- Email: [email protected]
Ultrafast photodissociation dynamics of cyanobenzene near the ionization threshold
28 December 2001
Chemical Physics Letters 350 (2001) 495±501 www.elsevier.com/locate/cplett
Ultrafast photodissociation dynamics of cyanobenzene near the ionization threshold Xiu-Ping Hong, Wei-Kan Chen, Po-Yuan Cheng
*
Department of Chemistry, National Tsing Hua University, Hsinchu 30043, Taiwan, ROC Received 25 September 2001; in ®nal form 6 November 2001
Abstract The photodissociation of cyanobenzene C6 H5 CN at a high energy of 9:6 eV near the ionization threshold has been investigated using femtosecond time-resolved laser-induced ¯uorescence (LIF) spectroscopy. Cyanobenzene molecules were excited by three-photon excitation at 388.6 nm and the temporal evolution of the free CN(X) fragment formation was probed in real time by monitoring the CN X ! B LIF signal. The results revealed that the CN(X) products are formed on three very dierent timescales, suggesting that the dissociation proceeds through at least three dissociation pathways at such a high energy. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction The cyano radical (CN) is known to play an important role in the chemistry of many dierent extraterrestrial environments [1,2]. It has been proposed that the reactive CN radical can react with many unsaturated hydrocarbons to form a variety of cyano organic compounds that have been identi®ed in interstellar clouds and planetary atmospheres [3±5]. A recent laboratory study [6] employing the crossed molecular beams technique has shown that the reaction of CN X 2 R with benzene to form cyanobenzene C6 H5 CN is barrierless and exothermic. On the other hand, recent astrochemical studies have also suggested that polycyclic aromatic hydrocarbons (PAHs) are
*
Corresponding author. Fax: +886-3-571-1082. E-mail address: [email protected] (P.-Y. Cheng).
ubiquitous and abundant in many extraterrestrial environments [7]. The combination of these results has led to a hypothesis that cyanobenzene and other cyano aromatics may exist in such environments [6]. Accordingly, the photodissociation of cyano aromatic compounds at high energies may play an important role in astrochemistry because high-energy radiation is omnipresent in most extraterrestrial environments. From the fundamental point of view, the ±CN group is often considered as a `pseudohalogen', and therefore it is intriguing to compare the photochemistry of cyanobenzene with the more widely studied halobenzenes. The unsaturated substituent ±CBN may give rise to low-lying electronic excited states that are not present in its halide counterparts and lead to new photochemical pathways [8]. The C6 H5 ±CN bond energy is 533 kJ=mol [9], much stronger than typical C6 H5 ±X bond energies in halobenzenes due to the
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 1 3 4 7 - 1
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extensive conjugation between the CN group and the phenyl ring. Although the photochemistry of cyanobenzene in the S1 energy region has attracted some attention because of its relation to the intramolecular charge-transfer reaction [10,11], the photodissociation dynamics at higher energies has not yet been well studied. The only report, to the best of our knowledge, is by Park et al. [9] who have studied the photodissociation of cyanobenzene at 193 nm by measuring the internal state distributions of the CN fragments and the product translational energy distributions. They concluded that the dissociation occurs on a triplet surface and that the reaction time is 4 ps < s < 100 ns [9]. In this work, we studied the photodissociation of cyanobenzene using femtosecond (fs) pump± probe laser-induced ¯uorescence (LIF) spectroscopy. The reaction under investigation is Ph±CN ! Ph CN X, where Ph denotes the phenyl radical C6 H5 . The molecules were excited through a three-photon transition at 388.6 nm to a total energy of 9:6 eV. The temporal evolution of the CN(X) product formation was then probed in real time by monitoring the CN X ! B LIF signal using a delayed probe pulse also at 388.6 nm. Our results revealed that the CN(X) photofragments are produced through three very dierent temporal processes, providing important implications to the photodissociation dynamics of cyanobenzene at high energies. 2. Experimental The fs laser system employed in this work consisted of a self-mode-locked Ti:sapphire laser (Spectra Physics, Tsunami) and a chirp-pulse regenerative ampli®er (`CPA', Spectra Physics, Spit®re). The oscillator output wavelength was tuned to around 777 nm and the output pulses were selectively ampli®ed in the CPA, producing fs pulses of 1 mJ=pulse in energy and 120 fs FWHM in duration at a repetition rate of 1 kHz. The CPA output was then split into two parts by a 30/70 beamsplitter. The transmitted and re¯ected beams were both frequency-doubled in
thin BBO crystals to produce fs pulses centered at 388.6 nm for the detection of CN radicals. Neutral density ®lters were placed in the beam paths to adjust the laser irradiance such that the pulses in one beam are much more intense than those in the other beam. Hereafter, we will refer to the more intense pulses as the `pump' and the weaker ones the `probe'. For the transients presented here the probe laser was attenuated to about 10± 20 times weaker than the pump laser, resulting in a negligible negative-time signal in comparison with the positive-time signal due to the multiphoton pumping scheme employed here. A polarizer was also placed in the probe beam path to set its polarization at the magic angle (54.7°) with respect to that of the pump in order to minimize any rotational coherence eect. The probe beam was sent through a computer-controlled optical delay line. The pump and probe beams were then collinearly recombined via a neutral density beamsplitter and focused into a gas cell through a f 500 mm lens. The typical pulse energies before entering the cell were 5±20 lJ for the pump and 0:5±1:0 lJ for the probe. The cross-correlation of the pump and probe pulses was 200 fs FWHM. Cyanobenzene (Aldrich, 99%) was subjected to several freeze-pump-thaw cycles prior to use and its vapor was slowly ¯owed into the cell through a needle valve to maintain a pressure of 450 mTorr in the cell during the experiments. The laser-induced ¯uorescence was collected at right angle to the direction of the laser beams through a conventional LIF detection system. A 35 nm (FWHM) bandpass ®lter centered at 405 nm and a long-pass ®lter (380 nm cut-o) were placed in the light collection system to reduce the scattered light and to ensure that only ¯uorescence photons in the 388±422 nm spectral region can be detected eciently. The signal from the photomultiplier tube was ampli®ed and then sent to a boxcar gate integrator for signal averaging. The gate was typically 100 ns in duration and was delayed from the laser pulse to avoid interference from the scattered light signal. The transients were obtained by monitoring the ¯uorescence signal while the pump vs. probe delay time was scanned.
