ICN has since evolved, and its dissociation dynamics are far better understood than those of BrCN. Halpern and Jackson reported the first molecular beam studies of BrCN dissociation [7–9]. They initially measured the nascent CN quantum state distributions from the photolysis of BrCN at 193 nm using LIF. The results showed less than 6% of the population in the vibrational levels higher than v00 = 0. The rotational distribution in the v00 = 0 level was peaked sharply at about N00 = 65, with a cutoff at about N00 = 80. Fisher et al. [11] studied the photolysis of BrCN at a number of excimer wavelengths and they found that the population of vibrationally excited CN fragments decreases with increasing energy. They reported about 16% of the CN fragments are vibrationally excited at 193 nm. Using a supersonic beam, Heaven et al. [22] examined the production of CN from the photolysis of BrCN at 193 nm. They observed the production of v00 = 0 and v00 = 1 CN radicals in the ratio of 4:1. However, the same process was studied later by Lu et al. [9]. They reported no significant difference between
C. Huang et al. / Chemical Physics Letters 426 (2006) 242–247
the measurements in the cooled supersonic beam and thermal effusive beam, suggesting that the rotational distribution is dominated by the fragmentation process and can be treated in the nonrotating frame of the parent molecule. Based on the observation of highly rotationally excited CN products, they concluded that the upper electronic state of BrCN dissociation is bent. Vector correlation studies of the dissociation of BrCN between 206 and 260 nm were performed by the Jackson group [4]. Their results showed there are at least two potential energy surfaces involved in the photodissociation process, one which yields CN + Br (2P1/2) (Br*) and another that produces CN + Br(2P3/2) (Br). At shorter wavelengths, the transition to the surface yielding Br* is dominant, while at the longer wavelength, dissociation to Br prevails. The vector correlations and correlation angles indicate the J is primarily perpendicular to l and v, and v is predominantly parallel to l. Wannenmacher et al. inferred that the perpendicular contribution was greater for the ground state Br than the Br* product. More recently, using brute force orientation, Kong and coworkers [13,23–25] studied the photodissociation processes of ICN and BrCN. Evidence of a perpendicular component for BrCN photodissociation at 213 nm was observed. Furthermore, using the oriented BrCN molecules, they could determine that the initial transition was roughly 33% perpendicular in nature. In the present work, we show one-color photodissociation of BrCN at 234 nm, and two-color dissociation at 193 nm, probing both Br and Br*. The experiments were carried out in a supersonic molecular beam combined with the DC slice ion imaging technique. Virtually all earlier work has relied upon CN detection. Our study, employing halogen atom detection, allows correlation of the product CN rovibrational distribution to the particular spin–orbit state of the Br product and affords additional insight into the dissociation dynamics. 2. Experimental The overall experimental set-up employed in the DC slice imaging approach have been described in detail elsewhere [26,27]. Briefly, a pulsed supersonic molecular beam of BrCN seeded 3% in Ar is expanded into the source chamber and collimated by a skimmer. The beam entered into a velocity mapping electrode assembly optimized for DC slice imaging, and was intersected at right angles by two counter-propagating laser beams. The photolysis laser light (193 nm) was generated by an ArF excimer system running at 30 Hz (GAM EX10/300). In order to produce the linearly polarized light, the laser output was first allowed to propagate through a total of eight fused silica windows set at Brewster’s angle before being focused into the interaction region using a 40 cm focal length lens. The photolysis laser power was approximately 0.5 mJ/ pulse. The probe laser was provided by frequency tripling the output of a dye laser (Continuum Jaguar, LDS 698 dye) pumped by the 532 nm harmonic of a Nd:YAG laser
243
(Quanta Ray PRO 290). The probe beam (0.5 mJ/pulse) was then focused through a second 30 cm focal length lens. The Br(2P3/2) and Br(2P1/2) atomic photofragments produced in the dissociation event were probed using the following two (2 + 1) REMPI schemes: 2hm
hm
Br 4p5 ð2 P3=2 Þ ! Br 4p4 6pð4 P3=2 Þ ! Brþ 2hm
hm
Br 4p5 ð2 P1=2 Þ ! Br 4p4 6pð4 D1=2 Þ ! Brþ
233:68 nm 234:02 nm
For the photodissociation of BrCN at 234 nm, the probe laser was used for photolysis as well. Following ionization, the Br+ ions were accelerated through the multi-lens velocity mapping assembly and impacted upon a dual microchannel plate array of 70 mm diameter, which was coupled to a P-47 phosphor screen. The front of the MCP assembly is held at ground and the back plate was pulsed to gate the center slice of the photofragment ion cloud at a specific mass by application of a high voltage pulse (+2.2 kV/+1 kV bias, 80 ns width) using a commercial pulser (DEI PVX-4140). In this particular instance, a repeller electrode held at +700 V was used in conjunction with three additional focusing lenses in the velocity mapping scheme to stretch the photofragment ion cloud along the time-of-flight axis to around 400 ns. The resulting image was recorded using a CCD camera (Sony XCST50, 768 · 494 pixels) in conjunction with the IMACQ Megapixel acquisition program [28] recently developed in our group that enabled high-resolution real time ion counting with sub-pixel precision. The newly developed Megapixel imaging program IMAN also was used to analyze the data. The translational energy distributions were determined by the appropriate direct integration of the sliced images [26,29]. The anisotropy parameters (b values) were derived by analyzing the sliced images, as a function of radius, r (and thus recoil velocity, v), in terms of the function IðhÞ / 1 þ b2 P 2 ðcos hÞ þ b4 P 4 ðcos hÞ where h is the angle between the photolysis laser polarization and the direction of photofragment recoil, and Pn(x) is the nth Legendre polynomial in x. b2 takes limiting values of +2 and 1 in the case of prompt dissociation following a pure parallel or a pure perpendicular excitation of the parent molecule, respectively. The b4 parameters were included for the ground state Br atom to account for the possibility of alignment in the product atom. However, we found satisfactory fits with this term fixed at 0, so we have neglected atomic alignment in our analysis to-date and report only results for b2 (hereinafter referred to simply as b). 3. Results and discussion DC sliced images of Br and Br* atoms resulting from photolysis of BrCN molecules at the wavelengths around 234 nm are shown in Fig. 1. The corresponding total translational energy release distributions and recoil-dependent anisotropy parameters are shown in Fig. 2. The dissociation
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V=0 50 45 40
2.0 V=1
P (E) (arb.) / β (E)
Br
J = 50 45 40 V=2 55 50 45
1.0
V=3 50 45 40
0 0.0
0.4
0.8
1.2
1.6
2.0
Translational Energy (eV) V=1 30
2.0
20
0
P (E) (arb.) / β (E)
Br*
Fig. 1. DC sliced images of Br and Br* resulting from the BrCN photodissociation at the wavelengths of 233.68 and 234.02 nm, respectively.
