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Jia-nan Fan, Ting-ting Cui, Zheng-bo Qin, Xian-feng Zheng, Zhi-feng Cui. Experimental and Theoretical Study on p-Chlorofluorobenzene in the S0, S1 and D0 States[J]. Chinese Journal of Chemical Physics , 2020, 33(4): 401-410. doi: 10.1063/1674-0068/cjcp2001005
Citation: Jia-nan Fan, Ting-ting Cui, Zheng-bo Qin, Xian-feng Zheng, Zhi-feng Cui. Experimental and Theoretical Study on p-Chlorofluorobenzene in the S0, S1 and D0 States[J]. Chinese Journal of Chemical Physics , 2020, 33(4): 401-410. doi: 10.1063/1674-0068/cjcp2001005

Experimental and Theoretical Study on p-Chlorofluorobenzene in the S0, S1 and D0 States

doi: 10.1063/1674-0068/cjcp2001005
More Information
  • The geometric structures and vibration frequencies of $ para $-chlorofluorobenzene ($ p $-ClFPh) in the first excited state of neutral and ground state of cation were investigated by resonance-enhanced multiphoton ionization and slow electron velocity-map imaging. The infrared spectrum of S$ _0 $ state and absorption spectrum for S$ _1 $$ \leftarrow $S$ _0 $ transition in $ p $-ClFPh were also recorded. Based on the one-color resonant two-photon ionization spectrum and two-color resonant two-photon ionization spectrum, we obtained the adiabatic excited-state energy of $ p $-ClFPh as 36302$ \pm $4 cm$ ^{-1} $. In the two-color resonant two-photon ionization slow electron velocity-map imagin spectra, the accurate adiabatic ionization potential of $ p $-ClFPh was extrapolated as 72937$ \pm $8 cm$ ^{-1} $ via threshold ionization measurement. In addition, Franck-Condon simulation was performed to help us confidently ascertain the main vibrational modes in the S$ _1 $ and D$ _0 $ states. Furthermore, the mixing of vibrational modes between S$ _0 $$ \rightarrow $S$ _1 $ and S$ _1 $$ \rightarrow $D$ _0 $ has been analyzed.
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Experimental and Theoretical Study on p-Chlorofluorobenzene in the S0, S1 and D0 States

doi: 10.1063/1674-0068/cjcp2001005

Abstract: The geometric structures and vibration frequencies of $ para $-chlorofluorobenzene ($ p $-ClFPh) in the first excited state of neutral and ground state of cation were investigated by resonance-enhanced multiphoton ionization and slow electron velocity-map imaging. The infrared spectrum of S$ _0 $ state and absorption spectrum for S$ _1 $$ \leftarrow $S$ _0 $ transition in $ p $-ClFPh were also recorded. Based on the one-color resonant two-photon ionization spectrum and two-color resonant two-photon ionization spectrum, we obtained the adiabatic excited-state energy of $ p $-ClFPh as 36302$ \pm $4 cm$ ^{-1} $. In the two-color resonant two-photon ionization slow electron velocity-map imagin spectra, the accurate adiabatic ionization potential of $ p $-ClFPh was extrapolated as 72937$ \pm $8 cm$ ^{-1} $ via threshold ionization measurement. In addition, Franck-Condon simulation was performed to help us confidently ascertain the main vibrational modes in the S$ _1 $ and D$ _0 $ states. Furthermore, the mixing of vibrational modes between S$ _0 $$ \rightarrow $S$ _1 $ and S$ _1 $$ \rightarrow $D$ _0 $ has been analyzed.