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3. Results and discussion Transient CN X ! B LIF signal was readily observed upon excitation of cyanobenzene vapor at 388.6 nm under the experimental conditions described above. Fig. 1 shows the observed LIF signal as a function of pump±probe delay time for three timescales. This transient ¯uorescence signal was assigned to the CN(X) fragments arising from the photodissociation of cyanobenzene on the bases of following reasons: (1) the signal was at the maximum when the probe laser wavelength was set at the bandhead of the CN X ! B 0; 0 transition near 388.6 nm; (2) the spectral ®lters used in the light collection system allowed only photons in the CN B ! X 0; 0 ¯uorescence spectral region to enter the photomultiplier tube; (3) the observed ¯uorescence lifetime is 40 ns; shorter (not longer) than the literature value of 63 ns for the CN B state under collisionless conditions [12]. The discrepancy is due to the relatively high pressure 450 mTorr of gas used here. In fact, it has been reported that the lifetime of CN B state measured in a 700 mTorr BrCN vapor is 33 ns [13], qualitatively consistent with our observations. These combined evidences unambiguously indicate that the observed transient ¯uorescence is due to free CN(X) radicals produced in the photodissociation of cyanobenzene. The observed transients apparently consist of several rise components of dierent timescales. The signal ®rst rises rapidly within the ®rst ps (see Fig. 1A) and then increases at a slower pace up to 20 ps (see Fig. 1B). The second component is then followed by an even slower rise that does not reach a constant level even at the longest delay time of 2.2 ns available here (see Figs. 1C and 2B). Since it is not energetically possible for one 388 nm photon 308 kJ=mol to eliminate CN from cyanobenzene, the observed CN fragments must be a consequence of multi-photon excitation. We have carried out pump-laser irradiance dependence measurements, shown in Fig. 2A, and found that the dependence exponent n of the transient CN LIF signal is close to three around the typical pump-laser pulse energy used here 10±20 lJ= pulse for delay times shorter than 500 ps. This strongly suggests that a three-photon excitation is
Fig. 1. CN(X) transients recorded by monitoring the CN X ! B LIF signal as a function of the pump±probe delay time upon multi-photon excitation of cyanobenzene at 388.6 nm for three dierent timescales. The three transients have been normalized to the same maximum intensity for the sake of clarity. The open circles are the experimental data points and the solid lines are results of nonlinear least-squares ®ts to a sum of a delta function at the zero delay time and a delayed molecular response function, both convoluted with a 200 fs FWHM Gaussian response function. The molecular response function consists of four exponential-rise components with the longest one being ®xed at 10 ns in the ®ts. A single set of time constants were used to obtain the best ®ts simultaneously for all three transients. The time constants obtained from the ®ts are indicated in the ®gures and the amplitude ratio obtained from ®tting the transient in (C) is A1 : A2 : A3 16 : 33 : 51.
responsible for producing the observed transients. On the other hand, we also noted that the irradiance dependence at longer times > 500 ps is slightly lower than that at shorter times (see Fig.
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Fig. 2. (A) Pump-laser irradiance dependences of the transient CN(X) LIF signal arising from the photodissociation of cyanobenzene at () 0.6 ps, () 10 ps, and (}) 900 ps. We assumed that the dependence of the transient signal S on the laser irradiance I follows S rI n . The data points were plotted on a log±log scale and the slope of the best-®tted straight line gave the dependence exponent n. (B) CN(X) transients obtained with three dierent pump-laser pulse energies; (a) 16 lJ=pulse, (b) 5 lJ=pulse, and (c) 2:5 lJ=pulse. Note that the timescale shown here is much longer than that in Fig. 1C. The three transients have been normalized to the same intensity at 2:2 ns for the sake of clarity.