V=0
1.0
0 0.0
30
0.4
0.8
20 0
1.2
1.6
Translational Energy (eV)
energy of BrCN is 3.60 ± 0.05 eV. Considering the spin– orbit splitting, 0.46 eV, the dissociation limit for Br* is 4.06 ± 0.05 eV, so the available energy at 234 nm is 1.70 eV for the Br product and 1.24 eV for the Br* product. To our knowledge, although there have been many previous studies, this is the first imaging experiment to examine the photodissociation processes of BrCN, and the first of any kind to detect the Br product. As a result, the images show features not seen in previous studies. The earlier vector correlation experiments [4] indicate that for the N00 = 40 rotational level of CN radical, the branching ratio was 100% Br* at a photolysis wavelength of 206 nm, 90% Br* at 222 nm, and 0% at 260 nm. The trend implies that the dissociation pathway is shifting to excited state Br* production at higher energy. However, it should be emphasized that this was an indirect measurement based on rotational profiles in CN. In any case, this trend is in contrast to that seen for ICN, which shows maximum branching to I* near the peak of the A-band absorption. However, in our imaging experiments, we observe significant yield of both spin– orbit states of Br at both 234 and 193 nm. In addition, the vibrational distributions of CN cofragments are quite distinct for Br and Br* images in both cases. For the results at 234 nm, there are four rings in the Br image. These are clearly associated with CN (v = 0–3). We used Gaussian fitting to the 0–15 data, where the v = 2, 3 peaks are well resolved, to determine the width and centers of the rotational distributions (Fig. 2) then adjusted the
Fig. 2. Total translational energy distribution (solid black line) and translational energy-dependent b parameters (dashed blue line) obtained from the images in Fig. 1 (234 nm dissociation). The coincident CN cofragment product state energies are shown on the spectra. Gaussian fits to the vibrational distribution are shown as thin blue lines. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
amplitudes to fit the full integrated translational energy distributions. The vibrational distribution coincident with Br peaks at v = 0 and decreases monotonically: the vibrational branching ratios are 1:0.31:0.13:0.05 for v = 0–3, respectively. However, the Br* image shows only two rings, corresponding to v = 0 and 1 of the electronic ground state of CN radical, and in this case the vibrational distribution is inverted, with a v = 0:1 ratio of 0.46:1. The rotational excitation is high in all vibrational states, and this is consistent with earlier LIF measurements of the CN product. The maximum rotational distribution is J = 45–50 for all CN product vibrational levels formed in coincidence with Br. The rotational excitation is considerably lower for Br* than for the Br product. The peak is at J 25 for both the v = 0 and 1 vibrational states, consistent with less bending in the excited state, as discussed below. The angular distributions are predominantly parallel for both Br and Br*, with slightly lower anisotropy for the Br channel. There is only a modest dependence of the b parameter on the translational energy, although we note
C. Huang et al. / Chemical Physics Letters 426 (2006) 242–247 V=0
P (E) (arb.) / β (E)
2.0
Br
J = 70
60 50
1.0
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Translational Energy (eV) 1.5 J = 70
Br* P (E) (arb.) / β (E)
the angular anisotropy for Br produced in coincidence with CN (v = 2, 3) is significantly lower (b = 1.1) than that for CN (v = 0, 1) (b = 1.4). We also carried out the experiment at 193 nm. Here, the available energy is 2.83 and 2.39 eV for the Br and Br* channels, respectively. This photolysis wavelength has been studied by many groups, as described in Section 1. The previous results implied sole production of Br* + CN (X2R+). However, we clearly detect both Br and Br* at 193 nm. Fig. 3 shows sliced images for Br and Br* resulting from 193 nm photodissociation of BrCN. The translational energy distributions and recoil-dependent b parameters indicate that v = 0 of the CN radical is dominant for both the Br and Br* images, as shown in Fig. 4. The peak of the rotational distributions are J = 63 and 58 for Br and Br* images, respectively, again consistent with earlier LIF measurements. In the Br* image, we also observed production of v = 1 of CN (X2R+) of about 10%, which falls between the 6% and 16% vibrational excitation reported by Jackson’s group and Fisher et al., respectively. However, this is for the Br* channel only. We have not determined the branching fraction; this will be pursued in the future as part of a more detailed investigation throughout the A-band. The cool vibrational excitation and high rotational excitation of the CN radical are in good agreement with the previous laser-induced-fluorescence (LIF) results. The angular distributions at 193 nm do not show a significant recoilenergy dependence, but in this case the Br* anisotropy (b 0.7) is substantially lower than that for Br (b 1.0). We now turn to the discussion of photodissociation dynamics. The high rotational excitation of the CN fragments arises from bending in upper electronic states of BrCN, as has long been known. Based on the observed
245
60
50 40 V=0
1.0
0.5 V=1
0 0.0
0.5
1.0
1.5
2.0
2.5
Translational Energy (eV) Fig. 4. Total translational energy distribution (solid black line) and translational energy-dependent b parameters (dashed blue line) obtained from the images in Fig. 3 (193 nm dissociation). The coincident CN cofragment product state energies are shown on the spectra. Gaussian fits to the vibrational distribution are shown as thin blue lines. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
peaks in the rotational profiles and the corresponding well-defined recoil velocities, we may assume initially negligible angular momentum to determine the impact parameters for the dissociation event. We equate the magnitude of the CN rotational angular momentum with the classical expression for the asymptotic orbital angular momentum hJ CN ¼ L ¼ lvb
Fig. 3. DC sliced images of Br and Br* from 193 nm photodissociation of BrCN.