Jia-nan Fan, Ting-ting Cui, Zheng-bo Qin, Xian-feng Zheng, Zhi-feng Cui. Experimental and Theoretical Study on p-Chlorofluorobenzene in the S0, S1 and D0 States[J]. Chinese Journal of Chemical Physics , 2020, 33(4): 401-410. doi: 10.1063/1674-0068/cjcp2001005
Citation: Jia-nan Fan, Ting-ting Cui, Zheng-bo Qin, Xian-feng Zheng, Zhi-feng Cui. Experimental and Theoretical Study on p-Chlorofluorobenzene in the S0, S1 and D0 States[J]. Chinese Journal of Chemical Physics , 2020, 33(4): 401-410. doi: 10.1063/1674-0068/cjcp2001005
  • Halogenated hydrocarbons, which play a vital role in the human life, are the important raw materials of pesticides, refrigerants, fire extinguishers, chemical dyes and so on. However, the derivatives of halogenated hydrocarbons are highly toxic, quite stable and hard degradation [1]. They can be enriched in animals through the food chain, which will cause the accumulation of residues and endanger human health and ecological environment, such as carcinogenesis [2], destruction of the ozone layer [3], and so on. Chlorinated hydrocarbons are ubiquitous pollutants that exist in the environment as herbicide, preservative, and disinfectant [4]. Some chlorinated aromatic hydrocarbons are widely used in medicine, industry, and agriculture [5]. They also commonly exist in potable water as disinfection because of chlorination [6]. In addition, many chlorinated hydrocarbons are extremely stable and easy to accumulate [7]. The photodissociation dynamics of halogenated aromatic hydrocarbons is closely related to atmospheric chemistry and has practical significance for environmental protection, especially in the protection of ozone layer [8].

    In recent decades, many new techniques have been used in the experimental and theoretical studies on halogenated hydrocarbons. Murakami $ et $ $ al $. [9] studied the fluorobenzene and chlorobenzene by means of multi-photoionization technology to obtain the two-photon absorption spectra. Gaber $ et $ $ al $. [10] drew conclusion that the argon atom shifted towards the chlorine atoms during excitation for the ortho- and the meta- isomer while it stayed in the middle of the ring for the $ para $- isomer from the REMPI spectra, and obtained the binding energy in the ground state of ion from the MATI spectra. Borg $ et $ $ al $. [11] used femtosecond laser spectroscopy and high order $ ab $ $ initio $ CASCF/CASPT2 quantum chemical calculation to study the photochemistry of six different fluorobromobenzene compounds with low excited states, and discussed the influence of the position and number of substituents on the dissociation mechanism and degree of bromobenzene. In addition, the quantum chemical calculations of the potential energy surfaces of monohalogenated benzene [12], dibromobenzene [13], bromofluorobenzene [14] and 1, 3, 5-tribromobenzene [15] showed that the main dissociation channels involved excitation from the lowest excited singlet (non-localized ($ \pi\pi^* $) state to the triple antibond ($ \pi\sigma^* $ or n$ \sigma^* $)) state on the C-X bond. Liu $ et $ $ al $. combined the femtosecond pump-probe method with time-of-flight mass spectroscopy and photoelectron velocity mapping technique to study the photodissociation dynamics of o-dichlorobenzene in its lowest excited singlet state [16].

    Recently, resonance-enhanced multiphoton ionization (REMPI) and slow electron velocity-map imaging (SEVI) based on laser technology have become powerful tools to obtain the information of excited and cationic electronic states of molecules [17, 18, 19]. The advantages of this technique which is widely used in the study of vibrational energy levels of aryl molecules are high resolution and high efficiency. For example, Hammond $ et $ $ al $. [17] applied SEVI to the experiment of molecular spectroscopy and dynamics of toluene. Its resolution can be comparable to ZEKE spectroscopy.

    A number of halogenated hydrocarbons and their derivatives have been studied recently. Typically, it was reported that the dissociation of ortho-, meta-, and $ para $-chlorofluorobenzene and their van der Waals clusters in supersonic jets were studied via the combination of REMPI and time of flight (TOF) mass spectrometry by Numata $ et $ $ al $. [20]. Tuttle $ et $ $ al $. investigated the S$ _1 $$ \leftarrow $S$ _0 $ electronic transition of $ p $-ClFPh via REMPI spectroscopy [21]. The adiabatic ionization potential (AIP) of $ p $-ClFPh was 9.011$ \pm $0.008 eV, which was reported through the equilibrium measurement method in 1978 [22]. In addition, Kemp $ et $ $ al $. reported AIP of $ p $-ClFPh as 9.0408$ \pm $0.0006 eV and analyzed the vibrational structures in ZEKE spectra [23]. In 2018, our group reported the vibrations of meta-bromofluorobenzene in the first excited state (S$ _1 $) and the cationic ground state (D$ _0 $), also gave the adiabatic excited-state energy and AIP [24]. Meanwhile, in view of a lack of unambiguously vibrational modes in S$ _0 $ state and UV absorption spectra nature of $ p $-ClFPh, which could be useful for the environmental monitoring, we also have performed the infrared and UV absorption spectra studies on $ p $-ClFPh.