2A), implying that a very slow component of lower pump-laser irradiance dependence may be present. To further clarify this point, we have measured CN(X) transients, shown in Fig. 2B, at three different pump-laser irradiances for a longer delay time of 2.2 ns. It is clear that these transients are strongly pump-laser irradiance dependent around
the pump-pulse energies used here and there exists a very slow component that becomes predominant under the very low pump-laser irradiance conditions. This extremely slow component has a lower pump-laser irradiance dependence than those of the faster components, making the overall irradiance dependence at longer times > 500 ps become lower than three. Since one 388 nm photon is not energetically capable of eliminating CN from cyanobenzene, we assigned this extremely long component to the photodissociation of cyanobenzene at the two-photon excitation level. Although it is not possible to determine the rise time of this two-photon component due to the limited optical delay available here, we estimated that it must be longer than 10 ns. On the other hand, the transient signal at shorter times < 500 ps is essentially dominated by the photodissociation of cyanobenzene at the three-photon excitation level. It is worth mentioning here that the success of separating the three-photon and the two-photon dissociation dynamics simply lies in the fact that they occur in very dierent timescales. Moreover, although four-photon excitation is also likely to occur, it ionizes cyanobenzene below its cation dissociation threshold (see Fig. 3) and thus does not produce any interfering CN(X) signal. One also notes that there is a sharp temporal feature at the zero delay time. This `initial spike' has a higher pump- and probe-laser irradiance dependence than those of the subsequent rise features. Under higher irradiance conditions, this sharp feature became much larger and we found that it resembles the cross-correlation trace of the pump and probe pulses in terms of the temporal width and position. Thus, this initial spike serves as a convenient internal reference to the zero delay time. We assigned this initial spike to the enhanced multi-photon absorption occurring when the pump and probe pulses overlap in time, which leads to the production of the ¯uorescent CN B state directly. The three transients shown in Fig. 1 were ®tted simultaneously to a multi-exponential rise function using a single set of time constants with the longest one being ®xed at 10 ns for the two-photon component described above. We also found that it is necessary to include an initial delay of 150 fs
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Fig. 3. Schematic diagram showing qualitative potential energy curves along the Ph±CN dissociation coordinate and energetics of some relevant reactant and product states. State energy levels are drawn to scale based on the best-known literature values. The cyanobenzene T1 zero-point level (ZPL) lies at about 3.35 eV above that of the S0 state, as determined by a phophorescence spectroscopic study in solid matrices [16]. The cyanobenzene S1 ZPL energy (4.53 eV) was taken from jet spectroscopic measurements [10], and the S2 ZPL energy (6.24 eV) was taken from an ab inito study using the CASPT2 method [17]. The lowest excited state ZPL energy of the phenyl radical (2.43 eV) was obtained from absorption spectroscopic measurements in low-temperature solid matrices [18]. The ionization potential of the phenyl radical (8.1 eV) was taken from photoionization measurements [19]. Curved arrows illustrate the dissociation pathways that are responsible for producing the observed CN(X) transient, as described in the text.
in order to ®t the very early part < 0:5 ps of the transient shown in Fig. 1A with acceptable quality. Careful analyses revealed that, in addition to the 10 ns two-photon component, the observed transients consist of at least three other exponentialrise components that are due to the three-photon dissociation. Trial ®ts using a smaller number of components yielded de®nitely unsatisfactory
499
results. The three time constants obtained from the ®ts are: s1 0:35 0:05 ps, s2 9:6 1:5 ps, and s3 180 80 ps. Their relative weights are A1 : A2 : A3 16 : 33 : 51. Note that the exact value assumed for the two-photon component does not aect the results of the ®t signi®cantly. For example, a ®t with the two-photon component ®xed at 100 ns, which is the upper limit given by Park et al. [9], gave almost identical values for s1 and s2 but shortened s3 to 130 ps. These small variations in the time constants do not aect the general picture described below. The total energy reached by the three-photon excitation at 388.6 nm amounts to 9.57 eV, just 0.16 eV below the ionization threshold of cyanobenzene at 9.73 eV [14]. It is likely that the initial state prepared in the excitation step is a high-lying Rydberg state. The energies of Rydberg states in the vicinity of the ionization threshold can be approximated by the Rydberg formula: En IP R= 2 n d [15], where n is the principle quantum number of the Rydberg series, IP is the ionization potential, R 13:6 eV is the Rydberg constant, and d is the quantum defect. For high n's the quantum defect is much smaller than n [15]; and the formula gives n 10 for the initial state prepared by three-photon excitation of cyanobenzene at 388.6 nm. The initial delay of 150 fs and the ®rst rise component of 350 fs observed in the CN(X) formation suggest that this initially excited Rydberg state is short-lived but does not undergo direct dissociation. The multiple-rise behavior of the CN(X) formation reveals the complexity involved in the photodissocation of cyanobenzene at such a high energy near the ionization threshold. Although the results obtained in the present work are not sucient to fully unravel the mechanism, some key features involved in the reaction can still be inferred. Indeed, the tri-exponential rise behavior of the CN(X) product formation immediately suggests that there are at least three dierent dissociation pathways leading to the production of CN(X). Fig. 3 shows a schematic diagram for the energetics of some relevant reactant and product states based on the best-known literature values [9,10,14,16±18]. Although there are ®ve energetically accessible product channels along the Ph±CN
