Photodissociation Dynamics of Diacetylene Rydberg States - The

Article pubs.acs.org/JPCA

Photodissociation Dynamics of Diacetylene Rydberg States Hongzhen Wang,# Shengrui Yu,# Shu Su, Dongxu Dai, Kaijun Yuan,* and Xueming Yang* State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China ABSTRACT: The state-selective photodissociation of diacetylene (C4H2) was studied in the wavelength range of 127.5−164.4 nm by high-resolution Rydberg H atom time-of-flight spectroscopy measurements. In the wavelength region, two Rydberg series nR and nR′ were state-selectively excited using tunable vacuum-ultraviolet laser radiation. In all photolysis wavelengths, two decay channels with different dissociation dynamics were observed. In one channel, the characteristic and isotropic translational energy distributions with a peak around 1800 cm−1 can be found, suggesting statistical dissociation through internal conversion (IC) from the Rydberg state to the ground state and then dissociation on the ground-state surface. In contrast to this, in the second channel, nonstatistical and anisotropic translational energy distributions were observed, possibly through IC to the excited repulsive state. The vibrational progressions of C4H (A2Π) products have also been observed and assigned to the CCC bend and CC stretch progressions in the second channel at 3R Rydberg states. have been indicated.14 Excitation of one electron from HOMO to LUMO orbitals (1πg → 2πu*) gives rise to Δu, Σu+, and Σu− states.15 The experimentally observed spectrum of C4H2 between 195 and 225 nm shows structured vibrational progressions, which was assigned to a 1Δu ← 1Σg+ forbidden electronic transition.11 At wavelengths higher than 200 nm, no free radicals were detected, and polymeric material was found to coat the inside of the reaction cell.16,17 It was suggested that a metastable state has been invoked as the precursor to the formation of long-chain polyyne species, well below the dissociation threshold of C4H2. Recently, Zwier and coworkers18,19 extensively investigated the UV photochemistry of C4H2 through reactions in a pulsed nozzle. The formation of various larger hydrocarbons has been found following excitation to the 1Δu state. The photochemistry is thought to proceed from a metastable excited state of diacetylene, which is assumed to account for the observed chemistry in models of Titan’s atmosphere. However, the recently measured submicrosecond lifetimes suggested that reactions of metastable C4H2* are likely to be less important in Titan’s atmosphere than previously believed.9,20,21 The second spectrum region, between 115 and 175 nm, shows a succession of several strong absorption peaks, which belong to the R and R′ Rydberg series with vibrational excitation. At the equilibrium geometry of the ground electronic state, the lowest-energy Rydberg states of diacetylene are expected to arise from 1πg → 3s and 1πg → 3p excitations.15 However, the 1πg → 3s transition must be forbidden in one-

1. INTRODUCTION Diacetylene (C4H2) is one of the largest molecular species identified in planetary atmospheres1−4 and is considered as a precursor to higher polyynes and polycyclic aromatic hydrocarbons in interstellar environments.5−7 It has been recognized as a key species in Titan’s atmosphere because it absorbs light at much longer wavelengths, where the solar flux is intense, than acetylene, ethane, or other important trace constituents of the atmosphere.8 It is now known that the threshold wavelength for single-photon dissociation of diacetylene is about 5.77 eV (215 nm).9 At the photolysis energy higher than 5.77 eV, molecule C4H2 should be photodissociated to C2, C2H, and C4H radicals that can induce catalytic hydrogen abstraction cycles. Thus, understanding the photodissociation dynamics of C4H2 is quite important to reveal the factors driving the chemistry of Titan’s atmosphere. In the region below the first ionization limit, the thermochemical thresholds of different photodissociation processes are shown as follows C4 H 2 → C4 H + H D0 = 5.77 eV (1) C4 H 2 → C2 + C2H 2

C4 H 2 → C2H + C2H

D0 = 6.06 eV

D0 = 6.93 eV

(2) (3)

Among the above channels, previous theoretical studies have shown that H loss channel dominates (>68%) at the photolysis energy of 7−10 eV.9,10 In the last 30 years, the photochemistry of C4H2 has been investigated in a series of photolysis wavelengths.11−13 Diacetylene in its ground electronic state has a linear (D∞h) geometrical structure and a (1πu)4(1πg)4(2πu*)0(1σu*)0(1σg*)0(2πg*)0 electronic configuration, where the occupied σ orbitals have been suppressed and the low-lying unfilled MOs © XXXX American Chemical Society