ð1Þ
where JCN is the CN rotational quantum number, L is the orbital angular momentum, l is the reduced mass for the Br–CN system, v is the total relative recoil speed corresponding to that rotational state, and b is the impact parameter. For the ground state Br at 234 nm, we find ˚ , while for Br* at 234 nm, we find b 0.22 A ˚. b 0.44 A * For both Br and Br products at 193 nm, we find ˚ . As discussed further below, this may reflect difb 0.5 A ferent bending characteristics for the excited states leading to the different product channels. At wavelengths around 234 nm, the CN radicals are significantly vibrationally excited. However, this vibrational excitation decreases with increasing energy, which is inconsistent with an impulsive origin for the vibrational excitation. Instead, the energy dependence of the vibrational excitation appears to track
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the spin–orbit branching reported by Jackson and coworkers, suggesting perhaps a related origin in the dynamics on the electronic states involved. The photodissociation of ICN has been studied in greater detail [6,11,2,30–32], so a comparison of our results for BrCN photodissociation to this established case is instructive. For ICN, the analogous A-band continuum is shifted to longer wavelength, peaking at 250 nm rather than 210 nm in BrCN. The angular distribution of the photodissociation of ICN was first measured by Ling and Wilson [2]. The results implied that the products did not come from a pure parallel transition. They explained this deviation by following four possible factors: (1) rotational velocity of ICN; (2) a mixture of parallel and perpendicular transitions; (3) a bent of configuration of ICN during dissociation; and (4) the lifetime of ICN excited state. Wittig and coworkers [30] measured rotational and angular distributions of CN (X2R+) following photodissocation of ICN at 266 nm, at the red end of the absorption analogous to the 234 nm region in BrCN. The rotational excitation of CN in the I* channel was found to be much lower than in the I channel, just as we see for BrCN. The anisotropy parameter, b, for the I* and I channel were 1.6 and 1.3, respectively, consistent with the BrCN result at 234 nm. Based on these observations, they concluded that the initial excitation is via a parallel transition to a state which correlates with I* + CN products and the production of I + CN fragments is due to a nonadiabatic transition in the interaction region. The theoretical calculations [33–35] indicate there are five potential surfaces, of symmetry 3 P0þ , 1P1 (A 0 , A00 ), 3P1 (A 0 , A00 ) involved in the photodissociation processes. The properties of these potential surfaces are summarized as follows: (i) the transition dipole to the dominant electronically excited state (3 P0þ ) is parallel but there is also some amplitude for transition to the 1P1 and 3P1 states with perpendicular character, (ii) all of the excited electronic states are bent in the Franck–Condon region. At larger I–CN distance, the 3 P0þ state becomes linear, and (iii) the conical intersection between the 3 P0þ and 1 P1 surfaces occurs at larger I–CN distance. Black [31], and subsequently Hall and coworkers [32] reported detailed vector correlation studies of ICN photodissociation throughout the A-band in an effort to look more deeply into the diabatic and adiabatic dynamics underlying the dissociation. Although they detected the CN product, the high-resolution Doppler measurements combined with polarization sensitivity to the rotational alignment allowed them to disentangle these issues. They showed that I* production occurs both through a parallel diabatic path from the 4A 0 3 P0þ state and a perpendicular adiabatic path involving the 5A 0 1P1 state. For the I product, they found an additional perpendicular contribution from the A00 1P1 state [32]. The nonadiabatic interactions are strongly dependent on the bending angle, and the angular dependence of the coupling and the distinct product correlations are responsible for the different anisotropy parameters for each spin–orbit product.