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dissociation coordinate at the total energy of 9:6 eV, only those producing the ground-state CN radicals can be detected in the present study. The initially prepared high-lying Rydberg state probably decay rapidly to the lower states through several dierent routes in a cascading manner via various intermediate states. During these processes, some states that are dissociative along the Ph±CN dissociation coordinate may be encountered and dissociation takes place in these states in competition with other decay channels. Two such states, denoted as Xa and Xb in Fig. 3, are assumed to possess such dissociative characteristics from mixing with repulsive con®gurations, such as 1 pPh ; rC±CN and 3 r; r C±CN , which can lead to the formation of CN(X) products. It is plausible that the initially prepared state ®rst undergoes an ultrafast internal conversion to the Xa state which subsequently dissociates rapidly along the Ph±CN bond to form C6 H5 A CN X. The initial delay and the ®rst rise of 350 fs observed in the CN(X) transient can be attributed to these two consecutive steps. While in the Xa state, the system can also decay through several internal conversion (IC) and intersystem crossing (ISC) steps to other lower states. When the system reaches the Xb state, it undergoes fast dissociation to form C6 H5 X CN X and gives rise to the second component of 9.8 ps. When the system ®nally reaches the lowest triplet state, T1 , it then slowly decomposes to form C6 H5 X CN X by overcoming a relatively high barrier, as illustrated in Fig. 3, and gives rise to the slowest component of 180 ps. The assignment of the slowest component to the T1 -dissociation is supported by simple RRKM calculations, which gave a time constant of 240 ps for the Ph±CN dissociation on the T1 surface in the case of no exit barrier. On the other hand, similar calculations predicted a dissociation time of 200 ns for the Ph±CN dissociation that takes place in the S0 state, i.e. three orders of magnitude slower than the experimental value. Thus, although some decay routes may indeed reach the S0 state, the dissociation timescale on the S0 surface is probably too long to be detected here. It should be noted here that other competing decay channels, such as the H-atom elimination, ring isomerization reactions, and excited-state CN
elimination channels, might also exist since the total excitation energy of 9:6 eV is certainly greater than their reaction thresholds. Although these channels may compete with the dissociation pathways described above and in¯uence the branching ratios, their existence is fully compatible with the general mechanism. The rise time of each component observed in the CN(X) transient simply re¯ects the lifetime of the corresponding dissociative state, which is determined not only by the dissociation step leading to the formation of CN(X) but also by all other decay channels. The complexity involved in the photodissociation of cyanobenzene at 9:6 eV bears some similarities to those observed for the photodissociation of halobenzenes [20,21]. For instance, Ichimura et al. [20] studied the photodissociation of chlorobenzene at 193 nm and found that the product translational energy distribution consists of three components. They suggested that the breaking of the Ph±Cl bond occurs through three channels: (1) a direct dissociation or fast predissociation; (2) a channel via vibrationally excited triplet levels; and (3) a channel via highly excited vibrational levels of the ground electronic state. This dissociation mechanism is indeed very similar to that proposed here for the photodissociation of cyanobenzene, suggesting that the photochemistry is largely determined by the aromaticity of Ph±Cl and Ph±CN. Finally, for the two-photon excitation component the total energy is essentially the same as that reached by 193 nm one-photon excitation employed by Park et al. [9]. The initial excitation reaches the S2 state, as indicated in Fig. 3, and the available energy for Ph±CN dissociation is only 86 kJ=mol. The present setup only allowed us to deduce a lower limit of 10 ns for the time constant of this two-photon component. In combination with the conclusion of Park et al. [9], one obtains 10 ns < s < 100 ns for the photodissociation of cyanobenzene at 6:4 eV. We believe that the dissociation at this energy 6:4 eV is likely to occur on the T1 surface following rapid IC and/or ISC steps from the initial state. Dissociation on the S0 surface is possible but is probably too slow to be observed in the present study [9].
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4. Conclusions
References
In this work, we have investigated the photodissociation of cyanobenzene at a high energy of 9:6 eV near the ionization threshold by monitoring the temporal evolution of the CN(X) fragments. The results revealed that the CN(X) products are formed on three very dierent timescales: s1 0:35 0:05 ps, s2 9:6 1:5 ps, and s3 180 80 ps. We proposed that the initially excited Rydberg state decays rapidly via a series of lower intermediate states in which the molecules dissociate to produce CN(X) at dierent timescales. RRKM rate calculations suggested that the slowest component of 180 ps is due to the dissociation on the lowest triplet surface. Photodissociation at the two-photon excitation level at 6:4 eV was also studied and a lower limit of 10 ns was determined for the dissociation time. Since the present study measured only the formation of the CN(X) products, the complete dynamics involved in the photodissociation of cyanobenzene at this high energy cannot be fully unraveled. A more complex mechanism involving other competing decay pathways is closer to reality. However, we believe that the general mechanism described in this work has already shed some light on the essential features and time scales of the complex dynamics involved in the photodissociation of cyano aromatic compounds.
[1] D.W. Clarke, J.P. Ferris, Origins Life Evol. Biosphere 27 (1997) 225. [2] D.A. Howe, T.J. Millar, Mon. Not. R. Astron. Soc. 244 (1990) 444. [3] R.I. Kaiser, D. Stranges, Y.T. Lee, A.G. Suits, Astrophys. J. 477 (1997) 982. [4] N. Balucani, O. Asvany, Y. Osamura, L.C.L. Huang, Y.T. Lee, R.I. Kaiser, Planet. Space Sci. 48 (2000) 447. [5] M.P. Berstein, S.A. Sandford, L.J. Allamandola, Astrophys. J. 476 (1997) 932. [6] N. Balucani, O. Asvany, A.H.H. Chang, S.H. Lin, Y.T. Lee, R.I. Kaiser, H.F. Bettinger, P.V.R. Schleyer, H.F. Schaefer III, J. Chem. Phys. 111 (1999) 7457. [7] F. Salama, E. Bakes, L.J. Allamandola, A.G.G.M. Tielens, Astrophys. J. 458 (1996) 621. [8] I.R. Lee, W.K. Chen, Y.C. Chung, P.Y. Cheng, J. Phys. Chem. A 104 (2000) 10596. [9] J. Park, C.F. Yu, R. Bersohn, Chem. Phys. 134 (1989) 421. [10] A. Mordzinski, A.L. Sobolewski, D.H. Levy, J. Phys. Chem. A 101 (1997) 8221. [11] K. Sakota, K. Nishi, K. Ohashi, H. Sekiya, Chem. Phys. Lett. 322 (2000) 407. [12] W.M. Jackson, J. Chem. Phys. 61 (1974) 4177. [13] C.K. Luk, R. Bersohn, J. Chem. Phys. 58 (1973) 2153. [14] M. Araki, S. Sato, K. Kimura, J. Phys. Chem. 100 (1996) 10542. [15] G. Herzberg, in: Molecular Spectra and Molecular Structure III. Electronic Spectra and Electronic Structure of Polyatomic Molecules, Van Nostrand Reinhold, New York, 1966, p. 341. [16] G.L. Lebel, J.D. Laposa, J. Mol. Spectrosc. 41 (1972) 249. [17] A.L. Sobolewski, W. Domcke, Chem. Phys. Lett. 250 (1996) 428. [18] J.G. Radziszewski, Chem. Phys. Lett. 301 (1999) 565. [19] Y.L. Sergeev, M.E. Akopyan, F.I. Vilesov, Opt. Spectrosc. 32 (1972) 121. [20] T. Ichimura, Y. Mori, H. Shinohara, N. Nishi, Chem. Phys. 189 (1994) 117. [21] T. Ichimura, Y. Mori, H. Shinohara, N. Nishi, J. Chem. Phys. 107 (1997) 835.