Received: September 11, 2015 Revised: October 21, 2015

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DOI: 10.1021/acs.jpca.5b08865 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

3. RESULTS AND DISCUSSION 3.1. Photodissociation of C4H2 at 3R Rydberg States. Time-of-flight (TOF) spectra of the H atom products were obtained from the photodissociation of C4H2 using the experimental technique described above. Figure 1 shows the

photon excitation because the gerade ground state of a centrosymmetric molecule must populate ungerade excited state. The 1πg → 3p orbit promotions will give Rydberg states of u symmetry. For example, the strong peaks at 164.41, 159.10, and 154.04 nm, which arise from 1Σu ← 1Σg+, can be assigned to 3R2°, 3R21, and 3R22, respectively (notation: 3R: n = 3 level of the R Rydberg series and 2n means the molecule has the vibrational excitation ν2 = n; ν2 is the CC stretching vibration). Photodissociation of C4H2 in this region is more complex. Glicker et al.16 studied the primary dissociative processes of C4H2 photodissociation at the wavelength 147 nm. The quantum yield was determined to be 0.2, 0.1, and 0.03 for channels 1−3, respectively. Recently, Silva and co-workers9 investigated the H elimination of C4H2 using DC slice ion imaging detection at 243, 212, and 121.6 nm photolysis. The slow, isotropic product velocity distributions at all three wavelengths investigated were reported. They attributed the products observed when exciting at the two near-UV excitation wavelengths to dissociation following two-photon absorptions. So far, however, no experimental study has been carried out on those Rydberg states in the VUV region between 127 and 164 nm. In our previous paper,22 we reported the photodissociation dynamics of C4H2 at 164.41 nm, and two distinct dissociation pathways have been observed. Here, we further report the experimental results of photodissociation in a series of C4H2 Rydberg states.

2. EXPERIMENTAL METHODS In this work, photodissociation dynamics of C4H2 have been studied using the H atom Rydberg tagging time-of-flight (HRTOF) technique combined with a narrow-band, tunable VUV light source. This technique has been used to investigate several photodissociation processes of small molecules, like CH4,23 C2H2,24 H2O,25−30 and HNCO.31 Briefly, the H atom products in the photolysis region from the photodissociation of C4H2 in the molecular beam were excited from the ground state to a high Rydberg state (n ≈ 50) via a two-step excitation. The first excitation step was made by the 121.6 nm coherent light generated using the difference four-wave mixing (DFWM) of 212.5 and 845 nm in a Kr cell in which two photons of 212.5 nm were in resonance with a Kr 4p-−5p[1/2,0] transition.32 In the second excitation step, the H atom products were then sequentially excited to a high Rydberg state using 365 nm radiation. The VUV photolysis source was also generated using DFWM in the same mixing cell for generating 121.6 nm.33 Because 121.6 nm also generates H atom signals, background subtraction was achieved by alternating the photolysis laser on and off. The neutral Rydberg tagged H products then flew ∼74 cm to reach a MCP detector and were ionized by field ionization (∼2 kV/cm). The detection axis, the molecular beam, and the photolysis laser beam were mutually perpendicular. Great efforts have been made on the optimization of the performance of the pulsed valve. A short beam pulse with a fast rise time (∼80 μs) is very important to minimize the C4H2 cluster in the molecular expansion. C4H2 was prepared via the method of Armitage et al.34 1,4Dichloro-2-butyne (Aldrich), aqueous potassium hydroxide, and dioxane were heated between 80 and 100 °C under reflux. The evolved gas was dried over CaCl2 and collected in a liquid nitrogen trap without further purification.

Figure 1. TOF spectra of the H atom products from the photodissociation of C4H2 at 164.41 (A), 159.10 (B), and 154.04 nm (C) with photolysis laser polarization parallel and perpendicular to the detection axis.

TOF spectra of the H atom products at the photolysis wavelength of 164.41, 159.10, and 154.04 nm, which have been assigned to the 3R2°, 3R21, and 3R22 transitions, respectively,35 with the photolysis laser polarization parallel (the angle between the linear polarization and the TOF axis, θ = 0°) and perpendicular (θ = 90°) to the flight path. TOF spectra of the H atom products at the magic angle (θ = 54.7°) polarization were also measured to ensure that the intensity ratio of the TOF spectra obtained at parallel and perpendicular polarization is correct. It is clear that two distinct features have been observed in the parallel direction at the photolysis wavelengths between 154 and 164 nm. One is a strong peak with partially resolved structures at the earlier arriving time, and the other is a broad component underlying the resolved progressions and extending up to the arriving time limit, while the spectra in the perpendicular direction at three wavelengths present broad continuum distributions with some small peaks at the beginning of the spectra. These TOF spectra can be converted into the total translational energy distribution spectra of the photodissociation products (H and C4H) in the centerof-mass frame through conservation of momentum by using ET = B