It is notable that the spin–orbit correlated anisotropy changes significantly from 234 nm, where the Br* product is near-limiting parallel but the Br is substantially less parallel, to 193 nm where both products show substantial perpendicular contributions. This is quite distinct from the behavior of ICN throughout the A-band, in which case the I* shows significantly higher b values for all correlated rotational levels [32]. For BrCN at 234 nm, the difference in the anisotropy for Br and Br* is consistent with much greater bending in the course of the dissociation for the Br product, with corresponding deviation from axial recoil reducing the magnitude of b. This notion is supported by the larger impact parameters we infer for that channel. At 193 nm, the smaller impact parameters indicate that reduced anisotropy results from direct participation of perpendicular components in the dissociation rather than bending, in support of observations by Kong and coworkers [13]. Although it is likely that the general features discussed for ICN are operative for BrCN, it is clear that the reduced spin–orbit coupling, the associated shifts in the energy levels, and the altered interactions among the electronic states leads to some qualitative differences in its behavior. To account in detail for the distinct dynamics in BrCN, a full treatment will require detailed wavelength dependent spin–orbit branching ratio measurements and high quality ab initio calculations. Acknowledgements The authors thank Ms. Suk Kyoung Lee for her assistance with the figures. This work was supported by the National Science Foundation under award number CHE-0415393. References [1] A.J. Yencha, A.E.R. Malins, G.C. King, Chem. Phys. Lett. 392 (2004) 202. [2] J.H. Ling, K.R. Wilson, J. Chem. Phys. 63 (1975) 101. [3] A.P. Baronavski, J.R. McDonald, Chem. Phys. Lett. 45 (1977) 172. [4] E.A.J. Wannenmacher, H. Lin, W.H. Fink, A.J. Paul, W.M. Jackson, J. Chem. Phys. 95 (1991) 3431. [5] F. Shokoohi, S. Hay, C. Wittig, Chem. Phys. Lett. 110 (1984) 1. [6] I. Nadler, H. Reisler, C. Wittig, Chem. Phys. Lett. 103 (1984) 451. [7] J.A. Russell, I.A. McLaren, W.M. Jackson, J.B. Halpern, J. Phys. Chem. 91 (1987) 3248. [8] J.B. Halpern, W.M. Jackson, J. Phys. Chem. 86 (1982) 3528. [9] R. Lu, J.B. Halpern, W.M. Jackson, J. Phys. Chem. 88 (1984) 3419. [10] S. Hay, F. Shokoohi, S. Callister, C. Wittig, Chem. Phys. Lett. 118 (1985) 6. [11] W.H. Fisher, R. Eng, T. Carrington, C.H. Dugan, S.V. Filseth, C.M. Sadowski, Chem. Phys. 89 (1984) 457. [12] W.S. Felps, K. Rupnik, S.P. McGlynn, J. Phys. Chem. 95 (1991) 639. [13] K.J. Franks, H. Li, W. Kong, J. Chem. Phys. 111 (1999) 1884. [14] E. Wrede et al., J. Chem. Phys. 114 (2001) 2629. [15] T.P. Rakitzis et al., Chem. Phys. Lett. 364 (2002) 115. [16] M. Beckert, E.R. Wouters, M.N.R. Ashfold, J. Chem. Phys. 119 (2003) 9576. [17] M. Beckert, S.J. Greaves, M.N.R. Ashfold, Phys. Chem. Chem. Phys. 5 (2003) 308.
C. Huang et al. / Chemical Physics Letters 426 (2006) 242–247 [18] T.P. Rakitzis, P.C. Samartzis, R.L. Toomes, T.N. Kitsopoulos, J. Chem. Phys. 121 (2004) 7222. [19] S. Manzhos, C. Romanescu, H. Loock, J. Chem. Phys. 121 (2004) 11802. [20] M.J. Sabety-Dzvonik, R.J. Cody, J. Chem. Phys. 64 (1976) 4794. [21] M.J. Sabety-Dzvonik, R.J. Cody, J. Chem. Phys. 66 (1977) 125. [22] M. Heaven, T.A. Miller, V.E. Bondybey, Chem. Phys. Lett. 84 (1981) 1. [23] K.J. Franks, H. Li, R.J. Hanson, W. Kong, J. Phys. Chem. A 102 (1998) 7881. [24] H. Li, K.J. Franks, W. Kong, Chem. Phys. Lett. 300 (1999) 247. [25] K.J. Franks, H. Li, S. Kuy, W. Kong, Chem. Phys. Lett. 302 (1999) 151. [26] D. Townsend, M.P. Minitti, A.G. Suits, Rev. Sci. Instrum. 74 (2003) 2530.
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[27] D. Townsend et al., Science 306 (2004) 1158. [28] W. Li, S.D. Chambreau, S.A. Lahankar, A.G. Suits, Rev. Sci. Instrum. 76 (2005) 63106. [29] A.G.S.A. Komissarov, M.P. Minitti, G.E. Hall, J. Chem. Phys. 124 (2006) 014303. [30] I. Nadler, D. Mahgerefteh, H. Reisler, C. Wittig, J. Chem. Phys. 82 (1985) 3885. [31] J.F. Black, J. Chem. Phys. 98 (1993) 6853. [32] M.L. Costen, S.W. North, G.E. Hall, J. Chem. Phys. 111 (1999) 6735. [33] J. Qian, D. Tannor, Y. Amatatsu, K. Morokuma, J. Chem. Phys. 101 (1994) 9597. [34] S. Yabushita, K. Morokuma, Chem. Phys. Lett. 175 (1990) 518. [35] Y. Amatatsu, S. Yabushita, K. Morokuma, J. Chem. Phys. 100 (1994) 4894.