Acknowledgements This work was supported by the National Science Council of the Republic of China (NSC 892113-M-007-051) and by the Ministry of Education of the Republic of China through the Project for Academic Excellence.
Chemical Physics Letters 350 (2001) 495±501 www.elsevier.com/locate/cplett
Ultrafast photodissociation dynamics of cyanobenzene near the ionization threshold Xiu-Ping Hong, Wei-Kan Chen, Po-Yuan Cheng
*
Department of Chemistry, National Tsing Hua University, Hsinchu 30043, Taiwan, ROC Received 25 September 2001; in ®nal form 6 November 2001
Abstract The photodissociation of cyanobenzene C6 H5 CN at a high energy of 9:6 eV near the ionization threshold has been investigated using femtosecond time-resolved laser-induced ¯uorescence (LIF) spectroscopy. Cyanobenzene molecules were excited by three-photon excitation at 388.6 nm and the temporal evolution of the free CN(X) fragment formation was probed in real time by monitoring the CN X ! B LIF signal. The results revealed that the CN(X) products are formed on three very dierent timescales, suggesting that the dissociation proceeds through at least three dissociation pathways at such a high energy. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction The cyano radical (CN) is known to play an important role in the chemistry of many dierent extraterrestrial environments [1,2]. It has been proposed that the reactive CN radical can react with many unsaturated hydrocarbons to form a variety of cyano organic compounds that have been identi®ed in interstellar clouds and planetary atmospheres [3±5]. A recent laboratory study [6] employing the crossed molecular beams technique has shown that the reaction of CN X 2 R with benzene to form cyanobenzene C6 H5 CN is barrierless and exothermic. On the other hand, recent astrochemical studies have also suggested that polycyclic aromatic hydrocarbons (PAHs) are
*
Corresponding author. Fax: +886-3-571-1082. E-mail address: [email protected] (P.-Y. Cheng).
ubiquitous and abundant in many extraterrestrial environments [7]. The combination of these results has led to a hypothesis that cyanobenzene and other cyano aromatics may exist in such environments [6]. Accordingly, the photodissociation of cyano aromatic compounds at high energies may play an important role in astrochemistry because high-energy radiation is omnipresent in most extraterrestrial environments. From the fundamental point of view, the ±CN group is often considered as a `pseudohalogen', and therefore it is intriguing to compare the photochemistry of cyanobenzene with the more widely studied halobenzenes. The unsaturated substituent ±CBN may give rise to low-lying electronic excited states that are not present in its halide counterparts and lead to new photochemical pathways [8]. The C6 H5 ±CN bond energy is 533 kJ=mol [9], much stronger than typical C6 H5 ±X bond energies in halobenzenes due to the
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 1 3 4 7 - 1
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X.-P. Hong et al. / Chemical Physics Letters 350 (2001) 495±501
extensive conjugation between the CN group and the phenyl ring. Although the photochemistry of cyanobenzene in the S1 energy region has attracted some attention because of its relation to the intramolecular charge-transfer reaction [10,11], the photodissociation dynamics at higher energies has not yet been well studied. The only report, to the best of our knowledge, is by Park et al. [9] who have studied the photodissociation of cyanobenzene at 193 nm by measuring the internal state distributions of the CN fragments and the product translational energy distributions. They concluded that the dissociation occurs on a triplet surface and that the reaction time is 4 ps < s < 100 ns [9]. In this work, we studied the photodissociation of cyanobenzene using femtosecond (fs) pump± probe laser-induced ¯uorescence (LIF) spectroscopy. The reaction under investigation is Ph±CN ! Ph CN X, where Ph denotes the phenyl radical C6 H5 . The molecules were excited through a three-photon transition at 388.6 nm to a total energy of 9:6 eV. The temporal evolution of the CN(X) product formation was then probed in real time by monitoring the CN X ! B LIF signal using a delayed probe pulse also at 388.6 nm. Our results revealed that the CN(X) photofragments are produced through three very dierent temporal processes, providing important implications to the photodissociation dynamics of cyanobenzene at high energies. 2. Experimental The fs laser system employed in this work consisted of a self-mode-locked Ti:sapphire laser (Spectra Physics, Tsunami) and a chirp-pulse regenerative ampli®er (`CPA', Spectra Physics, Spit®re). The oscillator output wavelength was tuned to around 777 nm and the output pulses were selectively ampli®ed in the CPA, producing fs pulses of 1 mJ=pulse in energy and 120 fs FWHM in duration at a repetition rate of 1 kHz. The CPA output was then split into two parts by a 30/70 beamsplitter. The transmitted and re¯ected beams were both frequency-doubled in