m ⎞⎛ d ⎞ 2 1 ⎛ mH⎜1 + H ⎟⎜ ⎟ 2 ⎝ mR ⎠⎝ t ⎠

(4) DOI: 10.1021/acs.jpca.5b08865 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A where mH and mR are the mass of the H atom and the cofragment C4H, d is the path length from the interaction region to the detector (d = ∼74 cm), and t is the measured TOF. Figure 2 shows the translational energy spectra from

E int(C4 H) = hν + E int(C4 H 2) − D0(H−C4H) − E T

(5)

where hν is the photon energy of the photolysis laser and Eint (C4H2) is the internal energy of the C4H2 molecule, which was set to about zero because the C4H2 molecule is cooled down to very low temperature in the supersonic expansion. Using this equation, the product translation energy distribution can be converted to the C4H product internal energy distribution. From eq 5, if the maximum translational energy limit (Emax T ) in the product translational energy distribution corresponds to the C4H ground rovibrational state, that is, Eint (C4H) = ∼0, D0(H−C4H) should be equal to hν − Emax T . (We also can use the first sharp peak, which can be assigned as C4H (A, (ν′2, ν′6, ν′7) = (000)) and Eint (C4H(A, 000) = ∼ 222 cm−1). The extrapolated onset of the maximum in Figure 3 yields D0(H−

Figure 2. Translation energy distributions (ET) from the photodissociation of C4H2 at 164.41 (A), 159.10 (B), and 154.04 nm (C) with photolysis laser polarization parallel and perpendicular to the detection axis.

Figure 3. Translational energy spectrum for photodissociation of C4H2 at 164.41 nm (magic angle 54.7°). The red and blue curves fitted to the spectra represent the signals from two distinct dissociation pathways. (Reprinted with permission from ref 22, copyright 2013.)

C4H2 photodissociation at three photolysis wavelengths. It is clear that the translational energy spectra show a bimodal distribution. One is a statistically distributed translational energy profile with the peak at low translational energy (∼1800 cm−1), and the other is a series of strong peaks with well-resolved progression at the high translational energy region. The structured component is less pronounced in the perpendicular direction, which means that this component is strongly anisotropic. The translational-energy-dependent angular anisotropy parameter β is shown in Figure 6. It is obvious that the slow component shows an isotropic angular distribution with the β value of around 0, while the fast component with resolved structures shows a large β value of about 1.5 near the energetic limit. The energy-dependent branching ratios of the two components cause the β value to be 0−1.5 at the translational energy of 9000−12000 cm−1. As reported in our previous paper,22 the slow and statistical translational energy profile, with the peak at about 1800 cm−1 and extending to the energetic limit, probably comes from the dissociation process on the ground state following internal conversion (IC) from the initially excited Rydberg state to the ground state S0. Such a dissociation time is long enough to allow for complete internal energy randomization. Because the momentum and energy are conserved in the photodissociation process, the product internal energy (Eint) deposited into the C4H product can be determined

C4H) = 45938 ± 100 cm−1(5.69 ± 0.2 eV). The error limit is due to the uncertainty in determining the threshold (half-width of the first peak), the photolysis laser line width, and the resolution of the TOF spectrometer. Our present result is very close to the recent theoretical value obtained by Suits et al.,9 D0(H−C4H) = 5.77 eV. From the translational energy distributions (Figures 3−5), peak spaces of the structural component are ∼220 cm−1 at around 14000 cm−1. From the previous spectroscopic studies,36−38 the first electronic state of C4H (A2Π) would lie very close to the ground state C4H (X2Σ+). The photoelectron spectrum studies39 also reported the frequencies of ν′7 (CCC bend) = 179 cm−1 for the X2Σ+ state and ν′7 = 220 cm−1 for the A2Π state; the latter frequency is closer to our observation. This means that the structures can be assigned to the vibrational progression the of ν′7 mode of the electronic excited C4H (A2Π) radicals. As reported in our previous paper,22 the two main peaks in Figure 3 have been assigned to the A2Π (ν′2 = 0, ν′6 = 0, ν′7) and A2Π (1, 1, ν′7) levels, where ν′2 is CC stretch vibration (∼2081 cm−1) and ν′6 is the H−CC bending vibration (∼401 cm−1). The similar assignment also can be found in Figures 4 and 5. It is interesting that the most intense peaks are A2Π (1, 1, ν′7) states and A2Π (2, 2, ν′7) states, with the photolysis laser tuned to the 3R21 and 3R22 transitions, respectively. It means that the vibrational energy of C