DC slice ion imaging of the ultraviolet photodissociation of BrCN Cunshun Huang a, Wen Li b
a,b
, Ruchira Silva a, Arthur G. Suits
a,b,*
a Department of Chemistry, Wayne State University, Detroit, MI 48202, USA Department of Chemistry, Stony Brook University, Stony Brook, NY 11794, USA
Received 21 April 2006; in final form 12 May 2006 Available online 10 June 2006
Abstract The photodissociation of jet-cooled BrCN molecules has been investigated by DC slice ion imaging of the Br atom product at wavelengths of 193 and 234 nm. The images reveal that both the ground state (2P3/2) and spin–orbit excited (2P1/2) bromine atoms are produced at both photolysis energies studied. The translational energy measurements show that at 193 nm the CN fragments are vibrationally cold, while the vibrational excitation is much higher in dissociation at 234 nm. Furthermore, the CN vibrational excitation in the spin–orbit excited Br product channel is higher at both wavelengths studied. 2006 Elsevier B.V. All rights reserved.
1. Introduction The cyanogen halides are an interesting class of molecules because they behave both chemically and physically much like diatomic halogen molecules [1]. This is due to the strength of the C–N bond (7.76 eV), as well as the very large electron affinity of the CN radical. The cyanogen halides have been a favorite system for those interested in photodissociation dynamics dating to the earliest days of photofragment translational spectroscopy [2]. The CN radical can be detected by laser-induced-fluorescence (LIF) [3–13], while halogen atoms can be detected by resonance-enhanced ionization (REMPI) time-of-flight spectroscopy [2,14–19]. Both methods make the products excellent probes for studying the dynamics of these dissociation processes. The first absorption band in BrCN is a weak continuum beginning at about 250 nm and extending below 185 nm, with a peak around 210 nm. Although the excited states have not been clearly identified in the case of BrCN, at least two states are involved based on the production of ground state and spin–orbit excited Br as discussed below. For *
Corresponding author. Address: Department of Chemistry, Wayne State University, Detroit, MI 48202, USA. E-mail address: [email protected] (A.G. Suits). 0009-2614/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.05.124
ICN, a particularly well-studied and closely related system, the A-band continuum is shifted about 40 nm to the red. Using LIF, Baronavski and McDonald [3] detected CN fragments created in the dissociation of ICN at 266 nm. Sabety-Dzvonik and Cody [20,21] measured the nascent CN distributions from flashlamp photolysis of ICN and ClCN. An extensive literature on ICN has since evolved, and its dissociation dynamics are far better understood than those of BrCN. Halpern and Jackson reported the first molecular beam studies of BrCN dissociation [7–9]. They initially measured the nascent CN quantum state distributions from the photolysis of BrCN at 193 nm using LIF. The results showed less than 6% of the population in the vibrational levels higher than v00 = 0. The rotational distribution in the v00 = 0 level was peaked sharply at about N00 = 65, with a cutoff at about N00 = 80. Fisher et al. [11] studied the photolysis of BrCN at a number of excimer wavelengths and they found that the population of vibrationally excited CN fragments decreases with increasing energy. They reported about 16% of the CN fragments are vibrationally excited at 193 nm. Using a supersonic beam, Heaven et al. [22] examined the production of CN from the photolysis of BrCN at 193 nm. They observed the production of v00 = 0 and v00 = 1 CN radicals in the ratio of 4:1. However, the same process was studied later by Lu et al. [9]. They reported no significant difference between
C. Huang et al. / Chemical Physics Letters 426 (2006) 242–247
the measurements in the cooled supersonic beam and thermal effusive beam, suggesting that the rotational distribution is dominated by the fragmentation process and can be treated in the nonrotating frame of the parent molecule. Based on the observation of highly rotationally excited CN products, they concluded that the upper electronic state of BrCN dissociation is bent. Vector correlation studies of the dissociation of BrCN between 206 and 260 nm were performed by the Jackson group [4]. Their results showed there are at least two potential energy surfaces involved in the photodissociation process, one which yields CN + Br (2P1/2) (Br*) and another that produces CN + Br(2P3/2) (Br). At shorter wavelengths, the transition to the surface yielding Br* is dominant, while at the longer wavelength, dissociation to Br prevails. The vector correlations and correlation angles indicate the J is primarily perpendicular to l and v, and v is predominantly parallel to l. Wannenmacher et al. inferred that the perpendicular contribution was greater for the ground state Br than the Br* product. More recently, using brute force orientation, Kong and coworkers [13,23–25] studied the photodissociation processes of ICN and BrCN. Evidence of a perpendicular component for BrCN photodissociation at 213 nm was observed. Furthermore, using the oriented BrCN molecules, they could determine that the initial transition was roughly 33% perpendicular in nature. In the present work, we show one-color photodissociation of BrCN at 234 nm, and two-color dissociation at 193 nm, probing both Br and Br*. The experiments were carried out in a supersonic molecular beam combined with the DC slice ion imaging technique. Virtually all earlier work has relied upon CN detection. Our study, employing halogen atom detection, allows correlation of the product CN rovibrational distribution to the particular spin–orbit state of the Br product and affords additional insight into the dissociation dynamics. 2. Experimental The overall experimental set-up employed in the DC slice imaging approach have been described in detail elsewhere [26,27]. Briefly, a pulsed supersonic molecular beam of BrCN seeded 3% in Ar is expanded into the source chamber and collimated by a skimmer. The beam entered into a velocity mapping electrode assembly optimized for DC slice imaging, and was intersected at right angles by two counter-propagating laser beams. The photolysis laser light (193 nm) was generated by an ArF excimer system running at 30 Hz (GAM EX10/300). In order to produce the linearly polarized light, the laser output was first allowed to propagate through a total of eight fused silica windows set at Brewster’s angle before being focused into the interaction region using a 40 cm focal length lens. The photolysis laser power was approximately 0.5 mJ/ pulse. The probe laser was provided by frequency tripling the output of a dye laser (Continuum Jaguar, LDS 698 dye) pumped by the 532 nm harmonic of a Nd:YAG laser
243
(Quanta Ray PRO 290). The probe beam (0.5 mJ/pulse) was then focused through a second 30 cm focal length lens. The Br(2P3/2) and Br(2P1/2) atomic photofragments produced in the dissociation event were probed using the following two (2 + 1) REMPI schemes: 2hm
hm
Br 4p5 ð2 P3=2 Þ ! Br 4p4 6pð4 P3=2 Þ ! Brþ 2hm
hm
Br 4p5 ð2 P1=2 Þ ! Br 4p4 6pð4 D1=2 Þ ! Brþ
233:68 nm 234:02 nm
For the photodissociation of BrCN at 234 nm, the probe laser was used for photolysis as well. Following ionization, the Br+ ions were accelerated through the multi-lens velocity mapping assembly and impacted upon a dual microchannel plate array of 70 mm diameter, which was coupled to a P-47 phosphor screen. The front of the MCP assembly is held at ground and the back plate was pulsed to gate the center slice of the photofragment ion cloud at a specific mass by application of a high voltage pulse (+2.2 kV/+1 kV bias, 80 ns width) using a commercial pulser (DEI PVX-4140). In this particular instance, a repeller electrode held at +700 V was used in conjunction with three additional focusing lenses in the velocity mapping scheme to stretch the photofragment ion cloud along the time-of-flight axis to around 400 ns. The resulting image was recorded using a CCD camera (Sony XCST50, 768 · 494 pixels) in conjunction with the IMACQ Megapixel acquisition program [28] recently developed in our group that enabled high-resolution real time ion counting with sub-pixel precision. The newly developed Megapixel imaging program IMAN also was used to analyze the data. The translational energy distributions were determined by the appropriate direct integration of the sliced images [26,29]. The anisotropy parameters (b values) were derived by analyzing the sliced images, as a function of radius, r (and thus recoil velocity, v), in terms of the function IðhÞ / 1 þ b2 P 2 ðcos hÞ þ b4 P 4 ðcos hÞ where h is the angle between the photolysis laser polarization and the direction of photofragment recoil, and Pn(x) is the nth Legendre polynomial in x. b2 takes limiting values of +2 and 1 in the case of prompt dissociation following a pure parallel or a pure perpendicular excitation of the parent molecule, respectively. The b4 parameters were included for the ground state Br atom to account for the possibility of alignment in the product atom. However, we found satisfactory fits with this term fixed at 0, so we have neglected atomic alignment in our analysis to-date and report only results for b2 (hereinafter referred to simply as b). 3. Results and discussion DC sliced images of Br and Br* atoms resulting from photolysis of BrCN molecules at the wavelengths around 234 nm are shown in Fig. 1. The corresponding total translational energy release distributions and recoil-dependent anisotropy parameters are shown in Fig. 2. The dissociation
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V=0 50 45 40
2.0 V=1
P (E) (arb.) / β (E)
Br
J = 50 45 40 V=2 55 50 45
1.0
V=3 50 45 40
0 0.0
0.4
0.8
1.2
1.6
2.0
Translational Energy (eV) V=1 30
2.0
20
0
P (E) (arb.) / β (E)
Br*
Fig. 1. DC sliced images of Br and Br* resulting from the BrCN photodissociation at the wavelengths of 233.68 and 234.02 nm, respectively.
