Article pubs.acs.org/JPCA
Photoisomerization of Vibrationally Hot Tetramethylethylene Produced by Ultrafast Internal Conversion from the Excited State Motoki Sato, Shunsuke Adachi, and Toshinori Suzuki* Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan S Supporting Information *
ABSTRACT: Isomerization of tetramethylethylene (TME) following ultrafast internal conversion was investigated using time-resolved photoelectron spectroscopy with vacuum-ultraviolet probe pulses. The difference photoelectron spectrum at τ = 15 ps was reasonably well reproduced using a linear combination of static photoelectron spectra of TME and its isomers. The isomers were produced as a consequence of unimolecular reaction of vibrationally hot TME, created by internal conversion from the excited state.
I. INTRODUCTION A variety of photochemical reactions occur after internal conversion to the ground electronic state. For example, ethylene undergoes ultrafast internal conversion as a cis− trans isomerization process, and ultimately decomposes to produce H2, C2H2, and other fragments.1 Methyl-substituted ethylenes have similar ultrafast internal conversion processes, suggesting that various isomerization and dissociation reactions may occur. For example, the results of photolysis studies on tetramethylethylene (TME) in the gas2 and liquid3 phases indicate that a significant fraction of photoexcited TME isomerizes into 2,3-dimethyl-1-butene (2,3-DM-1-B), 3,3dimethyl-1-butene (3,3-DM-1-B), and 1,1,2-trimethylcyclopropane (1,1,2-TMCP). It has been reported that thermal decomposition of TME at 700−720 K produces predominantly 3,3-DM-1-B, which differs from the products generated via UV and vacuum UV (VUV) photolysis of TME.4 These classic experiments are not free from collisional effects; therefore, it is interesting to revisit the photochemical dynamics of these molecules under collision-free conditions using modern experimental techniques. One of such techniques is timeresolved photoelectron spectroscopy (TRPES) using VUV probe pulses,5 which enables ionization of reactants/products from their ground states and identification of their chemical structures. Here, we describe the application of TRPES using VUV pulses to study TME photoisomerization following internal conversion. Ultrafast internal conversion from the π3s Rydberg state via the ππ* state of the substituted ethylenes has been previously studied by Stolow and colleagues,6 while this study focuses on the isomerization process after the ultrafast internal conversion. © XXXX American Chemical Society
II. EXPERIMENTAL METHODS The 90 nm light source used in this study has been described in detail previously.7 The pulse energy was 0.2 μJ at a laser repetition rate of 1 kHz, and the photon energy was 13.6 eV. Deep-ultraviolet pump pulses at 205 nm, the fourth harmonic of a Ti:sapphire laser, were obtained using nonlinear crystals. The probability of excitation of TME by the pump pulse was kept low to minimize undesired multiphoton processes by maintaining the pulse energy below 0.1 μJ. A continuous supersonic molecular beam of TME (>96%, Tokyo Chemical Industry Co.) was formed inside a magnetic-bottle time-offlight photoelectron spectrometer by coexpanding TME with helium carrier gas from a nozzle. The 205 nm and 90 nm pulses were focused onto the molecular beam, and the pump (probe) pulse arrives first at positive (negative) delays. The photoelectrons generated were detected using a multichannel plate at the end of a 1 m long flight tube, and the signal was amplified by a preamplifier and averaged using an analog-to-digital converter. The pump−probe cross-correlation time was estimated to be 150 fs using photoionization of nitric oxide. A probe pulse with the photon energy of 4.5 eV (the third harmonic of a Ti:sapphire laser) was also prepared for a comparative study. Special Issue: A: Piergiorgio Casavecchia and Antonio Lagana Festschrift Received: January 13, 2016 Revised: March 26, 2016
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DOI: 10.1021/acs.jpca.6b00410 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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III. RESULTS AND DISCUSSION Figure 1a shows the photoelectron kinetic energy (PKE) distribution measured using two-color photoionization of TME
transition with a maximum at 185 nm and a weak transition to the π3s Rydberg state at longer wavelengths.2,8 The excited state lifetimes of the π3s Rydberg state were measured by TRPES,6 whereas that of the ππ* valence state at 184.9 nm was estimated to be 3 to 4 times shorter from a fluorescence study.9 The lifetime observed in this study at an excitation wavelength of 205 nm was between these values, as shown in the inset in Figure 1b. While the PKE region shown in Figure 1a is free from a onecolor background signal, a strong time-independent photoionization signal of unexcited TME in the ground electronic state was observed at lower PKE values ( 5 ps are mainly due to the excited state dynamics and the unimolecular reaction of highly vibrationally excited TME, created by internal conversion from the excited state, respectively.