  • The experimental apparatus consists of a self-made time-of-flight mass spectrometer and velocity map imaging spectrometry which has been reported previously [25]. $ p $-ClFPh (99%) purchased from J&K Scientific was used without further purification. A supersonic molecular beam was produced by expanding the sample seeded in argon (99.999%) with a backing pressure of 4 bar through the orifice (0.5 mm diameter) of a pulsed valve (Parker, General Valve series 9) running at 10 Hz. After collimation by the skimmer with a 0.5 mm diameter, the $ p $-ClFPh supersonic molecular beam entered the interaction region between the repeller and extractor plates.

    Two different dye lasers were used to excite and ionize the molecules. The excitation laser pulse ($ \omega_1 $) was generated by frequency-doubled of the dye laser output (Sirah) pumped by the second harmonic output of Nd:YAG laser (Spectra-Physics). The ionization laser pulse ($ \omega_2 $) was generated by frequency-double of the output of dye laser (ND6000, Continuum) pumped by Nd:YAG laser (Powerlite Precision II, Continuum). Coumarins 540A and 503 were used to ionize $ p $-ClFPh. The laser bandwidth was approximate by 0.2 cm$ ^{-1} $, and the duration of the laser pulse was about 6-8 ns. Calibration of the fundamental wavelength was done with the wavemeter (SHR, Solarlaser, $ \sim $0.1 cm$ ^{-1} $).

    The one-color two-photon (2$ \omega_1 $) experiment was performed using a tunable frequency-doubling dye laser (Sirah). The two-color two-photon ($ \omega_1 $+$ \omega_2 $) experiment was performed near the S$ _1 $$ \leftarrow $S$ _0 $ transition of $ p $-ClFPh using a tunable frequency-doubling dye laser (Sirah) and another dye laser (ND6000, Continuum). The produced ions were perpendicularly accelerated by time-of-flight mass spectrometry. Ion signals were accumulated and analyzed by a multichannel scaler (MCS, SRS, SR245). The time-gated mass spectra were averaged for 100 laser shots for each wavelength. In addition, wavelength was scanned at 0.3 cm$ ^{-1} $ spacing. The region below 565 cm$ ^{-1} $ was recorded as a two-color spectrum, with the ionizing photon being 37978 cm$ ^{-1} $, while the remainder of the spectrum was recorded as a one-color spectrum. This was necessary as it is impossible to ionize via the S$ _1 $$ \leftarrow $S$ _0 $ transition in a (1+1) REMPI scheme in the first region of $ \sim $350 cm$ ^{-1} $ [20].

    In the two-color two-photon ($ \omega_1 $+$ \omega_2 $) ionization experiment, the pulse energy of the excitation laser ($ \omega_1 $) was held below 10 $ μ $J to prevent the one-color two-photon ionization process. The photoelectron signal was practically absent when only one of two laser pulses was applied to the system. Both laser pulses were linearly polarized with their E vectors perpendicular to the time-of-flight axis. The delay time between the excitation laser, the ionization laser and the pulse valve were controlled by two digital delay/pulse generators (DG535, SRS). Photoelectrons were accelerated along the time-of-flight axis in the velocity mapping condition and projected onto a home-made position-sensitive detector (50 mm diameter) coupled with a personal computer-interfaced CCD camera (Basler Scott, 782$ \times $582 pixels) system in conjunction with the photo-counting mode software interface embedded in LabVIEW code. The SEVI images were taken at low electric field condition (38 V/cm), and reconstructed through the BASEX program [26].

    Infrared spectra of liquid phase were measured at room temperature. The infrared spectra of $ p $-ClFPh were recorded using a FT-IR spectrometer (Vector 36, Brucker) at 0.4 cm$ ^{-1} $ resolution and equipped with potassm bromide window. The UV absorption spectra were achieved from SHIMADZU UV-3600Plus. The resolution of UV-3600Plus is 0.1 nm. The $ p $-ClFPh was dissolved in ethanol, and the concentration was 1.9$ \times $10$ ^{-7} $ mol/L.