V=0
1.0
0 0.0
30
0.4
0.8
20 0
1.2
1.6
Translational Energy (eV)
energy of BrCN is 3.60 ± 0.05 eV. Considering the spin– orbit splitting, 0.46 eV, the dissociation limit for Br* is 4.06 ± 0.05 eV, so the available energy at 234 nm is 1.70 eV for the Br product and 1.24 eV for the Br* product. To our knowledge, although there have been many previous studies, this is the first imaging experiment to examine the photodissociation processes of BrCN, and the first of any kind to detect the Br product. As a result, the images show features not seen in previous studies. The earlier vector correlation experiments [4] indicate that for the N00 = 40 rotational level of CN radical, the branching ratio was 100% Br* at a photolysis wavelength of 206 nm, 90% Br* at 222 nm, and 0% at 260 nm. The trend implies that the dissociation pathway is shifting to excited state Br* production at higher energy. However, it should be emphasized that this was an indirect measurement based on rotational profiles in CN. In any case, this trend is in contrast to that seen for ICN, which shows maximum branching to I* near the peak of the A-band absorption. However, in our imaging experiments, we observe significant yield of both spin– orbit states of Br at both 234 and 193 nm. In addition, the vibrational distributions of CN cofragments are quite distinct for Br and Br* images in both cases. For the results at 234 nm, there are four rings in the Br image. These are clearly associated with CN (v = 0–3). We used Gaussian fitting to the 0–15 data, where the v = 2, 3 peaks are well resolved, to determine the width and centers of the rotational distributions (Fig. 2) then adjusted the
Fig. 2. Total translational energy distribution (solid black line) and translational energy-dependent b parameters (dashed blue line) obtained from the images in Fig. 1 (234 nm dissociation). The coincident CN cofragment product state energies are shown on the spectra. Gaussian fits to the vibrational distribution are shown as thin blue lines. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
amplitudes to fit the full integrated translational energy distributions. The vibrational distribution coincident with Br peaks at v = 0 and decreases monotonically: the vibrational branching ratios are 1:0.31:0.13:0.05 for v = 0–3, respectively. However, the Br* image shows only two rings, corresponding to v = 0 and 1 of the electronic ground state of CN radical, and in this case the vibrational distribution is inverted, with a v = 0:1 ratio of 0.46:1. The rotational excitation is high in all vibrational states, and this is consistent with earlier LIF measurements of the CN product. The maximum rotational distribution is J = 45–50 for all CN product vibrational levels formed in coincidence with Br. The rotational excitation is considerably lower for Br* than for the Br product. The peak is at J 25 for both the v = 0 and 1 vibrational states, consistent with less bending in the excited state, as discussed below. The angular distributions are predominantly parallel for both Br and Br*, with slightly lower anisotropy for the Br channel. There is only a modest dependence of the b parameter on the translational energy, although we note
C. Huang et al. / Chemical Physics Letters 426 (2006) 242–247 V=0
P (E) (arb.) / β (E)
2.0
Br
J = 70
60 50
1.0
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Translational Energy (eV) 1.5 J = 70
Br* P (E) (arb.) / β (E)
the angular anisotropy for Br produced in coincidence with CN (v = 2, 3) is significantly lower (b = 1.1) than that for CN (v = 0, 1) (b = 1.4). We also carried out the experiment at 193 nm. Here, the available energy is 2.83 and 2.39 eV for the Br and Br* channels, respectively. This photolysis wavelength has been studied by many groups, as described in Section 1. The previous results implied sole production of Br* + CN (X2R+). However, we clearly detect both Br and Br* at 193 nm. Fig. 3 shows sliced images for Br and Br* resulting from 193 nm photodissociation of BrCN. The translational energy distributions and recoil-dependent b parameters indicate that v = 0 of the CN radical is dominant for both the Br and Br* images, as shown in Fig. 4. The peak of the rotational distributions are J = 63 and 58 for Br and Br* images, respectively, again consistent with earlier LIF measurements. In the Br* image, we also observed production of v = 1 of CN (X2R+) of about 10%, which falls between the 6% and 16% vibrational excitation reported by Jackson’s group and Fisher et al., respectively. However, this is for the Br* channel only. We have not determined the branching fraction; this will be pursued in the future as part of a more detailed investigation throughout the A-band. The cool vibrational excitation and high rotational excitation of the CN radical are in good agreement with the previous laser-induced-fluorescence (LIF) results. The angular distributions at 193 nm do not show a significant recoilenergy dependence, but in this case the Br* anisotropy (b 0.7) is substantially lower than that for Br (b 1.0). We now turn to the discussion of photodissociation dynamics. The high rotational excitation of the CN fragments arises from bending in upper electronic states of BrCN, as has long been known. Based on the observed
245
60
50 40 V=0
1.0
0.5 V=1
0 0.0
0.5
1.0
1.5
2.0
2.5
Translational Energy (eV) Fig. 4. Total translational energy distribution (solid black line) and translational energy-dependent b parameters (dashed blue line) obtained from the images in Fig. 3 (193 nm dissociation). The coincident CN cofragment product state energies are shown on the spectra. Gaussian fits to the vibrational distribution are shown as thin blue lines. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
peaks in the rotational profiles and the corresponding well-defined recoil velocities, we may assume initially negligible angular momentum to determine the impact parameters for the dissociation event. We equate the magnitude of the CN rotational angular momentum with the classical expression for the asymptotic orbital angular momentum hJ CN ¼ L ¼ lvb
Fig. 3. DC sliced images of Br and Br* from 193 nm photodissociation of BrCN.