with the 205 nm pump and 13.6 eV probe pulses; the spectrum is an average of those obtained for positive delay times of 1.5− 3.0 ps. Also shown in Figure 1a is the PKE spectrum measured using 4.5 eV probe pulses, averaged over positive delay times of 1.5−3.0 ps. The two spectra are essentially the same, except that they are shifted from each other by 9.1 eV due to the difference in probe photon energy, and they are consistent with the photoelectron spectrum measured previously by Wu et al.6 for photoionization from the π3s Rydberg state to the ground cationic state of TME at longer pump wavelength of 216−232 nm. Therefore, this ionization signal is assigned to the π3s Rydberg state [Figure 2a].
electron binding energy (eBE) is the difference between the probe photon energy (13.6 eV) and the observed PKE [Figure 3]. The PKE distribution at short delay times (τ < 5 ps) is explained by a bleach signal (blue) of the ground state and enhanced signal (red) of the excited electronic state(s) of TME, which are most simply explained using Koopmans’ picture of photoionization. The orbitals with eBEs of 8.5−10.0 eV are the π orbitals; the positive (red) and negative (blue) signals correspond to ionization of the π electrons in the neutral excited state [Figure 2b] and the ground state [Figure 2c] of TME, respectively. The spectral features for τ > 5 ps are mainly due to highly vibrationally excited TME, created by internal conversion from the excited state, and isomer products. In Figure 4, we compare the observed difference spectrum with a simulated spectrum prepared using static photoelectron spectra of the reactant (R; TME) and two possible products (P1, 2,3-DM-1-B; P2, 3,3DM-1-B). Figure 4a shows photoelectron spectra of R, P1, and P2, obtained using 90 nm pulses. The relative photoionization cross sections of the three molecules were estimated from the photoionization signal intensities and vapor pressures of the compounds (see Supporting Information for more details). Figure 4b presents a calculated difference spectrum assuming the formation of equal amounts of P1 and P2 species. While the P1 and P2 bands were too close in energy to be discriminated clearly with our experimental resolution, they were well
Figure 2. Ionization schemes of TME in our study. Photoemission from (a) the 3s Rydberg orbital and (b) π orbital in the excited state and (c) photoemission from the π orbital in the ground state.
Figure 1b shows the time profile of the photoelectron intensity averaged over the energy region hatched in Figure 1a, representing the π3s Rydberg excited state dynamics. The profile was well approximated using a single exponential decay curve with a time constant of 2.6 ps (red dashed curve). The ultraviolet absorption spectrum of TME exhibits a strong ππ* B
DOI: 10.1021/acs.jpca.6b00410 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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from the profile shown in Figure 1b. The ground state bleach signal almost vanishes at τ ∼ 6 ps owing to repopulation by internal conversion from the excited state; however, it exhibits gradual depopulation due to unimolecular decay of highly vibrationally excited states. At short time delays, both the signals at eBE of 9.5 eV and of the excited state reveal depopulation of the π3s state; however, the two time profiles deviate from one another gradually, because the signal at eBE of 9.5 eV starts to reveal the formation of the isomerized products. In other words, the difference between the signals at eBE of 9.5 eV and of the excited state is due to the formation of the isomers. Within the delay time range of this study (τ < 20 ps), the signal amplitudes at eBEs of 9.5 and 8.5 eV increased linearly, and the unimolecular reaction of vibrationally hot TME did not have time to complete. The interpretation described above is supported by curve fittings of the two time profiles of photoelectron intensities at eBEs of 9.5 and 8.5 eV using the following equation:
Figure 4. Predicted and experimental difference spectra. (a) Photoelectron spectra of the reactant (R; TME) and two possible products (P1, 2,3-DM-1-B; P2, 3,3-DM-1-B). (b) Predicted difference spectrum. (c) Experimental difference spectrum at τ = 15 ps.
I(t ) = Aex exp( −kext ) + Agr {kext + exp( −kext ) − 1} + A 0
(1)
The first and second terms correspond to the contributions of the excited state dynamics and ground state reaction of vibrationally hot TME, respectively (see Supporting Information for more details). The excited state lifetime τex = (kex)−1 was fixed to be 2.6 ps [Figure 1b]. The last constant term A0 was added to account for variation in the PKE spectrum due to vibrational excitation; however, hot TME exhibited a photoelectron spectrum very similar to that of cold TME. For τ ≫ τex, eq 1 yields I(t) ∼ Agrkext + A0, and thus describes well the linear increase by the unimolecular reaction of vibrationally hot TME. In other words, the time scale for isomerization τiso is longer than the delay time range of this study (20 ps). It is also worth mentioning the previous work by Nakashima et al., in which they reported the formation time of an allylic radical from hot TME to be 59 ns.11 On the basis of their results and those obtained in this study, it seems that the time scale for fragmentation τfrag is much longer than τiso. If so, it is reasonable not to consider the fragmentation processes in our study, although several photofragments have been identified in photolysis studies on TME in the gas phase.2,11 It would be interesting to determine τiso and τfrag experimentally to compare the product branching and isomerization rate constants with RRKM calculations in a future study. The photochemical reaction of TME examined in this study is summarized in Figure 6.