    The infrared and absorption spectrum were simulated by the Gaussian 09 program package [27]. Geometry optimization and harmonic vibrational frequency calculations of $ p $-ClFPh in the S$ _0 $, S$ _1 $, and D$ _0 $ states were also performed via the Gaussian 09 program package. The B3LYP method was adopted for the calculations of S$ _0 $ and D$ _0 $ states [28], while the method with configuration interaction singles (CIS) was applied to the S$ _1 $ state. The basis set aug-cc-pVTZ was utilized in all the optimized calculations. The stationary points were characterized as the energy minimum by verifying that all the corresponding frequencies were real. The calculated vibrational frequencies were scaled by a certain factor to approximately correct the combined errors stemming from the basis-set incompleteness and vibrational anharmonicity. Moreover, the FC-Lab II suite of programs has been utilized to predict the vibrational intensity distribution in the REMPI and SEVI spectra of $ p $-ClFPh [29] in order to assign the vibrational modes [30]. The resulting REMPI and SEVI stick spectra have been convoluted by a lorentzian profile with a FWHM of 4 cm$ ^{-1} $ in REMPI spectrum and 8 cm$ ^{-1} $ in SEVI spectrum, respectively. The vibrational modes of S$ _1 $ and D$ _0 $ states in $ p $-ClFPh were compared with the ones of S$ _0 $ state in $ p $-ClFPh, respectively, via a generalized Duschinsky matrix approach using FC-Lab II.

  • FIG. 1 shows the optimized structures of $ p $-ClFPh in S$ _0 $, S$ _1 $ and D$ _0 $ states. The $ p $-ClFPh with thirty normal modes belongs to the C$ _{\rm{2v}} $ point group and the Cartesian coordinates are generalized in Table S1 (supplementary materials). The theoretical frequencies and experimental vibrations for S$ _0 $, S$ _1 $ and D$ _0 $ states are listed in Table S2 (supplementary materials) for comparison. A scaling factor of 0.97 is used. Numerous studies have shown that a great number of S$ _1 $$ \leftarrow $S$ _0 $ transitions mainly correspond to the electron-excited $ \pi^* $$ \leftarrow $$ \pi $ transitions as shown in FIG. 2. Specifically, LUMO$ \leftarrow $HOMO transition of $ p $-ClFPh mainly contributes to S$ _1 $$ \leftarrow $S$ _0 $ transition that corresponds to the excitation of $ \pi^* $$ \leftarrow $$ \pi $. As a consequence, the weakening of the $ \pi $ bond leads to the expansion of the aromatic ring in S$ _1 $ state.

    Figure 1.  Geometric configuration and labeling of the atoms for $ p $-ClFPh in neutral ground state S$ _0 $ and ionic ground state D$ _0 $ using the optimized B3LYP/aug-cc-pVTZ structure, and first excited state S$ _1 $ at the level of RCIS-B3LYP/aug-cc-pVTZ.

    Figure 2.  The highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) for $ p $-ClFPh.

  • The infrared spectrum of $ p $-ClFPh is obtained through the FT-IR spectrometer and compared with the simulated infrared spectrum at the level of B3LYP/aug-cc-pVTZ. The theoretical results are in excellent agreement with the experimental ones as shown in FIG. 3. The intense peaks appearing at 632, 823, 1088, 1229 and 1487 cm$ ^{-1} $ well match theoretical values of 624, 829, 1082, 1212 and 1477 cm$ ^{-1} $. The weak peaks appear at 1012, 1152, 1267, 1286, 1402, 1593, 3076, and 3103 cm$ ^{-1} $, agreeing well with the calculated ones of 1002, 1141, 1274, 1278, 1389, 1583, 3099, and 3103 cm$ ^{-1} $. The peaks located at 632, 1012, 1088, and 1152 cm$ ^{-1} $ are associated with the vibrations of the benzene ring, and the specific descriptions of the vibrations are summarized in Table S3 (supplementary materials).

    Figure 3.  Infrared spectrum of $ p $-ClFPh: (a) experimental values, and (b) theoretical values with a scaling factor of 0.97.