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the strong vibronic coupling of the X and A states of the C4H radical. Such a phenomenon has also been observed in photoelectron spectra of the C4H− anion studied by Neumark and co-workers.39 To clarify this, we still need further theoretical calculations. These fast structured components are surely one-photon excitation to the Rydberg state, followed by radiationless transfer to a repulsive potential energy surface (S1) that correlates to H + C4H (A) products. The measured β value (∼+1.5) implies a parallel excitation, 1Σu ← X1Σ+g and that the radiationless transfer should be fast. The continuum probably arises from a σu* ← πg excitation, and the resulting 1Πu state will correlate with H + C4H (A) products. Such processes are also observed in VUV photolysis of C2H241 and HCN.42 Therefore, from the translational energy distributions and angular distributions, two dissociation pathways can be obtained. When C4H2 molecules are excited to the 3R Rydberg states, some molecules will dissociate to H + C4H (X2Σ+) on the ground state S0 through IC from the Rydberg state to the S0 state (pathway P1). This slow component with β ≈ 0 is consistent with unimolecular dissociation whose time scale is longer than the rotational period of the C4H2 molecule. The other molecules will dissociate on the excited repulsive state S1 (predissociation from 3R states to the excited repulsive state, pathway P2). The fast component with β ≈ 1.5 should be from a rapid dissociation process whose time scale is shorter than one rotational period of the parent molecule. The deconvolution results are shown in Figures 3−5. The branching ratios of the C4H radicals from the fast dissociation process (pathway P2) are estimated to be ∼0.2, 0.25, and 0.18 with the photolysis wavelength of 164.41, 159.10, and 154.04 nm, respectively. The average translational energy release of the fast component is ⟨f T⟩ ≈ 0.76, 0.68, and 0.60, and for the slow component, ⟨f T⟩ ≈ 0.27, 0.25, and 0.23 with the photolysis wavelength of 164.41, 159.10, and 154.04 nm, respectively. It is interesting that the overall dissociation dynamics of C4H2 are similar to each other as the vibrational excitation of the parent molecule increases. The observed difference is that the deposited translational energy deceases and internal energy increases in both pathways as the vibrational excitation of the parent molecule increases. 3.2. Photodissociation of C4H2 at Other Rydberg States. Besides dissociation at 3R Rydberg states, the tunable VUV source was used to dissociate diacetylene at a number of other transitions between 127 and 144 nm. Figure 7 shows the translational energy spectra of the H atom products at photolysis wavelengths of 135.76, 130.38, and 127.50 nm, which can be assigned to the 4R2°, 5R2°, and 6R2° transitions, respectively. Dissociation from the nR2° state produces spectra similar to those from the 3R state. As in all other spectra, a statistically distributed translational energy profile with the peak at low translational energy (∼1800 cm−1) and extending to the maximum available energy is observed, and such a slow component has the same polarization dependence (β ≈ 0), as shown in Figure 9. This means that the dominate dissociation pathway is also P1 in these Rydberg states. However, the fast component of translational spectra is different from that observed in the 3R Rydberg state. The fast component of spectra taken following excitation of the 4R2° states has a positive β value with the maximum of 1.3, similar to that observed in the 3R Rydberg state, which means that the dissociation process is quite fast, while those of the 5R2° and 6R2° states have a negative β value with the maximum of −0.8. The negative angular anisotropy parameter indicates the

Figure 4. Translational energy spectrum for photodissociation of C4H2 at 159.10 nm (magic angle 54.7°). The red and blue curves fitted to the spectra represent the signals from two distinct dissociation pathways. The observed structures can be assigned to the C4H (A2Π) radicals with vibrational excitation (ν′2, ν′6, ν′7), ν′2 −CC stretch, ν′6 −HCC bend, and ν′7 −CCC bend.