ð1Þ
where JCN is the CN rotational quantum number, L is the orbital angular momentum, l is the reduced mass for the Br–CN system, v is the total relative recoil speed corresponding to that rotational state, and b is the impact parameter. For the ground state Br at 234 nm, we find ˚ , while for Br* at 234 nm, we find b 0.22 A ˚. b 0.44 A * For both Br and Br products at 193 nm, we find ˚ . As discussed further below, this may reflect difb 0.5 A ferent bending characteristics for the excited states leading to the different product channels. At wavelengths around 234 nm, the CN radicals are significantly vibrationally excited. However, this vibrational excitation decreases with increasing energy, which is inconsistent with an impulsive origin for the vibrational excitation. Instead, the energy dependence of the vibrational excitation appears to track
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the spin–orbit branching reported by Jackson and coworkers, suggesting perhaps a related origin in the dynamics on the electronic states involved. The photodissociation of ICN has been studied in greater detail [6,11,2,30–32], so a comparison of our results for BrCN photodissociation to this established case is instructive. For ICN, the analogous A-band continuum is shifted to longer wavelength, peaking at 250 nm rather than 210 nm in BrCN. The angular distribution of the photodissociation of ICN was first measured by Ling and Wilson [2]. The results implied that the products did not come from a pure parallel transition. They explained this deviation by following four possible factors: (1) rotational velocity of ICN; (2) a mixture of parallel and perpendicular transitions; (3) a bent of configuration of ICN during dissociation; and (4) the lifetime of ICN excited state. Wittig and coworkers [30] measured rotational and angular distributions of CN (X2R+) following photodissocation of ICN at 266 nm, at the red end of the absorption analogous to the 234 nm region in BrCN. The rotational excitation of CN in the I* channel was found to be much lower than in the I channel, just as we see for BrCN. The anisotropy parameter, b, for the I* and I channel were 1.6 and 1.3, respectively, consistent with the BrCN result at 234 nm. Based on these observations, they concluded that the initial excitation is via a parallel transition to a state which correlates with I* + CN products and the production of I + CN fragments is due to a nonadiabatic transition in the interaction region. The theoretical calculations [33–35] indicate there are five potential surfaces, of symmetry 3 P0þ , 1P1 (A 0 , A00 ), 3P1 (A 0 , A00 ) involved in the photodissociation processes. The properties of these potential surfaces are summarized as follows: (i) the transition dipole to the dominant electronically excited state (3 P0þ ) is parallel but there is also some amplitude for transition to the 1P1 and 3P1 states with perpendicular character, (ii) all of the excited electronic states are bent in the Franck–Condon region. At larger I–CN distance, the 3 P0þ state becomes linear, and (iii) the conical intersection between the 3 P0þ and 1 P1 surfaces occurs at larger I–CN distance. Black [31], and subsequently Hall and coworkers [32] reported detailed vector correlation studies of ICN photodissociation throughout the A-band in an effort to look more deeply into the diabatic and adiabatic dynamics underlying the dissociation. Although they detected the CN product, the high-resolution Doppler measurements combined with polarization sensitivity to the rotational alignment allowed them to disentangle these issues. They showed that I* production occurs both through a parallel diabatic path from the 4A 0 3 P0þ state and a perpendicular adiabatic path involving the 5A 0 1P1 state. For the I product, they found an additional perpendicular contribution from the A00 1P1 state [32]. The nonadiabatic interactions are strongly dependent on the bending angle, and the angular dependence of the coupling and the distinct product correlations are responsible for the different anisotropy parameters for each spin–orbit product.
It is notable that the spin–orbit correlated anisotropy changes significantly from 234 nm, where the Br* product is near-limiting parallel but the Br is substantially less parallel, to 193 nm where both products show substantial perpendicular contributions. This is quite distinct from the behavior of ICN throughout the A-band, in which case the I* shows significantly higher b values for all correlated rotational levels [32]. For BrCN at 234 nm, the difference in the anisotropy for Br and Br* is consistent with much greater bending in the course of the dissociation for the Br product, with corresponding deviation from axial recoil reducing the magnitude of b. This notion is supported by the larger impact parameters we infer for that channel. At 193 nm, the smaller impact parameters indicate that reduced anisotropy results from direct participation of perpendicular components in the dissociation rather than bending, in support of observations by Kong and coworkers [13]. Although it is likely that the general features discussed for ICN are operative for BrCN, it is clear that the reduced spin–orbit coupling, the associated shifts in the energy levels, and the altered interactions among the electronic states leads to some qualitative differences in its behavior. To account in detail for the distinct dynamics in BrCN, a full treatment will require detailed wavelength dependent spin–orbit branching ratio measurements and high quality ab initio calculations. Acknowledgements The authors thank Ms. Suk Kyoung Lee for her assistance with the figures. This work was supported by the National Science Foundation under award number CHE-0415393. References [1] A.J. Yencha, A.E.R. Malins, G.C. King, Chem. Phys. Lett. 392 (2004) 202. [2] J.H. Ling, K.R. Wilson, J. Chem. Phys. 63 (1975) 101. [3] A.P. Baronavski, J.R. McDonald, Chem. Phys. Lett. 45 (1977) 172. [4] E.A.J. Wannenmacher, H. Lin, W.H. Fink, A.J. Paul, W.M. Jackson, J. Chem. Phys. 95 (1991) 3431. [5] F. Shokoohi, S. Hay, C. Wittig, Chem. Phys. Lett. 110 (1984) 1. [6] I. Nadler, H. Reisler, C. Wittig, Chem. Phys. Lett. 103 (1984) 451. [7] J.A. Russell, I.A. McLaren, W.M. Jackson, J.B. Halpern, J. Phys. Chem. 91 (1987) 3248. [8] J.B. Halpern, W.M. Jackson, J. Phys. Chem. 86 (1982) 3528. [9] R. Lu, J.B. Halpern, W.M. Jackson, J. Phys. Chem. 88 (1984) 3419. [10] S. Hay, F. Shokoohi, S. Callister, C. Wittig, Chem. Phys. Lett. 118 (1985) 6. [11] W.H. Fisher, R. Eng, T. Carrington, C.H. Dugan, S.V. Filseth, C.M. Sadowski, Chem. Phys. 89 (1984) 457. [12] W.S. Felps, K. Rupnik, S.P. McGlynn, J. Phys. Chem. 95 (1991) 639. [13] K.J. Franks, H. Li, W. Kong, J. Chem. Phys. 111 (1999) 1884. [14] E. Wrede et al., J. Chem. Phys. 114 (2001) 2629. [15] T.P. Rakitzis et al., Chem. Phys. Lett. 364 (2002) 115. [16] M. Beckert, E.R. Wouters, M.N.R. Ashfold, J. Chem. Phys. 119 (2003) 9576. [17] M. Beckert, S.J. Greaves, M.N.R. Ashfold, Phys. Chem. Chem. Phys. 5 (2003) 308.
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