    Table Ⅰ.  Experimental and calculated absorption wavelength ($ \lambda $), excited-state energies ($ E $) and oscillator strengths ($ f $) of $ p $-ClFPh using TD and CIS methods and aug-cc-pVTZ basis set.

  • In order to determine the low-lying excited state of $ p $-ClFPh, both TD-B3LYP and CIS with a basis set of aug-cc-pVTZ calculations are performed to compare with the experimental results. FIG. 4 shows the experimental and theoretical UV absorption spectra in the range of 140-320 nm. In the experiment we observe the absorption peaks at 244 and 208 nm. The calculated vertical excited-state energies, oscillator strengths ($ f $) and wavelengths are given in Table Ⅰ. The TD-B3LYP calculation for $ p $-ClFPh yields three electronic transitions ($ f $$ > $0.01) in this energy window: the first one is 189.75 nm ($ f $ = 0.3927), the second one with moderate oscillator strength is located at 217.10 nm ($ f $ = 0.0938) and the third one with lower oscillator strength is located at 246.98 nm ($ f $ = 0.0219). The CIS computation predicts six electronic transitions ($ f $$ > $0.01): there are two strong ones distributed at 168.02 nm ($ f $ = 1.5023) and 170.23 nm ($ f $ = 1.0080), the other four absorptions ($ f $$ > $0.01) with lower oscillator strength are located at 156.97 nm ($ f $ = 0.0564), 157.66 nm ($ f $ = 0.0448), 209.86 nm ($ f $ = 0.0262) and 166.75 nm ($ f $ = 0.0162), respectively. The Multiwfn program is used to analyze the main contributions of the orbital transitions [31]. According to the absorption spectrum, the first absorption band is mainly assigned to the transition of LUMO$ \leftarrow $HOMO (near 81% using the TD-DFT method and 67% using CIS method).

    Figure 4.  Experimental and simulated UV absorption spectra of $ p $-ClFPh in ethanol solution.

    Table Ⅱ.  The comparison of experimental and calculated normal modes observed in the S$ _1 $ state of $ p $-ClFPh.

  • FIG. 5 shows the spectrum of $ p $-ClFPh ranging from 0 to 1400 cm$ ^{-1} $ in S$ _1 $ state. The origin of the S$ _1 $$ \leftarrow $S$ _0 $ transition is established at 36302$ \pm $4 cm$ ^{-1} $. The result agrees well with the result reported by Tuttle $ et $ $ al $. (36275$ \pm $2 cm$ ^{-1} $) [21]. Cvitaš $ et $ $ al $. observed the value of band origin at 36275.10 cm$ ^{-1} $ [32]. Numata $ et $ $ al $. measured the band origin at 36272 cm$ ^{-1} $ in the fluorescence excitation spectrum [20]. These reported values are close to our measured value. The S$ _1 $ band assignment based on RCIS with a basis set of aug-cc-pVTZ is quite appropriate in Table Ⅱ, as the theoretical values well match with the experimental ones.

    Figure 5.  The experimental spectrum and Franck-Condon simulated spectrum of $ p $-ClFPh within $ \sim $1500 cm$ ^{-1} $ upon the S$ _1 $$ \leftarrow $S$ _0 $ electronic transition. The band origin of $ p $-ClFPh is located at 36302 cm$ ^{-1} $.

    The intense peaks appearing at 344 and 795 cm$ ^{-1} $ are in good agreement with the previously reported values of 344 and 795 cm$ ^{-1} $ [33], which are allocated to modes 7b$ ^1 $ and 17a$ ^1 $ (see Table S4 in supplementary materials). To observe 795 cm$ ^{-1} $ band, we see that there are two intense bands here. Tuttle $ et $ $ al $. assigned the 9$ ^1 $/29$ ^2 $ vibrational modes in $ para $-fluorotoluene [33], which prompts us to consider the similar situation herein. In fact, we can identify the 14$ ^1 $ transition at 396 cm$ ^{-1} $, and then the 14$ ^2 $ transition is expected to be close to 17a$ ^1 $, so the peak at 795 and 799 cm$ ^{-1} $ are possibly assigned to modes 17a$ ^1 $ and 14$ ^2 $.