thin BBO crystals to produce fs pulses centered at 388.6 nm for the detection of CN radicals. Neutral density ®lters were placed in the beam paths to adjust the laser irradiance such that the pulses in one beam are much more intense than those in the other beam. Hereafter, we will refer to the more intense pulses as the `pump' and the weaker ones the `probe'. For the transients presented here the probe laser was attenuated to about 10± 20 times weaker than the pump laser, resulting in a negligible negative-time signal in comparison with the positive-time signal due to the multiphoton pumping scheme employed here. A polarizer was also placed in the probe beam path to set its polarization at the magic angle (54.7°) with respect to that of the pump in order to minimize any rotational coherence eect. The probe beam was sent through a computer-controlled optical delay line. The pump and probe beams were then collinearly recombined via a neutral density beamsplitter and focused into a gas cell through a f 500 mm lens. The typical pulse energies before entering the cell were 5±20 lJ for the pump and 0:5±1:0 lJ for the probe. The cross-correlation of the pump and probe pulses was 200 fs FWHM. Cyanobenzene (Aldrich, 99%) was subjected to several freeze-pump-thaw cycles prior to use and its vapor was slowly ¯owed into the cell through a needle valve to maintain a pressure of 450 mTorr in the cell during the experiments. The laser-induced ¯uorescence was collected at right angle to the direction of the laser beams through a conventional LIF detection system. A 35 nm (FWHM) bandpass ®lter centered at 405 nm and a long-pass ®lter (380 nm cut-o) were placed in the light collection system to reduce the scattered light and to ensure that only ¯uorescence photons in the 388±422 nm spectral region can be detected eciently. The signal from the photomultiplier tube was ampli®ed and then sent to a boxcar gate integrator for signal averaging. The gate was typically 100 ns in duration and was delayed from the laser pulse to avoid interference from the scattered light signal. The transients were obtained by monitoring the ¯uorescence signal while the pump vs. probe delay time was scanned.
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3. Results and discussion Transient CN X ! B LIF signal was readily observed upon excitation of cyanobenzene vapor at 388.6 nm under the experimental conditions described above. Fig. 1 shows the observed LIF signal as a function of pump±probe delay time for three timescales. This transient ¯uorescence signal was assigned to the CN(X) fragments arising from the photodissociation of cyanobenzene on the bases of following reasons: (1) the signal was at the maximum when the probe laser wavelength was set at the bandhead of the CN X ! B 0; 0 transition near 388.6 nm; (2) the spectral ®lters used in the light collection system allowed only photons in the CN B ! X 0; 0 ¯uorescence spectral region to enter the photomultiplier tube; (3) the observed ¯uorescence lifetime is 40 ns; shorter (not longer) than the literature value of 63 ns for the CN B state under collisionless conditions [12]. The discrepancy is due to the relatively high pressure 450 mTorr of gas used here. In fact, it has been reported that the lifetime of CN B state measured in a 700 mTorr BrCN vapor is 33 ns [13], qualitatively consistent with our observations. These combined evidences unambiguously indicate that the observed transient ¯uorescence is due to free CN(X) radicals produced in the photodissociation of cyanobenzene. The observed transients apparently consist of several rise components of dierent timescales. The signal ®rst rises rapidly within the ®rst ps (see Fig. 1A) and then increases at a slower pace up to 20 ps (see Fig. 1B). The second component is then followed by an even slower rise that does not reach a constant level even at the longest delay time of 2.2 ns available here (see Figs. 1C and 2B). Since it is not energetically possible for one 388 nm photon 308 kJ=mol to eliminate CN from cyanobenzene, the observed CN fragments must be a consequence of multi-photon excitation. We have carried out pump-laser irradiance dependence measurements, shown in Fig. 2A, and found that the dependence exponent n of the transient CN LIF signal is close to three around the typical pump-laser pulse energy used here 10±20 lJ= pulse for delay times shorter than 500 ps. This strongly suggests that a three-photon excitation is
Fig. 1. CN(X) transients recorded by monitoring the CN X ! B LIF signal as a function of the pump±probe delay time upon multi-photon excitation of cyanobenzene at 388.6 nm for three dierent timescales. The three transients have been normalized to the same maximum intensity for the sake of clarity. The open circles are the experimental data points and the solid lines are results of nonlinear least-squares ®ts to a sum of a delta function at the zero delay time and a delayed molecular response function, both convoluted with a 200 fs FWHM Gaussian response function. The molecular response function consists of four exponential-rise components with the longest one being ®xed at 10 ns in the ®ts. A single set of time constants were used to obtain the best ®ts simultaneously for all three transients. The time constants obtained from the ®ts are indicated in the ®gures and the amplitude ratio obtained from ®tting the transient in (C) is A1 : A2 : A3 16 : 33 : 51.
responsible for producing the observed transients. On the other hand, we also noted that the irradiance dependence at longer times > 500 ps is slightly lower than that at shorter times (see Fig.