separated from the R band. The calculated spectrum [Figure 4b] agrees reasonably well with the experimental difference spectrum measured at τ = 15 ps [Figure 4c], although the photoionization cross sections of the products were larger than that of the reactant, so that the positive signal appears larger than the negative signal. Thus, the positive and negative signals were confirmed to be due to the products and depleted reactant, respectively. Note that internal conversion creates highly vibrationally excited states in the ground electronic state, and it is possible that the photoelectron spectrum of highly vibrationally excited molecules deviates from that of molecules in thermal equilibrium in terms of spectral shape and intensity. However, as we have previously shown,5 the influence of vibrational excitation on the shape of a difference photoelectron spectrum seems small. Pullen et al. have also shown that the influence of the vibrational temperature on a difference absorption spectrum is small for 1,3-cyclohexadiene.10 Figure 5 shows the time profiles of photoelectron intensities averaged over the positive (eBE = 9.5 ± 0.1 eV; red squares) and negative (eBE = 8.5 ± 0.1 eV; blue circles) signals (shown in Figure 3). Also shown with black squares is the photoelectron intensity of the π3s Rydberg state extracted
IV. CONCLUSIONS Ultrafast internal conversion and subsequent photoisomerization in the ground state of TME was studied using TRPES with VUV probe pulses. Upon photoexcitation by a 205 nm pump pulse, excited TME decays exponentially to the ground state with a time constant of 2.6 ps. Subsequently, TME isomers, such as 2,3-DM-1-B and 3,3-DM-1-B, are produced as a consequence of the unimolecular reaction of highly vibrationally excited TME. The difference photoelectron spectrum at τ = 15 ps was reasonably well reproduced using a linear combination of static photoelectron spectra of TME and its isomers. It is likely that the typical time scale for isomerization τiso is longer than the delay time range of this study (20 ps), and that for fragmentation τfrag is even longer than τiso. The photoelectron bands of the two isomers of TME were close in
Figure 5. Time profiles of photoelectron intensities at eBEs of 9.5 and 8.5 eV, and of the excited state shown in Figure 1b. Representative error bars are shown for selected data points. The dotted line at τ = 5 ps is provided to help distinguish the two contributions (see Figure 3 caption). C
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(7) Adachi, S.; Horio, T.; Suzuki, T. Generation of intense singleorder harmonic pulse in the vacuum ultraviolet region using a deep ultraviolet driving laser. Opt. Lett. 2012, 37, 2118−2120. (8) Rijkenberg, R.; Buma, W.; van Walree, C.; Jenneskens, L. Isolated building blocks of photonic materials: High-resolution excited-state photoelectron spectroscopy of jet-cooled tetramethylethylene and 1,1 ′-bicyclohexylidene. J. Phys. Chem. A 2002, 106, 5249−5262. (9) Hirayama, F.; Lipsky, S. Fluorescence of mono-olefinic hydrocarbons. J. Chem. Phys. 1975, 62, 576−583. (10) Pullen, S.; Anderson, N.; Walker, L.; Sension, R. The ultrafast photochemical ring-opening reaction of 1,3-cyclohexadiene in cyclohexane. J. Chem. Phys. 1998, 108, 556−563. (11) Nakashima, N.; Ikeda, N.; Shimo, N.; Yoshihara, K. Direct measurements of formation rate constants of allylic radical from hot olefins formed by internal conversion. I. J. Chem. Phys. 1987, 87, 3471−3481. Figure 6. Photochemical reaction of TME examined in this study.
energy and unresolved from each other; therefore, the chemical structure of the products could not be unambiguously identified in the present study. However, if the isomer bands are well separated, time-resolved photoelectron spectroscopy using VUV pulses enables identification of the products in photochemical isomerization reactions.
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ASSOCIATED CONTENT
S Supporting Information *
. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b00410. Further information on the photoelectron spectra of the reactant (TME) and two possible products (2,3-DM-1-B and 3,3-DM-1-B), and the derivation of equation 1 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported in part by the Research Foundation for Opto-Science and Technology and by the Amada Foundation.
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REFERENCES
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