    The weak bands appearing at 261, 396, 546, 765, 863, 1142 and 1340 cm$ ^{-1} $ agree with previously reported values of 265, 397, 546, 762, 861, 1139 and 1342 cm$ ^{-1} $ [33], which are assigned to the modes 10a$ ^1 $, 14$ ^1 $, 6a$ ^1 $, 10b$ ^1 $, 1$ ^1 $, 19b$ ^1 $ and 13$ ^1 $ (see Table S4 in supplementary materials). Besides these peaks, some extremely weak bands are also identified through Franck-Condon simulation. The peaks located at 488, 703, 1109 and 1197 cm$ ^{-1} $ are tentatively assigned to the modes 16b$ ^1 $, 5$ ^1 $, 19a$ ^1 $ and 8b$ ^1 $. Additionally, we firstly mark the peak at 1033 cm$ ^{-1} $ as mode 12$ ^1 $. We also find that the combination vibrations of 16a$ ^1 $(10a$ ^1 $) and 18b$ ^1 $3$ ^1 $(19a$ ^1 $) occur at 623 and 1232 cm$ ^{-1} $ through Franck-Condon simulation. The positions and relative intensities of these bands are well reproduced by Franck-Condon simulation to confirm the above assignment in FIG. 5.

  • We determine AIP through extrapolation to the ionization threshold according to the formula:

    where $ r_\max $ is the radial distance corresponding to the maximum photoelectron kinetic energy (being associated with D$ _0 $$ \leftarrow $S$ _1 $ origin transition), $ h\nu_{\rm{pump}} $ is the energy of the pump photon, $ h\nu_{\rm{probe}} $ is the energy of the probe photon, and the AIP is the adiabatic ionization potential. As shown in FIG. 6, it can be inferred from the vertical intercept that the AIP is 72937$ \pm $8 cm$ ^{-1} $ ((9.0428$ \pm $0.0010) eV). The measured value agrees fairly well with the previously reported one [23].

    Figure 6.  The total photon energy was expressed as the squares of the radial positions of the corresponding 0$ ^0 $ peak of the $ p $-ClFPh cation. Data points were marked with empty circles. The linear regression can extrapolate this peak position to a zero radius, thereby the AIP is derived by the arrow.

    Table Ⅲ.  The comparison of experimental and calculated normal modes observed in the D$ _0 $ state of $ p $-ClFPh.

    FIG. 7(a-g) show SEVI spectra of $ p $-ClFPh recorded at seven different ionization wavelengths. The distribution of the vibrational level in the D$ _0 $ state is performed by comparing the experimental with calculated vibrational frequencies which are listed in Table Ⅲ (all vibrational mode descriptions for S$ _0 $, S$ _1 $, and D$ _0 $ states of $ p $-ClFPh are summarized in Table S2 in supplementary materials). In order to ascertain the experimentally observed frequencies, Franck-Condon simulation is predicted for comparison as shown in FIG. 8. Based on Franck-Condon analysis, the position and vibrational intensity of theoretical simulation are in general agreement with the experimental results, and the vibrational mode distributions of $ p $-ClFPh cation in D$ _0 $ state are given. The intense peaks appearing at 382, 765, 832 and 1200 cm$ ^{-1} $ agree with previously reported values of 373, 782, 821 and 1176 cm$ ^{-1} $, which are allocated to modes 7b$ ^1 $, 10a$ ^1 $, 10b$ ^1 $ and 18a$ ^1 $. Besides, weak bands located at 645 and 1140 cm$ ^{-1} $ agree with previously reported values of 658 and 1122 cm$ ^{-1} $, which are tentatively assigned to modes 6b$ ^1 $ and 8b$ ^1 $. The position and relative intensity of these bands are well reproduced by Franck-Condon simulation to confirm the above assignment (FIG. 8). These observed frequencies are also summarized in Table Ⅲ. Besides these peaks, the bands appearing at 419 and 998 cm$ ^{-1} $ are in good agreement with the previously reported values of 421 and 987 cm$ ^{-1} $, which are assigned to the modes 14$ ^1 $ and 17a$ ^1 $. The peaks observed in SEVI spectrum of $ p $-ClFPh and vibrational modes are listed in Table Ⅳ.