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Fig. 2. (A) Pump-laser irradiance dependences of the transient CN(X) LIF signal arising from the photodissociation of cyanobenzene at () 0.6 ps, () 10 ps, and (}) 900 ps. We assumed that the dependence of the transient signal S on the laser irradiance I follows S rI n . The data points were plotted on a log±log scale and the slope of the best-®tted straight line gave the dependence exponent n. (B) CN(X) transients obtained with three dierent pump-laser pulse energies; (a) 16 lJ=pulse, (b) 5 lJ=pulse, and (c) 2:5 lJ=pulse. Note that the timescale shown here is much longer than that in Fig. 1C. The three transients have been normalized to the same intensity at 2:2 ns for the sake of clarity.
2A), implying that a very slow component of lower pump-laser irradiance dependence may be present. To further clarify this point, we have measured CN(X) transients, shown in Fig. 2B, at three different pump-laser irradiances for a longer delay time of 2.2 ns. It is clear that these transients are strongly pump-laser irradiance dependent around
the pump-pulse energies used here and there exists a very slow component that becomes predominant under the very low pump-laser irradiance conditions. This extremely slow component has a lower pump-laser irradiance dependence than those of the faster components, making the overall irradiance dependence at longer times > 500 ps become lower than three. Since one 388 nm photon is not energetically capable of eliminating CN from cyanobenzene, we assigned this extremely long component to the photodissociation of cyanobenzene at the two-photon excitation level. Although it is not possible to determine the rise time of this two-photon component due to the limited optical delay available here, we estimated that it must be longer than 10 ns. On the other hand, the transient signal at shorter times < 500 ps is essentially dominated by the photodissociation of cyanobenzene at the three-photon excitation level. It is worth mentioning here that the success of separating the three-photon and the two-photon dissociation dynamics simply lies in the fact that they occur in very dierent timescales. Moreover, although four-photon excitation is also likely to occur, it ionizes cyanobenzene below its cation dissociation threshold (see Fig. 3) and thus does not produce any interfering CN(X) signal. One also notes that there is a sharp temporal feature at the zero delay time. This `initial spike' has a higher pump- and probe-laser irradiance dependence than those of the subsequent rise features. Under higher irradiance conditions, this sharp feature became much larger and we found that it resembles the cross-correlation trace of the pump and probe pulses in terms of the temporal width and position. Thus, this initial spike serves as a convenient internal reference to the zero delay time. We assigned this initial spike to the enhanced multi-photon absorption occurring when the pump and probe pulses overlap in time, which leads to the production of the ¯uorescent CN B state directly. The three transients shown in Fig. 1 were ®tted simultaneously to a multi-exponential rise function using a single set of time constants with the longest one being ®xed at 10 ns for the two-photon component described above. We also found that it is necessary to include an initial delay of 150 fs
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Fig. 3. Schematic diagram showing qualitative potential energy curves along the Ph±CN dissociation coordinate and energetics of some relevant reactant and product states. State energy levels are drawn to scale based on the best-known literature values. The cyanobenzene T1 zero-point level (ZPL) lies at about 3.35 eV above that of the S0 state, as determined by a phophorescence spectroscopic study in solid matrices [16]. The cyanobenzene S1 ZPL energy (4.53 eV) was taken from jet spectroscopic measurements [10], and the S2 ZPL energy (6.24 eV) was taken from an ab inito study using the CASPT2 method [17]. The lowest excited state ZPL energy of the phenyl radical (2.43 eV) was obtained from absorption spectroscopic measurements in low-temperature solid matrices [18]. The ionization potential of the phenyl radical (8.1 eV) was taken from photoionization measurements [19]. Curved arrows illustrate the dissociation pathways that are responsible for producing the observed CN(X) transient, as described in the text.
in order to ®t the very early part < 0:5 ps of the transient shown in Fig. 1A with acceptable quality. Careful analyses revealed that, in addition to the 10 ns two-photon component, the observed transients consist of at least three other exponentialrise components that are due to the three-photon dissociation. Trial ®ts using a smaller number of components yielded de®nitely unsatisfactory
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results. The three time constants obtained from the ®ts are: s1 0:35 0:05 ps, s2 9:6 1:5 ps, and s3 180 80 ps. Their relative weights are A1 : A2 : A3 16 : 33 : 51. Note that the exact value assumed for the two-photon component does not aect the results of the ®t signi®cantly. For example, a ®t with the two-photon component ®xed at 100 ns, which is the upper limit given by Park et al. [9], gave almost identical values for s1 and s2 but shortened s3 to 130 ps. These small variations in the time constants do not aect the general picture described below. The total energy reached by the three-photon excitation at 388.6 nm amounts to 9.57 eV, just 0.16 eV below the ionization threshold of cyanobenzene at 9.73 eV [14]. It is likely that the initial state prepared in the excitation step is a high-lying Rydberg state. The energies of Rydberg states in the vicinity of the ionization threshold can be approximated by the Rydberg formula: En IP R= 2 n d [15], where n is the principle quantum number of the Rydberg series, IP is the ionization potential, R 13:6 eV is the Rydberg constant, and d is the quantum defect. For high n's the quantum defect is much smaller than n [15]; and the formula gives n 10 for the initial state prepared by three-photon excitation of cyanobenzene at 388.6 nm. The initial delay of 150 fs and the ®rst rise component of 350 fs observed in the CN(X) formation suggest that this initially excited Rydberg state is short-lived but does not undergo direct dissociation. The multiple-rise behavior of the CN(X) formation reveals the complexity involved in the photodissocation of cyanobenzene at such a high energy near the ionization threshold. Although the results obtained in the present work are not sucient to fully unravel the mechanism, some key features involved in the reaction can still be inferred. Indeed, the tri-exponential rise behavior of the CN(X) product formation immediately suggests that there are at least three dierent dissociation pathways leading to the production of CN(X). Fig. 3 shows a schematic diagram for the energetics of some relevant reactant and product states based on the best-known literature values [9,10,14,16±18]. Although there are ®ve energetically accessible product channels along the Ph±CN