Figure 5. Translational energy spectrum for photodissociation of C4H2 at 154.04 nm (magic angle 54.7°). The red and blue curves fitted to the spectra represent the signals from two distinct dissociation pathways. The observed structures can be assigned to the C4H (A2Π) radicals with vibrational excitation (ν′2, ν′6, ν′7), ν′2 −CC stretch, ν′6 −HCC bend, and ν′7 −CCC bend.

the parent molecule (3R2n, ν2 = n) directly deposits to the radical C4H with the ν′2 vibrational excitation. The fast component of the translational energy distribution from C4H2 photodissociation with a large β value suggests that the dissociation from the electronically excited Rydberg state 3R has a prompt dissociation pathway. The equilibrium geometry of the ground state C4H2 is linear, so is the C4H (X2Σ+) and C4H (A2Π) product.36 Schwell et al.40 also reported that the low-lying excited states of the centrosymmetric diacetylene ion are linear and belong to the D∞h symmetry group as does the group state of the neutral species. Rydberg states of dicaetylene converging to these ion states will also be linear. This means that the observed bending and stretching vibrational excitation of C4H (A2Π) product may comes from D


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Figure 6. Anisotropy parameter β as a function of the total translational energy for the photodissociation of C4H2 at 164.41, 159.10, and 154.04 nm.



Figure 9. Anisotropy parameter β as a function of the total translational energy for the photodissociation of C4H2 at wavelengths between 144.55 and 127.50 nm.

prompt dissociation on the repulsive surface through IC from the nR Rydberg state to the repulsive state (S1). The surface hopping possibility decreases because the energy gap between the initially excited Rydberg state and the repulsive state increases with the excitation energy increasing. Figure 8 shows the translational energy spectra of the H atom products at photolysis wavelengths of 144.55, 132.86, and 128.66 nm, corresponding to the 3R′2°, 4R′2°, and 5R′2° transitions, respectively. The spectra also have two components; the slow component shows a statistically distributed

perpendicular transition followed by a direct dissociation on a repulsive potential energy surface. Few structures have been observed in these three Rydberg states in the high translational energy region. This is because much more internal energy obscures the structures at high excitation energy. It is interesting that the branching ratio of theh fast component of nR (n > 3) states is obviously smaller than that observed in the 3R state, and the maximum is ∼0.08 (4R state). The branching ratio of the fast component decreases as the excitation energy increases. This is because the fast component comes from a E



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translational energy profile with the peak at low translational energy (∼1800 cm−1) and extending up to the energetic limit; the fast component is only observed in the parallel direction, and partially resolved structures in this part can be observed at the 3R′2° state. The branching ratio of the fast component obviously decreases as the excitation energy increases. The translational-energy-dependent anisotropy is similar to that observed in the 3R state. These behaviors also lead to the assumption that there are two pathways for photodissociation; the first channel is connected with vibrational excited C4H (A) radicals giving maximum signal at parallel polarization. The other channel shows a more statistical distribution of translational energy and gives isotropic signal. As observed in the nR state, the branching ratio of the fast component decreases as the excitation energy increases. It is worth mentioning that the translational energy spectra at shorter photolysis wavelengths are more and more similar to that observed at 121.6 nm photolysis studied by Suits and coworkers.9 This means that at shorter wavelengths, the statistical dissociation from the ground state would be much more pronounced.

REFERENCES

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4. CONCLUSIONS There are two distinct dissociation channels observed in the VUV photolysis of diacetylene. In one channel I, a highly structured C4H internal energy spectrum is observed following formation of vibrationally excited C4H fragments as a result of a rapid dissociation and dynamical constraints in the dissociation pathway. Symmetry arguments point to the production of C4H (A2Π) radicals. The population of C4H quantum states is found to depend strongly on the excited electronic level of diacetylene and also on the excitation of the CC stretch vibration within the different Rydberg states. As the excitation energy increases, the high internal energy of the C4H radical obscures the vibrational structure. In channel II, a characteristic translational energy profile with isotropic angular distribution is observed. This is interpreted as an IC to the ground state and then dissociation on the ground state. In the all-photolysis wavelength that we used, this channel always dominates. No information about high-lying dissociative potential energy surfaces or bound surfaces was found. For a more detailed analysis of the present results and understanding of the dynamics, such information is valuable.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.Y.). *E-mail: [email protected] (X.Y.). Author Contributions #

H.W. and S.Y. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are very grateful for the support of this work by the National Natural Science Foundation of China (No. 21133006, 2013CB834604), the Chinese Academy of Sciences, and the Ministry of Science and Technology. Y.S.R. is supported by the China Postdoctoral Science Foundation (No. 2014M551810 and 2015T80659). F


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Photodissociation Dynamics of Diacetylene Rydberg States - The

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