    Figure 7.  2C-R2PI SEVI images (left column) and corresponding spectra (right column) of $ p $-ClFPh. After the ionization of $ p $-ClFPh in the 0$ ^0_0 $ level in S$ _1 $, the recorded ionization wavelength is (a) 271.42 nm, (b) 269.87 nm, (c) 267.32 nm, (d) 266.82 nm, (e) 266.06 nm, (f) 265.31 nm, (g) 263.81 nm. The photoelectron images are reconstructed images by inverse Abel transformation. The double arrows indicate the directions of the laser polarization.

    Figure 8.  Franck-Condon simulation of $ p $-ClFPh in D$ _0 $ state at the level of aug-cc-pVTZ (blue line) in comparison with experimental spectra (black: 263.81nm, purple: 266.06 nm, green: 267.32 nm).

    Table Ⅳ.  The vibration assignments of $ p $-ClFPh observed in the D$ _0 $ state (in cm$ ^{-1} $).

  • In order to comprehend the multimode mixing mechanism of vibrations (the Duschinsky rotation) in S$ _0 $, S$ _1 $ and D$ _0 $ states during the excitation and ionization processes, the Duschinsky mixing analysis is performed. FIG. 9(a, b) show the Duschinsky matrices of $ p $-ClFPh representing the vibrational mode relationship between S$ _0 $ and S$ _1 $ states, and between S$ _1 $ and D$ _0 $ states, respectively. The corresponding information of FIG. 9(a, b) is also summarized in Table Ⅱ and Table Ⅲ. The redder their color is which means that the vibrational mode description is purer. In FIG. 9(a), we can find that most of the 30 normal vibrational modes in the S$ _1 $ state are similar to those in the S$ _0 $ state and only 6 of 30 normal vibrational modes appear slightly Duschinsky mixing. In those modes, 4 of S$ _1 $ modes are a two-mode mixing in S$ _0 $ state, and 2 out of S$ _1 $ modes are a three-mode mixing in S$ _0 $ state with quite Duschinsky mixing. From Table Ⅱ, it is not difficult to see that Q$ _{22} $, Q$ _{23} $, Q$ _{26} $ significantly contribute to the mode mixing, with mainly CH vibration and benzene ring vibration.

    Figure 9.  The matrix representations of the Duschinsky analysis correlating (a) the ground state S$ _0 $ normal modes and the excited state S$ _1 $ normal modes of $ p $-ClFPh, and (b) the excited state S$ _1 $ normal modes and the ground state D$ _0 $ normal modes of $ p $-ClFPh.

    Compared to FIG. 9(a), the multi-mode mixing in FIG. 9(b) is slightly serious. There are 20 out of D$ _0 $ modes exhibiting a ground state character ($ \geq $95%) (see Table Ⅲ). Also 6 of 30 normal vibrational modes appear to be slightly Duschinsky mixing. Specifically, 3 of D$ _0 $ modes are a two-mode mixing in S$ _1 $ state, and 3 out of D$ _0 $ modes are a three-mode mixing in S$ _1 $ state with quite Duschinsky mixing. Among them, Q$ _{22} $ and Q$ _{23} $ make significant contribution to the mode mixing, being associated with CH vibration and benzene ring vibration.

    The photoelectron angular distribution (PAD) is obtained by integrating the intensity of the Abel-inverted image. The PADs in two-photon ionization with linearly polarized light are generally described by the function [34, 35]:

    where $ \theta $ is the angle between the electron velocity vector and the laser polarization direction in the laboratory frame, $ k $ is a normalization constant proportional to the total photoionization cross-section, $ \beta_2 $ and $ \beta_4 $ are the anisotropy parameters associated with the second- and fourth-order Legendre polynomials $ P_2 $ and $ P_4 $, respectively, which can be determined by fitting Eq.(2).

    The PAD is determined by photoelectron scattering wave, which varies with the vibration dynamics of photoelectron kinetic energy (PKE) and ionized cations. The energy dependence of anisotropy parameters on PKE is shown in FIG. 10. The $ \beta_2 $ decreases with the increase of PKE, and the measured $ \beta_2 $($ E $) for D$ _0 $$ \leftarrow $S$ _1 $ ionization process is negative when the PKE$ > $1200 cm$ ^{-1} $ above the ionization threshold.