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dissociation coordinate at the total energy of 9:6 eV, only those producing the ground-state CN radicals can be detected in the present study. The initially prepared high-lying Rydberg state probably decay rapidly to the lower states through several dierent routes in a cascading manner via various intermediate states. During these processes, some states that are dissociative along the Ph±CN dissociation coordinate may be encountered and dissociation takes place in these states in competition with other decay channels. Two such states, denoted as Xa and Xb in Fig. 3, are assumed to possess such dissociative characteristics from mixing with repulsive con®gurations, such as 1 pPh ; rC±CN and 3 r; r C±CN , which can lead to the formation of CN(X) products. It is plausible that the initially prepared state ®rst undergoes an ultrafast internal conversion to the Xa state which subsequently dissociates rapidly along the Ph±CN bond to form C6 H5 A CN X. The initial delay and the ®rst rise of 350 fs observed in the CN(X) transient can be attributed to these two consecutive steps. While in the Xa state, the system can also decay through several internal conversion (IC) and intersystem crossing (ISC) steps to other lower states. When the system reaches the Xb state, it undergoes fast dissociation to form C6 H5 X CN X and gives rise to the second component of 9.8 ps. When the system ®nally reaches the lowest triplet state, T1 , it then slowly decomposes to form C6 H5 X CN X by overcoming a relatively high barrier, as illustrated in Fig. 3, and gives rise to the slowest component of 180 ps. The assignment of the slowest component to the T1 -dissociation is supported by simple RRKM calculations, which gave a time constant of 240 ps for the Ph±CN dissociation on the T1 surface in the case of no exit barrier. On the other hand, similar calculations predicted a dissociation time of 200 ns for the Ph±CN dissociation that takes place in the S0 state, i.e. three orders of magnitude slower than the experimental value. Thus, although some decay routes may indeed reach the S0 state, the dissociation timescale on the S0 surface is probably too long to be detected here. It should be noted here that other competing decay channels, such as the H-atom elimination, ring isomerization reactions, and excited-state CN
elimination channels, might also exist since the total excitation energy of 9:6 eV is certainly greater than their reaction thresholds. Although these channels may compete with the dissociation pathways described above and in¯uence the branching ratios, their existence is fully compatible with the general mechanism. The rise time of each component observed in the CN(X) transient simply re¯ects the lifetime of the corresponding dissociative state, which is determined not only by the dissociation step leading to the formation of CN(X) but also by all other decay channels. The complexity involved in the photodissociation of cyanobenzene at 9:6 eV bears some similarities to those observed for the photodissociation of halobenzenes [20,21]. For instance, Ichimura et al. [20] studied the photodissociation of chlorobenzene at 193 nm and found that the product translational energy distribution consists of three components. They suggested that the breaking of the Ph±Cl bond occurs through three channels: (1) a direct dissociation or fast predissociation; (2) a channel via vibrationally excited triplet levels; and (3) a channel via highly excited vibrational levels of the ground electronic state. This dissociation mechanism is indeed very similar to that proposed here for the photodissociation of cyanobenzene, suggesting that the photochemistry is largely determined by the aromaticity of Ph±Cl and Ph±CN. Finally, for the two-photon excitation component the total energy is essentially the same as that reached by 193 nm one-photon excitation employed by Park et al. [9]. The initial excitation reaches the S2 state, as indicated in Fig. 3, and the available energy for Ph±CN dissociation is only 86 kJ=mol. The present setup only allowed us to deduce a lower limit of 10 ns for the time constant of this two-photon component. In combination with the conclusion of Park et al. [9], one obtains 10 ns < s < 100 ns for the photodissociation of cyanobenzene at 6:4 eV. We believe that the dissociation at this energy 6:4 eV is likely to occur on the T1 surface following rapid IC and/or ISC steps from the initial state. Dissociation on the S0 surface is possible but is probably too slow to be observed in the present study [9].
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4. Conclusions
References
In this work, we have investigated the photodissociation of cyanobenzene at a high energy of 9:6 eV near the ionization threshold by monitoring the temporal evolution of the CN(X) fragments. The results revealed that the CN(X) products are formed on three very dierent timescales: s1 0:35 0:05 ps, s2 9:6 1:5 ps, and s3 180 80 ps. We proposed that the initially excited Rydberg state decays rapidly via a series of lower intermediate states in which the molecules dissociate to produce CN(X) at dierent timescales. RRKM rate calculations suggested that the slowest component of 180 ps is due to the dissociation on the lowest triplet surface. Photodissociation at the two-photon excitation level at 6:4 eV was also studied and a lower limit of 10 ns was determined for the dissociation time. Since the present study measured only the formation of the CN(X) products, the complete dynamics involved in the photodissociation of cyanobenzene at this high energy cannot be fully unraveled. A more complex mechanism involving other competing decay pathways is closer to reality. However, we believe that the general mechanism described in this work has already shed some light on the essential features and time scales of the complex dynamics involved in the photodissociation of cyano aromatic compounds.
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Acknowledgements This work was supported by the National Science Council of the Republic of China (NSC 892113-M-007-051) and by the Ministry of Education of the Republic of China through the Project for Academic Excellence.