    Figure 10.  The photoelectron angular anisotropy parameters ($ \beta_2 $ and $ \beta_4 $) obtained from the images pumping by S$ _1 $ $ 0^0_0 $ level as shown in FIG. 6 as a function of PKE. The solid and hollow symbols indicate $ \beta_2 $ and $ \beta_4 $ values, respectively.

    Near the ionization threshold, the value $ \beta_2 $ of $ para $-difluorobenzene decreases rapidly with the increase of PKE until it becomes negative, and it is suggested that the behavior of PAD trend near the ionization threshold is a consequence of the shape resonance. They concluded that the shape resonance was caused by the MO $ \pi^* $ orbital with high order angular momentum quantum number ($ l $$ > $1) [36, 37, 38]. This resonance was related to a $ \pi_{\rm{g}}^* $ (LUMO)$ \leftarrow $$ \pi_{\rm{u}} $ (HOMO) transition, which clearly showed the essential characteristics of d$ \pi_{\rm{g}} $ [39]. FIG. 11 shows the LUMO $ \pi^* $ orbital of $ para $-difluorobenzene. To compare with the case of $ p $-ClFPh (see FIG. 2), $ \pi^* $ antibonding (S$ _1 $ state) is similar to $ para $-difluorobenzene. The LUMO $ \pi^* $ orbital of $ p $-ClFPh in the ionization of D$ _0 $$ \leftarrow $S$ _1 $ exhibits a clear $ l $ = 3 character. Considering the shape resonance from the perspective of trapping by a centrifugal barrier due to the relatively high ($ l $ = 3) in this case, a significantly larger centrifugal barrier exists in the state and the value of the continuous resonant wavefunction effectively captures the electron at a short range. Therefore, it indicates that this behavior of PAD trend near threshold in $ p $-ClFPh should be a consequence of the shape resonance. All values of $ \beta_2 $ and $ \beta_4 $ for the following excitation of various vibrational intermediate states are available in Table S5 (supplementary materials).

    Figure 11.  The LUMO $ \pi^* $ orbital of $ para $-difluorobenzene.

  • We perform high-level theoretical prediction of the spectra and energy properties of $ p $-ClFPh. The infrared spectrum in S$ _0 $ state of $ p $-ClFPh is also studied, the vibrational modes of which are identified. According to the UV absorption spectrum, we get the transition probability mainly from HOMO to LUMO transition. Based on REMPI and SEVI spectroscopy and theoretical calculations, the geometric structures and vibrational frequencies of $ p $-ClFPh in the first excited state of neutral and ground state of cationic are studied in detail. The REMPI spectrum of $ p $-ClFPh gives the S$ _1 $$ \leftarrow $S$ _0 $ electronic transition energy, (36302$ \pm $4) cm$ ^{-1} $. The S$ _1 $ band assignment based on RCIS with a basis set of aug-cc-pVTZ is quite appropriate in the aid of Franck-Condon simulation. In the 2C-R2PI SEVI spectra, the accurate AIP is obtained to be (72937$ \pm $8) cm$ ^{-1} $ of $ p $-ClFPh through extrapolation to the ionization threshold. The vibrational frequencies in D$ _0 $ state calculated at the B3LYP/aug-cc-pVTZ level are in excellent agreement with the experimental ones and give the unambiguous assignment. More quite Duschinsky mixing existing in the D$ _0 $$ \leftarrow $S$ _1 $ ionization compared to that in the S$ _1 $$ \leftarrow $S$ _0 $ excitation indicates that more rotation of the vibrational coordinates occurs for the D$ _0 $$ \leftarrow $S$ _1 $ ionization than for the S$ _1 $$ \leftarrow $S$ _0 $ excitation.

    Supplementary materials: DFT calculated coordinates, vibrational frequencies and corresponding desriptions of $ p $-ClFPh in S$ _0 $, S$ _1 $ and D$ _0 $ states are shown.

  • This work was supported by the National Natural Science Foundation of China (No.11674003, No.21873003, No.21503003, No.11704004, and No.61475001), Anhui Natural Science Foundation (No.1908085QA17). We also acknowledge additional support from Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) (No.U1501501) and Super Computation Center of Shenzhen.

Reference (39)

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